How are esters converted back to the parent drug?
The ester bond is fairly easily broken under the right conditions. If the molecule is dissolved in water, this can occur by a simple chemical reaction, yielding the parent drug and a carboxylic acid. For example, if the steroid used is testosterone propionate, testosterone and propionic acid are released. Carboxylic acids are safe and natural in the body in reasonable amounts. It should not be thought that these are strong acids because they are not: they are acids in the same sense that, e.g., Vitamin C or lactic acid are acids. Furthermore, the amount of carboxylic acid present at any time is extremely low.
The carboxylic acids do not have any activities of interest. Once the ester group is removed, it has done its job, and the parent drug acts in its normal manner.
Besides the simple chemical hydrolysis described above, the esters can be removed by enzymes in the blood called esterases, though water still is required for the reaction. The great majority of hydrolysis occurs with the help of these enzymes or by non-specific reactions with proteins. These reaction cannot take place while the esterified steroid is dissolved in fat. Thus, while the esterified steroids are dissolved in fat, they are protected from hydrolyis, and thus serve as a depot for the drug, giving extended duration of action.
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