While most investors had their eyes glued all-day to the FDA’s website awaiting a scheduled decision on Chelsea Therapeutics International Ltd.’s droxidopa (Northera) it surprised everyone by giving the green light to Novato, Calif.-based Biomarin Pharmaceutical Inc.’s Vimizim (elosulfase alfa), two weeks ahead of its PDUFA date of Feb 28.

Vimizim, according to the agency represents the first FDA-approved treatment for Mucopolysaccharidosis Type IVA (Morquio A syndrome).

Morquio A syndrome is a rare, autosomal recessive lysosomal storage disease caused by a deficiency in N-acetylgalactosamine-6-sulfate sulfatase (GALNS).

Vimizim is intended to replace the missing GALNS enzyme and whose absence leads to problems with bone development, growth and mobility. There are approximately 800 patients with Morquio A syndrome in the U.S.

While the early approval caught many by surprise, it wasn’t entirely unexpected. Back in November it sailed through the FDA Endocrinologic and Metabolic Drugs Advisory Committee meeting with positive voting, with the only concern expressed in the briefing documents was the “modest” six-minute walk test (6MWT) improvements in Vimizim patients, and committee members seemed to question the endpoint, though Aldurazyme (laronidase) for mucopolysaccharidosis I, another enzyme replacement therapy (ERT) from Biomarin, was approved on the strength of 6MWT data. (See BioWorld Today, Nov. 20, 2013.)

The safety and effectiveness of Vimizim were established in a clinical trial involving 176 participants with Morquio A syndrome, ranging in age from 5 to 57 years.

Patients treated with the product showed greater improvement in a six-minute walk test than participants treated with placebo.

On average, patients walked 22.5 meters farther compared to the patients who received placebo. (See BioWorld Today, Nov. 6, 2012.)

The surprising FDA decision came so late in the day that the company’s share price wasn’t affected.

Biomarin’s shares (NASDAQ:BMRN) closed the day at $75.81, down 12 cents.