Early studies with finasteride a type 2 5areductase inhibitor

Initial studies with finasteride were conducted in normal volunteers to determine the biochemical efficacy and safety profile of the drug after single doses or with multiple daily dosing.

18.3.1 Effects on serum androgens and gonadotropins

Administration of finasteride markedly reduces circulating DHT levels in adult men (~70% below baseline), and inhibition of DHT formation is maintainedwith chronic dosing (Gormley 1995; Gormley etal. 1990; Stoner et al. 1994). Because finasteride is a selective inhibitor of the type 2 5aR enzyme in man, complete inhibition of DHT formation does not occur with treatment. Subsequent studies confirmed that the residual level of circulating DHT (~30% of total circulating DHT) observed with finasteride administration derives from the activity of the type 1 5aR enzyme (see Section 18.8), which is unaffected by finasteride. Because conversion of testosterone to DHT is reduced with finasteride administration, metabolism of testosterone is reduced, thereby leading to a small (~15%) increase in serum testosterone. Since testosterone is the primary substrate for estradiol formation by aromatasein men (Fig. 18.1), there is a similar small (~15%) increase in circulating estradiol levels. Because both testosterone and estradiol increase to a similar extent with finasteride treatment, the ratio of circulating testosterone to estradiol is unaltered. Finasteride treatment for six months produced no clinically significant effects on sex hormone-binding globulin or free hormone (testosterone) levels (Tenover etal. 1989).

Despite the marked reduction in circulating levels of DHT with finasteride treatment, circulating levels of the pituitary gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), remain within normal limits in treated men, with mean levels either unchanged or increased slightly. Provocative testing of the hypothalamic-pituitary axis, using gonadotropin-releasing hormone (GnRH), demonstrates that the response of LH and FSH to GnRH stimulation is not affected by finasteride treatment (Rittmaster etal. 1992).

18.3.2 Effects on other hormones

Finasteride administration has no effect on circulating levels of Cortisol (basal or adrenocorticotropic hormone [ACTH]-stimulated), prolactin, thyroid-stimulating hormone or thyroxine, and no effect on glucose tolerance is observed.

18.3.3 Effects on hematologic parameters

No effects on hematologic or clinical chemistry safety parameters are observed with finasteride treatment.

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