Drug and dietary interactions of the new and emerging oral anticoagulants.

Oral warfarin is associated with extensive food and drug interactions, and there is a need to consider such interactions with the new oral anticoagulants (OACs) dabigatran etexilate, rivaroxaban and apixaban. A literature survey was conducted using PubMed, EMBASE and recent abstracts from thrombosis meetings to identify publications related to food, drug and dietary supplement interaction studies with dabigatran etexilate, rivaroxaban and apixaban. Clinical experience regarding food interactions is currently limited. Regarding drug-drug interactions, dabigatran requires caution when used in combination with strong inhibitors or inducers of P-glycoprotein, such as amiodarone or rifampicin. Rivaroxaban (and possibly apixaban) is contraindicated in combination with drugs that strongly inhibit both cytochrome P450 3A4 and P-glycoprotein, such as azole antimycotics, and caution is required when used in combination with strong inhibitors of only one of these pathways. Important drug interactions of the new OACs that can lead to adverse clinical reactions may also occur with non-steroidal anti-inflammatory drugs and antiplatelet drugs, such as aspirin and clopidogrel. Over-the-counter (OTC) medications and food supplements (e.g. St. John's Wort) may also interact with the new OACs. Given the common long-term use of drugs for some chronic disorders, the frequent use of OTC medications and the need for multiple treatments in special populations, such as the elderly people, it is essential that the issue of drug interactions is properly evaluated. New OACs offer significant potential advantages to the field of venous thromboprophylaxis, but we should not fail to appreciate their lack of extensive clinical experience.

Inhibition of tissue factor-activated platelets by low-molecular-weight heparins and glycoprotein IIb/IIIa receptor antagonist.

Thrombotic disorders can lead to vascular distress and platelet activation eventually resulting in the rupture of the lesions where a sizable amount of tissue factor (TF) is generated during the pathogenesis of arterial diseases. Since low-molecular-weight heparins (LMWHs) and platelet glycoprotein (GP) IIb/IIIa inhibitors are clinically used for the management of acute coronary syndrome (ACS), studies were taken to determine the effects of these agents on TF-mediated activation of platelets. Freshly drawn native whole blood (WB) from normal healthy volunteers (n = 6) supplemented with a predetermined amount of TF was incubated with equivalent anti-Xa adjusted amounts of various LMWHs at 0.01-1.0 U/ml and tirofiban from 10 to 100 ng/ml. Platelet activation was assessed by measuring the expression of P-selectin (CD62) and the generation of platelet aggregates. At 0.01 U/ml, enoxaparin exhibited a stronger inhibition of TF-induced platelet activation compared to ardeparin and dalteparin. At 0.1 U/ml, these LMWHs produced a comparable inhibition of total P-selectin expression, and at 1.0 U/ml, a marked inhibition was noted. Since enoxaparin produced the best concentration-dependent inhibition of P-selectin expression (saline: 76 +/- 10% vs. 1.0 U/ml enoxaparin: 18 +/- 7%; P < .02) and platelet aggregate formation (saline: 63 +/- 7% vs. 1.0 U/ml enoxaparin: 35 +/- 6%, P < .035), this agent was used for additional studies. Unlike enoxaparin, tirofiban produced a weak concentration-dependent inhibition of platelet activation. At 100 ng/ml, tirofiban produced a 40% inhibition of P-selectin expression and about 60% inhibition of platelet aggregate formation. To elucidate the potential interaction between tirofiban and enoxaparin, the effect of 10 and 100 ng/ml tirofiban was studied with enoxaparin-supplemented WB in a 0.01-1.0 U/ml range. Additive effects between these two agents were noted only at lower concentrations. Thus, at therapeutic concentrations (0.8-1.2 U/ml), enoxaparin itself was capable of inhibiting TF-mediated activation of platelets to > 70%; whereas tirofiban failed to produce such concentration-dependent inhibition. This suggests that the simultaneous administration of GPIIb/IIIa receptor antagonist with LMWH may not have any added benefit in the clinical management of patients with ACS.

Laboratory monitoring of pentasaccharide in a dog model of hemodialysis.

