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Long read but good general to detailed information. I don't expect anyone to finish reading this in one day. Copy it, take your time and look up definitions on words that you don't understand. This is a useful tool and resource for those who truly want to understand what is going on. Reading is FUNdamental
I will post on Estrogens next week.
ANDROGENS
Chapter 2 - David J Handelsman MB BS, FRACP, PhD
INTRODUCTION
Testosterone is the principal androgen in the circulation of mature male mammals. An androgen, or male sex hormone, is defined as a substance capable of developing and maintaining masculine sexual characteristics (including the genital tract, secondary sexual characteristics and fertility) and the anabolic status of somatic tissues. Testosterone and synthetic androgens based on its structure are used clinically at physiological doses for androgen replacement therapy and, at higher doses, for pharmacological androgen therapy. The principal goal of androgen replacement therapy is to restore a physiological pattern of androgen exposure to the body. At present, such treatment is restricted to the major natural androgen, testosterone, and aims to deliver testosterone to replicate physiological circulating testosterone levels. Thus, an understanding of the normal physiology of testosterone is required as a basis for androgen pharmacology 1.
TESTOSTERONE: PHYSIOLOGY
Biosynthesis
Testosterone is synthesized by an enzymatic sequence of steps from cholesterol 2 (figure 1) within the 500 million Leydig cells located in the interstitial (intertubular) compartment which constitutes about 5% of mature testis volume 3. The cholesterol is predominantly formed by de novo synthesis from acetate although preformed cholesterol either from intracellular cholesterol ester stores or extracellular supply from circulating low-density lipoproteins also contributes 2. Testosterone biosynthesis involves 2 multi-functional cytochrome P450 complexes involving hydroxylations and side-chain scissions (cholesterol side-chain cleavage [C20 and C22 hydroxylation & C20,22 lyase] and 17-hydroxylase/17,20 lyase) together with 3 and 17 ß-hydroxysteroid dehydrogenases and D (4,5) isomerase. The highly tissue selective regulation of the 17,20 lyase activity (active in gonads but inactive in adrenals) independently of 17-hydroxylase activity (active in all steroidogenic tissues) when both activities reside in a single, multifunctional protein remains to be fully explained. Testicular testosterone secretion is principally governed by LH through its regulation of the rate-limiting conversion of cholesterol to pregnenolone within Leydig cell mitochondria by the cytochrome-P450 cholesterol side-chain cleavage enzyme complex located on the inner mitochondrial membrane. Cholesterol supply is governed by key proteins including sterol carrier protein 2 4 which facilitates cytoplasmic transfer of cholesterol to mitochondria as well as StAR 5 and peripheral benzodiazepine receptor 6 which govern cholesterol transport across mitochondrial membrane. All subsequent enzymatic steps are located in the Leydig cell endoplasmic reticulum. The high testicular production rate of testosterone creates both high local concentrations (up to 1 mg/gm tissue) and rapid turn-over (200 times per day) of intratesticular testosterone.
Figure 1. Pathways of testosterone biosynthesis and action. In men, testosterone biosynthesis occurs almost exclusively in mature Leydig cells by the enzymatic sequences illustrated. Cholesterol originates predominantly by de novo synthesis pathway from acetyl-CoA with luteinizing hormone regulating the rate?limiting step, the conversion of cholesterol to pregnenolone within mitochondria, while remaining enzymatic steps occur in smooth endoplasmic reticulum. The D5 and D4 steroidal pathways are on the left and right, respectively. Testosterone and its androgenic metabolite, dihydrotestosterone, exert biological effects directly through binding to the androgen receptor and indirectly through aromatization of testosterone to estradiol, which allows action via binding to the estrogen receptor. The androgen and estrogen receptors are members of the steroid nuclear receptor superfamily with highly homologous structure differing mostly in the C-terminal ligand binding domain. The LH receptor has the structure of a G-protein linked receptor with its characteristic seven transmembrane spanning helical regions and a large extracellular domain which binds the LH molecule which is a dimeric glycoprotein hormone consisting of an a subunit common to other pituitary glycoprotein hormones and a b subunit specific to LH. Most sex steroids bind to sex hormone binding globulin (SHBG) which binds tightly and carries the majority of testosterone in the bloodstream.
Secretion
Testosterone is secreted at adult levels during 3 epochs of male life - transiently during the first trimester of intrauterine life (coinciding with genital tract differentiation) and again during neonatal life (with unknown physiological significance) and continually after puberty to maintain virilisation. After middle age, circulating total and free testosterone levels decline gradually as gonadotrophin and SHBG levels increase 7-9 with these trends being exaggerated by the coexistence of chronic illness 8-12. These changes are attributable to impaired hypothalamic regulation of testicular function 13-17 as well as Leydig cell attrition 3 and dysfunction 18, 19 so that multiple functional defects are operative throughout the hypothalamo-pituitary-testicular axis20, 21.
Testosterone and other lipophilic steroids leave the testis by diffusing down a concentration gradient across cell membranes into the bloodstream with smaller amounts secreted into lymphatics and tubule fluid. After puberty, over 95% of circulating testosterone is derived from testicular secretion with the remainder arising from metabolic conversion of precursors, predominantly secreted by the adrenal cortex, of low intrinsic androgenic potency such as dehydroepiandrosterone (DHEA), its sulphate (DHEAS) and androstenedione. These weak androgens constitute a large reservoir of precursors for extragonadal conversion to bioactive sex steroids in extra-gonadal tissues including liver, kidney, muscle and adipose tissue. Endogenous adrenal androgens contribute negligibly to direct virilisation of men 22 and residual circulating androgens following medical or surgical castration have negligible biological effect on androgen-sensitive prostate cancer 23. Conversely, however, adrenal androgens make a proportionately larger contribution to the much (~10-fold) lower circulating testosterone concentrations in children and women in whom blood testosterone is derived about equally from direct gonadal secretion and indirectly from peripheral interconversion of adrenal androgen precursors. Exogenous DHEA at physiological replacement doses of 50 mg orally per day 24 is incapable of providing adequate androgen replacement in men while producing hyperandrogenism in women 25.
Hormone production rates can be calculated from either estimating metabolic clearance rate (from bolus injection or steady-state isotope infusion) and mean circulating testosterone levels 26 or by estimation of testicular arterio-venous differences and testicular blood-flow rate 27. These methods give consistent estimates for testosterone production rate of 3-10 mg per day 28, 29 with interconversion rates of ~4% to dihydrotestosterone (DHT) 29, 30 and 0.2% to estradiol 31 under the assumption of steady-state conditions (hours to days). These steady-state methods are a simplification which neglects diurnal rhythm 32, episodic fluctuation in circulating testosterone levels over shorter periods (minutes to hours) entrained by pulsatile LH secretion 33 and postural influence on hepatic blood flow 28. The major known determinants of testosterone metabolic clearance rate are circulating SHBG concentration 34 and hepatic blood flow 28.
Transport
Testosterone circulates in blood at concentrations above its aqueous solubility by binding to circulating plasma proteins. Testosterone binds avidly to sex-hormone binding globulin (SHBG), a dimeric glycoprotein of 95 kD with a single high-affinity androgen binding site and identical with testicular androgen binding protein 35. SHBG is secreted by the liver so its circulating levels are particularly vulnerable to first-pass effects of oral drugs most notably exogenous sex steroids. Circulating SHBG (and thereby total testosterone) concentrations are characteristically decreased (androgens, glucocorticoids) or increased (estrogens, thyroxine) by supraphysiological hormone concentrations at the liver such as produced by oral administration or by parenteral high-dose injections of hormones. In contrast, endogenous sex steroids as well as parenteral administration, which maintain physiological hormone concentrations (transdermal, depot implants), have minimal or no effects on SHBG levels. Other modifiers of circulating SHBG levels include up-regulation by acute or chronic liver disease and androgen deficiency and down-regulation by obesity, protein-losing states and genetic SHBG deficiency 36. Under physiological conditions, 60-70% of circulating testosterone is SHBG-bound with the remainder bound to lower-affinity, high capacity binding sites (albumin, a1-acid glycoprotein, transcortin) and 2% remaining non-protein bound. According to the free hormone hypothesis 37-39, the "free" (non-protein bound) fraction is the most biologically active with the loosely protein-bound testosterone constituting a larger "bioavailable" fraction of circulating testosterone. Nevertheless, "free" and/or "bioavailable" fractions would be accessible not only to sites of bioactivity but also more accessible to inactivation at sites of degradative metabolism, so the net significance of such derived measures of testosterone depends on empirical clinical evaluation for which there is minimal evidence. Free testosterone levels can be measured by the reference methods of tracer equilibrium dialysis or ultrafiltration methods or calculated by a variety of nomograms based on immunoassays of total testosterone and SHBG. Some estimates of free testosterone, notably the direct analog assay 40-42 and the "free testosterone index" 43, are clearly invalid. The clinical utility of various derived measures of testosterone remain to be established.
Circulating testosterone levels demonstrate distinct circhoral and diurnal rhythms. Circhoral LH pulsatility entrains some pulsatility in blood testosterone levels 33 although delays in testosterone secretion and buffering effects of the circulating steroid binding proteins cause marked dampening. Diurnal patterns of morning peak testosterone levels and nadir levels in afternoon are evident in younger men although this pattern is lost in some ageing men 32 possibly due to increased circulating SHBG levels, reduced testosterone secretion and/or neuroendocrine defects 16. Consequently, is it conventional to standardise testosterone measurements to morning blood samples on at least two different days.
Metabolism
Testosterone undergoes metabolism to both bioactive metabolites and to inactivated oxidised and conjugated metabolites for urinary and/or biliary excretion. A small proportion of circulating testosterone is metabolised to biologically active metabolites in specific target tissues to modulate biological effects. This includes both an activation pathway and a diversification pathway of androgen action.
The amplification pathway involves conversion of a small fraction (~4%) of circulating testosterone to a more potent androgen, DHT 29, 30. DHT has higher binding affinity to the androgen receptor and 3-10-fold greater molar biopotency than testosterone. In vitro, DHT is a more potent androgen than T due to its higher binding affinity 44 and more efficient transactivation of the androgen receptor 45, 46. Testosterone is converted to DHT, the most potent natural androgen, by the 5-a reductase enzyme, which exists in two forms (I & II), each specified by distinct genes 47 with type 1 5a reductase expressed in liver, skin, and brain whereas type 2 5a reductase is characteristically expressed strongly in the prostate but also to lower levels in other tissues such as skin (hair follicles) and liver 47. The functional predominance of prostatic expression of the type 2 5a reductase made it feasible to develop a relatively prostate-specific 5a reductase inhibitor, finasteride 48. This occurs extensively within the prostate stroma due to presence of type II 5-a reductase which converts >95% of testosterone entering the gland into the more potent androgen DHT 49. DHT circulates at ~10% of blood testosterone concentrations, partly (50-80%) due to spill-over from the pool of prostatic DHT 50, 51. Genetic mutations disrupting type II 5-a reductase lead to disorders of sexual differentiation involving the external genitalia and accessory glands originating from the urogenital sinus 52, which is developmentally dependent upon local amplification of testosterone to DHT.
The diversification pathway of androgen action involves a quantitatively small proportion (0.2%) of testosterone being converted to estradiol 31 which then acts via estrogen receptors. This diversification pathway of androgen action is governed by the cytochrome P450 enzyme (CYP19) aromatase 53. In eugonadal men, most (~80%) circulating estradiol is derived from extratesticular aromatisation. The biological importance of aromatisation in male physiology is highlighted by the striking developmental defects in bone and some other tissues of a man 54 and mouse line 55 harbouring genetic mutations that inactivate the estrogen receptor a. By contrast, genetic inactivation of the estrogen receptor b has little effect on male phenotype 56. (See chapter on Estrogens in Male Reproduction).
Testosterone is metabolised to inactive metabolites in the liver, kidney, muscle and adipose tissue. Inactivation is predominantly by hepatic mixed function oxidases leading to oxidative degradation at most oxygen moieties of the molecule and ultimately hepatic conjugation to glucuronides, which are rendered sufficiently hydrophilic for renal excretion.
Metabolic clearance rate of testosterone is reduced by increases in circulating SHBG levels 34 or decreases in hepatic blood flow (eg posture) 28 or function. Theoretically drugs that influence hepatic mixed function oxidase activity could alter metabolic inactivation of testosterone but empirical examples are few. Rapid hepatic metabolic inactivation of testosterone leads to both a low oral bioavailability 57-60 and a short duration of action when injected parenterally 61, 62. These limitations dictate the need for parenteral depot testosterone formulations (eg injectable testosterone esters, testosterone implants or transdermal testosterone) to achieve sustained androgenic effects, oral delivery systems which involve portal bypass (buccal 63, 64, sublingual 63, 65, gut lymphatic 58, 66) or active synthetic androgens 61, 62.
