(Reuters) - The Food and Drug Administration on Friday approved Mitsubishi Tanabe Pharma Corp’s treatment for fatal neurological disorder amyotrophic lateral sclerosis (ALS), marking the first such U.S. regulatory approval in more than two decades.

The drug, known chemically as edaravone, has been sold by Japan-based Mitsubishi Tanabe in Japan and South Korea since 2015.

In the United States, the only other approved ALS medicine, generic riluzole, modestly slows the progression of the disease in some people.

After six months of treatment with edaravone on top of standard-of-care, data showed the intravenous drug reduced the rate of functional decline in patients by about a third, Dr Jean Hubble, VP of medical affairs, at Mitsubishi Tanabe’s U.S. unit MT Pharma America Inc (MTPA), said.

ALS, whose cause is largely unknown, garnered international attention when New York Yankees player Lou Gehrig abruptly retired from baseball in 1939, after being diagnosed with the disease.

In 2014, ALS returned to the spotlight with the “Ice Bucket Challenge,” which involved people pouring ice-cold water over their heads, posting a video on social media, and donating funds for research on the condition, whose sufferers include British physicist Stephen Hawking.

The rare progressive condition attacks nerve cells located in the brain and spinal cord responsible for controlling voluntary muscles.

Eventually, the brain’s ability to start and control voluntary movement is lost, and the patient succumbs to the disease - usually three to five years from the onset of symptoms.

The FDA was expected to make its decision on edaravone by June 16. To be sold under the brand name Radicava, the drug should be available in the United States by August, MTPA Chief Commercial Officer Tom Larson said.

Radicava would cost $1,086 per infusion, Mitsubishi Tanabe Pharma said on Friday.

If taken annually for 12 months or 13 cycles, according to the dosing and administration per the label, the cost before government discount would be $145,524, the company added.

Another promising drug for ALS is being developed by French drugmaker AB Science SA ABS.PA, which in March reported positive late-stage data on its drug, masitinib. The drug is now under European review.

More than 6,000 people in the United States are diagnosed with ALS each year, according to the ALS Association.