Thrombolysis by a fibrin-specific antibody Fab'-urokinase conjugate.

The unidirectional crosslinking agent N-succinimidyl 3-(2-pyridyldithio)propionate (SPDP) was used to covalently couple fibrin-specific antibody Fab' to low-molecular-weight urokinase by means of the sulfhydryl groups of the inter-heavy-chain disulfide bonds. At enzymatic activities equal with respect to the substrate pyroglutamyl-glycyl-arginyl-p-nitroanilide-hydrochloride, the Fab'-urokinase conjugate lysed fibrin monomer linked to sepharose with a potency 95 times that of urokinase (which is essentially identical to the potency previously reported for an intact antibody-urokinase conjugate). The Fab'-urokinase conjugate also lysed human thrombi in fresh citrated plasma with a maximal potency 4.4 times that of uncoupled urokinase. These findings indicate that univalent antibody binding is sufficient to enhance fibrinolysis and that the increased potency of antifibrin-urokinase conjugates can also be demonstrated when cross-linked thrombi are used as substrate in the presence of plasma.