Abemaciclib and Niraparib Before Surgery for the Treatment of Hormone Receptor Positive HER2 Negative Breast Cancer

PRIMARY OBJECTIVES: I. To determine the maximum-tolerated dose (MTD) and/or recommended phase 2 dose of the combination of abemaciclib and niraparib tosylate monohydrate (niraparib). II. To assess safety and tolerability of the combination of abemaciclib and niraparib in early stage HR+ breast cancer. SECONDARY OBJECTIVES: I. To determine clinical response to treatment. II. To determine pathologic response to treatment. III. To determine feasibility of combination as determined by no delay to standard of care breast surgery. EXPLORATORY OBJECTIVE: I. To assess occurrence of secondary myelodysplastic syndrome (MDS)/acute myeloid leukemia (AML) malignancy. OUTLINE: This is a phase 1 dose-escalation study of abemaciclib in combination with niraparib followed by a dose-expansion study. Patients receive abemaciclib orally (PO) twice daily (BID) and niraparib PO once daily (QD). Treatment repeats every 28 days for up to 2-4 cycles in the absence of disease progression or unacceptable toxicity. Patients who complete 4 cycles undergo standard of care mastectomy or lumpectomy. Patients demonstrating progressive disease after only 2 cycles are switched to receive standard of care chemotherapy prior to undergoing mastectomy or lumpectomy. Patients are followed up at 30 days after date of surgery, every 3 months for the first 6 months, every 6 months for 2 years, then annually for up to 5 years from date of surgery.