BRCA 1/2-Mutation Related and Sporadic Breast and Ovarian Cancers: More Alike than Different

No longer is histology solely predictive of cancer treatment and outcome. There is an increasing influence of tumor genomic characteristics on therapeutic options. Both breast and ovarian cancers are at higher risk of development in patients with BRCA 1/2-germline mutations. Recent data from The Cancer Genome Atlas and others have shown a number of genomic similarities between triple negative breast cancers (TNBCs) and ovarian cancers. Recently, poly (ADP-ribose) polymerase (PARP) inhibitors have shown promising activity in hereditary BRCA 1/2-mutated and sporadic breast and ovarian cancers. In this review, we will summarize the current literature regarding the genomic and phenotypic similarities between BRCA 1/2-mutation related cancers, sporadic TNBCs, and sporadic ovarian cancers. We will also review Phase I, II, and III data using PARP inhibitors for these malignancies and compare and contrast the results with respect to histology.


INTRODUCTION
BRCA1 and 2 proteins play integral functions in DNA homologous recombination repair (HRR). In normal cells, the HRR pathway is activated in response to DNA double-stranded breaks (1). In BRCA 1/2-deficient cells, HRR is faulty secondary to loss of BRCA function, and therefore, other more error-prone DNA repair pathways are activated. These less perfect mechanisms are felt to be accountable, in part, for carcinogenesis. Similarly, tumors with defective HRR mechanisms are more susceptible to the direct DNA damaging effects of chemotherapy.
Homologous recombination repair dysfunction can be exploited as a therapeutic strategy by the use of poly (ADP-ribose) polymerase (PARP) inhibitors, which inhibit PARP proteins, most commonly PARP1 and 2. As part of the base excision repair (BER) pathway, PARP1 attaches long polymers of ADP-ribose on itself, so that, XRCC1 and other repair proteins have the ability to rapidly locate single-stranded DNA breaks (2)(3)(4). Newer evidence reveals that the exact role of PARP1 in the BER pathway is perhaps more indirect and not yet clearly defined (5). Recent studies have also shown that PARP1 is more versatile, and has been implicated in other DNA repair pathways, such as the non-homologous end-joining (NHEJ) repair pathway (6,7).
Several mechanisms by which PARP inhibition in HRRdeficient cells lead to cell death have been investigated. Most notably, the concept of synthetic lethality explains combinatory lethal effects of BER and HR repair dysfunction, whereas alone, HR or BER pathway disruptions are not lethal to the tumor cell (8). Additionally, other potential mechanisms have been explored including trapping of inhibited PARP1 at sites of DNA damage preventing other repair proteins access, failure to initiate HRR by PARP-dependent BRCA1 recruitment, and activation of the errorprone NHEJ repair pathway leading to genomic instability and subsequent cell death (9). Knowledge of PARP activity has led to effective treatment strategies for BRCA 1/2-germline mutation related tumors.

BRCA 1/2 -MUTATED OVARIAN AND BREAST CANCER
BRCA 1/2-mutation related ovarian and breast cancers account for 5-10% of all female ovarian and breast cancers (10,11). Ovarian cancers in the setting of BRCA 1/2-germline mutations can present with more aggressive, high-grade histologies, but are frequently responsive to chemotherapy, particularly platinumbased regimens, leading to an improved 5 years survival (12). The chemotherapy-sensitive mechanism is felt to be related to the intimate relationship between BRCA 1/2 proteins and defective HRR, as discussed above. Recent studies have demonstrated that women with BRCA-related ovarian cancers fare much better than sporadic ovarian cancers (13)(14)(15)(16). A study, published by the National Israeli Study of Ovarian Cancer, showed women with BRCA mutations had a median survival of 55.7 months compared to 37.9 months in sporadic ovarian cancers (p = 0.002) (15). This may be in part explained by the standard use of carboplatin-based therapies for ovarian malignancies as the DNA damage induced by the platinum should be more efficacious in the DNA repair-deficient BRCA-related tumors.
