Olaparib has few of the adverse effects of conventional chemotherapy, inhibits PARP, and has antitumor activity in cancer associated with the BRCA1 or BRCA2 mutation. (ClinicalTrials.gov number, NCT00516373.)

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...

Poly(ADP-ribose) polymerase activation is an immediate cellular response to metabolic-, chemical-, or ionizing radiation-induced DNA damage and represents a new target for cancer therapy. In this article, we disclose a novel series of substituted 4-benzyl-2 H-phthalazin-1-ones that possess high inhibitory enzyme and cellular potency for both PARP-1 and PARP-2. Optimized compounds from the series also demonstrate good pharmacokinetic profiles, oral bioavailability, and activity in vivo in an SW620 colorectal cancer xenograft model. 4-[3-(4-Cyclopropanecarbonylpiperazine-1-carbonyl)-4-fluorobenzyl]-2 H-phthalazin-1-one (KU-0059436, AZD2281) 47 is a single digit nanomolar inhibitor of both PARP-1 and PARP-2 that shows standalone activity against BRCA1-deficient breast cancer cell lines. Compound 47 is currently undergoing clinical development for the treatment of BRCA1- and BRCA2-defective cancers.

Inhibition of PARP is a promising therapeutic strategy for homologous recombinationdefi cient tumors, such as BRCA1-associated cancers. We previously reported that BRCA1-defi cient mouse mammary tumors may acquire resistance to the clinical PARP inhibitor (PARPi) olaparib through activation of the P-glycoprotein drug effl ux transporter. Here, we show that tumorspecifi c genetic inactivation of P-glycoprotein increases the long-term response of BRCA1-defi cient mouse mammary tumors to olaparib, but these tumors eventually developed PARPi resistance. In a fraction of cases, this resistance is caused by partial restoration of homologous recombination due to somatic loss of 53BP1. Importantly, PARPi resistance was minimized by long-term treatment with the novel PARP inhibitor AZD2461, which is a poor P-glycoprotein substrate. Together, our data suggest that restoration of homologous recombination is an important mechanism for PARPi resistance in BRCA1-defi cient mammary tumors and that the risk of relapse of BRCA1-defi cient tumors can be effectively minimized by using optimized PARP inhibitors. SIGNIFICANCE:In this study, we show that loss of 53BP1 causes resistance to PARP inhibition in mouse mammary tumors that are defi cient in BRCA1. We hypothesize that low expression or absence of 53BP1 also reduces the response of patients with BRCA1-defi cient tumors to PARP inhibitors.Cancer Discov; 3(1);[68][69][70][71][72][73][74][75][76][77][78][79][80][81]

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.