Regulation
During sexual differentiation early in intrauterine life, Leydig cell testosterone secretion precedes ontogeny of pituitary gonadotropin secretion. Testosterone is required for masculine sexual differentiation and is secreted by fetal Leydig cells autonomously of gonadotropin stimulation in most mammals 67. Higher primate placenta secretes a chorionic gonadotropin during early fetal life but whether this drives human fetal Leydig cell steroidogenesis is uncertain 68. After birth, testicular testosterone output is primarily regulated by pituitary LH secretion, which stimulates Leydig cell steroidogenesis via increasing substrate (cholesterol) availability, activating rate-limiting steroidogenic enzymes and cholesterol transport proteins and enhancing testicular blood-flow. LH is a dimeric glycoprotein consisting of an a subunit common to hCG, FSH and TSH and a ß subunit providing distinctive biological specificity for each dimeric glycoprotein hormone by virtue of its specific binding to the LH/hCG, FSH or TSH receptors 69. These cell surface receptors are highly homologous members of the heptahelical, G-protein linked family of membrane receptors. Functionally hCG is a natural, long-acting analog of LH as their ß subunits are nearly identical except that hCG has a C-terminal peptide extension of 31 amino acids containing 4 O-linked, terminally sialic acid capped carbohydrate side-chains conferring greater resistance to degradation, which prolongs circulating residence time and biological activity compared with LH 70. LH receptors are located exclusively on Leydig cell surface membranes and utilize signal transduction mechanisms involving both cAMP 71 and calcium 72 as second messengers to cause protein kinase-dependent phosphorylation of specific proteins as well as direct interactions with DNA transcription which ultimately maintain testosterone secretion.
Driven by brief episodic bursts of hypothalamic secretion of GnRH into the pituitary portal bloodstream, pituitary gonadotropes secrete LH episodically in pulses of high amplitude at about hourly intervals with little intervening interpulse basal LH secretion so that circulating LH levels are distinctly pulsatile 73. This pattern maintains Leydig cell sensitivity to LH as more continuous exposure causes desensitisation 2. Additional factors regulating testosterone secretion include paracrine factors originating within the testis to influence Leydig cell function usually via indirect effects on Sertoli cells and blood vessels, respectively 74. These include inhibin, activin, GnRH, FSH, prolactin, prostaglandins E2 and F2a, growth hormone, insulin-like and other growth factors as well as partially uncharacterised factors secreted by Sertoli cells. LH also influences testicular testosterone output by stimulation of Leydig cell secretion of vasoactive factors that promote testicular blood-flow.
Testosterone participates in a negative testicular feedback cycle through its inhibition of hypothalamic GnRH and, consequently, pituitary gonadotropin secretion. Such negative feedback involves both testosterone effects on androgen receptors as well as aromatisation to estradiol within the hypothalamus. The small proportion (20%) of circulating estradiol directly secreted from the testes means that estradiol derived from the bloodstream is minimally regulated physiologically so that it is unlikely to participate significantly in the acute negative feedback regulation of gonadotropin secretion in men.
Action
The binding of testosterone or its analogs to the androgen receptor causing activation is the primary mechanism of biological androgen action. In addition, testosterone is subject to metabolic activation to its bioactive metabolites, DHT and estradiol. The enzyme 5a-reductase 47 acts as a local androgen amplification mechanism by converting testosterone to the most potent natural androgen, DHT. Further, testosterone also exhibits bioactivity through conversion to estradiol by the enzyme aromatase 53. Aromatisation results in diversification of androgen action by facilitating differing effects mediated via the estrogen receptor. The quantitative importance of direct effects on the androgen receptor relative to indirect effects via active metabolites varies between androgens and target tissues, as do the androgenic thresholds and dose-response characteristics for each tissue.
The androgen receptor is specified by a single gene located at Xq11-12, expressing a protein of 919 amino acids, which resides in the nucleus 75. Androgen binding to the C-terminal hormone binding domain causes a conformational change in the androgen receptor protein and dimerisation to facilitate receptor binding to segments of DNA featuring a characteristic palindromic motif known as an androgen-response element. Ligand binding leads to shedding of heat-shock proteins which act as a chaperone for the unliganded androgen receptor. Specific binding of the dimerised, ligand-bound androgen receptor complex to tandem androgen-response elements initiates gene transcription so that the androgen receptor acts as a ligand-activated transcriptional factor. Mutations in the androgen receptor are relatively common leading to a wide spectrum of effects from functionally silent polymorphisms to androgen insensitivity syndromes that have phenotypes proportionate to the variable degree of blockade of androgen action 75. See also chapter on Androgen Physiology: Receptor and Metabolic Disorders.
Testosterone is converted to the more potent androgen DHT by the specific enzyme 5-a reductase which originates from 2 genes expressed in the testis, prostate, liver, kidney and skin 47. Congenital 5-a reductase deficiency due to mutation of the enzyme protein 76 leads to a distinctive form of genital ambiguity resulting in under-masculinisation of genetic males who may be raised as females, but in whom puberty leads to marked virilisation including phallic growth and, occasionally, masculine gender reorientation 77, although prostatic development remains rudimentary 78. This remarkable natural history reflects the predominant expression of 5-a reductase in urogenital sinus tissues, largely restricting DHT effects to those structures. This amplification mechanism for androgen action is exploited by the development of azasteroid 5-a reductase inhibitors 79 that capitalise on the restricted expression of 5-a reduction to block testosterone action on urogenital sinus tissue derivatives, notably the prostate, without blocking all peripheral androgenic action.
Although only a small proportion (0.2%) of testosterone is converted to estradiol by aromatase, the much higher molar potency of estradiol makes aromatisation a potentially important mechanism of diversifying androgen action in various tissues via effects on the estrogen receptor. Complete estrogen resistance due to a non-functional mutated estrogen receptor-a, once believed to be incompatible with life, has been reported in a man 54 who had a distinctive phenotype including an eunuchoidal habitus and severe osteopenia. A similar osteopenic phenotype was observed in mice with a non-functioning estrogen receptor-a produced by homologous recombination 55. The importance of estrogen to development of male bone is further highlighted by reports of men with complete genetic estrogen deficiency due to a non-functional mutated aromatase enzyme 80, 81. Men with aromatase deficiency had not only the same phenotype as in estrogen resistance but demonstrated significant bone maturation with estrogen treatment. These observations suggest the importance of aromatisation of testosterone to estradiol for development of some tissues, notably bone. Nevertheless other observations indicate that androgens and androgen receptors have important additional effects on bone. These include the greater mass of bone in men 82 despite very low circulating estradiol concentrations compared with young women, the failure of tfm rats having no functional androgen receptors but normal estradiol and estrogen receptors to maintain bone mass of normal males 83 and the ability of a non-aromatisable androgen to increase bone mass in estrogen-deficient women 84. Further studies are needed to fully understand the significance of aromatisation in maintaining androgen action in mature animals.
PHARMACOLOGY OF ANDROGENS
Indications for androgen therapy
Androgen therapy can be classified as physiological or pharmacological according to the dosage and objectives of treatment. Androgen replacement therapy aims to restore tissue androgen exposure in androgen deficient men to levels comparable with eugonadal men. Using the natural androgen, testosterone, and dosage limited to ensure blood testosterone levels within the eugonadal range, androgen replacement therapy aims to restore the full spectrum of androgen effects while replicating the safety experience of eugonadal men of similar age. Androgen replacement therapy is unlikely to prolong life, as androgen deficiency does not shorten life expectancy 85. In contrast, pharmacological androgen therapy utilizes androgens without restriction on androgen type or dosage but aiming primarily to produce androgen effects on muscle, bone, brain or other tissues. In this context, pharmacological androgen therapy requires evaluation by the efficacy, safety, and cost-effectiveness criteria as for any other drug. Many older uses of pharmacological androgen therapy are now relegated to second-line therapies as more specific treatments are developed. For example, erythropoietin has largely supplanted androgen therapy for anemia due to marrow or renal failure, whereas better first-line treatments for endometriosis and advanced breast cancer have similarly relegated androgen therapy to second-line status 86.
Androgen replacement therapy
The main specific clinical indication for testosterone is as androgen replacement therapy for hypogonadal men. The prevalence of male hypogonadism requiring androgen therapy in the general community can be estimated from the known prevalence of Klinefelter's syndrome (1.5-2.5 per 1000 male births 87) as Klinefelter's syndrome accounts for 35-50% of men requiring androgen replacement therapy (Handelsman, unpublished). The estimated prevalence of 5 per 1000 men in the general community makes androgen deficiency the commonest hormonal deficiency disorder among men. Although not shortening life expectancy 85, androgen deficiency is associated with preventable morbidity and a suboptimal quality of life. Due to its variable and often subtle clinical features, androgen deficiency remains under-diagnosed, thereby denying hypogonadal men simple and effective medical treatment with often striking benefits.
Hypogonadism of any cause may require androgen replacement therapy depending only on whether the deficit in endogenous testosterone production is sufficient to cause clinical and/or biochemical manifestations of androgen deficiency. The clinical features of androgen deficiency vary according to the severity, chronicity and epoch of life at presentation. These include ambiguous genitalia, microphallus, delayed puberty, sexual dysfunction, infertility, osteoporosis, anemia, flushing, muscular ache, lethargy, lack of stamina or endurance, easy fatigue or incidental biochemical diagnosis 88. As the underlying disorders are mostly irreversible, life-long treatment is usually required. Androgen replacement therapy can rectify most clinical features of androgen deficiency apart from inducing spermatogenesis 89. When fertility is required in gonadotropin-deficient men, spermatogenesis can be initiated by treatment with pulsatile GnRH 90 (if pituitary gonadotrope function is intact 91) or gonadotropins 92 to substitute for pituitary gonadotropin secretion with endogenous LH or exogenous hCG, respectively, acting via Leydig cell LH receptors to stimulate endogenous testosterone production. Where spermatogenesis remains persistently suboptimal, FSH may subsequently be added 92. Once fertility is no longer required, androgen replacement therapy usually reverts to the simpler and cheaper use of testosterone while preserving the ability subsequently to reinitiate spermatogenesis by gonadotropin replacement 92-94.
The potential role for androgen replacement therapy in men with partial or subclinical androgen deficiency states remain to be fully evaluated. Biochemical features of Leydig cell dysfunction, notably persistently elevated LH with low-normal testosterone and/or a high LH/T ratio are observed in ageing men 95 as well as in men with testicular dysfunction associated with male infertility 96 or after chemotherapy-induced testicular damage 97-99. While it is plausible that such features denote mild androgen deficiency, the clinical benefits remain uncertain. A carefully controlled study failed to show substantial benefit of androgen replacement therapy in mild Leydig cell failure following cancer treatment 100.
The prospect of reversing some aspects of male ageing by androgen replacement therapy has long been of interest and more recently the subject of clinical trials. The consensus from population-based cross-sectional as well as longitudinal studies is that circulating testosterone concentrations fall by ~1% per annum from mid-life onwards, a fall accelerated by the presence of concomitant chronic disease 7, and associated with decreases in tissue androgen levels 101. Consequently, a number of randomised, placebo-controlled clinical trials have been undertaken to determine if androgen supplementation ameliorates any age-related changes in bone, muscle and other androgen?dependent tissues. The best available evidence shows no benefits on bone density, muscular strength or consistent effects on quality of life 102, 103. Other studies have shown consistent, small changes in body composition but no consistent benefits in muscle, bone or quality of life measures 104-111. How any putative benefits will balance against potential long-term risks of accelerating cardiovascular or prostatic disease remains to be determined from larger and longer studies including disease outcome rather than surrogate variables. At present, androgen treatment for ageing men cannot be recommended as routine treatment. Nevertheless, androgen replacement therapy may be used even in older men who have severe androgen deficiency if contraindications such as prostate cancer are excluded.
Hormonal male contraception can be considered a form of androgen replacement therapy, since all currently envisaged regimens aiming to suppress spermatogenesis by inhibiting gonadotropin secretion, using testosterone either alone or together with a progestin or a GnRH antagonist (see also chapter on Male Contraception). As a consequence, exogenous testosterone is required to replace endogenous testosterone secretion.
Pharmacological androgen therapy
The objectives of pharmacological androgen therapy are, ideally, to improve mortality and morbidity due to an underlying disease. Mortality benefits require androgens modifying the natural history of an underlying disease, a goal not yet achieved for any non-gonadal disorder. Morbidity benefits are more realistic in aiming to improve quality of life by enhancing muscle, bone, brain or other androgen-sensitive function.
Pharmacological uses of androgens include treatment of anemia due to marrow or renal failure, osteoporosis, estrogen-receptor positive breast cancer, hereditary angioedema (C1 esterase inhibitor deficiency), immunological, pulmonary and muscular diseases (reviewed 86). These older applications now represent mostly second line, empirical therapies that are eventually rendered obsolete by more specific treatments for the underlying conditions. For example, recombinant erythropoietin 112 largely supplants androgen therapy for anemias, which acts primarily by stimulating endogenous erythropoietin secretion 113, 114. For historical reasons, pharmacological androgen therapy has often involved synthetic, orally active 17?a alkylated androgens despite their hepatotoxicity 115. In treating angioedema, oral 17-a alkylated androgens increase circulating C1-esterase inhibitor levels, which reverses the secondary depletion of serum complement factors (C4 & C2) and prevents attacks 116, 117. These benefits may derive from hepatic effects of the 17-a alkyl androgens rather than androgen action per se 118. Otherwise, the safer parenteral testosterone formulations are favoured for long-term clinical use although the balance between risks and benefits may vary where life-threatening diseases are present.
Contrary to accepted wisdom 119, pharmacological testosterone doses administered to eugonadal men for 10 weeks increase muscular size and strength 120; whether such effects enhance skilled muscular performance or can be sustained for longer periods remain unclear. Androgens transiently increase positive nitrogen balance and body weight presumably due to increased appetite and lean mass 121-124; however, androgen therapy has no proven role in sustaining improved nitrogen balance during catabolic states (eg burns, post-operative, post-trauma, malignant cachexia, malnutrition, glucocorticoid excess) or in preventing muscular atrophy during limb immobilisation 123, 125. Recent placebo-controlled clinical studies have revived interest in use of androgens for cachexic states including HIV/AIDS 126-128 but the objective functional benefits remain modest.
