The U.S. Food and Drug Administration (FDA) approved Roche Holding AG's multiple sclerosis (MS) drug Ocrevus, putting the potential blockbuster drug back on track after a delay by regulators over manufacturing issues.

The injectable drug, known chemically as ocrelizumab becomes the first U.S. approved medicine for primary progressive form of the neurological disease (PPMS).

The drug's entry into the U.S. market was delayed by three months when the FDA in late December pushed back its deadline for an approval a decision, saying the commercial manufacturing process for Ocrevus was not yet ready.

Ocrevus, an immuno suppressive drug that reduces the immune system's assault on the body's own neurons, could become a mainstay treatment for MS and help offset falling revenue from Roche's top-selling cancer drugs Rituxan, Avastin and Herceptin that will soon face competition from cheaper biosimilar versions.

Analysts forecast annual sales exceeding $3 billion by 2021, according to Reuters data.

Roche rival Novartis AG is also developing its own MS treatment, BAF312 for secondary progressive multiple sclerosis, and expects regulatory approval in the first half of 2017 depending on the outcome of talks with regulators.

Other companies are also seeking new drug alternatives for different forms of MS, including GeNeuro SA.

Roche said in the PPMS study, patients who got Ocrevus were 47 percent more likely than those who received placebo to have no evidence of disease progression.