Keywords

13.1 Introduction

Recent researches have provided new insights into the effective treatments for breast cancer, which is the most common cancer in women worldwide. Clinically treated according to its subtype, breast cancer has four subtypes identified as follows: (1) hormone receptor positive/human epidermal growth factor receptor 2 (HER2) negative, (2) hormone receptor positive/HER2 positive, (3) hormone receptor negative/HER2 negative, and (4) hormone receptor negative/HER2 negative. The last subtype is also called triple-negative breast cancer (TNBC), one of the most aggressive types of breast cancer. Unlike hormone receptor-positive (luminal-like) subtypes, there are no targeted therapies available for patients with TNBC, which shows aggressive behaviors. Therefore, many researchers are investigating the molecular background of TNBCs, with a particular focus on BRCA1/BRCA2 mutations.

In this chapter, we will discuss TNBC and the effects of BRCA mutations in this type of cancer. The roles of poly(ADP-ribose) polymerase (PARP) inhibitors in breast cancer treatment will also be elucidated.

13.2 TNBC

TNBC is defined based on immunohistochemical staining criteria. In the clinical setting, TNBC is defined to be estrogen receptor (ER) negative, progesterone receptor (PgR) negative, and human epidermal growth factor receptor 2 (HER2) negative. However, TNBC remains a heterogeneous disease that includes several intrinsic subtypes. Moreover, TNBC is known for its highly aggressive behavior and poor prognosis compared with other breast cancer subtypes [1], such as ER-positive, PgR-positive, and/or HER2-positive diseases.

13.2.1 Molecular Biological Features of TNBC

TNBC accounts for approximately 15% of all breast cancers. Compared with other subtypes, TNBC tends to occur in younger patients and exhibit large tumor burden, high nuclear grade, low BCL-2 expression, and high p53 and/or Ki-67 expression.

In 2000, Perou et al. performed a complementary DNA microarray gene profiling analysis in breast cancer and identified different molecular patterns, called “molecular portraits,” among breast cancers [2]. In this analysis, they classified breast cancers into five different intrinsic subtypes: luminal A, luminal B, HER2-enriched, basal-like, and normal. Seventy-five percent of clinically proven TNBC can be classified into the basal-like subtype. In a later publication, researchers confirmed that among TNBCs, 80% were the basal-like subtype, 3% were the luminal subtype, and 9% were the HER2-enriched subtype [3].

Among basal-like subtypes, molecules such as cytokeratin 5/6, vimentin, and laminin have been shown to be highly expressed, whereas Bcl-2 has been shown to exhibit low expression [4]. Moreover, loss of phosphatase and tensin homolog (PTEN) and the disappearance of phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA) expression, retinoblastoma (RB) 1 mutations, or KRAS mutations are commonly observed in basal-like TNBC [5, 6].

Mutations or deletions in the BRCA gene (BRCA1 and BRCA2) are also found in TNBCs. Among basal-like subtypes, 75% have been reported to be BRCA1-associated breast cancers [7], whereas 19.5% of all TNBCs show BRCA germ line mutations [8].

13.2.2 Optimal Strategies for Treatment with Currently Approved Agents

Despite the findings of molecular subtypes among TNBC, no predictive values of the molecular subtypes have been established. Treatment is therefore selected from currently recommended agents that are approved in general breast cancer population.

Anthracyclines and taxanes remain the primary therapeutic approaches for TNBC, although there is limited evidence of success in patients treated with anthracycline- and/or taxane-containing regimens in the perioperative setting [9]. Patients who show primary or acquired resistance to key drugs may be given further chemotherapeutic agents that are not crossresistant, such as capecitabine, eribulin, gemcitabine, or vinorelbine [10,11,12]. The use of multidrug regimens in patients with metastatic cancer is controversial, and guidelines, such as those issued by European Breast Cancer Conference [13], recommend sequential monotherapy for advanced breast cancer. In cases where the aggressive nature of the disease calls for the need to stabilize the symptoms and reduce the risk of inner organ dysfunction, which is often noted in patients with TNBC, a multidrug regimen may be recommended rather than a single-drug regimen. Other agents that are sometimes used in TNBC therapy include platinum-based regimens [14,15,16] and PARP inhibitors (which are being investigated). The use of these agents has been supported by the strong association of TNBC with germ line BRCA1 mutations.

