The purpose of this study is to evaluate the efficacy of Pixantrone + Rituximab compared to Gemcitabine + Rituximab in patients with relapsed or refractory diffuse large B-cell lymphoma (DLBCL), or follicular grade 3 lymphoma.

Eligible patients will be randomized to treatment with pixantrone plus rituximab or gemcitabine plus rituximab in up to six 28-day cycles. At the time patients experience progressive disease during study treatment, early follow- up, or intermediate follow-up, they enter the survival follow up period. Patients who complete study treatment or discontinue study treatment for any other reason will participate in the follow-up periods.

Early Follow-Up: After treatment completion or discontinuation, patient will enter a 24-week follow-up period.

Intermediate Follow-Up: After completing the 24-week early follow-up period, patient will enter an additional 72-week follow-up period.

Survival Follow-Up: All patients will be monitored for survival.

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.