Anti-thrombotic activity of KBT-3022 in experimental models of thrombosis

Inhibitors of platelet signal transduction as anti-aggregatory drugs

In the treatment and prevention of cardiovascular diseases, inhibition of platelet aggregation is of fundamental importance. Inhibition of platelet aggregation can be achieved by either inhibition of membrane receptors or by interception of signalling pathways. While receptor antagonism provides high specificity, the inhibition of platelet signal transduction is more effective. The effectiveness results from the inhibition of platelets, regardless of the cause of activation. These common pathway inhibitors are either intercepting platelet activating mechanisms or amplifying the action of endogenous platelet inhibitors. The physiological anti-aggregants are the endothelial factors NO and prostacyclin, which elevate intracellular cGMP or cAMP content, respectively. By administration of NO-releasing agents, prostacyclin analogues or other cyclic nucleotide elevating drugs the platelet anti-aggregatory action of endothelial factors can be effectively mimicked. Besides antiplatelet activity these drugs also act on vascular smooth muscle causing relaxation and therefore vasodilation, an additional beneficial effect. Inhibition of phosphodiesterases causes elevation of platelet cyclic nucleotide content and thus inhibits platelet aggregation and causes vasodilation. Another relevant target for anti-aggregatory treatment is the arachidonic acid metabolic pathway. This pathway can be intercepted by blockade of either cyclooxygenase-1 (COX-1) or thromboxane synthase. Inhibition of these enzymes may be further amplified by additional antagonism of the thromboxane receptor thus not only preventing formation of thromboxane but also activation of thromboxane receptor by thromboxane precursors, which were particularly effective in clinical trials. In vivo these precursors may be metabolised to prostacyclin in the endothelium and consequently provide additional platelet anti-aggregatory activity. A rather new target for platelet anti-aggregatory treatment is the ecto-nucleotidase CD-39 which limits the plasma level of nucleotides. While several of the novel anti-aggregatory drugs were disappointing in clinical studies combinations of drugs with different effector enzymes showed potent antithrombotic efficacy.

Modulation of vasoconstriction by clopidogrel and ticlopidine

Clopidogrel is an antiplatelet drug which has undergone extensive clinical trials in the management of stroke and other arterial disorders related to platelet activation. This agent is believed to produce the inhibition of ADP mediated direct and indirect actions leading to platelet adhesion/aggregation and other activation processes. Several other observed pharmacologic actions suggest that this drug may have additional sites of action. Ticlopidine also belongs to the same class of ADP receptor inhibitors and is extensively used for stroke prevention. To study the vasomodulatory action of clopidogrel and ticlopidine, the drugs were administered intravenously into canines at a dose of 10 mg/kg. Thirty minutes later femoral and pulmonary arteries were removed and taken for isolated tissue preparations. The intravenous injection of clopidogrel and ticlopidine caused significant vasomodulatory actions in both femoral and pulmonary ring preparations showing a marked desensitization to serotonin, endothelin-1, serum, and platelet rich plasma/arachidonic acid mixtures. In contrast, when the drugs were added directly to the organ bath containing femoral or pulmonary ring preparations from untreated animals, both clopidogrel and ticlopidine did not produce any effect on contractile response induced by serotonin, endothelin-1, serum, and platelet rich plasma/ arachidonic acid mixtures. These data suggest that endogenous transformation of clopidogrel and ticlopidine plays an important role in producing their vasomodulatory actions. Furthermore, these observations indicate that both clopidogrel and ticlopidine also modulate the vascular sites which may be contributory to the observed clinical effects.

The in vitro and in vivo pharmacological profiles of a platelet glycoprotein IIb/IIIa antagonist, NSL-9403

The in vitro and in vivo pharmacological profiles of NSL-9403 [orotyl-serylarginyl-glycyl-asparatyl-tryptophane], a platelet glycoprotein IIb/IIIa (GpIIb/IIIa) antagonist, has been studied. NSL-9403 inhibited platelet aggregation of human platelet-rich plasma (PRP) with IC50 values of 4.3 +/- 0.4 microM (collagen) and 1.8 +/- 0.3 microM (ADP), which was about 100 times more potent than RGDS. It also inhibited the binding of fibrinogen to activated platelets. Ex vivo collagen and ADP-induced platelet aggregation in a guinea pig was inhibited after a bolus intravenous administration of NSL-9403 at 1.25 mg/kg and above. NSL-9403 had an anti-thrombotic effect in in vivo thrombosis models. In a platelet agonist-induced pulmonary embolic sudden death model, where a bolus injection of collagen and epinephrine induced sudden death in mice, intravenous administration of NSL-9403 before an injection of collagen and epinephrine inhibited this platelet-agonist induced death in a dose dependent manner. In an arterio-venous shunt, infusion of NSL-9403 at 3 mg/kg/hour prevented an increase in circulation pressure due to thrombus formation in the shunt circuit and platelet loss. Infusion of NSL-9403 at 1 to 10 mg/kg/hour produced a complete inhibition of platelet-dependent arterial thrombosis in a dog femoral arterial thrombosis model. Thus NSL-9403 is a potent inhibitor or platelet aggregation in vitro and a potent anti-thrombotic agent in vivo with a relatively short duration of action.

