Lurbinectedin, a selective inhibitor of oncogenic transcription, in patients with pretreated germline BRCA1/2 metastatic breast cancer: results from a phase II basket study

Background Lurbinectedin, a selective inhibitor of oncogenic transcription, has shown preclinical antitumor activity against homologous recombination repair-deficient models and preliminary clinical activity in BRCA1/2 breast cancer. Patients and methods This phase II basket multitumor trial (NCT02454972) evaluated lurbinectedin 3.2 mg/m2 1-h intravenous infusion every 3 weeks in a cohort of 21 patients with pretreated germline BRCA1/2 breast cancer. Patients with any hormone receptor and human epidermal growth factor receptor 2 status were enrolled. The primary efficacy endpoint was overall response rate (ORR) according to RECIST v1.1. Secondary endpoints included duration of response (DoR), progression-free survival (PFS), overall survival (OS) and safety. Results Confirmed partial response (PR) was observed in six patients [ORR = 28.6%; 95% confidence interval (CI) 11.3% to 52.2%] who had received a median of two prior advanced chemotherapy lines. Lurbinectedin was active in both BRCA mutations: four PRs in 11 patients (36.4%) with BRCA2 and two PRs in 10 patients (20.0%) with BRCA1. Median DoR was 8.6 months, median PFS was 4.1 months and median OS was 16.1 months. Stable disease (SD) was observed in 10 patients (47.6%), including 3 with unconfirmed response in a subsequent tumor assessment [ORR unconfirmed = 42.9% (95% CI 21.8% to 66.0%)]. Clinical benefit rate (PR + SD ≥ 4 months) was 76.2% (95% CI 52.8% to 91.8%). No objective response was observed among patients who had received prior poly (ADP-ribose) polymerase inhibitors. The most common treatment-related adverse events (AEs) were nausea (61.9%), fatigue (38.1%) and vomiting (23.8%). These AEs were mostly grade 1/2. The most common grade 3/4 toxicity was neutropenia (42.9%: grade 4, 23.8%: with no febrile neutropenia). Conclusions This phase II study met its primary endpoint and showed activity of lurbinectedin in germline BRCA1/2 breast cancer. Lurbinectedin showed a predictable and manageable safety profile. Considering the exploratory aim of this trial as well as previous results in other phase II studies, further development of lurbinectedin in this indication is warranted.


INTRODUCTION
Breast cancer with BRCA1/2 mutations is emerging as a distinctive group of breast cancers that present at an earlier age with hallmarks of genomic instability and accumulation of DNA damage. [1][2][3] Two poly (ADP-ribose) polymerase inhibitors (PARPi) are available as the therapeutic option (olaparib and talazoparib), but many patients do not derive benefit because of multiple primary and secondary resistance mechanisms and toxicities. 4 Novel class of agents are needed to be developed beyond the current PARPi or the central protein kinases ataxia telangiectasia mutated (ATM) and ataxia telangiectasia and Rad3-related (ATR) inhibitors.
Lurbinectedin is a selective inhibitor of oncogenic transcription that leads to cell apoptosis 5 and also inhibits activated transcription in tumor-associated macrophages. 6 Lurbinectedin has antitumor activity against homologous recombination repair-deficient (HRD) cell lines. 7,8 The mechanism involves the irreversible stalling of elongating RNA polymerase II (Pol II) on the DNA template and its specific degradation by the ubiquitin/proteasome machinery. Subsequently, recruitment of DNA repair factors including xeroderma pigmentosum complementation group F nuclease induces the accumulation of doublestrand breaks and apoptosis as downstream events. 9 These effects are increased in HRD cells. In fact, in BRCA2-mutated cells, this could be related to the concurrence of deficient DNA repair and formation of R-loops that occurs during the elongation step of transcription by RNA polymerase II. 10,11 In a basket, open-label, phase II study (ClinicalTrials.gov identifier: NCT02454972), nine cohorts of patients with different tumor types were treated with lurbinectedin. Based on the results in the small-cell lung cancer (SCLC) cohort, 12 approval of lurbinectedin was obtained in this indication first in the United States 13 and later in other countries (Canada, Australia, Singapore and Arab Emirates). This report focuses on the outcomes in the germline BRCA1/2 breast cancer cohort. This cohort was evaluated because, in a previous phase II study, lurbinectedin had shown antitumor activity in patients with advanced breast cancer and germline BRCA1/2 pathogenic variants: overall response rate (ORR) was 41% and median overall survival (OS) was 20.0 months compared to 9% and 12.5 months, respectively, in patients with BRCA1/2 wild-type or unknown status. 14

