Abstract 3890: Interspecies comparison of in vitro endoxifen metabolism

Endoxifen (N-desmethyl-4-hydroxytamoxifen) is an active metabolite of tamoxifen being developed for use in breast cancer prevention and therapy. To optimize model selection for preclinical toxicology and pharmacology studies, the in vitro metabolism of endoxifen was compared using commercially available pools of hepatocytes from humans, rats, dogs, rabbits, minipigs, and non-human primates. Endoxifen (10 μM) was incubated with hepatocytes for 0, 2, or 4 hours. Cells were harvested, extracted, and analyzed by LC-MS/MS to quantitate levels of parent compound and major metabolites. Although substantial differences in patterns of endoxifen metabolism were seen in different species, endoxifen glucuronide and endoxifen sulfate were the primary metabolites identified in all species. Endoxifen was relatively stable when incubated with human hepatocytes: after 2 and 4 hours of incubation, endoxifen levels were > 20-fold greater than levels of endoxifen glucuronide and > 5-fold greater than levels of endoxifen sulfate. Levels of endoxifen glucuronide and endoxifen sulfate in human hepatocytes were similar at both time points. By contrast, endoxifen was extensively glucuronidated (but not sulfated) in both rabbit and minipig hepatocytes: after 4 hours of incubation, levels of endoxifen glucuronide in rabbit and minipig hepatocytes were > 5 times the level of parent drug. Levels of endoxifen sulfate were very low in both rabbits and minipigs. Rats and dogs demonstrated patterns of endoxifen metabolism that were intermediate to those of humans, rabbits, and minipigs. In rat hepatocytes, levels of parent drug were slightly greater than levels of endoxifen glucronide at 4 hours; little endoxifen sulfate was generated in rat hepatocytes. In dog hepatocytes, levels of endoxifen and endoxifen glucuronide were comparable at 4 hours; sulfation of endoxifen was greater in dogs than in other species, as levels of endoxifen sulfate at 4 hours were approximately 2/3 of the levels of parent drug and endoxifen glucuronide. Although endoxifen was somewhat less stable in hepatocytes isolated from non-human primates than in human hepatocytes, endoxifen metabolism in hepatocytes from non-human primates demonstrated the greatest similarity to metabolism in human hepatocyte pools: at 4 hours, levels of parent drug in monkey hepatocytes were approximately 4 times the levels of endoxifen glucuronide and 8 times the levels of endoxifen sulfate. These data demonstrate that endoxifen is quite stable when incubated with hepatocytes from humans and non-human primates. By contrast, endoxifen is rapidly and extensively glucuronidated in hepatocytes from rabbits and minipigs. Rats and dogs, the two species used most commonly in preclinical safety studies, both demonstrate substantially more rapid endoxifen metabolism than do humans. [Supported by HHSN261201500024I from the National Cancer Institute, DHHS.] Citation Format: Miguel Muzzio, Zhihua Huang, Elizabeth R. Glaze, David L. McCormick. Interspecies comparison of in vitro endoxifen metabolism [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3890.