TPS2609 Background: The DNA damage response (DDR) pathway is a key element of cellular integrity. Platinum compounds form covalent bonds with purine bases causing DNA cross-links that stall replication forks halting transcription. Poly (ADP-ribose)polymerase-1 (PARP-1) plays a pivotal role in DDR and base-excision repair. Ataxia-telangiectasia-related (ATR) protein kinase is also central to DDR and homologous recombination, activating a series of phosphorylation cascades culminating in cell cycle arrest to allow time for DNA repair. Veliparib (ABT-888) is a PARP 1/2 inhibitor (PARPi) with clinical evidence of antitumor activity in combination with cisplatin in BRCA mutation carriers (Rodler et al, Cancer Res. 2011). VX-970 is a potent ATR inhibitor, with antitumor activity across a range of cell lines in combination with DNA damaging agents, including cisplatin (Huntoon et al, Cancer Res. 2013). In this trial, we will evaluate whether the combination of veliparib + VX-970 impairs DNA repair, inducing a “BRCA null”-like phenotype leading to potentiation of the antitumor activity of cisplatin. Methods: Open label phase I trial of the veliparib+VX-970+cisplatin combination, following a 3+3 design, with dose limiting toxicities defined during cycle 1. Estimated enrollment: 24 patients (pts); Dana Farber and MD Anderson planned as additional sites. Drug administration over a 21-day cycle: VX- intravenously (IV) on Days 2 and 9; Veliparib orally twice daily (q12 hours ± 1 hour) Days 1-3 and 8-10; cisplatin 40 mg/m² IV Day 1 (with Day 8 added from DL3 onwards). Pts must be ≥ 18 years of age; have histologically confirmed solid tumors that have progressed on standard of care therapy known to prolong survival or without known standard, ECOG PS ≤2, and life expectancy ≥3 months. Pts with treated brain metastasis with stable disease ≥4 weeks without requiring steroids or anti-seizure medication are eligible. Exclusion criteria include a prolonged QTc interval, and sensory/motor neuropathy ≥grade 2 by CTCAE v.4. At this time, cohort 3 has enrolled 1 of 3 planned pts. Assessment of DDR and apoptosis biomarkers at the maximally tolerated dose using a validated and quantitative immunofluorescence assay is planned. Clinical trial information: NCT02723864.