Veliparib, Pembrolizumab, and Combination Chemotherapy in Treating Patient With Locally Advanced Rectal Cancer

PRIMARY OBJECTIVE: I. To demonstrate an absolute improvement in neoadjuvant rectal cancer (NAR) score for the experimental regimen as compared to concurrently randomized control patients. SECONDARY OBJECTIVES: I. To compare overall survival (OS). II. To compare disease-free survival (DFS). III. To compare the rate of pathologic complete response (pCR) (nodes and tumor). IV. To compare the rate of sphincter preservation. TERTIARY OBJECTIVES: I. To compare the proportion of patients who have a tumor resection overall, conditional on beginning induction chemotherapy, and conditional on beginning chemoradiotherapy. II. To compare time from initiation of chemoradiotherapy to surgery in the subset of patients with tumor resection. EXPLORATORY OBJECTIVES: I. To estimate the rate of disease progression during chemotherapy (prior to chemoradiation). II. To compare the rate of clinical complete response rate (cCR). III. To compare the rate of negative circumferential margin. IV. To compare the rate of completion of all cycles of neoadjuvant chemotherapy. V. To compare the rate of completion of full course of chemoradiation. VI. To compare the toxicity and safety between interventions. VII. To explore the correlative molecular predictors of response and distant failure. VIII. To explore the relationship between radiographic findings and pathologic outcomes. OUTLINE: Patients are sequentially randomized to treatment arms, initially ARM I or ARM II and then later either ARM I or ARM III. ARM I: Patients receive mFOLFOX6 regimen consisting of oxaliplatin intravenously (IV) over 2 hours, leucovorin calcium IV over 2 hours on day 1, and fluorouracil IV continuously over 46-48 hours on days 1-2. Treatment repeats every 2 weeks for 8 cycles in the absence of disease progression or unacceptable toxicity. 3-4 weeks after last does of mFOLFOX6 patient undergo radiation therapy (RT) and receive capecitabine orally (PO) twice daily (BID) Monday-Friday for 5 weeks in the absence of disease progression or unacceptable toxicity. ARM II: Patients receive mFOLFOX6 regimen consisting of oxaliplatin IV over 2 hours, leucovorin calcium IV over 2 hours on day 1, and fluorouracil IV continuously over 46-48 hours on days 1-2. Treatment repeats every 2 weeks for 8 cycles in the absence of disease progression or unacceptable toxicity. 3-4 weeks after last does of mFOLFOX6 patient undergo RT and receive capecitabine PO BID and veliparib PO BID Monday-Friday for 5 weeks in the absence of disease progression or unacceptable toxicity. ARM III: Patients receive mFOLFOX6 regimen consisting of oxaliplatin IV over 2 hours, leucovorin calcium IV over 2 hours on day 1, and fluorouracil IV continuously over 46-48 hours on days 1-2. Treatment repeats every 2 weeks for 8 cycles in the absence of disease progression or unacceptable toxicity. 3-4 weeks after last does of mFOLFOX6 patient undergo RT and receive capecitabine PO BID Monday-Friday for 5 weeks. They also receive pembrolizumab IV over 30 minutes every 3 weeks beginning on day 1 of RT for up to 6 cycles in the absence of disease progression or unacceptable toxicity. After completion of study treatment, patients are followed up at 30 days and every 6 months for up to 3 years.