Labeling proteins with Tc-99m via hydrazinonicotinamide (HYNIC): optimization of the conjugation reaction

At present there is considerable interest in labeling peptides with Tc-99m for the development of target specific radiopharmaceuticals for imaging purposes. In the present study the conjugation of the bifunctional coupling agent succinimidyl-hydrazinonicotinamide (S-HYNIC) was studied and optimized in a series of peptides [molecular weight (MW) 6.5–14.3 kDa]. Aprotinin (MW 6.5 kDa), cytochrome C (MW 12.4 kDa), α-lactalbumin (MW 14.2 kDa), and lysozyme (MW 14.3 kDa) were conjugated with S-HYNIC via the ϵ amino groups of their lysine residues. The effects of molar conjugation ratio, reaction temperature, pH, and protein concentration were studied. Reaction products were analyzed both with respect to the HYNIC-substitution ratio (spectrophotometrically) as well as to the labeling efficiency silica gel-instant thin layer chromatography (SG-ITLC) and molecular size fast performance liquid chromatography (FPLC). The effects of conjugation on biological activity were studied in three proteins binding to receptors on leukocytes: interleukin-8 (MW 8.5 kDa), interleukin-1α (MW 17 kDa), and interleukin-1 receptor antagonist (MW 17 kDa). The labeling efficiency of aprotinin, cytochrome c, α-lactalbumin, and lysozyme conjugated under optimal conjugation conditions exceeded 90%. Specific activities obtained were up to 7.5 MBq/μg. Conjugation was most efficient at 0°C (as compared to 20 and 40°C), at pH 8.2 (as compared to 6.0, 7.2, and 9.5), and at protein concentrations ≥ 2.5 mg/mL. In general, efficiency increased with increasing molar conjugation ratio (protein-HYNIC-ratio 1:3 < 1:6 < 1:15 < 1:30). For the receptor binding proteins, biological activity was preserved only under the mildest conjugation conditions. For each of these proteins an inverse relation between labeling efficiency and receptor binding capacity was found. Labeling proteins with 99mTc using S-HYNIC is easy, rapid, and efficient, and preparations with high specific activity can be obtained. However, biological activity of proteins may be lost at high HYNIC-substitution ratios. With the proteins tested here a careful balancing of reaction conditions resulted in acceptable, although suboptimal, labeling efficiencies and preservation of biological activity.