99mTc-neolactosylated human serum albumin for imaging the hepatic asialoglycoprotein receptor

Tc-99m-Labeled diethylenetriaminepentaacetic acid (DTPA)-coupled neogalactosyl human serum albumin (GSA) is used as an imaging agent for asialoglycoprotein receptor of the liver. However, its labeling is inconvenient because it should be incubated for 30 min at 50 degreesC. In addition, the conjugated DTPAs can cause decrease of pI and denaturation of protein. Therefore, we developed an improved agent Tc-99m-neolactosyl human serum albumin (LSA) which contains a terminal galactose. LSA was synthesized by conjugating lactose to human serum albumin by the formation of a Schiff's base and successive reduction with sodium cyanoborohydride. The number of conjugated lactose molecules per LSA was 40.7 +/- 12.3. To simplify the labeling procedure, we used a direct labeling method that adopts a high affinity Tc-99m binding site concept in antibody labeling. The produced LSA was reduced by beta-mercaptoethanol to generate sulfhydryl groups and purified by PD-10 size-exclusion column. The number of generated sulfhydryl groups per LSA was 21.9 +/- 3.0. Medronate and stannous chloride were added to the reduced LSA and freeze-dried. Finally, Tc-99m-pertechnetate (37 MBq, 1 mL) was added to the vial and incubated for 10 min at room temperature. The labeling efficiency of Tc-99m-LSA was higher than 98%, and the stability in human serum at 37 degreesC for 24 h was over 90%. Biodistribution study using balb/c mice and imaging study using SD rats showed high initial liver uptake and slow increase in the intestine due to hepatobiliary excretion after metabolism in the hepatocytes. Negligible spleen uptake was found while Tc-99m-tin colloid showed significant amount of spleen uptake due to reticuloendothelial uptake. In conclusion, an improved agent, Tc-99m-LSA, for imaging asialoglycoprotein receptor of the liver was successfully developed which showed a simple labeling procedure, high labeling efficiency, high stability, and high initial liver uptake.