In the BEGONIA phase 1b/2 trial, durvalumab plus trastuzumab deruxtecan produced unusually deep and durable responses as a first treatment for HER2-low metastatic breast cancer. The study pairs an antibody-drug conjugate—trastuzumab deruxtecan—with an immune checkpoint inhibitor, aiming to extend benefit in a subgroup historically limited by weaker HER2 biology. While BEGONIA is a platform study focused on response quality in early clinical development, the reported depth and durability stand out in HER2-low oncology where meaningful improvements have been harder to sustain. For the broader field, the combination supports continued exploration of AIC plus ADC strategies in breast cancer subsets defined by HER2 expression intensity.