Survival, treatment patterns, and costs of HER2+ metastatic breast cancer patients in Ontario between 2005 to 2020

Background To enable the integration of novel therapies, it is critical to understand current long-term outcomes in HER2-positive metastatic breast cancer (mBC), including survival, treatment patterns, and costs. We sought to define these outcomes among patients with mBC in Ontario. Methods We conducted a retrospective population-level study in Ontario women diagnosed with breast cancer of any stage between January 1, 2005 and December 31, 2019, with follow-up until December 31, 2020. HER2-positivity was based on receipt of a HER2-targeted therapy (HER2-TT) in the first line (1L) metastatic setting. Administrative databases at ICES were used to assess outcomes. Results In Ontario, 2557 patients were diagnosed with mBC and received a HER2-TT, and of these 1606 were diagnosed with early-stage (stage I-III) that became metastatic (recurrent), while 951 were diagnosed with late stage/de novo mBC (stage IV). The average age of all patients was 54.8 years ± 12.7 years. Treatment regimens that included pertuzumab and trastuzumab (cohort name: pert_tras ) were the most frequently used HER2-TT for 1L mBC (51.4%), while T-DM1 was the most frequent therapy (87.5%) in second line (2L). The median overall survival (mOS) from initiation of 1L pert_tras was not reached, whereas mOS from initiation of T-DM1 in 2L was 18.7 months. The overall mean cost per patient on pert_tras during 1L was $267,282. The main cost drivers were the cost of systemic therapy, followed by cancer clinic visits, with a mean cost per patient at $158,961 and $73,882, respectively. Conclusion The baseline characteristics and treatment patterns for patients who received HER2-TT in our study align with previously reported results. However, the mOS observed for 2L T-DM1 was shorter than that found in pivotal, clinical trial literature. As expected, anti-cancer systemic therapy costs were the main contributor to the over quarter-million dollar mean cost per patient on pert_tras in 1L.