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First Study of PGx Need in Primary Care Reveals Opportunity, Data Limitations

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Because pharmacogenomics can guide the prescription of several commonly prescribed drugs, people familiar with the field might expect it to prove useful to primary-care physicians. An article printed this month in the journal Pharmacogenomics takes a first crack at this notion, but more research will likely be required before genotyping becomes a common practice in primary care.

What the study showed, among other things, is that almost one-third of study patients were prescribed at least one of a collection of drugs known to elicit adverse events linked to genetic profiles. But as the first pharmacogenomics-related study in the vast arena of primary care, it also revealed a paucity of understanding where the field might eventually have impact. Without concrete genetic data, among other factors, physicians may be unlikely to incorporate pharmacogenomics into prescribing habits.

"I think [pharmacogenomics is] something [that is] a little bit more accepted and progressive in higher-risk disease states, but really hasn't been thought about in primary care quite as much," Gloria Grice, the paper's first author and a researcher at the St. Louis College of Pharmacy, told Pharmacogenomics Reporter last week. "We wanted to see how many patients are on [these drugs] — whether it would be worthwhile to have genetic testing in the primary care setting."


"I think the people who would have the most resistance up front would probably be third-party payors, because it's very expensive to do these kinds of tests, and I'm sure they won't want to pay for it — though over time, they'd probably see the cost savings."

The paper, entitled "Defining the Opportunity for Pharmacogenetic Intervention in Primary Care," documents the prescription of drugs over one year to 607 adult patients visiting three St. Louis clinics, charting the frequency with which 16 drugs were prescribed — drugs whose adverse-event risks can be estimated with pharmacogenetic testing. The study also examines the association of several patient demographic variables with the prescription of these drugs, but it does not explore what effect a physician's knowledge of patients' genetic profiles would have on treatment choices.

About 29 percent of patients visiting three primary-care offices in the St. Louis area had taken one or more drugs in the previous year whose adverse reactions have the potential to be anticipated through genomic testing, according to the study, which was conducted by Howard McLeod's group at the Washington University School of Medicine in St. Louis and the St. Louis College of Pharmacy. The researchers sought 16 drugs in patient interviews and in patient records that had previously been associated in research with adverse events that vary with patient genetics [see sidebar].

(The current printing of the journal article inaccurately states that about 29 percent of these patients had taken more than one of these drugs during the period studied. In fact, the patients were prescribed at least one of the drugs.)

The authors wrote that their study was the first to examine the potential of pharmacogenomics in the primary-care setting, with previous efforts limited to high-risk drugs or those having a narrow therapeutic window, "with little focus on other drugs that are widely used."

Without any knowledge of how many patients face a higher risk of adverse drug events, the fact that about 29 percent of patients take these drugs "wouldn't influence me at all" in deciding whether to adopt pharmacogenetic testing, said David Campbell, a primary-care physician and president and CEO of the Institute for Research and Education in Family Medicine in St. Louis. Campbell's institute provided some of the data used in the study, but he was not aware of the focus of the research or its findings, he said.

"You'd have to tie it in and correlate it with doing the genetic sampling," Campbell said. "If you have a genetic abnormality and you take this medicine, what's going to happen in what percentage of cases?"

The next studies of pharmacogenomics in primary care should produce those data, said Grice. "We know that these medications cause problems, we know these patients are on them; how many of these patients have these problems?" she said. "And then the next step really would be to do something more interventional, along the lines of exposing primary-care patients to genetic testing prior to making clinical decisions for chronic problems," and comparing decisions made with and without the benefit of genetic knowledge.

Cataloging Adverse Events

Of 16 drugs whose adverse events are known to vary by genotype, the percentage of each taken over the course of one year by 609 St. Louis primary-care patients is listed below.

Drug
Percent of patients
Ibuprofen
60.3
Naproxen
26.2
Fluoxetine
6.4
Metoprolol
5.9
Diltiazem
5.1
Warfarin
4.6
Erythromycin
4.0
Verapamil
3.3
Nortriptyline
1.8
Imipramine
1.3
Phenytoin
0.98
Carbamazepine
0.8
Piroxicam
0.33
Theophylline
0.33
Isoniazid
0
Rifampicin
0
SOURCE: Grice GR, Seaton TL, Woodland AM, and McLeod HL: Defining the Opoortunity for Pharmacogenetic Intervention in Primary Care. Pharmacogenomics 7(1), 61-65 (2006).

For pharmacogenomics to enter primary-care practice, certain conditions would have to be fulfilled, said Campbell. "I think [primary-care use] probably requires more continuing medical education about the importance of it," he said. "The other thing is that you have to make sure that it's going to be something that various insurance [companies] are going to recognize the importance of and cover, because it's going to be pretty hard to convince the patient — particularly when the risk [of adverse events] is still probably relatively small — to pursue testing that may get a little pricey," he said.

"I don't know how [primary care physicians] are going to feel about [pharmacogenomics]. If I had to guess, I'd say they would love it," said Grice. "I think the people who would have the most resistance up front would probably be third-party payors, because it's very expensive to do these kinds of tests, and I'm sure they won't want to pay for it — though over time, they'd probably see the cost savings," she added.

The demographics associated with patients taking these types of drugs in the primary-care setting did not reveal that genetic testing efforts should be concentrated on individuals of a particular ethnicity or gender. But perhaps unsurprisingly, the study revealed a significant association between age and the use of this class of medications.

"There is actually a good chunk of patients who visit their doctor routinely … that are on a medication that's got genetic variability," said Grice. The median age of patients who had not taken any of these drugs in the previous year was 43.4 years old, versus 53.2 years old among patients who had taken one or more during the same period.

— Chris Womack ([email protected])

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