ImmunoPET imaging of B-cell lymphoma using 124I-anti-CD20 scFv dimers (diabodies)

Rapid clearing engineered antibody fragments for immunoPET promise high sensitivity at early time points. Here, tumor targeting of anti-CD20 diabodies (scFv dimers) for detection of low-grade B-cell lymphomas were evaluated. In addition, the effect of linker length on oligomerization of the diabody was investigated. Four rituximab scFv variants in the V-L-V-H orientation with different linker lengths between the V domains (scFv-1, scFv-3, scFv-5, scFv-8), plus the scFv-5 with a C-terminal cysteine (Cys-Db) for site-specific modification were generated. The scFv-8 and Cys-Db were radioiodinated with I-124 for PET imaging, and biodistribution of I-131-Cys-Db was carried out at 2, 4 10 and 20 h. The five anti-CD20 scFv variants were expressed as fully functional dimers. Shortening the linker to three or one residue did not produce higher order of multimers. Both I-124-labeled scFv-8 and Cys-Db exhibited similar tumor targeting at 8 h post injection, with significantly higher uptakes than in control tumors (P < 0.05). At 20 h, less than 1% ID/g of I-131-labeled Cys-Db was present in tumors and tissues. Specific tumor targeting and high contrast images were achieved with the anti-CD20 diabodies. These agents extend the repertoire of reagents that can potentially be used to improve detection of low-grade lymphomas.