Gestonorone caproate is a potent, long-acting, and pure progestogen, possessing no androgenic, anabolic, antiandrogenic, estrogenic, antiestrogenic, glucocorticoid, mineralocorticoid, or teratogenic effects. It is approximately 20 to 25 times more potent than progesterone or hydroxyprogesterone caproate in animal bioassays when all are given by subcutaneous injection. In humans, 100 or 200 mg intramuscular gestonorone caproate has been said to be equivalent to 1,000 mg intramuscular hydroxyprogesterone caproate. Hence, gestonorone caproate is approximately 5- to 10-fold more potent than hydroxyprogesterone caproate in humans. The biological effects of gestonorone caproate in women have been studied.
Like other potent progestins, gestonorone caproate possesses potent antigonadotropic activity and is capable of markedly suppressing the gonadal production and circulating levels of sex hormones such as testosterone and estradiol. A clinical study found that 400 mg/week intramuscular gestonorone caproate suppressed testosterone levels by 75% in men, while orchiectomy as a comparator reduced testosterone levels by 91%. Levels of luteinizing hormone, conversely, remained unchanged. In general, progestogens can maximally suppress testosterone levels by about 70 to 80%. In accordance with its lack of glucocorticoid activity, gestonorone caproate has no anticorticotropic effects, and does not influence the secretion of adrenocorticotropic hormone.
17α-Hydroxyprogesterone has weak progestogenic activity, but C17α esterification results in higher progestogenic activity. Of a variety of different esters, the caproate (hexanoate) ester was found to have the strongest progestogenic activity, and this formed the basis for the development of gestonorone caproate, as well as other caproate progestogen esters such as hydroxyprogesterone caproate.
Gestonorone caproate has been found to decrease the weights of the prostate gland and seminal vesicles by 40 to 70% in adult male rats. It has been shown in canines to mediate these effects both via its antigonadotropic effects and by direct actions in these tissues. Gestonorone caproate decreases the uptake of testosterone into the prostate gland. It has also been found to have direct antiproliferative effects on human ovarian cancer cells in vitro.
Gestonorone caproate has been reported to act to some extent as a 5α-reductase inhibitor, similarly to progesterone.