| USAN: | Clopidogrel Bisulfate |
| Brand Name: | Plavix (Sanofi-Aventis/Bristol-Myers Squibb) |
| Molecular Weight: | 419.90 (Parent 321.06) |
| FDA Approval: | 1993 |
| Drug Class: | Anti-Platelet |
| Indications: | Antithrombotic, Blood Thinner |
| Mechanism of Action: | Antagonist of the P2Y12 Purinergic Receptor |
■ 1 History
Three types of blood cells exist in the human body: red blood cells, white blood cells, and platelets. Red blood cells, 45% of the blood, transport oxygen from the lungs to other body parts. White cells, less than 1% of the blood, defend us against bacterial and viral invasions. Platelets, also less than 1% of the blood (55% of the remaining blood is plasma), are small cell fragments that are involved in helping the blood clot, a process known as blood coagulation. Coagulation takes place when the enzyme thrombin elicits platelets and fibrin, a blood protein. Without platelets, coagulation at the site of an injury does not occur and uncontrolled bleeding ensues. Individuals with no ability to clot have a genetic condition called hemophilia. These individuals must periodically administer a clotting factor to their blood to prevent constant bleeding.
Conversely, thrombosis, the formation of blood clots inside blood vessels, can block coronary arteries and constrict vital oxygen supplies, resulting in a heart attack or stroke. Coronary thrombosis is a life-threatening blood clot in the artery. Deep-vein thrombosis (DVT) is commonly associated with long-distance air travel, when passengers are confined to cramped spaces for many hours.
In contrast with thrombosis, in which the clot is stationary, embolus is when an object such as a clot migrates from one part of the body through blood circulation and causes blockage. A pulmonary embolism occurs when emboli travel to the lungs. Approximately 90% of heart attacks and 80% of strokes are caused by blood clots, which kill some 200,000 hospital patients in the US each year.
Anticoagulants (blood thinners) are the drugs of choice to prevent and treat both thrombosis and embolism. To date, heparin, warfarin, and aspirin have all been widely used as blood thinners to prevent blood clots from forming.
Heparin is one of the oldest medicines still in widespread clinical use. Heparin was extracted in 1916 by Jay McLean from dog’s liver in the laboratories of William Howell at the Johns Hopkins University.1 Nearly a century after its discovery, heparin is still extensively used in clinics during hemodialysis, vascular surgery, and organ transplantation. Large dose heparin injections are routinely used to prevent blood clots in patients undergoing kidney dialysis or heart surgery. Heparin is also used to coat stents, flexible mesh metal cylinders that act as scaffolding to prevent an artery from collapsing after an obstruction has been cleared in a procedure called angioplasty, eliminating the need for a patient to take anti-clotting drugs. Heparin prevents formation of blood clots that block the artery at the site of the cleared obstruction.
Heparin works by binding to the active sites on the surface of the plasma protein antithrombin, converting this “sleeping” serine protease inhibitor antithrombin III (also known as AT III) to a potent anticoagulant.
Protamine sulfate has been used as an antidote in cases of heparin overdose to reverse heparin’s anticoagulant effects by binding to it.
Today, there are three types of heparins in use:
While heparins have to be given intravenously (it decomposes in the gut), Warfarin (5), available since the early 1950s, is an oral anticoagulant. In 1939, Karl Paul Link at the University of Wisconsin–Madison isolated dicumarol (4) from spoiled sweet clover hay that killed many cows. Cumarol (4) was produced when the sweet clovers were oxidized during fermentation. Warfarin (5, Coumadin), a synthetic analog of dicumarol, is more potent than dicumarol. It also has both higher water solubility and greater bioavailability than dicumarol. Furthermore, warfarin (5) and other coumarin analogs have an antidote—vitamin K, which is a key player in the blood clotting cascade. Warfarin’s mechanism of action (MOA), in turn, is via inhibition of the vitamin K epoxide reductase as a vitamin K antagonist.3–5
Warfarin (5) is still one of the most prescribed oral anticoagulants. However, it is associated with skin necrosis and hair loss. Moreover, gauging the dosage of warfarin not only depends on factors such as a patient’s age and weight, but also on factors such as genetic polymorphism in the genes encoding CYP2C9, the main enzyme responsible for the metabolism of the S-warfarin (the more potent of the two enantiomers), as well as the level of vitamin K epoxide reductase complex, subunit 1 (VKORC1).6 Even a slight change in dosage can mean the difference between too little, which would not be effective in preventing blood clots, and too much, which can cause dangerous internal bleeding. There is a dire need for more optimal oral anticoagulants.
