Cardiac Safety and Efficacy of SB3 Trastuzumab Biosimilar for ERBB2-Positive Early Breast Cancer: Secondary Analysis of a Randomized Clinical Trial

IMPORTANCE Trastuzumab has been the standard of care for the treatment of patients with ERBB2-positive breast cancer; however, cardiac events have been reported. This long-term follow-up study provides clinical evidence supporting the similarity of a trastuzumab biosimilar (SB3) to reference trastuzumab (TRZ). OBJECTIVE To compare cardiac safety and efficacy between SB3 and TRZ for patients with ERBB2-positive early or locally advanced breast cancer after up to 6 years of follow-up. DESIGN, SETTING, AND PARTICIPANTS This prespecified secondary analysis of a randomized clinical trial, conducted from April 2016 to January 2021, included patients with ERBB2-positive early or locally advanced breast cancer from a multicenter double-blind, parallel-group, equivalence phase 3 randomized clinical trial of SB3 vs TRZ with concomitant neoadjuvant chemotherapy who completed neoadjuvant and adjuvant treatment. INTERVENTIONS In the original trial, patients were randomized to either SB3 or TRZ with concomitant neoadjuvant chemotherapy for 8 cycles (4 cycles of docetaxel followed by 4 cycles of fluorouracil, epirubicin, and cyclophosphamide). After surgery, patients continued SB3 or TRZ monotherapy for 10 cycles of adjuvant treatment per previous treatment allocation. Following neoadjuvant and adjuvant treatment, patients were monitored for up to 5 years. MAIN OUTCOMES AND MEASURES The primary outcomes were the incidence of symptomatic congestive heart failure and asymptomatic, significant decrease in left ventricular ejection fraction (LVEF). The secondary outcomes were event-free survival (EFS) and overall survival (OS). RESULTS A total of 538 female patients were included (median age, 51 years [range, 22-65 years]). Baseline characteristics were comparable between the SB3 and TRZ groups. Cardiac safety was monitored for 367 patients (SB3, n=186; TRZ, n=181). Median follow-up was 68 months (range, 8.5-78.1 months). Asymptomatic, clinically significant LVEF decreases were rarely reported (SB3, 1 patient [0.4%]; TRZ, 2 [0.7%]). No patient experienced symptomatic cardiac failure or death due to a cardiovascular event. Survival was evaluated for the 367 patients in the cardiac safety cohort and an additional 171 patients enrolled after a protocol amendment (538 patients [SB3, n=267; TRZ, n=271]). No difference was observed in EFS or OS between treatment groups (EFS: hazard ratio [HR], 0.84; 95% CI, 0.58-1.20; P=.34; OS: HR, 0.61; 95% CI, 0.36-1.05; P=.07). Five-year EFS rates were 79.8% (95% CI, 74.8%-84.9%) in the SB3 group and 75.0% (95% CI, 69.7%-80.3%) in the TRZ group, and OS rates were 92.5% (95% CI, 89.2%-95.7%) in the SB3 group and 85.4% (95% CI, 81.0%-89.7%) in the TRZ group. CONCLUSIONS AND RELEVANCE In this secondary analysis of a randomized clinical trial, SB3 demonstrated cardiac safety and survival comparable to those of TRZ after up to 6 years of follow-up in patients with ERBB2-positive early or locally advanced breast cancer.