It's never a good day when drugs don't appear to work, and that was the case Thursday for an experimental combination of AstraZeneca medicines designed to treat lung cancer.
AstraZeneca shares fell 15 percent after the British drugmaker said its highly anticipated clinical trial, called MYSTIC, failed to show its two-drug combination extended progression-free survival, a measure of how long patients live without their cancer growing, compared with standard-of-care chemotherapy.
In London trading, it was the stock's biggest drop ever.
The combination was seen as AstraZeneca's ticket to catch up to leaders in immunotherapy, a new way of treating cancer that unleashes the power of the body's own immune system. The British drug giant had been seen as behind in a race dominated by Bristol-Myers and Merck, with Roche, Pfizer and others on their tails.
The results Thursday don't bode well for AstraZeneca's approach, and they affected other stocks as well.