Background: M6620 (M), a potent ATR inhibitor, has synergistic activity with cisplatin (C) in multiple preclinical models, resulting in DNA damage and antitumor activity. We hypothesize that inhibition of both homologous recombination and base excision repair through the combination of M6620 and veliparib (V, a potent inhibitor of PARP1/2) would result in accumulation of lethal double stranded breaks induced by cisplatin and increased antitumor activity and initiated a phase-1 dose escalation trial of this combination in patients (pts) with advanced solid tumors (NCT02723864). Methods: This is a standard 3+3 dose escalation design with 21-day cycles. M is given IV day 2 (D2) and D9; V orally twice daily D1-3 and D8-10; C IV D1 (and D8 from dose level 3 [DL3] onwards) at 40 mg/m² (with option of holding after cycle 6). Primary objectives: safety; tolerability; maximum tolerated dose (MTD). Secondary objectives: pharmacodynamic (PD) biomarkers; antitumor activity. Dose-limiting toxicity (DLT) evaluated during cycle 1, response using RECIST 1.1. Results: Thirty-seven patients enrolled, median 5 lines of prior therapy (Range 1-12). MTD: V 200 mg, M 210 mg/m², C 40 mg/m² (DL6). DLT: grade (gr) 4 thrombocytopenia (DL4), hypophosphatemia (not resolved in 24 hrs., DL3), infusion reaction (DL7). Common gr 3/4 toxicities: anemia (41%), thrombocytopenia (30%), neutropenia (27%), hyponatremia (11%) and hypophosphatemia (8%). Of 34 pts evaluable for response: 2 partial response (PR) (6%, 1 confirmed & 1 transient), 22 stable disease (SD) (65%, median 4 cycles, range 2-11). Of 24 pts with PR/SD, 22 had prior platinum chemotherapy. After July 2018, V was held for gr ≥2 anemia until improved to gr 1, followed by dose reduction. Of 5 pts treated in this period, 3 required V held. PD analysis of circulating tumor cells is underway to elucidate biomarkers of DNA damage and apoptosis. Conclusions: This combination of M6620, veliparib and cisplatin is safe, with activity seen in pts having received prior platinum. The most common toxicity was anemia, which prevented adequate delivery of veliparib. While MTD was established in a heavily pretreated population, consideration should be given to continued dose escalation in pts who have received fewer prior lines of therapy. Funded in part by NCI Contract No. HHSN261200800001E. Clinical trial information: NCT02723864