Bristol-Myers Squibb Co said its blockbuster Opdivo immunotherapy failed to slow progression of symptoms in previously untreated patients with advanced lung cancer, a major setback expected to benefit Merck & Co's rival medicine.

Shares of Bristol-Myers fell almost 16 percent to $63.54 after the company announced failure of the late-stage trial. Shares of Merck gained 8.7 percent to $62.89.

"This is the worst-case scenario" for Opdivo and Bristol-Myers, Leerink analyst Seamus Fernandez said in a research note. "There appears to be no silver lining."

Opdivo had been considered the clear leader in a costly new class of medicines called checkpoint inhibitors that work by taking the brakes off the immune system. Its second-quarter sales of $840 million were more than double those of Merck's Keytruda.

Opdivo won approval in March 2015 for treatment of patients with advanced non-small cell lung cancer who had previously failed to benefit from chemotherapy. Six months later, Keytruda received approval for a similar patient population.

Meanwhile, both companies raced to complete large trials to see if their drugs, which each cost about $150,000 a year, would also benefit patients who had never previously been treated for the most common form of lung cancer. The trials compared the drugs against chemotherapy.

"Obviously the results of this trial are disappointing," Bristol-Myers Chief Executive Giovanni Caforio said in an interview, saying that the data suggests there is no role in the foreseeable future for Opdivo as a standalone medicine for previously untreated lung cancer.

Patients in the Opdivo trial had tumors with low levels of PD-L1, a protein whose presence may help identify those most likely to benefit from checkpoint inhibitors. Those in the successful Merck trial, by contrast, were only enrolled if they were shown to have high levels of PD-L1 and represented a smaller percentage of the lung cancer population.

Caforio said the best hope now for Opdivo in front-line treatment of lung cancer is combination therapy. The company is conducting a late-stage study with more than 2,000 patients in which Opdivo will be combined with Bristol-Myers' other approved immunotherapy, Yervoy. The data is expected by early 2018.

"That study will provide the definitive answer of the role of combination immunotherapy in the front-line lung cancer setting," he said.

Data in June from a large clinical study showed Keytruda was effective in newly treated patients, not only in delaying progression of lung cancer symptoms but also in prolonging survival. The trial was stopped so that all patients could receive Keytruda.

The data spurred hopes that U.S. regulators would soon expand approved use of the Merck drug. Complete data for the study has not yet been released.

Now that Bristol-Myers' drug has failed in that effort, Merck's medicine could have a better chance to catch up with Opdivo's far larger sales.

Sanford Bernstein analyst Tim Anderson said Opdivo, Keytruda and similar drugs in development had been expected to capture peak annual combined sales of $10 billion to $15 billion from lung cancer, with Bristol-Myers winning the lion's share.

"Today's news very much helps level the playing field and is a major boost for Merck," Anderson said in a research note.

Opdivo and Keytruda are also approved to treat advanced melanoma and are being studied against a wide range of other types of cancer.