PKMYT1 inhibition induces DNA damage and synergizes with immune checkpoint blockade in CCNE1 -amplified gastroesophageal adenocarcinoma

Purpose CCNE1 amplification, found in approximately 10% of gastroesophageal adenocarcinoma (GEA), drives chromosomal instability and is associated with an immune-cold tumor microenvironment. Recent studies suggest that PKMYT1 inhibition is synthetic lethal in CCNE1 -amplified cancers; however, its role in GEA is largely uncharacterized. We investigated the therapeutic activity of PKMYT1 inhibition in CCNE1 -amplified GEA and its potential combination with immunotherapy. Experimental Design We evaluated lunresertib (RP-6306), a selective first-in-class PKMYT1 inhibitor, in CCNE1 -amplified and wild-type GEA cell lines to define its effects on cell viability, DNA damage, and gene expression changes. To investigate the role of PKMYT1 blockade on immune microenvironment modulation, we used a syngeneic murine model of CCNE1; Trp53 -/- GEA xenografts treated with lunresertib, anti-PD-1, or the combination. Results Lunresertib was selectively cytotoxic in CCNE1 -amplified GEA cell lines in cell viability and clonogenic assays. Mechanistically, treatment with lunresertib induced DNA damage, evidence by increased γH2AX expression and DNA micronuclei formation, and activated pro-inflammatory signaling pathways by gene set enrichment analysis. In a CCNE1 -amplified GEA murine model, PKMYT1 inhibition promoted T cell infiltration, reduced myeloid cells, and synergized with anti-PD-1 therapy to induce tumor regression. Conclusions Our results suggest that CCNE1 amplification is a potentially actionable target in GEA and support the development of combination therapeutic strategies utilizing PKMYT1 inhibition and PD-1 blockade. Translational Relevance Gastroesophageal adenocarcinoma (GEA) is an aggressive malignancy with poor prognosis and few actionable molecular alterations. CCNE1 amplification is present in approximately 10% of patients with GEA and represents a potential therapeutic vulnerability through synthetic lethality with PKMYT1 inhibition. We demonstrate that targeting CCNE1 -amplified GEA through PKMYT1 inhibition with lunresertib is selectively cytotoxic, induces DNA damage, and activates inflammatory signaling pathways. In a syngeneic murine model of CCNE1 -amplified gastric cancer, we further identified that PKMYT1 inhibition induced T cell infiltration and tumor microenvironment remodeling, and synergized with anti-PD-1 therapy to drive tumor regression. Together, these results support further development of PKMYT1 inhibition in combination with PD-1 blockade for CCNE1 -amplified GEA.