This is a single arm window of opportunity trial conducted in patients with early stage triple negative breast tumors to evaluate if treatment with a Poly(ADP-ribose) polymerase (PARP) inhibitor will increase expression of programmed cell death-1 with ligand (PD-L1) in triple negative breast tumors.

This is a single arm window of opportunity trial conducted in patients with early stage triple negative breast tumors. Patients who are planning to undergo surgery as part of their initial treatment will be eligible for this study. They will be treated with single agent rucaparib for 3 weeks and then proceed to surgery. Core-biopsies obtained at the time of diagnosis and tumor from the surgical resection will be assessed for change in expression of PD-L1 by Immunohistochemical assay (IHC).

Inclusion Criteria:

1. Have histologically documented triple negative breast cancer (TNBC) (defined as ER expression ≤10% by IHC, progesterone receptor (PR) expression≤10% by IHC and HER2 0 or 1+ by IHC or Fluorescence in situ hybridization (FISH) ratio <2 or human epidermal growth factor receptor 2 (HER2) gene copy number of <6)
2. Early stage breast cancer (stage I-III) and not be candidate for neoadjuvant chemotherapy
3. Be informed of the investigational nature of the study and all pertinent aspects of the trial
4. Have Eastern Cooperative Oncology Group (ECOG) performance status of 0-2
5. Have the ability to understand and the willingness to sign a written informed consent document in accordance with institutional and federal guidelines
6. Be ≥ 21 years of age
7. Have serum creatinine < 1.5 x institutional upper limit of normal (IULN) or a calculated creatinine clearance ≥ 30ml/min (calculated by Cockcroft Gault equation), bilirubin ≤ 2.0, and an serum glutamic oxaloacetic transaminase (SGOT)/s erum glutamic pyruvic transaminase (SGPT)/alkaline phosphatase ≤ 2.0 x IULN
8. Have adequate bone marrow function (ANC >1000, Platelets >100,000/ml, Hemoglobin >10gm/dL)
9. Women of childbearing potential or male patients of reproductive potential with female partners of childbearing potential must not consider getting pregnant and must avoid pregnancy during the study and for at least 6 months after the last dose of rucaparib. Female and male patients of reproductive potential must practice highly effective methods of contraception with their partners, if of reproductive potential, during treatment and for 6 months following last dose of rucaparib

Exclusion Criteria:

1. Ongoing or prior treatment with a PARPi for breast cancer or other malignancies
2. Receiving concurrent anti-neoplastic therapy for their breast cancer or another malignancy
3. Known documented or suspected hypersensitivity to the components of the study drug or analogs.
4. Pre-existing gastrointestinal disorders or defects (like duodenal stent etc) that would, in the opinion of the investigator, interfere with absorption of rucaparib