Abstract 3044: PKMYT1 inhibition is synthetically lethal with CCNE1 overexpression and synergizes with standard of care chemotherapy

Abstract BACKGROUND: CCNE1 is frequently overexpressed in ovarian, endometrial, and colorectal cancers due to gene amplification, reduced degradation (via FBXW7 mut/del), or increased gene transcription. High levels of CCNE1 dysregulate the G1/S checkpoint, increase replication stress, and amplify dependence on the G2/M checkpoint to ensure proper DNA damage repair before mitosis. Additionally, high CCNE1 levels are associated with poor prognosis and response to therapy, highlighting the need for improved treatments for CCNE1 high cancers. PKMYT1 is a protein kinase that regulates the G2/M checkpoint. The data presented here demonstrate that PKMYT1 inhibition, which inactivates the G2/M checkpoint and pushes cells into mitosis, is both synthetically lethal in CCNE1-high cancer cells and synergizes with standard of care chemotherapy in vitro and in vivo. METHODS: CCNE1-high cell lines were selected based on CCNE1 gene amplification or FBXW7 LOF. Sensitivity of CCNE1-high cell lines to single agent PKMYT1 inhibition was assessed by measuring the cell cycle, DNA damage, apoptosis, and cell viability. CCNE1-high cancer cell viability was also measured after combined treatment of PKMYT1 inhibition and standard of care chemotherapy. Bliss synergy scores were used to determine synergy. In vivo efficacy of PKMYT1 inhibition alone and in combination with chemotherapy was assessed using CCNE1-high human xenograft models. RESULTS: In CCNE1-high cancer cells, PKMYT1 inhibition disrupts the cell cycle by increasing entry into mitosis, thereby inducing DNA damage and apoptosis. Non-tumorigenic cells are insensitive to PKMYT1 inhibition. In CCNE1-high xenograft models, PKMYT1 inhibition exhibits significant single agent tumor growth inhibition, and its efficacy correlates with increased mitotic entry. Importantly, PKMYT1 inhibition synergizes with standard of care chemotherapies, namely those targeting the S and G2 phases of the cell cycle, in CCNE1-high cancers both in vitro and in vivo. CONCLUSION: The data presented here strongly support targeting CCNE1-high cancer cells with a combination of PKMYT1 inhibition and chemotherapy. CCNE1-high cancer cells, which accumulate DNA damage in S phase due to a dysregulated G1/S checkpoint, are highly susceptible to PKMYT1 inhibition due to the propagation of this increased DNA damage into mitosis. Additionally, chemotherapies targeting S/G2 phases, including nucleoside analogs and topoisomerase inhibitors, combine with PKMYT1 inhibition by further enhancing replication stress. Importantly, non-tumorigenic cells are insensitive to PKMYT1 inhibition due to intact G1/S checkpoint and DNA damage repair pathways. In summary, these data highlight PKMYT1 inhibition as a promising approach to selectivity target CCNE1-high cancer cells and enhance the effectiveness of standard of care chemotherapies while sparing non-tumorigenic cells. Citation Format: Natalie Hill, Jeremy Fournier, Srijita Bhowmik, Jenny Zhu, Saranya Chandrasekar, Ester Fernandez-Salas, Samantha S. Hodge. PKMYT1 inhibition is synthetically lethal with CCNE1 overexpression and synergizes with standard of care chemotherapy [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 3044.