Pharmacological androgen treatment has been advocated for treatment of estrogen-resistant menopausal symptoms such as loss of energy or libido 129 but remains controversial 130. The similarity of circulating testosterone concentrations in women, children and orchidectomized men with normal adrenal function indicates that the term androgen deficiency is not applicable to women 131. Few safety and efficacy studies have included placebo controls that are essential for evaluation of subjective endpoints 132, 133. High androgen doses suitable for androgen replacement in men 134-136 produces markedly supraphysiological blood testosterone concentrations and virilization 137, 138. Lower but still supraphysiological testosterone doses increases bone density in menopausal women 139. The efficacy of add-on androgen therapy for estrogen-resistant menopausal symptoms has been evaluated in one randomised, placebo-controlled study of physiological testosterone doses in estrogen-treated menopausal women 140. This study demonstrated benefits of transdermal testosterone restricted to a post-hoc subgroup analysis who attained mildly supraphysiological testosterone concentrations 141. In addition to risk of virilization, safety issues concerning androgen effects on cardiovascular disease and hormone-dependent cancers in women remain to be resolved.
Androgen misuse and abuse
Misuse of androgens involves medical prescription without a valid clinical indication and androgen abuse is the use of androgens for non-medical purposes. Medical misuse of androgens include prescribing androgens for male infertility or sexual dysfunction in non-androgen deficient men, where there is no likely benefit. The epidemic of androgen ("anabolic steroid") abuse, began in the 1950's, a product of the Cold War 142, 143. It has escalated, being fostered by the rich financial rewards in elite competitive sport. For decades, androgen abuse has been cultivated by underground folklore among athletes and trainers, particularly in power sports and body-builders, that "anabolic steroids" enhance sports performance. Based largely on speculation promulgated in pseudo-scientific underground publications, this folklore promotes the use of prodigious androgen doses in combination ("stacking") regimens. Although the effects of androgen abuse on muscular performance had long been doubted, based on studies concluding that claimed performance benefits were primarily a placebo response involving motivation, training and diet effects 119, 144, 145, a pivotal randomised, placebo-controlled clinical study showed that supraphysiological testosterone doses (600 mg testosterone enanthate weekly) for 10 weeks increases muscular size and strength 120. These effects reflect the linear dose-response relationships of muscular size and strength in eugonadal men with testosterone dose without obvious plateau over this period 146. Nevertheless, whether pharmacological androgen effects on muscle are sustained for longer periods, whether they enhance skilled muscular performance and if they extend to older men and those with wasting syndromes, remain to be further clarified.
Progressively, the epidemic of androgen abuse has spread from elite power athletes to recreational and cosmetic users wishing to augment body-building as well as to occupational users who work in security-related professions. As an illicit activity, the extent of androgen abuse in the general community is difficult to estimate although point estimates of prevalence are more feasible in captive populations such as high schools. The prevalence of self-reported lifetime ("ever") use is estimated to be 66 in the USA 147, 58 in Sweden 148, 32 in Australia 149 and 28 in South Africa 150 per 1000 boys in high school with much lower prevalence among girls. Voluntary self-report of androgen abuse understates drug usage among weightlifters 151 and prisoners 152, 153. Abusers consume androgens from many sources including veterinary, inert or counterfeit preparations, obtained mostly through illicit sales by underground networks with a small proportion obtained from compliant doctors. Although highly sensitive urinary drug screening methods for synthetic androgens have been adopted by international sporting bodies and legislation has been introduced by some governments to tightly regulate clinical use of androgens, the epidemic of androgen abuse driven by user demands shows little signs of abating 154.
Androgen abuse is associated with reversible depression of spermatogenesis and fertility 155-159, gynecomastia 160, hepatotoxicity due to 17a-alkylated androgens 161, HIV and hepatitis from needle sharing 162-167, local injury and sepsis from injections 168, over-training injuries 169 and mood and/or behavioral disturbances 170, 171. The medical consequences of androgen abuse for the cardiovascular system has been reviewed 172-174 but only anecdotal reports are available relating to prostate diseases 175, 176. Few controlled clinical studies of cardiovascular 177-179 or prostatic 180 effects of androgen abuse, and no systematic, population-based studies, are available so that the overall risks remain ill-defined although some evidence suggests minimal differences in life expectancy comparing users with other elite athletes 181. More definitive studies are required but, at present, largely anecdotal information suggests that serious short-term medical dangers is limited, considering the extent of androgen abuse, that androgens are not physically addictive 182, 183, and that most androgen abusers eventually discontinue drug use. Following cessation of prolonged use of high dose androgens, recovery of the hypothalamo-pituitary-testicular axis may be delayed for months creating a transient gonadotropin deficiency state 159, 184, 185. This may lead to temporary androgen deficiency symptoms which eventually abate without requiring additional hormonal treatments, which would further delay recovery and perpetuate the drug abuse cycle. The most effective approach for medical intervention to prevent and/or halt androgen abuse is yet to be defined but educational programs 186 as well as support and encouragement, comparable with smoking cessation programs, may be appropriate.
Practical goals of androgen replacement therapy
The goal of androgen replacement therapy is to replicate the physiological actions of endogenous testosterone usually for the remainder of life. This requires rectifying the deficit and maintaining androgenic/anabolic effects on bone 187, 188, muscle 189, blood?forming marrow 190, sexual function 191, 192 and other androgen?responsive tissues. The ideal preparation for long?term androgen replacement therapy should be safe, effective, convenient and inexpensive with long?acting depot properties due to reproducible, zero?order, release kinetics. Androgen replacement therapy usually employs testosterone rather than synthetic androgens for reasons of safety and ease of monitoring and aims to maintain physiological testosterone levels 193. The practical goal of androgen replacement therapy is therefore to maintain stable, physiological testosterone levels for prolonged periods using convenient depot testosterone formulations that facilitate compliance and avoid either supranormal or excessive fluctuation of androgen levels.
Pharmacological features of androgens
The major features of the clinical pharmacology of testosterone are its short circulating half-life/transit time and low oral bioavailability, both largely attributable to rapid hepatic conversion to biologically inactivate oxidised and glucuronidated excretory metabolites. The pharmaceutical development of practical testosterone preparations has been geared to overcoming these limitations. This has led to development of parenteral depot formulations (injectable, implantable, transdermal), products to bypass the hepatic portal system (sublingual, buccal, gut lymphatic absorption) and orally active synthetic androgens.
Androgens are defined pharmacologically by their binding and activation of the androgen receptor 75. Testosterone is the model androgen featuring a 19 carbon, 4 ring steroid structure with two oxygens (3-keto, 17ß-hydroxy) including a D4 non-aromatic A ring. Testosterone derivatives (see figure 2) have been developed to enhance intrinsic androgenic potency, prolong duration of action and/or improve oral bioavailability of synthetic androgens. Major structural modifications of testosterone include 17ß-esterification, 19-nor methyl, 17-a alkyl, 1-methyl, 7-a methyl and D-homo-androgens 194.
Figure 2. Pathways of Testosterone Action. In men, most (>95%) testosterone is produced under LH stimulation through its specific receptor, a heptahelical G-protein coupled receptor located on the surface membrane of the steroidogenic Leydig cells. The daily production of testosterone (5-7 mg) is disposed along one of four major pathways. The direct pathway of testosterone action is characteristic of skeletal muscle in which testosterone itself binds to and activates the androgen receptor. In such tissues there is little metabolism of testosterone to biologically active metabolites. The amplification pathway is characteristic of the prostate and hair follicle in which testosterone is converted by the type 2 5a reductase enzyme into the more potent androgen, dihydrotestosterone. This pathway produces local tissue-based enhancement of androgen action in specific tissues according to where this pathway is operative. The local amplification mechanism was the basis for the development of prostate-selective inhibitors of androgen action via 5a reductase inhibition, the forerunner being finasteride. The diversification pathway of testosterone action allows testosterone to modulate its biological effects via estrogenic effects that often differ from androgen receptor mediated effects. The diversification pathway, characteristic of bone and brain, involves the conversion of testosterone to estradiol by the enzyme aromatase which then interacts with the estrogen receptors a and/or b. Finally the inactivation pathway occurs mainly in the liver with oxidation and conjugation to biologically inactive metabolites that are excreted by the liver into the bile and by the kidney into the urine.
The identification of a single gene and protein for the androgen receptor 195 explains the physiological observation that, at equivalent doses, all androgens have essentially similar effects 118. Consequently the term "anabolic steroid", referring to an idealised androgen lacking virilising features but maintaining myotrophic properties, is a false distinction and perpetuates an obsolete terminology. Better understanding of the metabolic activation of androgens via 5a-reduction and aromatisation in target tissues has however led to the concept of designer androgens with tissue-specific actions analogous to the development of synthetic estrogen partial agonists with tissue specificity 196.
Formulation, route and dosage
Unmodified testosterone
Testosterone implants
Implants of fused crystalline testosterone provide stable, physiological testosterone levels for up to 6 months following a single implantation procedure 197. Typically, four 200 mg pellets are inserted under the skin of the lateral abdominal wall or hip using office-type minor surgery including a local anaesthetic. No suture or antibiotic is required and the pellets are fully biodegradable so do not require removal. This old testosterone formulation 198 has near ideal depot properties with testosterone being absorbed by simple dissolution from a solid reservoir into extracellular fluid at a rate governed by the solubility of testosterone in the extracellular fluid. The long duration of action makes it popular among younger androgen deficient men as reflected by a high continuation rate 199. The major limitations of this form of testosterone administration are the cumbersome implantation procedure and extrusion of a single pellet after 5-10% of procedures. Extrusions are more frequent among men with less subdermal fat and who undertake vigorous physical activities. However, neither surface washing 200 or antibiotic impregnation 201 nor varying the site of implantation 202 prevent extrusions. Other side-effects are rare (bleeding or infection <1%) 203. The simple, non-exclusive technology of testosterone pellets is commercially unattractive and has therefore limited its marketing availability.
Transdermal testosterone
Delivery of testosterone across the skin has long been of interest 61. In recent decades, testosterone in adhesive dermal patches and gels has been developed that can maintain physiological testosterone levels by daily application. The first transdermal patch was developed for application to the scrotum where the thin, highly vascular skin facilitates steroid absorption 204, 205. The scrotal patches are effective during long-term use 206 and there is minimal skin irritation 207, 208. However they are relatively large, require shaving for adhesion and disproportionately increase blood DHT levels due to 5-a reduction of testosterone during transdermal passage. Subsequently, a smaller patch for non-scrotal skin was developed 209 which is also effective during long-term use 210. The smaller size and application to less permeable dermal sites required inclusion of absorption enhancers that cause skin irritation 207, 208 of varying severity 211. This skin irritation may be prevented or ameliorated by topical corticosteroid cream 212 but discontinuation rates due to dermal intolerance are substantial (10-20%).
Dermal testosterone 213 or DHT 214, 215 gels developed in Europe are now more widely available 216-219. They must be applied daily on the trunk and the volatile hydroalcoholic gel base evaporates rapidly and is non-irritating to the skin. A potential problem is the transfer of androgen to the female partner by skin contact 220 although washing off excess gel after a short time may reduce this risk 221.
Testosterone microspheres
Suspensions of biodegradable microspheres, consisting of poly-glycolide-lactide matrix similar to absorbable suture material and laden with testosterone, can deliver stable, physiological levels of testosterone for 2-3 months following intramuscular injection 222, 223. Recent findings 224 suggest that the practical limitations of microsphere technology such as loading capacity, large injection volumes and batch variability may be overcome.
Oral testosterone
Micronized oral testosterone has low oral bioavailability requiring high daily doses (200-400 mg) to maintain physiological testosterone levels 57. This heavy androgen load causes prominent hepatic enzyme induction although testosterone itself is not hepatotoxic 225. Although effective in small studies, micronized oral testosterone is not commercially available and is little used.
Alternative oral formulations designed to bypass the portal route of absorption redeveloped 63 recently include testosterone in a sublingual cyclodextrin formulation 65, 226, 227 or a buccal lozenge 64. These products require multiple daily dosing to maintain physiological testosterone levels making them unlikely to be useful for long-term androgen replacement.
Testosterone esters
Injectable
The most widely used testosterone formulation is intramuscular injection of testosterone esters, formed by 17-b esterification of testosterone with fatty acids of various aliphatic and/or aromatic chain lengths, injected in a vegetable oil vehicle. This depot formulation relies on retarded release of the testosterone ester from the oil vehicle injection depot since esters undergo rapid hydrolysis by ubiquitous esterases to liberate free testosterone into the circulation. The pharmacokinetics and pharmacodynamics of androgen esters is therefore primarily determined by ester side-chain length, volume of oil vehicle and site of injection via hydrophobic physico-chemical partitioning of the androgen ester between the hydrophobic oil vehicle and the aqueous extracellular fluid 228.
Testosterone propionate with a short aliphatic side-chain ester has a brief duration of action requiring injections of 25-50 mg at 1-2 day intervals for androgen replacement. In contrast testosterone enanthate has a longer duration of action so that it is routinely administered at doses of 200-250 mg per 10-14 days for androgen replacement therapy in hypogonadal men 229-232. Other testosterone esters (cypionate, cyclohexanecarboxylate) have virtually identical pharmacokinetics making them pharmacologically equivalent to testosterone enanthate 232, the most widely used ester. Mixtures of short and longer acting testosterone esters are available but lack a convincing rationale and remain far from desirable zero-order kinetics release profiles.
