Synthesis and in vitro evaluation of a series of diketopiperazine inhibitors of plasminogen activator inhibitor-1.

We have synthesized and evaluated a series of diketopiperazine-based inhibitors of PAI-1. These studies resulted in the identification of 34 which inhibited PAI-1 in vitro with an IC(50)=0.2 microM. The synthesis and SAR of these compounds are described.

Polymerization of plasminogen activator inhibitor-1.

The activity of the serine proteinase inhibitor (serpin) plasminogen activator inhibitor-1 (PAI-1) is controlled by the intramolecular incorporation of the reactive loop into beta-sheet A with the generation of an inactive latent species. Other members of the serpin superfamily can be pathologically inactivated by intermolecular linkage between the reactive loop of one molecule and beta-sheet A of a second to form chains of polymers associated with diverse diseases. It has long been believed that PAI-1 is unique among active serpins in that it does not form polymers. We show here that recombinant native and latent PAI-1 spontaneously form polymers in vitro at low pH although with distinctly different electrophoretic patterns of polymerization. The polymers of both the native and latent species differ from the typical loop-A-sheet polymers of other serpins in that they readily dissociate back to their original monomeric form. The findings with PAI-1 are compatible with different mechanisms of linkage, each involving beta-strand addition of the reactive loop to s7A in native PAI-1 and to s1C in latent PAI-1. Glycosylated native and latent PAI-1 can also form polymers under similar conditions, which may be of in vivo importance in the low pH environment of the platelet.

Inhibition of plasminogen activator inhibitor-1 activity by two diketopiperazines, XR330 and XR334 produced by Streptomyces sp.

Two diketopiperazines, XR334 (1) and the novel compound XR330 (2), were isolated from the lyophilised biomass of an unidentified Streptomyces sp. Their structures were elucidated on the basis of spectroscopic studies and confirmed by chemical synthesis. Both compounds inhibited plasminogen activator inhibitor-1 activity in an amidolytic assay of tissue plasminogen activator mediated plasmin generation. Compound 1 also enhanced fibrinolysis ex vivo and protected against thrombus formation in the rat. These diketopiperazines represent the first low molecular weight inhibitors of plasminogen activator inhibitor-1, a physiological regulator of fibrinolysis.

Evaluation of a low molecular weight modulator of human plasminogen activator inhibitor-1 activity.

A critical component in the regulation of thrombus formation and clearance is the balance between tissue plasminogen activator (tPA) and plasminogen activator inhibitor type-1 (PAI-1). An increase in the plasma concentration of PAI-1 has been proposed as a risk factor in thrombotic disease. Inhibition of PAI-1 activity may have utility in the treatment of thromboembolic disease. We report here the evaluation of three diketopiperazine-based low molecular weight inhibitors of PAI-1 activity (XR334, XR1853 and XR5082). In vitro these compounds reversed the inhibitory effects of PAI-1 against both tPA and urokinase (UK) (IC50: 5 to 80 muM). In contrast, other serpin-serine protease interactions, including alpha 1-antitrypsin-trypsin, alpha 2-antiplasmin- plasmin and antithrombin-thrombin, were not affected, neither did these inhibitors affect global tests of haemostasis. In the light of this promising in vitro profile these compounds were evaluated in a standard radioisotopic assay of clot lysis in whole rat blood following intravenous administration. In this assay these compounds dose-dependently enhanced fibrinolysis ex vivo. After intravenous bolus administration XR334, XR1853 and XR5082 at 5 mg/kg increased clot lysis by 32.0 +/- 5.1% SEM (n = 25, p < 0.01), 36.7 +/- 3.5% SEM (n = 36, p < 0.01) and 60.0 +/- 2.8% SEM (n = 17, p < 0.01) respectively compared to vehicle. Intravenous infusion of these compounds (1 mg/kg/min for 20 min) significantly prolonged (approximately twofold) the time to blood vessel occlusion in the rat electrically-stimulated carotid artery thrombosis model. Thus, these low molecular weight inhibitors of PAI-1 activity enhanced fibrinolysis ex vivo and protected against thrombus formation in the rat.

