Source: Medicines & Healthcare Products Regulatory Agency (GB) Revision Year: 2018 Publisher: Besins Healthcare (UK) Limited, 1st Floor, 28 Poland Street, London, W1F 8QN, United Kingdom
Pharmacotherapeutic group: Androgens
ATC code: G03BA03
Endogenous androgens, principally testosterone, secreted by the testes and its major metabolite DHT, are responsible for the development of the external and internal genital organs and for maintaining the secondary sexual characteristics (stimulating hair growth, deepening of the voice, development of the libido); for a general effect on protein anabolism; for development of skeletal muscle and body fat distribution; for a reduction in urinary nitrogen, sodium, potassium, chloride, phosphate and water excretion.
Testosterone does not produce testicular development: it reduces the pituitary secretion of gonadotropins.
The effects of testosterone in some target organs arise after peripheral conversion of testosterone to estradiol, which than binds to oestrogen receptors in the target cell nucleus e.g. the pituitary, fat, brain, bone and testicular Leydig cells.
The percutaneous absorption of testosterone ranges from approximately 9% to 14% of the applied dose.
Following percutaneous absorption, testosterone diffuses into the systemic circulation at relatively constant concentrations during the 24 hour cycle.
Serum testosterone concentrations increase from the first hour after an application, reaching steady state from day two. Daily changes in testosterone concentrations are then of similar amplitude to those observed during the circadian rhythm of endogenous testosterone. The percutaneous route therefore avoids the blood distribution peaks produced by injections. It does not produce supra-physiological hepatic concentrations of the steroid in contrast to oral androgen therapy.
Administration of 5 g of Testogel produces an average testosterone concentration increase of approximately 2.5 ng/ml (8,7 nmol/l) in plasma.
When treatment is stopped, testosterone concentrations start decreasing approximately 24 hours after the last dose. Concentrations return to baseline approximately 72 to 96 hours after the final dose.
The major active metabolites of testosterone are dihydrotestosterone and estradiol.
Testosterone is excreted, mostly in urine, and in faeces as conjugated testosterone metabolites.
Testosterone has been found to be non-mutagenic in vitro using the reverse mutation model (Ames test) or hamster ovary cells. A relationship between androgen treatment and certain cancers has been found in studies on laboratory animals. Experimental data in rats have shown increased incidences of prostate cancer after treatment with testosterone.
Sex hormones are known to facilitate the development of certain tumours induced by known carcinogenic agents. No correlation between these findings and the actual risk in human beings has been established.
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