Influence of solvent on the availability of testosterone propionate from oily, intramuscular injections in the rat

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Abstract
The suggestion that the biological response to an oily intramuscular injection of testosterone ester is regulated by rapid accumulation of the steroid in body fat, followed by a slow release, has been tested by comparing the release rates of 14C-labelled testosterone propionate from different solvents following intramuscular injection into rats. Disappearance from the injection site was rectilinearly related to in-vitro partition coefficients, but elimination of radioactivity in urine and faeces was significantly longer, and the same for all four solvents. Testosterone and testosterone propionate were found in equal concentration in body fat, 2 and 3 days after injection, but their concentrations were too low to form an effective depot. It is suggested that the delay in release, and the independence of the delay on the nature of the solvent is a consequence of biliary recycling of testosterone.