Biosimilar trastuzumab-dkst monotherapy versus trastuzumab monotherapy after combination therapy: Toxicity, efficacy, and immunogenicity from the phase 3 Heritage trial.

110Background: The Heritage trial is a multicenter, double-blind, randomized, parallel-group, phase 3 study (NCT02472964) evaluating efficacy and safety of trastuzumab-dkst (Ogivri), a trastuzumab biosimilar, vs trastuzumab, in combination with taxane as first-line therapy for patients with HER2+ metastatic breast cancer. The primary endpoint, overall response rate on combination therapy at week 24, was previously reported (Rugo et al, JAMA 2017). Methods: Eligible patients were randomized 1:1 to trastuzumab-dkst or trastuzumab, combined with taxane. After 24 weeks, patients with responding or stable disease received monotherapy as per randomization. Here, we describe secondary endpoints of safety and immunogenicity during monotherapy and cumulative through 48 weeks; progression-free survival (PFS) and event-based overall survival (OS) will be presented in the future. Results: 500 patients were randomized, 342 continued treatment after 24 weeks, and 214 continued through 48 weeks. Treatment-emergent adver...