Compartmentalized Actions of the Plasminogen Activator Inhibitors, PAI-1 and Nsp, in Ischemic Stroke

Tissue plasminogen activator (tPA) is a multifunctional protease. In blood tPA is best understood for its role in fibrinolysis, whereas in the brain tPA is reported to regulate blood-brain barrier (BBB) function and to promote neurodegeneration. Thrombolytic tPA is used for the treatment of ischemic stroke. However, its use is associated with an increased risk of hemorrhagic transformation. In blood the primary regulator of tPA activity is plasminogen activator inhibitor 1 (PAI-1), whereas in the brain, its primary inhibitor is thought to be neuroserpin (Nsp). In this study, we compare the effects of PAI-1 and Nsp deficiency in a mouse model of ischemic stroke and show that tPA has both beneficial and harmful effects that are differentially regulated by PAI-1 and Nsp. Following ischemic stroke Nsp deficiency in mice leads to larger strokes, increased BBB permeability, and increased spontaneous intracerebral hemorrhage. In contrast, PAI-1 deficiency results in smaller infarcts and increased cerebral blood flow recovery. Mechanistically, our data suggests that these differences are largely due to the compartmentalized action of PAI-1 and Nsp, with Nsp deficiency enhancing tPA activity in the CNS which increases BBB permeability and worsens stroke outcomes, while PAI-1 deficiency enhances fibrinolysis and improves recovery. Finally, we show that treatment with a combination therapy that enhances endogenous fibrinolysis by inhibiting PAI-1 with MDI-2268 and reduces BBB permeability by inhibiting tPA-mediated PDGFRα signaling with imatinib significantly reduces infarct size compared to vehicle-treated mice and to mice with either treatment alone.

Druggable Hot Spots in the Schistosomiasis Cathepsin B1 Target Identified by Functional and Binding Mode Analysis of Potent Vinyl Sulfone Inhibitors

Schistosomiasis, a parasitic disease caused by blood flukes of the genus Schistosoma, is a global health problem with over 200 million people infected. Treatment relies on just one drug, and new chemotherapies are needed. Schistosoma mansoni cathepsin B1 (SmCB1) is a critical peptidase for the digestion of host blood proteins and a validated drug target. We screened a library of peptidomimetic vinyl sulfones against SmCB1 and identified the most potent SmCB1 inhibitors reported to date that are active in the subnanomolar range with second order rate constants (k_2nd) of ∼2 × 10⁵ M^–1 s^–1. High resolution crystal structures of the two best inhibitors in complex with SmCB1 were determined. Quantum chemical calculations of their respective binding modes identified critical hot spot interactions in the S1′ and S2 subsites. The most potent inhibitor targets the S1′ subsite with an N-hydroxysulfonic amide moiety and displays favorable functional properties, including bioactivity against the pathogen, selectivity for SmCB1 over human cathepsin B, and reasonable metabolic stability. Our results provide structural insights for the rational design of next-generation SmCB1 inhibitors as potential drugs to treat schistosomiasis.

Drug Targeting of Plasminogen Activator Inhibitor-1 Inhibits Metabolic Dysfunction and Atherosclerosis in a Murine Model of Metabolic Syndrome

OBJECTIVE

Enhanced expression of PAI-1 (plasminogen activator inhibitor-1) has been implicated in atherosclerosis formation in humans with obesity and metabolic syndrome. However, little is known about the effects of pharmacological targeting of PAI-1 on atherogenesis. This study examined the effects of pharmacological PAI-1 inhibition on atherosclerosis formation in a murine model of obesity and metabolic syndrome. Approach and Results: LDL receptor-deficient (ldlr^-/-) mice were fed a Western diet high in cholesterol, fat, and sucrose to induce obesity, metabolic dysfunction, and atherosclerosis. Western diet triggered significant upregulation of PAI-1 expression compared with normal diet controls. Addition of a pharmacological PAI-1 inhibitor (either PAI-039 or MDI-2268) to Western diet significantly inhibited obesity and atherosclerosis formation for up to 24 weeks without attenuating food consumption. Pharmacological PAI-1 inhibition significantly decreased macrophage accumulation and cell senescence in atherosclerotic plaques. Recombinant PAI-1 stimulated smooth muscle cell senescence, whereas a PAI-1 mutant defective in LRP1 (LDL receptor-related protein 1) binding did not. The prosenescent effect of PAI-1 was blocked by PAI-039 and R2629, a specific anti-LRP1 antibody. PAI-039 significantly decreased visceral adipose tissue inflammation, hyperglycemia, and hepatic triglyceride content without altering plasma lipid profiles.

