18F-FFNP

This phase II trial tests the accuracy of functional imaging (FFNP)-positron emission tomography (PET)/computed tomography (CT) to predict response to abemaciclib plus endocrine therapy. Abemaciclib is a drug used to treat certain types of hormone receptor positive (HR+), HER2 negative breast cancer. Abemaciclib blocks certain proteins, which may help keep tumor cells from growing. Endocrine therapy adds, blocks, or removes hormones that can cause cancer to grow. FFNP PET imaging is a form of x-ray that uses FFNP as an imaging agent that may provide more precise information about the location of tumors that "light up" with FFNP than a PET scan alone can provide.