Clopidogrel and Genotyping

Introduction

A major problem with clopidogrel is that it can cause gastrointestinal bleeding. A combination product of clopidogrel and omeprazole was developed by Cogentus Pharmaceuticals, Inc to address this issue. In 2007, the company had favorable reviews of their program from both the United States Food and Drug Administration (FDA) and the European Medicines Evaluation Agency. A clinical study was needed to demonstrate the validity of the hypothesis that the combination product could reduce gastrointestinal bleeding without adversely affecting the cardiovascular protective effects of clopidogrel. The COGENT (Clopidogrel and the Optimization of Gastrointestinal Events Trial) study was developed to accomplish this task.¹
COGENT was a randomized, double-blind, double-dummy, placebo-controlled, parallel-group study of the fixed dose combination of clopidogrel and omeprazole (75 mg/20 mg) versus clopidogrel (75 mg). The patients had presented with an acute coronary syndrome or had undergone placement of a coronary stent. It was intended to recruit 5000, but the sponsor lost financing and was bankrupted, as a consequence of the events described below. A total of 3761 patients were included in the analyses. A positive effect of omeprazole on gastrointestinal events was evident, but the study had fewer major adverse cardiovascular events than anticipated, so one could say only that if an effect of omeprazole on the performance of clopidogrel did occur, it was very modest.

The Metabolism of Clopidogrel

The established metabolism of clopidogrel was that it was a CYP3A substrate.² The 2-oxo-clopidogrel formed opened hydrolytically to the active thio acid.³ This mechanism was challenged by results from the OCLA study by Gilard.⁴ Omeprazole significantly decreased the inhibitory effect that clopidogrel had on platelet aggregation. As omeprazole is a CYP2C19 inhibitor, it was presumed that clopidogrel was a CYP2C19 substrate. This conclusion was supported by some in vitro metabolic work.⁵ However, this work made use of cDNA-derived supersomes. In the original poster from Sankyo, Japan, the use of human hepatosomes was also described and these data were consistent with clopidogrel being a CYP3A substrate. However, the hepatosome data were subsequently ignored in the full paper on the metabolism of clopidogrel, and the primary role of CYP2C19 was reaffirmed.⁶ A very complex metabolism was proposed, involving multiple CYPs and multiple intermediates. The major metabolic process that clopidogrel undergoes is hydrolysis by lipases to clopidogrelic acid that is completely inactive. Only about 15% of the dose proceeds on to the active thiol acid metabolite. It was therefore difficult to understand the Kazui metabolism that involved two oxidation steps and one reduction step. It seemed unlikely that lipases wouldn’t hydrolyze the intermediates to inactive carboxylic acids.
A significant error was the failure of Kazui et al to appreciate that cDNA supersomes can yield false-negative results, particularly with drugs such as clopidogrel that have a high first-pass metabolism in the enterocyte. A subsequent in vitro metabolic study using hepatosomes confirmed that clopidogrel is primarily a CYP3A substrate.⁷ CYP3A is the principal CYP in the enterocyte and there is no CYP2C19 present in the enterocyte. As indicated above, after the first-pass effect, the 2-oxo-clopidogrel that is formed opens to the active thiol acid quite rapidly, which then binds to the platelet P2Y12 receptor.

The Boxed Warning From the FDA

The FDA intervened and despite there being only in vitro data, the clopidogrel label was encumbered by a “black box.” This regulatory action was not well received by physicians — particularly by invasive cardiologists, to whom it was suggested that the CYP2C19 genetics of the patient should be obtained prior to the procedure. An expert consensus document was developed by the American College of Gastroenterology, the American Heart Association, and the American College of Cardiology Foundation.⁸ An important point made in this document was that, “it is not established that changes in these surrogate endpoints translate into clinically meaningful differences.”

Platelet Aggregation Studies

Another important error was the assumption that platelet aggregation correlated with cardiovascular events. Several large meta-analyses have been conducted.^9-11 There is no association of the concomitant administration of proton-pump inhibitors to patients treated with clopidogrel and an increase in major adverse cardiovascular event (MACE) rate. A population-based cohort study in western Denmark of approximately three million people identified 13,001 patients with coronary stents. The use of proton-pump inhibitors did not modify the cardioprotective effect of clopidogrel in these patients.¹²
The platelet aggregation studies are convincing that there is an association with the patient’s CYP2C19 genotype and platelet aggregation, but why is there no clinical signal? The clue is in the drug-drug interaction studies. The effect of concomitant administration of omeprazole on the active metabolite exposure is very modest (a reduction in area under the curve [AUC] of 40%) and large group sizes of 65 subjects had to be employed to obtain a statistically significant result.¹³ An interaction with a CYP3A inhibitor, such as grapefruit juice, requires only 14 subjects, as there is a six-fold reduction in exposure.¹⁴
The claim that CYP2C19 is the major metabolic pathway of clopidogrel is in error because one used a very sensitive method, such as platelet aggregation. Therefore, a minor pathway in the metabolism of clopidogrel can be detected. However, as the drug interaction studies show, the major pathway is via CYP3A.

