Shares in Vertex Pharmaceuticals have taken a hit after the company had to take the rather embarrassing step of correcting previously-announced interim mid-stage results of a combination cystic fibrosis treatment.

Vertex stock had climbed over 70% since May 7 when it reported interim analysis of an ongoing Phase II study of VX-809 and Kalydeco (ivacaftor). At the time, the firm claimed that 17 patients (ie 46%) with two copies of the F508del mutation experienced an improvement in a measure of lung function of 5 percentage points or more. It was also reported at the time that 11 patients, or 30%, experienced an improvement of 10% or more.

However, Vertex has restated those results, saying they were the result of a misinterpretation between the firm and its outside statistical vendor concerning the type of responder analysis performed. As such, the data presented earlier this month “were relative improvements, not absolute improvements as originally reported".

The actual absolute improvements were lower: 13 patients, or 35%, improved 5 percentage points, and another 7 patients, or 19%, improved 10% or more. As previously announced,  none of the patients treated with placebo (0/11) achieved a 5 percentage-point or more mean absolute improvement in lung function from baseline to day 56. Safety results are the same as those announced on May 7.

The news resulted in Vertex stock falling nearly 11% to $57.80, having been down nearly 18% earlier in the day. However Chris Wright, head of the company's global medicines development and medical affairs, noted that even with the restated figures, "the improvements in lung function seen to date in this study exceeded our expectations".

He added that "we’re continuing to move forward as quickly as possible toward a pivotal study of VX-809 and Kalydeco in people with two copies of the F508del mutation". Vertex also presented new data which shows that the mean absolute improvement in lung function for all patients on the combo was 8.5% compared with placebo.

Analysts still believe that the combination can be come a huge seller. The US Food and Drug Administration approved Kalydeco at the end of January for a rare type of CF that affects about 4% of patients, while the combo is being tested for a form of the disease that accounts for almost half of CF cases.