Anticoagulant activity and pharmacokinetic properties of a sub-cutaneously administered mixed micellar formulation of argatroban in experimental animals

We have studied the anticoagulant properties of a novel mixed micellar formulation containing 14 mg/ml argatroban administered by the sub-cutaneous (s.c.) route to rats, rabbits, dogs and primates. Blood samples were taken at various times post-treatment for the determination of the thrombin time (TT), Ecarin clotting time (ECT) and the activated partial thromboplastin time (aPTT). Plasma levels of argatroban were determined in the dog and primate. Mixed micelles alone (0.15 M sodium glycocholate and 0.15 M egg lecithin) were without effect on the clotting parameters. The mixed micellar formulation of argatroban dose-dependently increased all three clotting parameters in the rat (1-4 mg/kg), the rabbit (1 and 2 mg/kg), the dog (1 and 2 mg/kg) and the primate (0.25 and 0.5 mg/kg). In each case the TT was the most sensitive parameter, followed by the ECT and the aPTT. The duration of action of argatroban in each species was dose dependent and varied from 3 h in the rat to 6 h in the dog. In the latter, the mixed micelle formulation had a significantly increased plasma half-life and mean residence time without affecting the overall area under the curve. The increases in the clotting time were strongly correlated with the plasma levels of argatroban and were linear across the range of concentrations obtained in the dog and the primate, although the aPTT plasma concentration response curve was very flat. Species differences were noted between the increase in clotting time for a given plasma concentration, with the primate being more sensitive than the dog (e.g. 4.7 times more so in terms of the ECT). Thus, a mixed micellar formulation of argatroban, which markedly enhances its solubility, could be useful as a potential anticoagulant for sub-cutaneous administration.

Activity of a sub-cutaneously administered novel mixed micellar formulation of argatroban in rat and rabbit models of venous thrombosis

We studied the antithrombotic activity of a mixed micellar formulation containing 14 mg/ml argatroban administered by the subcutaneous (s.c.) route in rat and rabbit models of venous thrombosis. The effects on bleeding time in the rat tail transection bleeding time test were also studied. In a tissue factor-dependent arterio-venous shunt model, argatroban treatment led to dose-dependent reduction in thrombus weight with an estimated ID50 of 1.8 mg/kg s.c. In the same model, heparin had an estimated ID50 of 179 IU/kg. The antithrombotic activity of argatroban was accompanied by increases in the thrombin and ecarin clotting times but not the aPTT, whereas heparin increased the thrombin time and aPTT but not the ecarin clotting times. Argatroban also inhibited thrombus formation in a rabbit model of thromboplastin + stasis induced thrombosis in the rabbit jugular vein with an estimated ID50 of 1 mg/kg s.c. When tested in the rat tail transection bleeding time test, the mixed micellar formulation of argatroban caused significant increases in the bleeding time as from 8 mg/kg s.c., while heparin significantly increased the bleeding time at 800 U/kg. Mixed micellar argatroban appears to have a superior safety margin to heparin in terms of antithrombotic efficacy and bleeding risk. Thus, a mixed micellar formulation of argatroban, which markedly enhances its solubility, could be useful as a potential antithrombotic agent for subcutaneous administration.

Inhibition by Argatroban, a specific thrombin inhibitor, of platelet activation by fibrin clot-associated thrombin

Clot-associated thrombin retains amidolytic activity, and is resistant to inhibition by heparin, but not to low molecular weight thrombin inhibitors. We show that clot-associated thrombin induces platelet aggregation, is resistant to heparin:antithrombin III, less so to recombinant hirudin (rHV2Lys47) but not to argatroban, an active-site directed thrombin inhibitor. Fibrin clots prepared with human fibrinogen and thrombin were used to aggregate rabbit washed platelets assessed by single platelet counting, thromboxane B2 (TXB2) immunoassay and scanning electron microscopy. Fibrin clots decreased platelet counts, and released TXB2. Electron microscopy showed platelet aggregates on the clot surface. Argatroban concentration-dependently inhibited such aggregation with IC50s of 21 nM and 13 nM versus aggregation and TXB2 release respectively. The IC50s of Argatroban against fluid-phase thrombin producing similar aggregation were 12 nM (aggregation) and 33 nM (TXB2). rHV2Lys47 was less active against clot-induced aggregation (IC50 = 1.8 nM) than against fluid-phase thrombin (IC50 = 0.06 nM). Heparin had an IC50 of 0.02 mU/ml against aggregation induced by fluid-phase thrombin, but much greater concentrations are required to inhibit clot-induced aggregation (IC50 = 48 mU/ml). These data provide a basis for the superiority of direct-acting thrombin inhibitors over heparin in platelet rich thrombi.

