Plasma protein binding of Tc-99m-labeled hydrazino nicotinamide derivatized polypeptides and peptides

6-Hydrazinopyridine-3-carboxylic acid (HYNIC) constitutes one of the most a ttractive reagents to prepare Tc-99m-labeled polypeptides and peptides of v arious molecular weights in combination with two tricine molecules as colig ands. Indeed, Tc-99m-HYNIC-conjugated IgG showed biodistribution of radioac tivity similar to that of In-111 DTPA-conjugated IgG. However, recent studi es indicated significant plasma protein binding when the Tc-99m labeling pr ocedure was expanded to low molecular weight peptides. In this study, pharm acokinetics of Tc-99m-HYNIC-conjugated IgG, Fab and RC160 using tricine wer e compared with their radioiodinated counterparts to evaluate this Tc-99m-l abeling method. In mice, [Tc-99m](HYNIC-IgG)(tricine)(2) and [Tc-99m](HYNIC -Fab)(tricine)(2) showed persistent localization of radioactivity in tissue s when compared with their I-125-labeled counterparts. [Tc-99m](HYNIC-IgG)( tricine)(2) eliminated from the blood at a rate similar to that of I-125-la beled IgG, while [Tc-99m](HYNIC Fab)(tricine)(2) showed significantly slowe r clearance of the radioactivity than I-125-labeled Fab. On size-exclusion HPLC analyses, little changes were observed in radiochromatograms after inc ubation of [Tc-99m](HYNIC-IgG)(tricine)(2) in murine plasma. However, [Tc-9 9m](HYNIC-Fab)(tricine)(2) and [Tc-99m](HYNIC-RC160)(tricine)(2) demonstrat ed significant increases in the radioactivity in higher molecular weight fr actions in plasma. Formation of higher molecular weight species was reduced when [Tc-99m](HYNIC-RC160)(tricine)(2) was stabilized with nicotinic acid (NIC) to generate [Tc-99m](HYNIC-RC160)(tricine)(NIC). [(TC)-T-99m](HYNIC-R C160)(tricine)(NIC) also demonstrated significantly faster clearance of the radioactivity from the blood than [Tc-99m](HyNIC-RC160)(tricine)(2). These findings suggested that one of the tricine coligands in Tc-99m-HYNIC-label ed (poly)peptides would he replaced with plasma proteins to generate higher molecular weight species that exhibit slow blood clearance. In addition, t he molecular sizes of parental peptides played an important role in the pro gression of the exchange reaction of one of the tricine coligands with plas ma proteins. (C) 2001 Elsevier Science Inc. All rights reserved.