Improved Responses and Cures with Less Hematologic Toxicity Using Anti-CD20 Bispecific Antibody Pretargeted Radionuclides Compared to Directly Radiolabeled IgG in a NHL Model.

Directly radiolabeled anti-CD20 IgGs approved for the treatment of follicular and transformed non-Hodgkin lymphoma produce higher response rates than rituximab. However, these treatments result in severe and protracted hematologic toxicity, which is directly related to the slow blood clearance of these radiolabeled products. Pretargeting procedures separate radionuclide-targeting from the slow antibody-targeting step, and because the radiolabeled compound is small in size, it clears rapidly and thoroughly from the blood and body in just a few hours, minimizing radiation exposure. We have developed a novel recombinant and humanized Tri-Fab bispecific antibody (bsMAb) for pretargeting using the modular Dock-and-Lock procedure. This bsMAb divalently binds to CD20, with the monovalent Fab binding to a unique peptide hapten (HSG) that carries the therapeutic radionuclide. Nude mice bearing established (0.5 to 0.9 cm³) subcutaneous Ramos human B-cell lymphomas were given a single dose of either a ⁹⁰Y-humanized anti-CD20 IgG (150 and 175 μCi; 50 μg) or a bsMAb-pretargeted ⁹⁰Y-DOTA-HSG-peptide (150, 250, 500, and 700 μCi). Tissue counting data derived 24 h after injecting the radiolabeled product showed similar tumor uptake between the IgG and pretargeted peptide, but tumor/blood ratios were >1000:1 for the pretargeted peptide vs. 1:1 for the IgG. At both ⁹⁰Y-IgG doses, blood counts decreased ∼90% for 2 weeks after treatment, while the pretargeted groups only decreased ∼60% at the 700 μCi dose to as low as a 25% decrease at the 250 μCi dose, with full recoveries occurring within 2 weeks of the nadir. The severe hematologic toxicity also resulted in 2/10 and 4/10 deaths within 2 weeks of the 150- and 175-μCi ⁹⁰Y-IgG doses, respectively. In the remaining animals, after experiencing an initial response, all tumors progressed to ≥1.5 cm³ within 3 weeks of treatment. Only 1/10 animals given 500 μCi of the pretargeted dose showed tumor progression, with all others in this group and the 700-μCi-group showing no evidence of disease. Earlier studies have shown 40%, 59%, and 90% cure rates after 14 to 18 weeks evaluation with 150, 250, and 500 μCi, respectively, of the bsMAb-pretargeted ⁹⁰Y-peptide (n = 10 to 22 animals). These studies demonstrate that bsMAb pretargeting significantly improves response rates, with durable cures and without the excessive hematologic toxicity commonly associated with directly radiolabeled antibodies.

(Supported in part from USPHS grant P01-CA103985.)