Treatment with ACR-2316, a potential first- and best-in-class WEE1/PKMYT1 inhibitor, combined with anti-PD-L1 induces complete tumor regression with durable immune memory

Introduction: ACR-2316 is a potent and selective dual WEE1/PKMYT1 inhibitor rationally designed using Acrivon's generative phosphoprotemics AP3 platform. Currently advancing in a Phase 1 clinical study in AP3-identified solid tumor types, ACR-2316 was engineered for superior single-agent activity and high selectivity resulting in potent DNA damage and complete tumor regression across preclinical in vivo models. This study demonstrates that ACR-2316 not only damages the nuclear and mitochondrial genomes, but also stimulates the innate immune system, leading to complete tumor regression and lasting immune memory in mice when combined with PD-L1 blockade. Results: To investigate the mechanisms of immune activation by ACR-2316, we performed AP3-based proteomic profiling of xenograft tumors from ACR-2316 treated mice and observed strong upregulation of innate immune signaling pathways, including type I interferon. These findings were further validated at the cellular level, where ACR-2316 treatment led to the activation of double stranded RNA and DNA sensing machinery RIG-I, MDA5 and cGAS. Furthermore, we found evidence of mitochondrial DNA fragmentation with ACR-2316 treatment, suggesting that this may serve as an additional immune sensor.In a syngeneic colorectal cancer model, ACR-2316 monotherapy resulted in dose-dependent tumor growth inhibition. In combination with anti-PD-L1, ACR-2316 exhibited striking synergy, leading to complete tumor regression in mice. To assess the durability of this response, tumor cells were re-injected into tumor-free mice that were previously treated with the combination therapy. All animals remained tumor-free for over 200 days through four sequential tumor re-challenges, demonstrating strikingly robust and durable immune memory. To dissect the mechanism of this durable immunity, we systematically depleted key immune cell subsets in tumor re-challenged mice. While depletion of either CD4 + or CD8 + T cells alone did not enable tumor growth, co-depletion of both subsets resulted in tumor formation. This suggests that the immune memory generated by the combination treatment of ACR-2316 and anti-PD-L1 is co-dependent on both CD4 + and CD8 + T cell subsets. Conclusions: Our findings reveal the dual role of ACR-2316 in inducing tumor intrinsic DNA damage and promoting immune activation through multiple immune sensing mechanisms, resulting in permanent immune memory co-dependent on CD4+ and CD8+ T cell subsets. This provides a strong rationale for combining ACR-2316 with immune checkpoint inhibitors in the clinical setting. ACR-2316 is in a phase 1 monotherapy trial and has already shown initial clinical activity with tumor shrinkage and a confirmed partial response during dose escalation across solid tumors predicted by our AP3 platform to be sensitive to ACR-2316.