Evaluation of PAI-039 [{1-benzyl-5-[4-(trifluoromethoxy)phenyl]-1H-indol-3-yl}(oxo)acetic acid], a novel plasminogen activator inhibitor-1 inhibitor, in a canine model of coronary artery thrombosis

Specific inhibition on PAI-1 reduces the dose of Alteplase for ischemic stroke treatment

Administration of recombinant tPA (rtPA, or trade name Alteplase®) is an FDA-approved therapy for acute ischemic stroke (AIS), but poses the risk of hemorrhagic complications. Recombinant tPA can be rapidly inactivated by the endogenous inhibitor, plasminogen activator inhibitor 1 (PAI-1). In this work, we study a novel treatment approach that combines a PAI-1 inhibitor, PAItrap4, with a reduced dose of rtPA to address the hemorrhagic concern of rtPA. PAItrap4 is a highly specific and very potent protein-based inhibitor of PAI-1, comprising of a variant of uPA serine protease domain, human serum albumin, and a cyclic RGD peptide. PAItrap4 efficiently targets and inhibits PAI-1 on activated platelets, and also possesses a long half-life in vivo. Our results demonstrate that PAItrap4 effectively counteracts the inhibitory effects of PAI-1 on rtPA, preserving rtPA activity based on amidolytic and clot lysis assays. In an in vivo murine stroke model, PAItrap4, together with low-dose rtPA, enhances the blood perfusion in the stroke-affected areas, reduces infarct size, and promotes neurological recovery in mice. Importantly, such treatment does not increase the amount of cerebral hemorrhage, thus reducing the risk of cerebral hemorrhage. In addition, PAItrap4 does not compromise the normal blood coagulation function in mice, demonstrating its safety as a therapeutic agent. These findings highlight this combination therapy as a promising alternative for the treatment of ischemic stroke, offering improved safety and efficacy.

Elevated postischemic tissue injury and leukocyte-endothelial adhesive interactions in mice with global deficiency in caveolin-2: role of PAI-1

Ischemia/reperfusion (I/R)-induced rapid inflammation involving activation of leukocyte-endothelial adhesive interactions and leukocyte infiltration into tissues is a major contributor to postischemic tissue injury. However, the molecular mediators involved in this pathological process are not fully known. We have previously reported that caveolin-2 (Cav-2), a protein component of plasma membrane caveolae, regulated leukocyte infiltration in mouse lung carcinoma tumors. The goal of the current study was to examine if Cav-2 plays a role in I/R injury and associated acute leukocyte-mediated inflammation. Using a mouse small intestinal I/R model, we demonstrated that I/R downregulates Cav-2 protein levels in the small bowel. Further study using Cav-2-deficient mice revealed aggravated postischemic tissue injury determined by scoring of villi length in H&E-stained tissue sections, which correlated with increased numbers of MPO-positive tissue-infiltrating leukocytes determined by IHC staining. Intravital microscopic analysis of upstream events relative to leukocyte transmigration and tissue infiltration revealed that leukocyte-endothelial cell adhesive interactions in postcapillary venules, namely leukocyte rolling and adhesion were also enhanced in Cav-2-deficient mice. Mechanistically, Cav-2 deficiency increased plasminogen activator inhibitor-1 (PAI-1) protein levels in the intestinal tissue and a pharmacological inhibition of PAI-1 had overall greater inhibitory effect on both aggravated I/R tissue injury and enhanced leukocyte-endothelial interactions in postcapillary venules in Cav-2-deficient mice. In conclusion, our data suggest that Cav-2 protein alleviates tissue injury in response to I/R by dampening PAI-1 protein levels and thereby reducing leukocyte-endothelial adhesive interactions.NEW & NOTEWORTHY The role of caveolin-2 in regulating ischemia/reperfusion (I/R) tissue injury and the mechanisms underlying its effects are unknown. This study uses caveolin-2-deficient mouse and small intestinal I/R injury models to examine the role of caveolin-2 in the leukocyte-dependent reperfusion injury. We demonstrate for the first time that caveolin-2 plays a protective role from the I/R-induced leukocyte-dependent reperfusion injury by reducing PAI-1 protein levels in intestinal tissue and leukocyte-endothelial adhesive interactions in postcapillary venules.

