Abstract
Trastuzumab deruxtecan (T-DXd) is an antibody-drug conjugate successfully used to treat HER2-low and HER2-positive metastatic breast cancer, but resistance consistently develops. Using multivariate Cox proportional hazards in a real-world cohort of 2,799 patients with breast cancer, we aimed to identify clinically relevant T-DXd resistance mechanisms. In patients with samples collected prior to T-DXd treatment, higher expression of ERBB2 (HER2) and lower expression of ABCC1 (an ATP-binding cassette transporter involved in drug efflux) were significantly associated with longer T-DXd-related overall survival (OS); ABCC1 predicted OS independently of HER2. Furthermore, mutations in several genes were enriched in post-T-DXd samples compared to unmatched T-DXd-naïve samples, including ERBB2, NFE2L2 (a transcriptional activator of ABCC1), and KEAP1 (a negative regulator of NFE2L2), indicating plausible resistance mechanisms related to HER2 target levels and ABCC1-mediated drug efflux. Identifying such resistance mechanisms might lead to improved methods of precision oncology and novel therapeutic approaches to overcome resistance.