In Part A of the study, patients will be randomized 1:1 to receive denintuzumab mafodotin plus RCHOP (rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone) or denintuzumab mafodotin plus RCHP (rituximab, cyclophosphamide, doxorubicin, and prednisone) to assess the safety of these 2 combination regimens. Part B of the study is designed to evaluate the antitumor activity and safety of denintuzumab mafodotin in combination with either RCHOP or RCHP (Experimental Arm) compared with RCHOP alone (Comparator Arm).