Abstract 4460: Aromatase inhibition by active tamoxifen metabolites

Abstract

Abstract Introduction: The mechanism of tamoxifen action in the treatment of breast cancer is believed to be via active metabolites that act as potent antagonists at the estrogen receptors. Despite this, few data have been published that relate the concentrations of these metabolites to their expected clinical effects. Since anti-estrogenic clinical effects may be brought about not only by estrogen antagonism but also by reduced estrogen concentrations, we tested the direct effects of tamoxifen and its active metabolites on aromatase activity. Methods: The activity of recombinant human aromatase was measured by following the generation of fluorescent metabolite from 7-methoxy-4-trifluoromethylcoumarin (MFC) in microsomal incubations in the presence and absence of multiple concentrations of tamoxifen, endoxifen, N-desmethyl-tamoxifen and 4-hydroxy-tamoxifen. Inhibition of aromatase was determined at enzyme concentrations set at 7.5 nM by measuring the percentage of enzyme activity remaining. Results: With MFC concentrations at the Km (25 µM), the IC50s for endoxifen and N-desmethyl-tamoxifen were 6.1 µM and 20.7 µM respectively. At concentrations up to 50 µM, no appreciable inhibition by tamoxifen or 4-hydroxy-tamoxifen was observed. Detailed characterization of inhibition by endoxifen and N-desmethyl-tamoxifen across a range of substrate concentrations demonstrated non-competitive kinetics for both inhibitors. The Ki values for endoxifen and N-desmethyl-tamoxifen were 4.0 µM and 15.9 µM respectively. Preincubation of these tamoxifen metabolites with aromatase did not demonstrate any irreversible effect on enzyme activity. Discussion: Among tamoxifen and its three major metabolites, only endoxifen and N-desmethyl-tamoxifen were found to be potent inhibitors of aromatase. This enzyme is the major source of estrogen in humans, and changes in its activity may thus alter estrogen-regulated physiology. Inhibition by these tamoxifen metabolites may contribute to changes in serum and tissue estrogen concentrations, and may explain in part the variability in bone density, serum lipid concentration, severity of musculoskeletal pain and breast cancer recurrence observed in patients taking tamoxifen. Relationships between tamoxifen metabolite concentrations and clinical outcomes may be complex, and mechanisms that underlie the multiple clinical effects of tamoxifen include the possibility of aromatase inhibition by endoxifen and N-desmethyl-tamoxifen. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4460. doi:10.1158/1538-7445.AM2011-4460