The U.S. Food and Drug Administration (FDA) recently issued a table of gene-drug interactions for which genetic testing is appropriate for helping to determine safe doses of particular drugs for individuals.
Such testing—called pharmacogenetic testing—help healthcare practitioners determine whether a person's genetic makeup will influence how they respond to a certain medication or class of medications. This information can help healthcare practitioners tailor the amount of medication prescribed (which may be lower or higher than the usual dose) or prescribe an alternate drug if necessary. A healthcare practitioner may order testing prior to prescribing certain drug therapies, or when a patient who has started taking a drug doesn't respond to therapy or experiences side effects.
Pharmacogenetic tests are performed on blood or saliva samples, or swabs from the inside of the cheek (buccal swab). They detect changes (variants) in genes that affect response to drugs. These genetic variants can alter, for example:
- Enzymes that break down (metabolize) drugs within the body
- Enzymes that convert a drug to a more active form
- Targets of drugs
- Immune system responses to drugs
(For more detail, see the article on Pharmacogenetic Tests.)
Not everyone who is prescribed medication needs pharmacogenetic testing and not every drug has an applicable pharmacogenetic test associated with it. Generally, healthcare practitioners prescribe most medications according to established practices and do not order pharmacogenetic testing for most people or for most drugs. Usually these tests are reserved for drugs that are known to produce certain responses in people with particular genetic variants.
The FDA's table, published on February 25, 2020, includes certain established gene-drug interactions that already appear in FDA-approved drug labeling and some additional gene-drug interactions that are both consistent with the current FDA labeling and backed by what the agency considers sufficient scientific evidence based on published literature.
The table lists medications and associated genetic variants in three sections:
- The first section includes interactions between specific drugs and genetic variants that the agency says have sufficient evidence to warrant pharmacogenetic testing to determine proper doses for individuals. Common drugs in this section include the HIV drug abacavir, the pain drug tramadol, and the antiplatelet drug clopidogrel, which is prescribed to prevent heart attacks and strokes.
- Included in the second part of the table are drugs and genes with evidence that indicates some potential impact on safety. Examples include the cholesterol-lowering drug simvastatin and the gout medication allopurinol.
- The third section lists genetic variants that are associated with changes in concentrations of drugs and/or drug metabolites but may not require dose adjustments and may not benefit from pharmacogenetic testing. These include the cancer drug tamoxifen and some antidepressants, such as amitriptyline.
The table comes after years of the FDA determining how the agency should approach pharmacogenetic testing and concerns about such testing. An October 2018 FDA safety alert for physicians and patients called into doubt results from some pharmacogenetic tests and software used to make treatment recommendations. The alert gave as an example certain pharmacogenetic tests used for determining antidepressant doses. It said that the relationship between genetic variations and the effectiveness of antidepressant medication had not been established.
In 2019, the FDA sent a warning letter to one genomics laboratory that offered pharmacogenetic testing, saying that the agency was unaware of any data establishing that certain tests offered by the lab were able to predict patients' responses to drugs as claimed. Similarly, the agency reached out to other labs about how they communicated interpretation of pharmacogenetic results as they relate to certain drugs.
In a statement accompanying the table, the FDA says it is a way to both balance patient access to pharmacogenetic tests based on good science and protect public health. "Consistent with our mission to protect and promote public health, we believe it is important to take steps now to help ensure that claims being made for pharmacogenetic tests offered today are grounded in sound science to avoid inappropriate management of patients' medications—and to do so through approaches that both protect patients and advance the development of analytically and clinically validated pharmacogenetic tests," the statement says. Analytical and clinical validation are rigorous lab processes that are run to ensure that tests perform as intended.
The FDA says its table is not comprehensive. The agency continues to review scientific evidence, including studies that support various medical guidelines, and will update the table periodically. Meanwhile, the FDA has asked for public comment from various interested parties, including scientists, medical professionals, patients, and industry experts about specific gene-drug associations or other information that should be added or removed from the table.