Abstract: STUDY ON A DIRECT METHOD FOR~(99)Tc~m LABELING OF MONOCLONAL ANTIBODYⅠ.DIRECT LABELING OF Fab(3H11)FRAGMENTS WITH~(99)Tc~mSang Lixin;Wu Yonghui;Liu Bin;Liu Yuanfang(Department of Technical Physics,Peking University,Beijing 100871)Zhang Meiying;Lin Baohe(Beijing Institute for Cancer Research,Beijing 100034)Abstract:A direct method for technetium labeling of monoclonal antibody based on Schwarz method with some amelioration for labeling both intact MoAb and its fragments is investigated.This method employs 1000 molar excess of 2-mercaptoethanol(2-ME):MoAb to reduce a large fraction of disulfide bonds to free sulfhydryl groups,The MoAb is incubated for 30 minutes at room temperature.After an excess of 2-ME has been separated from reduced MoAb by Sephadex G50 centrifugation columns,a 500-1000 molar excess of ascorbic acid(AA):MoAb is introduced to MoAb solution to protect the free-SH against oxidation,Then solutions of MDP and pertechnetate are added to the reduced antibody and left in place to react for 20 minutes at room temperature.The radiolabeling yields have been obtained to be greater than 90％ for intact MoAb and 60％-82％for 3H11 Fab fragments.A high specific activity,0.96MBq/g,has been achieved.The immunoreactivity is retained.Biodistribution of the Tc-labeled antibodies is measured in tumor bearing nude mice.T/N ratios slightly better than those reported previously have been obtained.The data show that this method is applicable to tumor localization.Keywords:Direct labeling,~(99)Tc~m,Fab fragments,Free sulfhydryl groups