Recent advances have been the development of newer injectable testosterone esters including testosterone undecanoate in an oil vehicle and testosterone buciclate (trans-4-n-butyl cyclohexane carboxylate, 20 AET-1), a novel insoluble testosterone ester in an aqueous suspension. Testosterone undecanoate in an oil vehicle has a strikingly longer (8-12 weeks) duration of action 233-235 making it a promising advance as a depot form of testosterone. Similarly, testosterone buciclate produces prolonged slow testosterone release due to steric hindrance of ester side-chain hydrolysis. This produces low physiological levels of testosterone for up to 4 months following injection in non-human primates 236 as well as hypogonadal 237 and eugonadal 238 men.
Oral testosterone undecanoate
Oral testosterone undecanoate, an oleic acid suspension of the ester in 40 mg capsules, is administered as 160-240 mg in 3-4 doses per day. The hydrophobic, long aliphatic chain ester in an oil vehicle favours preferential absorption into chylomicrons entering the gastrointestinal lymphatics and largely bypassing hepatic first-pass metabolism during portal absorption. Testosterone undecanoate has low and erratic oral bioavailability, short duration of action and causes gastrointestinal intolerance. While it has well established safety 66, its limitations in efficacy make it a second choice 230, 231 unless parenteral therapy has to be avoided (eg bleeding disorders or anticoagulation, children) 239). It is widely marketed but not available in the USA.
Synthetic androgens
Most oral androgens are hepatotoxic 17-a alkylated androgens (methyltestosterone, fluoxymesterone, oxymetholone, oxandrolone, ethylestranol, stanozolol, methandrostenolone, norethandrolone, danazol) which are unacceptable for long-term androgen replacement therapy. The 1-methyl androgen, mesterolone, is functionally an orally active DHT analog free of hepatotoxicity but is not used for androgen replacement due to the need for multiple daily dosing and its poorly described pharmacology 240. Another potent, synthetic androgen free of hepatotoxicity, 7a-methyl 19-nortestosterone (MENT), is under development as a depot androgen 241 for androgen replacement 242 and male contraception 243. As a nandrolone derivative, MENT has tissue-specific selectivity in being susceptible to aromatisation but not 5a-reduction 244 thereby representing a fore-runner of designer androgens based on metabolic selectivity. Whether eliminating intra-prostatic androgen amplification by inhibition of 5a reduction can prevent or retard prostate disease is being examined in a large-scale chemoprevention study of an oral 5a-reductase inhibitor 245, 246. Preliminary evidence raise doubts about this hypothesis 247. If inhibition of 5a reductase is effective for prostate cancer chemoprevention, novel synthetic androgens refractory to 5a reductive amplification may become commercially attractive for clinical development.
Choice of preparation
The choice of testosterone formulation for androgen replacement therapy depends on physician experience and patient preference involving factors such as convenience, availability, familiarity, cost and tolerance for frequent injections. Preparations of testosterone or its esters are favoured over synthetic androgens for all androgen applications by virtue of assured safety and efficacy, ease of dose-titration and assay monitoring. The hepatotoxicity of synthetic 17-a alkylated androgens 115 makes them unsuitable for long?term androgen replacement therapy and this obsolete class of androgen is being progressively withdrawn from marketing and clinical usage in most countries.
Cross-over studies indicate that patients prefer formulations with stable testosterone levels and smoother clinical effects (eg implants 231; transdermal patches 248) to the wide fluctuations in testosterone levels and effects with intramuscular injections of testosterone esters in an oil vehicle 229, 231, 249.
There are few well established formulation or route-dependent differences between various testosterone formulations once adequate doses are administered. As with estrogen replacement, testosterone effects on SHBG may be viewed as manifestations of hepatic overdosage 36 so that oral 17-a alkylated androgens and testosterone undecanoate cause prominent lowering of SHBG levels due to marked first?pass hepatic effects while intramuscular testosterone ester injections cause transient falls which mirror testosterone levels and long-acting depot testosterone formulations (eg testosterone buciclate, implants & microspheres) have minimal effects 223, 231, 237, 250. Long-acting depot testosterone preparations with zero?order release patterns 197, 224, 235, 237 which are also convenient and affordable are likely to supplant the present injectable testosterone esters as the mainstays of androgen replacement therapy.
Side-effects of androgen therapy
Serious adverse effects from androgens are uncommon and are mostly due to either inappropriate treatment or the hepatotoxicity of the 17-a alkylated androgens. Virtually all androgenic side-effects are rapidly reversible on cessation of treatment apart from inappropriate virilization in children or women when voice deepening and/or terminal body hair may be irreversible.
Steroidal effects
Androgen replacement activates physical and mental activity to enhance mood, behaviour and libido thereby reversing their impairment during androgen deficiency 251. In healthy eugonadal men, however, administration of additional androgen has negligible effects on mood or behaviour 252-257. This contrasts with androgen abusers among whom high levels of background psychological disturbance 170, drug habituation 182 and anticipation 258 predispose to behavioural disturbances reported during this form of drug abuse 251, 259. Idiosyncratic hypomanic episodes have been reported in a small minority of young having supraphysiological doses of testosterone in some 260-262 but not all 252, 253, 255, 256 clinical studies.
Excessive or undesirable androgenic effects may be experienced during androgen therapy due to intrinsic androgenic effects in inappropriate settings (eg virilization in women or children). In some untreated hypogonadal men, particularly older men, initiation of androgen treatment with standard doses occasionally produces an intolerable increase in libido and erection frequency. More gradual acclimatisation to full androgen doses with counselling of men and their wives may be useful in such situations.
Seborrhea and acne are commonly associated with high blood testosterone levels, particularly among androgen abusers taking injectable testosterone esters. It has a predominantly truncal distribution in men in contrast to the predominantly facial distribution of adolescent acne. Acne is uncommon during androgen replacement therapy being restricted to a few susceptible individuals treated with intramuscular testosterone esters, probably related to their generation of transient supra-physiological testosterone concentrations in the days following injection 229. Acne is rare with depot testosterone products that maintain steady-state physiological blood testosterone levels. Androgen-induced acne is usually adequately managed with topical measures and/or broad-spectrum antibiotics with switch to steady-state delivery avoiding supraphysiological peaks of plasma testosterone. Increased body hair and temporal hair loss or balding may also be seen.
Weight gain reflecting anabolic effects on muscle mass is also common. Gynecomastia is a feature of androgen deficiency in men but may appear during androgen replacement therapy especially during use of aromatisable androgens such as testosterone that increase circulating estradiol levels at times when androgenic effects are inadequate (eg too low or infrequent dose or unreliable compliance with treatment).
Testosterone and its esters lower total and LDL cholesterol while HDL levels are minimally altered 105, 263-265 reversing the hyperlipidemic effects of chronic androgen deficiency 266. In contrast, oral 17-a alkylated androgens markedly lower HDL cholesterol levels due to prominent reduction in hepatic lipoprotein levels by first-pass hepatic effects related to the oral route of administration 267. The potential long-term consequences for cardiovascular disease of such pharmacological changes are unknown although low testosterone levels represent another of the constellation of inter-related epidemiological risk factors for atherosclerotic cardiovascular disease 268-270.
Hepatotoxicity
Hepatotoxicity is a well-recognised but uncommon side-effect of 17-a alkylated but not with other androgens 115. Biochemical hepatotoxicity, involving either a cholestatic or hepatitic pattern, usually abates with cessation of steroid ingestion. Hepatic tumors related to androgen usage include peliosis hepatis (blood-filled cysts), adenoma or carcinoma. Prolonged use of 17-a alkylated androgens, if unavoidable, requires regular clinical examination and biochemical monitoring of hepatic function. If biochemical abnormalities are detected treatment with 17-a alkylated androgens should cease and safer androgens may be substituted without concern. Where structural lesions are suspected, radionuclide scan, ultrasound or abdominal CT scan should precede hepatic biopsy during which severe bleeding may be provoked in peliosis hepatis. As equally effective and safer alternatives exist, the hepatotoxic 17-a alkylated androgens should not be used for androgen replacement therapy.
Formulation-related
Complications related to testosterone formulations are related to mode of administration or idiosyncratic reactions to constitutents. Intramuscular injections of oil vehicle may cause local pain, bleeding or bruising and, rarely, coughing fits or fainting possibly due to oil microembolisation 271. Inadvertent subcutaneous administration of the oil vehicle is highly irritating and may cause pain, inflammation or even dermal necrosis. Allergy to vegetable oil vehicle of testosterone ester injections (sesame, castor, arachis) is very rare and even patients allergic to peanuts may tolerate arachis (peanut) oil without incident. Oral testosterone undecanoate frequently causes gastrointestinal intolerance due to the oleic acid suspension vehicle. Testosterone implants may be associated with extrusion of implants or with bleeding, infection or scarring at implant sites 199. Parenteral injection of newer testosterone esters 237 or biodegradable microspheres 223 involves a large injection volume that may cause discomfort. Non-scrotal transdermal patches frequently cause skin irritation with a significant minority (10-20%) unable to use truncal patches. Topical steroid-impregnated gels may transfer androgens through topical skin-to-skin contact 220.
Monitoring of androgen replacement therapy
Monitoring of androgen replacement therapy involves primarily clinical observations to optimise androgen effects and recognise side-effects. Once well established, androgen replacement therapy requires only very limited, judicious use of biochemical testing or hormone assays. Testosterone and its esters are sufficiently safe not to require routine toxicological monitoring.
Clinical monitoring depends upon observation of serial improvement in the key presenting features of androgen deficiency. Androgen deficient patients may report subjective improvements in energy, well?being, psychosocial drive, initiative and assertiveness as well as in sexual activity (especially libido and ejaculation frequency), increased sexual hair and muscular strength and endurance. Patients become familiar with their own leading androgen deficiency symptoms and these appear in predictable sequence and at consistent symptomatic thresholds. Objective measures of androgen action would be desirable, but suitably sensitive biochemical indices are not yet available for most androgen?responsive tissues 272. The main biochemical measures available for monitoring of androgenic effects include hemoglobin and trough reproductive hormone (testosterone, LH & FSH) levels. Hemoglobin rises by about 10-20 g/L when androgen dosage is adequate 190, 273). Occasionally excessive hemoglobin responses may create polycythemia requiring venesection and/or anti-coagulation together with temporary interruption or change to more steady-state testosterone delivery systems that avoid supraphysiological peak testosterone concentrations. Circulating testosterone and gonadotropin levels relative to testosterone doses, usually trough levels prior to next dose, can be helpful in establishing adequacy of depot testosterone regimens. In the presence of normal testosterone negative feedback on hypothalamic GnRH and pituitary LH secretion (ie men with hypergonadotropic hypogonadism), plasma LH levels are elevated in rough proportion to the degree of androgen deficiency. In severe androgen deficiency, virtually castrate LH levels may be present and, conversely, circulating LH levels provide a sensitive and specific index of tissue testosterone effects 197, 229. Suppression of LH into the eugonadal range indicates adequate androgen replacement therapy, whereas persistent non?suppression after the first few months of treatment is an indication of inadequate dosage or pattern of testosterone levels. In hypogonadotropic hypogonadism, however, impaired hypothalamo-pituitary function diminishes circulating LH levels regardless of androgen effects, so LH levels do not reflect tissue androgenic effects.
Plasma testosterone measurements are of most importance for diagnosis, during initiation and for evaluating adequacy of treatment. During depot testosterone treatment where quasi-steady-state plasma testosterone levels are achieved, trough plasma testosterone levels may detect patients whose treatment is suboptimal and where the dose and/or treatment interval need modification. Plasma testosterone levels are not helpful for routine monitoring androgen therapy using any synthetic androgens or oral testosterone undecanoate. Serial evaluation of bone density (especially vertebral trabecular bone) by dual photon absorptiometry at 1-2 year intervals may be helpful in verifying the adequacy of tissue androgen effects 187, 188.
The World Health Organisation has developed guidelines for the therapeutic use of androgens in men 274. Although chronic androgen deficiency protects against prostate disease 78, 275, the prostate of androgen-deficient men receiving androgen replacement therapy is restored to, but does not exceed, age-appropriate norms 276, 277. Furthermore, as there is no prospective relationship between endogenous androgen concentrations and subsequent development of prostate cancer 278-280, maintaining physiological testosterone concentrations should ensure no higher rates of prostate disease than eugonadal men of similar age. Similarly, low blood testosterone concentration is a risk factor for cardiovascular disease 268, 269 so that androgen replacement therapy is likely to improve risk of cardiovascular disease. Hence during androgen replacement therapy aiming to maintain physiological testosterone concentrations, surveillance for cardiovascular and prostate disease should be comparable with, but need be no more intensive than, eugonadal men of equivalent age 270. The effects of pharmacological androgen therapy, where androgen dosage is not necessarily restricted to eugonadal limits, on cardiovascular and prostate disease are difficult to predict and surveillance then depends on the nature, severity and life-expectancy of the underlying disease.
Contraindications and precautions for androgen replacement therapy
Contraindications to androgen replacement therapy are prostate or breast cancer as tumors may be androgen responsive and pregnancy where transplacental passage of androgens may disturb fetal sexual differentiation. Precautions and/or careful monitoring of androgen use is required in (i) initiating treatment in older men who may experience intolerable changes in libido, (ii) competitive athletes who may be disqualified, (iii) women of reproductive age, especially those who use their voice professionally, who may become irreversibly virilized, (iv) in patients with bleeding disorders or during anti?coagulation when parenteral administration may cause severe bruising or bleeding, (v) sex steroid-sensitive epilepsy or migraine, (vi) those with cardiac or renal failure or severe hypertension susceptible to fluid overload from sodium and fluid retention, (vii) men with obstructive sleep apnea which may be exacerbated by exogenous androgens 281. Excessive androgen doses prior to completion of puberty risks premature epiphyseal closure leading to foreshortened final adult stature and/or precocious sexual development.
