Chemical modification to reduce renal uptake of disulfide-bonded variable region fragment of anti-Tac monoclonal antibody labeled with Tc-99m

The anti-Tac disulfide-bonded variable region fragment (dsFv) is a genetica lly engineered, 25 kDa, murine monoclonal antibody fragment that recognizes the alpha subunit of the interleukin-2 receptor (IL-2R alpha). The dsFv ra diolabeled with the tetrafluorophenyl ester (TFP) of [Tc-99m]mercaptoacetyl triglycine ([Tc-99m]MAG3-TFP) showed rapid tumor uptake and fast blood clea rance in mice, resulting in high tumor-to-nontumor background ratios. Howev er, its high renal uptake was a problem. In this study, we tested the effec t of lowering the isoelectric point (pI) of dsFv to <9.3 on renal and tumor uptake. To lower the pi, dsFv was acylated simultaneously with both [Tc-99 m]MAG3-TFP and TFP-glycolate. The acylation of dsFv decreased its pi and it s immunoreactivity inversely proportional to the molar ratio of TFP-glycola te to dsFv, whereas the conjugation of [Tc-99m]MAG3-TFP alone did not. When biodistribution studies were performed in nude mice, the effect of the low ered pi was reflected primarily in decreased kidney uptake and whole-body r etention, with its highest effect seen at the earliest time point (15 min) after injection. In tumor-bearing nude mice, glycolated [Tc-99m]MAG3-dsFv w ith a pi range of 4.9 to 6.5 accumulated selectively into IL-2 receptor-pos itive SP2/Tac tumor similar to that of the control [I-125]dsFv labeled by t he Iodo-Gen method, whereas its renal uptake was 25% of [I-125]dsFv at 15 m in. At 90 min, the ratios of tumor to receptor-negative SP2/0 tumor, liver, kidney, stomach, and blood had peaked at 10.9, 8.5, 0.3, 5.0, and 6.2, res pectively, for the glycolated [Tc-99m]MAG3-dsFv. The corresponding ratios f or [125I]dsFv were 3.7, 5.0, 0.1, 1.5, and 2.1, respectively.