Nonetheless, TNBC shows an aggressive behavior and very poor prognosis with limited treatment options. A biomarker-based understanding of molecular targets is required to facilitate further improvements in treatment strategies for TNBC.

13.3 BRCA Mutations

13.3.1 Functions and Mechanisms of BRCA

BRCA was first discovered in the 1990s and has been one of the most notorious and well-known cancer-related genes identified to date. It was originally considered as a tumor-suppressor gene [17]. However, further evidence shows that BRCA proteins are involved in the maintenance of genomic integrity. Therefore, instead of functioning as “gatekeeper” proteins of tumor suppressor, the BRCA family of proteins acts as “caretaker” proteins of DNA repair. Moreover, BRCA proteins are known to function in concert with other proteins, such as RAD50/Mre11 and RAD51, which play important roles in repairing DNA breaks caused by ionizing radiation [18].

During DNA replication, DNA molecules are particularly vulnerable to breakage in the single-stranded molecule portions that have not yet undergone replication near the replication fork. When an accidental breakage of the still unreplicated single-stranded DNA occurs at the replication forks, the resulting breaks are functionally equivalent to double-stranded breaks occurring in an already formed double helix. These double-stranded breaks are usually fixed by homology-directed repair (HDR). At sites of stalled replication forks where double-stranded breaks are observed, BRCA1 is located along with proliferating cell nuclear antigen (PCNA) and other DNA repair proteins, including RAD50 and RAD51 [19]. BRCA2 protein is also found at the same location, providing evidence of its collaboration in the DNA repair process [20]. When cells lack either BRCA1 or BRCA2, all types of HDR are compromised.

In mice, genetic disruption of BRCA1 function causes death during early embryogenesis, whereas mutant germ line alleles of BRCA2 cause only partial loss of function, which results in susceptibility to lymphoid malignancies and unusual chromosomal aberrations [18]. In humans, mutant germ line alleles of either BRCA1 or BRCA2 lead to a natural susceptibility to breast and ovarian carcinomas [21]. In ovarian cancer, an estimated 70–80% of cases are caused by BRCA mutations. Some somatic mutations in BRCA2 are associated with prostate and colon carcinomas. Additionally, female cells lacking BRCA1 function cannot properly inactivate one of the two X chromosomes. The mechanism of X-inactivation is essential in cells of early female embryos and must persist in all linear descendants. How this loss of BRCA function intersects with its DNA repair functions and how BRCA1 mutation inclines to generate cancer primarily in women remain unknown.

13.3.2 BRCA Mutations in TNBCs

According to an analysis published by the International Breast Cancer Linkage Consortium, 0.12% of the general population carries BRCA1 germ line mutations [22]. In patients with breast cancer, approximately 5% of patients are BRCA mutation carriers. According to a retrospective study, among patients with BRCA mutations, 57.1% have the clinical TNBC subtype [23]. Additionally, 19.5% of TNBCs have been shown to have germ line BRCA mutations [8]. When the population is narrowed down to those who have familial breast cancers, defined as breast cancer with a family history of one or more first- or second-degree relatives with breast cancer that does not fit the hereditary breast cancer definition, almost half of cancers are associated with germ line transmission of BRCA1 or BRCA2 mutations. In addition to germ line mutations, methylation of BRCA1 is also known to be frequently found in TNBCs [24]. In all, the findings have shown that BRCA mutations are highly associated with TNBCs.

13.4 Function of PARP1

Among the many backup mechanisms required for proper repair or maintenance of the genome, poly(ADP-ribose) (PAR) synthesis is one of the earliest responses to DNA strand breakage. PARP1 is an abundant and stable component of chromatin and facilitates DNA repair by binding to DNA breaks and attracting other repairing proteins [25,26,27,28,29]. It is comprised of three functional domains: the amino-terminal DNA-binding domain which is important for binding of PARP1 to DNA breaks, the central automodification domain which allows the enzyme to PARate itself, and the C-terminal catalytic domain which transfers ADP-ribose subunits from NAD+ to protein acceptors (Fig. 13.1) [30]. Among the seven main pathways used for DNA repair, PARP plays an important role in base excision repair (BER). At sites of single-stranded DNA breaks in which PARP binds to the DNA, PARP is activated and converts nicotinamide adenine dinucleotide (NAD) into ADP-ribose polymers (PAR) by attracting XRCC1, a scaffold protein that interacts with and recruits, stabilizes, or stimulates multiple enzymatic components involved in single-stranded breakage. For short patch repair and long patch repair at lesions that are more difficult to repair, the breakage goes through a single-stranded break intermediate and then arrives at a ligation stage to yield repaired DNA. PARP1 and PARP3 are among the 17 PARP isoforms that are also involved in double-stranded break repair [31].

