UPDATE 2-Shire ADHD drug approved for US adults, shares jump

(Adds company, analyst comments, updates shares)

By Mark Potter

LONDON, April 23 (Reuters) - Britain's Shire Plc (SHP.L) received a boost on Wednesday when its biggest drug hope, Vyvanse for attention deficit hyperactivity disorder (ADHD), was approved by U.S. regulators to treat adults.

Shares in Britain's third-biggest drugmaker jumped as much as 7 percent to 983 pence after the earlier-than-expected decision from the U.S. Food and Drug Administration (FDA).

Shire had expected to receive a so-called approvable letter from the FDA, which would have signalled its intention to back the medicine, pending further information. The straight approval raised hopes that Vyvanse sales this year would receive a boost.

Shire SAID in February that its forecast for Vyvanse sales $350 million to $400 million this year assumed the drug would be approved to treat U.S. adults by the middle of this year.

A spokeswoman said on Wednesday that Shire would start marketing Vyvanse to adults in "the next several weeks," but declined to comment on whether this would affect sales expectations for this year.

Shares in Shire, which is due to report first-quarter results on Friday, closed up 6.6 percent at 979 pence, valuing the business at about 5.7 billion pounds ($11.3 billion).

The U.S. Centres for Disease Control and Prevention has said that about 7.8 percent of all school-aged children, or about 4.4 million U.S. children, have been diagnosed with ADHD at some point in their lives, Shire said.

The disorder, characterised by impulsive behaviour and difficulty in concentrating, is also estimated to affect 4.4 percent of U.S. adults aged 18-44, Shire said, citing results from the National Comorbidity Survey Replication.

MARKET SHARE

"The outcome wasn't assured and that's why there's a positive reaction," said Landsbanki analyst Shawn Manning of the approval. He said it was likely to have some impact on Vyvanse sales this year, but cautioned it might not be that large.

Although Shire has not been able to market Vyvanse to adults until now, doctors have been free to prescribe the drug to adults and many have already done so.

Some analysts have also been disappointed by Vyvanse's growth rate since its launch in July 2007 as a treatment for ADHD in children aged 6 to 12 years.

Vyvanse is key to Shire's future as the firm hopes it will replace its top-selling ADHD drug, Adderall XR, which could face generic competition from April 2009.