Companion Diagnostics Hit the Big Time

By Mark Terry

Companion diagnostics recently came on the scene for healthcare providers in a big way with a sensitivity test for the anticoagulant drug warfarin (Coumadin). Companion diagnostics, also known as personalized medicine, is, simply put, a laboratory test that indicates whether a patient has a genetic sensitivity to a particular drug.

Warfarin is used to treat strokes, prevent heart attacks, and treat blood clots. The reason the warfarin sensitivity test made such a splash is the sheer number of patients receiving warfarin treatment. According the F.D.A., approximately twenty million people take the drug in the U.S., and two million Americans start warfarin treatment each year. Roughly forty-three thousand cases a year end up in the emergency room due to warfarin-related adverse effects. The only medication with more complications and adverse side effects is insulin.

Michael McGarrity, chief marketing officer of Nanosphere, Inc., the first company to receive FDA clearance for a warfarin sensitivity test, compares companion diagnostics to Goldilocks and the Three Bears. In the case of warfarin, if it's prescribed and the patient metabolizes it too slowly, there's no effect. If the patient metabolizes it too quickly, it becomes toxic and can have adverse side effects such as bleeding or blood clots. But, if it's dosed properly, it's just right. McGarrity says, "Most companion diagnostics is that kind of story: it's not going to do anything for you, kill you, or work just great."

Michael Murphy, president and CEO of ParagonDx , a company that manufactures warfarin sensitivity test kits, thinks this new age of companion diagnostics means nurses will have new things to look for in their patient population.

"There is an answer for a problem they might have been seeing all along," he says, "but they might not have known it had a genetic component. They can go back to the doctor and report what they're seeing and possibly help the patient out by getting them access to a genetic test that can help explain why they have bruising all over their body or unusual bleeding."

The warfarin sensitivity test, like most companion diagnostics, is a molecular diagnostic test. In the case of the warfarin test, people have two genes, CYP2C9 and VKORC1, that are involved in drug metabolism. Specific mutations in those genes indicate the individual's sensitivity to the drug. It was only recently that the FDA suggested the test be used in all patients receiving warfarin. But, the test is not picking up steam the way industry experts had hoped because most insurers do not cover it.

Companion diagnostics is also being used to test patients' sensitivity to several other drugs. HER-2/neu testing is available for susceptibility to the breast cancer drug Herceptin. Breast cancers that are HER2+ (Human Epidermal growth factor Receptor 2) are treated differently than those that are not. HER2+ cancer is aggressive, and it typically responds to Herceptin. Tumors that are not HER2+ do not respond to Herceptin. Physicians use the HER-2/neu test as a guide in selecting treatment.

Another test called Trofile, produced by Monogram Bioscience, is being used to identify specific receptors on the HIV virus' surface that makes it susceptible or resistant to certain AIDS drugs called CCR5 antagonists. An example of that type of drug is Pfizer's maraviroc, brand name Selzentry or Celsentri (outside the U.S.).

"I think nurses are, a lot of time, the eyes and ears for doctors," says Murphy, "I think they need to know about these genetic tests so they can look for the problems and recommend testing to the doctors on behalf of their patients."

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