* Implantable device effectively reduced migraines
* Therapy known as peripheral occipital nerve stimulation
* Study presented at International Headache meeting
* Shares end 2.6 percent lower (Adds analyst comment, closing stock price)
By Debra Sherman
CHICAGO, June 23 (Reuters) - A pacemaker-like device made by St. Jude Medical Inc STJ.N helped reduce patients' debilitating migraine headaches, the company said on Thursday, but Wall Street analysts said the trial results were likely not strong enough to win U.S. regulatory approval.
St. Jude, which makes implantable heart defibrillators, heart valves and other medical products, said it was not clear when the device would get U.S. Food and Drug Administration approval for the treatment of chronic migraines. The device currently is available in Europe to treat chronic pain.
Migraines are disabling headaches that can last for hours or days at a time and for which there are few treatments. Last year, the FDA approved Allergan's (AGN.N) injectable anti-wrinkle drug Botox to treat migraines.
The St. Jude system takes a different approach. It works by delivering mild electrical pulses from a device implanted in the chest, much like a pacemaker, to leads threaded under the skin at the back of the head. The pulses stimulate the occipital nerves.
Results from the 157-patient study, presented at the International Headache Congress in Berlin, showed a statistically significant improvement in cutting the number of headache days per month and helping in quality of life.
But the FDA wanted to see an even greater rate of migraine improvement for patients in the study using the device versus those who were in a control group.
A St. Jude spokesman said the company will meet with FDA officials later this year to determine what steps it needs to take to win U.S. approval.
He said the company believes the market for this could exceed $1 billion.
The failure to meet study goals, coupled with a rating downgrade by a Bernstein Research analyst who raised concerns about growth in its core market for heart defibrillators, sent St. Jude's shares down as much as 4.8 percent on Thursday.
"We believe the failed U.S. migraine trial combined with the weakening cardiac rhythm management market start to raise questions regarding St. Jude Medical's growth trajectory," said Goldman Sachs analyst David Roman.
About 10 percent of adults worldwide suffer from migraines, according to the World Health Organization. The condition affects almost 28 million Americans, or some 13 percent of the population, according to the National Headache Foundation.
The St. Jude-funded study -- the largest of its kind -- followed patients who suffered 26 headaches per month on average. Participants were implanted with the company's Genesis neurostimulator and randomly assigned to an active or control group for 12 weeks.
The active group got stimulation immediately, while the control group did not receive stimulation until after the first 12 weeks. After one year, 66 percent of patients reported excellent or good pain relief, St. Jude said.
Patients who received stimulation reported a 28 percent decrease in their number of headache days, or seven fewer days per month, compared with the placebo group, which reported a 4 percent decrease, or one fewer day per month.
At 12 weeks, 53 percent of patients in the active group ranked their relief as excellent or good, compared with 17 percent in the placebo group.
St. Jude has filed for European approval of the Genesis neurostimulation system for managing of pain and disability associated with chronic migraine and expects to begin a limited introduction in Europe later this year.
"The story isn't that the primary endpoint (was) missed -- it was just an outdated endpoint -- the story is how much better the patients did on this therapy than anything we've seen before. For the first time, we know this thing works," said JPMorgan analyst Michael Weinstein.
"The chronic migraine market represents a large area of unmet medical need, and St. Jude is well positioned to capture a first-mover advantage," he said.
St. Jude rival Medtronic Inc (MDT.N) said it continued to explore the use of neurostimulation technology for treating chronic migraine headaches, but would not give details of its plans or timeline.
St. Jude shares closed 2.6 percent lower at $47.40 on the New York Stock Exchange on Thursday, off an earlier low at $46.33. (Reporting by Debra Sherman, editing by Michele Gershberg, Gunna Dickson and Matthew Lewis)