Recently, the British Journal of Clinical Pharmacology published the first clinical guideline developed by the UK Centre of Excellence for Regulatory Science and Innovation in Pharmacogenomics (CERSI PGx), titled “CYP2C19 genotype testing for clopidogrel: A guideline developed by the UK Centre of Excellence for regulatory science and innovation in pharmacogenomics (CERSI PGx)”. This milestone document focuses on the clinical value of CYP2C19 genotyping in guiding clopidogrel therapy.
About CERSI PGx
CERSI PGx is one of seven UK government backed regulatory science and innovation centres launched in January 2025. Led by the University of Liverpool, it is jointly funded by Innovate UK, the Medical Research Council (MRC), the Medicines and Healthcare products Regulatory Agency (MHRA), and the Office for Life Sciences (OLS). The centre aims to accelerate the safe and effective integration of pharmacogenomics (PGx) into the National Health Service (NHS) by addressing key implementation barriers. This guideline marks the very first clinical guidance issued since CERSI PGx was established.
Why CYP2C19 Matters for Clopidogrel
CYP2C19 is a key member of the cytochrome P450 enzyme family, responsible for the metabolic activation or inactivation of many drugs. Genetic polymorphisms in CYP2C19 lead to significant inter individual differences in drug metabolism, affecting efficacy and safety.
Clopidogrel is a widely used antiplatelet agent for preventing thrombotic events in coronary artery disease, ischaemic stroke, peripheral artery disease, and atrial fibrillation. As a prodrug, clopidogrel requires metabolic activation by CYP2C19. The guideline classifies individuals into ultrarapid, rapid, normal, intermediate, and poor metabolisers based on CYP2C19 genotype. Carriers of loss of function alleles (e.g., CYP2C192 and *3*) – intermediate and poor metabolisers – cannot effectively activate clopidogrel, leading to insufficient platelet inhibition and an increased risk of recurrent thrombosis.
The frequency of the CYP2C192 allele is approximately 15% in Europeans, 30% in South Asians, and as high as 60% in indigenous Oceanian populations.
Key Recommendation: Universal CYP2C19 Testing for Clopidogrel
The guideline states that, regardless of the indication, all patients for whom clopidogrel is being considered should undergo CYP2C19 genotyping. Based on the results, antiplatelet therapy should be optimised:
-Poor metabolisers should avoid clopidogrel and preferentially use alternative drugs that do not depend on CYP2C19 metabolism, such as ticagrelor or prasugrel.
-Intermediate metabolisers should also consider alternative agents or adjusted regimens rather than simply increasing the clopidogrel dose.
In the UK, clopidogrel is approved for secondary prevention of atherothrombotic events, for moderate to high risk transient ischaemic attack (TIA) or mild ischaemic stroke, and for preventing atherothrombotic and thromboembolic events in atrial fibrillation.
Beyond Clopidogrel: Other Drugs Where CYP2C19 Genotyping Is Critical
The value of CYP2C19 genotyping extends far beyond clopidogrel. As a major drug metabolising enzyme, CYP2C19 also plays a critical role in the metabolism of voriconazole, several antidepressants, and proton pump inhibitors (PPIs). Multiple international and national guidelines recommend genotype guided individualisation for these drugs.
1. Antidepressants (SSRIs)
Selective serotonin reuptake inhibitors (SSRIs) – such as sertraline, citalopram, and escitalopram – are first line agents for depression and are primarily metabolised by CYP2C19. CYP2C19 enzyme activity directly determines the plasma concentration of these drugs. Poor metabolisers have a 30%–60% reduction in drug clearance, which predisposes them to adverse effects such as QT interval prolongation and sedation. Ultrarapid metabolisers often have subtherapeutic plasma concentrations, leading to delayed treatment response and an increased risk of drug discontinuation.