Fig. 13.1
figure 1

Function of PARP1 in DNA repair (Reprinted by permission from Macmillan Publishers Ltd: Nat Rev Cancer. (10(4): 293–301), copyright (2010))

Abbreviations: ATM, ataxia telangiectasia mutated; BER, base excision repair; BRCT, BRCA1 carboxy-terminal repeat motif; DNA-PKcs, DNA-protein kinase catalytic subunit; DSB, double-stranded break; HR, homologous recombination; NHEJ, nonhomologous end joining; NLS, nuclear localization signal; PPi, inorganic pyrophosphate; SSB, single-stranded break; Zn, zinc finger [30]

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For cells that lack BRCA1 or BRCA2 function, PARP acts as a backup system to maintain the genome and plays a critical role following accidental breaks that occur at replication forks during the S phase. Consequently, the cells become highly sensitive to killing by pharmacologic inhibitors of PARP1 [32]. However, Parp−/− mice are viable and fertile, which explains the redundant DNA repair systems. Therefore, PARP inhibition has little if any effect on normal tissues.

13.5 PARP Inhibitors and Their Effects on Cancer

PARP inhibitors exhibit competitive inhibition with NAD by blocking the catalytic PARP domain. PARP inhibitors show single-stranded DNA breakage repair activity, inducing apoptosis through accumulation of damaged DNA in the cells. By inhibiting PARP1, the repair phenomenon can be trapped at the single-stranded intermediate state, thereby blocking ligation. PARP inhibitors bind to the catalytic site and prevent the release of PARP1 from DNA by “trapping” PARP1 at the site and removing PARP1 from the normal catalytic cycle [27, 28, 33]. When BER does not function properly, single-stranded breaks are left unrepaired, leading to the formation of double-stranded breaks due to stalling of the replication fork. Since double-stranded breaks are repaired by either nonhomologous end joining or homologous recombination, inhibition of PARP alone does not lead to efficient cell death. Therefore, for PARP inhibitors to exert beneficial effects on DNA repair, another repair pathway other than BER must be functionally damaged by PARP inhibition.

13.5.1 Synthetic Lethality

Synthetic lethality was introduced nearly a century ago by geneticists. It involves the combined knockout of two genes, which leads to a lethal form of genetic interactions that can selectively kill cancer cells while sparing normal cells [34]. The concept of synthetic lethality involving PARP and BRCA is related to the observation that both proteins are normally nonessential but critical for the survival of cancer cells. The most striking evidence of synthetic lethality is the use of PARP inhibitors in homologous recombination-defective tumors [32, 35]. As BRCA1 and BRCA2 are associated with homologous recombination, PARP inhibitors have been used for monotherapy in treating patients with BRCA1- or BRCA2-mutated cancers. Other genes associated with homologous recombination are RAD51, RAD54, DSS1, PRA1, NBS, ATR, ATM, CHK1, CHK2, FANCD2, FANCA, and FANCC. Cells with a deficiency in one of these genes show sensitivity to PARP inhibitors, confirming the concept of synthetic lethality [33].

13.6 Clinical Application of PARP Inhibitors in Cancer

In PARP1-knockout mice, deficiencies in PARP1 function result in impaired DNA repair, which consequently leads to a higher sensitivity to anticancer agents. It indicates that PARP1 inhibition may induce sensitivity to DNA damage by anticancer agents and therefore act as a radiosensitizer or chemosensitizer in the treatment of cancers. PARP1 is also known for its strong activation by radiotherapy or DNA methylating anticancer agents. Based on available evidence, along with the development of PARP inhibitors in patients with germ line BRCA mutations, new therapeutic approaches using PARP inhibitors combined with DNA-damaging anticancer agents have been evaluated. Approximately 30 years ago, small-molecule nicotinamide analogs were found to enhance the cytotoxicity of dimethyl sulfate, a DNA-damaging agent, by inhibiting PARylation [36,37,38]. Subsequently, clinical PARP inhibitors, including veliparib, rucaparib, olaparib, and niraparib, were developed. A more potent second-generation PARP inhibitor, talazoparib, has also been developed [39]. The difference among these agents is the ability to “trap” PARP1, an essential mechanism of PARP inhibitors. Talazoparib is approximately 100 times more potent than niraparib and is therefore more potent than olaparib and rucaparib [40]. The chemical structures of clinical PARP inhibitors and the ability of each PARP inhibitor to “trap” PARP1 is thought to broadly correlate with its cytotoxic potency [33]. Among currently available PARP inhibitors, olaparib (Lynparza) was the first to be approved by the US Food and Drug Administration (FDA) for treating patients with germ line BRCA mutations in advanced ovarian cancer in February 2014. The development of olaparib in breast cancer will be further discussed in this chapter.

