Comparison of a tetravalent single-chain antibody-streptavidin fusion protein and an antibody-streptavidin chemical conjugate for pretargeted anti-CD20 radioimmunotherapy of B-cell lymphomas

The efficacy of radioimmunotherapy (RIT) for patients with relapsed non-Hodgkin lymphoma (NHL) is limited by nonspecific delivery of radiation to normal tissues due to the long circulating half-life of radiolabeled anti-CD20 antibodies (Abs). Pretargeted RIT using a covalent conjugate of the 1F5 anti-CD20 Ab with streptavidin (SA) has been shown to augment the efficacy of RIT and decrease toxicity compared with a directly labeled 1F5 Ab. We have engineered a tetravalent single-chain 1F5 (scFV)(4)SA fusion protein and compared it to the 1F5-SA conjugate. Athymic mice bearing Ramos lymphoma xenografts received either the conjugate or fusion protein, followed 20 hours later by a biotin-N-acetyl-galactosamine clearing agent, followed 4 hours later by In-111-DOTA-biotin. After 24 hours, 11.4% +/- 2.1% of the injected dose of radionuclide was present per gram of tumor (% ID/g) using 1F5 (scFv)(4)SA compared with 10.8% +/- 2.5% ID/g with 1F5 Ab-SA. Superior tumor-to-normal organ ratios of radioactivity were consistently seen using the fusion protein compared with the chemical conjugate (eg, tumor-to-blood ratio > 65:1 after 48 hours with the fusion protein, but < 7:1 with the conjugate). More than 90% of lymphoma-bearing mice could be cured with minimal toxicity using either reagent followed by 1200 mu Ci (44.4 MBq) Y-90-DOTA-biotin.