CONCLUSIONS

Pharmacological targeting of PAI-1 inhibits atherosclerosis in mice with obesity and metabolic syndrome, while inhibiting macrophage accumulation and cell senescence in atherosclerotic plaques, as well as obesity-associated metabolic dysfunction. PAI-1 induces senescence of smooth muscle cells in an LRP1-dependent manner. These results help to define the role of PAI-1 in atherosclerosis formation and suggest a new plasma-lipid-independent strategy for inhibiting atherogenesis.

High-affinity binding of plasminogen-activator inhibitor 1 complexes to LDL receptor-related protein 1 requires lysines 80, 88, and 207

It is well-established that complexes of plasminogen-activator inhibitor 1 (PAI-1) with its target enzymes bind tightly to low-density lipoprotein (LDL) receptor-related protein 1 (LRP1), but the molecular details of this interaction are not well-defined. Furthermore, considerable controversy exists in the literature regarding the nature of the interaction of free PAI-1 with LRP1. In this study, we examined the binding of free PAI-1 and complexes of PAI-1 with low-molecular-weight urokinase-type plasminogen activator to LRP1. Our results confirmed that uPA:PAI-1 complexes bind LRP1 with ∼100-fold increased affinity over PAI-1 alone. Chemical modification of PAI-1 confirmed an essential requirement of lysine residues in PAI-1 for the interactions of both PAI-1 and uPA:PAI-1 complexes with LRP1. Results of surface plasmon resonance measurements supported a bivalent binding model in which multiple sites on PAI-1 and uPA:PAI-1 complexes interact with complementary sites on LRP1. An ionic-strength dependence of binding suggested the critical involvement of two charged residues for the interaction of PAI-1 with LRP1 and three charged residues for the interaction of uPA:PAI-1 complexes with LRP1. An enhanced affinity resulting from the interaction of three regions of the uPA:PAI-1 complex with LDLa repeats on LRP1 provided an explanation for the increased affinity of uPA:PAI-1 complexes for LRP1. Mutational analysis revealed an overlap between LRP1 binding and binding of a small-molecule inhibitor of PAI-1, CDE-096, confirming an important role for Lys-207 in the interaction of PAI-1 with LRP1 and of the orientations of Lys-207, -88, and -80 for the interaction of uPA:PAI-1 complexes with LRP1.

Dual-reporter high-throughput screen for small-molecule in vivo inhibitors of plasminogen activator inhibitor type-1 yields a clinical lead candidate

Plasminogen activator inhibitor type-1 (PAI-1) is a serine protease inhibitor (serpin) implicated in numerous pathological processes, including coronary heart disease, arterial and venous thrombosis, and chronic fibrotic diseases. These associations have made PAI-1 an attractive pharmaceutical target. However, the complexity of the serpin inhibitory mechanism, the inherent metastability of serpins, and the high-affinity association of PAI-1 with vitronectin in vivo have made it difficult to identify pharmacologically effective small-molecule inhibitors. Moreover, the majority of current small-molecule PAI-1 inhibitors are poor pharmaceutical candidates. To this end and to find leads that can be efficiently applied to in vivo settings, we developed a dual-reporter high-throughput screen (HTS) that reduced the rate of nonspecific and promiscuous hits and identified leads that inhibit human PAI-1 in the high-protein environments present in vivo Using this system, we screened >152,000 pure compounds and 27,000 natural product extracts (NPEs), reducing the apparent hit rate by almost 10-fold compared with previous screening approaches. Furthermore, screening in a high-protein environment permitted the identification of compounds that retained activity in both ex vivo plasma and in vivo Following lead identification, subsequent medicinal chemistry and structure-activity relationship (SAR) studies identified a lead clinical candidate, MDI-2268, having excellent pharmacokinetics, potent activity against vitronectin-bound PAI-1 in vivo, and efficacy in a murine model of venous thrombosis. This rigorous HTS approach eliminates promiscuous candidate leads, significantly accelerates the process of identifying PAI-1 inhibitors that can be rapidly deployed in vivo, and has enabled identification of a potent lead compound.

