AstraZeneca plc (AZN - Get Report)  suffered their biggest-even decline Thursday after the drugmaker revealed a "disappointing" failure in the trial of its key cancer therapy.

AstraZeneca shares plunged more than 16% in London trading, the biggest decline on record, to change hands at a December 2016 low of 4,295 pence each. Its U.S.-listed shares were down $5.27 apiece, or 15%, in early trading to $28.68 per share.

The news didn't bode well for another drug company that is also exploring a similar treatment, including Bristol Myers Squibb Co. (BMY - Get Report) , which repored second quarter earnings of 74 cents per share, in-line with the analyst estimates.

"The read through is an obvious negative read-through for BMY's combo of Opdivo (nivolumab; anti-PD-1) + Yervoy (ipilimumab; anti-CTLA-4) in the ongoing CheckMate-227 trial," wrote Geoffrey C. Porges, of Leerink Partners, in a note Thursday. "While there are several key differences in trial design between MYSTIC and CM-227, they are unlikely to result in meaningfully different outcomes."

Shares of Bristol Myers were down almost 6% to $52.90 piece.

AstraZeneca was hoping to prove that its combination of two immunotherapy drugs, durvalumab and tremelimumab, can help previously untreated patients with advanced lung cancer. The company Thursday said that the combination "did not meet a primary endpoint of progression-free survival compared to chemotherapy."  

"While the results from the MYSTIC trial for progression free survival in first-line Stage IV non-small cell lung cancer compared with standard of care are disappointing, the trial was designed to assess overall survival and we look forward to evaluating the remaining primary endpoints of overall survival for both mono- and combination therapy," said chief medical officer Sean Bohen.

The failure overshadowed better-than-expected second quarter earnings from the U.K.'s second-largest pharma group despite continued pressure in the U.S. from loss of exclusivity of key products.

AstraZeneca said earnings per share for the three months ending in June hit $0.87, beating a FactSet consensus of $0.80, while revenue for the period came in largely in-line with estimates at $5.05 billion but down 10% against the same quarter last year. Meanwhile product sales were down 10% year-on-year coming in at $4.94 billion in the quarter beating analysts' estimates of $4.87 billion.

Editor's pick: This article was originally published at 3:20 am ET and has been updated.

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