Contrary to the more convincing outcomes in BRCA 1/2related ovarian cancers, the outcomes of BRCA mutation-related breast cancers are less clear. Women with BRCA1 mutations typically develop breast cancer at an earlier age than BRCA2-related and sporadic breast cancers. BRCA1-related breast cancers tend to also be higher grade, hormone receptor-negative, and HER-2-negative, or "triple negative" (17), and also frequently express a basal phenotype (18)(19)(20)(21)(22)(23)(24)(25)(26). Patients with BRCA-mutated breast cancers generally respond to therapy as well as sporadic cancers; however, the risk of second ipsilateral or contralateral primaries may be as high as 3-5% per year, compared to 0.5-1% per year risk, seen in sporadic breast cancers (17). In contrast to ovarian cancer, platinum chemotherapy is not standardly administered to patients with breast cancer. The use of platinum agents has been evaluated in a small series which have demonstrated high efficacy in breast cancer in particular in the setting of a BRCA mutation. Silver et al. evaluated the use of neoadjuvant platinum-containing chemotherapy in patients with triple negative breast cancer (TNBC) (N = 28), and found those more likely to be platinum-sensitive were those with low BRCA1 gene expression (27). Likewise, in BRCA-mutated breast cancer patients who received cisplatin in the neoadjuvant setting showed a high rate of pathologic complete response (pCR) in a small series. Ten of 12 patients achieved pCR (83%). When non-platinum-containing regimens were used, the pCR rate was 14% (28). These studies highlight the rationale to further explore the use of platinumcontaining regimens, specifically for patients with TNBC and BRCA mutations.

BRCAness: SPORADIC TRIPLE NEGATIVE BREAST CANCERS
Triple negative breast cancers account for~20% of all breast cancers and are associated with an aggressive clinical picture (20,25,29). Due to lack of hormone receptor or HER-2 expression, and no other known target for tailored therapy, the only current treatment option is chemotherapy. Over 80% of hereditary BRCA1-mutated cancers are TNBCs. Several studies have investigated a potential role for BRCA1 inactivation in sporadic TNBC given the similar clinical outcomes and histological characteristics among these cancers and hereditary BRCA1-mutated breast cancers. Breast cancers developing in patients with BRCA1 mutations, in addition to frequently being triple negative, also often express basal markers (18-22, 25, 26). Gene microarray expression profiling has shown considerable similarities between BRCA1-mutated tumors and basal tumors (25). This shared phenotype has been termed "BRCAness" (26). What is unknown is whether the basal phenotype is a result of the BRCA loss or if the BRCA loss results in the basal phenotype (6).
Recently, Lehmann and colleagues delved further into the characterization of TNBC. They performed an analysis of gene expression profiles of 587 TNBC cases and identified six separate subtypes of TNBC. These six subtypes were: basal-like 1 (BL1), basal-like 2 (BL2), immunomodulatory (IM), mesenchymal (M), mesenchymal stem-like (MSL), and luminal androgen receptor (LAR) subtype. Additional analysis of TNBC cell lines, representative of each of these identified subsets, revealed differential responses to various therapeutic agents. Both the BL1 and BL2 groups showed increased gene expression involved in DNA damage response, and showed higher response to cisplatin (30). In a follow-up study, Masuda et al. presented neoadjuvant chemotherapy response data in each of the aforementioned TNBC subtypes (31). In 130 TNBC patients, who received standard anthracyclineand taxane-based chemotherapy, the BL1 subtype achieved a pCR most frequently (52%). In contrast, the pCR in the BL2 subtype was 0%. The molecular differences in BL1 and BL2 may explain these differential responses. Specifically, the BL1 subtype involves the cell cycle, DNA replication reactome, and the BRCA pathway, among others, whereas the BL2 subtype involves growth factor, glycolysis, and gluconeogenesis pathways. This work demonstrates that even within "basal-like breast cancer (BLBC)," there may be a great deal of heterogeneity.
Telli and colleagues recently presented a study evaluating gemcitabine, carboplatin, and iniparib, a compound initially believed to have PARP inhibitory effects, in the neoadjuvant treatment of triple negative and BRCA-mutated breast cancer (32). This study demonstrated a pCR of 36% overall, with a pCR in BRCA 1/2mutation carriers of 47%. Furthermore, patients who were both triple negative and had a BRCA 1/2-mutation, had a pCR of 56%. Although only 10 patients were classified as BL1 or BL2, there were an equal number of responders and non-responders to the neoadjuvant platinum regimen. It is also notable that only one patient classified as basal-like had a known BRCA mutation, whereas, there were BRCA-mutated tumors that were classified as IM, M, MSL, and unspecified (32). Although basal-like TNBC has become nearly synonymous with BRCAness, this study found that the basal-like subtype of TNBC was neither particularly responsive to the treatment combination, nor had a higher number of BRCA-germline mutations. In this study, the homologous recombination deficiency (HRD) score appeared to be more predictive of platinum response, as compared to TNBC intrinsic subtyping (30). The HRD assay has been developed to evaluate for loss of heterozygosity (LOH), which has been shown to be predictive of response to platinum in BRCA-related and sporadic cancers (33). While, this data is hypothesis-generating and thought-provoking, larger, prospective studies will be needed before any formal conclusions can be drawn.