The influence of therapeutic blocking of Gp IIb/IIIa on platelet alpha-granular fibrinogen.

Recent evidence suggests that platelet alpha-granule fibrinogen (fg) is derived from the plasma pool. Since platelets from patients with Type I Glanzmann's thrombasthenia (GT) are deficient in intracellular fibrinogen (fg) it was hypothesized that Gp IIb/IIIa could mediate the uptake of fg. To study the potential role of Gp IIb/IIIa in intracellular fg trafficking, the influence of therapeutic blocking of Gp IIb/IIIa on platelet fg was studied in 12 patients with stable ischaemic heart disease. Patients were either given a single intravenous dose of the monoclonal antibody 7E3 Fab (n = 4) or a combination of bolus and continuous infusion up to 24 (n = 3), 36 (n = 3) or 96 h (n = 2). All patients showed grossly prolonged bleeding times with a significant reduction of ex-vivo ADP induced aggregation. Although, surface Gp IIb/IIIa binding sites were consistently reduced in all patients, there was a variable but delayed decrease in platelet fg relative to vWf:Ag in only six out of the 12 patients studied. The reduction in fg appeared dependent upon both dosage and duration of Gp IIb/IIIa blockade. The study provides further evidence for the novel role of Gp IIb/IIIa in the intracellular trafficking of fg to platelet and megakaryocytic alpha-granules.

Platelet-neutrophil interactions: their significance.

In the vicinity of an acute inflammatory response both cellular and non-cellular elements may interact to modify the overall response. Evidence suggests that leukocytes may play an active role in the modulation of platelet function and vice-versa. This interaction may be abnormal in certain pathological states. Neutrophils have been found to alter platelet behaviour by several mechanisms. These include transcellular metabolism of eicosanoids. Neutrophils utilize platelet-derived arachidonate to increase leukotriene synthesis. Other arachidonate metabolites result from platelet-neutrophil interaction and these differ quantitatively and qualitatively from those arising from either cell-type alone. Another mechanism is the release of a nitric oxide-like factor by neutrophils. Nitric oxide inhibits platelet adhesion and aggregation via guanylate cyclase stimulation. Neutrophils, under different conditions, are potent inducers of platelet calcium flux, aggregation and secretion. This activity is mediated by a neutrophil-derived protease, most likely to be cathepsin G. The interaction of platelets with neutrophils may help to explain some of the pathophysiological events associated with different clinical states.

Differentiation-linked activation of the respiratory burst in a monocytic cell line (U937) via Fc gamma RII. A study of activation pathways and their regulation.

Activation of the respiratory burst in the monocytic cell line U937 by cross-linking human 40-kDa FcR for IgG (Fc gamma RII) with the IgG1 mAb, CIKM5, is dependent on the maturation state of the cell. Addition of anti-Fc gamma RII to undifferentiated cells does not activate the respiratory burst but differentiation with human rIFN-gamma (200 U/ml) for 13 to 15 days results in maximal stimulation by this agonist, with half-maximal responses in cells incubated for 10 to 12 days. During maturation the development of responsiveness to cross-linking Fc gamma RII occurs later than the development of responsiveness to the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (maximal responses at 7 to 9 days), or the chemotactic peptide FMLP (half-maximal responses at 7 to 9 days). The late development of maximal Fc gamma RII responses is not associated with either increased Fc gamma RII expression, enhanced calcium mobilization induced by anti-Fc gamma RII, changes in protein kinase C activity (PKC) or a switch in PKC isotype expression. Activation of the respiratory burst via Fc gamma RII may not be mediated by activation of PKC as the kinase inhibitors 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride and N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride inhibited the Fc gamma RII response by less than 20% at concentrations which inhibit the 12-O-tetradecanoylphorbol-13-acetate-induced respiratory burst by more than 80%. IFN-gamma U937 cells did not metabolize incorporated arachidonate into eicosanoids when stimulated with anti-Fc gamma RII, suggesting that eicosanoids do not mediate activation of the respiratory burst, and this was confirmed by the lack of inhibition by the specific 5'-lipoxygenase and glutathione S-transferase inhibitor, piriprost, and the cyclo-oxygenase inhibitor, indomethacin. In addition there was no significant release of radiolabeled arachidonate in response to anti-Fc gamma RII. The response to anti-Fc gamma RII is inhibited by pertussis toxin, suggesting that signal transduction is via a GTP-binding protein. Agents that elevate intracellular cAMP increased the magnitude of the cAMP transients stimulated by anti-Fc gamma RII and also inhibited the respiratory burst. FMLP responses showed a similar pattern of sensitivity to this range of inhibitors, suggesting that both Fc gamma RII and FMLP receptor share common regulatory mechanisms. However, the termination of the respiratory burst activated via Fc gamma RII and FMLP receptor is independently regulated, in that after FMLP-induced activation there is no subsequent inhibition of the Fc gamma RII-mediated response and vice versa.

