of Work: Disulfide stabilized variable-region fragment (dsFv) of anti-Tac monoclonal antibody was previously radiolabeled with Tc-99m to detect scintigraphically hematologic malignancies that express IL- 2alpha: receptors. Tc-99m labeled dsFv has proven to rapidly target IL-2 receptor-positive human tumors (ATAC4 and SP2/Tac) xenografted in nude mice, whereas it cleared rapidly from blood and most organs, thereby resulting in high tumor-to-nontumor tissue ratios. However, this radiopharmaceutical is excreted primarily through the kidneys and its high kidney uptake (70% injected dose at 15 min) was a limitation. To improve the property of this radiopharmaceutical with respect to the high renal uptake, two approaches have been investigated: insertion of readily metabolizable ester bond between dsFv and the radiolabel and lowering isoelectric point (pI 10) of dsFv. The second approach has been more successful: lowering pI by conjugating glycolate molecules to lysine residues of dsFv. During FY 97, the second approach was extended to test the effects of different acylating agents such as tetrafluorophenyl lactate and succinic anhydride on the lowering of the pI and also on the biodistribution of the acylated Tc-99m-labeled dsFv. The effect of the lactate conjugation on the pI was similar to that of the glycolate conjugation, whereas the acylation reaction with succinic anhydride was more efficient in lowering the pI, perhaps because this reaction neutralizes one positive charge on an amino group of dsFv and at the same time adds one negative charge to dsFv. Comparative biodistribution studies in nude mice, between these acylated Tc-99m- labeled dsFv preparations with a similar pI range (pI 5-7), indicate that the acylated Tc-99m-labeled dsFv preparations were excreted readily through the kidneys and that the renal uptake was three to four times lower than that of the control Tc-99m labeled dsFv. This study suggests that the renal uptake of dsFv involves interactions between positively charged dsFv and negatively charged phospholipid bilayers of renal parenchymal cell membranes. The study also indicates that the lowering of the pI decreases the renal uptake. In conclusion, this study indicates that the high renal uptake can be optimized by chemical modifications of dsFv and that a small difference in the molecular structure of the acylating agents does not cause an appreciable difference in renal clearance.
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