In sporadic basal tumors, there are data that show reduced BRCA1 mRNA expression. It is felt that epigenetic modification of the BRCA gene, such as promoter hypermethylation, is responsible for this (34)(35)(36). Interestingly, no tumors showed both BRCA1 mutation and BRCA1 promoter methylation suggesting that these events are mutually exclusive in The Cancer Genome Atlas (TCGA) research network data (37). The association between BRCA1-mutated and BLBCs provides an important rationale to include this frequently encountered patient population in studies geared toward manipulation of the characteristic faulty DNA repair mechanisms in BRCA1-mutated tumors. As we move into an era where genomic analyses of tumors is becoming the norm, it will be important to link the genome, methylome, and proteome to clinical characteristics and outcomes.

BRCAness: SPORADIC HIGH-GRADE SEROUS OVARIAN CANCERS
Similarly, there are many commonalities among BRCA 1/2mutated cancers and sporadic epithelial ovarian cancers (EOCs). Although only 5-10% of ovarian cancers are directly attributable to a germline mutation in BRCA1 or 2, there is a growing body of evidence to suggest that additional mechanisms of BRCA dysfunction are involved in the pathogenesis of ovarian cancer (26,38,39). One study demonstrated alterations of BRCA1 and/or 2 in up to 82% of examined ovarian cancers (n = 92) (40). Methylation of the BRCA1 promoter has been demonstrated in up to 14% of sporadic breast and up to 30% of sporadic ovarian cancers (26,35,(41)(42)(43)(44)(45)(46). LOH has been described in ovarian tumors and Frontiers in Oncology | Cancer Molecular Targets and Therapeutics may have multiple possible mechanisms leading to malignancy including co-existing LOH of BRCA1 and p53, and hypermethylation acting in a synergistic fashion (33,(47)(48)(49)(50)(51). In contrast, BRCA2 methylation has not been found to be a significant contributor (39,52). Identifying and manipulating these BRCA-like deficiencies in DNA repair in sporadic ovarian cancers is of great importance and provides rationale for including these patients in clinical trials designed for BRCA-related malignancies.
Another important mechanism of BRCAness in ovarian cancers is the presence of somatic mutations in BRCA1 and 2 (53). Hennessy and colleagues performed BRCA1/2 sequencing on 235 unselected ovarian cancers and found that 19% of the sample had detectable mutations in BRCA1 (N = 31) or BRCA2 (N = 13). In the 28 samples, where germline DNA was also available, 42.9% of the BRCA1 mutations and 28.6% of the BRCA2 mutations were somatic. Of interest, somatic BRCA 1/2-mutations in breast cancer appear to be less frequent. In the TCGA BLBC cohort, about 20% had either germline (N = 12) or somatic (N = 8) BRCA 1/2-mutations. Another study evaluated 77 TNBC samples and only one harbored a somatic BRCA mutation (54). This potentially explains the seemingly higher activity of single agent PARP inhibitors, discussed later, in sporadic ovarian cancer as compared to sporadic TNBC.

GENOMIC SIMILARITIES: BASAL-LIKE BREAST CANCER AND HIGH-GRADE SEROUS OVARIAN CANCERS
The Cancer Genome Atlas network recently published findings again demonstrating the four distinct molecular signatures in breast cancer from diverse genetic and epigenetic alterations: luminal A, luminal B, basal-like, and HER-2 enriched subtypes (55). Strikingly, BLBCs were notably different than the other three subtypes based on comprehensive analyses using multiple platforms. As expected, these cancers also often (80%) lacked expression of ER, PR, and HER-2 identifying as TNBCs. Specifically, most BLBCs showed a high frequency of TP53 deleterious mutations (80%), as well as, loss of RB1 and BRCA1. PIK3CA mutations (~9%) were also a common feature of BLBC. Analyses also highlighted increased MYC activation as a BLBC characteristic.
