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

Abstract

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.

B-cell maturation antigen (BCMA) in multiple myeloma: rationale for targeting and current therapeutic approaches

“…Other unique BCMA-targeted therapies are being investigated for treatment of MM. These include an anti-BCMA mAb conjugated to an antitumor maytansine derivative via a non-cleavable linker (AMG 224, under clinical study); combination therapy with an antibody-coupled Tcell receptor (ACTR087) plus an anti-BCMA antibody (SEA BCMA); a BCMA-and CD16A-directed tetravalent antibody that engages natural killer cells (AFM26); anti-BCMA recombinant immunotoxins; a heteroclitic BCMA peptide encapsulated nanoparticle-based cancer vaccine; and an antibody-based scaffold that binds CD3, BCMA, and programmed cell death ligand 1 [89][90][91][92][93]. Antimyeloma therapies targeting or incorporating APRIL, the primary ligand for BCMA, have also been developed.…”

Critical Issues in the Development of Immunotoxins for Anticancer Therapy

“…89 Clinical trials of the various deimmunized ITs with better results are anticipated (Table 1). 71,90 Aside from the heterologous toxin moiety, the antibody-based targeting moiety could be immunogenic, 86 especially a moiety based on murine and chimeric antibodies used in the firstand second-generation ITs. 72 To address this problem, humanized and fully human antibodies are utilized in a recent version of ITs.…”

Anti-BCMA Immunotoxins: Design, Production, and Preclinical Evaluation

“…The activity of anti-BCMA immunotoxin LMB-70 was investigated in a more relevant bone marrow mouse model. To generate the bone marrow mouse model, the myeloma cell lines were engineered by transfecting lentivirus expressing GFP-Luciferase genes [ 41 ]. When myeloma-Luc cells were injected intravenously into the tail vein of NSG mice, cells grew and populated the bone marrow as assessed by bioluminescence imaging and showed a growth pattern similar to those in myeloma patients.…”

“…Anti-BCMA immunotoxin LMB-70 was tested for anti-tumor efficacy in the mouse bone marrow xenograft model with two myeloma cell lines. In one model, H929-luc cells were used and the immunotoxin was given IV every other day starting on day 8 after cell injection [ 41 ]. In contrast to the subcutaneous xenograft model, LMB-70 produced a complete response in all treated mice when tested in mouse bone marrow xenograft harboring H929-luc myeloma cells with a dose of 1.5 mg/kg QODX5 ( Figure 5 ).…”