VS-6766+Abema+Fulv in Met HR+/HER- BC

This is single-arm open-label phase 1/2 trial assessing the safety and efficacy of the new drug, VS-6766, plus abemaciclib and fulvestrant in treating metastatic hormone receptor-positive and HER2-negative (HR+/HER2-) breast cancer. VS-6766 is a unique small molecule drug that targets and interrupts a pathway that allows cancer cells to grow.The U.S. Food and Drug Administration (FDA) has not approved VS-6766 as a treatment for any disease. The FDA has approved abemaciclib and fulvestrant as a treatment option for breast cancer. The study is divided into three study periods: a screening period; a treatment period; and a post-treatment follow-up period. Participants will receive study treatment for as long there are no serious side effects and the disease does not get worse. It is expected that about 63 people will take part in this research study. Verastem Oncology, a biopharmaceutical company, is supporting this research study by providing VS-6766 and funding to conduct this trial. Eli Lilly, a pharmaceutical company, is supporting this research study by providing the drug, abemaciclib.