Whereas target-specific drugs are available for treating ERBB2-overexpressing and hormone receptor-positive breast cancers, no tailored therapy exists for hormone receptor-and ERBB2-negative (''triple-negative'') mammary carcinomas. Triple-negative tumors account for 15% of all breast cancers and frequently harbor defects in DNA double-strand break repair through homologous recombination (HR), such as BRCA1 dysfunction. The DNA-repair defects characteristic of BRCA1-deficient cells confer sensitivity to poly-(ADP-ribose) polymerase 1 (PARP1) inhibition, which could be relevant to treatment of triple-negative tumors. To evaluate PARP1 inhibition in a realistic in vivo setting, we tested the PARP inhibitor AZD2281 in a genetically engineered mouse model (GEMM) for BRCA1-associated breast cancer. Treatment of tumor-bearing mice with AZD2281 inhibited tumor growth without signs of toxicity, resulting in strongly increased survival. Long-term treatment with AZD2281 in this model did result in the development of drug resistance, caused by up-regulation of Abcb1a/b genes encoding P-glycoprotein efflux pumps. This resistance to AZD2281 could be reversed by coadministration of the P-glycoprotein inhibitor tariquidar. Combination of AZD2281 with cisplatin or carboplatin increased the recurrence-free and overall survival, suggesting that AZD2281 potentiates the effect of these DNA-damaging agents. Our results demonstrate in vivo efficacy of AZD2281 against BRCA1-deficient breast cancer and illustrate how GEMMs of cancer can be used for preclinical evaluation of novel therapeutics and for testing ways to overcome or circumvent therapy resistance.breast cancer ͉ drug resistance ͉ P-glycoprotein ͉ GEMM ͉ DNA repair P oly(ADP-ribose) polymerase 1 (PARP1) is involved in surveillance and maintenance of genome integrity and functions as a key molecule in the repair of DNA single-strand breaks (SSBs) (1-3). Inactivation of SSB repair by PARP1 inhibition during S-phase induces DNA double-strand breaks (DSBs) and may thus confer synthetic lethality to cells with defective homology-directed DSB repair (4, 5). Mutations in BRCA1 or BRCA2 predispose to hereditary breast and ovarian cancer, which accounts for 3-5% of all breast cancers and a greater proportion of ovarian cancers (6). BRCA1 and BRCA2 function is critical for homologous recombination (HR) (6, 7), and BRCA-deficient cells appear to be highly sensitive to PARP inhibition, resulting in increased genomic instability, cell cycle arrest, and apoptosis (4, 5). PARP1 inhibition might, therefore, be a specific therapy for cancers with defects in BRCA1/2 or other HR pathway components (clinically relevant PARP inhibitors are reviewed in ref. 8). Recently, Donawho et al. (9) have reported that the PARP inhibitor ABT-888 in combination with platinum drugs or cyclophosphamide, but not alone, causes regression of BRCA1-deficient MX-1 xenografts. However, this study uses only a single BRCA1-mutated tumor line without isogenic controls to address the impact of BRCA1 mutation on response...

Background: Breast cancer cells deficient for BRCA1 are hypersensitive to agents inducing DNA double-strand breaks (DSB), such as bifunctional alkylators and platinum agents. Earlier, we had developed a comparative genomic hybridisation (CGH) classifier based on BRCA1 -mutated breast cancers. We hypothesised that this BRCA1-like CGH classifier could also detect loss of function of BRCA1 due to other causes besides mutations and, consequently, might predict sensitivity to DSB-inducing agents. Patients and methods: We evaluated this classifier in stage III breast cancer patients, who had been randomly assigned between adjuvant high-dose platinum-based (HD-PB) chemotherapy, a DSB-inducing regimen, and conventional anthracycline-based chemotherapy. Additionally, we assessed BRCA1 loss through mutation or promoter methylation and immunohistochemical basal-like status in the triple-negative subgroup (TN subgroup). Results: We observed greater benefit from HD-PB chemotherapy versus conventional chemotherapy among patients with BRCA1-like CGH tumours [41/230 = 18%, multivariate hazard ratio (HR) = 0.12, 95% confidence interval (CI) 0.04–0.43] compared with patients with non-BRCA1-like CGH tumours (189/230 = 82%, HR = 0.78, 95% CI 0.50–1.20), with a significant difference (test for interaction P = 0.006). Similar results were obtained for overall survival ( P interaction = 0.04) and when analyses were restricted to the TN subgroup. Sixty-three percent (20/32) of assessable BRCA1-like CGH tumours harboured either a BRCA1 mutation ( n = 8) or BRCA1 methylation ( n = 12). Conclusion: BRCA1 loss as assessed by CGH analysis can identify patients with substantially improved outcome after adjuvant DSB-inducing chemotherapy when compared with standard anthracycline-based chemotherapy in our series.

Purpose: To assess efficacy of the novel, selective poly(ADP-ribose) polymerase-1 (PARP-1) inhibitor AZD2281against newly established BRCA2-deficient mouse mammary tumor cell lines and to determine potential synergy between AZD2281and cisplatin. Experimental Design: We established and thoroughly characterized a panel of clonal cell lines from independent BRCA2-deficient mouse mammary tumors and BRCA2-proficient control tumors. Subsequently, we assessed sensitivity of these lines to conventional cytotoxic drugs and the novel PARP inhibitor AZD2281. Finally, in vitro combination studies were done to investigate interaction between AZD2281and cisplatin. Results: Genetic, transcriptional, and functional analyses confirmed the successful isolation of BRCA2-deficient and BRCA2-proficient mouse mammary tumor cell lines.Treatment of these cell lines with 11 different anticancer drugs or with g-irradiation showed that AZD2281, a novel and specific PARP inhibitor, caused the strongest differential growth inhibition of BRCA2-deficient versus BRCA2-proficient mammary tumor cells. Finally, drug combination studies showed synergistic cytotoxicity of AZD2281 and cisplatin against BRCA2-deficient cells but not against BRCA2-proficient control cells. Conclusion: We have successfully established the first set of BRCA2-deficient mammary tumor cell lines, which form an important addition to the existing preclinical models for BRCA-mutated breast cancer. The exquisite sensitivity of these cells to the PARP inhibitor AZD2281, alone or in combination with cisplatin, provides strong support for AZD2281as a novel targeted therapeutic against BRCA-deficient cancers.

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