Executives had previously warned that the late-stage clinical study combining its drug candidate, called VX-661, with its approved drug Kalydeco might not prove effective for about 150 patients enrolled in the study. Each of those patients has two separate genetic mutations linked to CF.

The announcement sent shares of the Boston-based biotech company down more than 2 percent in after-hours trading.

Vertex Pharmaceuticals Inc. said Monday it is ending a clinical study testing a two-drug combination therapy on a small group of cystic fibrosis patients after an independent board concluded the experimental treatment wasn’t showing meaningful benefit.

In a statement, Vertex chief medical officer Jeffrey Chodakewitz said that this CF population has “a form of the disease that is particularly difficult to treat.” But he said Vertex remains hopeful those patients will be able to benefit from a three-drug combination -- including VX-661 and Kalydeco -- that Vertex is also developing.

VX-661 is considered Vertex’s lead development drug, which the company is using in combination with other approved and experimental medicines to treat CF patients with different mutations. Its two approved drugs, Kalydeco and Orkambi, treat CF patients with certain mutations, but there are many patients with other mutations who currently have no treatments.


Vertex told investors at the J.P. Morgan Healthcare Conference last January that it was initiating four global clinical trials of the two-drug combination on different patient populations. The other three studies, involving a total of about 900 patients, are continuing.

If they are successful, Vertex plans to apply to the Food and Drug Administration in the second half of next year for approval of the combination treatment for the other groups.

Analysts said the setback, which was identified in a planned “futility study” by an independent data monitoring group of clinicians involved in the clinical trial, won’t alter the company’s overall strategy of developing treatments for CF patients with different genetic mutations. The obstructive lung disease affects about 75,000 people worldwide, including 30,000 in the United States.


Robert Weisman can be reached at robert.weisman@globe.com. Follow him on Twitter @GlobeRobW.