AstraZeneca's combination of two injectable immunotherapy drugs failed to help patients as hoped in a closely watched advanced lung cancer trial, sending its shares plunging on Thursday.
The so-called MYSTIC study was the most anticipated clinical experiment in the pharmaceutical industry this year and the news saw the shares tumble more than 16 percent, wiping $14 billion off the company's value in their biggest ever daily fall.
The study was seen as key to proving the value of the group's new drug pipeline and its future as an independent company, after it spurned a $118 billion takeover attempt by Pfizer in 2014.
Uncertainty about the MYSTIC outcome had been heightened recently by speculation that Chief Executive Pascal Soriot might be considering a highly paid new job as head of Israel-based Teva Pharmaceutical Industries.
Soriot declined to comment directly on what he described as "rumors" on Thursday, while company insiders said he would have had to make a statement if he had firm plans to leave.
"I'm not a quitter," Soriot said, adding he was proud to lead AstraZeneca and was committed to delivering on the strategy of returning the company to growth. "The only thing I can tell you is I am here today."
Initial results from MYSTIC found the combination of durvalumab and tremelimumab was no more effective at stopping disease progression than chemotherapy in patients expressing a protein called PD-L1 on 25 percent or more of their cancer cells.
Immunotherapy drugs are designed to help the body's immune cells kill cancer and PD-L1 levels are widely used as a benchmark to determine if they are likely to work for individual patients.