Two organizations recently issued a joint clinical alert to help physicians interpret a new black box warning for Plavix (clopidogrel). The warning, mandated in March by the Food & Drug Administration (FDA), addressed reduced effectiveness of Plavix in patients who are poor metabolizers of its active ingredient.
Plavix is used to prevent blood clots after a recent heart attack or stroke, and in people with certain disorders of the heart or blood vessels. The medication keeps the platelets in the blood from coagulating to prevent unwanted blood clots that can occur with certain heart or blood vessel conditions. There is no way for doctors to measure how well Plavix is working. Generally, if patients avoid another heart attack or stroke, it is thought to be effective.
According to the FDA, Plavix does not have its anti-platelet effects until it is metabolized into its active form by the liver enzyme, CYP2C19. People who have reduced functioning of their CYP2C19 liver enzyme cannot effectively convert Plavix to its active form. As a result, Plavix may be less effective in altering platelet activity in those people. These “poor metabolizers” may not receive the full benefit of Plavix treatment and may remain at risk for heart attack, stroke, and cardiovascular death.
According to the FDA, it is estimated that 2 percent to 14 percent of the U.S. population are poor metabolizers. The agency recommends that health care professionals consider alternative dosing of Plavix for these patients, or consider using other anti-platelet medications. Tests are available to assess CYP2C19 genotype to determine if a patient is a poor metabolizer.
The FDA left decisions about what to do with the information provided by the black box up to individual health care providers.
According to an article posted on TCTMD.com, e American College of Cardiology Foundation (ACCF) and American Heart Association (AHA) issued a joint clinical alert on June 28, 2010 addressing this issue and providing recommendations for practice. According to the alert, health care practitioners should:
• Emphasize to patients the importance of compliance with Plavix therapy, as compliance greatly impacts the drugs effectiveness. Without compliance, genetic and platelet function testing are irrelevant.
• Adhere to guideline recommendations in terms of using Plavix.
• Realize that there are other drugs out there that are alternative. Prasugrel, for example, is one option, but its benefits must be weighed against its bleeding risks.
As to testing, the alert points out that the evidence base is insufficient to recommend either routine genetic or platelet function testing at the present time. There is also no information that routine testing improves outcome in large subgroups of patients. Because of this, clinical judgment is required in regards to testing, the alert says.