Inclusion Criteria
1. Pathologic biopsy proven adenocarcinoma of the prostate
2. At least one of the following criteria:
* cN1 on conventional or PET imaging
* Grade group 5
* Grade group 4
* Grade group 3 and PSA ≥20 ng/mL
* High probability of Radiographic T3 on MRI AND Grade group ≥2
* Grade Group 3 AND PSA ≥10 ng/mL AND ≥50% positive biopsy cores
3. Age ≥ 18
4. ECOG \< 1
5. Adequate organ and marrow function as defined per protocol.
6. Use of highly effective contraception (e.g. condoms) for the duration of treatment and a minimum of 120 days thereafter. Men must also agree not to donate sperm for the duration of the study participation, and for at least 120 days thereafter.
7. International Prostate Symptoms Score (IPSS) ≤ 20
8. Medically fit for treatment and agreeable to follow-up
9. Ability to understand and the willingness to sign a written informed consent
10. Tissue available for MiOncoSeq testing to assign DNA repair deficiency status
Exclusion Criteria
1. Clinical or radiographic evidence of distant metastatic disease by CT/bone scan
2. Clinical or radiographic evidence of high probability of clinical T4 disease
3. Prostate gland size \>80 cc measured by ultrasound or MRI
4. Prominent median lobe assessed by treating physician
5. Lack of tissue from biopsy to be sent for correlative studies
6. Any prior treatment for prostate cancer (incudes history of TURP within 5 years of enrollment, chemotherapy, radiation therapy, or anti-androgen therapy)
7. Prohibited within 30 days prior to administration to study treatment: spironolactone and other investigational drug therapies.
8. Prohibited 3 months before participant registration and during administration of study treatment: non-steroidal anti-androgens (e.g., bicalutamide, flutamide, nilutamide), steroidal antiandrogens (megestrol acetate, cyproterone acetate), oral ketoconazole, chemotherapy, immunotherapy, estrogens, radiopharmaceuticals.
9. History of prior pelvic radiation therapy
10. Concurrent treatment with strong CYP3A4 inducers such as phenytoin, carbamazepine, rifampin, rifabutin, rifapentine, phenobarbital
11. Enrollment concurrently in another investigational drug study within 1 month of registration
12. History of another active malignancy within the previous 3 years except for adequately treated skin cancer or superficial bladder cancer
13. History of or active Crohn's disease or ulcerative colitis
14. Contraindication to or inability to tolerate MRIs
15. Patients with severe depression
16. Uncontrolled diabetes or known HbA1c\>10
17. Any gastrointestinal disorder affecting absorption
18. Active pituitary or adrenal dysfunction
19. Patients with significant cardiovascular disease potentially including severe / unstable angina, recent history of myocardial infarction, clinically significant heart failure, cerebrovascular disease, venous thromboembolic events, clinically significant arrhythmias)
20. Uncontrolled hypertension with persistently elevated systolic blood pressure \>160 mmgHg or diastolic blood pressure \>100 mmHg despite anti-hypertensive agents.
21. Prolonged QTc \>450 ms or any ECG changes that interfere with QT interval interpretation
22. Major surgery within 1 month of registration
23. History of myelodysplastic syndrome or leukemia
24. A known hypersensitivity to niraparib, abiraterone acetate, leuprolide, and/or prednisone
25. Active infection or other medical condition that would be a contraindication to prednisone use
26. Patients with known active hepatitis or chronic liver disease including cirrhosis
27. Any condition that in the opinion of the investigator would preclude participation in this study
