Cancer Therapy: Preclinical Pretargeted Radioimmunotherapy Using Genetically Engineered Antibody-Streptavidin Fusion Proteins for Treatment of Non-Hodgkin Lymphoma

Purpose: Pretargeted radioimmunotherapy (PRIT) using streptavidin (SAv)-biotin technology can deliver higher therapeutic doses of radioactivity to tumors than conventional RIT. However, "endogenous" biotin can interferewith the effectiveness of this approachbyblocking binding of radiolabeled biotin to SAv. We engineered a series of SAv FPs that downmodulate the affinity of SAv for biotin, while retaining high avidity for divalent DOTA-bis-biotin to circumvent this problem. Experimental Design: The single-chain variable region gene of themurine 1F5 anti-CD20 antibody was fused to thewild-type (WT) SAv gene and tomutant SAv genes, Y43A-SAv and S45A-SAv. FPswere expressed, purified, and compared in studies using athymic mice bearing Ramos lymphoma xenografts. Results:Biodistribution studies showeddelivery ofmore radioactivity to tumors ofmice pretargetedwith mutant SAv FPs followed by In-DOTA-bis-biotin [6.2 1.7% of the injected dose per gram (%ID/gm) of tumor 24 hours after Y43A-SAv FP and 5.6 2.2%ID/g with S45A-SAv FP] than in mice on normal diets pretargeted with WT-SAv FP (2.5 1.6%ID/g; P 1⁄4 0.01). These superior biodistributions translated into superior antitumor efficacy inmice treated withmutant FPs and Y-DOTA-bis-biotin [tumor volumes after 11days: 237 66mmwith Y43A-SAv, 543 320mmwith S45A-SAv, 1129 322mmwithWT-SAv, and 1435 212 mm with control FP (P < 0.0001)]. Conclusions: Genetically engineered mutant-SAv FPs and bis-biotin reagents provide an attractive alternative to current SAv-biotin PRIT methods in settings where endogenous biotin levels are high. Clin Cancer Res; 17(23); 1–10. 2011 AACR.