(Reuters) - U.S. health regulators have expanded the use of one of Vertex Pharmaceuticals Inc’s cystic fibrosis drugs to include patients aged between 12 months and 24 months.

The expanded approval makes Kalydeco the first medicine to treat the underlying cause of the genetic disease in this age group among patients with specific mutations in the CF transmembrane regulator (CFTR) gene, Vertex said on Wednesday.

The drug is currently approved in the United States to treat patients aged 2 and older whose CFTR gene has certain mutations.

There are few treatment options for the 70,000 cystic fibrosis patients worldwide who rarely live beyond 40 and possess a defective gene that leads to the build-up of thick mucus which clogs the lungs and other organs.

The approval is based on data from an ongoing late-stage safety study in 25 children who have one of 10 mutations in the CFTR gene, the company said.

The drug’s revenue rose 33 percent to $253 million in the latest quarter and accounted for about 34 percent of total cystic fibrosis drug sales.

Kalydeco is designed to help hydrate and clear mucus from the airways and is available as tablets and granules that can be ingested with certain food and liquids.

European regulators will evaluate expanding the drug’s label and are expected to make a decision in the first half of 2019, Vertex said.