Discovery and development of direct thrombin inhibitors

Direct thrombin inhibitors (DTIs) are a class of anticoagulant drugs that can be used to prevent and treat embolisms and blood clots caused by various diseases. They inhibit thrombin, a serine protease which affects the coagulation cascade in many ways. DTIs have undergone rapid development since the 90's. With technological advances in genetic engineering the production of recombinant hirudin was made possible which opened the door to this new group of drugs. Before the use of DTIs the therapy and prophylaxis for anticoagulation had stayed the same for over 50 years with the use of heparin derivatives and warfarin which have some well known disadvantages. DTIs are still under development, but the research focus has shifted towards factor Xa inhibitors, or even dual thrombin and fXa inhibitors that have a broader mechanism of action by both inhibiting factor IIa (thrombin) and Xa. A recent review of patents and literature on thrombin inhibitors has demonstrated that the development of allosteric and multi-mechanism inhibitors might lead the way to a safer anticoagulant.USA: FDA never gave approval Direct thrombin inhibitors (DTIs) are a class of anticoagulant drugs that can be used to prevent and treat embolisms and blood clots caused by various diseases. They inhibit thrombin, a serine protease which affects the coagulation cascade in many ways. DTIs have undergone rapid development since the 90's. With technological advances in genetic engineering the production of recombinant hirudin was made possible which opened the door to this new group of drugs. Before the use of DTIs the therapy and prophylaxis for anticoagulation had stayed the same for over 50 years with the use of heparin derivatives and warfarin which have some well known disadvantages. DTIs are still under development, but the research focus has shifted towards factor Xa inhibitors, or even dual thrombin and fXa inhibitors that have a broader mechanism of action by both inhibiting factor IIa (thrombin) and Xa. A recent review of patents and literature on thrombin inhibitors has demonstrated that the development of allosteric and multi-mechanism inhibitors might lead the way to a safer anticoagulant. Anticoagulation therapy has a long history. In 1884 John Berry Haycraft described a substance found in the saliva of leeches, Hirudo medicinalis, that had anticoagulant effects. He named the substance ‘Hirudine’ from the Latin name. The use of medicinal leeches can be dated back all the way to ancient Egypt. In the early 20th century Jay McLean, L. Emmet Holt Jr. and William Henry Howell discovered the anticoagulant heparin, which they isolated from the liver (hepar). Heparin remains one of the most effective anticoagulants and is still used today, although it has its disadvantages, such as requiring intravenous administration and having a variable dose-response curve due to substantial protein binding. In the 1980s low molecular-weight heparin (LMWH) were developed. They are derived from heparin by enzymatic or chemical depolymerization and have better pharmacokinetic properties than heparin. In 1955 the first clinical use of warfarin, a vitamin K antagonist, was reported. Warfarin was originally used as a rat poison in 1948 and thought to be unsafe for humans, but a suicide attempt suggested that it was relatively safe for humans. Vitamin K antagonists are the most commonly used oral anticoagulants today and warfarin was the 11th most prescribed drug in the United States in 1999 and is actually the most widely prescribed oral anticoagulant worldwide. Warfarin has its disadvantages though, just like heparin, such as a narrow therapeutic index and multiple food and drug interactions and it requires routine anticoagulation monitoring and dose adjustment. Since both heparin and warfarin have their downsides the search for alternative anticoagulants has been ongoing and DTIs are proving to be worthy competitors. The first DTI was actually hirudin, which became more easily available with genetic engineering. It is now available in a recombinant form as lepirudin (Refludan) and desirudin (Revasc, Iprivask). Development of other DTIs followed with the hirudin analog, bivalirudin, and then the small molecular DTIs. However, such DTIs were also having side effects such as bleeding complications and liver toxicity, and their long-term effects were in doubt. When a blood vessel ruptures or gets injured, factor VII comes into contact with tissue factors which starts a process called the blood coagulation cascade. Its purpose is to stop bleeding and repair tissue damage. When this process is too active due to various problems the risk of blood clots or embolisms increases. As the name indicates the cascade is a multi-step procedure where the main product thrombin is made by activating various proenzymes (mainly serine proteases) in each step of the cascade. Thrombin has multiple purposes, but mainly it converts soluble fibrinogen to an insoluble fibrin complex. Furthermore, it activates factors V, VIII and XI, all by cleaving the sequences GlyGlyGlyValArg-GlyPro and PhePheSerAlaArg-GlyHis, selectively between Arginine (Arg) and Glycine (Gly). These factors generate more thrombin. Thrombin also activates factor XIII that stabilizes the fibrin complex and therefore the clot and it stimulates platelets, which help with the coagulation. Given this broad action of thrombin it stands as a good drug target for anticoagulant drugs such as heparin, warfarin and DTIs and antiplatelet drugs like aspirin. Thrombin is in the serine protease family. It has 3 binding domains in which thrombin-inhibition drugs bind to. Those proteases have a deep narrow gap as an active binding site that consists of two β-barrel subdomains that make up the surface gap which binds substrate peptides. The surface in the gap seems to have limiting access to molecules by steric hindrance, this binding site consists of 3 amino acids, Asp-102, His-57 and Ser-195. Thrombin also has two exosites (1 and 2). Thrombin is a little different from other serine proteases as exosite 1 is anion-binding and binds to fibrin and other similar substrates while exosite 2 is a heparin-binding domain.