Construction and Characterization of a Novel Ribonuclease Immunotoxin Consisting of Two Ranpirnase (Rap) Molecules Fused to an Internalizing Anti-CD74 Humanized IgG4 Antibody.

Ranpirnase (Rap), isolated from frog (Rana pipiens) oocytes, is a monomeric ribonuclease (MW 11800) that kills cells by degrading t-RNA upon internalization. Previous studies indicated that the cytotoxicity of Rap could be enhanced more than 10,000-fold when the enzyme is chemically conjugated to an internalizing antibody. Here we describe the construction and characterization of 2L-Rap-hLL1-γ4P, composed of two Rap molecules fused to hLL1, an internalizing anti-CD74 humanized monoclonal antibody. To reduce unwanted cytotoxicity, the IgG1 constant region of hLL1 was replaced with an IgG4 that contains a proline mutation in the hinge region. 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. The fusion protein was characterized by a variety of techniques, including SE-HPLC, SDS-PAGE, in vitro transcription translation (IVTT) assay using luciferase reporter system, and competition ELISA to measure the binding affinity for CD74. The in vitro potency was determined in non-Hodgkin’s lymphoma (Daudi) and multiple myeloma (MC/CAR) cell lines by MTS tetrazolium dye reduction assay. In vivo pharmacokinetics and biodistribution of radiolabeled 2L-Rap-hLL1- γ4P was compared to radiolabeled hLL1 mAb in naïve mice and in vivo therapeutic efficacy of 2L-Rap-hLL1- γ4P was determined in a xenograft model of Burkitt’s non-Hodgkin’s lymphoma (Daudi). Purified 2L-Rap-hLL1- γ4P was shown to be a single peak by SE-HPLC and its 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). Reducing-SDS-PAGE of 2L-Rap-hLL1- γ4P revealed the presence of 3 bands, one corresponding to the heavy chain and the other two appearing to be derived from the Rap-fused light chains (38,526 and 36,700 by MS). Occurrence of the 2 light chains was shown to be due to glycosylation of Rap at the N69 residue. The binding affinity of 2L-Rap-hLL1- γ4P for CD74 was indistinguishable from that of hLL1. Both 2L-Rap-hLL1- γ4P and hLL1 bound to CD74 with subnanomolar affinity. The EC₅₀ of RNase activity, as measured by the IVTT assay, was 300 pM for 2L-Rap-hLL1- γ4P and 30 pM for recombinant Rap (expressed in E. coil). In in vitro cytotoxicity assays, 2L-Rap-hLL1- γ4P was significantly cytotoxic against Daudi (EC₅₀ 280 pM) and the myeloma cell line, MC/CAR (EC₅₀ 50 nM). In contrast, free Rap or naked hLL1 did not demonstrate significant cytotoxicity at the concentrations tested. In vivo, the pharmacokinetic profile of 2L-Rap-hLL1- γ4P was almost identical to that of naked hLL1. Both 2L-Rap-hLL1- γ4P and hLL1 showed biphasic clearance from the circulation; the α and β half-life (t_1/2) of 2L-Rap-hLL1- γ4P were 5 h and 119 h, respectively, and those of hLL1 were 4 h and 125 h, respectively. In tissue biodistribution studies, no significant difference was observed between 2L-Rap-hLL1- γ4P and hLL1 with regards to normal tissue uptake. Early efficacy results in the Daudi Burkitt’s non-Hodgkin’s lymphoma xenograft model demonstrate that treatment with a single dose of 2L-Rap-hLL1- γ4P as low as 1 μg/mouse significantly improves survival in comparison to untreated control mice (P<0.0001).