Dihydrotestosterone

Biological activity

DHT has an affinity (K_d) of 0.25 to 0.5 nM for the human AR, which is about 2- to 3-fold higher than that of testosterone (K_d = 0.4 to 1.0 nM) and 15–30 times higher than that of adrenal androgens. The dissociation rate of DHT from the AR is 5-fold slower than that of testosterone. The EC₅₀ of DHT for activation of the AR is 0.13 nM, which is about 5-fold higher than that of testosterone (EC₅₀ = 0.66 nM). In bioassays, DHT has been found to be 2.5- to 10-fold more potent than testosterone.

The terminal half-life of DHT in the body (53 minutes) is longer than that of testosterone (34 minutes), and this may account for some of the difference in their potency. A study of transdermal DHT and testosterone treatment reported terminal half-lives of 2.83 hours and 1.29 hours, respectively.

Sexual development

During male embryogenesis DHT has an essential role in the formation of the male external genitalia, while in the adult male DHT acts as the primary androgen in the prostate gland, seminal vesicles, skin, and hair follicles.

An example illustrating the significance of DHT for the development of secondary sex characteristics is congenital 5α-reductase type II deficiency. This genetic mutation can result in pseudohermaphroditism. The condition typically presents with underdeveloped male genitalia and prostate. Males with this condition are often raised as girls due to their lack of conspicuous male genitalia. At the onset of puberty, although their DHT levels remain very low, their testosterone levels elevate normally. Their musculature develops like that of other male adults. After puberty, men with this condition have a large deficiency of pubic and body hair and reportedly no incidence of androgenic alopecia (pattern hair loss). They also reportedly have no incidence of prostate cancer.

Unlike other androgens such as testosterone, DHT cannot be converted by the enzyme aromatase into an estrogen like estradiol. Therefore, it is frequently used in research settings to distinguish between the effects of testosterone caused by binding to the AR and those caused by testosterone's conversion to estradiol and subsequent binding to and activation of estrogen receptors.

Pathology

DHT produced locally at the site of hair follicles by 5α-reductase, and not systemic DHT, is the primary causal factor in male androgenic alopecia, although the pathology regarding this phenomenon is poorly understood. In the case of female androgenic alopecia, on the other hand, the situation is more complex, and DHT is only one of several possible causes. Women with increased levels of DHT may develop symptoms of hyperandrogenism such as certain androgynous masculine secondary sex characteristics, including a deepened voice and facial hair. In men, prostate growth and differentiation are highly dependent on androgens, especially DHT, and DHT is involved in the pathogenesis of benign prostatic hyperplasia (BPH) and prostate cancer.

Management

5α-Reductase inhibitors like finasteride and dutasteride, which inactivate the 5α-reductase enzyme and block the formation of DHT, are commonly used for the treatment of two DHT-related conditions, androgenic alopecia and BPH. Both finasteride and dutasteride are approved for the treatment of BPH and androgenic alopecia. Dutasteride is three times more potent than finasteride in inhibiting the type II enzyme and 100 times more potent than finasteride in inhibiting the type I form of the DHT-producing enzyme. Both finasteride and dutasteride are potent inhibitors of the third isotype of the enzyme.

Acne, hirsutism (excessive hair growth), and seborrhea are also DHT-related conditions, and 5α-reductase inhibitors may be used to treat these conditions as well. In addition, antiandrogens like cyproterone acetate, spironolactone, and bicalutamide, as well as estrogens like ethinylestradiol (which are functional antiandrogens), may also be used to treat these conditions.

Biosynthesis

DHT is synthesized from testosterone by the enzyme 5α-reductase. In males, approximately 5% of testosterone undergoes 5α-reduction into DHT.

Metabolism

DHT is inactivated in the liver and extrahepatic tissues like the skin into 3α-androstanediol and 3β-androstanediol by the enzymes 3α-hydroxysteroid dehydrogenase and 3β-hydroxysteroid dehydrogenase, respectively. These metabolites are in turn converted, respectively, into androsterone and epiandrosterone, then conjugated (via glucuronidation and/or sulfation), released into circulation, and excreted in urine.

Unlike testosterone, DHT cannot be aromatized into an estrogen, and for this reason, has no propensity for estrogenic effects.

Levels

Serum DHT levels are about 10% of those of testosterone, but levels in the prostate gland are 5- to 10-fold higher than those of testosterone due to a more than 90% conversion of testosterone into DHT by locally expressed 5α-reductase. For this reason, and in addition to the fact that DHT is much more potent as an AR agonist than is testosterone, DHT is considered to be the major androgen of the prostate gland.