PAI-1, the Plasminogen System, and Skeletal Muscle

The plasminogen system is a critical proteolytic system responsible for the remodeling of the extracellular matrix (ECM). The master regulator of the plasminogen system, plasminogen activator inhibitor-1 (PAI-1), has been implicated for its role in exacerbating various disease states not only through the accumulation of ECM (i.e., fibrosis) but also its role in altering cell fate/behaviour. Examination of PAI-1 has extended through various tissues and cell-types with recent investigations showing its presence in skeletal muscle. In skeletal muscle, the role of this protein has been implicated throughout the regeneration process, and in skeletal muscle pathologies (muscular dystrophy, diabetes, and aging-driven pathology). Needless to say, the complete function of this protein in skeletal muscle has yet to be fully elucidated. Given the importance of skeletal muscle in maintaining overall health and quality of life, it is critical to understand the alterations—particularly in PAI-1—that occur to negatively impact this organ. Thus, we provide a comprehensive review of the importance of PAI-1 in skeletal muscle health and function. We aim to shed light on the relevance of this protein in skeletal muscle and propose potential therapeutic approaches to aid in the maintenance of skeletal muscle health.

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.

Effects of a high-fat diet on spontaneous metastasis of Lewis lung carcinoma in plasminogen activator inhibitor-1 deficient and wild-type mice

This study investigated the effects of a high-fat diet on spontaneous metastasis of Lewis lung carcinoma (LLC) in plasminogen activator inhibitor-1 deficient (PAI-1^−/−) and wild-type mice. The high-fat diet increased the number of pulmonary metastases by 60% (p<0.01), tumor cross-sectional area by 82% (p<0.05) and tumor volume by 130% (p<0.05) compared to the AIN93G diet. Deficiency in PAI-1 reduced the number of metastases by 35% (p<0.01) compared to wild-type mice. In mice fed the high-fat diet, PAI-1 deficiency reduced tumor cross-sectional area by 52% (p<0.05) and tumor volume by 61% (p<0.05) compared to their wild-type counterparts; however, PAI-1 deficiency affected neither area nor volume in mice fed the AIN93G diet. Adipose and plasma concentrations of PAI-1 were significantly higher in high-fat fed wild-type mice than in their AIN93G-fed counterparts. Adipose and plasma PAI-1 were not detectable in PAI-1^−/− mice regardless of the diet. Mice deficient in PAI-1 showed significantly greater plasma concentrations of monocyte chemotactic protein-1, tumor necrosis factor-α, leptin, vascular endothelial growth factor, tissue inhibitor of metalloproteinase-1 and insulin compared to wild-type mice, indicating a compensatory overproduction of inflammatory cytokines, angiogenic factors and insulin in the absence of PAI-1. We conclude that PAI-1 produced by the host, including that by adipose tissue, promotes high-fat enhanced metastasis of LLC.

Inhibition of PAI-1 antiproteolytic activity against tPA by RNA aptamers

Plasminogen activator inhibitor-1 (PAI-1; SERPINE1) inhibits the plasminogen activators: tissue-type plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA). Elevated levels of PAI-1 have been correlated with an increased risk for cardiovascular disease. Pharmacologically suppressing PAI-1 might prevent, or successfully treat PAI-1 related vascular diseases. This can potentially be accomplished by using small RNA molecules (aptamers). This study's goal is to develop RNA aptamers to a region of PAI-1 that will prevent the ability of PAI-1 to interact with the plasminogen activators. The aptamers were generated through a systematic evolution of ligands via exponential enrichment approach that ensures the creation of RNA molecules that bind to our target protein, PAI-1. In vitro assays were used to determine the effect of these aptamers on PAI-1's inhibitory activity. Three aptamers that bind to PAI-1 with affinities in the nanomolar range were isolated. The aptamer clones R10-4 and R10-2 inhibited PAI-1's antiproteolytic activity against tPA and disrupted PAI-1's ability to form a stable covalent complex with tPA. Increasing aptamer concentrations correlated positively with an increase in cleaved PAI-1. To the best of our knowledge, this is the first report of RNA molecules that inhibit the antiproteolytic activity of PAI-1.

Targeting plasminogen activator inhibitor-1 inhibits angiogenesis and tumor growth in a human cancer xenograft model

Cancers of the urinary bladder result in aggressive and highly angiogenic tumors for which standard treatments have only limited success. Patients with advanced disease have a 5-year survival rate of less than 20%, and no new anticancer agent has been successfully introduced into the clinic armamentarium for the treatment of bladder cancer in more than 20 years. Investigations have identified plasminogen activator inhibitor-1 (PAI-1), a serine protease inhibitor, as being highly expressed in several malignancies, including bladder cancer, in which high expression is associated with a poor prognosis. In this study, we evaluated PAI-1 as a potential therapeutic target for bladder cancer. PAI-1 expression was manipulated in a panel of cell lines and functional inhibition was achieved using the small molecule tiplaxtinin. Reduction or inhibition of PAI-1 resulted in the reduction of cellular proliferation, cell adhesion, and colony formation, and the induction of apoptosis and anoikis in vitro. Treatment of T24 xenografts with tiplaxtinin resulted in inhibition of angiogenesis and induction of apoptosis, leading to a significant reduction in tumor growth. Similar results were obtained through evaluation of the human cervical cancer HeLa cell line, showing that PAI-1-mediated effects are not restricted to tumor cells of bladder origin. Collectively, these data show that targeting PAI-1 may be beneficial and support the notion that novel drugs such as tiplaxtinin could be investigated as anticancer agents.

