Clopidogrel is FDA approved for the medical management of unstable angina (UA)/non-ST-segment elevation myocardial infarction (NSTEMI), ST-segment elevation myocardial infarction (STEMI) in patients receiving fibrinolytic therapy, and for secondary prevention in recent myocardial infarction (MI), recent stroke, and peripheral arterial disease.
FDA-approved indications for clopidogrel include:
Clopidogrel is an irreversible inhibitor of the platelet P2Y12 adenosine diphosphate receptor. Inhibition of this receptor prevents the downstream activation of the glycoprotein IIb/IIIa receptor complex which leads to reduced platelet aggregation. Clopidogrel is an inactive prodrug that requires enzymatic activation via a variety of CYP enzymes, including the CYP2C19 and CYP3A4 enzymes, through a two-step process of bioactivation. Genetic polymorphisms to these enzymes can influence response to therapy. The most commonly discussed genetic polymorphism related to clopidogrel is that of one or both alleles of the CYP2C19 enzyme. Patients with any loss of function allele will not effective metabolize clopidogrel which leads to the inability to inhibit platelet activity. For example, patients who are homozygous for these non-functioning alleles often demonstrate the poorest metabolism, and subsequent activation of clopidogrel, as indicated by high on-treatment platelet reactivity via platelet function testing. Typically, in a normal metabolizer, the drug has a bioavailability of 50%, with only 15% of an oral dose becoming active via esterase hydrolysis with the CYP enzymes. Active clopidogrel inhibits the platelet for the life of the platelet (7 to 10 days). However, platelet function can begin to return as new platelets turn over, and a return of full function is often seen by 5 days. Given this factor, clopidogrel should be held at least 5 days before any surgical procedure. It should be noted this decision should not occur in isolation, especially in patients who are high-risk (e.g., patients with recent stenting for ACS). Thus, in these situations, it is imperative to consult with the primary prescriber for the clopidogrel.
Clopidogrel is only supplied in a tablet formulation, and thus all doses are given via an oral route. Doses of clopidogrel can be administered without respect to meals. Due to the dependence of CYP2C19 for metabolism for activation, it is recommended that administration of agents that inhibit CYP2C19 metabolism should be avoided (e.g., omeprazole, lansoprazole). Furthermore, agents that slow down gastrointestinal motility can delay absorption (e.g., opioid agents) and are recommended to be avoided in the acute setting.
Regarding pregnancy risk factor, clopidogrel has a risk factor B classification, meaning no evidence of risk. However, most of the data supporting this have been from animal models which found no adverse events in reproduction studies. It is not known if clopidogrel is secreted into the breast milk. At the moment, the expert recommendation is to stop nursing or stop the drug in these situations.
Below are the typical dosing regimens for clopidogrel:
Of note, there is no adjustment required for renal or hepatic impairment.
Bleeding is the most common side effect reported and can occur at varying degrees of severity and any site. Risk factors for bleeding include age older than 75 years, recent bleeding event, low body weight, or use of medications (e.g., non-steroid anti-inflammatory agents or warfarin) that can increase the risk of bleeding. If bleeding should occur, the risk/benefit of continuing therapy should occur with the primary prescriber of the clopidogrel. There is currently no reversal agent for clopidogrel therapy. Theoretically, hemostasis could be restored via exogenous platelet administration; however, data exploring this strategy are mixed. The use of platelets should be reserved for severe, life-threatening bleeding.
The other most common adverse effect is rash/pruritus. In cases of mild-to-moderate hypersensitivity (e.g., rash), the patient can be given a course of a steroid burst while being on maintained on therapy. Other options to manage these patients or scenarios include desensitization and switching to an alternative agent with a different structure (e.g., ticagrelor). Rarely, clopidogrel has been associated with thrombotic thrombocytopenia (TTP). A patient who develops TTP while on clopidogrel should be treated with urgent plasmapheresis.
Clopidogrel is contraindicated in patients who have had anaphylaxis to clopidogrel or its components or have active bleeding.
Patients on clopidogrel should be monitored for signs of bleeding, both visibly and via laboratory testing (hemoglobin and hematocrit).
As noted, several patients can have genetic polymorphisms to the CYP enzymes. The CYP2C19 enzyme has been the most studied regarding medication metabolism and response. Genetic testing may be considered in patients prior to initiating therapy in patients at high risk for adverse outcomes (e.g., PCI patients with increased risk of stent thrombosis). However, based on available data, the most optimal dose has been determined. Furthermore, after drug administration, the use of platelet function testing can be used to determine response. There are various consensus opinions to the defined threshold for non-responsiveness and optimal strategy for management.
Overdose following clopidogrel administration may result in bleeding complications. Based on animal studies, a single dose of 1500 to 2000 mg/kg was lethal to mice and rats, and 3000 mg/kg was lethal in baboons.
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