TARGETING OF ANTIBODY-CONJUGATED PLASMINOGEN ACTIVATORS TO THE PULMONARY VASCULATURE

Thrombolytic therapy has not been widely used for pulmonary embolism d ue to less than optimal results with conventional plasminogen activato rs. We propose a new approach to deliver plasminogen activators to the luminal surface of the pulmonary vasculature to potentially improve d issolution of pulmonary thromboemboli. Our previous studies have docum ented that a monoclonal antibody (mAb) to angiotensin-converting enzym e (anti-angiotensin-converting enzyme mAb 9B9) accumulates in the lung s of various animal species after systemic administration. We coupled I-125-labeled biotinylated plasminogen activators (single-chain urokin ase plasminogen activator, tissue-type plasminogen activator and strep tokinase) to biotinylated mAb 9B9, using streptavidin as a cross-linke r. The fibrinolytic activity of plasminogen activators was not changed significantly by either biotinylation or by coupling to streptavidin. Antibody-conjugated plasminogen activators bind to the antigen immobi lized in plastic wells and provide lysis of fibrin clots formed in the se wells. Therefore, antibody-conjugated plasminogen activators bound to their target antigen retain their capacity to activate plasminogen. One hour after i.v. injection of mAb 9B9-conjugated radiolabeled biot inylated single-chain urokinase plasminogen activator, biotinylated ti ssue-type plasminogen activator or biotinylated-streptokinase in rats, the level of radiolabel was 7.4 +/- 0.81, 5.9 +/- 0.4 and 3.6 +/- 0.4 % of injected dose/g (ID/g) of lung tissue vs. 0.5 +/- 0.01, 0.3 +/- 0 .01 and 0.6 +/- 0.3% ID/g after injection of the same activators conju gated with control mouse IgG (P < .01 in all cases). Injection of mAb 9B9-conjugated radiolabeled plasminogen activator led to its rapid pul monary uptake with a peak value 6.2 +/- 1.2% ID/g attained 3 hr after injection. One day later, 2.2 +/- 0.5% of the injected radioactivity w as found per gram of lung tissue, although the blood level was 0.13 +/ - 0.03% ID/g (lung/blood ratio 16.7 +/- 0.3). Therefore, conjugation o f plasminogen activators with anti-angiotensin-converting enzyme mAb 9 B9 provides their specific targeting to and prolonged association with the pulmonary vasculature. These results provide a basis for study of the local pulmonary fibrinolysis by mAb 9B9-conjugated plasminogen ac tivators.