Antihirudin antibodies develop in 40% to 74% of patients receiving lepirudin after 4 days or more of treatment. Of note, fatal anaphylaxis has been described with lepirudin, particularly in patients who are treated again within 3 months of a previous exposure to this agent. In contrast, argatroban does not appear to be immunogenic.

Argatroban is a small-molecule direct thrombin inhibitor that, unlike lepirudin, is not immunogenic. It is a univalent competitive inhibitor of thrombin and binds only to the catalytic site of thrombin via a noncovalent bond to form a reversible complex. Argatroban has a plasma half-life of 39 to 52 minutes and is extensively metabolized by the liver into four mostly inactive metabolites. Renal dysfunction, age, and gender do not alter the elimination half-life of the drug.

Currently argatroban is approved in the United States for the prophylaxis and treatment of thrombosis in patients with HIT. The recent recommended dose of argatroban in the treatment of HIT for patients in the intensive care unit (ICU) is 0.5 µg/kg/min and the dose should be adjusted to maintain the activated partial thromboplastin time (aPTT) at 1.5 to 3.0 times the patient’s baseline, with the maximum infusion rate at 10 µg/kg/min. A general guideline is to increase the infusion rate by 20% and recheck the aPTT in 4 hours. If supratherapeutic, it is recommended to decrease the infusion rate by 50% and recheck the aPTT in 4 hours. Argatroban increases the a PTT and activated clotting time (ACT) in a dose-dependent manner.

Monitoring Argatroban Clinical Activity :

The best method to monitor therapy with DTIs has not been clearly established. The anticoagulation effect of direct thrombin inhibition is typically monitored using the activated partial thromboplastin time. Although dose-dependent increases also occur in the prothrombin time (PT) and international normalized ratio (INR), these are not dependable markers of activity. It must be stressed that an elevated PT does not mean there is a clinically significant alteration in the extrinsic pathway. The ecarin clotting time better reflects the actual plasma concentration of DTIs, but this test is not widely available. Recombinant hirudins and argatroban can be monitored with the use of the aPTT and bivalirudin with the ACT.

Since direct thrombin inhibitors also increase the PT and INR, a reliable means to monitor warfarin therapy during co administration of a direct thrombin inhibitor is important. Efforts to characterize argatroban‘s effect on the INR during concurrent warfarin therapy have been reported. In one study, argatroban 1 to 2 µg/kg/min and warfarin 2.5 to 5.0 mg/day were administered concurrently to healthy subjects over 6 days. Because of its short half-life, argatroban was temporarily discontinued daily to simulate warfarin monotherapy. Within 4 hours of stopping the argatroban, mean INRs decreased almost twofold although mean functional factor X levels remained unchanged. Subsequent in vitro studies using plasma from warfarin-treated patients demonstrated a linear relationship between the INR in the presence versus absence of exogenous argatroban.

This underscores the importance of postponing warfarin initiation pending substantial resolution of HIT (platelets greater than 100,000/dL) to avoid warfarin-induced microvascular thrombosis. This complication appears to be caused by depletion of the vitamin K dependent natural anticoagulant, protein C. This complication is usually seen when the INR rises above 3.5 as this represents a surrogate marker for protein C depletion.

A chromogenic-based method can be used to determine factor X levels as a monitor of the oral anticoagulation from warfarin without effect from argatroban. Also, patients receiving argatroban cannot be evaluated for potential coagulation abnormalities with routine functional (clot-based) assays for fibrinogen, factor levels, or protein C. Argatroban acts as an inhibitor in these assays, causing a dose-dependent false decrease of fibrinogen and factor levels, and a false increase of protein C.

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