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Gene Tests Could Predict if a Drug Will Work for a Patient


 

PGx checks are already strongly recommended or considered routine before some medications are prescribed. These include abacavir (Ziagen), an antiviral treatment for HIV that can have severe side effects in people with one gene variant.

The US Food and Drug Administration (FDA) recommends genetic testing for people with colon cancer before starting the drug irinotecan (Camptosar), which can cause severe diarrhea and raise infection risk in people with a gene variant that slows the drug’s elimination from the body.

Genetic testing is also recommended by the FDA for people with acute lymphoblastic leukemia before receiving the chemotherapy drug mercaptopurine (Purinethol) because a gene variant that affects drug processing can trigger serious side effects and raise the risk for infection at standard dosages.

“One of the key benefits of pharmacogenomic testing is in preventing adverse drug reactions,” Dr. Wiisanen said. “Testing of the thiopurine methyltransferase enzyme to guide dosing with 6-mercaptopurine or azathioprine can help prevent myelosuppression, a serious adverse drug reaction caused by lower production of blood cells in bone marrow.”

When, Why, and How to Test

“A family doctor should consider a PGx test if a patient is planning on taking a medication for which there is a CPIC guideline with a dosing recommendation,” said Teri Klein, PhD, professor of biomedical data science at Stanford University in California, and principal investigator at PharmGKB, an online resource funded by the NIH that provides information for healthcare practitioners, researchers, and consumers about PGx. Affiliated with CPIC, it’s based at Stanford University.

You might also consider it for patients already on a drug who are “not responding or experiencing side effects,” Dr. Caudle said.

Here’s how four PGx experts suggest consumers and physicians approach this option.

Find a Test

More than a dozen PGx tests are on the market — some only a provider can order, others a consumer can order after a review by their provider or by a provider from the testing company. Some of the tests (using saliva) may be administered at home, while blood tests are done in a doctor’s office or laboratory. Companies that offer the tests include ARUP Laboratories, Genomind, Labcorp, Mayo Clinic Laboratories, Myriad Neuroscience, Precision Sciences Inc., Tempus, and OneOme, but there are many others online. (Keep in mind that many laboratories offer “lab-developed tests” — created for use in a single laboratory — but these can be harder to verify. “The FDA regulates pharmacogenomic testing in laboratories,” Dr. Wiisanen said, “but many of the regulatory parameters are still being defined.”)

Because PGx is so new, there is no official list of recommended tests. So you’ll have to do a little homework. You can check that the laboratory is accredited by searching for it in the NIH Genetic Testing Laboratory Registry database. Beyond that, you’ll have to consult other evidence-based resources to confirm that the drug you’re interested in has research-backed data about specific gene variants (alleles) that affect metabolism as well as research-based clinical guidelines for using PGx results to make prescribing decisions.

The CPIC’s guidelines include dosing and alternate drug recommendations for more than 100 antidepressants, chemotherapy drugs, the antiplatelet and anticlotting drugs clopidogrel and warfarin, local anesthetics, antivirals and antibacterials, pain killers and anti-inflammatory drugs, and some cholesterol-lowering statins such as lovastatin and fluvastatin.

For help figuring out if a test looks for the right gene variants, Dr. Caudle and Dr. Wright recommended checking with the Association for Molecular Pathology’s website. The group published a brief list of best practices for pharmacogenomic testing in 2019. And it keeps a list of gene variants (alleles) that should be included in tests. Clinical guidelines from the CPIC and other groups, available on PharmGKB’s website, also list gene variants that affect the metabolism of the drug.

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