* Late-stage study meets main goal
* Enrollment for chronic migraine study on track
* Shares fall as much as 26 pct to 2 1/2 yr-low (Adds details, analyst comment, shares)
June 27 (Reuters) - Alder BioPharmaceuticals Inc's late-stage study data on its experimental treatment to prevent episodic migraine fell short of investors' expectation, sending its shares plunging as much as 26 percent to a more than two-and-half year low.
Alder is among a clutch of drugmakers racing to develop a new, more effective migraine treatment by targeting calcitonin gene-related peptide (CGRP), a protein involved in pain-signaling during migraine.
Amgen Inc, which is developing its migraine drug in partnership with Novartis AG, submitted a U.S. marketing application in May for the treatment.
Eli Lilly Co and Teva Pharmaceutical Industries Ltd also reported positive late-stage trial data in the past two months.
In Alder's trial, patients receiving a 300 mg quarterly dose of its drug, eptinezumab, through infusion, experienced a reduction of 4.3 days from baseline in monthly migraine attacks, while those on the 100 mg saw a reduction of 3.9 days.
This compared with the average 3.2-day reduction for patients on placebo.
The mean reduction looked slightly numerically lower than other CGRP trials, highlighting the larger placebo rate for the intravenous (IV), Leerink Partners analyst Paul Matteis said.
Alder's treatment is likely to be the last to file for approval but could still capture a sizable market share, mainly due to its less-frequent dosing schedule and multiple modes of administration, analysts and experts have said.
While the approval of the IV is mostly derisked, Tuesday's trial data raises the question of what Alder will do with its self-administration plan as the higher 300 mg dose appears better than 100 mg, Leerink's Matteis said.
The study data met the main goal, Alder said.
Alder and others are aiming at the about 36 million Americans who suffer migraines, which are characterized by recurrent episodes of moderate-to-severe headaches accompanied by nausea, vomiting, and sensitivities to light and sound.
The lack of effective medicines and the sheer number of patients guarantee that each company's drug, if approved, will eventually generate at least $1 billion in sales, analysts have forecast.
Alder said it expects to complete enrollment later this year in another late-stage trial testing patients with chronic migraine.
The company's shares recouped some of their losses and were down 20 percent at $14.9 in early trading on Tuesday. (Reporting by Ankur Banerjee in Bengaluru; Editing by Sriraj Kalluvila)