An experimental drug has shown promise in treating late-stage multiple sclerosis, reducing annual rates of relapse of major symptoms, its maker reports.

The drug ocrelizumab reduced relapses by almost 50 percent compared to an older drug, interferon beta-1a (Rebif), in three comprehensive clinical trials, its manufacturer Genentech announced.

Genentech, a member of the Swiss Roche group, said the results suggest the drug can be an important new opportunity for treating the debilitating disease.

"The results of these three pivotal trials have the potential to transform the treatment of MS," said chief medical officer Sandra Horning.

"Ocrelizumab is the first investigational medicine to significantly reduce disability progression in people with relapsing MS and people with primary progressive MS - a form of MS with no approved treatments," she said.

In two of the studies, which involved 1,656 patients suffering from relapsing multiple sclerosis - the most common form of the condition - the drug reduced the yearly rate of relapse of major symptoms and other markers of the disease's status, researchers said.

More than 250,000 people in the United States and more than 2.3 million people worldwide are afflicted by multiple sclerosis, in which the body's own immune system attacks the myelin sheaths that cover nerve fibers.

The disease, with symptoms including muscle weakness and cognitive impairment, usually arises between the ages of 20 and 40, and women are more commonly affected than men.

"This is potentially a big deal for our patients," Stephen Hauser, chief of neurology at the School of Medicine at the University of California San Francisco, said of the two studies in which he was the research leader.

Administered by infusion every six months, ocrelizumab works by targeting a protein called CD20 in immune-system B cells that can trigger an inflammatory process that attacks myelin, he explained.

In a third trial with 732 patients with primary progressive multiple sclerosis - a more serious form of the condition - ocrelizumab was more effective than a placebo in slowing the progression of the disabling disease, Genentech said.

Roche and Genentech say they plan to apply for regulatory approval for the drug early next year, which could see ocrelizumab on the market by 2017.