Antithrombin III Independent Anticoagulants

1. Antithrombin III Independent Anticoagulants Benedict R. Lucchesi, M.D., Ph.D. Department of Pharmacology University of Michigan Medical School

2. Antithrombin III Independent Anticoagulants • Hirudin • From the medicinal leech • Synthesized by recombinant DNA techniques • Direct inhibitor of thrombin • Lepirudin (Refludin™) • desulfohirudin - a recombinant hirudin* derived from yeast cells. • Hirugen (bivalirudin, Angiomax™) • Synthetic dodecapeptide derived from hirudin. • Argatroban • Arginine based compound • Weak competitive inhibitor of thrombin. • These compounds are used in those patients who have developed thrombocytopenia during treatment with heparin.

3. Lepirudin (Refludin™)

4. Lepirudin [rDNA] (Refludan™) • highly specific direct inhibitor of thrombin - [Leu1-Thr2]-63-desulfohirudin - a recombinant hirudin* derived from yeast cells. • polypeptide - 65 amino acids MW=6979.5 • identical to natural hirudin except for the substitution of leucine for isoleucine at the N-terminal end of the molecule and the absence of a sulfate group on tyrosine 63. • action independent of ATIII and not inhibited by platelet factor 4 • one molecule of lepirudin binds one molecule of thrombin - all thrombin-dependent coagulation pathways are affected • approved for clinical use in the treatment of heparin-induced thrombocytopenia type II. * Hirudin derived from the leech Hirudo medicinalis

5. Bivalirudin (Hirulog, Angiomax™)

6. Bivalirudin (Hirulog, Angiomax™) • Synthetic 20-amino acid peptide analog of naturally occurring hirudin • Bivalirudin is a specific and reversible direct thrombin inhibitor that binds to the catalytic site and the anion-binding exosite of circulating and clot-bound thrombin. • Inhibition of thrombin prevents activation of factors V, VIII, and XIII; conversion of fibrinogen to fibrin; platelet activation and aggregation).

7. Bivalirudin (Hirulog, Angiomax™) • The effects of bivalirudin are reversed as thrombin slowly cleaves the bilvalirudin-Arg3-Pro4 bond, resulting in recovery of thrombin active site function. • The onset of anticoagulant effect is immediate after direct IV injection of bivalirudin. • Bivalirudin therapy prolongs several coagulation assays: • activated clotting time (ACT), • activated partial thromboplastin time (aPTT), • thrombin time (TT), • prothrombin time (PT) • Coagulation times return to the normal range approximately 1—2 hours after discontinuance of the drug.

8. (A) Structure of bivalirudin and (B) bivalirudin / hirudin complexes • Bivalirudin consists of an active site-directed moiety linked by a poly-glycine spacer to a dodeca-peptide analogue of the carboxy terminal of hirudin. • Once bivalirudin complexes thrombin, it is converted from a noncompetitive inhibitor that interacts with both the active site and exosite 1 on thrombin to a competitive inhibitor that only binds to exosite 1.

9. Potential for enhanced protein C activation by bivalirudin. Fluid-phase thrombin is complexed and inhibited by bivalirudin. • Upon arrival to the microcirculation where thrombomodulin is concen-trated, the amino-terminal domain of bivalirudin is released, leaving only the carboxy-terminal domain bound to exosite 1 on thrombin. • The affinity of thrombomodulin for thrombin is higher than that of the carboxy-terminal dodecapeptide of bivalirudin, thus thrombin binds to thrombomodulin, where it activates protein C.

10. Argatroban

11. Argatroban • Argatroban, a synthetic piperidine carboxylic acid derivative of l-arginine, is an anticoagulant. • Commercially available argatroban is a racemic mixture of the R- and S-diastereoisomers in a ratio of approximately 65 to 35, with the S-isomer having about twice the thrombin-inhibitory potency of the R-isomer. • Argatroban, a highly selective, reversible, small-molecule direct thrombin inhibitor that binds rapidly to the catalytic site/a polar region of both circulating (free) and clot-bound thrombin.

12. Argatroban • Inhibition of thrombin prevents various steps in the coagulation process (e.g., activation of factors V, VIII, and XIII and of protein C; conversion of fibrinogen to fibrin; platelet activation and aggregation). • At infusion rates up to 40 mcg/kg per minute, argatroban produces dose-dependent increases inactivated partial thromboplastin time (aPTT) and several other coagulation assays (activated clotting time [ACT], prothrombin time [PT], and thrombin time [TT]). • Metabolized principally by the liver via hydroxylation and aromatization. • Does not induce antibody formationto itself nor does it interact with heparin-induced antibodies.

13. Argatroban • Administered by continuous IV infusion. • Before administering argatroban, all parenteral anticoagulants must be discontinued and a baseline activated partial thromboplastin time (aPTT) obtained. Supplied, as a concentrated drug (100 mg/ml), which must be diluted 100-fold prior to infusion. Should not be mixed with other drugs prior to dilution in a suitable intravenous fluid.