Similarities in the Biodistribution of Iodine-Labeled Anti-Tac Single-Chain Disulfide-Stabilized Fv Fragment and Anti-Tac Disulfide-Stabilized Fv Fragment

We evaluated the biodistribution and pharmacokinetics of two different iodine-labeled Fv fragments of anti-Tac monoclonal antibody (MAb) in normal and tumor-bearing nude mice. One was a disulfide-stabilized Fv fragment (dsFv), and the other was a single-chain disulfide-stabilized Fv fragment (scdsFv). The scdsFv is a newly developed type of Fv fragment superior to the dsFv in which the V H and V L are linked by covalent bonds through a spacer arm and by an internal disulfide bond. These modifications increase the yield of scdsFv. Both reagents recognize the alpha subunit of the interleukin-2 receptor (IL-2Rα). The biodistribution of the Fv fragments was evaluated in normal mice co-injected with 50 mg of l -lysine and in a no-lysine control group. Biodistribution was also evaluated in nude mice bearing subcutaneous tumor xenografts derived from IL-2Rα-positive ATAC4 cells and receptor-negative A431 cells. These mice were co-injected with 125 I-labeled anti-Tac scdsFv (6 μCi/0.7 μg) and 131 I-labeled anti-Tac dsFv (2 μCi/0.7 μg) or with 131 I-labeled anti-Tac scdsFv (6 μCi/0.7 μg) and 125 I-labeled anti-Tac dsFv (4 μCi/0.7 μg). The biodistribution of 125 I-labeled anti-Tac scdsFv and 131 I-labeled anti-Tac dsFv was very similar in all organs and the tumors. The renal uptake of both reagents was blocked effectively (<93%) and similarly by lysine. The scdsFv cleared slightly faster from the circulation than did the dsFv because there were more aggregates of dsFv than of scdsFv (3% vs. 1%, respectively). The scdsFv-to-dsFv ratio ranged from 0.79 to 1.20 in all organs at all time points we examined. In conclusion, the first biodistribution study of an scdsFv molecule shows that the scdsFv had a biodistribution very similar to that of the dsFv and seems to be a good alternative to the dsFv because of its higher production yield.