The effect of esterification on the release of testosterone and estradiol from silicone implants

Four experiments were conducted to determine the effect of esterification on the release of testosterone and estradiol from capsule-type silicone implants. In castrated male rats testosterone propionate and, testosterone enanthate implants increased (P < .05) blood testosterone concentrations and seminal vesicle weights move than empty and testosterone implants. Testosterone propionate and testosterone enanthate implants released 4 times and 10 times, respectively, more equivalent testosterone per cm(2) per day than testosterone implants. The increase in blood testosterone concentrations was proportional to the increase in implant release rate for testosterone propionate, but not for testosterone enanthate. Although estradiol-17 beta implants increased (P < .05) uterine weights of ovariectomized rats, uterine weights in estradiol benzoate implanted ovariectomized rats were 1.3 times greater than in estradiol-17 beta implanted rats. Estradiol benzoate implants released less than 0.5 times the amount of equivalent estradiol per cm(2) per day than estradiol-17 beta implants.