ZURICH (Reuters) - The U.S. Food and Drug Administration has granted breakthrough therapy designation for ocrelizumab (OCREVUS) for treating people with primary progressive multiple sclerosis (PPMS), Swiss drugmaker Roche said on Wednesday.

There are no approved treatments now for PPMS, a debilitating form of the disease marked by steadily worsening symptoms.

"Roche plans to pursue marketing authorization for both PPMS and relapsing multiple sclerosis (RMS), a more common form of the disease, and will submit data from three pivotal Phase III studies to global regulatory authorities in the first half of 2016," it said in a statement.

Roche believes ocrelizumab's potency and benign side effect profile make it unique, while industry analysts see it as a likely multibillion-dollar-a-year seller that will help the group diversify beyond its mainstay cancer business.

Breakthrough therapy designation expedites the development and review of medicines intended to treat serious or life-threatening diseases. Ocrelizumab is the first investigational medicine for MS to win the designation by the FDA, Roche said.

Clinical trials released in October showed Roche's big new drug hope cut multiple sclerosis relapses by nearly half compared with the older product Rebif, underscoring its potential in the main relapsing form of the disease.

Ocrelizumab also cut clinical disability by nearly a quarter in a separate study of people with PPMS, which affects around 15 percent of patients.

(Story refiles to remove extraneous characters from brand name OCREVUS)

(Reporting by Michael Shields; Editing by Joshua Franklin)