Primary results from EL1SSAR, a prospective phase IIIb study of first-line atezolizumab plus nab-paclitaxel therapy for patients with PD-L1-positive advanced triple-negative breast cancer

Background: IMpassion130 trial results led to approval of first-line atezolizumab plus nab-paclitaxel for PD-L1-positive advanced triple-negative breast cancer (aTNBC). The global single-arm EL1SSAR study (NCT04148911) enrolled a broader patient population to elucidate outcomes in routine practice. Methods: Patients with measurable PD-L1-positive aTNBC and no prior systemic therapy for advanced disease received atezolizumab (840 mg, days 1&15) plus nab-paclitaxel (100 mg/m², days 1,8,15) every 28 days until disease progression or unacceptable toxicity. PD-L1 status was assessed locally (mandatory) and centrally (optional) using the VENTANA PD-L1 (SP142) Assay. Patients with stable asymptomatic CNS metastases, ECOG PS 2 or selected autoimmune diseases were allowed. Primary endpoints were the incidence of grade ≥2 immune-mediated and grade ≥3 adverse events (AEs). Secondary endpoints included overall survival and investigator-assessed progression-free survival (PFS). PD-L1 testing concordance was an exploratory endpoint. Results: Among 182 treated patients, five had CNS metastases and three had ECOG PS ≥ 2. Patients received a median of six treatment cycles (range 1–63); 7% were treated for >3 years. Grade ≥2 immune-mediated AEs occurred in 12% of patients (95% CI 8%–18%) and grade ≥3 AEs in 47% (95% CI 39%–54%). There were no treatment-related deaths. Median PFS was 7.4 (95% CI 5.6–10.6) months (>2 years in 27 patients [15%]) and median overall survival was 27.0 (95% CI 22.0–33.8) months. Concordance between local and central PD-L1 testing was 67% (65/97). Median PFS was >11 months in patients with centrally confirmed PD-L1-positive status. Conclusions: Safety and efficacy were consistent with results from IMpassion130, which enrolled a more selected population.