CLINICAL PHARMACOLOGY
Mechanism of Action
Endogenous androgens, including testosterone and dihydrotestosterone (DHT), are responsible for the normal growth and development of the male sex organs and for maintenance of secondary sex characteristics. These effects include the growth and maturation of prostate, seminal vesicles, penis and scrotum; the development of male hair distribution, such as facial, pubic, chest and axillary hair; laryngeal enlargement; vocal cord thickening; and alterations in body musculature and fat distribution. Testosterone and DHT are necessary for the normal development of secondary sex characteristics. Male hypogonadism results from insufficient secretion of testosterone and is characterized by low serum testosterone concentrations. Signs/symptoms associated with male hypogonadism include erectile dysfunction and decreased sexual desire, fatigue and loss of energy, mood depression, regression of secondary sexual characteristics, and osteoporosis.
Male hypogonadism has two main etiologies. Primary hypogonadism is caused by defects of the gonads, such as Klinefelter Syndrome or Leydig cell aplasia, whereas secondary hypogonadism is the failure of the hypothalamus (or pituitary) to produce sufficient gonadotropins (FSH, LH).
Pharmacodynamics
No specific pharmacodynamic studies were conducted using ANDRODERM.
Pharmacokinetics
Absorption
ANDRODERM delivers physiologic amounts of testosterone, producing circulating testosterone concentrations that approximate the normal concentration range (300 - 1030 ng/dL) seen in healthy men. ANDRODERM provides a continuous daily dose of testosterone in a self-contained transdermal system. Following ANDRODERM application, testosterone is continuously absorbed during the 24-hour dosing period with a median (range) Tmax of 8 (4-12) hours.
Distribution
Circulating testosterone is primarily bound in the serum to sex hormone-binding globulin (SHBG) and albumin. Approximately 40% of testosterone in plasma is bound to SHBG, 2% remains unbound (free) and the rest is bound to albumin and other proteins.
Metabolism
Testosterone is metabolized to various 17-keto steroids through two different pathways. The major active metabolites of testosterone are estradiol and dihydrotestosterone (DHT).
During steady-state pharmacokinetic studies in hypogonadal men treated with ANDRODERM, the average DHT:T and E2:T ratios were approximately 1:10 and 1:200, respectively.
Excretion
There is considerable variation in the half-life of testosterone as reported in the literature, ranging from 10 to 100 minutes. About 90% of a dose of testosterone given intramuscularly is excreted in the urine as glucuronic and sulfuric acid conjugates of testosterone and its metabolites; about 6% of a dose is excreted in the feces, mostly in the unconjugated form. Inactivation of testosterone occurs primarily in the liver.
Upon removal of the ANDRODERM systems, serum testosterone concentrations decrease with an apparent half-life of approximately 70 minutes. Hypogonadal concentrations are reached within 24 hours following system removal. There is no accumulation of testosterone during continuous treatment.
Effect of Showering
In a two-way crossover study, the effects of showering on the pharmacokinetics of total testosterone following a single application of ANDRODERM 4 mg/day were assessed in 16 hypogonadal males. Showering 3 hours after application of ANDRODERM increased Cavg by 0.5% and decreased Cmax by 0.4% respectively, as compared to not showering. The systemic exposure to ANDRODERM was similar following applications with or without showering 3 hours after application.
NONCLINICAL TOXICOLOGY
Carcinogenesis, Mutagenesis, Impairment of Fertility
Testosterone has been tested by subcutaneous injection and implantation in mice and rats. In mice, the implant induced cervical-uterine tumors, which metastasized in some cases. There is suggestive evidence that injection of testosterone into some strains of female mice increases their susceptibility to hepatoma. Testosterone is also known to increase the number of tumors and decrease the degree of differentiation of chemically induced carcinomas of the liver in rats. Testosterone was negative in the in vitro Ames and in the in vivo mouse micronucleus assays. The administration of exogenous testosterone has been reported to suppress spermatogenesis in the rat, dog and non-human primates, which was reversible on cessation of the treatment.
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