Removal of the Boxed Warning

The FDA has a responsibility to warn of potential problems with drugs and to do so promptly. A boxed warning in the drug label is a reasonable vehicle to do that. However, in this case, the data are derived primarily from in vitro methods and the one piece of in vivo data, namely, the drug interaction study of clopidogrel and omeprazole that the FDA would not normally consider to be clinically relevant. It might have been wiser for the FDA to request further study of the matter.
Clopidogrel remains an important drug for the management of patients undergoing percutaneous coronary interventions. The presence of the black box in the clopidogrel label provides an avenue for a litigious patient who was not genotyped for CYP2C19 to sue. It is to be hoped that the FDA will respond as promptly in removing the black box as they did in imposing it.

References

1.    Bhatt DL, Cryer BL, Contant CF. Clopidogrel with or without omeprazole in coronary artery disease. New Engl J Med. 2010;363:1909-1917.
2.    Clarke TA, Waskell LA. The metabolism of clopidogrel is catalyzed by human cytochrome P-450 3A and is inhibited by atorvastatin. Drug Metab Dispos. 2003;31:53-59.
3.    Savi P, Pereillo JM, Uzabiaga MF, et al. Indentification and biological activity of the active metabolite of clopidogrel. Thromb Haemost. 2000;84:891-896.
4.    Gilard M, Arnaud B, Cornily JC, et al. Influence of omeprazole on the antiplatelet action of clopidogrel associated with aspirin: the randomized, double-blind OCLA (Omeprazole CLopidogrel Aspirin) study. J Am Coll Cardiol. 2008;51:256-260.
5.    Kurihara A, Hagihara K, Kazui M, Ishizuka T, Farid NA, Ikeda T. In vitro metabolism of antiplatelet agent clopidogrel: cytochrome P450 isoforms responsible for the two oxidation steps involved in the active metabolite formation. Drug Metab Rev. 2005;37(Suppl 2):99.
6.    Kazui M, Nishiya Y, Ishizuka T, et al. Identification of the human cytochrome P450 enzymes involved in the two oxidative steps in the bioactivation of clopidogrel to its pharmacologically active metabolite. Drug Metab Dispos. 2010;38:92-99.
7.    Bouman HJ, Schomig E, van Werkum JW, et al. Paraoxonase-1 is a major determinant of clopidogrel efficacy. Nat Med. 2011;17:110-116.
8.    Abraham NS, Hlatky MA, Antman EM, et al. ACCF/ACG/AHA 2010 expert consensus document on the concomitant use of proton pump inhibitors and thienopyridines: a focused update of the ACCF/ACG/AHA 2008 expert consensus document on reducing the gastrointestinal risks of antiplatelet therapy and NSAID use. J Am Coll Cardiol. 2010;56:2051-2066.
9.    Holmes MV, Perel P, Shah T, Hingorani AD, Casa JP. CYP2C19 genotype, clopidogrel metabolism platelet function and cardiovascular events. JAMA. 2011;306:2704-2714.
10.    Siller-Matula JM, Jilma B, Schror K, Christ G, Huber K. Effect of proton pump inhibitors on clinical outcome in patients treated with clopidogrel: a systematic review and meta-analysis. J Thromb Haemost. 2010;8:2624-2641.
11.    Bauer T, Bouman HJ, van Werkum JW, Ford NF, ten Berg JM, Taubert D. Impact of CYP2C19 variant genotypes on clinical efficacy of antiplatelet treatment with clopidogrel: systematic review and meta-analysis. BMJ. 2011;343:d4588.
12.    Schmidt M, Johansen MB, Robertson DJ, et al. Concomitant use of clopidogrel and proton pump inhibitors is not associated with major adverse cardiovascular events following coronary stent implantation. Aliment Pharmacol Ther. 2012;35:165-174.
13.    Angiolillo DJ, Gibson CM, Cheng S, et al. Differential effects of omeprazole and pantoprazole on the pharmacodynamics and pharmacokinetics of clopidogrel in healthy subjects: randomized, placebo-controlled, crossover comparison studies. Clin Pharmacol Ther. 2011;89:65-74.
14.    Holmberg MT, Tornio A, Neuvonen M, Neuvonen PJ, Backman JT, Niemi M. Grapefruit juice inhibits the metabolic activation of clopidogrel. Clin Pharmacol Ther. 2014;95:307-313.

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From Woodland Clinical Consulting LLC, Green Valley, Arizona and Rutgers-RWJ Medical School, New Brunswick, New Jersey.

Disclosure: The author has completed and returned the ICMJE Form for Disclosure of Potential Conflicts of Interest. The author reports no conflicts of interest regarding the content herein.

Address for correspondence: Neville F. Ford, MD, PhD, Woodland Clinical Consulting LLC, 5481 S. Acacia Creek Drive, Green Valley, AZ 85622 or Rutgers-RWJ Medical School, New Brunswick, NJ 08903. Email: neville@woodfieldclinical.com