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WASHINGTON, Apr 29 (Reuters) The first drug to treat a rare, debilitating muscle disease that can kill infants within a year won U S approval yesterday, health officials said.

Genzyme Corp. developed the drug, Myozyme, to treat Pompe disease, which afflicts fewer than 10,000 people worldwide. It causes severe muscle weakness and breathing trouble that can be fatal, but the progression can vary.

If the disease emerges in infancy, it usually causes an enlarged heart and kills within a baby's first year of life. In other cases symptoms appear during childhood, adolescence or adulthood, causing progressive respiratory failure.

Pompe disease is caused by an inherited enzyme deficiency that leads to a build-up of carbohydrates in muscles. Myozyme replaces that enzyme, known as acid alpha-glucosidase.

The Food and Drug Administration said it approved Myozyme after reviewing studies of 39 patients who received their first dose at ages ranging from one month to 3.5 years. The drug was given intravenously every two weeks for up to two years.

In one test of 18 children, all survived a year and only three needed ventilators to help them breathe. One died after 14 months of treatment and another after 25 months.

Without treatment, infants are expected to live with Pompe disease 18 months at most.

FDA officials said the children's progress was substantially better than would be expected without treatment, but the drug did not return them to normal health. Their long-term prognosis is unknown.

''There are improvements in motor function, but the children still remain behind in terms of developmental milestones,'' said Dr. Julie Beitz, head of the FDA office that reviewed Myozyme.

The drug's label will include a boxed warning, the strongest type for prescription drugs, about the possibility of life-threatening allergic reactions, the FDA said. Three percent of 280 patients who have received Myozyme treatment have experienced significant allergic reactions, Genzyme said.

The company is studying the drug's effects in people with late-onset Pompe disease.

Genzyme's chief executive has said Myozyme could become as big a seller as the company's Cerezyme drug, which is expected to generate more than $1 billion in sales this year. Cerezyme treats a rare condition called Gaucher disease, a painful build-up of fatty deposits in certain organs and bones.

Myozyme's price will be similar to that of the company's other enzyme replacement therapies, which cost about 200,000 dollars a year, Genzyme spokesman Bo Piela said. The drug will be shipped to pharmacies in two weeks.

The shares of Cambridge, Massachusetts-based Genzyme fell 3 cents to close at 61.16 dollars on Nasdaq.

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