Anti-BCMA immunotoxins produce durable complete remissions in two mouse myeloma models

Multiple myeloma (MM) is a B cell malignancy for which new treatments are urgently needed. The B cell maturation antigen (BCMA) is a lineage-restricted differentiation protein highly expressed on myeloma. Recombinant immunotoxins (RITs) are proteins composed of the Fv or Fab portion of an antibody fused to a bacterial toxin. We previously treated H929 myeloma s.c. tumors with anti-BCMA immunotoxins, very active on killing cultured cells, and observed tumor growth inhibition but not complete tumor responses. To determine if immunotoxins were more active against cells growing in the bone marrow (BM), the normal location of myeloma cells, we developed a BM mouse model that is more relevant to human disease. H929 cells were transfected with luciferase and GFP, enriched by flow, recycled through the BM of a mouse, and injected IV into nonobese diabetic scid γ mice (NSG) mice. A second myeloma mouse model used the MM.1S-GFP-luc cell line. Mice were treated IV with immunotoxins, and the tumor burden was assessed using bioluminescence imaging. We achieved complete durable remissions when treating mice with H929-GFP-luc cells with anti-BCMA RITs both leptomycin B-75 (LMB-75) [anti-BCMA-disulfide-stabilized (ds)-Fv-PE24] (where PE represents Pseudomonas exotoxin A) or LMB-70 (anti-BCMA-Fab-PE24) given every other day for 5-d (QOD×5) doses beginning on day 4 or day 8. Mice were disease free at 3 months; untreated mice became moribund around day 40. We also achieved long-term responses using the MM.1S-GFP-luc myeloma cell line. Treatment with an 1.5 mg/kg LMB-75 QOD×5 anti-BCMA RIT beginning on day 4 caused the complete disappearance of tumors for 80 days. To summarize, LMB-75 and LMB-70, our anti-BCMA RITs, induced complete durable responses in two myeloma models.