13.7 PARP Inhibitors in the Field of Breast Cancer

During clinical development, PARP inhibitors have been investigated in combination with DNA-damaging anticancer agents or radiotherapy, or as monotherapy, in cancers that show decreased BRCA1 or BRCA2 functions, mainly TNBC. In the field of breast cancer, BSI-201 was the first PARP1 inhibitor to be reported [41]. In a phase I (and Ib) trial, this compound showed safety and effectiveness and was later tested in a randomized phase II trial, which compared combined treatment of gemcitabine plus carboplatin (GC) plus BSI-201 and GC alone in patients with metastatic TNBC with two or less prior regimens [42, 43]. The progression-free survival (PFS) was 6.9 months versus 3.3 months, and overall survival was 9.2 months versus 5.7 months, indicating a statistically longer survival for the GC plus BSI-201 arm. The overall response rate was also higher in the GC plus BSI-201 arm (48% versus 16%, p = 0.002). There were high expectations for the phase III trial, but the primary endpoint was not achieved.

Alternatively, olaparib has been developed as another promising PARP inhibitor for the treatment of breast cancer, which will be discussed below.

13.8 Development of Olaparib (Lynparza) in Breast Cancer

Olaparib is a PARP1 inhibitor first discovered during a screening test for agents that induce sensitivity of cells to cytotoxic agents, such as topoisomerase I inhibitors and alkylating agents. It has showed antitumor activity in cells with homologous recombination deficiency, which implies its role as a promising agent for the treatment of BRCA-mutated cancer. Moreover, olaparib was first approved by the FDA for treatment of BRCA-mutated ovarian cancers. In this section, we will discuss the development of olaparib studies in the field of breast cancer.

13.8.1 Preclinical Study

Through an in vitro study, olaparib monotherapy demonstrated strong antitumor activity in breast cancer cells with BRCA1 mutations [44]. In an in vivo study of BRCA1−/− tumor-bearing mice, olaparib inhibited tumor growth without signs of toxicity, which significantly increased the survival rate. In a similar analysis with BRCA2−/− murine mammary epithelium, daily exposure to olaparib for 28 days caused significant regression or growth inhibition in 46 of 52 tumors [45]. The same analysis was conducted with olaparib in combination with carboplatin. Although no advantage over carboplatin monotherapy was observed, a significant increase in time to tumor relapse or death was observed if PARP inhibitors were continuously administered [46]. In combination therapy, temozolomide or dacarbazine plus olaparib was shown to have antitumor activity. Similarly, olaparib with topoisomerase I inhibitors or platinum agents also showed activity in vitro and in vivo.

13.8.2 Clinical Phase I Monotherapy Trials

Olaparib was first tested in early phase clinical trials for advanced solid tumors with no further standard therapy [47]. However, as the activity of this agent against BRCA-mutated cancers became clearer, the protocol was amended to include patients with BRCA mutations. Later, during the expansion phase, patients with BRCA mutations were specifically enrolled, and a total of 60 patients were eventually included. The dose of oral administration started at 10 mg either once daily or twice daily for 14 consecutive days in a 21-day cycle. During the higher dose phase, the drug was taken twice daily for 28 consecutive days. Dose-limiting toxicity was confirmed at doses of 400 and 600 mg twice daily. In the cohort receiving 400 mg, one patient experienced grade 3 agitation and grade 3 fatigue, and in the cohort receiving 600 mg, one patient experienced grade 4 thrombocytopenia and another patient experienced grade 3 somnolence. Overall, 21 patients with BRCA mutations were enrolled, and among the 19 patients with breast, ovarian, or prostate cancers, nine patients (47%) achieved a partial response, and 12 patients (63%) achieved a clinical benefit (partial response or stable disease). This was a surprisingly high response rate for a cohort that included patients with relapsed breast, ovarian, or prostate cancer. Furthermore, patients with BRCA mutations did not show higher incidences of adverse events than patients having wild-type BRCA.

