Elevated active plasminogen activator inhibitor 1 (PAI-1) has an adverse effect on the outcomes of intrapleural fibrinolytic therapy (IPFT) in tetracycline-induced pleural injury in rabbits. In order to enhance IPFT with prourokinase (scuPA), two mechanistically distinct approaches to targeting PAI-1 were tested: slowing its reaction with urokinase (uPA) and mAb-mediated PAI-1 inactivation. Removing positively charged residues at the "PAI-1 docking site" (¹⁷⁹RHRGGS¹⁸⁴->¹⁷⁹AAAAAA¹⁸⁴) of uPA results in a 60-fold decrease in the rate of inhibition by PAI-1. Mutant prourokinase (0.0625-0.5mg/kg; n=12) showed efficacy comparable to wt-scuPA and did not change IPFT outcomes (p>0.05). Notably, the rate of PAI-1 independent intrapleural inactivation of the mutant uPA was two times higher (p<0.05) than that of the wt-enzyme. Trapping PAI-1 in a "molecular sandwich" type complex with catalytically inactive S195A-tcuPA (0.1 and 0.5mg/kg) did not improve the efficacy of IPFT with scuPA (0.0625-0.5mg/kg; n=11). IPFT failed in the presence of MA-56A7C10 (0.5mg/kg; n=2), which forms a stable intrapleural "molecular sandwich" complex, allowing active PAI-1 to accumulate by blocking its transition to a latent form. In contrast, inactivation of PAI-1 by accelerating the active-to-latent transition mediated by mAb MA-33B8 (0.5mg/kg; n=2) improved the efficacy of IPFT with scuPA (0.25mg/kg). Thus, under conditions of slow (4-8h) fibrinolysis in tetracycline-induced pleural injury in rabbits, only the inactivation of PAI-1, but not a decrease in the rate of its reaction with uPA, enhances IPFT. Therefore, the rate of fibrinolysis, which varies in different pathologic states, could affect the selection of PAI-1 inhibitors to enhance fibrinolytic therapy.