Pharmacodynamics of tamoxifen and its 4-hydroxy and N-desmethyl metabolites: Activation of caspases and induction of apoptosis in rat mammary tumors and in human breast cancer cell lines

The antiestrogen tamoxifen (TAM) is extensively metabolized by cytochrome P -450 in humans and rodents. The active, estrogen receptor-binding metabolit es, 4-hydroxy TAM (OHT) and N-desmethyl TAM (DMT) have been well characteri zed. We showed that the s.c. injection of 1 mg/kg TAM in adult female Sprag ue Dawley rats bearing carcinogen-induced mammary tumors resulted in rapid serum decline of parent TAM but higher exposure of the metabolites, OHT and DMT. We found for the first time that the administration of TAM for a shor t time resulted in a delayed induction of caspase activity and apoptosis wi thin the mammary tumors. When TAM, OHT, or DMT was added to human breast ca ncer cell lines in culture, each elicited a time- and dose-dependent induct ion of caspase activity, preceding apoptosis. Importantly, pretreatment of the cells with a pharmacological inhibitor of caspases [benzyloxy Val-Ala-A sp-fluoromethyl ketone (z-VAD-fmk)] blocked apoptosis induced by all three of the compounds, implicating a critical role of caspases in TAM-, OHT-, or DMT-induced apoptosis. The results obtained from these studies suggest tha t one possible mechanism of inhibition of mammary carcinogenesis and tumor growth in vivo may be the induction of caspase-dependent apoptosis, and tha t the metabolites OHT and DMT may contribute to the antitumor effect of TAM .