Ocrevus, from Genentech, the first drug for an aggressive form of multiple sclerosis called primary progressive MS. Genentech via AP

A new multiple sclerosis drug approved by the U.S. Food and Drug Administration late Tuesday offers hope to patients with the most severe form of the progressive disease.

The intravenous drug, made by Genentech, is called ocrelizumab (Ocrevus). Given every six months, it worked best for patients with relapsing multiple sclerosis (MS) — the most common form of the disease, the FDA reported.

But Ocrevus also appeared to slow progression of a more severe type of the disease, called primary progressive MS.

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“If the side effect profile continues to hold up ... I think ocrelizumab will become the leading MS therapy,” said Dr. Steven Galetta, chairman of neurology at NYU Langone Medical Center in New York City. He wasn’t involved in the clinical trials that led to the drug’s approval.

“The drug offers the first option for patients with primary progressive MS,” he added.

However, side effects need to be monitored going forward. Of particular concern, Galetta said, are infections and tumors.

Other neurologists agreed that the drug represents a significant advance.

“In clinical trials, ocrelizumab showed promising results in slowing the disease activity of primary-progressive multiple sclerosis, as well as the more common, relapsing-remitting form of MS,” said Dr. Dhanashri Miskin, attending neurologist at Lenox Hill Hospital in New York City.

“The side effects also appear to be more reasonable compared to other drugs currently available. We still need to be cautious about potential long-term safety of the drug once it is widely prescribed,” Miskin added.

Another neurologist was equally enthusiastic.

“The magnitude of the benefits that we’ve seen with ocrelizumab in all forms of MS are really quite stunning,” said Dr. Stephen Hauser, whose committee oversaw the late-stage clinical trials.

“I think that this is a very big deal,” Hauser, chair of neurology at the University of California, San Francisco, told The New York Times.

Previously, without any effective treatment, patients with primary progressive MS faced paralysis and mental decline.

Although the new drug only modestly slowed decline in that group of patients, neurologists hailed that advance as a great first step.

“It’s a very encouraging result,” said Dr. Fred Lublin, director of the Center for Multiple Sclerosis at Mount Sinai Hospital in New York City.

Lublin, an investigator for the clinical trial, told The Times: “This sort of opens the door for us. Once we open that door, then we do better and better and better.”

Progress in this case is costly, however.

Genentech, which is owned by Switzerland-based Roche, reportedly said it will charge a list price of $65,000 a year.

Still, that’s 25 percent cheaper than the existing drug Rebif, which Ocrevus was stacked against in two clinical trials that preceded Ocrevus’s approval. Rebif is said to cost $86,000 a year.

Genentech said in a statement that it’s trying to counter the rise in MS-drug prices.

“Pricing Ocrevus 25 percent less than the comparator in our trials is an important first step,” the drug maker said.

Multiple sclerosis is believed to be an autoimmune disease, meaning the body mistakenly attacks itself. The Multiple Sclerosis Foundation estimates that more than 400,000 people in the United States have MS. Roughly 15 percent have the primary progressive form.

MS affects the brain and spinal cord by damaging the myelin sheath, the material that surrounds and protects nerve cells. Symptoms can include visual disturbances, muscle weakness, trouble with coordination and balance, and thinking and memory problems, according to the U.S. National Institute of Neurological Disorders and Stroke.

The FDA approval followed two clinical trials. One involved more than 700 patients with primary progressive multiple sclerosis. Researchers found that those taking Ocrevus had 24 percent less risk of their disability progressing compared with patients who were taking a placebo.

In the other trial, which involved relapsing MS, those taking Ocrevus saw a 47 percent reduction in their rate of relapses compared with patients taking Rebif.

Side effects included reactions at the injection site, upper respiratory infections and cold sores.

Ocrevus targets a specific type of B cell. In people with MS, these B cells appear to malfunction and play a role in central nervous system damage.