Platelet aggregation is inhibited by a nitric oxide-like factor released from human neutrophils in vitro.

Thrombin-induced platelet aggregation was inhibited in vitro by washed human neutrophils. Aggregation was inhibited in a neutrophil concentration dependent manner but glutaraldehyde fixed neutrophils had no significant effect on platelet aggregation. The neutrophil-derived inhibitory factor had the pharmacological profile of nitric oxide. Its action was potentiated by both superoxide dismutase and M&B22, 948, a selective cyclic guanosine monophosphate (cyclic GMP) phosphodiesterase inhibitor. Haemoglobin lessened this inhibitory action of neutrophils. L-Arginine, the substrate for nitric oxide formation, enhanced inhibition, whereas, L-canavanine, a structural analogue of L-arginine, prevented it. Nitric oxide release by neutrophils antagonized platelet ATP secretion and thromboxane B2 release. Inhibition was mediated by nitric oxide activation of guanylate cyclase with a subsequent rise in cyclic GMP. When neutrophils were stimulated with formyl-met-leu-phe, there was a further increase in platelet cyclic GMP. This was enhanced by superoxide dismutase, but lessened by haemoglobin. Leukotriene B4 stimulation of neutrophils promoted inhibition of platelet aggregation. Leukotriene B4 alone had no direct effect on thrombin-induced aggregation of platelets. Platelets, when incubated with neutrophils and stimulated with calcium ionophore A23187, increased leukotriene B4 production by neutrophils in a platelet concentration dependent manner. Platelets alone were unable to release leukotriene B4. The action of platelets in haemostasis is modified as they come into contact with neutrophils. This may be an important physiological mechanism.

Changes in haemostasis after stopping the combined contraceptive pill: implications for major surgery.

OBJECTIVE: To investigate the changes in haemostasis in the three months immediately after stopping the combined contraceptive pill. DESIGN: Prospective randomised study. SETTING: Family planning centre in London. SUBJECTS: 24 women aged 35-45 investigated before, during, and after six months' use of combined oral contraceptives containing 30 micrograms ethinyl oestradiol together with the progestogens desogestrel or gestodene. MAIN OUTCOME MEASURES AND RESULTS: Blood samples were taken immediately before and after six months of oral contraceptive use and one, two, four, six, eight, and 12 weeks after the pill had been stopped. During the six months of oral contraceptive use the plasma concentration of factor X and fibrinogen increased and that of antithrombin III decreased. Between two and six weeks after stopping the pill a rebound phenomenon occurred with plasma concentrations of antithrombin III increasing (mean change from baseline at two weeks 0.06 IU/l and at six weeks 0.10 IU/l) and fibrinogen decreasing (0.26 g/l change at two weeks and 0.40 g/l at six weeks). Factor X concentrations fell gradually and the values at eight weeks were not significantly different from those found before the combined pill was started. CONCLUSION: The combined pill should be stopped at least four weeks before major surgery, which carries the risk of postoperative thrombosis, to allow the potentially prothrombotic haemostatic changes that occur during its use to be corrected.

