Effects of tamoxifen on testosterone metabolism in postmenopausal women with breast cancer.

: Testosterone is a known estrogen precursor especially in postmenopausal women. Tamoxifen, an anti-estrogen, is used in the treatment of women with breast cancer in whom metastatic disease has been demonstrated. The action of Tamoxifen is thought to be to occupy the intracellular estrogen receptor sites in target tissues and thus block the action of the biologically active estrogen, estradiol. Effects of Tamoxifen on the production and metabolism of hormones have been postulated. We studied the kinetics of testosterone metabolism by the constant infusion of 3H-testosterone in six postmenopausal women with breast cancer prior to and during Tamoxifen therapy. The Tamoxifen did not produce any significant change in the metabolic clearance rate, the plasma concentration or the calculated blood production rate of testosterone. The only significant alteration in the conversion ratio of testosterone to metabolites was the reduction (p less than 0.02) in conversion to 5 alpha-dihydrotestosterone. A significant reduction in the plasma concentrations (p less than 0.05) of dehydroepiandrosterone and of luteinizing hormone (p less than 0.02) was found. Other steroid and peptide hormones did not show any significant changes. We conclude that Tamoxifen therapy has very little effect on the kinetics of testosterone metabolism in postmenopausal women with metastatic breast cancer.