The BLBC mutation spectrum reported in the TCGA was similar to that identified in previously described serous ovarian cancers (56) and BLBC were more similar to serous ovarian carcinomas than to other subtypes of breast cancer. One gene, in particular, TP53, had a >10% mutation frequency in both basal-like breast and serous ovarian cancers. As well, both tumors when compared to luminal showed increased BRCA1 inactivation, RB1 loss, cyclin E1 amplification, high expression of AKT3, and MYC amplification. These molecular commonalities strongly suggest shared driving events in tumorigenesis, and similarly, show support for shared treatment strategies for TNBCs and high-grade serous ovarian cancers. Of note, p53 mutations have been described to have high frequency in BRCA mutation-related cancers as well (57,58).   populations, as well as, those with malignancies sharing qualities of BRCAness or HRR-deficiency, such as basal-like or TNBC and serous ovarian cancer.

BRCA 1/2 -MUTATION STUDIES
The first published Phase I study evaluating PARP inhibitors in the clinic used olaparib (AZD2281) enrolling patients with varying malignancies (Tables 1 and 2) (62). An expansion cohort of BRCA-positive ovarian, breast, and prostate cancer patients was enrolled at the recommended Phase II dose of 400 mg twice daily. Nearly half of the evaluable patients had an objective response (19 patients, 47%). Results from this pivotal study showed olaparib was generally well tolerated. From here, two Phase II proof-ofconcept trials (ICEBERG 1 and 2) (Tables 1 and 2) confirmed activity in both BRCA-mutated ovarian and breast cancers, with olaparib at 400 mg twice daily [ORR 11/33 (33%) and 11/27 (41%), respectively], with low overall toxicities (63,64). Olaparib was also evaluated in patients with sporadic cancers displaying a presumed BRCAness phenotype. Gelmon et al. performed a non-randomized Phase II trial using olaparib in heavily treated high-grade serous or undifferentiated ovarian carcinomas and TNBCs (65) (Tables 1-4). Stratified by BRCA mutation status, both BRCA-mutated and BRCA-wild type ovarian carcinoma patients showed response to olaparib. In contrast, neither BRCAmutated nor sporadic breast cancer patients demonstrated significant response to olaparib. Potential explanations for these mixed results include that not all TNBCs have a BRCA-like phenotype, so there may have been some heterogeneity to this population (30).
In a population of BRCA-positive recurrent ovarian cancer patients with a platinum-free interval of ≤12 months, olaparib was compared to pegylated liposomal doxorubicin (PLD) in a randomized Phase II trial (N = 97) (66) ( Table 2). Progression free survival (PFS) was not statistically significantly different for olaparib 200 or 400 mg twice daily (combined or individually) versus PLD (PFS 6.5 versus 8.8 versus 7.1 months, respectively). Where the PFS and ORR were consistent with prior studies for olaparib at 400 mg twice daily, the efficacy of PLD was higher than expected when compared with previous trials. Toxicity profiles were distinct between olaparib (nausea, vomiting, and fatigue) and PLD (stomatitis and palmar-plantar erythrodysesthesia), and overall, the drugs were well tolerated. Although olaparib did not show an improvement in PFS over chemotherapy, these results show that targeted therapy with a PARP inhibitor is as effective as chemotherapy, with potential for improved tolerability.
Other PARP inhibitors have also been studied in clinical trials including niraparib (MK4827) in both BRCA-positive and sporadic tumors. This compound's mechanism of action includes PARP inhibition via a novel PARP trapping mechanism (67). A Phase I study utilizing niraparib monotherapy was recently published that established a maximum tolerated dose of 300 mg/day (N = 100) (68) ( Table 1). Dose-limiting toxicities (DLTs) were reported in the first cycle including grade 4 thrombocytopenia at a dose of 400 mg/day. Non-hematologic DLTs included grade 3 fatigue and grade 3 pneumonitis at lower doses (30 and 60 mg/day, respectively). Common treatment-related effects were anemia, nausea, fatigue, thrombocytopenia, anorexia, neutropenia, constipation, and vomiting, but were predominantly grade 1 or 2. There were anti-tumor responses seen in the BRCA-mutated breast and ovarian cancer population, and these were recorded at doses >60 mg/day. Results from this study show promise for this newer PARP inhibitor and currently there are multiple Phase III trials recruiting in BRCA-positive breast and ovarian, and sporadic ovarian cancer populations (NCT01905592, NCT01847274) (Tables 5 and 6).