TM5275 prolongs secreted tissue plasminogen activator retention and enhances fibrinolysis on vascular endothelial cells

INTRODUCTION

Elevated plasminogen activator inhibitor-1 (PAI-1) reduces fibrinolytic potential in plasma, contributing to thrombotic disease. Thus, inhibiting PAI-1 activity is clinically desirable. We recently demonstrated that tissue plasminogen activator (tPA) remains on the surface of vascular endothelial cells (VECs) after secretion in a heavy-chain dependent manner, which is essential for high fibrinolytic activity on the surface of VECs, and that PAI-1 dissociates retained tPA from the cell surface as a result of high-molecular weight complex formation. Based on the model whereby amounts of tPA and its equilibrium with PAI-1 dynamically change after exocytosis, we examined how TM5275, a newly synthesized small molecule PAI-1 inhibitor, modulated tPA retention and VEC surface-derived fibrinolytic activity using microscopic techniques.

MATERIALS AND METHODS

The effects of TM5275 on the kinetics of the secretion and retention of green fluorescent protein (GFP)-tagged tPA (tPA-GFP) on VECs were analyzed using total internal reflection fluorescence microscopy. The effects of TM5275 on the generation of plasmin activity were evaluated by both plasminogen accumulation and fibrin clot lysis on tPA-GFP-expressing VECs using confocal laser scanning microscopy.

RESULTS

TM5275 at concentrations of 20 and 100 μM significantly prolonged the retention of tPA-GFP on VECs by inhibiting tPA-GFP-PAI-1 high-molecular-weight complex formation. TM5275 enhanced the time-dependent accumulation of plasminogen as well as the dissolution of fibrin clots on and around the tPA-GFP-expressing cells.

CONCLUSIONS

The profibrinolytic effects of TM5275 were clearly demonstrated by the prolongation of tPA retention and enhancement of plasmin generation on the VEC surface as a result of PAI-1 inhibition.

Protumorigenic activity of plasminogen activator inhibitor-1 through an antiapoptotic function

BACKGROUND

Plasminogen activator inhibitor-1 (PAI-1) is a protease inhibitor but is paradoxically associated with poor outcomes in cancer patients. However, the mechanisms of its effects on tumor cells have not been explored.

METHODS

Endogenous PAI-1 in human tumor cell lines (HT-1080, A549, HCT-116, and MDA-MB-231) was suppressed by small interfering RNAs (siRNAs) and PAI-039, a small molecule inhibitor of PAI-1, and the effects on apoptosis were examined. Tumorigenicity of PAI-1 knockdown (KD) tumor cells was examined in immunodeficient PAI-1 wild-type and knockout (KO) mice (9-15 per group), and event-free survival was analyzed by the Kaplan-Meier method. The effect of PAI-1 suppression on HT-1080 xenotransplanted tumors was evaluated for cell proliferation, apoptosis, and angiogenesis. All statistical tests were two-sided.

RESULTS

Genetic and pharmacological inhibition of PAI-1 in the four tumor cell lines increased spontaneous apoptosis (mean fold increase relative to control: HT-1080, siRNA#1, mean = 4.0, 95% CI = 2.6 to 5.3, P < .001; siRNA#2, mean = 2.6, 95% CI = 2.4 to 2.9, P < .001, Student t test), which was blocked in the presence of recombinant PAI-1, a caspase-8 inhibitor, or Fas/FasL neutralizing antibodies and was partially attenuated by a plasmin inhibitor-aprotinin. PAI-1 KO mice implanted with PAI-1 KD HT-1080 cells had decreased tumorigenesis and prolonged survival compared with control mice (P = .002, log-rank test), and their tumors exhibited decreased cell proliferation and angiogenesis and increased apoptosis. Furthermore, five of 15 PAI-1 KO mice implanted with PAI-1 KD HT-1080 cells never developed tumors.

