Abstract 4472: Combination of medroxyprogesterone acetate (MPA) and a WEE1 or ATR inhibitor enhances anti-tumor activity of estrogen receptor-positive endometrial cancer cells.

Abstract Objectives: Hormonal therapy is an important treatment option for patients with early-stage, low-grade endometrial cancer who desire fertility preservation, as well as for those with advanced or recurrent disease resistant to chemotherapy and immune checkpoint inhibitors. Based on evidence that estrogen signaling regulates cell cycle progression via cyclin D1 transcription, the combination of endocrine therapy with cyclin-dependent kinase 4/6 inhibitors has become a pivotal strategy in the management of estrogen receptor-positive (ER+) recurrent breast cancer in recent years. However, clinical trials investigating similar approaches in ER+ recurrent endometrial cancer have shown only limited responses so far. The aim of this study is to develop a novel therapeutic strategy combining hormonal therapy with targeted therapy for ER+ endometrial cancer. Methods: The anti-tumor effects of combining hormonal therapies were assessed, using ER+ endometrial cancer cells, ARK1 and MFE280. Tamoxifen, letrozole, fulvestrant, and medroxyprogesterone acetate (MPA, 25 μM for MFE280; 30 μM for ARK1) were used as hormone therapies. As drugs for targeted therapies, either the ATR inhibitor (ATRi) AZD6738 (2.5 μM) or the WEE1 inhibitor (WEE1i) MK1775 (0.1 μM) was employed. The anti-tumor effect of ESR1 knockdown via siRNA was also examined. The synergistic effects of MPA and ATRi/WEE1i were evaluated by coefficient of drug interaction (CDI) methods. Results: Among these hormone therapies, only MPA showed a distinct dose-dependent anti-tumor effect, and thus it was selected for combination with molecular-targeted agents. MPA and ATRi synergistically decreased cell viability compared with control in both cell lines, reducing viability to 0.48-, 0.40-, and 0.16-fold in ARK1 and to 0.83-, 0.85-, and 0.56-fold in MFE280 by MPA monotherapy, ATRi monotherapy, and MPA-ATRi combination therapy, respectively. The CDI values of the MPA-ATRi combination were less than 1 (ARK1 = 0.81; MFE280 = 0.79). When ESR1 was knocked down, the cell viability under MPA-ATRi combination treatment was 0.37-fold in ARK1 and 0.74-fold in MFE280 relative to the control, suggesting that ESR1 knockdown attenuated the anti-tumor effects of the combination therapy. In contrast, MPA and WEE1i exhibited only additive effects in ARK1 cells, reducing viability to 0.39-, 0.71-, and 0.30-fold following MPA monotherapy, WEE1i monotherapy, and MPA-WEE1i combination therapy, respectively. The CDI value for the combination was approximately 1, consistent with an additive interaction. Conclusion: We found that the combination of MPA and ATRi markedly enhances anti-tumor activity in ER+ endometrial cancer, in an ERα expression-dependent manner. Future studies are needed to elucidate the mechanism of these synergistic effects. Citation Format: Yuki Takemoto, Yasuto Kinose, Li Han, Mai Koizumi, Yan Wang, Kanako Kasuya, Aasa Shimizu, Erika Nakatsuka, Mahiru Kawano, Kenjiro Sawada, Michiko Kodama. Combination of medroxyprogesterone acetate (MPA) and a WEE1 or ATR inhibitor enhances anti-tumor activity of estrogen receptor-positive endometrial cancer cells abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 4472.