Aspirin’s antiplatelet effect was first discovered by Harvey J. Weiss at Columbia University in 1967.7 Spurred by Armand Quick’s report that low-dose aspirin prolonged the prothrombin (clotting) time in normal subjects, Weiss gave 300-mg aspirin to ten healthy men and observed that aspirin indeed inhibited platelet aggregation and adenosine diphosphate (ADP) release. Meanwhile, many researchers reported that anti-inflammatory agents, including aspirin, inhibited aggregation of platelets in several animal species. Weiss proposed in his Lancet paper “the results suggest that these agents may have antithrombotic properties.”7
Despite aspirin’s popularity in treating almost every ill known to man (the world consumes 50 million pounds of aspirin a year); its MOA was not deciphered until 1971. It was found that aspirin works by inhibiting prostaglandin synthetase, explaining most of its antiplatelet, antipyretic, and anti-inflammatory properties.8 Thromboxane A2 (6, TxA2, Fig. 2.1), a prostaglandin, is known to promote clotting, and low-dose aspirin (81 mg baby aspirin) is currently used as a prophylaxis to decrease the risk of heart attack and occlusive stroke by inhibiting the biosynthesis of thromboxane A2.
Fig. 2.1. The Arachidonic Acid Cascade
AstraZeneca’s oral anticoagulant ximelagatran (8, Exanta), a direct thrombin inhibitor, became available in Europe in the early 2000s. Guided by the three-dimensional coordinates of human α-thrombin, medicinal chemists arrived at a dipeptide, melagatran (7). Unfortunately, melagatran is highly ionic, with an oral bioavailability of less than 3–7% in humans although its bioavailability was greater than 50% in dogs. Transforming the original carboxylic acid to in the corresponding ethyl ester and converting the original amidine, a strong base, to hydroxyamidine, a nearly neutral fragment, gave rise to ximelagatran (8). In essence, ximelagatran is a double pro-drug of melagatran (7) with an oral bioavailability of 18–20% in humans.9
One disadvantage of ximelagatran (8) is that there is no antidote if acute bleeding develops whereas warfarin (5) can be antagonized by vitamin K and heparins by protamine sulfate. Another drawback of ximelagatran (8) is its effect of severe liver toxicity in a small population of patients; this was one of the reasons why in 2004 the FDA rejected the drug for licensure in the US. In 2006, AstraZeneca withdrew ximelagatran (8) from the market after additional reports of liver damage surfaced.10
The brightest star in the universe of anticoagulants is Sanofi-Aventis’s clopidogrel bisulfate (1, Plavix), which was an improvement on one of their earlier drugs: ticlopidine hydrochloride (10, Ticlid).
In 1972, Maffrand of Porcor (later Sanofi) prepared some analogs of tinoridine (9, Nonflamin), a thienopyridine antiinflammatory drug discovered by the Japanese drug firm Yoshitomi. Maffrand’s thienopyridine analogs were not endowed with antiinflammatory properties at all; instead, they inhibited blood platelet aggregation. Maffrand’s group prepared and evaluated hundreds of similar compounds to determine which produced optimum antiplatelet aggregation. The fruit of their 5-year labor was Ticlid (10), whose marketing was approved in France in 1978.11
Unfortunately, thrombotic thrombocytopenic purpura (TTP), a rare but potentially fatal side effect, was observed in patients after Ticlid (10) reached the market.12 Because of TTP and other side effects, when Ticlid was approved by the FDA in 1991 the agency required it to carry a “black box” warning that it could cause life-threatening blood disorders. As a consequence, the sales of Ticlid were insignificant.
Efforts at Sanofi to seek a safer drug with equal or higher potency by minimizing Ticlid’s toxicities resulted in clopidogrel bisulfate (1, Plavix). Clopidogrel bisulfate (1) provides a significant improvement in the prevention of myocardial infarction, stroke and vascular death in patients with symptomatic atherosclerosis (ischemic stroke, myocardial infarction, or established peripheral arterial disease).13
In 1997, the FDA granted clearance to market Plavix (1) for the reduction of atherosclerotic events (myocardial infarction, stroke, vascular deaths) in patients with atherosclerosis documented by recent myocardial infarctions, recent stroke, or established peripheral arterial diseases. The drug also won approval by the European Commission in 1998. It quickly became widely used in angioplasty procedures to open clogged arteries in the heart and legs and to prevent strokes. Within the first year, more than three million patients in America had taken Plavix, partially because of its improved clinical outcomes and safety benefits in patients with acute coronary syndrome (ACS). More than 48 million Americans use Plavix on a daily basis.14,15 Plavix was the second best-selling drugs of all time from 2008 to 2011.