Rucaparib (CO-338/AG-014699, also previously PF-01367338) was recently evaluated in Phase I and II studies in advanced solid tumors, including BRCA-positive breast and ovarian cancers. The PARP inhibitor as monotherapy and in combinations with cytotoxic chemotherapy is under investigation. In a standard Frontiers in Oncology | Cancer Molecular Targets and Therapeutics   dose-escalation fashion, a Phase I/II study (Tables 1 and 2) is currently evaluating rucaparib monotherapy in advanced solid tumors (N = 29) including ovarian/primary peritoneal (N = 7) and breast (N = 17) cancer patients (69). Thus far, no DLTs at 360 mg twice daily (study not yet complete) have been reported. To date, two PRs were seen in one BRCA-positive ovarian cancer, and one BRCA-positive breast cancer patient at 300 mg daily dosing during the sixth week of therapy. Ten additional patients (ovarian N = 5, breast N = 4, and colorectal N = 1) have experienced stable disease (SD) at >12 weeks so far; seven of which are BRCApositive. Overall the disease control rate (PR + SD > 12 weeks) for ovarian cancer patients is 86% (6/7). Further results are anticipated from this study. These promising results to date have supported ARIEL2, a Phase II study of rucaparib in platinum-sensitive, relapsed, high-grade epithelial ovarian, fallopian tube, or primary peritoneal cancer patients, which is currently recruiting patients ( Table 6). BMN 673, a novel, highly potent PARP 1/2 inhibitor, demonstrated high efficacy in preclinical studies (70). BMN 673 elicits DNA repair biomarkers at much lower concentrations [PARP1 half maximal inhibitory concentration (IC 50 ) <1 nmol/L] than earlier generation PARP inhibitors, i.e., olaparib, veliparib, and rucaparib. Its anti-tumor activity has been tested in vitro and in xenograft cancer models, as monotherapy and in combination. Anti-tumor activity was seen in BRCA1, BRCA2, and PTEN deficient cells with a 20 to more than 200-fold greater potency than existing PARP 1/2 inhibitors. Synergism was also seen when BMN 673 was combined with temozolomide, SN38, or platinum drugs. Thus far, BMN 673 has been the most specific PARP inhibitor in its class.
The first in-human Phase I, clinical trial using BMN 673 in solid tumor patients was recently presented at ASCO 2013 (71) (Tables 1 and 2). Patients with advanced solid tumors defective in DNA repair, including BRCA-mutated breast (N = 6), and ovarian (N = 17) cancer patients, were eligible for the stage II expansion phase at the maximum tolerated dose of 1000 mcg daily. In total, 39 patients with advanced solid tumors were enrolled, including those tumors with deleterious BRCA mutations. Thrombocytopenia was dose-limiting and occurred in three patients at doses 900 or 1100 mcg daily. Most potential treatment-related adverse events (AEs) were grade 1/2 and included fatigue, nausea, flatulence, anemia, neutropenia, thrombocytopenia, and alopecia. Objective responses were seen in 11/17 BRCA-mutated ovarian/primary peritoneal cancer patients and 2/6 BRCA-mutated breast cancer patients. Based on these encouraging results, the recommended dose, 1000 mcg daily, will be studied in a Phase III trial in BRCA-carrier metastatic or locally advanced breast cancer patients (NCT01945775) ( Table 5).
In addition to the single agent studies described above, PARP inhibitors have been combined with chemotherapy in BRCA mutation-related malignancies. Lee et al. in a Phase I/Ib study, utilized olaparib, in combination with carboplatin, in a standard dose-escalation study design in BRCA 1/2-mutated breast and ovarian cancers (N = 45) (72) (Tables 1 and 2). The recommended Phase II dose was 400 mg twice daily for 14 days with carboplatin AUC 5. As noted in several other trials utilizing olaparib, and other PARP inhibitors, myelosuppression was frequently present with grade 3/4 AEs (neutropenia 42%), as well as, thrombocytopenia (20%), anemia (13%), carboplatin-hypersensitivity (9%), and fatigue (7%). Responses included one CR in a breast cancer patient that was durable (duration of 17 months), and a PR in 15/34 (44%) ovarian cancer (duration 3-28+ months) and 6/8 breast cancer (duration 5-24+ months) patients. Prolonged SD was seen in 14/34 (41%) ovarian cancer patients for as long as 25 months and for 11 months in a breast cancer patient. Remarkably, the overall clinical benefit rate was 100% in breast cancer patients and 85% in ovarian cancer patients. A summary of Phase I-III studies utilizing PARP inhibitors in BRCA 1/2-mutated breast and ovarian cancers can be found in Tables 1 and 2.

