Poly(ADP-ribose) polymerase (PARP) inhibitors are effective in germline BRCA1 or BRCA2 (BRCA1/2) mutation-associated metastatic breast cancer. However, studies evaluating PARP inhibitors plus platinum-based chemotherapy in germline BRCA1/2-wildtype triple-negative breast cancer are scarce. A large proportion of germline BRCA1/2-wildtype triple-negative breast cancer shows homologous recombination deficiency (HRD), resulting in a BRCA-like phenotype that might render sensitivity to PARP inhibitors. The S1416 trial assessed the efficacy of cisplatin combined with the PARP inhibitor veliparib in three predefined groups of metastatic breast cancer: germline BRCA1/2-mutated, BRCA-like, and non-BRCA-like.

S1416 was a randomised, double-blind, placebo-controlled, phase 2 trial conducted at 154 community and academic clinical sites across the USA. Eligible patients aged 18 years or older had metastatic or recurrent triple-negative breast cancer or germline BRCA1/2-associated metastatic or recurrent breast cancer, an Eastern Cooperative Oncology Group performance status of 0–2, and had received up to one line of chemotherapy for metastatic disease. Patients were randomly assigned (1:1) via the National Clinical Trials Network open interactive system with dynamic balancing on number of previous cytotoxic regimens for metastatic disease to receive intravenous cisplatin (75 mg/m², day 1) combined with either veliparib or matching placebo (300 mg orally twice a day, days 1–14) on a 21-day cycle. Investigators, patients, and the sponsors were masked to treatment assignment; the study statisticians were unmasked. Central testing after ran domisation classified patients as having mutated or wildtype germline BRCA1/2. A biomarker panel established a priori was used to classify patients with wildtype germline BRCA1/2 into BRCA-like and non-BRCA-like phenotype groups, with BRCA-like status based on at least one of the biomarkers: genomic instability score (≥42), somatic BRCA1/2 mutations, BRCA1 promoter methylation, or non-BRCA1/2 homologous recombination repair germline mutations. The primary endpoint was investigator-assessed progression-free survival, analysed separately for the three predefined biomarker groups with a prespecified α value for each analysis. Efficacy analyses were done by intention to treat and included all eligible patients. Safety analyses of toxicities attributed to treatment included all patients who received at least one dose of veliparib or placebo. The study is ongoing and registered with ClinicalTrials.gov, NCT02595905.

Between July 7, 2016, and June 15, 2019, 335 patients were enrolled and randomly assigned. 320 patients (n=162 to cisplatin plus veliparib, all women; and n=158 to cisplatin plus placebo, 157 women and one man) were eligible for efficacy evaluation. 247 patients were classified into the three biomarker groups: germline BRCA1/2-mutated (n=37), BRCA-like (n=101), and non-BRCA-like (n=109). 73 patients could not be classified due to missing biomarker information. Median follow-up was 11·1 months (IQR 5·6–20·8). In the germline BRCA1/2-mutated group, median progression-free survival was 6·2 months (95% CI 2·3–9·2) in the cisplatin plus veliparib group and 6·4 months (4·3–8·2) in the cisplatin plus placebo group (HR 0·79 [95% CI 0·38–1·67]; log-rank p=0·54). In the BRCA-like group, median progression-free survival was 5·9 months (95% CI 4·3–7·8) in the cisplatin plus veliparib group versus 4·2 months (2·3–5·0) in the cisplatin plus placebo group (HR 0·57 [95% CI 0·37–0·88]; p=0·010). In the non-BRCA-like group, median progression-free survival was 4·0 months (95% CI 2·5–4·7) in the cisplatin plus veliparib group versus 3·0 months (2·2–4·4) in the cisplatin plus placebo group (HR 0·89 [95% CI 0·60–1·33]; p=0·57). The most common grade 3 or worse adverse events attributed to treatment were neutropenia (71 [46%] of 155 patients in the cisplatin plus veliparib group vs 29 [20%] of 147 in the cisplatin plus placebo group), leukopenia (42 [27%] vs 11 [7%]), anaemia (35 [23%] vs 12 [8%]), and thrombocytopenia (29 [19%] vs four [3%]). Serious adverse events attributed to treatment occurred in 48 (31%) patients in the cisplatin plus veliparib group and 53 (36%) patients in the cisplatin plus placebo group. Treatment-related adverse events led to death in one patient in the cisplatin plus veliparib group (sepsis) and one patient in the cisplatin plus placebo group (acute kidney injury due to cisplatin plus heart failure from previous doxorubicin exposure).

The addition of veliparib to cisplatin significantly improved progression-free survival in patients with BRCA-like metastatic triple-negative breast cancer, but not in patients with non-BRCA-like metastatic breast cancer. PARP inhibitors combined with platinum-based chemotherapy should be explored further in BRCA-like triple-negative breast cancer.

National Cancer Institute and National Institute of General Medical Sciences (US National Institutes of Health); AbbVie; Myriad Genetics; the Biomarker, Imaging, and Quality of Life Studies Funding Program (awarded by the National Cancer Institute); and The University of Kansas Cancer Center.