(107) Tamoxifen Accumulation And Metabolism In The Mouse Ovary

Rita Li¹; Isaac Asante¹; Michael Awadalla²; Xiaohui Yu¹; Gulbahar Z Kutluturk³; Stan Louie¹; Sue Ann Ingles⁴; Trenton Place²; Rachel Danis²; Lynda K McGinnis²

Department of Clinical Pharmacy¹, University of Southern California, Los Angeles CA USA

Department of Obstetrics and Gynecology², University of Southern California, Los Angeles CA USA

Department of Molecular Pathology and Experimental Medicine³, University of Southern California, Los Angeles CA USA

Department of Population and Public Health Sciences⁴, University of Southern California, Los Angeles CA USA

Abstract Text: Tamoxifen (TAM) is a selective estrogen modulator (SERM) commonly used as a long-term adjuvant chemotherapeutic for reproductive-age women with estrogen-sensitive breast cancer. Despite its widespread use, little is known about the long-term impact on the female reproductive system. To address this, we developed and characterized a mouse model of long-term TAM treatments feeding mice TAM in their food (TAM-chow) which is less stressful than daily gavage or injections.  The serum and ovarian concentration of TAM were evaluated in adult female mice.   Two concentrations of TAM were tested: 250 mg/kg and 500 mg/kg in mouse chow.  Animals were weighed every 2 days and samples collected after 2- or 5-weeks of treatment. The serum and ovarian concentration of TAM and its metabolites (e.g., N-desmethyl-TAM, TAM-O and endoxifen) were measured by liquid chromatography mass spectrophotometry (LC-MS/MS). Correlation between serum and ovarian concentrations of TAM and its metabolites were evaluated.  Real-time PCR was used to measure expression of estrogen receptors (Esr1, Esr2, Gpr30) and cytochrome p450 (CYP) enzymes in whole ovaries. TAM-fed mice lost weight during the first few days of the study, but thereafter regained weight. Mice fed the 250 mg diet regained weight more rapidly than the 500 mg diet.  By 21 days, the 250 mg group was significantly heavier than 500 mg group and not different from controls. TAM concentration in serum was higher at 5-weeks (24.5ng/ml and 97.6ng/ml) versus 2-weeks (12.5ng/ml and 24.5ng/ml) for the 250 and 500 groups respectively. In contrast ovarian tissue TAM levels remained stable with 0.84ng/ml and 5.21ng/ml TAM at 5 weeks versus 1.31ng/mg and 6.73ng/mg at 2 weeks, respectively (P >0.05).   This study showed that TAM and desmethyl TAM are transported into ovaries; similarly, Z-endoxifen formed from CYP2D6 is also transported. However, e-endoxifen appears to be locally produced by the CYP3A found in the ovaries.  An examination of estrogen receptor expression in response to TAM revealed an increased expression of Esr1 at 2-weeks of TAM treatment. Esr1 declined by 5-weeks but was still over-expressed compared to untreated ovaries. In contrast Esr2 was sharply downregulated after 2-weeks of TAM treatment, then was significantly increased above controls at 5-weeks (P< 0.01).  Expression of the non-genomic estrogen receptor, Gpr30, was also evaluated and found that TAM treatment significantly increased Gpr30 expression for both 250 and 500 mg groups, remaining high at both 2-weeks and 5-weeks (P< 0.01).  Feeding mice TAM ad lib in mouse chow was effective in producing a mouse model to evaluate the long-term TAM impact on the murine reproductive system.  LC-MS/MS determined that TAM and metabolite exposure differed between serum and ovarian tissue indicating that serum levels may not accurately predict exposure levels experience by developing oocytes.