* Cuts LDL cholesterol by 25 pct vs 3 pct for placebo
* Study involved patients with rare cholesterol disorder
* 1 patient discontinues due to elevated liver enzymes
* Isis shares fall 6.1 pct; Genzyme edges up (Adds analyst comments, background, updates shares)
By Lewis Krauskopf
NEW YORK, May 20 (Reuters) - Genzyme Corp GENZ.O and Isis Pharmaceuticals Inc ISIS.O said on Wednesday their experimental cholesterol-lowering drug met the main goal in a late-stage study in patients with a rare but serious genetic cholesterol disorder.
The injected drug, mipomersen, cut levels of "bad" LDL cholesterol by 25 percent after 26 weeks of weekly treatment compared with 3 percent for those on placebo.
But Isis shares fell 6.1 percent after the data was released. The companies said six patients dropped out of the study before it finished, including one who discontinued because of elevated liver enzymes.
Carol Werther, an analyst with Summer Street Research, also said she was expecting mipomersen to cut LDL by 30 to 35 percent.
"The efficacy was under expectations and the dropout rate was higher than expected," Werther said, although she noted the data should support approval of mipomersen.
Other analysts said investors may be selling to lock in profits on Isis shares, which had risen more than 30 percent since early March. Shares of larger Genzyme rose 0.6 percent.
The patients in the Phase 3 study had homozygous familial hypercholesterolemia, a rare condition that hinders their ability to properly metabolize the fatty substance. It affects an estimated one in 1 million people.
Before the study began, their average LDL levels exceeded 400 milligrams per deciliter -- far above recommended levels -- despite being on statins and other lipid-lowering therapies.
"These are promising results for a very high-risk patient population that is in great need of new treatment options," Genzyme Chief Medical Officer Richard Moscicki said in a statement.
The trial enrolled 51 patients. Of the 34 patients treated with mipomersen, 28 completed the study, and one patient discontinued due to elevations in liver enzymes.
Based on the data, Genzyme plans to seek approval for mipomersen in the second half of 2010.
The companies are also conducting late-stage studies for other conditions, including patients with severe hypercholesterolemia, and patients at high risk for coronary heart disease.
JP Morgan analyst Geoff Meacham said in a research note the results should support approval for the "niche" use in patients with the rare disorder, but "safety remains a key concern" as the companies seek to expand its use to larger populations.
Cambridge, Massachusetts-based Genzyme, which specializes in drugs for rare diseases, struck a licensing deal with smaller biotechnology company Isis in January 2008 for rights to mipomersen.
Isis shares were down 95 cents at $14.53 on the Nasdaq on Wednesday morning. Genzyme shares were up 36 cents at $59.75. (Reporting by Lewis Krauskopf; Editing by Brian Moss and Matthew Lewis)