Pretargeted Versus Directly Targeted Radioimmunotherapy Combined with Anti-CD20 Antibody Consolidation Therapy of Non-Hodgkin Lymphoma

We determined whether therapeutic responses using a bispecific antibody that pretargeted Y-90-hapten-peptide radioimmunotherapy or a directly radiolabeled, humanized, Y-90-anti-CD20 IgG (veltuzumab) could be improved by combining these treatments with unlabeled humanized antibodies against CD22 (epratuzumab), CD74 (milatuzumab), or veltuzumab. Methods: Nude mice bearing established subcutaneous Ramos human Burkitt lymphoma were treated with antibodies alone or in combination with pretargeted radioimmunotherapy (PT-RAIT) or radioimmunotherapy, and tumor growth was monitored. Biodistribution studies examined the effect that predosing with unlabeled veltuzumab had on radioimmunotherapy and PT-RAIT targeting. Results: None of the unconjugated antibodies was effective against established and rapidly growing xenografts, but PT-RAIT, at approximately 30% of its maximum tolerated dose, and radioimmunotherapy alone, at its maximum tolerated dose, were able to arrest growth and even entirely ablate tumors in some animals. Only combinations with veltuzumab improved therapeutic responses, most significantly when a veltuzumab regimen (weekly, 1.0 mg followed by 3 x 0.5 mg) was initiated 1 wk after PT-RAIT or Y-90-veltuzumab. Biodistribution data indicated that when unlabeled veltuzumab (1.0 or 0.25 mg) was administered in advance of the radiolabeled veltuzumab or bispecific antibody injection, tumor uptake was significantly reduced (In-111-veltuzumab, 47% and 25%, respectively; In-111-hapten-peptide, 74% and 49%, respectively). Despite an approximately 50% decrease in radioactivity uptake in the tumor, antitumor responses were not diminished significantly for Y-90-veltuzumab, and in the case of PT-RAIT responses were improved. However, higher amounts of predosed veltuzumab reduced the effects of PT-RAIT. Conclusion: These studies suggest that administering unlabeled anti-CD20 IgG therapy after the radioactivity dose provides the best efficacy and that the amount of unlabeled anti-CD20 IgG administered as a predose to anti-CD20-targeted radionuclide therapy should be minimized.