The Food and Drug Administration wants ViroPharma Inc. to run another clinical trial of its drug Cinryze before it will consider a new approval for the drug, the Exton-based company said today.

Cinryze treats hereditary angioedema, a genetic disease that can cause dangerous swelling in the throat and extremities. The drug was approved in October as a routine preventive treatment for angioedema attacks, and ViroPharma is trying to gain additional marketing approval for the drug as a treatment for acute attacks.

The FDA wants a new clinical trial because the placebo-controlled study included in ViroPharma's application was not strong enough, ViroPharma said. The company did not say how long a new study might take.

It said the FDA did not mention any safety concerns in its complete response letter.

Cinryze was a product of anti-inflammatory drug developer Lev Pharmaceuticals Inc., of New York, which ViroPharma bought last year.

In premarket trading, shares of ViroPharma fell 73 cents, or 10.5 percent, to $6.20 after closing Wednesday at $6.93.