CONCLUSIONS

These data suggest that PAI-1 exerts a protective effect against tumor cell apoptosis by a mechanism that, in part, involves plasmin activation and inhibition of Fas/Fas-L-mediated apoptosis and may be a promising therapeutic target.

Therapeutic value of small molecule inhibitor to plasminogen activator inhibitor-1 for lung fibrosis

Fibrosis is a final stage of many lung diseases, with no effective treatment. Plasminogen activator inhibitor-1 (PAI-1), a primary inhibitor of tissue-type and urokinase-type plasminogen activators (tPA and uPA, respectively), plays a critical role in the development of fibrosis. In this study, we explored the therapeutic potential of an orally effective small molecule PAI-1 inhibitor, TM5275, in a model of lung fibrosis induced by transforming growth factor-β1 (TGF-β1), the most potent and ubiquitous profibrogenic cytokine, and in human lung fibroblasts (CCL-210 cells). The results show that an intranasal instillation of AdTGF-β1(223/225), an adenovirus expressing constitutively active TGF-β1, increased the expression of PAI-1 and induced fibrosis in murine lung tissue. On the other hand, treating mice with 40 mg/kg of TM5275 for 10 days, starting 4 days after the instillation of AdTGF-β1(223/225), restored the activities of uPA and tPA and almost completely blocked TGF-β1-induced lung fibrosis, as shown by collagen staining, Western blotting, and the measurement of hydroxyproline. No loss of body weight was evident under these treatment conditions with TM5275. Furthermore, we show that TM5275 induced apoptosis in both myofibroblasts (TGF-β1-treated) and naive (TGF-β1-untreated) human lung fibroblasts, and this apoptosis was associated with the activation of caspase-3/7, the induction of p53, and the inhibition of α-smooth muscle actin, fibronectin, and PAI-1 expression. Such an inhibition of fibrotic responses by TM5275 occurred even in cells pretreated with TGF-β1 for 6 hours. Together, the results suggest that TM5275 is a relatively safe and potent antifibrotic agent, with therapeutic potential in fibrotic lung disease.

Clinical utility of level-of-evidence-1 disease forecast cancer biomarkers uPA and its inhibitor PAI-1

The prognostic and/or predictive value of the cancer biomarkers, urokinase-type plasminogen activator (uPA) and its inhibitor (plasminogen activator inhibitor [PAI]-1), determined by ELISA in tumor-tissue extracts, was demonstrated for several cancer types in numerous clinically relevant retrospective or prospective studies, including a multicenter breast cancer therapy trial (Chemo-N0). Consequently, for the first time ever for any cancer biomarker for breast cancer, uPA and PAI-1 have reached the highest level of evidence, level-of-evidence-1. At present, two other breast cancer therapy trials, NNBC-3 and Plan B, also incorporating uPA and PAI-1 as treatment-assignment tools are in effect. Furthermore, small synthetic molecules targeting uPA are currently in Phase II clinical trials in patients afflicted with advanced cancer of the ovary, breast or pancreas.

Effects of specific chemical suppressors of plasminogen activator inhibitor-1 in cardiovascular diseases

INTRODUCTION

plasminogen activator inhibitor-1 (PAI-1) is critical in thrombus formation and inflammation. Although these are essential pathological features of cardiovascular diseases, the effects of PAI-1 inhibition against the development of cardiovascular remodeling have not been well studied.

AREAS COVERED

the review explores the therapeutic value of PAI-1 in the progression of various cardiovascular diseases. To date, the authors have reported that a novel PAI-1 inhibitor suppressed the development of experimental autoimmune myocarditis, vascular remodeling after arterial injury, and heart transplant rejection using rodent models. Pathologically, the PAI-1 inhibitor improved histological remodeling of myocardium and arteries with suppression of inflammation and thrombus formation.

EXPERT OPINION

PAI-1 inhibitors appear to exhibit potent effects on the prevention of adverse tissue remodeling. However, PAI-1 is a multifunctional protein and more research is needed to further elucidate the association between PAI-1 expression and cardiovascular disease.

Development of inhibitors of plasminogen activator inhibitor-1

Plasminogen activator inhibitor-1 (PAI-1) belongs to the serine protease inhibitor super family (serpin) and is the primary inhibitor of both the tissue-type (tPA) and urokinase-type (uPA) plasminogen activators. PAI-1 has been implicated in a wide range of pathological processes where it may play a direct role in a variety of diseases. These observations have made PAI-1 an attractive target for small molecule drug development. However, PAI-1's structural plasticity and its capacity to interact with multiple ligands have made the identification and development of such small molecule PAI-1 inactivating agents challenging. In the following pages, we discuss the difficulties associated with screening for small molecule inactivators of PAI-1, in particular, and of serpins, in general. We discuss strategies for high-throughput screening (HTS) of chemical and natural product libraries, and validation steps necessary to confirm identified hits. Finally, we describe steps essential to confirm specificity of active compounds, and strategies to examine potential mechanisms of compound action.

