May 8 (Reuters) - FDA staff reviewers questioned whether Vertex Pharmaceuticals Inc's experimental cystic fibrosis drug added any benefit over its already approved therapy, Kalydeco, in patients with the most common genetic mutation underlying the disease.

Vertex is seeking approval for the combination of the compound lumacafotor and Kalydeco to treat CF patients 12 years or older with the particular mutation.

In documents released on Friday, staff reviewers questioned whether Kalydeco alone had a positive effect on patients with the mutation, but were satisfied with the safety profile of the combination, to be called Orkambi. (http://1.usa.gov/1ceMWB6)

The review comes before a panel of independent advisers to the U.S. Food and Drug Administration makes a recommendation on approval, which the agency is not obliged to follow the recommendations, but typically does.

(Reporting by Vidya L Nathan and Natalie Grover in Bengaluru; Editing by Joyjeet Das)