The FDA has turned down an application to expand the use of Vertex Pharma's cystic fibrosis therapy Kalydeco.
In a complete response letter, the US regulatory authority said that it will need additional data before it can approve Kalydeco (ivacaftor) for CF patients aged two or more who have one of 23 genetic mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene.
The FDA indicated that the marketing application - based on preclinical data and a phase IIa clinical trial - cannot be approved "in its present form".
The news is a blow to Vertex' ambitions to boost sales of Kalydeco, which is currently its biggest product with sales of $632m last year, up more than a third on the prior year. The company has previously predicted sales would reach $670m-$690m in 2016 as it tries to build a sustainable business in CF.
Approval of the new label would have added around 1,500 additional eligible US patients for Kalydeco, which is currently approved for patients aged two or over with one of 10 different mutations, accounting for around 2,000 patients in the US and 4,000 internationally.
"Our intention with this submission was to rapidly bring Kalydeco to additional people with CF who we believe may benefit," said Vertex' chief medical officer Jeffrey Chodakewitz.
"We are disappointed by this decision and look forward to discussing with the FDA the next steps to bring Kalydeco to people with CF who have these residual function mutations," he added.
While certainly a setback to its plans for 2016, Vertex will take comfort from the strong performance of its follow-up CF combination therapy Orkambi (ivacaftor and lumacaftor), which brought in $220m in the fourth quarter - overtaking Kalydeco - despite only being approved in the US in July.
Analysts have predicted that Orkambi could become a $5bn-a-year product thanks to a list price of $259,000 a year, a view that has been reinforced by the news that some 4,500 patients had started treatment with the combination therapy by the end of 2015.
Kalydeco was the first drug to reach the market that is able to address the genetic malfunction behind the condition, rather than simply alleviate symptoms like other available treatments.
Taken twice-daily in tablet form, it restores the function of the mutated CFTR protein, which regulates salt and water transport in the body.