I will post on Estrogens next week.
ANDROGENS
Chapter 2 - David J Handelsman MB BS, FRACP, PhD
INTRODUCTION
Testosterone is the principal androgen in the circulation of mature male mammals. An androgen, or male sex hormone, is defined as a substance capable of developing and maintaining masculine sexual characteristics (including the genital tract, secondary sexual characteristics and fertility) and the anabolic status of somatic tissues. Testosterone and synthetic androgens based on its structure are used clinically at physiological doses for androgen replacement therapy and, at higher doses, for pharmacological androgen therapy. The principal goal of androgen replacement therapy is to restore a physiological pattern of androgen exposure to the body. At present, such treatment is restricted to the major natural androgen, testosterone, and aims to deliver testosterone to replicate physiological circulating testosterone levels. Thus, an understanding of the normal physiology of testosterone is required as a basis for androgen pharmacology 1.
TESTOSTERONE: PHYSIOLOGY
Biosynthesis
Testosterone is synthesized by an enzymatic sequence of steps from cholesterol 2 (figure 1) within the 500 million Leydig cells located in the interstitial (intertubular) compartment which constitutes about 5% of mature testis volume 3. The cholesterol is predominantly formed by de novo synthesis from acetate although preformed cholesterol either from intracellular cholesterol ester stores or extracellular supply from circulating low-density lipoproteins also contributes 2. Testosterone biosynthesis involves 2 multi-functional cytochrome P450 complexes involving hydroxylations and side-chain scissions (cholesterol side-chain cleavage [C20 and C22 hydroxylation & C20,22 lyase] and 17-hydroxylase/17,20 lyase) together with 3 and 17 ß-hydroxysteroid dehydrogenases and D (4,5) isomerase. The highly tissue selective regulation of the 17,20 lyase activity (active in gonads but inactive in adrenals) independently of 17-hydroxylase activity (active in all steroidogenic tissues) when both activities reside in a single, multifunctional protein remains to be fully explained. Testicular testosterone secretion is principally governed by LH through its regulation of the rate-limiting conversion of cholesterol to pregnenolone within Leydig cell mitochondria by the cytochrome-P450 cholesterol side-chain cleavage enzyme complex located on the inner mitochondrial membrane. Cholesterol supply is governed by key proteins including sterol carrier protein 2 4 which facilitates cytoplasmic transfer of cholesterol to mitochondria as well as StAR 5 and peripheral benzodiazepine receptor 6 which govern cholesterol transport across mitochondrial membrane. All subsequent enzymatic steps are located in the Leydig cell endoplasmic reticulum. The high testicular production rate of testosterone creates both high local concentrations (up to 1 mg/gm tissue) and rapid turn-over (200 times per day) of intratesticular testosterone.
Figure 1. Pathways of testosterone biosynthesis and action. In men, testosterone biosynthesis occurs almost exclusively in mature Leydig cells by the enzymatic sequences illustrated. Cholesterol originates predominantly by de novo synthesis pathway from acetyl-CoA with luteinizing hormone regulating the rate?limiting step, the conversion of cholesterol to pregnenolone within mitochondria, while remaining enzymatic steps occur in smooth endoplasmic reticulum. The D5 and D4 steroidal pathways are on the left and right, respectively. Testosterone and its androgenic metabolite, dihydrotestosterone, exert biological effects directly through binding to the androgen receptor and indirectly through aromatization of testosterone to estradiol, which allows action via binding to the estrogen receptor. The androgen and estrogen receptors are members of the steroid nuclear receptor superfamily with highly homologous structure differing mostly in the C-terminal ligand binding domain. The LH receptor has the structure of a G-protein linked receptor with its characteristic seven transmembrane spanning helical regions and a large extracellular domain which binds the LH molecule which is a dimeric glycoprotein hormone consisting of an a subunit common to other pituitary glycoprotein hormones and a b subunit specific to LH. Most sex steroids bind to sex hormone binding globulin (SHBG) which binds tightly and carries the majority of testosterone in the bloodstream.
Secretion
Testosterone is secreted at adult levels during 3 epochs of male life - transiently during the first trimester of intrauterine life (coinciding with genital tract differentiation) and again during neonatal life (with unknown physiological significance) and continually after puberty to maintain virilisation. After middle age, circulating total and free testosterone levels decline gradually as gonadotrophin and SHBG levels increase 7-9 with these trends being exaggerated by the coexistence of chronic illness 8-12. These changes are attributable to impaired hypothalamic regulation of testicular function 13-17 as well as Leydig cell attrition 3 and dysfunction 18, 19 so that multiple functional defects are operative throughout the hypothalamo-pituitary-testicular axis20, 21.
Testosterone and other lipophilic steroids leave the testis by diffusing down a concentration gradient across cell membranes into the bloodstream with smaller amounts secreted into lymphatics and tubule fluid. After puberty, over 95% of circulating testosterone is derived from testicular secretion with the remainder arising from metabolic conversion of precursors, predominantly secreted by the adrenal cortex, of low intrinsic androgenic potency such as dehydroepiandrosterone (DHEA), its sulphate (DHEAS) and androstenedione. These weak androgens constitute a large reservoir of precursors for extragonadal conversion to bioactive sex steroids in extra-gonadal tissues including liver, kidney, muscle and adipose tissue. Endogenous adrenal androgens contribute negligibly to direct virilisation of men 22 and residual circulating androgens following medical or surgical castration have negligible biological effect on androgen-sensitive prostate cancer 23. Conversely, however, adrenal androgens make a proportionately larger contribution to the much (~10-fold) lower circulating testosterone concentrations in children and women in whom blood testosterone is derived about equally from direct gonadal secretion and indirectly from peripheral interconversion of adrenal androgen precursors. Exogenous DHEA at physiological replacement doses of 50 mg orally per day 24 is incapable of providing adequate androgen replacement in men while producing hyperandrogenism in women 25.
Hormone production rates can be calculated from either estimating metabolic clearance rate (from bolus injection or steady-state isotope infusion) and mean circulating testosterone levels 26 or by estimation of testicular arterio-venous differences and testicular blood-flow rate 27. These methods give consistent estimates for testosterone production rate of 3-10 mg per day 28, 29 with interconversion rates of ~4% to dihydrotestosterone (DHT) 29, 30 and 0.2% to estradiol 31 under the assumption of steady-state conditions (hours to days). These steady-state methods are a simplification which neglects diurnal rhythm 32, episodic fluctuation in circulating testosterone levels over shorter periods (minutes to hours) entrained by pulsatile LH secretion 33 and postural influence on hepatic blood flow 28. The major known determinants of testosterone metabolic clearance rate are circulating SHBG concentration 34 and hepatic blood flow 28.
Transport
Testosterone circulates in blood at concentrations above its aqueous solubility by binding to circulating plasma proteins. Testosterone binds avidly to sex-hormone binding globulin (SHBG), a dimeric glycoprotein of 95 kD with a single high-affinity androgen binding site and identical with testicular androgen binding protein 35. SHBG is secreted by the liver so its circulating levels are particularly vulnerable to first-pass effects of oral drugs most notably exogenous sex steroids. Circulating SHBG (and thereby total testosterone) concentrations are characteristically decreased (androgens, glucocorticoids) or increased (estrogens, thyroxine) by supraphysiological hormone concentrations at the liver such as produced by oral administration or by parenteral high-dose injections of hormones. In contrast, endogenous sex steroids as well as parenteral administration, which maintain physiological hormone concentrations (transdermal, depot implants), have minimal or no effects on SHBG levels. Other modifiers of circulating SHBG levels include up-regulation by acute or chronic liver disease and androgen deficiency and down-regulation by obesity, protein-losing states and genetic SHBG deficiency 36. Under physiological conditions, 60-70% of circulating testosterone is SHBG-bound with the remainder bound to lower-affinity, high capacity binding sites (albumin, a1-acid glycoprotein, transcortin) and 2% remaining non-protein bound. According to the free hormone hypothesis 37-39, the "free" (non-protein bound) fraction is the most biologically active with the loosely protein-bound testosterone constituting a larger "bioavailable" fraction of circulating testosterone. Nevertheless, "free" and/or "bioavailable" fractions would be accessible not only to sites of bioactivity but also more accessible to inactivation at sites of degradative metabolism, so the net significance of such derived measures of testosterone depends on empirical clinical evaluation for which there is minimal evidence. Free testosterone levels can be measured by the reference methods of tracer equilibrium dialysis or ultrafiltration methods or calculated by a variety of nomograms based on immunoassays of total testosterone and SHBG. Some estimates of free testosterone, notably the direct analog assay 40-42 and the "free testosterone index" 43, are clearly invalid. The clinical utility of various derived measures of testosterone remain to be established.
Circulating testosterone levels demonstrate distinct circhoral and diurnal rhythms. Circhoral LH pulsatility entrains some pulsatility in blood testosterone levels 33 although delays in testosterone secretion and buffering effects of the circulating steroid binding proteins cause marked dampening. Diurnal patterns of morning peak testosterone levels and nadir levels in afternoon are evident in younger men although this pattern is lost in some ageing men 32 possibly due to increased circulating SHBG levels, reduced testosterone secretion and/or neuroendocrine defects 16. Consequently, is it conventional to standardise testosterone measurements to morning blood samples on at least two different days.
Metabolism
Testosterone undergoes metabolism to both bioactive metabolites and to inactivated oxidised and conjugated metabolites for urinary and/or biliary excretion. A small proportion of circulating testosterone is metabolised to biologically active metabolites in specific target tissues to modulate biological effects. This includes both an activation pathway and a diversification pathway of androgen action.
The amplification pathway involves conversion of a small fraction (~4%) of circulating testosterone to a more potent androgen, DHT 29, 30. DHT has higher binding affinity to the androgen receptor and 3-10-fold greater molar biopotency than testosterone. In vitro, DHT is a more potent androgen than T due to its higher binding affinity 44 and more efficient transactivation of the androgen receptor 45, 46. Testosterone is converted to DHT, the most potent natural androgen, by the 5-a reductase enzyme, which exists in two forms (I & II), each specified by distinct genes 47 with type 1 5a reductase expressed in liver, skin, and brain whereas type 2 5a reductase is characteristically expressed strongly in the prostate but also to lower levels in other tissues such as skin (hair follicles) and liver 47. The functional predominance of prostatic expression of the type 2 5a reductase made it feasible to develop a relatively prostate-specific 5a reductase inhibitor, finasteride 48. This occurs extensively within the prostate stroma due to presence of type II 5-a reductase which converts >95% of testosterone entering the gland into the more potent androgen DHT 49. DHT circulates at ~10% of blood testosterone concentrations, partly (50-80%) due to spill-over from the pool of prostatic DHT 50, 51. Genetic mutations disrupting type II 5-a reductase lead to disorders of sexual differentiation involving the external genitalia and accessory glands originating from the urogenital sinus 52, which is developmentally dependent upon local amplification of testosterone to DHT.
The diversification pathway of androgen action involves a quantitatively small proportion (0.2%) of testosterone being converted to estradiol 31 which then acts via estrogen receptors. This diversification pathway of androgen action is governed by the cytochrome P450 enzyme (CYP19) aromatase 53. In eugonadal men, most (~80%) circulating estradiol is derived from extratesticular aromatisation. The biological importance of aromatisation in male physiology is highlighted by the striking developmental defects in bone and some other tissues of a man 54 and mouse line 55 harbouring genetic mutations that inactivate the estrogen receptor a. By contrast, genetic inactivation of the estrogen receptor b has little effect on male phenotype 56. (See chapter on Estrogens in Male Reproduction).
Testosterone is metabolised to inactive metabolites in the liver, kidney, muscle and adipose tissue. Inactivation is predominantly by hepatic mixed function oxidases leading to oxidative degradation at most oxygen moieties of the molecule and ultimately hepatic conjugation to glucuronides, which are rendered sufficiently hydrophilic for renal excretion.
Metabolic clearance rate of testosterone is reduced by increases in circulating SHBG levels 34 or decreases in hepatic blood flow (eg posture) 28 or function. Theoretically drugs that influence hepatic mixed function oxidase activity could alter metabolic inactivation of testosterone but empirical examples are few. Rapid hepatic metabolic inactivation of testosterone leads to both a low oral bioavailability 57-60 and a short duration of action when injected parenterally 61, 62. These limitations dictate the need for parenteral depot testosterone formulations (eg injectable testosterone esters, testosterone implants or transdermal testosterone) to achieve sustained androgenic effects, oral delivery systems which involve portal bypass (buccal 63, 64, sublingual 63, 65, gut lymphatic 58, 66) or active synthetic androgens 61, 62.
Regulation
During sexual differentiation early in intrauterine life, Leydig cell testosterone secretion precedes ontogeny of pituitary gonadotropin secretion. Testosterone is required for masculine sexual differentiation and is secreted by fetal Leydig cells autonomously of gonadotropin stimulation in most mammals 67. Higher primate placenta secretes a chorionic gonadotropin during early fetal life but whether this drives human fetal Leydig cell steroidogenesis is uncertain 68. After birth, testicular testosterone output is primarily regulated by pituitary LH secretion, which stimulates Leydig cell steroidogenesis via increasing substrate (cholesterol) availability, activating rate-limiting steroidogenic enzymes and cholesterol transport proteins and enhancing testicular blood-flow. LH is a dimeric glycoprotein consisting of an a subunit common to hCG, FSH and TSH and a ß subunit providing distinctive biological specificity for each dimeric glycoprotein hormone by virtue of its specific binding to the LH/hCG, FSH or TSH receptors 69. These cell surface receptors are highly homologous members of the heptahelical, G-protein linked family of membrane receptors. Functionally hCG is a natural, long-acting analog of LH as their ß subunits are nearly identical except that hCG has a C-terminal peptide extension of 31 amino acids containing 4 O-linked, terminally sialic acid capped carbohydrate side-chains conferring greater resistance to degradation, which prolongs circulating residence time and biological activity compared with LH 70. LH receptors are located exclusively on Leydig cell surface membranes and utilize signal transduction mechanisms involving both cAMP 71 and calcium 72 as second messengers to cause protein kinase-dependent phosphorylation of specific proteins as well as direct interactions with DNA transcription which ultimately maintain testosterone secretion.
