ABSTRACT: DNA repair is essential for the survival of both normal and cancer cells. An elaborate set of signaling pathways detect single-strand and double-strand DNA breaks and mediate either DNA repair or apoptosis if the damage is too great to repair. Poly(adenosine diphosphate [ADP]–ribose) polymerases (PARPs) play a key role in the repair of base damage via the base excision repair pathway. Pharmacological inhibition of PARP induces cell death in tumors with mutations in certain DNA repair pathways—such as the BRCA pathways of double-strand break repair—and when combined with chemotherapies that cause DNA damage. PARP inhibitors are being investigated as a monotherapy for the treatment of patients with BRCA1/2 mutations; in the treatment of triple-negative breast cancer, because of its molecular similarities to BRCA1-mutated malignancies; and as a strategy to potentiate the DNA-damaging effects of chemotherapy and radiation. The aim of this article is to review the preclinical data and rationale for PARP inhibitor use in the aforementioned settings, as well as the current status of the clinical development of these agents in the treatment of breast cancer, along with future directions for research in this field. Trials have been identified via searches of PubMed, clinicaltrials.gov, and the Proceedings of the American Society of Clinical Oncology Annual Meeting and the San Antonio Breast Cancer Symposium.
DNA Repair and PARP Function
DNA repair is critical for the survival of cells. Estimations of the number of DNA damage events that occur on a daily basis are in the thousands. A number of DNA repair systems allow for repair and survival. While this is a desirable outcome for normal cells, DNA repair also allows cancer cells to survive the DNA injury posed by chemotherapy or radiation. Thus, there is a long-standing interest in impeding DNA repair as a potential strategy for enhancing the activity of chemotherapy and radiation in the treatment of cancer.
DNA damage results from a variety of exogenous and endogenous insults. Multiple types of DNA repair mechanisms exist; these include pathways that repair single-strand breaks and others that repair double-strand breaks. The pathways that are predominately involved in double-strand break repair are the nonhomologous end-joining and homologous recombination pathways. Homologous recombination is a highly accurate mechanism that repairs double-strand breaks in the S and G2 phases of the cell cycle. Integral to the function of the homologous recombination pathway are BRCA1 and BRCA2. Loss of function of these proteins via inherited gene mutations results in faulty homologous recombination. This is likely a key step in tumorigenesis in individuals with BRCA1/2 mutations who are predisposed to the development of breast, ovarian, and other cancers.
Base excision repair (BER) is the key pathway for the repair of damaged bases caused by endogenous DNA damage. Poly(adenosine diphosphate [ADP]–ribose) polymerases (PARPs) detect the single-strand breaks that are induced to remove damaged bases.[3,4] At least 17 members of the PARP family have been described to date, although PARP1 and PARP2 are the most relevant to BER.. PARPs also have a number of other key functions, including a role in the epigenetic regulation of chromatin and control of cell division via interaction with centromeres.
Central to the use of PARP inhibitors in the treatment of patients with malignancy related to BRCA1/2 mutations is the concept of synthetic lethality. Synthetic lethality refers to the situation in which two gene deficiencies that independently would not cause cell death are in fact lethal when they occur in combination. In the setting of persons with BRCA mutations, the presence of the BRCA mutation and subsequent nonfunctional homologous recombination alone are not enough to cause tumor cell death. Applying the synthetic lethality concept, it was hypothesized that inhibiting an additional DNA repair pathway—namely BER—with a PARP inhibitor could cause the death of BRCA-deficient tumor cells (Figure 1). Specifically, the loss of PARP1 function results in the accumulation of single-strand DNA breaks, which are subsequently converted to double-strand breaks by cellular transcription and replication. These double-strand breaks, which are typically repaired by homologous recombination or nonhomologous end-joining in normal cells, would accumulate in BRCA1- or BRCA2-deficient cells, leading to subsequent cell death.
