Clopidogrel is a prodrug of a platelet inhibitor used to reduce the risk of myocardial infarction and stroke. Clopidogrel is indicated to reduce the risk of myocardial infarction for patients with non-ST elevated acute coronary syndrome (ACS), patients with ST-elevated myocardial infarction, and in recent MI, stroke, or established peripheral arterial disease. It has been shown to be superior to aspirin in reducing cardiovascular outcomes in patients with cardiovascular disease and provides additional benefit to patients with acute coronary syndromes already taking aspirin.

Consensus-based therapeutic guidelines also recommend the use of clopidogrel rather than ASA for antiplatelet therapy in people with a history of gastric ulceration, as inhibition of the synthesis of prostaglandins by ASA can exacerbate this condition. In people with healed ASA-induced ulcers, however, those receiving ASA plus the proton-pump inhibitor (PPI) esomeprazole had a lower incidence of recurrent ulcer bleeding than those receiving clopidogrel. However, prophylaxis with proton-pump inhibitors along with clopidogrel following acute coronary syndrome may increase adverse cardiac outcomes, possibly due to inhibition of CYP2C19, which is required for the conversion of clopidogrel to its active form. The European Medicines Agency has issued a public statement on a possible interaction between clopidogrel and proton-pump inhibitors. However, several cardiologists have voiced concern that the studies on which these warnings are based have many limitations and that it is not certain whether an interaction between clopidogrel and proton-pump inhibitors is real.

Clopidogrel generally has a low potential to interact with other pharmaceutical drugs. Combination with other drugs that affect blood clotting, such as aspirin, heparins and thrombolytics, showed no relevant interactions. Naproxen did increase the likelihood of occult gastrointestinal bleeding, as might be the case with other nonsteroidal anti-inflammatory drugs. As clopidogrel inhibits the liver enzyme CYP2C19 in cellular models, it has been theorized that it might increase blood plasma levels of drugs that are metabolized by this enzyme, such as phenytoin and tolbutamide. Clinical studies showed that this mechanism is irrelevant for practical purposes.

In November 2009, the U.S. Food and Drug Administration (FDA) announced that clopidogrel should be used with caution in people using the proton-pump inhibitors omeprazole or esomeprazole, but pantoprazole appears to be safe. The newer antiplatelet agent prasugrel has minimal interaction with (es)omeprazole, hence might be a better antiplatelet agent (if no other contraindications are present) in people who are on these proton-pump inhibitors.

Clopidogrel is activated in the liver by cytochrome P450 enzymes, including CYP2C19. Due to opening of the thiophene ring, the chemical structure of the active metabolite has three sites that are stereochemically relevant, making a total of eight possible isomers. These are: a stereocentre at C4 (attached to the —SH thiol group), a double bond at C3—C16, and the original stereocentre at C7. Only one of the eight structures is an active antiplatelet drug. This has the following configuration: Z configuration at the C3—C16 double bond, the original S configuration at C7, and, although the stereocentre at C4 cannot be directly determined, as the thiol group is too reactive, work with the active metabolite of the related drug prasugrel suggests the R-configuration of the C4 group is critical for P2Y₁₂ and platelet-inhibitory activity.

Regards
Rutherford
Managing Editor
Pharmacy Practice and Education.