Driven by brief episodic bursts of hypothalamic secretion of GnRH into the pituitary portal bloodstream, pituitary gonadotropes secrete LH episodically in pulses of high amplitude at about hourly intervals with little intervening interpulse basal LH secretion so that circulating LH levels are distinctly pulsatile 73. This pattern maintains Leydig cell sensitivity to LH as more continuous exposure causes desensitisation 2. Additional factors regulating testosterone secretion include paracrine factors originating within the testis to influence Leydig cell function usually via indirect effects on Sertoli cells and blood vessels, respectively 74. These include inhibin, activin, GnRH, FSH, prolactin, prostaglandins E2 and F2a, growth hormone, insulin-like and other growth factors as well as partially uncharacterised factors secreted by Sertoli cells. LH also influences testicular testosterone output by stimulation of Leydig cell secretion of vasoactive factors that promote testicular blood-flow.
Testosterone participates in a negative testicular feedback cycle through its inhibition of hypothalamic GnRH and, consequently, pituitary gonadotropin secretion. Such negative feedback involves both testosterone effects on androgen receptors as well as aromatisation to estradiol within the hypothalamus. The small proportion (20%) of circulating estradiol directly secreted from the testes means that estradiol derived from the bloodstream is minimally regulated physiologically so that it is unlikely to participate significantly in the acute negative feedback regulation of gonadotropin secretion in men.
Action
The binding of testosterone or its analogs to the androgen receptor causing activation is the primary mechanism of biological androgen action. In addition, testosterone is subject to metabolic activation to its bioactive metabolites, DHT and estradiol. The enzyme 5a-reductase 47 acts as a local androgen amplification mechanism by converting testosterone to the most potent natural androgen, DHT. Further, testosterone also exhibits bioactivity through conversion to estradiol by the enzyme aromatase 53. Aromatisation results in diversification of androgen action by facilitating differing effects mediated via the estrogen receptor. The quantitative importance of direct effects on the androgen receptor relative to indirect effects via active metabolites varies between androgens and target tissues, as do the androgenic thresholds and dose-response characteristics for each tissue.
The androgen receptor is specified by a single gene located at Xq11-12, expressing a protein of 919 amino acids, which resides in the nucleus 75. Androgen binding to the C-terminal hormone binding domain causes a conformational change in the androgen receptor protein and dimerisation to facilitate receptor binding to segments of DNA featuring a characteristic palindromic motif known as an androgen-response element. Ligand binding leads to shedding of heat-shock proteins which act as a chaperone for the unliganded androgen receptor. Specific binding of the dimerised, ligand-bound androgen receptor complex to tandem androgen-response elements initiates gene transcription so that the androgen receptor acts as a ligand-activated transcriptional factor. Mutations in the androgen receptor are relatively common leading to a wide spectrum of effects from functionally silent polymorphisms to androgen insensitivity syndromes that have phenotypes proportionate to the variable degree of blockade of androgen action 75. See also chapter on Androgen Physiology: Receptor and Metabolic Disorders.
Testosterone is converted to the more potent androgen DHT by the specific enzyme 5-a reductase which originates from 2 genes expressed in the testis, prostate, liver, kidney and skin 47. Congenital 5-a reductase deficiency due to mutation of the enzyme protein 76 leads to a distinctive form of genital ambiguity resulting in under-masculinisation of genetic males who may be raised as females, but in whom puberty leads to marked virilisation including phallic growth and, occasionally, masculine gender reorientation 77, although prostatic development remains rudimentary 78. This remarkable natural history reflects the predominant expression of 5-a reductase in urogenital sinus tissues, largely restricting DHT effects to those structures. This amplification mechanism for androgen action is exploited by the development of azasteroid 5-a reductase inhibitors 79 that capitalise on the restricted expression of 5-a reduction to block testosterone action on urogenital sinus tissue derivatives, notably the prostate, without blocking all peripheral androgenic action.
Although only a small proportion (0.2%) of testosterone is converted to estradiol by aromatase, the much higher molar potency of estradiol makes aromatisation a potentially important mechanism of diversifying androgen action in various tissues via effects on the estrogen receptor. Complete estrogen resistance due to a non-functional mutated estrogen receptor-a, once believed to be incompatible with life, has been reported in a man 54 who had a distinctive phenotype including an eunuchoidal habitus and severe osteopenia. A similar osteopenic phenotype was observed in mice with a non-functioning estrogen receptor-a produced by homologous recombination 55. The importance of estrogen to development of male bone is further highlighted by reports of men with complete genetic estrogen deficiency due to a non-functional mutated aromatase enzyme 80, 81. Men with aromatase deficiency had not only the same phenotype as in estrogen resistance but demonstrated significant bone maturation with estrogen treatment. These observations suggest the importance of aromatisation of testosterone to estradiol for development of some tissues, notably bone. Nevertheless other observations indicate that androgens and androgen receptors have important additional effects on bone. These include the greater mass of bone in men 82 despite very low circulating estradiol concentrations compared with young women, the failure of tfm rats having no functional androgen receptors but normal estradiol and estrogen receptors to maintain bone mass of normal males 83 and the ability of a non-aromatisable androgen to increase bone mass in estrogen-deficient women 84. Further studies are needed to fully understand the significance of aromatisation in maintaining androgen action in mature animals.
PHARMACOLOGY OF ANDROGENS
Indications for androgen therapy
Androgen therapy can be classified as physiological or pharmacological according to the dosage and objectives of treatment. Androgen replacement therapy aims to restore tissue androgen exposure in androgen deficient men to levels comparable with eugonadal men. Using the natural androgen, testosterone, and dosage limited to ensure blood testosterone levels within the eugonadal range, androgen replacement therapy aims to restore the full spectrum of androgen effects while replicating the safety experience of eugonadal men of similar age. Androgen replacement therapy is unlikely to prolong life, as androgen deficiency does not shorten life expectancy 85. In contrast, pharmacological androgen therapy utilizes androgens without restriction on androgen type or dosage but aiming primarily to produce androgen effects on muscle, bone, brain or other tissues. In this context, pharmacological androgen therapy requires evaluation by the efficacy, safety, and cost-effectiveness criteria as for any other drug. Many older uses of pharmacological androgen therapy are now relegated to second-line therapies as more specific treatments are developed. For example, erythropoietin has largely supplanted androgen therapy for anemia due to marrow or renal failure, whereas better first-line treatments for endometriosis and advanced breast cancer have similarly relegated androgen therapy to second-line status 86.
Androgen replacement therapy
The main specific clinical indication for testosterone is as androgen replacement therapy for hypogonadal men. The prevalence of male hypogonadism requiring androgen therapy in the general community can be estimated from the known prevalence of Klinefelter's syndrome (1.5-2.5 per 1000 male births 87) as Klinefelter's syndrome accounts for 35-50% of men requiring androgen replacement therapy (Handelsman, unpublished). The estimated prevalence of 5 per 1000 men in the general community makes androgen deficiency the commonest hormonal deficiency disorder among men. Although not shortening life expectancy 85, androgen deficiency is associated with preventable morbidity and a suboptimal quality of life. Due to its variable and often subtle clinical features, androgen deficiency remains under-diagnosed, thereby denying hypogonadal men simple and effective medical treatment with often striking benefits.
Hypogonadism of any cause may require androgen replacement therapy depending only on whether the deficit in endogenous testosterone production is sufficient to cause clinical and/or biochemical manifestations of androgen deficiency. The clinical features of androgen deficiency vary according to the severity, chronicity and epoch of life at presentation. These include ambiguous genitalia, microphallus, delayed puberty, sexual dysfunction, infertility, osteoporosis, anemia, flushing, muscular ache, lethargy, lack of stamina or endurance, easy fatigue or incidental biochemical diagnosis 88. As the underlying disorders are mostly irreversible, life-long treatment is usually required. Androgen replacement therapy can rectify most clinical features of androgen deficiency apart from inducing spermatogenesis 89. When fertility is required in gonadotropin-deficient men, spermatogenesis can be initiated by treatment with pulsatile GnRH 90 (if pituitary gonadotrope function is intact 91) or gonadotropins 92 to substitute for pituitary gonadotropin secretion with endogenous LH or exogenous hCG, respectively, acting via Leydig cell LH receptors to stimulate endogenous testosterone production. Where spermatogenesis remains persistently suboptimal, FSH may subsequently be added 92. Once fertility is no longer required, androgen replacement therapy usually reverts to the simpler and cheaper use of testosterone while preserving the ability subsequently to reinitiate spermatogenesis by gonadotropin replacement 92-94.
The potential role for androgen replacement therapy in men with partial or subclinical androgen deficiency states remain to be fully evaluated. Biochemical features of Leydig cell dysfunction, notably persistently elevated LH with low-normal testosterone and/or a high LH/T ratio are observed in ageing men 95 as well as in men with testicular dysfunction associated with male infertility 96 or after chemotherapy-induced testicular damage 97-99. While it is plausible that such features denote mild androgen deficiency, the clinical benefits remain uncertain. A carefully controlled study failed to show substantial benefit of androgen replacement therapy in mild Leydig cell failure following cancer treatment 100.
The prospect of reversing some aspects of male ageing by androgen replacement therapy has long been of interest and more recently the subject of clinical trials. The consensus from population-based cross-sectional as well as longitudinal studies is that circulating testosterone concentrations fall by ~1% per annum from mid-life onwards, a fall accelerated by the presence of concomitant chronic disease 7, and associated with decreases in tissue androgen levels 101. Consequently, a number of randomised, placebo-controlled clinical trials have been undertaken to determine if androgen supplementation ameliorates any age-related changes in bone, muscle and other androgen?dependent tissues. The best available evidence shows no benefits on bone density, muscular strength or consistent effects on quality of life 102, 103. Other studies have shown consistent, small changes in body composition but no consistent benefits in muscle, bone or quality of life measures 104-111. How any putative benefits will balance against potential long-term risks of accelerating cardiovascular or prostatic disease remains to be determined from larger and longer studies including disease outcome rather than surrogate variables. At present, androgen treatment for ageing men cannot be recommended as routine treatment. Nevertheless, androgen replacement therapy may be used even in older men who have severe androgen deficiency if contraindications such as prostate cancer are excluded.
Hormonal male contraception can be considered a form of androgen replacement therapy, since all currently envisaged regimens aiming to suppress spermatogenesis by inhibiting gonadotropin secretion, using testosterone either alone or together with a progestin or a GnRH antagonist (see also chapter on Male Contraception). As a consequence, exogenous testosterone is required to replace endogenous testosterone secretion.
Pharmacological androgen therapy
The objectives of pharmacological androgen therapy are, ideally, to improve mortality and morbidity due to an underlying disease. Mortality benefits require androgens modifying the natural history of an underlying disease, a goal not yet achieved for any non-gonadal disorder. Morbidity benefits are more realistic in aiming to improve quality of life by enhancing muscle, bone, brain or other androgen-sensitive function.
Pharmacological uses of androgens include treatment of anemia due to marrow or renal failure, osteoporosis, estrogen-receptor positive breast cancer, hereditary angioedema (C1 esterase inhibitor deficiency), immunological, pulmonary and muscular diseases (reviewed 86). These older applications now represent mostly second line, empirical therapies that are eventually rendered obsolete by more specific treatments for the underlying conditions. For example, recombinant erythropoietin 112 largely supplants androgen therapy for anemias, which acts primarily by stimulating endogenous erythropoietin secretion 113, 114. For historical reasons, pharmacological androgen therapy has often involved synthetic, orally active 17?a alkylated androgens despite their hepatotoxicity 115. In treating angioedema, oral 17-a alkylated androgens increase circulating C1-esterase inhibitor levels, which reverses the secondary depletion of serum complement factors (C4 & C2) and prevents attacks 116, 117. These benefits may derive from hepatic effects of the 17-a alkyl androgens rather than androgen action per se 118. Otherwise, the safer parenteral testosterone formulations are favoured for long-term clinical use although the balance between risks and benefits may vary where life-threatening diseases are present.
Contrary to accepted wisdom 119, pharmacological testosterone doses administered to eugonadal men for 10 weeks increase muscular size and strength 120; whether such effects enhance skilled muscular performance or can be sustained for longer periods remain unclear. Androgens transiently increase positive nitrogen balance and body weight presumably due to increased appetite and lean mass 121-124; however, androgen therapy has no proven role in sustaining improved nitrogen balance during catabolic states (eg burns, post-operative, post-trauma, malignant cachexia, malnutrition, glucocorticoid excess) or in preventing muscular atrophy during limb immobilisation 123, 125. Recent placebo-controlled clinical studies have revived interest in use of androgens for cachexic states including HIV/AIDS 126-128 but the objective functional benefits remain modest.