Varying dosages of pentasaccharide (400-800 nmol/kg) were compared to a 250-U/kg single bolus dosage of unfractionated heparin (UFH) in a dog model of hemodialysis. Several laboratory assays were used to monitor the effects of pentasaccharide and UFH. The pentasaccharide did not produce any anticoagulant effects as measured by the activated partial thromboplastin time. However, in the anti-Xa chromogenic assay and the Heptest assays, there was a dose-dependent prolongation after pentasaccharide administration. In the group of dogs administered 800 nmol/kg of pentasaccharide, there was a 50% decrease in the thrombin antithrombin (TAT) complex level after 60 minutes on dialysis. In the UFH-treated dogs, wide variations in assays were observed. There was a marked elevation in the activated partial thromboplastin time and Heptest assays up to 6 hours after UFH administration. Both anti-Xa and anti-IIa activity was measured up to 4 hours. In the TAT assay, UFH was found to have a stronger effect in suppressing the formation of TAT in comparison to the pentasaccharide. These results suggest that pentasaccharide can be used as a replacement for UFH in a dog model of hemodialysis to keep the dialysis circuit patent. In addition, the anti-Xa-based assays such as the Heptest and the chromogenic anti-Xa assays can be used to monitor the effects of pentasaccharide in this model.

Antithrombin agents as anticoagulants and antithrombotics: implications in drug development.

The development of direct thrombin inhibitors goes back nearly four decades. Organic synthetic benzamidine derivatives were initially developed as direct antithrombin agents. Later, the structural analysis of fibrinogen, leading to the identification of thrombin cleavage sites, resulted in the recognition of specific peptide sequences where thrombin cleaved fibrinogen. These observations led to the development of synthetic peptide derivatives as inhibitors of thrombin. The leech salivary extract contained natural hirudin, the structural elucidation of which led to the development of a recombinant equivalent protein (r-hirudin). Understanding the biochemical actions of thrombin and the structure of various inhibitors prompted the development of hirulogs, a class of hybrid molecules with two sites of action. Currently, several of these thrombin inhibitors are being developed for various indications in both intravenous and subcutaneous protocols. The increased interest in thrombin inhibitors is also prompted by reports of heparin-induced thrombocytopenia (HIT) with heparin and the need to anticoagulate patients with alternate drugs. These agents produce a direct anticoagulant response by targeting thrombin. In addition, the amplification of the coagulation cascade by thrombin activation of factors V and VIII, stabilization of fibrin by activated factor XHI, and platelet activation is also inhibited by these thrombin inhibitors. Some of the synthetic thrombin inhibitors are also capable of inhibiting other enzymes in the coagulation cascade. Thrombin inhibitors therefore exert a complex effect on the coagulation network and should be carefully evaluated in clinical trials. These drugs can be used for prophylactic and therapeutic and surgical indications. However, the different thrombin inhibitors have shown distinct pharmacologic differences. There is now an interest in developing oral antithrombin inhibitors. Such issues as antagonism, laboratory monitoring, drug interactions, and long-term safety remain unresolved. Current research is focused on addressing these issues.

A simulated post-angioplasty low molecular weight heparin schedule in a non-human primate model.

Due to a variety of pathophysiologic processes the long-term success rate of percutaneous transluminal angioplasty (PTCA) is only 50-70%. Acute restenosis also occurs in 30-40% of patients. Currently, studies are in progress to investigate the influence of low molecular weight heparin (LMWH) prophylaxis on the patency rate after PTCA. Our aim was to determine an optimal schedule to start the LMWH prophylaxis after the routinely performed heparinization. The alterations of the hemostatic parameters during the drug regimen change-over were evaluated. Non-human primates (Macaca mulatta) were divided into 6 treatment groups (n = 3/group). Three groups received heparin i.v. at 15 U/kg to mimic the end phase of therapeutic treatment infusion given 12-24 hrs. after PTCA. Three groups received full heparinization (250 U/kg i.v.) to mimic patients without the above interim phase therapy. Following both regimens, LMWH (Mono-Embolex) (1 mg/kg s.c.) was started at various intervals. The group initially treated with 15 U/kg heparin exhibited a continued anticoagulant effect when LMWH was started 30 min. after the heparin injection. Whereas, when LMWH was started after 2 hrs. the measurable anticoagulant effect was lost during 1 and 3 hrs. after the heparin injection. When LMWH was started 2 or 4 hrs. after the 250 U/kg dose, the anticoagulant response was sustained. Aside from the anti-IIa and the anti-Xa activity, there was no significant difference in other coagulation parameters between these two regimens. The fibrinolytic system was not altered in the therapeutic heparinization group. However, after the initial bolus of 250 mg/kg heparin, the monkeys treated with LMWH exhibited higher t-PA and D-dimer levels. Although our data shows definite differences between the two drug treatment schedules, further studies are warranted before an optimal drug regimen can be suggested for clinical use.

The thrombolytic potency of LMW-heparin compared to urokinase in a rabbit jugular vein clot lysis model.