Characterization of a novel class of polyphenolic inhibitors of plasminogen activator inhibitor-1

Plasminogen activator inhibitor type 1, (PAI-1) the primary inhibitor of the tissue-type (tPA) and urokinase-type (uPA) plasminogen activators, has been implicated in a wide range of pathological processes, making it an attractive target for pharmacologic inhibition. Currently available small-molecule inhibitors of PAI-1 bind with relatively low affinity and do not inactivate PAI-1 in the presence of its cofactor, vitronectin. To search for novel PAI-1 inhibitors with improved potencies and new mechanisms of action, we screened a library selected to provide a range of biological activities and structural diversity. Five potential PAI-1 inhibitors were identified, and all were polyphenolic compounds including two related, naturally occurring plant polyphenols that were structurally similar to compounds previously shown to provide cardiovascular benefit in vivo. Unique second generation compounds were synthesized and characterized, and several showed IC(50) values for PAI-1 between 10 and 200 nm. This represents an enhanced potency of 10-1000-fold over previously reported PAI-1 inactivators. Inhibition of PAI-1 by these compounds was reversible, and their primary mechanism of action was to block the initial association of PAI-1 with a protease. Consistent with this mechanism and in contrast to previously described PAI-1 inactivators, these compounds inactivate PAI-1 in the presence of vitronectin. Two of the compounds showed efficacy in ex vivo plasma and one blocked PAI-1 activity in vivo in mice. These data describe a novel family of high affinity PAI-1-inactivating compounds with improved characteristics and in vivo efficacy, and suggest that the known cardiovascular benefits of dietary polyphenols may derive in part from their inactivation of PAI-1.

Novel bis-arylsulfonamides and aryl sulfonimides as inactivators of plasminogen activator inhibitor-1 (PAI-1)

Inactivators of plasminogen activator inhibitor-1 (PAI-1) have been identified as possible treatments for a range of conditions, including atherosclerosis, venous thrombosis, and obesity. We describe the synthesis and inhibitory activity of a novel series of compounds based on bis-arylsulfonamide and aryl sulfonimide motifs that show potent and specific activity towards PAI-1. Inhibitors containing short linking units between the sulfonyl moieties and a 3,4-dihydroxy aryl substitution pattern showed the most potent inhibitory activity, and retained high specificity for PAI-1 over the structurally-related serpin anti-thrombin III (ATIII).

Structure activity studies of a novel cytotoxic benzodiazepine

Analogues of Bz-423, a pro-apoptotic 1,4-benzodiazepine with potent activity in animal models of systemic lupus erythematosus and rheumatoid arthritis, have been designed, synthesized, and evaluated in cell-culture assays. The results of these experiments have defined the structural elements of this new cytotoxic agent required for activity.

Structure-activity relationships for inhibition of cysteine protease activity and development of Plasmodium falciparum by peptidyl vinyl sulfones

The Plasmodium falciparum cysteine proteases falcipain-2 and falcipain-3 appear to be required for hemoglobin hydrolysis by intraerythrocytic malaria parasites. Previous studies showed that peptidyl vinyl sulfone inhibitors of falcipain-2 blocked the development of P. falciparum in culture and exerted antimalarial effects in vivo. We now report the structure-activity relationships for inhibition of falcipain-2, falcipain-3, and parasite development by 39 new vinyl sulfone, vinyl sulfonate ester, and vinyl sulfonamide cysteine protease inhibitors. Levels of inhibition of falcipain-2 and falcipain-3 were generally similar, and many potent compounds were identified. Optimal antimalarial compounds, which inhibited P. falciparum development at low nanomolar concentrations, were phenyl vinyl sulfones, vinyl sulfonate esters, and vinyl sulfonamides with P(2) leucine moieties. Our results identify independent structural correlates of falcipain inhibition and antiparasitic activity and suggest that peptidyl vinyl sulfones have promise as antimalarial agents.