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Where Does Evidence for Pharmacogenetics Come From?

FDA, CPIC, and PharmGKB logo

One commonly asked question is, “Where does evidence for pharmacogenetics (PGx) come from?” You may have noticed that there are icons next to different medications, like citalopram, on the Genomind pharmacogenetic report.

These icons identify medications that have PGx guidance available, the Rx icon indicating FDA biomarker guidance and the notepad icon indicating a CPIC or DPWG biomarker guidance.

These are the principle organizations which make recommendations on associations for gene-drug pairs, which in turn were and are used to develop our pharmacogenetic test.

Does the FDA recognize pharmacogenetic associations?

The US Food and Drug Administration (FDA) is a federal regulatory agency responsible for protecting the public health by ensuring the safety, efficacy, and security of human and veterinary drugs, biological products, and medical devices, food, and more.

The FDA includes gene-drug associations in many mandatory product labels, or package inserts (PI).

Generally, these data have been provided by manufacturers, and have solid evidence behind them. However, many drugs are generic, and may receive little support from the manufacturers, so newer associations which may impact human health and disease often do not wind up in the PI.

You can view a list of Pharmacogenomic Biomarkers in Drug Labeling on the FDA website.

Recently, the FDA also created a Table of Pharmacogenetics Associations, which includes PGx associations that they have evaluated and for which they believe “there is sufficient scientific evidence to suggest that subgroups of patients with certain genetic variants, or genetic variant-inferred phenotypes are likely to have altered drug metabolism, and in certain cases, differential therapeutic effects, including differences in risks of adverse events”.

What is the role of the Clinical Pharmacogenetics Implementation Consortium (CPIC)?

The Clinical Pharmacogenetics Implementation Consortium (CPIC) is an international consortium of individual volunteers and a small, dedicated staff who facilitate the use of pharmacogenetic tests for patient care.

CPIC curates and posts peer-reviewed, evidence-based, and detailed gene/drug clinical practice guidelines. Some CPIC Guidelines support gene-drug associations that the FDA does not mention and are incorporated in Genomind’s pharmacogenetic report.

CPIC also has taken on the responsibility of standardizing the field of pharmacogenetics, including terminology and also genotype to phenotype conversion tables.

What is the role of the Dutch Pharmacogenetics Working Group (DPWG)?

The Dutch Pharmacogenetics Working Group (DPWG) was established by the Royal Dutch Pharmacists Association with the objective of developing pharmacogenomic-based therapeutic (dose) recommendations.

Up to now, DPWG has developed and published pharmacogenomic guidelines for over 80 drugs and are updated every 3 months.

Many PGx guidance recommendations are based on pharmacokinetic (PK) gene-drug interactions.

Taking citalopram as an example, all three organizations have similar guidance for individuals who are CYP2C19 poor metabolizers, recommending not to exceed a certain dose or considering a lower starting dose due to the risk of QT prolongation.

PGx Guidance Comparison Chart for Citalopram (FDA, CPIC, and DPWG)

Guidance on Citalopram Dosing for CYP2C19 Poor Metabolizers
Organization PGx Guidance
FDA1 Citalopram 20 mg/day is the maximum recommended dose in CYP2C19 poor metabolizers due to the risk of QT prolongation
CPIC2 Consider a 50% reduction of recommended starting dose and titrate to response or select alternative drug not predominantly metabolized by CYP2C19
DPWG3 Do not exceed the following daily doses (50% of the standard maximum dose):

1. adults up to 65 years: 20 mg as tablets or 16 mg as drops

2. adults 65 years or older: 10 mg as tablets or 8 mg as drops

In Conclusion

These organizations are important in the PGx world because they provide expert guidance on how to integrate PGx testing into clinical practice and support dose adjustments for specific medications based on significant PK gene-drug interactions.

Additionally, PharmGKB, the Pharmacogenetics Knowledge Base, is a NIH supported database of pharmacogenetics that curates all the guideline info from FDA, CPIC, and DPWG. PharmGKB is an extensive, nicely organized database of gene-drug associations and even ranks these associations with a Level of Evidence score. You can learn more about pharmacogenetics on the PharmGKB website.

As these organizations continue to make updates based on continual review of new scientific literature, we at Genomind continue to apply those changes accordingly to our test.

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  1. Allergan USA, Inc. Celexa (citalopram) [package insert]. U.S. Food and Drug Administration website. Revised January 2019. Accessed December 29, 2020.
  2. Hicks JK, Bishop JR, Sangkuhl K, Mueller DJ, Ji Y, Leckband SG, et al. Clinical Pharmacogenetics Implementation Consortium (CPIC) guideline for CYP2D6 and CYP2C19 genotypes and dosing of selective serotonin reuptake inhibitors. Clin Pharmacol Ther. 2015;98(2):127-134.
  3. Bank PCD, Caudle KE, Swan JJ, Gammal RS, Whirl-Carrillo M, Klein TE, et al. Comparison of the guidelines of the Clinical Pharmacogenetics Implementation Consortium and the Dutch Pharmacogenetics Working Group. Clin Pharmacol Ther. 2018;103(4):599-618.

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