Since the beginning of the clinical use of low molecular weight (LMW) heparins their thrombolytic or profibrinolytic potency has been a matter of controversial discussions. Regarding this problem, the aim of our study was to test a LMW-heparin (Sandoparin) in an in vivo model comparing its lytic activity to unfractionated heparin and urokinase at different doses. For this purpose a newly developed short-term rabbit jugular vein clot lysis model was developed. Urokinase infused at doses of 3300, 6600 and 10,000 U/kg to control animals for one hour showed a clear dose-dependent clot lysis. Test animals were injected with a bolus of 0.5 mg/kg of LMW-heparin followed by a constant infusion of either 0.5, 1.0 or 2.0 mg/kg for one hour. A similar dose-dependent effect was observed for LMW-heparin as for urokinase. Unfractionated heparin did not exhibit a dose-dependent lytic activity in this model. No lysis was found in rabbits treated with saline. These findings suggest that the LMW-heparin tested exhibits a dose-dependent in vivo lytic activity which can be compared to clinically effective doses of urokinase, and that this activity is not present with unfractionated heparin.

Comparison of recombinant hirudin and heparin as an anticoagulant in a cardiopulmonary bypass model.

Recombinant (r) hirudin is a potent thrombin-specific inhibitor originally derived from the natural hirudin of the leech (Hirudo medicinalis). We have studied the efficacy of r-hirudin compared to heparin in a dog model of cardiopulmonary bypass (CPB) surgery. Two administration regimens were used for r-hirudin: Group I received 1.0 mg/kg intracardiac (i.c.) bolus then intravenous (i.v.) bolus at 30 min (n = 10); Group II received 1.0 mg/kg (i.c.) bolus with 1.25 +/- 0.04 mg/kg/h (i.v.) infusion (n = 8). Group III was given heparin 1.66 mg/kg (i.c.) bolus (n = 9). Aspiration of blood from the chest cavity revealed no significant difference between the three groups. Measurement of fibrin deposits in the pump line filter revealed higher amounts in the r-hirudin groups (P = 0.02). Decreases in platelets, fibrinogen and haematocrit due primarily to haemodilution were the same in each group. The bleeding time was less prolonged for r-hirudin than for heparin (p less than 0.001). No antagonist for r-hirudin was used; however, due to its short half-life, all coagulation parameters returned to baseline within 30 min after CPB. Since r-hirudin has no effect on platelets, is a poor immunogen, does not require a plasma cofactor, and may not require an antagonist, it may provide an alternative anticoagulant to heparin in CPB. Additional studies are, however, needed to optimize the dose and to evaluate other clinical aspects of r-hirudin.

An objective perspective on recombinant hirudin: a new anticoagulant and antithrombotic agent.

Leeches have been in medical use for many years. Hirudin, the anticoagulant obtained from the medicinal leech has been purified, characterized and can now be produced by recombinant (r) technology. R-hirudin is a potent inhibitor of thrombin and is therefore a potentially valuable anticoagulant and antithrombotic drug. This article reviews the current status of r-hirudin in this role and compares the pharmacokinetics, mechanism of action and clinical efficacy of this agent with heparin. The methods available for laboratory assessment and clinical monitoring of r-hirudin and the possible ways of antagonizing its effects are also discussed. Finally, the potential clinical applications of r-hirudin are outlined, although further laboratory and clinical studies, together with a fall in the cost of this compound are required before r-hirudin can be more widely accepted as an anticoagulant and antithrombotic agent.

Molecular markers of contrast media-induced adverse reactions.

Currently used routine laboratory screening methods are not reliable in the prediction of contrast media-related adverse reactions. Cost-effective laboratory methods for various molecular markers of pathophysiologic activation have now become available. In vitro activation tests in high-risk patients may prove to be useful in the prediction of contrast-induced activation of various adverse reactions. If prior clinical evaluation of a patient suggests an ongoing pathologic process, profiling of the following molecular markers may prove to be useful and helpful in avoiding contrast-induced adverse reactions: Fibrinopeptide A, platelet factor 4, thromboxane B2, 5-HETE, physiologic inhibitors of proteases, B beta 15-42 related peptides, bradykinin/kininogen, and anaphylactozins. In patients suspected of reacting to contrast agents, a small dose of 1 to 5 ml can be injected as a bolus and molecular marker profiling on blood drawn after the infusion may prove to be useful screening tests. Additional clinical studies are needed to prove this. However, this screening test should be performed with extreme caution, since some of the patients are known to react to as little as 1 ml dose. The safety of newly developed nonionic contrast agents can be readily assessed by profiling various molecular markers of adverse reactions, which provide a more reliable and quantitative assessment of contrast-induced adverse reactions. Based on molecular marker profiling, various prophylactic agents can be given to high-risk patients to avoid contrast-induced adverse reactions.