Neoadjuvant Lenvatinib Combined With Letrozole in Hormone Receptor Positive Breast Cancer

Hypothesis: We hypothesize that combining a RET inhibitor such as lenvatinib with endocrine therapy may potentiate anti-tumor effects in ER+ breast cancers in the clinic. Given that RET inhibition is the primary mechanism of action of lenvatinib, we further hypothesize that RET expression in the breast tumor may be a biomarker that predicts for better response to the combination of a RET inhibitor and endocrine therapy in both a neoadjuvant (Part A) and a metastatic (Part B) setting. Primary Objectives 1. To confirm the dose of lenvatinib that can be safely combined with letrozole in the lead-in Phase Ib part of the study. 2. Part A (For patients with non-metastatic breast cancer undergoing neoadjuvant treatment) : To determine the overall clinical response (measured by ultrasound) and pathological complete response rate after 2 weeks of neoadjuvant single agent lenvatinib followed by 12 weeks of letrozole combined with lenvatinib and compare with historical controls treated with single agent letrozole. 3. Part B (For patients with metastatic breast cancer and who have tumor lesion that can be serially biopsied safely): to determine the clinical response (measured by RECIST criteria) after 2 weeks of single agent lenvatinib followed by continuous dual therapy of lenvatinib and letrozole, until time of progression or intolerability. Secondary Objectives 1. To evaluate the overall biological effects (Ki67 changes, histological response, apoptosis, RET and downstream targets such as AKT and ERK) of 2 weeks of single agent lenvatinib, and of letrozole + lenvatinib, in ER positive breast cancer, respectively. 2. To identify biological predictors for treatment response to letrozole + lenvatinib in ER positive breast cancer using pharmacokinetics, pharmacogenetics, genomics and proteomics strategies. 3. To compare the following parameters between RET positive versus RET negative, ER positive breast cancer: * Part A : i. Clinical response (measured by ultrasound) and biological effects of 2 weeks of single agent lenvatinib, ii. Clinical response (measured by ultrasound) and biological effects after 2 weeks of single agent lenvatinib followed by 12 weeks of letrozole combined with lenvatinib -Part B : i. Clinical response measured by RECIST criteria after 2 weeks of single agent lenvatinib followed by letrozole combined with lenvatinib ii. Progression-free and overall survival with letrozole combined with lenvatinib