The U.S. FDA has declined to approve Viatris and Mapi Pharma's once-a-month injection for treating relapsing forms of multiple sclerosis (MS), the companies said on Monday.

The companies were reviewing the content of the health regulator's complete response letter and would soon determine the appropriate next steps, they said.

The injection, GA Depot, is a long-acting injection version of glatiramer acetate, which was approved for relapsing forms of MS under brand name Copaxone, manufactured by Teva Pharmaceutical Industries.

MS is a neurological disease in which the immune system attacks brain cells, causing motor disabilities. It affects about 400,000 people in the United States, according to the National Institutes of Health.

Relapsing MS is a type of the disease where the symptoms appear sporadically, in the form of attacks. This is followed by a period of disease inactivity and weeks, months, or even years may pass before another attack occurs.