* Mydayis set for third-quarter launch
* ADHD drug treats adolescents and adults
* Drug bolsters Shire's position in attention deficit market
* Shares up 1.5 percent in London trading (Adds sales forecasts, shares, reaction)
By Natalie Grover and Ben Hirschler
June 21 (Reuters) - Shire has won U.S. approval for a long-acting attention deficit drug aimed at adolescents and adults, boosting its stock of medicines for the cognitive condition that affects millions of children and is being diagnosed more in older people.
The U.S. Food and Drug Administration (FDA) green light for Mydayis follows good results for Shire's new hereditary angioedema drug and helps offset the setback of early generic competition to a Shire drug for ulcerative colitis.
Shares in Dublin-based Shire rose 1.5 percent by 0800 GMT in London on Wednesday following the overnight FDA news.
The new drug Mydayis, previously known as SHP465, contains the same active ingredient as Shire's widely used attention deficit hyperactivity disorder (ADHD) treatment Adderall XR but is formulated to last up to 16 hours.
Adderall XR, which is also available in generic forms, manages symptoms for up to 12 hours.
"It's obviously not a new molecule but it does extend delivery," said Gregory Mattingly, who has been a study investigator in multiple Shire ADHD drug trials.
Shire, whose ADHD drugs Adderall XR and Vyvanse generated close to $2.4 billion in sales last year, plans to launch Mydayis in the third quarter of 2017.
The company has said Mydayis could have annual sales of $500 million by 2020, well above the $294 million consensus figure, as compiled by Thomson Reuters. Jefferies analysts noted that every ADHD drug launched by Shire has significantly exceeded initial market projections.
While Shire was once best-known for its ADHD medicines, analysts now view the business as essentially a cash-generating machine to help the company repay debt and fund investment in its large portfolio of drugs for rare diseases.
Mydayis is designed to help those patients, many of them college students or adults, who now take two doses of various ADHD treatments to control their symptoms throughout the day. The drug cannot be used by children aged 12 years or younger.
Shire first filed an application to market Mydayis in 2006 but faced multiple setbacks, including a requirement for additional studies to satisfy the FDA.
ADHD is characterised by inattention and/or hyperactivity-impulsivity that affects development. Estimates vary but recent data show up to 11 percent of American children are afflicted and more than half continue to suffer as adults.
Both Mydayis and Adderall XR contain amphetamine, a stimulant that elevates levels of dopamine - a neurotransmitter associated with motivation, attention and movement.
As a stimulant, such drugs carry a risk of abuse, can be poorly tolerated, and even be fatal in rare cases. Non-stimulant ADHD treatments have fewer side-effects but are typically less effective.
Other ADHD drug developers include Neos Theraputics Inc , Alcobra Ltd, Aevi Genomic Medicine and privately held NLS Pharma. (Editing by Bill Trott and David Clarke)