UPDATE 3-FDA asks for additional study on ViroPharma HAE drug

* Yet to decide on additional trial - CEO

* Largest opportunity for Cinryze is prophylaxis -CEO

* ViroPharma shares fall as much as 13 pct

* Dyax sees HAE treatment approval by 2009 end

* Dyax shares rise up to 15 pct (Adds analyst, CEO comment; background; adds closing share price)

By Vidya L Nathan

BANGALORE, June 4 (Reuters) - ViroPharma Inc (VPHM.O) said U.S. health regulators declined to approve an additional indication of its treatment for a rare genetic disease, asking the company to conduct another clinical study, sending shares of the biotechnology company down 13 percent.

Shares of Dyax Corp (DYAX.O), which is also awaiting the U.S. Food and Drug Administration's approval for its drug to treat the disease, rose as much as 15 percent on hopes that its drug now stands a better chance to hit the market first.

ViroPharma's Chief Executive Vincent Milano told Reuters that the company has not yet decided on conducting the additional trial.

Maxim Group analyst Yale Jen said the company may not move the program forward.

"Why work on something that potentially could have absolutely no reward or very low reward?"

The drug, Cinryze, was already approved last year as a preventive therapy for hereditary angioedema (HAE), and the company is seeking marketing approval for the drug to also treat acute attacks of the possibly life-threatening disease.

When the FDA approves the drug for preventive use and not for use in severe attacks of the same disease, it sends mixed signals about the drug's efficacy to physicians, WBB Securities analyst Steve Brozak said.

Cinryze has been in the market only for a few months as a therapy to prevent attacks of HAE, which causes swelling, particularly of the face and airways.

The health regulator had not cited any safety concerns in its complete response letter to the new application, the company said.

Cinryze was expected to earn between $40 million and $150 million in sales as a treatment for acute HAE, compared with sales estimates of $200 million to $250 million in the preventive indication.