Pharmacological androgen treatment has been advocated for treatment of estrogen-resistant menopausal symptoms such as loss of energy or libido 129 but remains controversial 130. The similarity of circulating testosterone concentrations in women, children and orchidectomized men with normal adrenal function indicates that the term androgen deficiency is not applicable to women 131. Few safety and efficacy studies have included placebo controls that are essential for evaluation of subjective endpoints 132, 133. High androgen doses suitable for androgen replacement in men 134-136 produces markedly supraphysiological blood testosterone concentrations and virilization 137, 138. Lower but still supraphysiological testosterone doses increases bone density in menopausal women 139. The efficacy of add-on androgen therapy for estrogen-resistant menopausal symptoms has been evaluated in one randomised, placebo-controlled study of physiological testosterone doses in estrogen-treated menopausal women 140. This study demonstrated benefits of transdermal testosterone restricted to a post-hoc subgroup analysis who attained mildly supraphysiological testosterone concentrations 141. In addition to risk of virilization, safety issues concerning androgen effects on cardiovascular disease and hormone-dependent cancers in women remain to be resolved.
Androgen misuse and abuse
Misuse of androgens involves medical prescription without a valid clinical indication and androgen abuse is the use of androgens for non-medical purposes. Medical misuse of androgens include prescribing androgens for male infertility or sexual dysfunction in non-androgen deficient men, where there is no likely benefit. The epidemic of androgen ("anabolic steroid") abuse, began in the 1950's, a product of the Cold War 142, 143. It has escalated, being fostered by the rich financial rewards in elite competitive sport. For decades, androgen abuse has been cultivated by underground folklore among athletes and trainers, particularly in power sports and body-builders, that "anabolic steroids" enhance sports performance. Based largely on speculation promulgated in pseudo-scientific underground publications, this folklore promotes the use of prodigious androgen doses in combination ("stacking") regimens. Although the effects of androgen abuse on muscular performance had long been doubted, based on studies concluding that claimed performance benefits were primarily a placebo response involving motivation, training and diet effects 119, 144, 145, a pivotal randomised, placebo-controlled clinical study showed that supraphysiological testosterone doses (600 mg testosterone enanthate weekly) for 10 weeks increases muscular size and strength 120. These effects reflect the linear dose-response relationships of muscular size and strength in eugonadal men with testosterone dose without obvious plateau over this period 146. Nevertheless, whether pharmacological androgen effects on muscle are sustained for longer periods, whether they enhance skilled muscular performance and if they extend to older men and those with wasting syndromes, remain to be further clarified.
Progressively, the epidemic of androgen abuse has spread from elite power athletes to recreational and cosmetic users wishing to augment body-building as well as to occupational users who work in security-related professions. As an illicit activity, the extent of androgen abuse in the general community is difficult to estimate although point estimates of prevalence are more feasible in captive populations such as high schools. The prevalence of self-reported lifetime ("ever") use is estimated to be 66 in the USA 147, 58 in Sweden 148, 32 in Australia 149 and 28 in South Africa 150 per 1000 boys in high school with much lower prevalence among girls. Voluntary self-report of androgen abuse understates drug usage among weightlifters 151 and prisoners 152, 153. Abusers consume androgens from many sources including veterinary, inert or counterfeit preparations, obtained mostly through illicit sales by underground networks with a small proportion obtained from compliant doctors. Although highly sensitive urinary drug screening methods for synthetic androgens have been adopted by international sporting bodies and legislation has been introduced by some governments to tightly regulate clinical use of androgens, the epidemic of androgen abuse driven by user demands shows little signs of abating 154.
Androgen abuse is associated with reversible depression of spermatogenesis and fertility 155-159, gynecomastia 160, hepatotoxicity due to 17a-alkylated androgens 161, HIV and hepatitis from needle sharing 162-167, local injury and sepsis from injections 168, over-training injuries 169 and mood and/or behavioral disturbances 170, 171. The medical consequences of androgen abuse for the cardiovascular system has been reviewed 172-174 but only anecdotal reports are available relating to prostate diseases 175, 176. Few controlled clinical studies of cardiovascular 177-179 or prostatic 180 effects of androgen abuse, and no systematic, population-based studies, are available so that the overall risks remain ill-defined although some evidence suggests minimal differences in life expectancy comparing users with other elite athletes 181. More definitive studies are required but, at present, largely anecdotal information suggests that serious short-term medical dangers is limited, considering the extent of androgen abuse, that androgens are not physically addictive 182, 183, and that most androgen abusers eventually discontinue drug use. Following cessation of prolonged use of high dose androgens, recovery of the hypothalamo-pituitary-testicular axis may be delayed for months creating a transient gonadotropin deficiency state 159, 184, 185. This may lead to temporary androgen deficiency symptoms which eventually abate without requiring additional hormonal treatments, which would further delay recovery and perpetuate the drug abuse cycle. The most effective approach for medical intervention to prevent and/or halt androgen abuse is yet to be defined but educational programs 186 as well as support and encouragement, comparable with smoking cessation programs, may be appropriate.
Practical goals of androgen replacement therapy
The goal of androgen replacement therapy is to replicate the physiological actions of endogenous testosterone usually for the remainder of life. This requires rectifying the deficit and maintaining androgenic/anabolic effects on bone 187, 188, muscle 189, blood?forming marrow 190, sexual function 191, 192 and other androgen?responsive tissues. The ideal preparation for long?term androgen replacement therapy should be safe, effective, convenient and inexpensive with long?acting depot properties due to reproducible, zero?order, release kinetics. Androgen replacement therapy usually employs testosterone rather than synthetic androgens for reasons of safety and ease of monitoring and aims to maintain physiological testosterone levels 193. The practical goal of androgen replacement therapy is therefore to maintain stable, physiological testosterone levels for prolonged periods using convenient depot testosterone formulations that facilitate compliance and avoid either supranormal or excessive fluctuation of androgen levels.
Pharmacological features of androgens
The major features of the clinical pharmacology of testosterone are its short circulating half-life/transit time and low oral bioavailability, both largely attributable to rapid hepatic conversion to biologically inactivate oxidised and glucuronidated excretory metabolites. The pharmaceutical development of practical testosterone preparations has been geared to overcoming these limitations. This has led to development of parenteral depot formulations (injectable, implantable, transdermal), products to bypass the hepatic portal system (sublingual, buccal, gut lymphatic absorption) and orally active synthetic androgens.
Androgens are defined pharmacologically by their binding and activation of the androgen receptor 75. Testosterone is the model androgen featuring a 19 carbon, 4 ring steroid structure with two oxygens (3-keto, 17ß-hydroxy) including a D4 non-aromatic A ring. Testosterone derivatives (see figure 2) have been developed to enhance intrinsic androgenic potency, prolong duration of action and/or improve oral bioavailability of synthetic androgens. Major structural modifications of testosterone include 17ß-esterification, 19-nor methyl, 17-a alkyl, 1-methyl, 7-a methyl and D-homo-androgens 194.
Figure 2. Pathways of Testosterone Action. In men, most (>95%) testosterone is produced under LH stimulation through its specific receptor, a heptahelical G-protein coupled receptor located on the surface membrane of the steroidogenic Leydig cells. The daily production of testosterone (5-7 mg) is disposed along one of four major pathways. The direct pathway of testosterone action is characteristic of skeletal muscle in which testosterone itself binds to and activates the androgen receptor. In such tissues there is little metabolism of testosterone to biologically active metabolites. The amplification pathway is characteristic of the prostate and hair follicle in which testosterone is converted by the type 2 5a reductase enzyme into the more potent androgen, dihydrotestosterone. This pathway produces local tissue-based enhancement of androgen action in specific tissues according to where this pathway is operative. The local amplification mechanism was the basis for the development of prostate-selective inhibitors of androgen action via 5a reductase inhibition, the forerunner being finasteride. The diversification pathway of testosterone action allows testosterone to modulate its biological effects via estrogenic effects that often differ from androgen receptor mediated effects. The diversification pathway, characteristic of bone and brain, involves the conversion of testosterone to estradiol by the enzyme aromatase which then interacts with the estrogen receptors a and/or b. Finally the inactivation pathway occurs mainly in the liver with oxidation and conjugation to biologically inactive metabolites that are excreted by the liver into the bile and by the kidney into the urine.
The identification of a single gene and protein for the androgen receptor 195 explains the physiological observation that, at equivalent doses, all androgens have essentially similar effects 118. Consequently the term "anabolic steroid", referring to an idealised androgen lacking virilising features but maintaining myotrophic properties, is a false distinction and perpetuates an obsolete terminology. Better understanding of the metabolic activation of androgens via 5a-reduction and aromatisation in target tissues has however led to the concept of designer androgens with tissue-specific actions analogous to the development of synthetic estrogen partial agonists with tissue specificity 196.
Formulation, route and dosage
Unmodified testosterone
Testosterone implants
Implants of fused crystalline testosterone provide stable, physiological testosterone levels for up to 6 months following a single implantation procedure 197. Typically, four 200 mg pellets are inserted under the skin of the lateral abdominal wall or hip using office-type minor surgery including a local anaesthetic. No suture or antibiotic is required and the pellets are fully biodegradable so do not require removal. This old testosterone formulation 198 has near ideal depot properties with testosterone being absorbed by simple dissolution from a solid reservoir into extracellular fluid at a rate governed by the solubility of testosterone in the extracellular fluid. The long duration of action makes it popular among younger androgen deficient men as reflected by a high continuation rate 199. The major limitations of this form of testosterone administration are the cumbersome implantation procedure and extrusion of a single pellet after 5-10% of procedures. Extrusions are more frequent among men with less subdermal fat and who undertake vigorous physical activities. However, neither surface washing 200 or antibiotic impregnation 201 nor varying the site of implantation 202 prevent extrusions. Other side-effects are rare (bleeding or infection <1%) 203. The simple, non-exclusive technology of testosterone pellets is commercially unattractive and has therefore limited its marketing availability.
Transdermal testosterone
Delivery of testosterone across the skin has long been of interest 61. In recent decades, testosterone in adhesive dermal patches and gels has been developed that can maintain physiological testosterone levels by daily application. The first transdermal patch was developed for application to the scrotum where the thin, highly vascular skin facilitates steroid absorption 204, 205. The scrotal patches are effective during long-term use 206 and there is minimal skin irritation 207, 208. However they are relatively large, require shaving for adhesion and disproportionately increase blood DHT levels due to 5-a reduction of testosterone during transdermal passage. Subsequently, a smaller patch for non-scrotal skin was developed 209 which is also effective during long-term use 210. The smaller size and application to less permeable dermal sites required inclusion of absorption enhancers that cause skin irritation 207, 208 of varying severity 211. This skin irritation may be prevented or ameliorated by topical corticosteroid cream 212 but discontinuation rates due to dermal intolerance are substantial (10-20%).
Dermal testosterone 213 or DHT 214, 215 gels developed in Europe are now more widely available 216-219. They must be applied daily on the trunk and the volatile hydroalcoholic gel base evaporates rapidly and is non-irritating to the skin. A potential problem is the transfer of androgen to the female partner by skin contact 220 although washing off excess gel after a short time may reduce this risk 221.
Testosterone microspheres
Suspensions of biodegradable microspheres, consisting of poly-glycolide-lactide matrix similar to absorbable suture material and laden with testosterone, can deliver stable, physiological levels of testosterone for 2-3 months following intramuscular injection 222, 223. Recent findings 224 suggest that the practical limitations of microsphere technology such as loading capacity, large injection volumes and batch variability may be overcome.
Oral testosterone
Micronized oral testosterone has low oral bioavailability requiring high daily doses (200-400 mg) to maintain physiological testosterone levels 57. This heavy androgen load causes prominent hepatic enzyme induction although testosterone itself is not hepatotoxic 225. Although effective in small studies, micronized oral testosterone is not commercially available and is little used.
Alternative oral formulations designed to bypass the portal route of absorption redeveloped 63 recently include testosterone in a sublingual cyclodextrin formulation 65, 226, 227 or a buccal lozenge 64. These products require multiple daily dosing to maintain physiological testosterone levels making them unlikely to be useful for long-term androgen replacement.
Testosterone esters
Injectable
The most widely used testosterone formulation is intramuscular injection of testosterone esters, formed by 17-b esterification of testosterone with fatty acids of various aliphatic and/or aromatic chain lengths, injected in a vegetable oil vehicle. This depot formulation relies on retarded release of the testosterone ester from the oil vehicle injection depot since esters undergo rapid hydrolysis by ubiquitous esterases to liberate free testosterone into the circulation. The pharmacokinetics and pharmacodynamics of androgen esters is therefore primarily determined by ester side-chain length, volume of oil vehicle and site of injection via hydrophobic physico-chemical partitioning of the androgen ester between the hydrophobic oil vehicle and the aqueous extracellular fluid 228.
Testosterone propionate with a short aliphatic side-chain ester has a brief duration of action requiring injections of 25-50 mg at 1-2 day intervals for androgen replacement. In contrast testosterone enanthate has a longer duration of action so that it is routinely administered at doses of 200-250 mg per 10-14 days for androgen replacement therapy in hypogonadal men 229-232. Other testosterone esters (cypionate, cyclohexanecarboxylate) have virtually identical pharmacokinetics making them pharmacologically equivalent to testosterone enanthate 232, the most widely used ester. Mixtures of short and longer acting testosterone esters are available but lack a convincing rationale and remain far from desirable zero-order kinetics release profiles.