This hypothesis was confirmed in two pivotal preclinical studies that demonstrated that loss of function of BRCA1 or BRCA2 conferred exquisite sensitivity to PARP inhibitors. Bryant et al observed that the PARP inhibitors NU1025 and AG14361 were profoundly cytotoxic in V-C8 (BRCA2-deficient) cells but did not affect V79 (BRCA2-expressing) cells.  Also, PARP inhibition affected survival of MCF7 (wild-type p53) and MDA-MB-231 (mutated p53) cells only when BRCA2 was depleted. In addition, the investigators also found significant response to AG14361 in xenograft tumor models of implanted BRCA2-deficient V-C8 cells. Farmer et al described increased sensitivity to PARP inhibitors KU0058684 and KU0058948 of mouse embryonic stem cells lacking wild-type BRCA1 or BRCA2. Of note, treatment with PARP inhibitors resulted in DNA damage, as indicated by the formation of gamma-H2AX foci, which occurs at sites of DNA damage in wild-type BRCA1- and BRCA2-deficient cells. However, repair of the DNA damage, as determined by measurement of nuclear RAD51 foci formation (which only occurs in the setting of BRCA-dependent homologous recombination), was only seen in the wild-type cells.
Of interest, sensitivity to PARP inhibition has been observed in cells with defects in homologous recombination other than BRCA deficiency. These additional defects include phosphatase and tensin homolog (PTEN) deficiency, ATM deficiency,[10,11] and Aurora A over-expression. For instance, Mendes-Pereira et al recently suggested that the previously reported association between PTEN deficiency and genomic instability is likely to result from defective homologous recombination. They found reduced activity of RAD51 and a reduced capacity to form nuclear RAD51 foci in response to DNA damage in PTEN-deficient colon and endometrial cancer cell lines. They also found that PTEN deficiency correlates with a five-fold decrease in the number of double-strand breaks repaired by homologous recombination. Several PTEN-deficient tumor cell lines and xenograft models were found to have increased sensitivity to the PARP inhibitor olaparib. These observations are of great interest, as they may broaden the population of patients who would potentially benefit from PARP inhibition, beyond the small population of patients with BRCA1/2 germline mutations.
Clinical Applications of PARP Inhibitors
BRCA1- and BRCA2-related breast cancer
Based on the above preclinical data on observed synthetic lethality in BRAC1/2-deficient cancers, a number of PARP inhibitors were developed for clinical use by various pharmaceutical companies (see Table 1—web site only). Fong et al reported the first phase I study using the oral PARP inhibitor olaparib (AZD2281; KU-0059436) . The drug was initially given in a standard dose-escalating fashion to 60 patients with various malignancies, with an expansion subsequently performed at the recommended phase II dose in a population of BRCA1/2-deficient patients. The drug was generally well tolerated and the dose-limiting toxicities observed were reversible (grade 3 mood alterations and somnolence at the 400-mg twice-daily dose, and grade 4 thrombocytopenia and grade 3 somnolence at the 600-mg twice-daily dose). Adverse effects were not different in the BRCA mutation carriers enrolled in the study than in the non-carriers. The expansion cohort for patients with BRCA mutations consisted of 22 subjects with primarily breast, ovarian, or prostate cancers, who received olaparib at a dose of 200 mg twice daily. No objective tumor responses were seen in subjects without a BRCA mutation; however 12 out of 19 evaluable BRCA mutation carriers (63%) had a clinical benefit from treatment with olaparib. Nine of these patients (47%) had objective responses by Response Evaluation Criteria In Solid Tumors (RECIST) criteria, including a complete response in a patient with BRCA2-mutated breast cancer. Some patients had durable responses of over 1 year. Correlative pharmacodynamic studies demonstrated reduction in PAR levels and induction of gamma-H2AX foci indicative of double-strand breaks in tumor specimens and peripheral blood mononuclear cells.
