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Abstract Background: Individuals with an inherited BRCA1 or BRCA2 (BRCA) mutation have an elevated risk of developing breast and ovarian cancer, and this BRCA defect is also present in a subset of sporadic tumors. Tumors do not have homologous recombination repair (HRR) to repair double-stranded breaks (DSBs), but they still can use other DNA repair pathways to survive. Therefore, we hypothesized that crosstalk between DNA repair pathways complements HRR in BRCA-deficient cells, leading to tumor survival. Treatment with either a poly(ADP)ribose polymerase (PARP) inhibitor (PARPi) or a platinum drug induced BRCA-deficient cell lethality by different mechanisms. In addition, combining both drugs has been reported to be synergistic in BRCA mouse models. However, the mechanism of this synergism and whether this synergism occurs in human cells are still unclear. Moreover, not all the patients respond to a PARPi chemotherapy alone. Hence, our objectives were: 1) to study the mechanism by which a PARPi combined with a platinum drug is synthetic lethal in human BRCA1 or BRCA-deficient cells; 2) to assess PARP activity as a biomarker for chemosensitivity. Methods: We compared the cytotoxic effect of ABT-888 (a PARPi) with cisplatin or carboplatin (platinum drugs) alone versus combinations by examining Brca-proficient and -deficient mouse embryonic stem (ES) cell survival using clonogenic assays. The effect of ABT-888 and carboplatin combination treatment on human cell growth was tested in HCC1937 (a triple negative BRCA1-deficient breast cancer line) and EUFA423F (a BRCA2-deficient Fanconi anemia cell line), along with their BRCA complemented lines. Synergism, antagonistic or additive drug interactions were assessed. Drug(s)-induced DSBs, HRR and apoptosis were examined and endogenous PARP activity was determined. Results: Each monotherapy killed or inhibited more Brca/BRCA-deficient cells than the corresponding Brca/BRCA-proficient cells. In Brca-deficient cells, the ABT-888 and carboplatin combination was more synergistic than ABT-888 and cisplatin. All paired BRCA-deficient or complemented cells had the same level of DNA damage following treatment with ABT-888 and/or Carboplatin. BRCA-deficient cells lack HRR, while BRCA-proficient cells use HRR. Failure to repair DSBs in BRCA-deficient cells triggered apoptosis, which was synergistic in the BRCA1-deficient cells. Moreover, higher endogenous PARP activity in Brca/BRCA-deficient cells than their respective Brca/BRCA proficient cells correlated with increased chemosensitivity. Conclusion: Our data suggest that combining ABT-888 and carboplatin will likely be more successful than monotherapy in treating most BRCA-associated cancers. In addition, high endogenous PARP activity could be used as a marker to identify PARPi susceptible tumors. Lack of HRR combined with a strong dependence on PARP activity for repair of platinum lesions led to synthetic lethality in BRCA-deficient cells. A randomized phase II trial has recently been initiated to test this drug combination in patients with BRCA-associated breast cancer and we are currently investigating γH2AX as a pharmacodynamic marker in hair follicles. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A102.