Comparison of outcomes in a population-based cohort of metastatic breast cancer patients receiving anti-HER2 therapy with clinical trial outcomes

Purpose Little data exist for comparing cardiac safety and survival outcomes of trastuzumab/pertuzumab or ado-T emtansine (TDM1) in metastatic breast cancer (MBC) patients enrolled in randomized clinical trial (RCT) vs the real-world. Methods This was a retrospective population-based cohort of all patients with MBC treated with trastuzumab/pertuzumab or TDM1 (2012-2017) in Ontario, Canada. Outcomes were incident heart failure (HF) and overall survival (OS). RCT data were obtained from digitizing survival curves and compared with cohort data using Kaplan-Meier analysis. Age-based comparison of outcomes was conducted for patients >= 65 years old vs younger than 65. Results The two cohorts composed of 833 and 397 patients treated with trastuzumab/pertuzumab and TDM1, of whom 5.5% and 7.6% had baseline HF, respectively. Incident HF following trastuzumab/pertuzumab or TDM1 was low (trastuzumab/pertuzumab 1.8 events/100 person years; TDM1 0.02 events/100 person years). The median OS was 39.2 and 56.4 months in the trastuzumab/pertuzumab population-based cohort and CLEOPATRA, respectively. The median OS was 15.4 and 30.9 months in the TDM1 population-based cohort and EMILIA, respectively. Cohort OS was significantly worse than RCT OS (trastuzumab/pertuzumab HR 1.67, 95% CI 1.37-2.03, p < 0.0001; TDM1 HR 2.80, 95% CI 2.27-3.44, p < 0.0001). Older patients had worse OS than younger patients for trastuzumab/pertuzumab (HR 1.60, 95% CI 1.19-2.16, p = 0.0018), but not for TDM1 (HR 1.16, 95% CI 0.81-1.66, p = 0.43). Conclusion HF incidence during trastuzumab/pertuzumab or TDM1 therapy in this real-world cohort was low. Survival in this cohort was worse compared to RCT, suggesting that recruitment of patients similar to the real-world population is required.