Following the success observed in phase I, two phase II trials with olaparib—ICEBERG (International Collaborative Expertise for BRCA Education and Research through Genetics) 1 and 2—were carried out in breast and ovarian cancers, respectively. In the breast cancer study, presented by Tutt et al, 54 women with BRCA1- or BRCA2-deficient breast cancer were assigned to receive olaparib at either 400 mg (n = 24; cohort 1) or 100 mg (n = 24; cohort 2) twice daily in a nonrandomized, sequential fashion. The 400-mg dose had been determined to be the maximum tolerated dose in the phase I study described above, while the 100-mg dose was shown to have clinical activity as well as pharmacodynamic activity without dose-limiting toxicity. BRCA mutation status was centrally determined for all patients. Eligibility requirements included treatment with at least one prior chemotherapy regimen, and the median number of prior therapies was three (range, one to five). Prior therapies included anthracycline and taxanes in the majority of patients; approximately a quarter of the patients had received platinum-containing therapies as well. The primary endpoint was objective response rate, which was 22% in the 100-mg twice-daily dose cohort and 41% at the 400-mg twice-daily dose. The clinical benefit rates were 26% and 52%, respectively. These response rates are quite remarkable for a biologically based therapy, particularly since they are similar to or better than the response rates expected with chemotherapy in anthracycline- and taxane-refractory patients. Responses were observed in heavily pretreated patients; the median response duration was 144 days at the 400-mg twice-daily dose and 141 days with the 100-mg twice-daily dose. There was no apparent difference in activity in patients with BRCA1 mutations and BRCA2 mutations, nor was activity related to the triple-negative status. As in the phase I study, olaparib was well tolerated and the most frequent adverse events at both dose levels were fatigue and nausea.
Although the authors recommended caution with interpretation of the improved response at the higher dose level, it was acknowledged that the lower dose appeared inferior in this trial as well as in the accompanying ovarian cancer trial. In that phase II study, in heavily pretreated BRCA-mutated ovarian cancer patients, a response rate of 33% was observed at the 400-mg twice-daily dose, and a rate of 13% at the 100-mg twice-daily dose. It was suggested that the higher doses potentially had greater tissue penetration leading to enhanced target inhibition in tumors, despite the seemingly adequate pharmacodynamic activity that was observed in the surrogates of peripheral blood mononuclear cells and hair follicles at the lower dose. Unfortunately, serial tumor biopsies were not obtained to confirm these findings in the target tissue. Of particular note, there was a suggestion of diminished response to PARP inhibition in platinum-resistant patients, as there were few responses observed in that population in either the phase I or phase II studies. Small patient numbers limit any further exploration of this observation; however, it will be an important consideration in future trials, particularly since mechanisms of platinum resistance may be similar to mechanisms of resistance to PARP inhibitors. Exploration of these findings will require a clear and consistent definition of platinum resistance across trials. At this time, phase III testing of olaparib will take place in ovarian cancer rather than in breast cancer.
There are currently a number of ongoing clinical studies in BRCA1/2 mutation carriers utilizing various PARP inhibitors both as single agents and in combination with chemotherapy. Early data with the PARP inhibitor MK-4827 have been presented that have also demonstrated single-agent activity in a population of BRCA-mutant breast cancer patients. In that trial, half of the patients with BRCA1/2 mutations either had a response by RECIST criteria or had prolonged stable disease. Clinical trials with other PARP inhibitors given as a single agent are enrolling, and data are anticipated soon. In BRCA1/2-related malignancies, it is rational to expect greater activity with the combination of PARP inhibitors and chemotherapy than with PARP inhibitor monotherapy; however, there are no mature data from trials examining this question—particularly data that examine it in a randomized fashion. These ongoing studies will help to answer the subsequent questions that will arise regarding the most effective combinations, sequencing of therapy, and role of maintenance therapy. Table 2 summarizes the trials that have been reported evaluating PARP inhibitors in BRCA1/2-related breast cancer.