Vasomodulatory action of clopidogrel and ticlopidine

Clopidogrel is a thienopyridine derived antiplatelet drug that has currently undergone extensive clinical trials in the management of various arterial disorders related to platelet activation. While the proposed mechanism of its pharmacologic action is believed to be the inhibition of ADP mediated direct and indirect actions on platelet adhesion/aggregation and other activation processes, several other observed pharmacologic actions suggest that this drug may also have additional sites of action. Ticlopidine also belongs to the same class of ADP receptor inhibitor and is extensively used for prevention of ischemic disorders. In order to investigate the vasomodulatory action of clopidogrel and ticlopidine, rabbit and rat isolated tissue preparation systems were used. Clopidogrel and ticlopidine were found to produce dose dependent vasomodulatory actions in rabbit or rat treated with 30 minutes intravenous administration. The aortas harvested from both the rabbits and rats treated with clopidogrel or ticlopidine exhibited marked desensitization to the serotonin, endothelin-1, serum and platelet rich plasma/arachidonic acid mixtures. Both control rabbit aortic rings and rat aortic strips did not produce any inhibition of the serotonin induced contraction. These data suggest that clopidogrel and ticlopidine plays an important role in producing these vasomodulatory actions. Furthermore these observations indicate that both the clopidogrel and ticlopidine also modulate the vascular sites which may be contributory to the observed clinical effects.

Antithrombotic effects in a rat model of aspirin-insensitive arterial thrombosis of desethyl KBT-3022, the main active metabolite of a new antiplatelet agent, KBT-3022

The antithrombotic effect of desethyl KBT-3022, which is the main active metabolite of the new antiplatelet agent, KBT-3022 (ethyl 2-[4,5-bis(4-methoxyphenyl)thiazol-2-yl] pyrrol-1-ylacetate; a cyclooxygenase inhibitor), was determined using a photochemically induced arterial thrombosis model in the rat femoral artery. Pretreatment with desethyl KBT-3022 (0.1, 0.3 and 1 mg/kg, i.v.) prolonged the time required to achieve thrombotic occlusion in the femoral artery and inhibited collagen-induced platelet aggregation in whole blood ex vivo, each in a dose-dependent manner. In all 6 rats used, particularly at the highest dose (1 mg/kg, i.v.) tested, cyclic variations in blood flow were hardly ever observed and complete cessation of blood flow did not occur during the 30-min observation time. BM-13505 (1, 3 and 10 mg/kg, i.v.), a thromboxane A2 receptor antagonist, also prolonged the time to occlusion, but cyclic variations in blood flow did occur. On the other hand, aspirin (10 and 30 mg/kg, i.v.) had little effect in terms of preventing thrombosis, although it inhibited collagen-induced platelet aggregation to the same extent as did desethyl KBT-3022. Desethyl KBT-3022 inhibited the thrombin-induced aggregation of washed platelets in a concentration-dependent manner (1-40 microM), whereas aspirin and BM-13505 did not. These findings suggest that the potent antithrombotic effect of desethyl KBT-3022 may be attributable in part to its additional ability to inhibit thrombin-induced platelet aggregation. Accordingly, thromboxane A2 and thrombin may be important thrombotic mediators in this rat model.

The mechanism of action of KBT-3022, a new antiplatelet agent

1. The mechanism of action of a new antiplatelet agent, KBT-3022 (ethyl 2-[4,5-bis(4-methoxyphenyl)thiazol-2-yl]pyrrol-1-ylacetate) and its active main metabolite, desethyl KBT-3022, was investigated. 2. KBT-3022 and desethyl KBT-3022 inhibited cyclooxygenase from ovine seminal gland with IC50 values of 0.69 and 0.43 microM, respectively. 3. At concentrations higher than those required for cyclooxygenase inhibition, desethyl KBT-3022 inhibited cAMP-phosphodiesterase, specific binding of U46619, and release of phosphatidic acid from thrombin-stimulated platelets. 4. Oral administration of KBT-3022 inhibited the production of thromboxane B2 during blood coagulation more potently than the production of 6-keto-prostaglandin F1 alpha from aortic strips in guinea pigs. 5. These findings suggest that KBT-3022 may inhibit platelet activation principally via the inhibition of cyclooxygenase by desethyl KBT-3022.

