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

This study tests the combination of lenvatinib (a targeted drug) and pembrolizumab (an immunotherapy) for women with hormone receptor-positive (ER+ or PR+) metastatic breast cancer that has stopped responding to hormone-blocking therapy. The study is open to women who carry a specific genetic variant called FGFR1 or FGF amplification. **You may be eligible if...** - You are 21 or older with metastatic or locally advanced breast cancer that is ER+ and/or PR+ (hormone receptor-positive) and HER2-negative - Your cancer has progressed despite at least 1 line of hormone therapy - You have not previously received lenvatinib - You have received at most 1 prior line of chemotherapy for advanced disease **You may NOT be eligible if...** - You have active brain metastases requiring treatment - You have significant cardiovascular disease or high blood pressure that is uncontrolled - You have active autoimmune disease or have had prior organ transplant - You are pregnant or breastfeeding Talk to your doctor to see if this trial is right for you.