Recent advances have been the development of newer injectable testosterone esters including testosterone undecanoate in an oil vehicle and testosterone buciclate (trans-4-n-butyl cyclohexane carboxylate, 20 AET-1), a novel insoluble testosterone ester in an aqueous suspension. Testosterone undecanoate in an oil vehicle has a strikingly longer (8-12 weeks) duration of action 233-235 making it a promising advance as a depot form of testosterone. Similarly, testosterone buciclate produces prolonged slow testosterone release due to steric hindrance of ester side-chain hydrolysis. This produces low physiological levels of testosterone for up to 4 months following injection in non-human primates 236 as well as hypogonadal 237 and eugonadal 238 men.
Oral testosterone undecanoate
Oral testosterone undecanoate, an oleic acid suspension of the ester in 40 mg capsules, is administered as 160-240 mg in 3-4 doses per day. The hydrophobic, long aliphatic chain ester in an oil vehicle favours preferential absorption into chylomicrons entering the gastrointestinal lymphatics and largely bypassing hepatic first-pass metabolism during portal absorption. Testosterone undecanoate has low and erratic oral bioavailability, short duration of action and causes gastrointestinal intolerance. While it has well established safety 66, its limitations in efficacy make it a second choice 230, 231 unless parenteral therapy has to be avoided (eg bleeding disorders or anticoagulation, children) 239). It is widely marketed but not available in the USA.
Synthetic androgens
Most oral androgens are hepatotoxic 17-a alkylated androgens (methyltestosterone, fluoxymesterone, oxymetholone, oxandrolone, ethylestranol, stanozolol, methandrostenolone, norethandrolone, danazol) which are unacceptable for long-term androgen replacement therapy. The 1-methyl androgen, mesterolone, is functionally an orally active DHT analog free of hepatotoxicity but is not used for androgen replacement due to the need for multiple daily dosing and its poorly described pharmacology 240. Another potent, synthetic androgen free of hepatotoxicity, 7a-methyl 19-nortestosterone (MENT), is under development as a depot androgen 241 for androgen replacement 242 and male contraception 243. As a nandrolone derivative, MENT has tissue-specific selectivity in being susceptible to aromatisation but not 5a-reduction 244 thereby representing a fore-runner of designer androgens based on metabolic selectivity. Whether eliminating intra-prostatic androgen amplification by inhibition of 5a reduction can prevent or retard prostate disease is being examined in a large-scale chemoprevention study of an oral 5a-reductase inhibitor 245, 246. Preliminary evidence raise doubts about this hypothesis 247. If inhibition of 5a reductase is effective for prostate cancer chemoprevention, novel synthetic androgens refractory to 5a reductive amplification may become commercially attractive for clinical development.
Choice of preparation
The choice of testosterone formulation for androgen replacement therapy depends on physician experience and patient preference involving factors such as convenience, availability, familiarity, cost and tolerance for frequent injections. Preparations of testosterone or its esters are favoured over synthetic androgens for all androgen applications by virtue of assured safety and efficacy, ease of dose-titration and assay monitoring. The hepatotoxicity of synthetic 17-a alkylated androgens 115 makes them unsuitable for long?term androgen replacement therapy and this obsolete class of androgen is being progressively withdrawn from marketing and clinical usage in most countries.
Cross-over studies indicate that patients prefer formulations with stable testosterone levels and smoother clinical effects (eg implants 231; transdermal patches 248) to the wide fluctuations in testosterone levels and effects with intramuscular injections of testosterone esters in an oil vehicle 229, 231, 249.
There are few well established formulation or route-dependent differences between various testosterone formulations once adequate doses are administered. As with estrogen replacement, testosterone effects on SHBG may be viewed as manifestations of hepatic overdosage 36 so that oral 17-a alkylated androgens and testosterone undecanoate cause prominent lowering of SHBG levels due to marked first?pass hepatic effects while intramuscular testosterone ester injections cause transient falls which mirror testosterone levels and long-acting depot testosterone formulations (eg testosterone buciclate, implants & microspheres) have minimal effects 223, 231, 237, 250. Long-acting depot testosterone preparations with zero?order release patterns 197, 224, 235, 237 which are also convenient and affordable are likely to supplant the present injectable testosterone esters as the mainstays of androgen replacement therapy.
Side-effects of androgen therapy
Serious adverse effects from androgens are uncommon and are mostly due to either inappropriate treatment or the hepatotoxicity of the 17-a alkylated androgens. Virtually all androgenic side-effects are rapidly reversible on cessation of treatment apart from inappropriate virilization in children or women when voice deepening and/or terminal body hair may be irreversible.
Steroidal effects
Androgen replacement activates physical and mental activity to enhance mood, behaviour and libido thereby reversing their impairment during androgen deficiency 251. In healthy eugonadal men, however, administration of additional androgen has negligible effects on mood or behaviour 252-257. This contrasts with androgen abusers among whom high levels of background psychological disturbance 170, drug habituation 182 and anticipation 258 predispose to behavioural disturbances reported during this form of drug abuse 251, 259. Idiosyncratic hypomanic episodes have been reported in a small minority of young having supraphysiological doses of testosterone in some 260-262 but not all 252, 253, 255, 256 clinical studies.
Excessive or undesirable androgenic effects may be experienced during androgen therapy due to intrinsic androgenic effects in inappropriate settings (eg virilization in women or children). In some untreated hypogonadal men, particularly older men, initiation of androgen treatment with standard doses occasionally produces an intolerable increase in libido and erection frequency. More gradual acclimatisation to full androgen doses with counselling of men and their wives may be useful in such situations.
Seborrhea and acne are commonly associated with high blood testosterone levels, particularly among androgen abusers taking injectable testosterone esters. It has a predominantly truncal distribution in men in contrast to the predominantly facial distribution of adolescent acne. Acne is uncommon during androgen replacement therapy being restricted to a few susceptible individuals treated with intramuscular testosterone esters, probably related to their generation of transient supra-physiological testosterone concentrations in the days following injection 229. Acne is rare with depot testosterone products that maintain steady-state physiological blood testosterone levels. Androgen-induced acne is usually adequately managed with topical measures and/or broad-spectrum antibiotics with switch to steady-state delivery avoiding supraphysiological peaks of plasma testosterone. Increased body hair and temporal hair loss or balding may also be seen.
Weight gain reflecting anabolic effects on muscle mass is also common. Gynecomastia is a feature of androgen deficiency in men but may appear during androgen replacement therapy especially during use of aromatisable androgens such as testosterone that increase circulating estradiol levels at times when androgenic effects are inadequate (eg too low or infrequent dose or unreliable compliance with treatment).
Testosterone and its esters lower total and LDL cholesterol while HDL levels are minimally altered 105, 263-265 reversing the hyperlipidemic effects of chronic androgen deficiency 266. In contrast, oral 17-a alkylated androgens markedly lower HDL cholesterol levels due to prominent reduction in hepatic lipoprotein levels by first-pass hepatic effects related to the oral route of administration 267. The potential long-term consequences for cardiovascular disease of such pharmacological changes are unknown although low testosterone levels represent another of the constellation of inter-related epidemiological risk factors for atherosclerotic cardiovascular disease 268-270.
Hepatotoxicity
Hepatotoxicity is a well-recognised but uncommon side-effect of 17-a alkylated but not with other androgens 115. Biochemical hepatotoxicity, involving either a cholestatic or hepatitic pattern, usually abates with cessation of steroid ingestion. Hepatic tumors related to androgen usage include peliosis hepatis (blood-filled cysts), adenoma or carcinoma. Prolonged use of 17-a alkylated androgens, if unavoidable, requires regular clinical examination and biochemical monitoring of hepatic function. If biochemical abnormalities are detected treatment with 17-a alkylated androgens should cease and safer androgens may be substituted without concern. Where structural lesions are suspected, radionuclide scan, ultrasound or abdominal CT scan should precede hepatic biopsy during which severe bleeding may be provoked in peliosis hepatis. As equally effective and safer alternatives exist, the hepatotoxic 17-a alkylated androgens should not be used for androgen replacement therapy.
Formulation-related
Complications related to testosterone formulations are related to mode of administration or idiosyncratic reactions to constitutents. Intramuscular injections of oil vehicle may cause local pain, bleeding or bruising and, rarely, coughing fits or fainting possibly due to oil microembolisation 271. Inadvertent subcutaneous administration of the oil vehicle is highly irritating and may cause pain, inflammation or even dermal necrosis. Allergy to vegetable oil vehicle of testosterone ester injections (sesame, castor, arachis) is very rare and even patients allergic to peanuts may tolerate arachis (peanut) oil without incident. Oral testosterone undecanoate frequently causes gastrointestinal intolerance due to the oleic acid suspension vehicle. Testosterone implants may be associated with extrusion of implants or with bleeding, infection or scarring at implant sites 199. Parenteral injection of newer testosterone esters 237 or biodegradable microspheres 223 involves a large injection volume that may cause discomfort. Non-scrotal transdermal patches frequently cause skin irritation with a significant minority (10-20%) unable to use truncal patches. Topical steroid-impregnated gels may transfer androgens through topical skin-to-skin contact 220.
Monitoring of androgen replacement therapy
Monitoring of androgen replacement therapy involves primarily clinical observations to optimise androgen effects and recognise side-effects. Once well established, androgen replacement therapy requires only very limited, judicious use of biochemical testing or hormone assays. Testosterone and its esters are sufficiently safe not to require routine toxicological monitoring.
Clinical monitoring depends upon observation of serial improvement in the key presenting features of androgen deficiency. Androgen deficient patients may report subjective improvements in energy, well?being, psychosocial drive, initiative and assertiveness as well as in sexual activity (especially libido and ejaculation frequency), increased sexual hair and muscular strength and endurance. Patients become familiar with their own leading androgen deficiency symptoms and these appear in predictable sequence and at consistent symptomatic thresholds. Objective measures of androgen action would be desirable, but suitably sensitive biochemical indices are not yet available for most androgen?responsive tissues 272. The main biochemical measures available for monitoring of androgenic effects include hemoglobin and trough reproductive hormone (testosterone, LH & FSH) levels. Hemoglobin rises by about 10-20 g/L when androgen dosage is adequate 190, 273). Occasionally excessive hemoglobin responses may create polycythemia requiring venesection and/or anti-coagulation together with temporary interruption or change to more steady-state testosterone delivery systems that avoid supraphysiological peak testosterone concentrations. Circulating testosterone and gonadotropin levels relative to testosterone doses, usually trough levels prior to next dose, can be helpful in establishing adequacy of depot testosterone regimens. In the presence of normal testosterone negative feedback on hypothalamic GnRH and pituitary LH secretion (ie men with hypergonadotropic hypogonadism), plasma LH levels are elevated in rough proportion to the degree of androgen deficiency. In severe androgen deficiency, virtually castrate LH levels may be present and, conversely, circulating LH levels provide a sensitive and specific index of tissue testosterone effects 197, 229. Suppression of LH into the eugonadal range indicates adequate androgen replacement therapy, whereas persistent non?suppression after the first few months of treatment is an indication of inadequate dosage or pattern of testosterone levels. In hypogonadotropic hypogonadism, however, impaired hypothalamo-pituitary function diminishes circulating LH levels regardless of androgen effects, so LH levels do not reflect tissue androgenic effects.
Plasma testosterone measurements are of most importance for diagnosis, during initiation and for evaluating adequacy of treatment. During depot testosterone treatment where quasi-steady-state plasma testosterone levels are achieved, trough plasma testosterone levels may detect patients whose treatment is suboptimal and where the dose and/or treatment interval need modification. Plasma testosterone levels are not helpful for routine monitoring androgen therapy using any synthetic androgens or oral testosterone undecanoate. Serial evaluation of bone density (especially vertebral trabecular bone) by dual photon absorptiometry at 1-2 year intervals may be helpful in verifying the adequacy of tissue androgen effects 187, 188.
The World Health Organisation has developed guidelines for the therapeutic use of androgens in men 274. Although chronic androgen deficiency protects against prostate disease 78, 275, the prostate of androgen-deficient men receiving androgen replacement therapy is restored to, but does not exceed, age-appropriate norms 276, 277. Furthermore, as there is no prospective relationship between endogenous androgen concentrations and subsequent development of prostate cancer 278-280, maintaining physiological testosterone concentrations should ensure no higher rates of prostate disease than eugonadal men of similar age. Similarly, low blood testosterone concentration is a risk factor for cardiovascular disease 268, 269 so that androgen replacement therapy is likely to improve risk of cardiovascular disease. Hence during androgen replacement therapy aiming to maintain physiological testosterone concentrations, surveillance for cardiovascular and prostate disease should be comparable with, but need be no more intensive than, eugonadal men of equivalent age 270. The effects of pharmacological androgen therapy, where androgen dosage is not necessarily restricted to eugonadal limits, on cardiovascular and prostate disease are difficult to predict and surveillance then depends on the nature, severity and life-expectancy of the underlying disease.
Contraindications and precautions for androgen replacement therapy
Contraindications to androgen replacement therapy are prostate or breast cancer as tumors may be androgen responsive and pregnancy where transplacental passage of androgens may disturb fetal sexual differentiation. Precautions and/or careful monitoring of androgen use is required in (i) initiating treatment in older men who may experience intolerable changes in libido, (ii) competitive athletes who may be disqualified, (iii) women of reproductive age, especially those who use their voice professionally, who may become irreversibly virilized, (iv) in patients with bleeding disorders or during anti?coagulation when parenteral administration may cause severe bruising or bleeding, (v) sex steroid-sensitive epilepsy or migraine, (vi) those with cardiac or renal failure or severe hypertension susceptible to fluid overload from sodium and fluid retention, (vii) men with obstructive sleep apnea which may be exacerbated by exogenous androgens 281. Excessive androgen doses prior to completion of puberty risks premature epiphyseal closure leading to foreshortened final adult stature and/or precocious sexual development.
