TUESDAY, Dec. 6, 2016 (HealthDay News) -- An experimental drug for infants with a rare and fatal illness shows promise, researchers report.

There is no treatment for spinal muscular atrophy type 1 (SMA-1), a degenerative neuromuscular disease that occurs in about one in every 11,000 births.

According to background information in the new study, babies with the condition are born with two faulty copies of a gene that's essential to the transfer of signals from the spinal cord to muscles. Over time, muscles atrophy, until even breathing becomes difficult.

Most infants with the genetic condition die by their second birthday, the researchers said.

"I've seen so many kids die with this disease," said pediatric neurologist and study co-author Dr. John Day. He directs the Neuromuscular Disorders Clinic at Lucile Packard Children's Hospital Stanford in California.

But a new phase 2 clinical trial of 20 babies with the condition found that the drug nusinersen was safe and improved muscle function and nerve activity in most of the infants.

As the researchers explained, nusinersen binds to the faulty gene, allowing it to function properly.

The results of the new study -- funded by Ionis Pharmaceuticals Inc and drug maker Biogen -- have prompted a larger phase 3 clinical trial. That's the last phase needed to submit the drug to the U.S. Food and Drug Administration for approval.

The first infant to receive the drug was 7-month-old Zoe Harting. Day said that before starting to take the drug in 2013, she couldn't sit up or roll over, couldn't move her legs or lift her arms when she was lying down, and struggled to swallow.

John Harting, Zoe's father, said her symptoms were apparent early on.

Watching Zoe with a cousin who was only a week younger, "It was really noticeable that they were very different," Harting said. "Her cousin was rolling over and Zoe was almost immobile."

"It was really, really hard," he said. One neurologist, "basically told us she would not live past 2, and that we could only hold her, love her and let her die."

But the new drug -- which Zoe started in June of 2013 -- seems to have changed all that. Now age 4, Zoe is gradually improving and can eat and talk, gets around in a motorized wheelchair, goes to preschool, and can even play catch with her father. Her parents recently bought her a recumbent bicycle in the hope it will help strengthen her legs.

"She continues to slowly gain motor skills; it's quite unexpected and rewarding," Day said in a Stanford news release.

"It's a world of difference," Harting added.

However, not every infant fared so well. As published Dec. 5 in The Lancet, four of the babies in the trial died of the disease during the study, including one who died too early to be included in the final analysis.

All the infants experienced problems associated with their disease, and 16 had 77 serious problems such as difficulty breathing or respiratory infections. Those complications were considered by the investigators not to be drug-related, however.

One infant developed mild neutropenia (low white blood cell count) and another had mild vomiting possibly linked to the drug.

Nevertheless, muscle skills improved in 16 of the 20 babies, with the biggest improvements in the ability to grasp, kick and sit. Overall muscle function improved in 14 of 18 babies.

"While our results are promising, this drug does not represent a cure," study lead author Dr. Richard Finkel, of the Nemours Children's Hospital, said in a journal news release.

"We observed remarkable improvement in muscle function -- for instance, some babies undergoing treatment developed the ability to sit and roll over independently, and improved their head control, kicking, grasping, standing and even walking," he said. However, "although this type of improvement has not been observed before in infants with infantile-onset spinal muscular atrophy, the drug did not restore normal levels of muscle function," he said.

"It's important to interpret these findings carefully as our study is relatively small and open-label, but this is an important first step," he added.

The results "a major milestone on the journey towards a viable therapy," but "need to be interpreted cautiously," due to the study limitations, Thomas Gillingwater of the University of Edinburgh, wrote in an accompanying editorial.

More information

The U.S. National Institute of Neurological Disorders and Stroke has more about spinal muscular atrophy.