Antithrombin agents: the new class of anticoagulant and antithrombotic drugs

Management of thrombotic and cardiovascular disorders in the new millenium

Anticoagulants and antithrombotic drugs have played a key role in the prophylaxis, treatment and surgica/interventional management of thrombotic and cardiovascular disorders. There are several newer drugs which are currently developed for the anticoagulant management of cardiovascular diseases in both the medical and surgical indications. These include the low molecular weight heparins (LMWHs), antithrombin agents such as the Hirudin, Hirulog and Argatroban and indirect and direct anti-Xa drugs, represented by Pentasaccharide (Arixtra) and DX 9065a, respectively. Several other agents such as the natural and recombinant anti-Xa drugs and anti-tissue factor agents are also developed. The antiplatelet agents include Clopidogrel, Cilostazol, Anplag and GP IIb/IIIa inhibitors. For the subcutaneous indications, unfractionated heparin is gradually replaced by the low molecular weight heparins (LMWHs). LMWHs such as the Enoxaparin and Dalteparin are commonly used for the management of acute coronary syndrome. These drugs have been approved for the treatment of unstable angina and are currently undergoing rigorous trials for interventional indications. Arixtra is also developed for various subcutaneous indications. However, it exhibits lower anticoagulant effects and may not be optimal for intravenous and interventional purposes. At a higher dosage when administered intravenously the LMWHs produce varying degrees of anticoagulation at relatively lower activated clotting times (150-200). Several studies in vascular and cardiovascular interventions have shown that even at a relatively lower anticoagulant level the LMWHs are as effective as unfractionated heparin at the recommended dosages which produce a relatively higher level of anticoagulation (ACT > 200 secs.). Thus, these agents are currently developed for interventional and surgical indications. It should be emphasized that different LMWHs produce different degrees of anticoagulation and should therefore be individually optimized for a given interventional or surgical purposes. At a relatively high dosage the levels of LMWHs can be measured by using the ACT and APTT. When administered with such GP IIb/IIIa inhibitors as the Abciximab, Aggrastat or Eptifibratide, these drugs may require dosage adjustment However, since the introduction of the front loading of Clopidogrel, the unqualified use of GP IIb/IIIa is debated. LMWHs will find expanded indications in both the medical and surgical management of patients with cardiovascular disorders including atrial fibrillation and congestive heart failure. The only approved anti-Xa drug is represented by a synthetic heparinomimetic, namely, Arixtra. This drug is given for the prophylaxis of post orthopedic indications. This agent is undergoing additional clinical trials in the management of coronary artery diseases. Because of the dependence on antithrombin III (AT) and the sole anti-Xa effects, it has a narrow therapeutic index and its efficacy in this indication may be limited. Additional clinical trials are needed at this time to validate the clinical potential of this drug. The antithrombin agents (Hirudin, Hirulog and Argatroban) were initially developed for arterial indications. However, these agents are approved as a substitute anticoagulant in patients with heparin induced thrombocytopenia (HIT) and PCI. Currently an of these agents are being developed for surgical and interventional use. However, since there is no available antidote at this time, the development is somewhat limited. The antithrombin agents may be useful in patients with HIT which require further clinical validation. Many other anti-Xa agents are also developed. Most of these can be given parenterally. However, the clinical data is somewhat limited. Similarly, several of the new antiplatelet drugs can be administered parenterally and may be useful in CAD. Since most of these newer anticoagulant and antithrombotic drugs are mono-therapeutic their therapeutic index is rather limited. Only in combination these agents can mimic heparins. At this time it is safe to state that heparin and its LMW derivatives will remain the anticoagulant of choice for cardiovascular indications until these newer agents have been validated in extended clinical trials in polytherapeutic settings.

Anticoagulants for atrial fibrillation: why is the treatment rate so low?

The incidence of atrial fibrillation (AF) is increasing in many countries along with aging demographics. Atrial fibrillation is clearly associated with an increased rate of stroke. Numerous large clinical trials have shown that dose-adjusted warfarin can reduce the stroke rate in these patients, particularly in the elderly, and clear guidelines for the use of anticoagulants in such patients have been published. However, many studies show that treatment rates remain disappointingly low (< or = 50%). Numerous barriers to the use of dose-adjusted warfarin exist, including practical, patient-, physician-, and healthcare system-related barriers. These include the complex pharmacokinetics of warfarin, the need for continuous prothrombin time monitoring and dose adjustments, bleeding events, noncompliance, drug interactions, and increased costs of monitoring and therapy. Possible solutions to this problem are discussed and include improved patient and physician education, the use of anticoagulation clinics, new approaches to AF, and potential treatment improvements through use of newer anticoagulants.

