Lys and Arg in UBI:: A specific site for a stable Tc-99m complex?

The aim of this study was to help establish if ubiquicidin peptide 29-41 fragment (UBI) contains a specific site for Tc-99m labeling by a new direct method under alkaline conditions. Since this peptide does not have cysteine residues, it is possible that neighboring arginine and lysine in the peptide amino acid sequence (Thr-Gly-Arg-Ala-Lys-Arg-Arg-Met-Gln-Tyr-Asn-Arg-Arg) could be a specific coordination site to form a stable Tc-99m-UBI complex. Following direct labeling, the in vitro stability of Tc-99m-UBI was compared to UBI radiolabeled by one indirect method using HYNIC/tricine and HYNIC/tricine/EDDA. Radiochemical purity of Tc-99m-UBI averaged 97% compared to 88% for Tc-99m-HYNIC-UBI/tricine and 98% for Tc-99m-HYNIC-UBI/tricine/EDDA. Both Tc-99m-HYNIC-UBI (tricine or EDDA) and Tc-99m-UBI showed stability in human serum and solutions of cysteine. Tc-99m-UBI radiochemical purity 24 h after dilution in 0.9% NaCl was greater than 90% at pH 9 and greater than 95% at pH 6.5. Under one set of experimental conditions, in vitro binding to bacteria of Tc-99m-UBI was 35% and identical to that of Tc-99m-HYNIC-UBI/tricine and Tc-99m-HYNIC-UBI/tricine/EDDA at 32% and 31% respectively. The biodistribution of Tc-99m-UBI in mice showed a rapid renal clearance. To help identify the site(s) of 99'Tc binding following direct labeling, molecular mechanics and quantum-mechanical calculations were performed which showed that the amine groups of Arg(7) and Lys are the most probable site. The calculations show that these groups can form a square pyramid with two water molecules for the Tc cation (dxysp(3)). It will be necessary to isolate and characterize the Tc-99(V)(O)-UBI (H2O)(n) complex to confirm these results. (C) 2003 Elsevier Inc. All rights reserved.