The Effect of Long-term Oral Tamoxifen and Metabolism in the Mouse Ovary

Tamoxifen (TAM) is a selective estrogen receptor modulator (SERM) widely used as long-term chemotherapy for premenopausal women with estrogen-sensitive breast cancer. We developed a murine model of chronic TAM exposure by administering TAM admixed in mouse chow (TAM-chow), a less stressful and physiologically relevant alternative to daily gavage or injections. Mice received either 36mg/kg/day or 72mg/kg/day for 2 or 5 weeks. Body weights were monitored every 2-days; blood and tissue samples collected at endpoints. TAM and key metabolites (N-desmethyl-TAM (NDM-TAM), 4-hydroxytamoxifen (4-OH-TAM), and endoxifen isomers,) were quantified in serum and ovarian tissues using LC-MS/MS. The concentrations of TAM and metabolites showed dose-dependent trends: serum TAM levels increased with time, while ovarian levels remained relatively stable. Correlation analyses suggest that TAM, NDM-TAM, and Z-endoxifen are transported into the ovary with linear serum-ovary concentration relationships. 4-OH-TAM showed a significant monotonic association, but the relationship was not well described by a specific functional model. In contrast, E-endoxifen may reflect local ovarian metabolism involving CYP-mediated pathways. Estrogen receptor expression in ovarian tissue was dynamically regulated, where Esr1 increased significantly after 2-weeks and returned toward baseline by 5-weeks, while Esr2 showed a biphasic response with initial downregulation followed by upregulation at 5-weeks. Estrogen receptor Gpr30 was elevated across treatment groups, indicating sustained upregulation. Taken together, the TAM-chow model effectively simulates long-term oral TAM treatment, enabling detailed evaluation of ovarian TAM pharmacokinetics, metabolism, and estrogen signaling alterations. This approach offers a robust platform to study chronic TAM effects on female reproductive biology under clinically relevant conditions.