Antithrombotic actions of argatroban in rat models of venous, 'mixed' and arterial thrombosis, and its effects on the tail transection bleeding time

1. The antithrombotic action of argatroban, a synthetic thrombin inhibitor, was studied in three models of thrombosis in the rat, and in the tail transection bleeding time test. Heparin was studied as a reference anticoagulant. 2. In the model of venous thrombosis induced by thromboplastin followed by stasis of the abdominal vena cava, argatroban had an ED50 of 125 micrograms kg-1, when administered as an i.v. bolus 5 min prior to the thromboplastin injection: the ED50 of heparin was 42 micrograms kg-1, where ED50 is the dose which reduces the weight of the thrombus by 50% compared with that of the control animals. When the two compounds were administered by continuous i.v. infusion, argatroban (ED50 = 1.5 micrograms kg-1 min-1) had the same potency as heparin (ED50 = 1.2 micrograms kg-1 min-1). 3. Argatroban was active in the arterio-venous shunt model with an ED50 of 0.6 mg kg-1 when the compound was given as a bolus. The ED50 of heparin was 0.04 mg kg-1 under the same conditions. The two compounds had ED50 values of 6 micrograms kg-1 min-1 (argatroban) and 3 micrograms kg-1 min-1 (heparin), when administered by continuous i.v. infusion. 4. When tested against occlusive arterial thrombus formation by electrical stimulation of the left carotid artery, both compounds given as either an i.v. bolus or a continuous infusion led to dose-dependent increases in the duration of post-lesion vessel patency. Heparin bolus was more active than argatroban on a weight basis, in that 2 mg kg-1 gave a similar increase in the time to occlusion as 8 mg kg-1 argatroban. As in the other models, when given as continuous infusions, argatroban (111% increase in time to occlusion at 20 tg kg-1, min-1) had similar activity to that of heparin (180% increase at 25 jg kg-1 min-1) on a weight basis. Hoever, the antithrombotic effects of argatroban were accompanied by only moderate changes in the coagulation parameters (thrombin time and activated partial thromboplastin time, APTT), whereas, even at a subthreshold dose of heparin (12.5 pg kg-1 min-1), both the thrombin time and the APTT were greater than 150 s.5. Infusions of both compounds caused dose-dependent increases in the tail transection bleeding time,with the dose of argatroban that doubles the bleeding time (11 I g kg-1 min-1) being five times greater than that of heparin (EDI, = 2.2 fig kg-1 min-1).6. These data show that, when administered as an intravenous infusion, argatroban is a potent antithrombotic agent in rat models of venous 'mixed' and arterial thrombosis, this effect can be obtained with a lower degree of systemic anticoagulation than with heparin in the arterial model, and argatroban has a lower haemorrhagic potential than that of heparin.

Effects of the synthetic thrombin inhibitor argatroban on fibrin- or clot-incorporated thrombin: comparison with heparin and recombinant Hirudin

The inhibitory effects of argatroban on clot- or fibrin-bound human thrombin were studied using the thrombin-specific chromogenic substrate S2238 (200 microM). These effects were compared to those of recombinant hirudin (rHV2 Lys 47) and the heparin/antithrombin III complex. Argatroban concentration-dependently inhibited the cleavage of S2238 by a thrombin solution, which had been titrated to give the same change in OD405 nm as fibrin-bound thrombin, with an IC50 of 1.1 microM with 90% inhibition at 8 microM. rHV2 Lys 47 and heparin had IC50 values of 1.2 nM and 0.003 U/ml respectively under these conditions. However, when the compounds were tested against fibrin-bound thrombin, argatroban had an IC50 of 2.8 microM with 65% inhibition at 8 microM, whereas rHV2 Lys 47 had an IC50 of 23 nM (with only 56% inhibition at 200 nM), and heparin had an IC50 of 0.5 +/- 0.38 U/ml (with only 58% inhibition at 5 U/ml); i. e. the two compounds were 19 and 168 times less active against fibrin-bound thrombin than against thrombin in solution. The differences between the inhibitory effects of the compounds against thrombin bound to a plasma clot were even more striking in that the IC50 of argatroban was increased from 1.1 (vs. thrombin in solution) to 2.7 microM, while, although rHV2 Lys 47 and heparin had IC50 values of 2.8 nM and 0.004 U/ml against thrombin in solution, they had little (32% inhibition by 4 microM rHV2 Lys 47) or no effect (even at 5.0 U/ml heparin) against the amidolytic activity of a plasma clot.(ABSTRACT TRUNCATED AT 250 WORDS)

[Pulmonary emphysema induced by elastase: influence of the dose and effect of added collagenase]

The endotracheal deposition of pancreatic porcine elastase (EPP) at 40 u X kg-1 body weight provokes a typical pulmonary emphysema (alveoli disruption) in rats. This emphysema is significant (p less than 0.001) when quantitatively compared to a placebo (100% increase of the mean linear intercept, ILM). The EPP-treated lungs are very heterogeneous and the size of the alveoli vary as much as 30% versus 19% in controls. The emphysema is more effective at a 80 u X kg-1 dose and the individual disparities are reduced (dose effect). When bacterial collagenase (200 u X kg-1 body weight) is added to EPP, the pulmonary abnormalities (disruptions, ILM) are in no way increased (no enzymatic synergy, repair by collagenesis?). In contrast, alveolar dilation is slightly reduced 8 weeks after enzyme administration (p less than 0.05): EPP is not altered in vitro by collagenase and despite several hypothesis, the moderating effect of collagenase in vivo still remains unexplained. This result suggests that the joint presence of several proteases is not necessarily an aggravating factor in the etiopathogenesis of emphysema.