13.8.3 Clinical Phase II Monotherapy Trials

To date, three phase II trials of olaparib monotherapy have been published in the field of breast cancer. The first trial was an international collaborative trial undertaken in six countries. This trial included patients with advanced breast cancer with BRCA1 or BRCA2 mutations who had been given at least one prior chemotherapy regimen [48]. The study was comprised of two different dosage cohorts: 400 mg twice daily (phase I maximum tolerated dose) and 100 mg twice daily (a dose that showed activity in the phase I trial). Objective responses were observed in 11 of 27 patients (41%; 95% confidence interval [CI]: 25–59) in the first cohort and 6 of 27 patients (22%; 95% CI: 11–41) in the latter cohort. The toxicities were mainly at low grade. Therefore, these results provided positive evidence for the concept of PARP inhibition in BRCA-deficient breast cancers. The second trial was a multicenter trial conducted in Canada and included patients with recurrent high-grade serous or poorly differentiated ovarian carcinoma or TNBC, regardless of BRCA1 or BRCA2 mutation status [49]. Patients received olaparib 400 mg twice daily. Ninety-one patients were enrolled (65 with ovarian cancer and 26 with breast cancer), and among the 63 evaluable patients, objective responses were observed in 7 of 17 patients (41%; 95% CI: 22–64) with BRCA1 or BRCA2 mutations and 11 of 46 patients (24%; 95% CI: 14–38) without mutations. Although no objective responses were reported in patients with breast cancer, 30% of patients achieved stable disease for at least 8 weeks, with a median PFS of 54 days. The third phase II trial was an international collaborative trial that enrolled patients with germ line BRCA1 or BRCA2 mutations with recurrent breast, ovarian, pancreatic, or prostate cancer [50]. Patients with breast cancer had to have at least three prior-chemotherapy regimens for metastatic disease. Olaparib was administered at 400 mg twice daily. Among the 298 patients treated and evaluated, an objective response was achieved in 78 of 298 patients (26.2%; 95% CI: 21.3–31.6) and in eight of 62 patients (12.9%; 95% CI: 5.7–23.9) with breast cancer. Stable disease was observed in 47% (95% CI: 34.0–59.9) of patients with breast cancer. Table 13.1 summarizes the phase II trials that included patients with breast cancer.

Table 13.1 Clinical phase II studies of olaparib monotherapy in breast cancer
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13.8.4 Clinical Phase III Monotherapy Trials

Three phase III trials of olaparib monotherapy have been initiated in patients with germ line BRCA mutation-positive breast cancer. They are OlympiA (NCT02032823), Neo-Olympia (D081EC00005), and OlympiAD (NCT0000622). OlympiA is a randomized double-blind study which assesses the efficacy of olaparib at a dose of 300 mg twice daily. In this study, olaparib was administered with and without placebo as adjuvant treatment in patients with BRCA1/BRCA2 mutations and high-risk HER2-negative breast cancer. The patients were divided into two groups, with one completing definitive local treatment and the other undergoing either neoadjuvant or adjuvant chemotherapy. Neo-Olympia is a randomized three-arm trial comparing olaparib monotherapy at a dose of 300 mg twice daily, placebo therapy plus weekly paclitaxel (80 mg/m2), and olaparib therapy at a dose of 100 mg twice daily plus weekly paclitaxel (80 mg/m2) in the neoadjuvant setting in patients with BRCA1/BRCA2 mutations and operable, locally advanced, or inflammatory breast cancer. OlympiAD is a randomized open-label trial which assesses the efficacy of olaparib at a dose of 300 mg twice daily. It compares olaparib monotherapy with treatment of physician’s choice (TPC) of capecitabine, vinorelbine, or eribulin in patients with BRCA1/BRCA2 mutations and metastatic breast cancer. Two of the trials began enrolment in 2014, and findings from the OlympiAD trial were recently reported at the 2017 ASCO Annual Meeting [51]. At 77% data maturity, PFS was significantly longer in the olaparib arm [7.0 vs 4.2 months, hazard ratio (HR) 0.58; 95% CI: 0.43–0.80; p = 0.0009] with a higher objective response rate of 59.9% in the olaparib arm compared to 28.8% in the TPC arm (HR 0.57; 95CI: 0.40–0.83). The safety profile of olaparib was consistent with prior studies. These promising results were the first to demonstrate improved outcomes with a PARP inhibitor in breast cancer. Table 13.2 summarizes the phase III trials that included patients with breast cancer.