Medical use

DHT is available in pharmaceutical formulations for medical use as an androgen or anabolic-androgenic steroid (AAS). When used as a drug, it is referred to as androstanolone (INN) or as stanolone (BAN). The availability of pharmaceutical DHT is limited; it is not available in the United States or Canada, but is available in certain European countries, including the United Kingdom, Germany, France, Spain, Italy, Belgium, and Luxembourg. Brand names of DHT include Anaboleen, Anabolex, Anaprotin (UK), Andractim (formerly AndroGel-DHT) (FR, BE, LU), Androlone, Apeton, Gelovit (ES), Neodrol, Ophtovital, (DE), Pesomax (IT), Stanaprol, and Stanolone, among others. The available formulations of DHT include buccal or sublingual tablets (Anabolex, Stanolone), topical gels (Andractim, Gelovit, Ophtovital), and, as esters in oil, injectables like dihydrotestosterone propionate (Pesomax) and dihydrotestosterone valerate (Apeton). Esters of DHT act as prodrugs of DHT in the body and have a long-lasting depot when given via intramuscular injection. Dihydrotestosterone benzoate (Ermalone-Amp, Hermalone, Sarcosan) and dihydrotestosterone enanthate (Anaboleen Depot) are additional DHT esters that are also available for medical use, while a few others, including dihydrotestosterone acetate, dihydrotestosterone butyrate, and dihydrotestosterone formate, were developed but never marketed.

Unlike testosterone and various synthetic AAS, DHT cannot be aromatized, and for this reason, poses no risk of estrogenic side effects like gynecomastia at any dosage. In addition, DHT cannot be metabolized by 5α-reductase (as it is already 5α-reduced), and for this reason, is not potentiated in so-called androgenic tissues like the skin, hair follicles, and prostate gland. This provides exogenous DHT with a greater ratio of anabolic to androgenic effects compared to testosterone, and DHT may be less prone to producing certain skin and hair-related side effects like acne, oily skin, seborrhea, hirsutism (excess facial/body hair growth), and androgenic alopecia (pattern hair loss), as well as prostate enlargement (which can lead to benign prostatic hyperplasia) and an increased risk of prostate cancer.

Pharmaceutical DHT is used mainly in the treatment of male hypogonadism. It was under development in a topical formulation for the treatment of cachexia in cancer patients, and reached phase III clinical trials for this indication, but ultimately was not introduced for this purpose. Although DHT itself has not been approved for the treatment of cachexia, an orally active synthetic derivative of DHT, oxandrolone (2-oxa-17α-methyl-DHT), is approved and used for this indication.

Chemistry

DHT is a 5α-androstane (C19) steroid with a ketone group at the C3 position and a hydroxyl group at the C17β position. It is the derivative of testosterone in which the double bond between the C4 and C5 positions has been reduced or hydrogenated.

Derivatives

Several C17β ester prodrugs of DHT, including androstanolone benzoate, androstanolone enanthate, androstanolone propionate, and androstanolone valerate, have been developed and introduced for medical use as AAS.

Synthetic derivatives of DHT used as AAS include mesterolone (1α-methyl-DHT), drostanolone (2α-methyl-DHT), metenolone (1β-methyl-δ¹-DHT), stenbolone (2-methyl-δ¹-DHT), epitiostanol (2α,3α-epithio-3-deketo-DHT), mepitiostane (a 17-ether prodrug of epitiostanol), 1-testosterone (dihydroboldenone; Δ¹-DHT), mesabolone (a 17-ether prodrug of Δ¹-DHT), prostanozol (a 17-ether prodrug of the non-17α-methylated analogue of stanozolol), and bolazine (an azine dimer prodrug of a drostanolone-like AAS), as well as the 17α-alkylated derivatives mestanolone (17α-methyl-DHT), methasterone (2α,17α-dimethyl-DHT), oxandrolone (2-oxa-17α-methyl-DHT), oxymetholone (2-hydroxymethylene-17α-methyl-DHT), stanozolol (a 2,3-pyrazole A ring-fused derivative of 17α-methyl-DHT), furazabol (a 2,3-furan A ring-fused derivative of 17α-methyl-DHT), androisoxazole (a 2,3-isoxazole A ring-fused derivative of 17α-methyl-DHT), methylstenbolone (2,17α-dimethyl-δ¹-DHT), methyl-1-testosterone (methyldihydroboldenone; 17α-methyl-δ¹-DHT), methylepitiostanol (2α,3α-epithio-3-deketo-17α-methyl-DHT), desoxymethyltestosterone (3-deketo-17α-methyl-δ²-DHT), and mebolazine (an azine dimer prodrug of a methasterone-like AAS).