The 2023 Clinical Pharmacogenetics Implementation Consortium (CPIC) guideline states that poor metabolisers taking citalopram or escitalopram have an elevated risk of QT prolongation and recommends a 50% dose reduction. The 2021 Dutch Pharmacogenetics Working Group (DPWG) guideline advises that poor metabolisers should receive a maximum dose of escitalopram reduced by 50%, and that ultrarapid metabolisers should avoid escitalopram altogether. For sertraline, the DPWG recommends a daily dose not exceeding 75 mg in poor metabolisers.
Importantly, the recently published Chinese Expert Consensus on Pharmacogenomic Testing in Psychiatry (2025) – developed by the Precision Medicine Collaboration Group of the Chinese Society of Psychiatry – explicitly includes recommendations for CYP2C19 genotyping. Consensus statement notes that dose adjustment recommendations from international guidelines such as CPIC and DPWG for drug metabolising enzymes (including CYP2C19) can be referenced for Chinese populations. Therefore, CYP2C19 genotyping before initiating SSRI therapy (e.g., escitalopram) enables dose optimisation or switching to alternative drugs not metabolised by CYP2C19, thereby achieving precision treatment, improving response rates, and reducing adverse events.
2. Proton Pump Inhibitors (PPIs)
Proton pump inhibitors – including omeprazole, lansoprazole, and pantoprazole – are widely used for acid related disorders such as gastroesophageal reflux disease and peptic ulcers. Their metabolism is also highly dependent on CYP2C19. Patients with different CYP2C19 genotypes show significant variability in response to PPIs. Carriers of loss of function alleles (*2, *3) have markedly increased drug exposure, which may enhance acid suppression but also raise the risk of adverse effects. In contrast, normal metabolisers have relatively lower plasma concentrations and may experience weaker acid suppression, although inter individual variation remains substantial.
The 2020 CPIC guideline for PPIs advises that ultrarapid metabolisers taking omeprazole or similar drugs metabolise the drug too quickly, resulting in insufficient plasma concentrations and poor acid suppression. In these patients, the dose should be increased and therapeutic response monitored. For poor metabolisers, drug clearance is slow and plasma concentrations may be elevated; while efficacy may be better, the potential for drug toxicity is increased. Dose reduction and monitoring of response are reasonable considerations. Therefore, for patients initiating PPI therapy or those experiencing poor response or adverse effects, CYP2C19 genotyping is recommended to guide individualised dosing, optimise efficacy, and minimise adverse events.
3. Voriconazole
Voriconazole is a broad spectrum antifungal agent used to treat serious fungal infections such as invasive aspergillosis. It has a narrow therapeutic window: excessively high plasma concentrations increase the risk of hepatotoxicity and visual disturbances, while low concentrations result in treatment failure. The metabolism of voriconazole is primarily mediated by CYP2C19, and genetic polymorphisms have a profound impact on its plasma concentration.
The CPIC published a dedicated guideline on CYP2C19 and voriconazole in 2016. It states that ultrarapid metabolisers have reduced voriconazole trough concentrations and often fail to reach target therapeutic levels. Poor metabolisers have elevated trough concentrations and a significantly increased risk of adverse reactions. The CPIC guideline provides specific dosing recommendations based on genotype. For example, adult ultrarapid metabolisers should receive alternative first line agents that do not depend on CYP2C19 metabolism, such as isavuconazole, liposomal amphotericin B, or posaconazole. Therefore, CYP2C19 genotyping before voriconazole therapy enables individualised dosing and reduces the incidence of drug related adverse events.
Clinical Significance: Making Medication More Reliable
The newly released guideline once again places CYP2C19 genotyping at the forefront of precision medicine. However, it is important to recognise that the clinical applications of CYP2C19 genotyping go far beyond clopidogrel – from voriconazole (antifungal) and SSRIs (antidepressants) to proton pump inhibitors for acid suppression. The CYP2C19 genotype acts as a “compass” for drug therapy.
As precision medicine gains wider acceptance, an increasing number of authoritative guidelines are incorporating CYP2C19 genotyping into routine medication workflows. For patients, knowing their own CYP2C19 genotype helps them understand their individual drug response profile and enables shared decision making with their physician to develop a more suitable treatment plan. For clinicians, integrating objective genetic test results into prescribing decisions is a powerful means to improve treatment quality and ensure patient safety.
