ENDURE Cancer is a program of the Entertainment Industry Foundation, administered by AACR. Footnotes Conflict of Interest: The authors have no conflicts of interest to disclose. Author Contributions BHL, JTP, and CMR BI 2536 designed and conceived the study.BHL, EEG, VES, and BAT designed and performed experiments. BHL, AN, N. in SCLC, we investigated combination therapy with talazoparib and found marked synergy and efficacy or status. Conclusions is a relevant predictive biomarker of sensitivity to PARP inhibitor monotherapy in SCLC and we identify combinatorial therapy with TMZ as a particularly promising therapeutic strategy that warrants further clinical investigation. (6). These promising preclinical data in SCLC supported the inclusion of a SCLC cohort in a phase I study of the PARP inhibitor talazoparib with promising initial signals of efficacy (7). PARP1 was first described as a regulator of base BI 2536 excision repair, but has since been implicated in the function of homologous recombination (HR), non-homologous end joining and microhomology-mediated end joining (5, 8). There are at least two distinct mechanisms of action of PARP inhibitors: enzymatic inhibition and the more recently recognized PARP trapping (9, 10). Inhibition of PARP enzymatic activity was initially thought to explain the synthetic lethality observed with PARP inhibitors in breast and ovarian cancers with mutations. These mutations lead to deficiencies in HR, leaving these cancers highly dependent on PARP mediated repair (11, 12). The PARP inhibitor olaparib was recently approved for treatment of germline mutant ovarian cancer patients based on the results of a randomized phase II study (13). In addition, the FDA granted olaparib Breakthrough Therapy designation for treatment of patients with castrate resistant metastatic prostate cancer harboring and mutations. However, mutations in are notably rare ( 3%) in SCLC based on recent comprehensive genomic analyses (4, 14). PARP trapping is a distinct mechanism of action of PARP BI 2536 inhibitors, whereby the inhibitor/PARP complex becomes fixed on the DNA at sites of single-strand breaks, leading to a failure to repair, and, with replication, induction of multiple double strand breaks. PARP trapping may be responsible for synergy between PARP inhibitors and DNA damaging agents that increase the prevalence of single-strand breaks. Further, this mechanism may be operant in cancers without defined HR deficiencies (9). The various PARP inhibitors in clinical development and clinical use vary in relative potency for both enzymatic inhibition and PARP trapping effects. Olaparib and talazoparib have comparable levels of catalytic inhibition, while talazoparib is ~100-fold more potent than olaparib at trapping PARP-DNA complexes (9, 10). Rucaparib appears to have activity similar to olaparib, while veliparib is less potent both BI 2536 in enzymatic inhibition and in trapping activity (9, 10). Beyond inactivating mutations in known mediators of HR such as and HR pathway disruptions in other known and unknown mediators of this pathway (15, 16). This has led to substantial interest in strategies for defining BRCAness, or HR deficiency (HRD), including using characteristic patterns of mutation and loss from whole exome sequencing data to generate HRD scores (17C19). Discovering novel mechanisms of HRD in sporadic tumors may broaden the therapeutic potential ETS2 of PARP inhibitors. In this study, we sought to define determinants of PARP inhibitor activity in SCLC, and also to evaluate the combinatorial activity of PARP inhibition with the DNA damaging agent TMZ. BI 2536 Recent studies have suggested that TMZ is a particularly effective agent in recurrent metastatic SCLC, with both systemic and central nervous system activity, leading to its inclusion in the National Comprehensive Cancer Network guidelines for standard treatment of this disease (20). We report (was recently reported to be actively recruited to sites of DNA damage, and to inhibit HR, strongly supporting these findings (21). We found the association between PARP trapping and cytotoxic activity is stronger in SCLC than in other tumor types, and focused subsequent analyses on the strongest PARP trapper, talazoparib. By CRISPR-Cas9 gene editing and shRNA approaches, we established that loss of confers resistance to talazoparib in SCLC cell lines. expression by immunohistochemistry (IHC) is associated with tumor.