■ 2 Pharmacology
2.1 Bioavailability, Metabolism, and Toxicology
Remarkably, despite clopidogrel’s (1) enormous commercial success, the identity of its active metabolite 12 was not known until 1999, when it was isolated after exposure of clopidogrel (1) or 2-oxo-clopidogrel (11) to human hepatic microsomes (Scheme 1).16 Metabolite 12 was determined to be an antagonist of the P2Y12 purinergic receptor; it prevents binding of ADP to the P2Y12 receptor. However, clopidogrel (1) itself is not active in vitro, but is activated in vivo by cytochrome P450 (CYP450)-mediated hepatic metabolism to give the active metabolite 12.17
As far as the pharmacokinetics (PK) of clopidogrel (1) is concerned, clopidogrel is rapidly absorbed with a Tmax reached at from 0.5 to 1.0 h.18 Because clopidogrel (1) is a prodrug, it takes 37 days for its effect on platelet aggregation to reach the steady state. Therefore, a common regimen entails a 300-mg loading dose (LD), followed by 75-mg maintenance dose (MD). Generally, Cmax for the active metabolite 12 is 70 ng/mL and 28 ng/mL for a 300-mg LD and a 75-mg MD, respectively. The corresponding area under the curve (AUC0–t) is 90 ng•h/mL and 29 ng•h/mL, respectively, for a 300-mg/75-mg LD/MD regimen. The concentrations of active metabolite 12 are typically below the assay limit (0.5 ng/mL) 2 to 4 h after dosing, therefore its half-life T1/2 could not be calculated.
Similarly, ticlopidine (10) is first oxidized by CYP450 in the liver to 2-oxo-ticlopidine (13), which is further oxidized to the active metabolite 14.18 Just like metabolite 12, metabolite 14 was also determined to be an antagonist of the P2Y12 purinergic receptor and prevents binding of ADP to the P2Y12 receptor. Note that clopidogrel (1) and ticlopidine (10) do not share a common active metabolite although both active metabolites 12 and 14 are P2Y12 receptor antagonists (Scheme 2).
Ticlopidine (10) was plagued by toxicity issues. In addition to TTP, it is known to induce severe bone marrow aplasia, cholestatic jaundice, and acute cholestatic hepatitis. In contrast, clopidogrel (1) is associated with fewer toxicity issues. Since CYP450 2C19 (CYP2C19) is involved in metabolizing clopidogrel (1) to its active metabolite 12, patients with certain genetic variations in the CYP2C19 gene may not benefit from taking clopidogrel (1). In addition, because proton pump inhibitors (see chap, 10) and antiepileptic drugs are often metabolized by CYP2C19, DDI occur when taken together with clopidogrel (1).19
2.2 Mechanism of Action
Platelets play an important role in thrombus formation by adhering to exposed subendothelial structures in response to vascular injury. In this way, they become rapidly activated by their interaction with thrombogenic substrates and generated or locally released agonists, including adenosine-5´-diphosphate (ADP), thromboxane A2 (6), and thrombin. On the other hand, ADP plays a key role in hemostasis as well as in the pathogenesis of arterial thrombosis because the pharmacological inhibition of ADP-induced platelet aggregation decreases the risk of arterial thrombosis. The transduction of the ADP signal involves its interaction with two platelet receptors, the Gq-coupled P2Y1 receptor and the Gi-coupled P2Y12 receptor, which belong to the family of purinergic P2 receptors. The concomitant activation of both the Gq and Gi pathways by ADP is necessary to elicit normal platelet aggregation (Fig. 2.2). In contrast to P2Y1, P2Y12 has a very selective tissue distribution, making it an attractive molecular target for therapeutic intervention. Indeed, metabolites 12 and 14 of thienopyridines ticlopidine (10) and clopidogrel (1) are both P2Y12 antagonists and they are shown to be efficacious antithrombotic agents in clinical practice either alone or in combination with other antithrombotic drugs.20
