Background: The multicenter, double-blind, randomized, parallel-group, phase 3 Heritage trial (NCT02472964) evaluated efficacy and safety of trastuzumab-dkst (Ogivri), a trastuzumab biosimilar, vs trastuzumab, plus taxane as first-line therapy for patients with HER2+ metastatic breast cancer. Overall response at week (wk) 24 and progression-free survival (PFS) at wk 48 have been reported (Rugo et al, JAMA 2017; ASCO 2018). Methods: Eligible patients were randomized 1:1 to trastuzumab-dkst or trastuzumab, plus taxane. After 24 wks, patients with responding/stable disease continued monotherapy per randomization. Safety and OS during maintenance, cumulative through 36 months of follow-up from last patient on study, are described. Results: 500 patients were randomized; 343 received monotherapy after 24 wks (trastuzumab-dkst, n = 179; trastuzumab, n = 164). 128 patients discontinued monotherapy (trastuzumab-dkst, n = 63; trastuzumab, n = 65); mean time to discontinuation was 19 months in both groups. Treatment-emergent adverse events (TEAEs) during monotherapy were similar for trastuzumab-dkst (69%) and trastuzumab (73%); most were low grade and serious TEAE rates were 6% in both groups. Cumulative rates of TEAEs of special interest were similar for hypersensitivity, pulmonary, and cardiac events (trastuzumab-dkst, 23%, 16%, and 5%; trastuzumab, 24%, 13%, and 5%). Incidences of left ventricular ejection fraction (LVEF) < 50% ≥1 time postbaseline (trastuzumab-dkst, 5%; trastuzumab, 3%) and LVEF < 50% postbaseline and ≥10% reduction (trastuzumab-dkst, 4%; trastuzumab, 3%) were low. At 36 months, 169 patients had received further lines of therapy, with similar distribution of HER2 treatments, endocrine therapies, and chemotherapies. Final median PFS was 11.1 months in both groups. Median duration of response was 9.9 and 9.8 months and median OS was 35.0 and 30.2 months for trastuzumab-dkst and trastuzumab, respectively: unstratified hazard ratio, 0.9 (95% CI, 0.70-1.17). Conclusions: Long-term safety data with similar median OS compared with originator trastuzumab further support the safety and efficacy profile of trastuzumab-dkst. Clinical trial information: NCT02472964