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Medical indications for the use of testosterone

Pharmacodynamics

A major androgen. Physiological effects are due to the combined action of testosterone itself and its metabolites with androgenic (dihydrotestosterone) and estrogenic (estradiol) activity. In target organs (prostate, seminal vesicles, skin and others) it is reduced by 5a-reductase to 5a-dihydrotestosterone, which specifically interacts with androgen receptors on the cell surface and penetrates into the nucleus. It is converted to estradiol in some tissues (including the hypothalamus).
The main effects of testosterone are: androgenic (growth and development of genitals, stimulation of their activity, manifestation of secondary sexual characteristics, providing erectile function), anabolic (maintenance of muscle mass and regulation of adipose tissue mass, stimulation of organ-specific protein synthesis in kidneys, liver, sebaceous and sweat glands; maintenance of bone tissue density), antigonadotropic (suppression of gonadotropin secretion), reproductive (maintenance of spermatogenesis), psychophysiological (maintenance of libido, formation of stereotypical sexual behaviour, mood, psychostimulant effect), haemopoietic (stimulation of erythropoietin production in kidneys, erythropoiesis in red bone marrow).

In men with hypogonadism androgen therapy leads to a clinically significant increase in plasma concentration of testosterone, dihydrotestosterone, androstenedione, a decrease in the content of globulin binding sex hormones (HSPG), luteinising and follicle-stimulating hormone, and in men with primary (hypergonadotropic) hypogonadism normalises the concentration of gonadotropin. Sexual function improves, libido increases, fat tissue mass decreases, non-fat body mass increases, bone mineral density increases, psychophysiological state improves.

Pharmacokinetics

Absorption of testosterone from mucous membranes and skin is high and depends on the dosage form. It is administered as testosterone undecanoate (orally, intramuscularly) and active testosterone (topically).
About 98% of circulating testosterone binds to hsps and albumin. Only the free fraction of testosterone is considered biologically active. Biotransformation to inactive metabolites occurs in the liver. After administration of labelled testosterone, about 90% of the radioactivity is detected in the urine as glucuronide and sulphate acid conjugates. Products detectable in urine include androsterone and ethiocholanolone. 6% of the active ingredient is detectable in the faeces after passing through the intestinal-hepatic circulation.

Testosterone formed from testosterone undecanoate by cleavage of the ester bond is metabolised and excreted by the same pathways as endogenous testosterone. Undecanoic acid is metabolised by β-oxidation in the same way as other aliphatic carboxylic acids.

Peculiarities of pharmacokinetics by different routes of administration

After ingestion, a significant portion of testosterone (in the form of undecanoate ester) is absorbed in the small intestine and enters the lymphatic system, thus partially bypassing the primary passage through the liver. From the lymphatic system, testosterone undecanoate enters the plasma. In the plasma and tissues, the natural male sex hormones testosterone and dihydrotestosterone are released as a result of hydrolysis.

A single administration of 80-160 mg of testosterone undecanoate results in a clinically significant increase in the total plasma concentration of testosterone, the maximum concentration is approximately 40 nmol/L and is reached 4-5 h after administration. The increased testosterone concentration persists for at least 8 h. The elimination half-life is from 10 to 100 min.

After intramuscular administration of testosterone undecanoate is gradually released from the depo-form (oil solution) and almost completely broken down by serum esterases into testosterone and undecanoic acid. An increase in serum testosterone concentration is noted as early as the next day.

According to two studies, in men with hypogonadism after a single intramuscular injection of 1000 mg of testosterone undecanoate, mean values of maximum testosterone concentration of 45 and 24 nmol/l are determined after 7 and 14 days, respectively. Postmaximal testosterone levels decrease with a half-life of approximately 53 days.

After repeated intramuscular injections of 1000 mg testosterone undecanoate to men with hypogonadism with an injection interval of 10 weeks, equilibrium concentrations were reached between the 3rd and 5th injection. The mean values of maximum and minimum testosterone concentrations at equilibrium were about 42 and 17 nmol/L, respectively. Postmaximal testosterone levels in serum decreased with a half-life of approximately 90 days, which corresponds to the rate of release of the substance from the depot.

When applied topically, the absorption rate of testosterone ranges from 9 to 14% of the applied dose. After absorption through the skin, testosterone enters the systemic bloodstream in relatively constant concentrations over a 24-hour cycle. Daily fluctuations in testosterone concentrations have the same amplitude as circadian changes in endogenous testosterone content. External administration of testosterone avoids the distribution peaks in blood that occur with the injectable route of administration. In contrast to oral androgen therapy, external administration of testosterone does not cause an increase in hepatic steroid concentrations above physiological norms.

External application of 50 mg of testosterone causes an average increase in its plasma concentration of approximately 2.5 ng/ml (8.7 nmol/l). After discontinuation of treatment, testosterone concentrations begin to decrease approximately 24 h after the last use. Concentrations return to baseline approximately 72-96 h after the last dose. The major active metabolites of testosterone when applied topically are dihydrotestosterone and estradiol.

Indications

Hormone replacement therapy for disorders associated with testosterone deficiency: post-castration syndrome, eunuchoidism, hypopituitarism, endocrine impotence, some forms of infertility associated with impaired spermatogenesis, menopausal symptoms in men (decreased libido and physical activity, deterioration of general well-being), masculinisation in transsexuals.
Testosterone therapy is indicated in men with osteoporosis caused by androgen deficiency.

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