PIP: This study investigated the changes in hemostasis in the 3 months immediately after cessation of the combined oral contraceptive (OC). This prospective, randomized study has based at a family planning center in London and 24 women, ages 35-45, were investigated before, during, and after 6 months use of these combined OCs containing 30 mcg ethinyl estradiol along with the progestogens desogestrel or gestodene. Blood samples were taken immediately before and after 6 months of OC use and 1,2,4,6,8, and 12 weeks after the pill had been stopped. During the 6 months of OC use, the plasma concentration of factor X and fibrinogen increased and that of antithrombin III decreased. Between 2-6 weeks after stopping the pill, a rebound phenomenon occurred with plasma concentrations of antithrombin III increasing (mean change from baseline at 2 weeks 0.06 IU/l and at 6 weeks 0.10 IU/l) and fibrinogen decreasing (0.26 g/l change at 2 weeks and 0.40 g/l at 6 weeks). Factor X concentrations fell gradually and the values at 8 weeks were not significantly different from those found before the combined pill was begun. The combined OC should be stopped at least 4 weeks prior to major surgery (which carries the risk of postoperative thrombosis) in order to allow the potentially prothrombotic hemostatic changes that occur during its use to reverse themselves.

Blood hyperviscosity and its relationship to progressive renal failure in patients with diabetic nephropathy.

Blood rheology was investigated in patients with diabetic nephropathy and progressive renal insufficiency, and compared with similar non-diabetic patients and healthy control subjects. Plasma viscosity and whole blood viscosity at standardized haematocrit were elevated to a comparable degree in the two patient groups, but erythrocyte deformability was normal. In diabetic patients, the rate of progression of renal failure showed weak, but significant, correlations with plasma viscosity (rs = 0.50, p = 0.005), standardized whole blood viscosity (rs = 0.41, p = 0.021), plasma fibrinogen (rs = 0.46, p = 0.010), C reactive protein (rs = 0.40, p = 0.023), and proteinuria (rs = 0.52, p = 0.003). Both plasma viscosity and plasma fibrinogen correlated significantly with proteinuria (rs = 0.45, p = 0.012 and 0.40, p = 0.027, respectively). Rheological abnormality is probably a manifestation of increased acute phase proteins, but it remains to be determined whether these are the cause or the effect of the renal injury. Abnormal blood rheology may be a risk factor for the progression of renal failure in diabetic nephropathy.

A plasma inhibitor of platelet aggregation in patients with Argentine hemorrhagic fever.

Hemorrhage in patients with Lassa fever is associated with the presence of a circulating plasma inhibitor of platelet aggregation. This study was to determine whether patients with Argentine hemorrhagic fever (AHF) develop a similar inhibitor. Normal platelets showed significantly weaker aggregation responses to a sub-maximal dose of adenosine diphosphate (ADP) when mixed with plasma from 10 patients with AHF (mean percent of control +/- 1 SE = 57.2 +/- 6.7%) compared to those mixed with plasma from 9 viral control patients (79.5 +/- 4.1%; P less than 0.05) and 9 febrile patients with septicemia (103.8 +/- 3%; P less than 0.001). Plasma from 3 patients with severe AHF inhibited in a dose-dependent fashion the aggregation responses of normal platelets to collagen, sodium arachidonate, a calcium ionophore (A23187), and ristocetin; none of 4 samples from convalescent AHF patients showed this inhibitory activity. The platelet inhibition was sudden in onset and unaffected by a 30 min pre-incubation, not neutralized by convalescent plasma with high titer antibody to Junin virus, and abolished after heating plasma from an AHF patient at 56 degrees C for 30 min. Hemorrhage in AHF is associated with the presence of a circulating inhibitor of platelet aggregation, and disturbed hemostasis in arenavirus-induced hemorrhagic fevers may have a common basis.

The in-vitro and ex-vivo effects of chloroquine sulphate on platelet function: implications for malaria prophylaxis in patients with impaired haemostasis.