Targeted inhibition of the serotonin 5HT2A receptor improves coronary patency in an in vivo model of recurrent thrombosis

BACKGROUND

Release of serotonin and activation of serotonin 5HT2A receptors on platelet surfaces is a potent augmentative stimulus for platelet aggregation. However, earlier-generation serotonin receptor antagonists were not successfully exploited as antiplatelet agents, possibly owing to their lack of specificity for the 5HT2A receptor subtype.

OBJECTIVE

To assess whether targeted inhibition of the serotonin 5HT2A receptor attenuates recurrent thrombosis and improves coronary patency in an in vivo canine model mimicking unstable angina.

METHODS

In protocol 1, anesthetized dogs were pretreated with a novel, selective inverse agonist of the 5HT2A receptor (APD791) or saline. Recurrent coronary thrombosis was then initiated by coronary artery injury+stenosis, and coronary patency was monitored for 3 h. Protocol 2 was similar, except that: (i) treatment with APD791 or saline was begun 1 h after the onset of recurrent thrombosis; (ii) template bleeding time was measured; and (iii) blood samples were obtained for in vitro flow cytometric assessment of platelet responsiveness to serotonin.

RESULTS

APD791 attenuated recurrent thrombosis, irrespective of the time of treatment: in both protocols, flow-time area (index of coronary patency; normalized to baseline coronary flow) averaged 58-59% (P<0.01) following administration of APD791 vs. 21-28% in saline controls. Moreover, the in vivo antithrombotic effect of APD791 was not accompanied by increased bleeding, but was associated with significant and selective inhibition of serotonin-mediated platelet activation.

CONCLUSION

5HT2A receptor inhibition with APD791, even when initiated after the onset of recurrent thrombosis, improves coronary patency in the in vivo canine model.

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).

Effects of pharmacological inhibition and genetic deficiency of plasminogen activator inhibitor-1 in radiation-induced intestinal injury

PURPOSE

To investigate effects of plasminogen activator inhibitor 1 (PAI-1) genetic deficiency and pharmacological PAI-1 inhibition with PAI-039 in a mouse model of radiation-induced enteropathy.

METHODS AND MATERIALS

Wild-type (Wt) and PAI-1(-/-) knockout mice received a single dose of 19 Gy to an exteriorized localized intestinal segment. Sham and irradiated Wt mice were treated orally with 1 mg/g of PAI-039. Histological modifications were quantified using a radiation injury score. Moreover, intestinal gene expression was monitored by real-time PCR.

RESULTS

At 3 days after irradiation, PAI-039 abolished the radiation-induced increase in the plasma active form of PAI-1 and limited the radiation-induced gene expression of transforming growth factor beta1 (TGF-beta1), CTGF, PAI-1, and COL1A2. Moreover, PAI-039 conferred temporary protection against early lethality. PAI-039 treatment limited the radiation-induced increase of CTGF and PAI-1 at 2 weeks after irradiation but had no effect at 6 weeks. Radiation injuries were less severe in PAI-1(-/-) mice than in Wt mice, and despite the beneficial effect, 3 days after irradiation, PAI-039 had no effects on microscopic radiation injuries compared to untreated Wt mice.

CONCLUSIONS

A genetic deficiency of PAI-1 is associated with amelioration of late radiation enteropathy. Pharmacological inhibition of PAI-1 by PAI-039 positively impacts the early, acute phase increase in plasma PAI-1 and the associated radiation-induced gene expression of inflammatory/extracellular matrix proteins. Since PAI-039 has been shown to inhibit the active form of PAI-1, as opposed to the complete loss of PAI-1 in the knockout animals, these data suggest that a PAI-1 inhibitor could be beneficial in treating radiation-induced tissue injury in acute settings where PAI-1 is elevated.