METHODS
The study protocol was approved by the independent local ethics committee of each participating center and was conducted in accordance with the Declaration of Helsinki, Good Clinical Practice guidelines and local regulations for clinical trials. Signed informed consent was obtained from all patients before their inclusion in the study.

Patient selection
Twenty-one patients with germline BRCA1/2 breast cancer were treated at 12 investigational sites in France (n ¼ 3), Spain (n ¼ 15), Switzerland (n ¼ 1) and the United States (n ¼ 2). Eligibility criteria included patients ! 18 years old with pathologically proven diagnosis of germline BRCA1/2 metastatic breast carcinoma; pretreated with one to three chemotherapy-containing lines [prior endocrine and/or human epidermal growth factor receptor 2 (HER2)-directed treatment were allowed]; measurable disease as per RECIST v.1.1; 15 Eastern Cooperative Oncology Group performance status 2; and adequate major organ function (including neutrophil count ! 2.0 Â 10 9 /l). Patients were excluded if they had: previously received lurbinectedin or trabectedin; prior or concurrent malignant disease unless in complete remission for >5 years; known central nervous system involvement (active or stable/treated disease); concomitant unstable or serious medical condition; or an impending need for radiotherapy.

Assessments
The primary objective of this study was to assess the antitumor activity of lurbinectedin in terms of ORR, the primary endpoint, as assessed by the investigators. Radiological tumor evaluation was carried out every 6 weeks (two cycles) until cycle 6, and every 9 weeks (three cycles) thereafter. Objective response was to be confirmed at least 4 weeks later. Secondary efficacy endpoints included disease control rate (ORR or stable disease), duration of response (DoR), progression-free survival (PFS) and OS.
Safety was evaluated in all patients who received at least one lurbinectedin infusion, complete or incomplete, by assessment of adverse events (AEs), clinical laboratory test results, physical examinations and vital signs. Laboratory tests were done weekly during cycles 1 and 2, and on day 1 of subsequent cycles. AEs were recorded and coded with the Medical Dictionary for Regulatory Activities (MedDRA) v.21.0. AEs and laboratory values were graded according to the National Cancer Institute-Common Toxicity Criteria for Adverse Events (NCI-CTCAE) v. 4.0. All patients were followed until recovery from any lurbinectedin-related AE.

Statistical methods
Up to 25 patients were to be recruited to test the null hypothesis that 1% or fewer patients would have a response (P 0.01) versus the alternative hypothesis that 10% or more patients would have a response (P ! 0.10). The variance of the standardized test was based on the null hypothesis. The type I error (alpha) associated with this one-sided test is 0.025 and the type II error (beta) is 0.2; hence, statistical power is 80%. With these assumptions, if the number of patients who achieve a confirmed response is ! 2, then this would allow the rejection of the null hypothesis.
Initially, 15 patients were to be included in the first stage. If at least one confirmed response occurred in the first 15 assessable patients, recruitment would continue up to 25 assessable patients. Two of the first 15 patients had confirmed partial response (PR) to lurbinectedin treatment. Recruitment continued up to 21 patients while evaluating response in the first 15 patients. As the six responses observed in these 21 patients surpassed the threshold of !2 confirmed responses established in the statistical hypothesis, no more patients were enrolled.
Descriptive statistics were used. Non-continuous variables are described in frequency tables using counts and percentages. Continuous variables are described by median, minimum and maximum. Binomial exact estimates and 95% confidence intervals (CIs) were calculated for the evaluation of the main endpoint (ORR). The KaplaneMeier method was used to analyze DoR, PFS and OS. SAS software (SAS Institute, Cary, NC) was used to generate statistical outputs.