Inhibitory effects of the new anti-platelet agent KBT-3022 and its metabolite on rabbit neutrophil function in vitro

The effects of the new anti-platelet agent KBT-3022, ethyl 2-[4,5-bis(4-methoxyphenyl)-thiazol-2-yl]pyrrol-1-ylacetate, and its metabolite desethyl KBT-3022 on rabbit neutrophil function were investigated in comparison with the effects of acetylsalicylic acid (ASA), ticlopidine hydrochloride (TP), cilostazol (CIL) and indomethacin (IM). The adhesion and migration of neutrophils induced by formyl-methionyl-leucyl-phenylalanine (fMLP) were inhibited by all the compounds tested, their rank order of potency being KBT-3022 = desethyl KBT-3022 > TP = CIL = IM > ASA. KBT-3022, desethyl KBT-3022, CIL and IM all suppressed fMLP-induced increases in the intracellular free Ca2+ concentration ([Ca2+]i) in neutrophils, their potencies correlating with their inhibitory effects on fMLP-induced adhesion and migration. KBT-3022 (1 microM), desethyl KBT-3022 (1-10 microM) and CIL (10 microM) but not IM significantly inhibited both neutrophil migration and the increase in [Ca2+]i induced by leukotriene B4 (LTB4). KBT-3022 (1 microM) and desethyl KBT-3022 (1 microM) suppressed the increase in [Ca2+]i induced by complement C5a. Although KBT-3022 and desethyl KBT-3022 did not influence [3H]LTB4 and [125I]C5a specific binding, [3H]fMLP specific binding was inhibited by desethyl KBT-3022 (IC50: 1.9 microM). Neutrophil adhesion and superoxide anion production stimulated by phorbol 12-myristate 13-acetate were partially inhibited by KBT-3022 (1 microM) and desethyl KBT-3022 (1-10 microM). These results suggest that KBT-3022 and desethyl KBT-3022 have a wider spectrum of action and are more potent inhibitors of neutrophil activation than ASA, TP, CIL and IM.

Effect of KBT-3022, a new cyclooxygenase inhibitor, on experimental brain edema in vitro and in vivo

The effect of KBT-3022 (ethyl 2-[4,5-bis(4-methoxyphenyl)thiazol-2-yl]pyrrol-1-ylacetate), a new cyclooxygenase inhibitor, on experimental brain edema was studied. In vitro, KBT-3022 (100 microM) and its metabolite desethyl KBT-3022 (10 and 100 microM), but neither acetylsalicylic acid nor indomethacin, inhibited arachidonic acid-induced swelling of guinea pig cortical slices. KBT-3022 (3-100 microM) and desethyl KBT-3022 (3-30 microM), but neither acetylsalicylic acid nor indomethacin, inhibited lipid peroxidation in guinea pig brain homogenate. In vivo, oral administration of KBT-3022 (1, 3 and 10 mg/kg) and indomethacin (10 and 30 mg/kg), but not acetylsalicylic acid, prevented brain edema induced by bilateral carotid occlusion and recirculation in gerbils. Indomethacin then prevented postischemic hyperthermia, but not KBT-3022. KBT-3022 (10 mg/kg) and indomethacin (30 mg/kg) inhibited lactate accumulation in gerbil brain after ischemia and recirculation. These results suggest that KBT-3022 prevents development of both cytotoxic edema in vitro and vasogenic edema in vivo.

Protective effect of KBT-3022, a new cyclooxygenase inhibitor, in cerebral hypoxia and ischemia

The protective effect of KBT-3022 (ethyl 2-[4,5-bis-(4-methoxyphenyl)thiazol-2-yl]pyrrol-1-ylacetate) , a new cyclooxygenase inhibitor, in cerebral hypoxia and ischemia was studied and compared with those of indomethacin and acetylsalicylic acid (ASA). Oral administration of KBT-3022 (3-100 mg/kg) and indomethacin (3 and 10 mg/kg) significantly prevented KCN-induced death in mice, while ASA (100 mg/kg) had no effect. KBT-3022 (3 and 10 mg/kg, p.o.) and indomethacin (10 mg/kg, p.o.) significantly prolonged the survival time of mice subjected to normobaric hypoxia, while ASA (100 mg/kg, p.o.) had no effect. KBT-3022 (3-30 mg/kg, p.o.) and indomethacin (3 mg/kg, i.p.) significantly ameliorated delayed neuronal death in the gerbil hippocampal CA1 sector after occlusion of bilateral carotid arteries for 5 min, while ASA (300 mg/kg, p.o.) had no effect. KBT-3022 (10 mg/kg, p.o.) significantly inhibited ATP depletion in the gerbil hippocampus after a 1-min occlusion of bilateral carotid arteries, but had no effect on ATP depletion after a 5-min occlusion and the recovery during recirculation. These results show that KBT-3022 exerts protective effects against cerebral anoxia and hypoxia and ameliorates delayed neuronal death in the hippocampus. KBT-3022 may therefore be useful for prophylaxis of ischemic cerebrovascular disorders.