Overcoming PD-1 resistance with a first-in-class dual-mode agent that transforms ‘cold' tumors to ‘hot'

Background: Immune checkpoint blockade fails in most colorectal cancers (CRCs) because they are immunologically ‘cold' and mismatch repair-proficient (pMMR). This stands in stark contrast to mismatch repair-deficient (dMMR) ‘hot' tumors, which are highly responsive; neoadjuvant anti-PD-1 achieves a 100% clinical complete response in early-stage dMMR CRC. This profound disparity underscores the urgent need for a strategy to therapeutically convert pMMR tumors into a dMMR-like state. Current approaches lack the coordinated mechanism necessary to induce and sustain this conversion at the cellular level. Methods: We have designed a panel of dual-mode agents to deliver two synergistic insults to individual tumor cells, directly and indirectly disrupting DNA mismatch repair (MMR) MSH2/6 genes. Using a real-time microsatellite instability (MSI) reporter screen, we identified lead compounds that mechanistically induce a high MSI (MSI-H) phenotype. The most promising candidate was subsequently evaluated in murine models of pMMR CRC resistant to PD-1 blockade. Results: We identified BDB025, a potent, first-in-class dual-mode agent that induces a robust MSI-H phenotype while simultaneously epigenetically silencing critical MMR genes. Crucially, this dMMR-like conversion is achieved at very low, sub-cytotoxic dosages, demonstrating a unique non-cell-death-mediated mechanism of action. This coordinated, two-pronged attack within the same cell overwhelms the DNA mismatch repair machinery, specifically MSH2/MSH6 , increases tumor mutational burden and T cell influx. Consequently, BDB025 converts ‘cold' pMMR tumors to ‘hot', ablates established tumors, and overcomes resistance to anti-PD-1 therapy, significantly outperforming standard agents. Conclusion: We demonstrate a paradigm-shifting therapeutic strategy: a single molecule that pharmacologically induces a synthetic dMMR state by co-opting two synergistic mechanisms of action. Our lead candidate, BDB025, effectively breaks PD-1 resistance, paving the way to extend the curative potential of immunotherapy to the vast population of patients with pMMR cancers. Keywords: PD-1 resistance, colorectal cancer, MSI-H, DNA mismatch repair, MSH2, MSH6