CARRBORO, NORTH CAROLINA — The Food and Drug Administration granted accelerated approval for a new intravenous enzyme replacement therapy manufactured by Denali Therapeutics for the treatment of Hunter syndrome, a rare progressive illness that affects about 500 people in the United States. The disease, also known as mucopolysaccharidosis type II, or MPS II, often causes death before age 21.

Hunter syndrome is characterized by a deficiency of an enzyme required to break down certain molecules. Over time, toxins accumulate in the bodies of children with the condition, progressively affecting organs including the heart and often affecting the brain, leading to dementia-like symptoms. Nearly all people affected by MPS II are boys, and life expectancy ranges from about 10 to 20 years.

The current standard of care for Hunter syndrome slows only the physical aspects of the illness. Denali's enzyme replacement therapy also targets cognitive decline, according to Dr. Joseph Muenzer, an expert in Hunter syndrome and related rare diseases. The drug does not reverse existing disease regressions, according to Muenzer, but could extend the lives of children with the condition and prevent many symptoms from developing if administered early.

"If we take a child very young and can treat them prior to damage, now the potential is almost unlimited," Muenzer said. "We don't know how well they'll do in the future, but they'll do dramatically different than they would have otherwise."

For Kim Stephens of Carrboro, North Carolina, the approval carried personal weight. Her son, Cole, is 15 years old and was diagnosed with Hunter syndrome when he was 2. Cole is deficient in the enzyme the disease strips away.

According to Stephens, before his disease regression, Cole was learning to read and could speak in full sentences. He initially could string together a few words, then could say only one word — "Mommy" — before becoming entirely nonverbal as the disease progressed. Despite being a teenager, Cole's developmental level is similar to that of a 3-year-old, according to his mother. He enjoys doing puzzles.

"I've been in fight-or-flight mode since his diagnosis," Stephens said. With the FDA's approval, she added: "I can breathe."