The role of plasminogen activator inhibitor 1 in renal and cardiovascular diseases

The 50 kDa glycoprotein plasminogen activator inhibitor 1 (PAI-1) is the major physiological inhibitor of tissue-type and urokinase-type plasminogen activator. These two molecules convert inactive plasminogen into its fibrin-degrading form, plasmin. Plasma and tissue concentrations of PAI-1 are extremely low under normal circumstances but increase under pathologic conditions. This increase is mediated by many factors, including reactive oxygen species. Increased PAI-1 activity is associated with an increased risk of ischemic cardiovascular events and tissue fibrosis. Whereas the antifibrinolytic property of PAI-1 derives mainly from its inhibition of serine proteases, its profibrotic actions seem to derive from a capacity to stimulate interstitial macrophage recruitment and increase transcription of profibrotic genes, as well as from inhibition of serine proteases. Despite studies in mice that lack or overexpress PAI-1, the biological effects of this molecule in humans remain incompletely understood because of the complexity of the PAI-1-plasminogen-activator-plasmin system. The cardioprotective and renoprotective properties of some currently available drugs might be attributable in part to inhibition of PAI-1. The development of an orally active, high-affinity PAI-1 inhibitor will provide a potentially important pharmacological tool for further investigation of the role of PAI-1 and might offer a novel therapeutic strategy in renal and cardiovascular diseases.

Effect of tiplaxtinin (PAI-039), an orally bioavailable PAI-1 antagonist, in a rat model of thrombosis

OBJECTIVE

To assess the antithrombotic and profibrinolytic effects of tiplaxtinin (PAI-039), an orally bioavailable antagonist of PAI-1, in rat models of thrombosis.

METHODS AND RESULTS

Carotid artery and vena cava vascular injury was produced by application of FeCl3 and blood flow was monitored using ultrasonic technology. To assess efficacy in a thrombosis prevention paradigm, PAI-039 was administered orally 90 min before injury (1-30 mg kg(-1)). To assess efficacy in a thrombosis treatment paradigm, vascular injury and stable thrombus formation were followed 4 h later by recovery and PAI-039 administration. PAI-039 prevented carotid artery occlusion in 20, 68 and 60% of animals pretreated with 0.3, 1.0 and 3.0 mg kg(-1), respectively. Time to occlusive thrombosis was increased from 18.2 +/- 4.6 min in controls to 32.5 +/- 8.7 (P = ns), 46.1 +/- 7.0 (P < 0.05), and 41.6 +/- 11.3 min (P < 0.05) in the respective PAI-039 treatment groups. In the vena cava protocol, PAI-039 pretreatment significantly reduced thrombus weight at PAI-039 doses of 3, 10 and 30 mg kg(-1). When PAI-039 was dosed in a treatment paradigm 4 h after stable arterial and venous thrombosis, a significant reduction in thrombus weight was observed 24 h later at PAI-039 doses of 3, 10 and 30 mg kg(-1). PAI-039 (10, 30 and 100 mg kg(-1)) had no effect on platelet aggregation in response to ADP or collagen and was not associated with increased bleeding or prolonged prothrombin time. In animals bearing no vascular injury, PAI-039 had no effect on circulating, low-levels of PAI-1 activity. In contrast, circulating PAI-1 activity increased 5-fold following the induction of vascular injury, which was completely neutralized by PAI-039.

CONCLUSIONS

PAI-039 exerts antithrombotic efficacy in rat models of arterial and venous vascular injury without effecting platelet aggregation.

The effects of pharmacological PAI-1 inhibition on thrombus formation and neointima formation after arterial injury

OBJECTIVE

Plasminogen activator inhibitor (PAI)-1 plays a role in neointimal formation after percutaneous coronary intervention (PCI), the effect of overexpression or lack of PAI-1 is controversial. Murine arterial injury models develop neointimal hyperplasia similar to that observed in clinical coronary arterial restenosis after PCI.

METHODS AND RESULTS

To clarify the role of PAI-1 in thrombus formation and neointimal formation after arterial injury, we used a specific PAI-1 inhibitor (IMD-1622) in a rat aorta-vein shunt model and a mouse arterial injury model. While the non-treated shunt model showed massive thrombus formation, IMD-1622 administration suppressed this. Injured arteries with vehicles showed significant neointimal formation with enhancement of adhesion molecules, fibrinogen accumulation and cell proliferation on day 28 after injury. However, intimal thickening and expression of these factors were suppressed in PAI-1 recipients.

CONCLUSION

A specific PAI-1 inhibitor prevents thrombus formation and arterial neointimal formation after arterial injury. Thus, PAI-1 plays a critical role in arterial remodeling after mechanical injury. PAI-1 regulation may be useful to prevent thrombus and neointimal formation after PCI.

