Methods to avoid adverse effect of circulating antigen on biodistribution of ¹²⁵I-labeled antiTac dsFv: Preinjection of intact antibody versus clearance of antigen with avidin-biotin system

The presence of circulating antigen may adversely affect the biodistribution of a radiolabeled antibody. The α subunit of the interleukin-2 receptor (IL-2Rα) is a cell-surface receptor that is overexpressed in various hematologic malignancies and in benign disorders. This receptor is cleaved from the cell surface and can be found in high concentrations in serum. Radiolabeled antiTac antibodies are being evaluated to target this receptor. Previous studies have shown that circulating soluble IL-2Rα (sIL-2Rα)adversely affected the biodistribution of radiolabeled antiTac disulfide-stabilized (ds)Fv. In this study, we compared blocking and clearing sIL-2Rα to see which better minimized its interference with the biodistribution of radiolabeled antiTac dsFv. Methods: Two models of sIL- 2Rα were used: one consisted of mice given intravenous sIL-2Rα and the other consisted of mice bearing SP2/Tac tumor xenografts (IL-2Rα positive), which shed sIL-2Rα. We biotinylated humanized antiTac monoclonal antibody (bt-HuTac) and radiolabeled it with ¹²⁵I. We then compared its biodistribution with that of humanized antiTac monoclonal antibody IgG (HuTac). We examined the biodistribution of an injected dose of ¹²⁵I- labeled antiTac dsFv after a preinjection of HuTac to block the sIL-2Rα epitope and after a preinjection of bt-HuTac, followed by an avidin chase. Result: The ¹²⁵I-labeled bt-HuTac cleared from the serum at a rate similar to that of HuTac. The avidin chase effectively cleared >92% of circulating¹²⁵I-labeled bt-HuTac within 20 min and was also effective in clearing sIL-2Rα. In comparison, HuTac prolonged the retention of¹²⁵I- labeled sIL-2Rα in the circulation, and the avidin chase decreased ¹²⁵I- labeled sIL-2Rα to <18% of control. Although the two-step antigen-clearing system effectively cleared the antigen from the circulation and improved the biodistribution of¹²⁵I- labeled dsFv, the HuTac preinjection method had a similar but longer lasting beneficial effect on ¹²⁵I-labeled dsFv biodistribution. Conclusion: Preinjection of either HuTac or bt-HuTac with avidin chase improved the biodistribution of subsequently administered ¹²⁵I-labeled antiTac dsFv by preventing the dsFv from binding to the sIL- 2Rα, but the HuTac blocking method is simpler and longer lasting.