Lurbinectedin treatment
A total of 188 cycles were administered to the 21 treated patients. The median number of cycles per patient was 6 (range 1-24 cycles), with 61.9% of patients having received !5 cycles. The median relative dose intensity was 98.5% (range 62.2%-103.6%).

Efficacy results
All 21 treated patients were assessable for efficacy. Confirmed PR was observed in six patients; these patients had received a median of two prior advanced chemotherapy lines. Therefore, ORR was 28.6% (95% CI 11.3% to 52.2%). SD was observed in 10 patients (47.6%), including 3 patients with unconfirmed response in a subsequent tumor assessment [ORR unconfirmed ¼ 42.9% (95% CI 21.8% to 66.0%)]. Six patients with SD (28.6%) reached SD ! 4 months. (Table 2). Overall, 76.2% of patients had reduction in target lesions during the treatment period ( Figure 1A).
Overall, 4.2% of cycles had dose delay due to treatmentrelated reasons (grade 2/3 neutropenia) in five patients (25.0%). Dose was reduced due to treatment-related reasons in 6.6% of cycles in seven patients (35.0%); the most common cause was neutropenia (grade 2, 3 or 4). Of note, the protocol stated that in case of grade 4 neutropenia, lurbinectedin dose had to be reduced instead of continuing at the same dose with G-CSF prophylaxis.
Most patients (n ¼ 19; 90.5%) discontinued treatment due to disease progression. Two of the 21 patients were receiving lurbinectedin treatment at the end of study date and they continued this therapy after the study under compassionate use.
Eleven deaths occurred during the study; all of them were due to progression of the patient's underlying malignant disease.

DISCUSSION
This cohort from a phase II exploratory basket study included 21 patients with germline BRCA1/2 metastatic breast carcinoma treated with lurbinectedin. The ORR was 28.6% (95% CI 11.3% to 52.2%) in patients who had received a median of two prior chemotherapy lines for advanced disease. These results (six confirmed PRs) were above the threshold of !2 confirmed responses established in the statistical hypothesis and the study met its primary endpoint. The results of this trial, together with those of a previous phase II trial, 14 show that lurbinectedin is active in this population, especially in patients with deleterious BRCA2 mutations. The ORR was 36.4% (4/11) in patients with BRCA2 mutation and 20.0% (2/10) in patients with BRCA1 mutation. Trabectedin, a related compound, also showed higher efficacy in BRCA2 breast cancer patients versus BRCA1 (ORR 33% versus 9%). 16 BRCA2 prevents the formation of RNAeDNA hybrids (R-loops) that occurs during the elongation step of transcription by RNA polymerase II. One hypothesis to explain the differential activity of trabectedin and lurbinectedin observed in BRCA2-compared with BRCA1mutated metastatic breast cancer is the concurrence of deficient DNA repair and the formation of R-loops. 14 The lower response observed here in comparison with that reported previously (41%; 95% CI 28% to 55%) 14 could be explained by differences in the number of prior chemotherapy lines for advanced disease (1, range 0-3 versus 2, range 0-3) and prior PARPi (24% versus 17%). Furthermore, all patients in the previous phase II study received doses higher than 3.2 mg/m 2 (3.5 mg/m 2 or 7.0 mg flat dose, equivalent to 4.0 mg/m 2 ). Nevertheless, the median PFS (4.1 months) was similar to that reported previously (4.6 months) 14 and the overall rate of unconfirmed response was 42.8%. Some limitations of the current study are the small size of the cohort evaluated, the absence of a central laboratory to confirm BRCA status and the lack of sampling during the study to perform pharmacodynamic studies.   In conclusion, the current efficacy results in patients with germline BRCA-mutated metastatic breast cancer show lurbinectedin as an active and safe agent in this population, especially in patients with BRCA2 mutations. These results are in line with findings in a previous phase II trial. Therefore, development of lurbinectedin in this indication is warranted. Pharmacogenomic and molecular analysis may help to select the patient population that could obtain a higher benefit with lurbinectedin treatment.