Enhanced clearance of Abeta in brain by sustaining the plasmin proteolysis cascade

The amyloid hypothesis states that a variety of neurotoxic beta-amyloid (Abeta) species contribute to the pathogenesis of Alzheimer's disease. Accordingly, a key determinant of disease onset and progression is the appropriate balance between Abeta production and clearance. Enzymes responsible for the degradation of Abeta are not well understood, and, thus far, it has not been possible to enhance Abeta catabolism by pharmacological manipulation. We provide evidence that Abeta catabolism is increased after inhibition of plasminogen activator inhibitor-1 (PAI-1) and may constitute a viable therapeutic approach for lowering brain Abeta levels. PAI-1 inhibits the activity of tissue plasminogen activator (tPA), an enzyme that cleaves plasminogen to generate plasmin, a protease that degrades Abeta oligomers and monomers. Because tPA, plasminogen and PAI-1 are expressed in the brain, we tested the hypothesis that inhibitors of PAI-1 will enhance the proteolytic clearance of brain Abeta. Our data demonstrate that PAI-1 inhibitors augment the activity of tPA and plasmin in hippocampus, significantly lower plasma and brain Abeta levels, restore long-term potentiation deficits in hippocampal slices from transgenic Abeta-producing mice, and reverse cognitive deficits in these mice.

S35225 is a direct inhibitor of Plasminogen Activator Inhibitor type-1 activity in the blood

The increased risk of thrombotic events associated with disease states such as diabetes and hypertension has been correlated with elevated circulating levels of Plasminogen Activator Inhibitor type-1 (PAI-1). In the present study we evaluate the benzothiophene derivative S35225 in comparison with two recently described inhibitors of PAI-1 activity Tiplaxtinin and WAY140312 on a panel of PAI-1 activity assays in vitro and in vivo. In a direct chromogenic assay, S35225 has an IC50 value of 44+/-0.9 microM similar to that of Tiplaxtinin (34+/-7 microM) and of WAY140312 (39+/-1 microM). In a clot lysis assay however, S35225 has a significantly lower IC50 value than Tiplaxtinin and WAY140312 (0.6+/-0.3 versus 22+/-5 and 16+/-2 microM respectively). Using a tPA capture assay to quantify active PAI-1 in rat or human plasma, neither WAY140312, nor Tiplaxtinin attained 50% inhibition of PAI-1 activity at the highest concentration tested (1 mM); S35225 has an IC50 value of 194+/-30 microM against active rat PAI-1 and 260+/-41 microM against active human PAI-1. The ability of the compounds to inhibit endogenous active PAI-1 in the rat following intravenous administration was also tested using the tPA capture assay. Only S35225 reduced circulating active PAI-1 levels in vivo (maximum inhibition of 76+/-5% at 10 mg/kg and 53+/-5% at 3 mg/kg). In contrast to Tiplaxtinin and WAY140312, S35225 is a direct inhibitor of PAI-1 activity in vitro in rat and human plasmas where vitronectin is constitutively present as well as in vivo in the blood after an intravenous administration in the rat.

Mechanism of inactivation of plasminogen activator inhibitor-1 by a small molecule inhibitor

The inactivation of plasminogen activator inhibitor-1 (PAI-1) by the small molecule PAI-1 inhibitor PAI-039 (tiplaxtinin) has been investigated using enzymatic analysis, direct binding studies, site-directed mutagenesis, and molecular modeling studies. Previously PAI-039 has been shown to exhibit in vivo activity in various animal models, but the mechanism of inhibition is unknown. PAI-039 bound specifically to the active conformation of PAI-1 and exhibited reversible inactivation of PAI-1 in vitro. SDS-PAGE indicated that PAI-039 inactivated PAI-1 predominantly through induction of PAI-1 substrate behavior. Preincubation of PAI-1 with vitronectin, but not bovine serum albumin, blocked PAI-039 activity while analysis of the reciprocal experiment demonstrated that preincubation of PAI-1 with PAI-039 blocked the binding of PAI-1 to vitronectin. Together, these data suggest that the site of interaction of the drug on PAI-1 is inaccessible when PAI-1 is bound to vitronectin and may overlap with the PAI-1 vitronectin binding domain. This was confirmed by site-directed mutagenesis and molecular modeling studies, which suggest that the binding epitope for PAI-039 is localized adjacent to the previously identified interaction site for vitronectin. Thus, these studies provide a detailed characterization of the mechanism of inhibition of PAI-1 by PAI-039 against free, but not vitronectin-bound PAI-1, suggesting for the first time a novel pool of PAI-1 exists that is vulnerable to inhibition by inactivators that bind at the vitronectin binding site.

Effect of pharmacologic plasminogen activator inhibitor-1 inhibition on cell motility and tumor angiogenesis

BACKGROUND

Plasminogen activator inhibitor-1 (PAI-1) is integrally involved in tumorigenesis by impacting on both proteolytic activity and cell migration during angiogenesis.

