Citation:

Laws, S., Matt Henderson, M. Hotchkiss, AND L. Mills. In Vitro Metabolism of Tamoxifen in Human, Rat, and Fish Microsomes. Presented at Society of Toxicology, 53rd Annual Meeting, Phoenix, AZ, March 23 - 27, 2014.

Impact/Purpose:

Poster presentation for Annual Meeting 2014 Society of Toxicology. This study investigates if species differences in the in vivo metabolism of an endocrine active pharmacuetical (tamoxifen) can be predicted using in vitro assay containing hepatic microsomes from fish, rat or human.

Description:

Results from an in vivo study comparing biologically-active metabolites in the plasma of Wistar rats and cunner fish (Tautogolabrus adspersus) treated with tamoxifen indicate notable differences in circulating metabolite concentrations between these two species. After a single oral dose of tamoxifen (25 mg/kg), the predominant metabolite observed from 1 to 72 hrs in rats was N-desmethyltamoxifen while in cunner it was 4-hydroxytamoxifen. The predominant metabolite in humans has been reported to be N-desmethyltamoxifen. To investigate if these species differences could be predicted using an in vitro assay, hepatic microsomes from cunner fish, Wistar rats, and humans were used. The experiment was designed to measure metabolic activity in all 3 species at 37° and 18°C, but in preliminary experiments, fish microsomes were shown to be inactive at 37°C. Thus, metabolism among all 3 species was compared at 18°C (1, 2, 3, 4 hrs), and rat and human microsomes were also compared at 37oC (0.5, 1, 1.5, 2 hrs). Final assay concentrations of microsomes (0.4 mg/mL), tamoxifen (10μM), and NADPH (30μM) were prepared in KPO4 (pH 7.4, 100mM). Metabolic activity was quenched by adding 0.5 mL of ice cold acetone. Samples were stored at -80°C until analyzed for tamoxifen and metabolites using LC-MS/MS. The highest concentrations of N-desmethyltamoxifen and 4-hydroxytamoxifen at all time points were observed in rat microsomes. In rat and human microsomes, N-desmethyltamoxifen was the predominant metabolite at both 18°C and 37°C. While formation of 4-hydroxytamoxifen was minimal with human microsomes, this metabolite doubled from 1 to 2 hrs in fish and rat microsomes (18°C). Unlike in vivo results, fish microsomes produced about twice as much N-desmethyltamoxifen as 4-hydroxytamoxifen during 4 hrs in the in vitro assays. Thus, though these in vitro results are similar to in vivo studies reported for human and rats, they do not reflect in vivo results observed for cunner fish. This abstract does not necessarily reflect U.S.EPA policy.

Record Details: