Methods for Estimating Long-Term Outcomes for Trastuzumab Deruxtecan in HER2-Positive Unresectable or Metastatic Breast Cancer After Two or More Anti-HER2 Therapies

Background DESTINY-Breast01 (NCT03248492) is a phase II single-arm trial evaluating trastuzumab deruxtecan (T-DXd) in adults with human epidermal growth factor receptor 2-positive (HER2+) unresectable or metastatic breast cancer (u/mBC) who have received two or more prior anti-HER2 therapies. Objectives Objectives were to explore approaches for estimating long-term overall survival (OS) with T-DXd from immature data (June 2020 data-cut; median follow-up 20.5 months), and compare predicted long-term outcomes with UK-recommended non-targeted therapies eribulin, capecitabine, and vinorelbine. Methods Two methods were used to model T-DXd long-term OS: (1) applying a hazard ratio (HR) to the OS curve for another HER2 targeted therapy (third-line trastuzumab emtansine [T-DM1]) with longer trial follow-up; and (2) extrapolating T-DXd OS data directly. Comparator OS was based on direct extrapolation of published data (comparison with vinorelbine OS was not possible). Quality-adjusted life years (QALYs) were calculated using a previously published model of utility. Results Both extrapolation methods demonstrated longer mean/median OS with T-DXd versus eribulin, and capecitabine (44.7/32.9 months [applying an HR to the T-DM1 OS curve]; 47.7/29.9 months [using direct extrapolation]; vs 11.3/9.2, and 17.8/13.6 months, respectively), translating to 2.3, 2.3, 0.6, and 0.9 discounted QALYs. Conclusion Alternative methods produced consistent results, showing T-DXd is associated with substantial gains in OS and QALYs versus eribulin, and capecitabine. Modelled median OS results were similar to a later data-cut (median of 29.1 months, March 2021 data-cut). The modelling approach in which an HR was applied to the T-DM1 OS curve informed a submission to the National Institute for Health and Care Excellence.