OBJECTIVES

We hypothesized that an orally active small molecule inhibitor of PAI-1 (PAI-039; tiplaxtinin) could affect smooth muscle cell (SMC) attachment and migration in vitro on a vitronectin matrix, and exhibit antiangiogenic activity in vivo.

METHODS

In vitro assays were used to assess the mechanism of inhibition of PAI-1 by PAI-039 using wild-type PAI-1 in the presence or absence of vitronectin and wild-type PAI-1 and specific PAI-1 mutants in SMC adhesion and migration assays. An in vivo tumor angiogenesis model was used to assess the effect of PAI-039 administration on neovascularization in a Matrigel implant.

RESULTS

PAI-039 dose-dependently inhibited soluble, but not vitronectin-bound, PAI-1. Cell adhesion assays using PAI-1 mutants unable to bind vitronectin (PAI-1K) or inactivate proteases (PAI-1R) further suggested that PAI-039 inactivated PAI-1 by binding near its vitronectin domain. In a tumor angiogenesis model, PAI-039 treatment of wild-type mice dose-dependently decreased hemoglobin concentration and endothelial cell staining within the Matrigel implant, indicating reduced angiogenesis, but exhibited no in vivo efficacy in PAI-1 null mice.

CONCLUSIONS

Administration of an orally active PAI-1 inhibitor prevented angiogenesis in a Matrigel implant. The lack of activity of PAI-039 against wild-type PAI-1 bound to vitronectin and PAI-1K suggests PAI-039 binding near the vitronectin-binding site. Our studies further substantiate a role for PAI-1 in cellular motility and tumor angiogenesis, and suggest for the first time that these effects can be modulated pharmacologically.

Modulation of adipose tissue development by pharmacological inhibition of PAI-1

OBJECTIVE

The effect of a novel small molecule plasminogen activator inhibitor (PAI-1) inhibitor on adipose tissue physiology was investigated.

METHODS AND RESULTS

In human preadipocyte cultures, PAI-039 inhibited both basal and glucose-stimulated increases in active PAI-1 antigen, yet had no effect on PAI-1 mRNA, suggesting a direct inactivation of PAI-1. Differentiation of human preadipocytes to adipocytes was associated with leptin synthesis, which was significantly reduced in the presence of PAI-039, together with an atypical adipocyte morphology characterized by a reduction in the size and number of lipid containing vesicles. In a model of diet-induced obesity, pair-fed C57 Bl/6 mice administered PAI-039 in a high-fat diet exhibited a dose-dependent reduction in body weight, epididymal adipose tissue weight, adipocyte volume, and circulating plasma active PAI-1. Plasma glucose, triglycerides, and leptin were also significantly reduced in drug-treated mice, and concentrations of PAI-039 associated with these physiological effects were near the in vitro IC50 for the inhibition of PAI-1.

CONCLUSIONS

Our results indicate that a small molecule inactivator of PAI-1 can neutralize glucose-stimulated increases in PAI-1 in human preadipocyte cultures, reduce adipocyte differentiation, and prevent the development of diet-induced obesity. These data suggest the pharmacological inhibition of PAI-1 could be beneficial in diseases associated with expansion of adipose tissue mass.

Enhanced spontaneous thrombolysis: A new therapeutic challenge

Spontaneous thrombolysis is an endogenous protective mechanism against lasting arterial thrombotic occlusion, which is implicated in the pathogenesis of myocardial infarction and acute coronary events. Novel therapies for coronary heart disease (CHD) targeting atherosclerosis and thrombosis, together with cardiovascular prevention programs targeting risk-factors and lifestyle provide evidence that CHD is preventable. Although reduced fibrinolytic activity is a recognized risk-factor for ischemic cardiovascular events, it has so far been neglected. Our knowledge of the fibrinolytic effect of drugs commonly used for CHD such as antiplatelet agents (aspirin, ticlopidine, clopidogrel), anti-diabetic biguanides (phenformin, metformin) or anti-hypertensive drugs is scanty and conflicting. This is mainly due to the lack of a global test of spontaneous thrombolysis, as opposed to fibrinolysis of plasma or whole blood, i.e. the assessment of various activators and inhibitors of the fibrinolytic system. A recently described technique allows the measurement of spontaneous thrombolysis, that is, lysis of an autologous platelet-rich thrombus in the absence of added plasminogen activators. Early results suggest that this test may have significant clinical potential both in identifying those at risk of fatal cardiac events and in finding new therapeutic avenues or lifestyles to improve spontaneous thrombolytic activity.