The Right Dose for the Right Person

Warfarin – the most widely used anticoagulant in the world, it’s a well-known drug even amongst non-experts. Phenytoin, an anticonvulsant, is also among the most commonly prescribed drugs in its class. But despite their ubiquity, it can be difficult to establish the minimal effective dose for a patient. This is especially true in situations where their genotypes might be affecting metabolism of the drug – for instance, in patients who exhibit mutations in genes coding for the cytochrome P450 enzyme CYP2C9.

For this reason, the Association for Molecular Pathology (AMP) Pharmacogenetics Working Group has developed a series of guidelines to help standardize clinical testing for these genes (1). The AMP-led working group included organizational representation from both the College of American Pathologists and the Clinical Pharmacogenetics Implementation Consortium (CPIC). To learn more, we spoke to Victoria M. Pratt, Associate Professor and Director of Pharmacogenetics and Molecular Genetics Laboratories at the Indiana University School of Medicine, AMP President, and PGx Working Group Chair.

What is the value of CYP2C9 genotyping?

The AMP PGx Working Group started with CYP2C19 and CYP2C9 genotyping panels due to the widespread adoption of these tests and our desire to help physicians, pharmacists, researchers, and other stakeholders better understand what these panels include and what the test results mean. The cytochrome P450 2C9 is one of the most abundant and important drug-metabolizing enzymes. It is involved in the phase I metabolism of many commonly prescribed medications, including the anticoagulant warfarin and the anticonvulsant phenytoin. The new CYP2C9 report focuses on testing that can be applied to all CYP2C9-related medications.

When is CYP2C9 genotyping most useful?

Because CYP2C9 testing is closely associated with the metabolism of warfarin and phenytoin, genotyping tests could be used to elucidate why a particular person reacts a certain way to medication. The CYP2C9 enzyme metabolizes the more potent S-warfarin enantiomer, and the CYP2C9 *2, *3, *5, *6, *8, and *11 alleles are associated with reduced S-warfarin clearance. Data consistently demonstrate reduced warfarin dose requirements in individuals who carry these variant alleles.

Patients with a reduced- or no-function CYP2C9 allele are more likely to have impaired phenytoin metabolism and require lower doses of the drug to prevent neurologic toxicity. Clinical Pharmacogenetics Implementation Consortium guidelines recommend consideration of lower phenytoin doses in patients who carry such alleles.

What are the key points of the new guidelines?

This new report offers a two-tier categorization of CYP2C9 alleles as an aid for designing CYP2C9 genotyping assays. Our goal is to promote standardization of pharmacogenomic gene/allele testing across clinical laboratories. Using criteria such as allele frequencies in different populations and ethnicities, the availability of reference materials, and other technical considerations, we recommended a Tier 1 list of a minimum set of alleles and their defining variants that should be included in all clinical CYP2C9 pharmacogenomic tests. We also defined a Tier 2 list of optional CYP2C9 alleles that do not currently meet one or more of the criteria for inclusion in Tier 1. These recommendations are not to be interpreted as restrictive, but to provide a reference guide.

What should diagnostic professionals know about CYP2C9 genotyping?

We considered a number of important factors, including the functional impact of the variants, allele frequencies in different populations and ethnicities, the availability of reference materials, as well as other technical considerations for pharmacogenomic testing. We highly recommend that clinical laboratory professionals include our Tier 1 set of alleles, which

• have been well-characterized and shown to significantly affect the function of the protein and/or gene leading to an alteration in a drug response phenotype,
• have an appreciable minor allele frequency in a population/ethnicity group, and
• have publicly available reference materials.

These recommendations should help to standardize testing and genotyping concordance between laboratories.

Pharmacogenetics is a rapidly changing field, and we intend to update these recommendation documents as new data and reference materials become available. AMP members are among the early adopters of molecular diagnostic testing in clinical settings, and we are committed to continuously improving professional practice and patient care. We recognize that there are additional alleles not listed in the guidelines – there are more than 60 CYP2C9 alleles currently listed in the PharmVar database – and we expect to add some of them to our Tier 1 or Tier 2 lists in the future as new data concerning functional impact, frequency, and reference materials becomes available.

We are also aware that our recommendations to include the alleles more prevalent among African and African American populations may be difficult to implement with currently available genotyping platforms. We concluded that failure to include these alleles could lead to inaccurate CYP2C9 phenotype prediction among individuals with known or unknown African ancestry and could potentially contribute to existing health care disparities in these populations.