Macro & Micro-Test’s CYP2C19 Genotyping Solution
Macro & Micro Test offers a CYP2C19 genotyping kit based on an improved amplification refractory mutation system (ARMS) combined with Taqman probes with the following features:
-Comprehensive allele coverage – detects CYP2C192, *3, and *17 without missing key variants.
-Robust quality control – incorporates negative/positive controls, an internal control, and UDG enzyme for four level quality control to ensure accurate results.
-Automated extraction – compatible with Macro & Micro-Test’s fully automated nucleic acid extractor, improving workflow efficiency.
-Broad compatibility – works with mainstream real time PCR instruments on the market, including the ABI 7500 Hongshi SLAN 96P.
-Automated result interpretation – dedicated analysis software (on ABI 7500, SLAN 96P, etc.) enables automatic interpretation of results, enhancing efficiency.
-POCT ready automation – HWTS AIO800 Fully Automated Nucleic Acid Amplification Analyzer enables “sample in, result out” operation.
With the continuous advancement of pharmacogenomics, CYP2C19 genotyping is expected to benefit an increasing number of patients, moving precision medicine from concept to routine clinical practice. The newly published CERSI PGx guideline reinforces the critical role of CYP2C19 testing not only for clopidogrel but also for a growing list of drugs, including antidepressants, proton pump inhibitors, and voriconazole. To facilitate the widespread adoption of genotype guided prescribing, reliable and user friendly testing solutions are essential. Macro & Micro-Test’s pharmacogenomic testing portfolio, featuring comprehensive allele coverage, robust quality control, and automation ready platforms, aims to support healthcare providers in implementing precision medicine and ultimately safeguarding patient health.
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References:
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2.Lee CR, Luzum JA, Sangkuhl K, et al. Clinical Pharmacogenetics Implementation Consortium Guideline for CYP2C19 Genotype and Clopidogrel Therapy: 2022 Update. Clin Pharmacol Ther. 2022. doi:10.1002/cpt.25261.
3.National Institute for Health and Care Excellence (NICE). CYP2C19 genotype testing to guide clopidogrel use after ischaemic stroke or transient ischaemic attack. Diagnostics guidance DG59. Published: 31 July 2024.
4.Precision Medicine Research Collaboration Group of the Chinese Society of Psychiatry. Expert consensus on pharmacogenomic testing in psychiatry (2025) [Zhonghua Jing Shen Ke Za Zhi]. Chinese Journal of Psychiatry. 2025;58(6):434-445. doi:10.3760/cma.j.cn11366120240611-00181
5.Dello Russo C, Frater I, Kuruvilla R, et al. CYP2C19 genotype testing for clopidogrel: A guideline developed by the UK Centre of Excellence for regulatory science and innovation in pharmacogenomics (CERSI-PGx). Br J Clin Pharmacol. 2025. DOI: 10.1093/bjcp/…
6.Moriyama B, Owusu Obeng A, Barbarino J, et al. Clinical Pharmacogenetics Implementation Consortium (CPIC) Guidelines for CYP2C19 and Voriconazole Therapy. Clin Pharmacol Ther. 2017;102(1):45-51. doi:10.1002/cpt.595.
7.Bousman CA, Stevenson JM, Ramsey LB, et al. Clinical Pharmacogenetics Implementation Consortium (CPIC) Guideline for CYP2D6, CYP2C19, CYP2B6, SLC6A4, and HTR2A Genotypes and Serotonin Reuptake Inhibitor Antidepressants. Clin Pharmacol Ther. 2023;114(1):51-68. doi:10.1002/cpt.2903.
8.Brouwer JMJL, Nijenhuis M, Soree B, et al. Dutch Pharmacogenetics Working Group (DPWG) guideline for the gene-drug interaction between CYP2C19 and CYP2D6 and SSRIs. Eur J Hum Genet. 2021. doi:10.1038/s41431-021-00894-2.
Post time: Apr-22-2026