Table 13.2 Clinical phase III studies of olaparib monotherapy in breast cancer
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13.8.5 Combination Therapy

Olaparib has been tested with several other agents, such as paclitaxel, temozolomide, dacarbazine, topotecan, bevacizumab, paclitaxel plus carboplatin, and newer agents (e.g., phosphoinositol 3-kinase [PI3K] inhibitors).

13.8.5.1 Paclitaxel Plus Olaparib

In a phase I/II trial, patients with advanced TNBC were treated with olaparib at a dose of 200 mg twice daily in combination with paclitaxel (90 mg/m2, days 1, 8, and 15) on a 28-day cycle [52]. Patients were treated with either first-line or second-line chemotherapy. The response rate was high, with seven (37%) out of 19 patients achieving an objective response. Although the toxicities were relatively well tolerated, severe neutropenia was observed at a greater frequency than expected. In the second cohort, the dose intensity of paclitaxel was not retained, even with the use of prophylactic granulocyte colony-stimulating factor.

13.8.5.2 Paclitaxel Plus Carboplatin Plus Olaparib

In a cohort of patients with advanced solid tumors including breast cancer, a phase I study was conducted to investigate the treatment of olaparib with either paclitaxel (80 mg/m2, days 1, 8, and 15) or carboplatin (AUC 4–5, day 1) or both paclitaxel (90–175 mg/m2, day 1) plus carboplatin (AUC 4–5, day 1; TC). Olaparib was given at a dose of 50–200 mg twice a day every day or 200–400 mg twice a day for 5 or 10 consecutive days [53]. The hematological toxicities were too strong to maintain the dose in the cohorts taking olaparib every day plus carboplatin or taking olaparib everyday plus TC. However, olaparib given at a dose of 100 mg twice a day every day in combination with PTX was well tolerated, as was olaparib given at 200 mg twice a day for 10 consecutive days plus TC. The overall objective response rate was 16.1% (14/87 patients), whereas the response rate in patients with BRCA1/BRCA2 mutations was 50% (6/12 patients).

13.8.5.3 Eribulin Plus Olaparib

Eribulin mesylate is a nontaxane inhibitor of microtubule dynamics of the halichondrin class of antitumor agents. Eribulin is currently recognized as a global standard treatment for metastatic or recurrent breast cancer following the use of anthracyclines and taxanes. Pooled analyses of two phase III trials of eribulin monotherapy in patients with metastatic or recurrent breast cancer suggested favorable survival benefits, particularly in patients with TNBC [11, 12]. In a cohort of patients with TNBC, a phase I/II trial was conducted in Japan to investigate the safety profiles and efficacy of olaparib in combination with eribulin under the assumption that this combination may be a favorable regimen for patients with metastatic or recurrent TNBC [54]. Patients who had received both anthracycline- and taxane-containing regimens were enrolled to be treated with eribulin at a dose of 1.4 mg/m2 (days 1 and 8) plus olaparib twice daily every day at a dose of 25–300 mg. The recommended phase II dose of olaparib was 300 mg twice daily. Pharmacokinetic (PK)/pharmacodynamic (PD) analysis also showed that the Cmax and area under the curve (AUC) of olaparib were dose dependent and that both parameters of eribulin and olaparib were not influenced by each other. An objective response was observed in seven of the 18 evaluable patients, indicating a relatively high response rate of 38.9% (95% CI: 17.3–64.3). Six patients maintained their responses for over a year, and the median PFS was 4.22 months (95% CI: 2.99–7.36). The most frequent adverse events were the occurrences of neutropenia (grade 3 or more: 83.3%), but the drug was overall well tolerated.

13.9 Development of Other PARP Inhibitors: Talazoparib

Talazoparib has a much higher potency for “trapping” PARP inhibitors than olaparib. In a recent phase I study, talazoparib has shown some promise in treating 13 early-stage patients with germ line BRCA1 or BRCA2 mutations. The patients were treated for 2 months with talazoparib before neoadjuvant chemotherapy and surgery [55]. Decreased tumor volume was observed in all 13 patients following the 2-month treatment with talazoparib, and the average volume reduction was 78% (range: 30–98%). The toxicity of this drug also proved to be well tolerated, as no grade 4 toxicities were observed, and only one patient required dose reduction due to grade 3 neutropenia. The study is ongoing, and researchers will next investigate the pathological response to talazoparib alone for 4–6 months.

