Study Evaluating the Antitumor Activity and Safety of Niraparib as Neoadjuvant Treatment in Participants With Breast Cancer

This is an open-label, single-arm pilot study evaluating the antitumor activity and safety of niraparib as neoadjuvant therapy in participants with Human epidermal growth factor receptor 2 (HER2)-negative and breast cancer susceptibility gene mutant (BRCAmut) localized breast cancer (primary tumor \>=1 centimeters \[cm\]). Breast magnetic resonance imaging (MRI), breast ultrasound, and tumor core biopsy will be performed at the screening (Days -28 to -1). Participants will receive niraparib (200 milligrams \[mg\] orally \[PO\]) treatment daily for 28 days (Cycle 1) and then will undergo breast ultrasound at the end of Cycle 1 on Day 28. Based on breast ultrasound reports, the participants will either discontinue the study (disease progression) or will continue niraparib treatment (complete response \[CR\], partial response \[PR\] or stable disease \[SD\]) for an additional cycle (Cycle 2). A breast MRI and breast ultrasound will be performed at the end of Cycle 2. Approximately 21 participants will be enrolled in this study and the study duration will be approximately 2 years.