Preclinical Safety and Efficacy of Two Novel Immunotoxins Consisting of Ranpirnase (Rap) Fused to an Internalizing Anti-CD74 Humanized IgG4 Antibody in Human Non-Hodgkin’s Lymphoma Xenografts.

Rap, an amphibian ribonuclease, is a single-chain protein of 104 amino acids that kills cells by degrading t-RNA upon internalization. CD74 is a rapidly internalizing type-II transmembrane chaperone molecule associated with HLA-DR, and has high expression on hematological malignancies including B-cell non-Hodgkin’s lymphoma (NHL) and multiple myeloma (MM). We have constructed and evaluated two novel immunotoxins, 2L-Rap-hLL1-γ 4P and 2L-Rap(N69Q)-hLL1-γ 4P, each composed of two Rap molecules fused to hLL1, an internalizing anti-CD74 humanized monoclonal antibody. The Rap gene was inserted at the N-terminus of the light chain in the expression vector of hLL1 and expressed in NS0 mouse myeloma cells. To reduce unwanted cytotoxicity, the CH1, CH2, CH3 and the hinge regions of the γ 1 chain of hLL1 were replaced with those of γ 4. Additionally, the serine residue in the hinge region was converted to proline to prevent the formation of IgG4 half-molecules. Noting that Rap contains a potential N-glycosylation site at the 69th residue of asparagine(N69), a variant of Rap, referred to as Rap(N69Q), was constructed by changing N to Q (glutamine) and this variant was used to make 2L-Rap(N69Q)-hLL1-γ 4P. Purified recombinant immunotoxins were shown to be a single peak by SE-HPLC and their MW determined by MALDI-TOF to be 177,150, which is in agreement with the MW of one IgG (150,000) plus two Rap molecules (24,000). In vitro, both immunotoxins retained RNase activity, specific binding to CD74, and were significantly more potent against CD74-positive NHL and MM cell lines (Daudi, Raji and MC/CAR) than naked hLL1 or non-specific control immunotoxin, 2L-Rap(N69Q)-hRS7(immunotoxin against EGP-1). In Raji and Daudi Burkitt’s lymphoma xenograft models, treatment with a single 5- to 50-μg dose of 2L-Rap-hLL1-γ 4P, given as early or delayed treatment, resulted in cures of most animals. Additionally, treatment with a single 15-μg dose of 2L-Rap(N69Q)-hLL1-γ 4P 1-day post injection of cells resulted in 100% cures. Treatment with 2L-Rap-hLL1-γ 4P or 2L-Rap(N69Q)-hLL1-γ 4P was significantly better than all controls, including saline, naked hLL1 and non-specific immunotoxin. The maximum tolerated dose of 2L-Rap-hLL1-γ 4P or 2L-Rap(N69Q)-hLL1-γ 4P in SCID or BALB/c mice was 50 μg/mouse and the dose-limiting toxicity was hepatic. In our preliminary studies, we have observed that treating animals with NSAID’s, such as indomethacin, can ameliorate the hepatoxicity of 2L-Rap-hLL1-γ 4P. All animals that were injected with 100 μg/mouse 2L-Rap-hLL1-γ 4P alone died with a median survival time of 7 days; however, animals treated with 1.25mg/kg indomethacin prior and post-treatment of 2L-Rap-hLL1-γ 4P survived the duration of study (day 40). Experiments to determine the possible causes of liver toxicity produced by 2L-Rap-hLL1-γ 4P and to determine the MTD of Rap-immunotoxins in mice after treatment with indomethacin are ongoing. In conclusion, we have constructed two CD74-targeted novel recombinant immunotoxins containing Rap molecules that have demonstrated curative therapeutic effects in animal models of human B-cell lymphoma, and thus could be potential therapeutics for CD74-postive lymphomas and myelomas.