Phase II Lenvatinib and Pembrolizumab in Endocrine Resistant Breast Cancer With Letrozole

This is a phase II study testing the combination of Pembrolizumab with Lenvatinib, a multi-kinase inhibitor that has activity against vascular endothelial growth factor receptors 1-3 (VEGFR1-3), fibroblast growth factor receptors 1-4 (FGFR1-4), ret protooncogene (RET), platelet-derived growth factor receptor-alpha (PDGFR-alpha) and KIT (a stem cell factor receptor), and letrozole, a non-steroidal aromatase inhibitor, in advanced hormone receptor (HR) positive human epidermal growth factor receptor 2 (HER2) negative breast cancer (BC) that has progressed on/after standard endocrine therapy.