Immunoreactivity of 111In and 131I fibrin-specific monoclonal antibody used for thrombus imaging☆

Abstract Immunoreactivity of radiolabeled F(ab′) 2 fragment of anti-fibrin T2G1s monoclonal antibody was determined by affinity chromatography using fibrin-coated Sepharose. This preparation is useful for thrombus detection in vivo by gamma camera imaging, provided a high percentage of immunoreactivity is retained after labeling. For 111 In labeling, DTPA/F(ab′) 2 molar ratios were varied from 1000 to 6600/1, with little effect on immunoreactivity. Immunoreactivity of the F(ab′) 2 fragment, labeled with imaging doses of 131 I and 111 In, remained high and ranged from 81% to 89%. Equilibrium binding analysis determined the affinity constants of 111 In- or 125 I-labeled T2G1s and its F(ab′) 2 and Fab fragments, to be equivalent ( K d ∼ 3 × 10 −8 M). This suggests that only one binding domain of T2G1s binds to fibrin even though there are two antigenic sites/mol of fibrin.