CYP2C19 genotype testing to guide clopidogrel use after ischaemic stroke or transient ischaemic attack

People who have had a stroke are at increased risk of further occlusive vascular events, such as recurrent stroke or myocardial infarction. For those with non-cardioembolic ischaemic stroke or transient ischaemic attack (TIA), the antiplatelet drug clopidogrel can be used to reduce this risk.

People with non-minor ischaemic stroke are normally offered clopidogrel after taking aspirin for 2 weeks. People with TIA or minor stroke may start clopidogrel immediately. Detailed guidance on current practice can be found in NICE's guidance on stroke and transient ischaemic attack in over 16s, NICE's guidance on clopidogrel and modified-release dipyridamole for the prevention of occlusive vascular events, and the Royal College of Physicians national clinical guideline for stroke.

Clopidogrel is a prodrug. The CYP2C19 gene encodes the CYP2C19 enzyme, which is needed to metabolise clopidogrel to its active form.

The CYP2C19 gene has many alternative versions (alleles) that produce variations of the CYP2C19 enzyme with different levels of activity. Each allele is given a star (*) number for identification. The function of each allele can be described as 'normal', 'completely absent', 'decreased' or 'increased', or the function may be uncertain.

Clopidogrel is less effective in people with alleles that produce CYP2C19 enzymes with completely absent or decreased function.

People with 2 loss-of-function alleles have no CYP2C19 enzyme activity. They cannot activate clopidogrel to its active form and are classed as 'poor metabolisers'. People with only 1 loss-of-function allele have reduced enzyme activity and are classed as 'intermediate metabolisers'.

Loss-of-function alleles are more common in certain ethnic groups, such as people with an Asian family background. The Clinical Pharmacogenetics Implementation Consortium's guideline for clopidogrel and CYP2C19 contains further information about the distribution of loss-of-function alleles by ethnic group.

CYP2C19 genotype testing can identify which CYP2C19 alleles a person has. This provides information on how well a person can metabolise clopidogrel and so can be used to guide antiplatelet treatment.

Clinical experts indicated that there are several methods that diagnostic genetic laboratories could use to implement CYP2C19 genotype testing. These include:

• Gene sequencing approaches, which determine the order of DNA bases in a particular DNA segment. In NHS laboratories, this could be done through Sanger sequencing or next-generation sequencing.

• Targeted genotyping assays, which are used to amplify and detect specific variants in target genomic DNA. The methods of detection, variants detected, equipment requirements and throughput capability vary between systems.
  
  Laboratory testing usually requires a blood sample.

The Genedrive System is a point-of-care gene amplification device used for qualitative in vitro molecular diagnostic tests. The company state that the test can detect the *2, *3, *4, *8, *17 and *35 alleles. The technology consists of:

• Genedrive System analyser, which is a rapid thermocycler capable of polymerase chain reaction (PCR) and isothermal-based amplification techniques.

• Genedrive CYP2C19 ID Kit, which includes an assay cartridge containing reagents, a sample collection cheek swab, a transfer capillary and a collection buffer. The cartridges can be stored at room temperature.

The test uses a single cheek swab to collect the sample. The company states that each cartridge takes less than 1 hour to run. The result of the test is automated. The person's diplotype (their specific pair of CYP2C19 alleles) and metaboliser status are displayed on the device. The company states that results can be transferred electronically to patient records by internet or third-party middleware, or printed with an optional label printer. External controls for all targeted alleles are available in a separate kit to check proper performance of the platform.

The Genomadix Cube CYP2C19 system (Genomadix) is a point-of-care DNA test used to detect the *2, *3 and *17 alleles of the CYP2C19 gene. The technology consists of:

• Genomadix Cube platform, which includes the Genomadix analyser thermal cycling instrument for PCR amplification, the user-interface software and a barcode scanner.

• Genomadix Cube test kit, which includes cheek swabs and a cartridge containing all the reagents needed to determine CYP2C19 genotype. The cartridges must be stored between −15°C and −80°C and used within 15 minutes of removal from the freezer.

Samples are run on the Genomadix Cube CYP2C19 system, which combines and automates DNA extraction, PCR amplification and fluorescence-based detection of CYP2C19 alleles. The test uses 3 cheek-swab samples, which are inserted into the reagent cartridge. The company states that the test takes 1 hour to run for each cartridge.

The test report will either display the detected diplotype or an inconclusive result. When the test result is inconclusive, the company state that the test should be repeated with new swabs and a new cartridge. Results are stored locally on a laptop connected to the device and can be exported as a PDF. Optional external controls are available to check proper performance of the platform as per local requirements for accreditation.