The purpose of this study is to evaluate the efficacy and safety of MDV9300 in patients with relapsed or refractory diffuse large B-cell lymphoma (DLBCL) that have achieved either stable disease or a partial remission following definitive salvage therapy.

Two cohorts of patients will be enrolled: a cohort treated with salvage chemotherapy but considered ineligible for autologous stem cell transplant (ASCT), and a cohort of patients who have received ASCT following salvage chemotherapy.

Inclusion Criteria:

• Age 18 years or older and willing and able to provide informed consent;
• Histologically confirmed relapsed or refractory CD20+ DLBCL, transformed indolent lymphoma (follicular or other), or primary mediastinal large B-cell lymphoma;
• Received prior treatment with a standard anthracycline and therapeutic anti-CD20 monoclonal antibody-based regimen;
• For transplant-ineligible patients, salvage therapy just prior to MDV9300 treatment must have resulted in a PR or stable disease;
• For post autologous stem cell transplant (ASCT) patients, salvage therapy plus ASCT just prior to MDV9300 treatment must have resulted in a PR or stable disease;
• Adequate bone marrow reserve as defined per protocol;
• Eastern Cooperative Oncology Group (ECOG) performance status ≤ 1. Patients with stable ECOG scores of 2 may be allowed with medical monitor approval.

Exclusion Criteria:

• Burkitt, mantle cell, follicular, or mucosa-associated lymphoid tissue lymphoma
• History of serious autoimmune disease;
• History of central nervous system involvement of lymphoma;
• Prior therapy with agents targeting immune coinhibitory receptors.