Pembrolizumab and Combination Chemotherapy in Treating Patients With Previously Untreated Diffuse Large B-cell Lymphoma or Grade 3b Follicular Lymphoma

PRIMARY OBJECTIVES: I. To measure the toxicity profile of pembrolizumab (MK-3475) when co-administered with full-course RCHOP (rituximab, cyclophosphamide, doxorubicin hydrochloride, vincristine sulfate, and prednisone) in subjects with previously untreated diffuse large B-cell lymphoma (DLBCL). SECONDARY OBJECTIVES: I. To assess clinical outcomes including response rate, event-free survival, and overall survival after MK-3475 + RCHOP induction for subjects with previously untreated DLBCL. TERTIARY OBJECTIVES: I. To measure baseline expression of proteins in the programmed death-1 (PD-1) family on tumor cells and coexisting immune infiltrates, using archival tissue when available. II. To measure peripheral blood T cell subsets before and after treatment using flow cytometry, and to measure baseline vitamin D (25-hydroxy, total). III. To explore relationships with these parameters and likelihood of response to therapy and outcomes. OUTLINE: Patients receive pembrolizumab intravenously (IV) over 30 minutes on day 1 and prednisone orally (PO) on days 1-5. Patients also receive rituximab IV, cyclophosphamide IV, doxorubicin hydrochloride IV, and vincristine sulfate IV on day 2 of course 1 and on day 1 of subsequent courses. Treatment repeats every 21 days for 6 courses in the absence of disease progression or unacceptable toxicity. After completion of the study treatment, patients are followed up to 5 years.