Eftilagimod alpha (IMP321) is currently being tested in the Phase IIb AIPAC trial (NCT02614833) in patients with hormone receptor (HR)-positive metastatic breast cancer (BC). In this video, Hans Wildiers, MD, PhD, University Hospitals Leuven, KU Leuven, Leuven, Belgium, comments on the rationale behind using eftilagimod alpha for the treatment of BC. In contrast to classical immunotherapy approaches that use monoclonal antibodies to target immune checkpoint molecules, eftilagimod alpha is a soluble LAG-3 protein that activates antigen-presenting cells (APCs) and therefore makes them more effective in presenting antigens to cytotoxic lymphocytes. In the AIPAC trial, patients are first treated with paclitaxel and then with eftilagimod alpha in order to create more tumor antigens for APCs and induce a CD8-mediated immune response to the tumor. This interview took place during the 36th Society for Immunotherapy of Cancer (SITC) Annual Meeting in Washington, D.C.