A phase II study of atezolizumab, pertuzumab, and high-dose trastuzumab for central nervous system metastases in patients with HER2-positive breast cancer

Abstract Purpose: Patients with HER2-positive breast cancer brain metastases have few effective systemic therapy options. In a prior study, pertuzumab with high-dose trastuzumab demonstrated a high clinical benefit rate (CBR) in the central nervous system (CNS) in patients with brain metastases. The current trial evaluated whether the addition of atezolizumab to this regimen would produce further improvements in CNS response. Patients and Methods: This was a single-arm, multi-center, phase II trial of atezolizumab, pertuzumab, and high-dose trastuzumab for patients with HER2-positive breast cancer brain metastases. Participants received atezolizumab 1200 mg IV every 3 weeks (q3w), pertuzumab (loading dose 840 mg IV, then 420 mg IV q3w), and high-dose trastuzumab (6 mg/kg IV weekly for 24 weeks, then 6 mg/kg IV q3w). The primary endpoint was CNS overall response rate (ORR) per Response Assessment in Neuro-Oncology Brain Metastases (RANO-BM) criteria. Key secondary endpoints included CBR, overall survival (OS), and safety and tolerability of the combination. Results: Among 19 enrolled participants, two had a confirmed intracranial partial response for a CNS-ORR of 10.5% (90% CI: 1.9%-29.6%). The study did not meet the prespecified efficacy threshold and was terminated early. The CBR was 42.1% at 18 weeks and 31.6% at 24 weeks. Seven patients (36.8%) required a dose delay or hold, and the most frequent any-grade adverse events were diarrhea (26.3%) and fatigue (26.3%). Conclusions: The addition of atezolizumab to pertuzumab plus high-dose trastuzumab does not result in improved CNS responses in patients with HER2-positive breast cancer brain metastases.