Trastuzumab emtansine for HER2-positive metastatic breast cancer: Outcomes from a whole-of-population Australian cohort

Purpose: We aim to describe the treatment patterns and overall survival (OS) outcomes in patients receiving trastuzumab emtansine (T-DM1) for HER2-positive metastatic breast cancer (HER2+MBC) in routine clinical care. Methods: Retrospective, whole-of-population cohort study of people initiating T-DM1 for HER2+MBC between October 2015 and May 2019 in Australia. We used dispensing claims to estimate time-to-T-DM1 initiation, duration of treatment, and treatments administered prior to and following T-DM1 therapy. We estimated OS from T-DM1 initiation and stratified results based on whether patients received first- or second-line T-DM1 treatment. We benchmarked outcomes to those reported in the pivotal, EMILIA trial. Results: 345 patients initiated T-DM1: 309 as second-line therapy for HER2+MBC and 36 as first-line therapy. 51% of patients had received endocrine therapy and 98% of second-line patients received pertuzumab prior to starting T-DM1. The median age was 57 years (53 in EMILIA); median time-to-T-DM1 initiation from start of HER2-targeted therapy for HER2+MBC was 11.6 months (IQR: 7.9–16.6); median duration of T-DM1 treatment was 6.5 months (3.1–13.5; 7.6 months in EMILIA), and median OS was 19.3 months (7.9–29.5; 29.9 months in EMILIA). Conclusions: Our findings highlight differences in patient characteristics (older, more previous pertuzumab therapy) and outcomes (shorter OS) from the T-DM1 pivotal trial and provide real-world estimates that can inform patient, clinician and policy, decisions around the use of HER2-targeted therapies in routine clinical care