Background: This signal finding study was designed to evaluate the clinical efficacy of a PARP inhibitor, talazoparib, in advanced stage squamous cell lung cancer harboring HRRD. Methods: Eligible patients (pts) identified through the parent S1400 screening platform were required to have a deleterious mutation in any of the study-defined HRR genes [ATM, ATR, BARD1, BRCA1, BRCA2, BRIP1, CHEK1, CHEK2, FANCA, FANCC, FANCD2, FANCF, FANCM, NBN (NBS1), PALB2, RAD51, RAD51B (RAD51L1), RAD54L, RPA1) defined as the full eligible population (FEP). The primary analysis population (PAP) is defined by a subset of genes [ATM, ATR, BRCA1, BRCA2, PALB2]. Pts have platinum sensitive disease (at least stable disease on platinum doublet) and progressed on most recent line of systemic therapy, a Zubrod performance status of 0-1, adequate organ function, and not have been previously exposed to a PARP inhibitor and not be on systemic therapy within 21 days of registration. A 2-stage design with exact 93% power and 1-sided 0.07 level type I error required enrollment of 40 patients in the PAP in order to rule out an ORR of 15% or less if the true ORR is 35% or greater. At least 3 or more responses were needed in the first 20 pts in order to proceed to full enrolment of 40 pts in the PAP. The total accrual goal was 60 FEP assuming 67% of patients would be in the PAP. Results: The study enrolled 51 patients of whom 47 are eligible and analyzable for response (FEP) with 24 in the PAP. In the FEP, median age 66.7 yrs; M/F 39/8 (83/17%); 85% White and 15% Black; 77% of the pts received at least 1 prior line of treatment for stage IV. The study was closed for futility with only one response in the PAP. In the PAP (n = 24, median age 68 yrs), ORR was 4% (95%CI: 0, 21) and DCR was 54% (95%CI: 33, 74); median PFS of 2.4 months (95%CI: 1.5-2.8) and median OS was 5.2 months (95%CI: 3.8-10, 7). There were five responders in the FEP with ORR of 11%; DCR of 53% and median DoR was 1.8 months (95% CI: 1.3, 4.2); median PFS was 2.5 months (95%CI: 1.6-3.0) and median OS was 5.7 months (95% CI: 4.5-8.7). The most frequent grade ≥3 adverse event in the FEP were: Anemia (14.9%), thrombocytopenia (12.8%); lymphopenia (8.5%) and nausea (6.4%). Conclusions: S1400G failed to show sufficient level of efficacy for talazoparib in a biomarker defined subset of squamous lung cancer with HRRD. There were no new safety signals and hematologic toxicities were the most frequent adverse events. Clinical trial information: NCT02154490