Comparison of Pixantrone + Rituximab With Gemcitabine + Rituximab in Patients With Aggressive B-cell Non-Hodgkin Lymphoma or Follicular Grade 3 Lymphoma Who Have Relapsed After Therapy and Are Not Eligible for Stem Cell Transplant

Inclusion Criteria: 1. Diagnosis of DLBCL (de novo DLBCL, or transformed from indolent lymphoma) or follicular grade 3 lymphoma on the basis of tissue biopsy. 2. Patients with de novo DLBCL must have received 1-3 treatment regimens for DLBCL. Patients with follicular grade 3 lymphoma must have received 1-3 treatment regimens for follicular lymphoma (any grade). Patients with DLBCL transformed from indolent lymphoma must have received at least 1-4 treatment regimens for NHL. 3. Received rituximab containing a multi-agent therapy for the treatment of NHL. 4. Not eligible for high-dose chemotherapy and stem cell transplant. 5. Patients with DLBCL transformed from indolent lymphoma must have had a complete or partial response to a therapy for NHL lasting at least 12 weeks. Exclusion Criteria: 1. Primary refractory de novo DLBCL or primary refractory follicular grade 3 lymphoma, defined as documented progression within 12 weeks of the last cycle of the first-line multi-agent regimen. 2. Prior treatment with cumulative dose of doxorubicin or equivalent exceeding 450 mg/m2 3. Any experimental therapy ≤ 28 days prior to randomization 4. Other malignancy within last 5 years except for the following: curatively treated basal cell/squamous cell skin cancer, carcinoma in situ of the cervix, superficial transitional cell bladder carcinoma, or in situ ductal carcinoma of the breast after complete resection 5. Any contraindication or known allergy or hypersensitivity to any study drugs 6. Concomitant therapy with any anticancer agents, immunosuppressive agents, other investigational anticancer therapies. Low-dose corticosteroids for the treatment of non cancer-related illnesses are permitted.