By Peter Loftus 

Merck & Co.'s new cancer drug Keytruda improved the survival odds of patients with the skin cancer melanoma compared with Bristol-Myers Squibb Co.'s Yervoy, in the first head-to-head study of a new wave of drugs that harness the body's immune system to destroy tumors.

As scientists reported the results Sunday at a medical conference, Merck also said it has filed for U.S. regulatory approval to market Keytruda as a treatment for lung cancer, which represents a bigger patient population than melanoma.

Bristol, Merck, Roche Holding AG and other drug makers are racing for a share of the burgeoning cancer immunotherapy market, which some analysts predict could reach $20 billion to $30 billion in annual sales in the next decade.

Both Yervoy and Keytruda are immunotherapies, but they work in different ways. Yervoy has become the standard first-line treatment for advanced melanoma since hitting the market in 2011, helping it to rack up $1.3 billion in sales last year. Keytruda, known as a PD-1 inhibitor, was approved last year to treat melanoma in patients whose disease worsened after trying Yervoy.

Yervoy was the first drug to reach the market for advanced melanoma shown to prolong life. It "became the gold standard against which everything else had to be measured," said Suzanne Topalian, director of the melanoma program at Johns Hopkins Kimmel Cancer Center.

With the new study showing Keytruda superior to Yervoy, "this is now expected to change the treatment landscape for melanoma," she said. "This is a high impact trial."

The study reported Sunday tested Keytruda against Yervoy in more than 830 patients whose melanoma had spread to other parts of the body. About one-third of patients received Keytruda once every two weeks, one-third received it every three weeks, and the remaining one-third received the standard four cycles of Yervoy.

One year after the start of treatment, 74% of patients who received Keytruda every two weeks and 68% of those who received it every three weeks were still alive, compared with 58% for those who received Yervoy. Measured another way, Keytruda reduced the risk of death by 31% to 37% versus Yervoy, according to the results, which were published online Sunday by the New England Journal of Medicine and presented at the annual meeting of the American Association for Cancer Research in Philadelphia.

The survival benefit prompted an independent monitoring committee to recommend in March that the study be stopped early so that patients who received Yervoy could be given the option of receiving Keytruda. The rates of serious side effects--including the inflammatory bowel disease colitis--were lower among patients who took Keytruda than those on Yervoy.

"The treatment paradigm for patients with advanced melanoma should be changed because a PD-1 antibody up front is beneficial," said Antoni Ribas, professor of oncology at UCLA's Jonsson cancer center, one of the leaders of the Merck-funded study and co-author of the NEJM paper. "It has a higher response rate and a better safety profile."

Merck plans around mid-year to apply for regulatory approval of Keytruda as a first-line treatment for metastatic melanoma. Roger Perlmutter, president of Merck's research arm, said the new study supports Keytruda as "the favored drug" in first-line treatment.

Bristol-Myers' Opdivo acts on the same PD-1 mechanism as Keytruda. Dr. Ribas said it is likely Opdivo is similarly more effective against melanoma than Yervoy, though it wasn't tested in the study reported Sunday.

But Yervoy may not be fully supplanted as a first-line treatment for metastatic melanoma. Bristol-Myers has been testing a regimen combining Yervoy with its own anti-PD-1 drug, Opdivo, for metastatic melanoma. It has shown promising survival benefits in prior studies, but with relatively high rates of adverse events. Results of a new study of the combination are due out Monday.

Fouad Namouni, head of Bristol-Myers's development program for Yervoy and Opdivo, said Yervoy "set a new standard in durable long-term survival," and the company now believes the future of melanoma treatment lies in combining immunotherapies including Yervoy and Opdivo.

In a separate study released Sunday, Keytruda significantly shrank tumors in 19% of patients with lung cancer in an early-stage trial. The tumor-shrinkage rate was higher, at 45%, among patients whose tumors substantially expressed a substance called PD-L1, which tumor cells use to interact with another substance, PD-1, which is present on immune cells, to escape destruction. The results suggest that the level of a tumor's PD-L1 expression could be used to predict which patients will respond best to drugs that block the PD-1/PD-L1 interaction, said Edward Garon, a UCLA oncologist who co-authored the study.

Merck has filed for U.S. regulatory approval of Keytruda to treat non-small-cell lung cancer.

Earlier this year, Bristol's Opdivo was approved by the Food and Drug Administration to treat a subtype of non-small cell lung cancer. On Friday, Bristol said a clinical trial of patients with another subtype of lung cancer was stopped early because Opdivo improved survival compared with chemotherapy.

Ron Winslow contributed to this article.

Write to Peter Loftus at peter.loftus@wsj.com

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