Although talazoparib can kill BRCA-mutated cells in vitro at a 200-fold lower dose than olaparib or rucaparib, the in vitro therapeutic ratio achieved in BRCA1-/BRCA2-defective cells is similar with that in wild-type cells for all three PARP inhibitors. Therefore, it is still too early to draw any conclusion regarding which PARP inhibitor is most effective. Table 13.3 shows the clinical trials conducted with PARP inhibitors in patients with breast cancer (excluding the clinical trial of olaparib monotherapy discussed above).

Table 13.3 Clinical studies of PARP inhibitors including breast cancer
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13.10 Acquired Resistance to PARP Inhibitors

Multiple potential mechanisms of resistance have been identified through in vitro experiments. Even though homologous recombination repair is defective, the restoration of homologous recombination repair in BRCA1-mutant tumor cells has been identified through loss of 53BP1 and REV7 proteins [56, 57]. Moreover, the loss of PARP1 [58] has been proposed to cause resistance, as with other proteins that are important for maintaining replication fork stability [59]. Secondary mutations in BRCA1 or BRCA2 can also occur, leading to restoration of sufficient homologous recombination repair function and resulting in PARP inhibitor resistance [60, 61]. Additionally, this secondary mutation is known to cause clinical resistance to platinum-based chemotherapy [62, 63].

13.10.1 Genetic Deficiencies Other Than BRCA1/BRCA2

Not long after the discovery that BRCA1 and BRCA2 mutant cells were highly susceptible to PARP inhibitors, deficiencies in a number of tumor-suppressor genes, such as ATM, ATR, PALB2, and FANC, which are all involved in homologous recombination repair, have been shown to confer sensitivity to PARP inhibitors [63, 64].

In an in vitro experiment, wild-type BRCA1/BRCA2 breast cancer cells (i.e., MCF-7 and ZR-75–1 cells) that were genetically manipulated to knockdown ATM expression were treated with olaparib [65]. ATM depletion sensitized both cell lines, as assessed by short- and long-term survival assays. These data indicated that ATM depletion could sensitize breast cancer cells to PARP inhibitors and that cancers, such as those arising in mutant ATM heterozygous carriers, may be potential targets for PARP inhibitors. A similar phenomenon has been discovered for other tumor cells, such as gastric cancer cell lines and colorectal cell lines, and studies have highlighted the clinical utility of ATM expression as a predictive marker for the sensitivity of gastric cancer cells to PARP inhibitors [66].

The Fanconi anemia (FA) repair pathway is also known to play a collaborative role with BRCA genes. Patients with FA have a high incidence of malignancies, and their cells show hypersensitivity to DNA cross-linking agents, such as mitomycin C (MMC) and cisplatin. Cancers with defective FA/BRCA pathways are likely to be more sensitive to these types of therapy or to treatments in which an additional repair mechanism is targeted, such as treatment with PARP inhibitors. In a recent study, researchers developed a new assay to identify patients with FA functional defects using FA triple-stain immunofluorescence (FATSI, FancD2/DAPI/Ki67) [67]. The study was also conducted to verify the safety and feasibility of veliparib as monotherapy and in combination with MMC. According to FATSI screening, 28.7% (185/643) of patients were FATSI-negative, demonstrating that a substantial number of tumors exhibited FA functional deficiency. Among the 61 FATSI-negative patients who received treatment, six antitumor responses were observed with five in the combination arm. However, some clinical benefits were observed, and a better understanding of this mechanism is needed.

13.11 Concluding Remarks and Future Perspectives

Many studies have investigated the use of PARP inhibitors in breast cancer, with a particular focus on TNBC with BRCA mutations. So far, one trial of olaparib monotherapy has shown promising results for breast cancer. However, given the relatively small size of the study, it is difficult to tell which subset of patients would benefit the most from olaparib. Determining the optimal use of PARP inhibitors within drug combinations has been challenging, and new biomarkers may be needed to identify appropriate populations who may benefit most from PARP inhibitors. In addition, resistance to PARP inhibitors can arise in advanced disease, and further studies are needed to elucidate the related mechanisms.