(Corrects to remove word "embargo" from headline)

By Deena Beasley

Oct 10 (Reuters) - Isis Pharmaceuticals Inc has begun enrolling infants in a pivotal Phase 3 trial of ISIS-SMNrx, an experimental treatment for spinal muscular atrophy, and it said ongoing mid-stage studies show that the drug increases muscle function in infants and children with the disease.

SMA is a rare, genetic disease that causes severe muscle atrophy and weakness in newborns and children. SMNrx is an "antisense" drug designed to interfere at the genetic level to regulate formation of disease-causing proteins.

Isis has a development agreement with Biogen Idec Inc , which has the option to license the SMA drug.

Without treatment, infants with SMA lived ventilator free for a median of 10.5 months, and fewer than 20 percent were alive after 18 months, according to a retrospective study conducted by Dr Richard Finkel, chief of the pediatric neurology division at Nemours Children's Hospital in Orlando, Florida, and a lead investigator on the Isis trials.

As of Sept. 2, a Phase 2 study of SMNrx showed a median, ventilator-free age of 16.3 months for infants initially treated with a 6 mg dose of the drug, and 11.6 months for infants given a 12 mg dose. Babies in the second group began treatment five months after the first, lower-dose group.

Data presented on Friday at the International World Muscle Society Congress in Berlin also show that the drug is increasing the muscle function of infants in the study, works best at the higher dose and is boosting levels of a key protein that is missing in SMA patients, Finkel said.

As of early September, three infants in the mid-stage study had died from respiratory infections and one was placed on permanent ventilation.

Finkel said investigators now have information from autopsies showing that the drug is able to distribute through the entire spinal cord and up into the brain.

"It seems to be doing the job that it is intended to do - alter the splicing of messenger RNA to allow for a more correct set of instructions to be produced inside the nerve cells," he said.

Isis is currently enrolling infants with SMA in a Phase 3 study and plans to initiate a second Phase 3 study in children with SMA later this year.

"One of the encouraging things for me is the longer these infants are treated with SMNrx the better the effect seems to be," said Dr Stanley Crooke, chief executive officer at Isis. "We were hoping that in healthier kids we would slow the disease ... but we are seeing significant improvements in muscle strength." (Reporting by Deena Beasley; Editing by Cynthia Osterman)