Lowering of pI by acylation improves the renal uptake of 99mTc-labeled anti-Tac dsFv:: effect of different acylating reagents

Anti-Tac disulfide-stabilized variable region fragment (dsFv) was labeled with Tc-99m by a preformed chelate approach using Tc-99m-MAG3-trifluorophenyl (TFP) ester. Simultaneously it was acylated with TFP-lactate or succinic anhydride to decrease the isoelectric point of dsFv (pI 10). Acylation of dsFv (0.04 mM) with the lactate at a 73 times molar excess reduced the pI to 5.0-6.7, whereas acylation with succinic anhydride at a 30 times molar excess reduced the pI to 4.9-8.7. Comparative biodistribution studies performed in mice (n = 5) showed the reduced renal accumulation of the Tc-99m proportional to the pI reduction. The effect of the pI on the reduced renal uptake was especially pronounced at 15 min postinjection. The reduced renal uptake was also reflected in the reduced whole-body retention, indicating that lowering the pI inhibited the tubular reabsorption of the labeled dsFv. Published by Elsevier Science Inc. All rights reserved.