* Merck's Victrelis may hamper HIV drugs
* Many people with hepatitis C also have HIV
* FDA changes label based on study, small trial
WASHINGTON, April 26 (Reuters) - U.S. drug regulators are changing the label for Merck & Co Inc's recently approved Victrelis treatment for hepatitis C to show it should not be taken with some widely used HIV medicines.
"Co-administration (of the two drugs) ... is not recommended at this time because of the possibility of reducing the effectiveness of the medicines, permitting the amount of HCV (hepatitis C virus) or HIV ... in the blood to increase," the U.S. Food and Drug Administration said on Thursday.
The FDA and Merck first warned about the issue in February based on the results of a drug interaction study, but a label change could further crimp sales of Victrelis.
Victrelis, approved last May, attacks the hepatitis C virus that over time can lead to chronic liver disease or liver failure. Many patients with hepatitis are also infected with the human immunodeficiency virus, or HIV, which weakens the immune system and is fatal without treatment.
The drug interaction study tested healthy volunteers who took Victrelis and the widely used HIV treatment Norvir (ritonavir) with one of three other anti-HIV pills. All of the HIV drugs work by blocking protease, an enzyme the virus needs in order to replicate.
The study found Victrelis reduced the concentrations of the HIV drugs in the blood.
Analysts have said the findings could crimp sales of Victrelis by as much as a quarter, since so many people are infected with both diseases.
The FDA said it is changing the label for Victrelis also based on a small clinical trial presented last month that studied a total of 98 people.
The trial results were not as conclusive as the drug interaction study. The trial tested people with both HIV and the hepatitis C virus, and gave some people Victrelis plus an older combination hepatitis C therapy, and others only the older combination treatment (peginterferon/ribavirin). All patients were given a type of anti-HIV pill.
Out of the 64 people taking Victrelis, three had a rebound in their HIV, while four of the 34 people taking only the older combo drug had a rebound of the virus.
Merck has said it still plans a larger drug-interaction trial of Victrelis with other HIV drugs.
The FDA said it would communicate any new information about taking Victrelis together with the HIV drugs when it becomes available.