Complete responders in patients with HER2-positive metastatic breast cancer: a real-world SONABRE study

Background: We aimed to describe real-world complete response (CR) rates, predictors of CR, and survival outcomes in patients treated with first-line pertuzumab, trastuzumab, and chemotherapy for HER2-positive (HER2+) metastatic breast cancer (MBC). Methods: Patients with HER2+ MBC who started first-line pertuzumab, trastuzumab, and chemotherapy in 2013-2021 were selected from the SONABRE Registry (NCT-03577197), involving eleven Dutch hospitals. CR was defined as no evidence of disease on imaging during first-line systemic therapy with or without local therapy (surgery and/or radiotherapy). Multivariable logistic regression was used to identify predictors for CR. Progression-free survival (PFS) and overall survival (OS) from the start of first-line pertuzumab, trastuzumab, and chemotherapy were computed with the Kaplan-Meier method. Results: We included 244 patients treated with first-line pertuzumab, trastuzumab, and chemotherapy, with a median follow-up duration of 71 months (interquartile range: 60-82 months). A CR during first-line systemic therapy was reached in 63 patients (26 %). Patients younger than 65 years (odds ratio (OR) = 5.24, 95 %CI:1.49-18.44, p = 0.01), those with de Novo MBC (OR = 2.48, 95 %CI:1.24-4.93, p = 0.01), or those with a single metastatic site (OR = 3.81, 95 %CI:1.83-7.96, p < 0.001) were more likely to reach CR than their counterparts. The 5-year PFS and OS rates were, respectively, 63 % (95 %CI:48 %-75 %) and 85 % (95 %CI:72 %-92 %) in patients with a CR and 10 % (95 %CI:6 %-16 %) and 36 % (95 %CI:28 %-44 %) in patients without a CR. Conclusions: In the real world, one in four patients treated with first-line pertuzumab, trastuzumab, chemotherapy, with or without local therapy, reached CR. Two third of patients with a CR were still free of progression after five years.