Monitoring of argatroban in ARG310 study: potential recommendations for its use in interventional cardiology

Argatroban is a peptidomimimetic synthetic direct thrombin inhibitor with reversible and specific properties resulting in predictable anticoagulant effects. Usually, argatroban therapy is monitored by the activated dotting time (ACT) or activated partial thromboplastin time (aPTT). While other global dotting tests for the monitoring of anticoagulants are useful, their applicability to antithrombin agents (particularly of argatroban at higher dosages) is rather questionable. In this study, we sought to compare the argatroban anticoagulant levels in patients undergoing percutaneous transluminal angioplasty (PTCA) and stenting procedures, utilizing both functional and absolute quantitation methods. Argatroban produced a comparable increase of ACT and HMT, 5 to 10 minutes after administration. The level of anticoagulation achieved (400-450 seconds with ACT and HMT) following a slow bolus of argatroban (350 microg/kg) was maintained throughout the procedure using 25 microg/kg/min infusion. Following discontinuation of argatroban at the end of the procedure, the ACTs and HMTs showed a comparable progressive reduction in the anticoagulant response, which reflected the elimination of argatroban 2 to 3 hours after the procedure. No significant differences between the three methods (Hemotec, Hemochron, and HMT) were noted at any given sampling time. Argatroban dosage at 350 microg/kg intravenous slow bolus followed by 25 microg/kg/min infusion was adequate to perform PTCA and stenting procedures. There was no incidence of bleeding complications. Absolute quantitation of argatroban levels in patient plasmas by a newly developed HPLC method was found to be quite comparable with the ecarin dotting time (ECT) results. The ECT system was found to be less sensitive when compared to other tests, and therefore, could be used as a point-of-care test during the PTCA/stenting procedures to monitor argatroban.

The thrombin receptor, PAR-1, causes transformation by activation of Rho-mediated signaling pathways

We utilized a cDNA expression library derived from the B6SutA(1) mouse myeloid progenitor cell line to search for novel oncogenes that promote growth transformation of NIH3T3 cells. A 2.2 kb transforming cDNA was recovered that encodes the wild type thrombin-stimulated G protein-coupled receptor PAR-1. In addition to its potent focus forming activity, constitutive overexpression of PAR-1 in NIH3T3 cells promoted the loss of anchorage- and serum-dependent growth. Although inhibitors of thrombin failed to block PAR-1 transforming activity, a PAR-1 mutant that cannot be cleaved by thrombin was nontransforming. Since the foci of transformed cells induced by PAR-1 bear a striking resemblance to those induced by activated RhoA, we determined if PAR-1 transformation was due to the aberrant activation of a specific Rho family member. Like RhoA, PAR-1 cooperated with activated Raf-1 and caused synergistic enhancement of transforming activity, induced stress fibers when microinjected into porcine aortic endothelial cells, stimulated the activity of the serum response factor and NF-kappaB transcription factors, and PAR-1 transformation was blocked by co-expression of dominant negative RhoA. Finally, PAR-1 transforming activity was blocked by pertussis toxin and by co-expression of the RGS domain of Lsc, implicating Galpha(i) and Galpha(12)/Galpha(13) subunits, respectively, as mediators of PAR-1 transformation. Taken together, these observations suggest that PAR-1 growth transformation is mediated, in part, by activation of RhoA.

Anticoagulant and antiprotease effects of a novel heparinlike compound from shrimp (Penaeus brasiliensis) and its neutralization by heparinase I

Heparin is usually obtained from mammalian organs, such as beef lung, beef mucosa, porcine mucosa, and sheep intestinal mucosa. Because of the increased use of heparin in the production of low-molecular-weight heparin (LMWH), there is a growing shortage of the raw material needed to produce LMWHs. A previous report described the structural features of a novel LMWH from the shrimp (Penaeus brasiliensis). In order to compare anticoagulant and antiprotease effects of this heparin, global anticoagulant tests, such as the prothrombin time, activated partial thromboplastin time, thrombin time, and Heptest, were used. Amidolytic anti-Xa and anti-IIa activities were also measured. The relative susceptibility of this heparin to flavobacterial heparinase was also evaluated. The United States Pharmacopeia (USP) potency of shrimp heparin (SH) was found to be 28 U/mg. SH produced a concentration-dependent prolongation of all of the clotting tests and exhibited marked inhibition of FXa and FIIa. Heparinase treatment resulted in a marked decrease of the anticoagulant effects and neutralized the in vitro anti-IIa actions. However, the anti-Xa activities were only partially neutralized. Protamine sulfate was only partially effective in neutralizing the anticoagulant and antithrombin effects of SH. SH also produced marked prolongation of activated clotting time, which was neutralized by heparinase but not by protamine sulfate. These results suggest that SH is a strong anticoagulant with comparable properties to mammalian heparins and can be used in the development of clinically useful antithrombotic-anticoagulant drugs.