Stikkelbroeck et al. (2001) investigated the prevalence of testicular tumors in 17 adolescent and adult male patients with CAH aged 16 to 40 years. In 16 of 17 patients, one or more testicular tumors ranging in maximal length from to cm were found on ultrasonography. In 6 patients, the testicular tumors were palpable. Undertreatment, defined as the presence of a salivary androstenedione level above the upper reference morning level, was found in 5 of 17 patients at the time of investigation. The other 12 patients were treated adequately or even overtreated at the time of investigation. Nevertheless, 11 of these 12 patients showed testicular tumors on ultrasonography. Tumor size was significantly larger in patients who were heterozygous or homozygous for deletion or conversion of the CYP21 gene than in patients who did not have this genotype. Impairment of Leydig cell function as manifested by decreased plasma levels of testosterone was found in 6 of 17 patients. Semen analysis in 11 patients revealed azoospermia in 3 patients and poor semen quality in 4 patients. The authors concluded that, when carefully sought for, testicular adrenal rest tumors are frequently present in adolescent and adult males with CAH and are often accompanied by impaired spermatogenesis and Leydig cell failure.
The mechanisms of variable response to tamoxifen have been the subject of much scrutiny in the published literature. Early studies attempting to link a clinical response to tamoxifen therapy with plasma tamoxifen concentrations reported no statistically significant differences in outcomes between women who received 20 mg of tamoxifen daily and those who received 40 mg of tamoxifen daily, even though women in the 40 mg tamoxifen group had higher plasma tamoxifen concentrations than those in the 20 mg tamoxifen group. These results have been reported as evidence that plasma tamoxifen concentration is not a predictor of clinical outcome. Because there is evidence that tamoxifen is converted to anti-estrogenic metabolites, one hypothesis is that altered patterns of metabolism of tamoxifen might contribute to inter-individual variability in effects (Jin et al, 2005). Plasma concentrations of the active tamoxifen metabolite endoxifen are associated with the cytochrome P450 (CYP) 2D6 genotype.
Cytochrome P450 2A6 (abbreviated CYP2A6 ) is a member of the cytochrome P450 mixed-function oxidase system, which is involved in the metabolism of xenobiotics in the body. CYP2A6 is the primary enzyme responsible for the oxidation of nicotine and cotinine . It is also involved in the metabolism of several pharmaceuticals, carcinogens, and a number of coumarin-type alkaloids. CYP2A6 is the only enzyme in the human body that appreciably catalyzes the 7-hydroxylation of coumarin , such that the formation of the product of this reaction, 7-hydroxycoumarin, is used as a probe for CYP2A6 activity.