How pharmacogenomics can inform patient care

Consider the following scenario: A patient presents to your pharmacy with the results of their 23andMe genetic test and would like to know how these results affect their medications. Their physician recommended they speak to you. You are grateful for their trust, but you are thinking: “I don’t know enough about different pharmacogenomics, or PGx, tests that are out there. I don’t know enough about PGx to make recommendations; the phone is ringing. I don’t have time for this.”

This article will give you some key points when assessing pharmacogenomics, or PGx, tests. Among the key points to consider are genes and variants of clinical significance (pharmvar.org is a good resource for this), the strength of evidence used to make medication recommendations and the context of your patient, including concurrent medications, disease states and such habits as smoking. 

Pharmacogenomics Testing 
There are two forms of PGx testing: direct-to-consumer, or DTC, and clinical. PGx testing analyzes a person’s DNA to predict the patient’s medication metabolism and response. Some are used clinically to help guide required dose and identify increased risk of side effects or lack of efficacy. Variety and lack of standardization are factors you likely are to encounter. 

Genotyping Versus Sequencing
PGx tests assess genetic variants in genes related to drug response (pharmacogenes). They typically are not disease-predictive tests, such as those for Huntington’s disease. 

There are two main types, depending on technologies used: genotyping or sequencing. Most PGx tests use genotyping, which means they look for known and specific variants. They can test only one gene or panel of genes. Their advantages over sequencing are being cheaper, quicker and easier to interpret, and are mostly used for reactive prescribing — patient already is on a medication.   

Sequencing tests (Sanger, NGS) identify all variants in a tested region. They are more expensive, take longer, and results may be difficult to interpret clinically. Their advantages are increased sensitivity, and are more optimal for preemptive prescribing, especially if clinically significant variants are present. 

DTC Versus Clinical Genetic Testing
How DTC tests compare with clinical tests depends on the testing platform in question and genes tested. So far, 23andMe is the only DTC testing platform approved by the Food and Drug Administration for pharmacogenomics testing, which uses a saliva sample to obtain a person’s DNA for the eight common genes, including CYP2D6. It analyzes about 17 variants of CYP2D6, whereas some clinical tests may double that. In general, be aware of “false negatives” — for example, CYP2D6 normal metabolizer, where genetic variants exist but are not identified.

Some DTC platforms test variants that may or may not have clinical significance with varying degrees to support their claims. In addition, not all DTC companies test for the same set of variants. Currently, there are ongoing efforts on recommendations for alleles that should be included in clinical versus direct-to-consumer testing. 

Clinical Evidence Behind Genes Tested 
Many genes are selected based on validation studies that assess clinical utility of drug-gene interaction that are reported in PGx studies. There are several international, curated databases on PGx variants and their guidelines. Efforts are taking place to assign function and determine dosing recommendations for these variants.  

I also would recommend becoming familiar with resources on pharmgkb.org, as well as the Clinical Pharmacogenetics Implementation Consortium, the Dutch Pharmacogenetics Working Group and other sources. 

In summary, resources are available to help you navigate the expanding field of pharmacogenomics testing. Incorporating PGx testing can help you elevate and differentiate your pharmacy practice through individualized medication management.