HEB DENIED BY FDA -Hemispherx Biopharma

* FDA asks company to conduct additional study
* FDA asks for safety and efficacy data * Shares fall 45 pct to 66 cents in extended trade

Dec 1 (Reuters) - Hemispherx Biopharma Inc said U.S. health regulators did not approve its experimental drug Ampligen for the treatment of chronic fatigue syndrome, sending its shares down 45 percent in after-hours trade.
In a complete response letter to the company, the U.S. Food and Drug Administration stated that the two primary clinical studies submitted with the new drug application did not provide credible evidence of efficacy.
The FDA recommended at least one additional clinical study which shows a convincing effect and confirms safety in the target population, Hemispherx said in a statement.
The FDA also asked the company to resolve outstanding inspection issues at some of its facilities producing Ampligen, that included the facility located in New Brunswick, New Jersey and one of its third party manufacturing facilities, Hollister-Stier Laboratories.
Hemispherx said it has not determined the impact of the recommendations on timelines and overall cost of the program.
The company said it is reviewing the complete response letter and will seek an expedited meeting with the FDA to discuss its recommendations.
Shares of the company fell 45 percent to 66 cents in after market trade. They closed at $1.20 Tuesday on the American Stock Exchange.