Alder BioPharmaceuticals Inc said its injection to prevent migraines had met the main goal in a mid-stage study, the results of which supported a dosing strategy of four doses a year.

About 38 million Americans suffer from migraines - intense headaches characterized by throbbing pain, sensitivity to light and nausea. The condition, in which attacks can last for days, is not curable.

Multiple doses of the drug, ALD403, were tested against a placebo in patients suffering from chronic migraines, those who experienced 15 or more headache days per month, of which at least eight were assessed as migraine days.

The two highest doses of the drug, 300 mg and 100 mg, brought about a 75 percent reduction in migraine days in 33 percent and 31 percent of patients respectively, meeting the main goal of the study.

Drugs taken to treat migraines don't always work, while the side-effects of preventative medicines typically approved for other conditions, including beta blockers and botox, discourage their use.

Prophylactic treatments lower headache frequency by about half in the 40 percent of sufferers who use them, the American Migraine Foundation estimates.

Alder joins companies such as Amgen Inc, Eli Lilly Co and Teva Pharmaceutical Industries Ltd that are developing similar drugs to target calcitonin gene-related peptide, or CGRP, a protein involved in pain-signaling during migraines.

Alder's data addresses outstanding questions about the drug's competitiveness on dosing and administration, indicating the antibody should be as good if not better than others in development, Jefferies analysts wrote in a note.

The drug is differentiated by how fast it acts and the durability of its effect, Chief Executive Randall Schatzman told Reuters in an interview.

"We're proposing that patients are dosed four times a year. Most other developers are looking at once-a-month dosing," he said, adding that separate early-stage data also showed the treatment could potentially be self-administered.

The drug is currently in a late-stage trial for use in frequent episodic migraine. A late-stage study for chronic migraine sufferers is slated to begin later this year.

If all goes well, ALD403 could win U.S. Food and Drug Administration approval by 2019, said Schatzman. Alder plans to independently sell the medicine in the United States to serve the 13 million patients eligible for migraine prevention therapy.

The Bothell, Washington-based company's stock was up about 30 percent in premarket trade.