Multiple Myeloma (MM) is an incurable plasma cell malignancy that afflicts 66,000 Americans, with 20,580 new cases diagnosed each year. The recent introduction of bortezomib, thalidomide, lenalidomide, and other novel agents for treatment of MM has significantly improved response rates, progression-free survival, and overall survival for this disease. However, despite these exciting advances, virtually all MM patients eventually die of disease progression after a median survival of only 5 years from diagnosis, emphasizing the continuing need for improved regimens for this disease. The efficacy of radioimmunotherapy (RIT) in the treatment of leukemia and lymphoma is well established. In this application we will investigate the safety and efficacy of pretargeted radioimmunotherapy (PRIT) directed against the myeloma-associated CD38 antigen in murine models of MM. In Aim 1, we will compare the biodistribution of radioactivity obtained using a directly radiolabeled anti-CD38 Ab (OKT10) with the biodistribution of PRIT using an engineered tetravalent OKT10 scFv4-streptavidin (SA) molecular fusion protein, followed by a dendrimeric clearing agent and then 90Y-labeled DOTA-biotin. Studies will be done in two complementary model systems, namely, athymic mice bearing subcutaneous MM xenografts and SCID-human mice bearing fresh human multiple myeloma cells growing in implanted human bone containing human stromal microenvironment. In Aim 2, we will compare the toxicities and therapeutic efficacies of conventional 90Y-DOTA-OKT10 with those of 90Y-DOTA-biotin pretargeted with the OKT10 scFv4- streptavidin (SA) fusion protein in the same two mouse models. In Aim 3, we will assess the impact of CD38 antigen upregulation induced by all-trans-retinoic acid on the biodistribution and therapeutic efficacy of radiolabeled DOTA-biotin targeted to mouse myeloma xenografts; and malignant plasma cells in the SCID-hu mouse myeloma model. In Aim 4, we will investigate the toxicity and efficacy of combination therapy using anti- CD38 pretargeted RIT, with and without lenalidomide and/or bortezomib, two of the most promising new agents for MM. We hypothesize that the PRIT strategies defined in this proposal will amplify the amount of radiation delivered to MM cells, decrease the radiation delivered to the liver, lungs, and other normal organs, improve remission and cure rates, prolong survival, and markedly attenuate toxicities compared to conventional RIT. We predict synergistic anti-tumor activity with combinations of PRIT and lenalidomide or bortezomib. We anticipate rapid translation of the results of these preclinical experiments into our clinical RIT program for MM.