Platelet aggregation responses were studied in platelet-rich plasma from six healthy volunteers before and 2 and 6 h after ingestion of 600 mg chloroquine sulphate. Apart from a mild reduction in height of aggregation response to 1 microgram ml-1 collagen 2 h post-drug ingestion (mean percentage of pre-drug values +/- s.e.m. = 87.8% +/- 4.0%; P = 0.04), no significant differences were observed in platelet responses to ADP (1 and 5 microM) or collagen (1 and 4 micrograms ml-1) at 2 or 6 h post-chloroquine compared to the pre-drug values. In vitro, drug concentrations approximately 1000 times greater than those used therapeutically were required for 50% inhibition of platelet aggregation and ATP release in response to 5 microM ADP, 1 microgram ml-1 collagen and 4 micrograms ml-1 collagen (IC50 concentrations +/- s.e.m. for inhibition of aggregation = 98.5 +/- 3.7, 53.5 +/- 56.4 and 113.0 +/- 6.2 mg l-1 respectively; IC50s +/- s.e.m. for inhibition of ATP release = 0.9 +/- 0.2, 14.7 +/- 4.0 and 23.0 +/- 5.3 mg l-1 respectively). These data provide no cause for concern in using chloroquine for malaria prophylaxis in patients with impaired haemostasis.

Plasma D dimer: a useful marker of fibrin breakdown in renal failure.

D dimer and other large fragments produced during the breakdown of crosslinked fibrin may be measured by enzyme immunoassay using monoclonal antibodies. In 91 patients with renal disease and varying degrees of renal dysfunction, plasma D dimer showed no correlation with renal function, whereas FgE antigen, a fibrinogen derivative which is known to be cleared in part by the kidney, showed a significant negative correlation with creatinine clearance. Plasma concentrations of D dimer were, however, increased in patients with chronic renal failure (244 +/- 31 ng/ml) (mean +/- SEM) and diabetic nephropathy (308 +/- 74 ng/ml), when compared with healthy controls (96 +/- 13 ng/ml), and grossly elevated in patients with acute renal failure (2,451 +/- 1,007 ng/ml). The results indicate an increase in fibrin formation and lysis, and not simply reduced elimination of D dimer by the kidneys, and are further evidence of activated coagulation in renal disease. D dimer appears to be a useful marker of fibrin breakdown in renal failure.

Platelet function and the bleeding time in progressive renal failure.

Bleeding time and platelet function tests were performed on 31 patients with progressive chronic renal failure (CRF) due to non-immunological (urological) causes, and compared with 22 healthy controls. Patients were classified as mild (plasma creatinine less than 300 mumol/l), moderate (300-600 mumol/l) or severe renal failure (greater than 600 mumol/l). Bleeding time was rarely prolonged in mild and moderate CRF and mean bleeding time significantly elevated only in severe CRF (p less than 0.005). Haematocrit was the only index which correlated with bleeding time (r = -0.40). Platelet counts, collagen stimulated thromboxane generation, and platelet aggregation responses to ADP, collagen and ristocetin were all either normal or increased in all three CRF groups, but thromboxane production in clotting blood was reduced. Plasma fibrinogen, C reactive protein and von Willebrand factor (vWF) were elevated in proportion to CRF. We found no evidence that defects in platelet aggregation or platelet interaction with vWF prolong the bleeding time in patients with progressive CRF.

Abnormal blood rheology in progressive renal failure: a factor in non-immune glomerular injury?

Chronic renal insufficiency progresses by a final common pathway of glomerular damage characterised by microvascular injury and glomerulosclerosis. In order to investigate the possible role of blood rheology in this process, rheological indices were compared between healthy controls and a group of patients with progressive renal failure due to renal diseases that were not considered to be immunologically mediated. Plasma viscosity was significantly increased in the renal insufficiency group (P less than 0.005), and correlated with raised plasma concentrations of fibrinogen (r = 0.63; P less than 0.005). Whole-blood viscosity corrected to a standard haematocrit of 0.45 was also raised. A weak but significant correlation was seen between plasma viscosity and 24-h urinary protein excretion (r = 0.50; P less than 0.005). Our data show that in chronic renal insufficiency, rheology is abnormal. Proteinuria correlates with plasma viscosity, which is consistent with the hypothesis that raised plasma viscosity leads to an increase in glomerular capillary pressure and thence glomerular permeability. Correction of rheological abnormalities might help to preserve kidney function and reduce proteinuria in these patients.