(Reuters) - Nearly 70 percent of children with potentially fatal acute Graft Versus Host Disease responded to treatment with an experimental stem cell therapy from Mesoblast Ltd, meeting the primary goal of a late-stage U.S. trial, the Australian company announced on Wednesday.

The study tested Mesoblast’s off-the-shelf stem cell product, remestemcel-L, in 55 children who did not respond to steroids after a bone marrow transplant triggered acute Graft Versus Host Disease (aGVHD).

On day 28 after receiving the first dose of remestemcel-L, 69 percent of children had experienced a complete or partial response, deemed a statistically significant increase over historical expectations of about 45 percent, according to data presented at a medical meeting. It was an open-label trial, meaning there was no comparator treatment or placebo.

Of 50 patients who had completed 100 days of follow-up after receiving at least one remestemcel-L infusion, the survival rate was 78 percent, a potential indicator of longer-term benefit. That contrasts with 100-day survival rates as low as 30 percent in patients who fail to respond to steroid therapy.

In patients who receive an allogeneic (from a healthy donor) bone marrow transplant for conditions such as blood cancers, donor cells may attack the recipient causing aGVHD, which activates an inflammatory response by the immune system and potential tissue damage in the skin, gut and liver. When severe and unresponsive to steroids, it is often fatal.

“These children ... suffer from a particularly aggressive and life-threatening disease for which there are currently no available treatments,” Dr. Joanne Kurtzberg, the study’s lead investigator from Duke University Medical Center, said in a statement.

“We are now seeing that children who receive remestemcel-L can have significant overall response rates and reduced early mortality,” she said, adding that the therapy was well tolerated.

The results were consistent with those from a prior remestemcel-L trial in 241 children treated under an expanded access protocol, where day 28 overall response rate correlated with day 100 survival, the company said.

Mesoblast said it believes that 6-month safety and efficacy data from this 55-patient Phase 3 trial may be sufficient for filing for accelerated U.S. approval.

There are about 30,000 allogeneic bone marrow transplants performed globally each year, some 25 percent in the pediatric population. Nearly half of all patients who receive such a transplant develop aGVHD, according to the Center for International Blood and Marrow Transplant Research. Death rates are extremely high in those who experience liver or gastrointestinal involvement.