Radioimmunotherapy in the pi-BCL1 B cell lymphoma model: efficacy depends on more than targeted irradiation alone.

We report the in vivo radioimmunotherapy (RIT) of a new variant of the BCL1 syngeneic mouse B-cell lymphoma model, π-BCL1, using a panel of monoclonal antibodies (MoAb) reactive with B cell-associated antigens (CD19, CD22, CD40, MHC II, and idiotype). MoAb were radiolabeled with 131I or used in conjunction with external beam irradiation. When administered early in disease (day 4) l31I-anti-MHC II MoAb produced long term disease free survivors as a result of targeted irradiation alone; the equivalent unlabelled MoAb was non-therapeutic. In contrast, 13lI-anti-CD40, and 131I-anti-Idiotype (Id) MoAb administered at day 4 despite targeting irradiation and having intrinsic therapeutic activity as unconjugated antibodies, protected mice for approximately 30 days. The 131I-anti-CD19 and anti-CD22 were therapeutically inactive. Treating later in the disease (day 14, after tumor inoculation) permitted study of the efficacy in the presence of an increased tumor load. An increased tumor burden brought about an expec...