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

The presence of circulating antigen may adversely affect the biodistributio n of a radiolabeled antibody. The alpha subunit of the interleukin-2 recept or (IL-2R alpha) is a cell-surface receptor that is overexpressed in variou s hematologic malignancies and in benign disorders. This receptor is cleave d from the cell surface and can be found in high concentrations in serum. R adiolabeled antiTac antibodies are being evaluated to target this receptor. Previous studies have shown that circulating soluble IL-2Ra (sIL-2R alpha) adversely affected the biodistribution of radiolabeled antiTac disulfide-s tabilized (ds)Fv. In this study, we compared blocking and clearing sIL-2R a lpha to see which better minimized its interference with the biodistributio n of radiolabeled antiTac dsFv Methods: Two models of sIL-2R alpha were use d: one consisted of mice given intravenous sIL-2R alpha and the other consi sted of mice bearing SP2/Tac tumor xenografts (IL-2Ra: positive), which she d sIL-2R alpha. We biotinylated humanized antiTac monoclonal antibody (bt-H uTac) and radiolabeled it with I-125. We then compared its biodistribution with that of humanized antiTac monoclonal antibody IgG (HuTac). We examined the biodistribution of an injected dose of I-125-labeled antiTac dsFv afte r a preinjection of HuTac to block the sIL-2R alpha epitope and after a pre injection of bt-HuTac, followed by an avidin chase. Result: The I-125-label ed bt-HuTac cleared from the serum at a rate similar to that of HuTac. The avidin chase effectively cleared >92% of circulating I-125-labeled bt-HuTac within 20 min and was also effective in clearing sIL-2R alpha. In comparis on, HuTac prolonged the retention of I-125-labeled sIL-2R alpha in the circ ulation, and the avidin chase decreased I-125-labeled sIL-2R alpha to <18% of control. Although the two-step antigen-clearing system effectively clear ed the antigen from the circulation and improved the biodistribution of I-1 25-labeled dsFv, the HuTac preinjection method had a similar but longer las ting beneficial effect on I-125-labeled dsFv biodistribution. Conclusion: P reinjection of either HuTac or bt-HuTac with avidin chase improved the biod istribution of subsequently administered I-125-labeled antiTac dsFv by prev enting the dsFv from binding to the sIL-2R alpha, but the HuTac blocking me thod is simpler and longer lasting.