Background: Debio 0123 is a highly selective and brain penetrant WEE1 inhibitor that has previously shown significant preclinical efficacy in combination with DNA damaging agents and is currently being investigated in several phase 1 clinical studies either as a monotherapy or in combination. WEE1 plays an important role in regulating the cell cycle during the S-phase and at the G2/M transition. Lunresertib (RP6306) is a selective and potent PKMYT1 inhibitor currently in phase 1 clinical studies as a monotherapy or in combination. Like WEE1, PKMYT1 regulates the G2/M transition, arresting cells with DNA damage to permit repair before progressing into mitosis. Herein, we present pre-clinical data that demonstrates the synergistic activity of the lunresertib and Debio 0123 combination in breast and ovarian tumor models. Methods: Knockdown studies were performed utilizing siRNA against PKMYT1 and WEE1 to explore potential synergistic relationship of these targets in different cancer cell types. In vitro, combination efficacy of the selective WEE1i (Debio 0123) and PKMYT1i (lunresertib) was assessed by cytotoxicity in cell lines across different cancer types. Additionally, combination efficacy of lunresertib and Debio 0123 was further assessed in vivo in subcutaneous breast and ovarian cancer cell line-derived and patient-derived xenograft models. Results: Knockdown of either PKMYT1 or WEE1 alone resulted in minor decrease in cell viability, however, concomitant knockdown of both targets resulted in no surviving cell populations. In vitro, combination of lunresertib and Debio 0123 resulted in significant synergy (Bliss) at low nM doses of both compounds. In vivo, in models of ovarian or breast cancer, monotherapy treatments with either Debio 0123 or lunresertib only resulted in minor tumor growth inhibition. Strikingly, the combination of Debio 0123 and lunresertib at all dose levels and schedules tested across all 5 models resulted in complete and sustained regression of all tumors. Conclusions: These results demonstrate, in preclinical models of ovarian and breast cancer, the WEE1 inhibitor, Debio 0123, is synergistic with the PKMYT1 inhibitor, lunresertib, and consistently leads to deep and sustained regressions in vivo. Citation Format: Luke Piggott, Diana Bianca Rocha Gomes, Paula Martinez Sanz, Violeta Serra. The WEE1 inhibitor Debio 0123 is synergistic with the PKMYT1 inhibitor lunresertib in preclinical models of ovarian and breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 2914.