The PKMYT1 inhibitor RP-6306 has synergistic efficacy with carboplatin in CCNE1 amplified tumor models

Background: PKMYT1 is an essential mitotic checkpoint kinase in cancer cells with amplification of the replication stress-inducing gene CCNE1. RP-6306 is a first-in-class highly potent and selective PKMYT1 inhibitor currently in clinical trials as a single agent (NCT04855656) and in combination with gemcitabine and irinotecan-based therapy (NCT05147350 and NCT05147272). PKMYT1 inhibition combined with replication stress induces premature mitosis that ultimately kills cells by induction of catastrophic DNA damage. Platinum-based chemotherapies also induce replication stress via a variety of mechanisms, and they have found broad use in the clinic, including ovarian cancer where CCNE1 amplification is relatively frequent and platinum efficacy is limited. Here we investigate the combination of carboplatin with RP-6306 in CCNE1-amplified tumor settings to understand the potential for synergy that could be further tested in clinic. Methods and Results: We demonstrate that combining carboplatin with the PKMYT1 inhibitor RP-6306 is synergistic in several preclinical models of CCNE1-amplified cancer. In vitro, strong synergistic effects are observed in a CCNE1-amplified ovarian model using low concentrations of carboplatin in cell growth inhibition assays. In vivo, this synergy manifests as regressions in an OVCAR-3 xenograft model at well-tolerated doses while single agent carboplatin or RP-6306 at the same doses shows growth inhibitory effects. Finally, we describe a mechanistic model for the effective combination of RP-6306 with carboplatin measuring the mitotic marker phospho-Histone H3 and DNA damage marker γH2AX. Conclusions: Together, these results provide a strong rationale for clinical development of this combination in CCNE1-amplified cancer that has therapeutic potential across several solid tumor types where platins are used. Conflict of interest: Other Substantive Relationships: All the authors of this abstract are employees of Repare Therapeutics Inc.