Remarkable response to a novel ATR inhibitor in a patient with poorly differentiated neuroendocrine carcinoma.

M6620 (formerly known as VX-970) is a potent inhibitor of ataxia telangiectasia and Rad3-related protein (ATR), a serine/threonine-specific protein kinase involved in activation of checkpoint signaling and promotion of cell cycle arrest in response to DNA damage (inhibition constant [Ki] <300 pM, IC50 of 20 nM). ATR inhibition enhances the cytotoxic effect of DNA damaging drugs and infrared radiation (IR) in many cancer cell lines and primary human tumors. M6620 is currently under investigation in early-phase clinical trials for the treatment of a number of malignancies. Below, we report a case of a patient with metastatic prostate cancer with clonal evolution to poorly differentiated large cell neuroendocrine carcinoma who developed an exceptional response to treatment with M6620 and cisplatin on a phase I trial VX12-970-001 (NCT02157792: An Open-Label, First-in-Human Study of the Safety, Tolerability, and Pharmacokinetics of VX-970 in Combination With Cytotoxic Chemotherapy) with over 20 months of non-CNS progression free survival. We will discuss the mechanism of action of M6620, rationale for enrolling the patient in this trial and hypothesize the reasons for this exceptional response.