SPORADIC BREAST AND OVARIAN CANCER TRIALS
The earliest trials reported for sporadic TNBCs evaluated iniparib (BSI-201) in combination with gemcitabine and carboplatin. The Phase II trials showed promising anti-tumor activity, prolonged median progression-free survival, and median overall survival (OS) with minimal overall toxicity (73). Disappointingly, the results were not significant in the Phase III trial (74). There are a number of potential explanations for the lack of efficacy seen in the Phase III study, including the heterogeneity within the subtypes of TNBC. Importantly, it was discovered that iniparib was actually not a PARP inhibitor, at physiologic concentrations. Rather, iniparib was shown to cause telomere-centric DNA damage (75).
There are also a number of reported and ongoing studies with "true" PARP inhibitors in sporadic TNBCs, although, only a few studies that have been published in final format. A Phase I/II study of mention explored the use of olaparib in combination with Frontiers in Oncology | Cancer Molecular Targets and Therapeutics www.frontiersin.org  Combination therapies with PARP inhibitors have also been investigated in sporadic ovarian and breast cancers, specifically with other novel targeted agents. Cediranib, an anti-angiogenesis agent, was studied with olaparib in recurrent epithelial ovarian or TNBCs (N = 28, 20 ovarian and 8 breast) (82) (Tables 1-4). Patients were enrolled to four dose levels and the recommended Phase II dose was cediranib 30 mg daily and olaparib 200 mg twice daily was based on one occurrence of grade 4 neutropenia (≥4 days) and one of grade 4 thrombocytopenia with dosages of cediranib 30 mg daily and olaparib 400 mg twice daily. Seventyfive percent of patients experienced grade 3 or higher toxicities with grade 3 hypertension and fatigue, occurring in 25 and 18% of subjects, respectively. Despite the frequent hematologic and non-hematologic toxicities, the ORR was 44% in the evaluable ovarian cancer population (N = 18). Sixty-one percent of ovarian patients had clinical benefit (including those with SD). None of the breast cancer patients experienced clinical response, but two patients had SD for >24 weeks. A summary of Phase I-III studies utilizing PARP inhibitors in sporadic breast and ovarian cancers can be found in Tables 3 and 4.

PLATINUM AND PARP INHIBITOR RESISTANCE
BRCA 1/2-deficient cancers are known to be hypersensitive to platinum agents which are thought account for, in part, better overall prognosis for those patients with BRCA 1/2-germline mutationrelated breast and ovarian cancer. Not all patients respond to platinum, however, and indeed, it is likely that the majority of tumors will eventually become platinum-resistant. Additionally, not all patients with BRCA 1/2-germline mutations or those with an expected BRCAness phenotype respond to PARP inhibition. Several mechanisms of resistance to both agents have been hypothesized and are likely to be multifactorial in etiology. Current evidence suggests that secondary mutations occur in the BRCA1 or BRCA2 gene restoring the wild type BRCA 1/2 open reading frame which may provide return of DNA repair through a functional HR pathway. These reversion mutations are thought to lead to platinum resistance, as well as PARP inhibitor resistance (83)(84)(85)(86)(87). It is imperative that these secondary mutations are identified to help modulate therapeutic management of these populations. Of interest, PARP inhibitor resistance may, in fact, not affect subsequent therapy response, including subsequent platinum regimens (88).

CONCLUSION
Poly (ADP-ribose) polymerase inhibitors have shown promising activity as both monotherapy and in combination with cytotoxic chemotherapy in BRCA 1/2-mutated cancers. More recently, this concept has been implicated in sporadic high-grade serous ovarian cancers and TNBCs. Like platinum agents, PARP inhibitors have been efficacious in this population. Published data from the TCGA network further support this therapeutic strategy by showcasing the genomic similarities between high-grade serous ovarian cancers and TNBCs. It may be worthwhile in the future to study new drug therapies in tandem in these two populations. New strategies are needed to combat tumor resistance mechanisms, such as secondary mutations that revert BRCA genes to wild type, to both platinum agents and PARP inhibitors. Future directions for PARP inhibition include when best to use these agents, in what combinations, and precisely, how to define the optimal populations that will get the most benefit.