(Reuters) - Merck & Co has pulled ahead of rivals in the race to combine immunotherapy with other drugs as a treatment for lung cancer, potentially giving it a major lift in the battle for the largest cancer market.

U.S. regulators have agreed to a speedy review of Merck’s application to combine its immune system-boosting drug Keytruda with chemotherapy as an initial therapy for advanced lung cancer, the U.S. drugmaker said.

Merck said the U.S. Food and Drug Administration (FDA) would decide by May 10 whether to approve its Keytruda combination treatment, sending the company’s stock 5 percent higher in early U.S. trading on Wednesday.

Shares in companies working on competing immunotherapy-based combinations for lung cancer fell back, with Bristol-Myers Squibb down 2 percent, while Roche and AstraZeneca lost around 0.5 percent.

Immunotherapy is revolutionizing some areas of cancer care but giving it on its own only seems to work better than chemotherapy in previously untreated lung cancer patients who have high levels of a protein called PD-L1.

Since just a quarter to a third of non-small cell lung cancer (NSCLC) patients have tumors with at least 50 percent of cells producing PD-L1, around 70 percent of the market is still up for grabs for successful combination products.

Analysts at Evercore ISI estimate the market for first-line lung cancer for all patients could be as high as $14 billion.

Although Merck presented good results from a mid-stage Phase II trial for its Keytruda-chemotherapy combination at conference in Denmark in October, many analysts had thought it would need to wait for data from a larger Phase III study before filing.

“This comes as an important surprise because if FDA approves the application, Merck would suddenly be catapulted ahead of all other (immunotherapy) competitors who are also pursuing competing combination regimens of their own,” said Bernstein analyst Tim Anderson.

Merck had not previously indicated that it was close to filing for the combination therapy and analysts had been looking for this news toward the end of 2017.

RISKS REMAIN

Jefferies analyst Jeffrey Holford agreed the early move was positive for Merck but said an approval was “high risk”, given several weaknesses in the small Phase II study.

Merck’s early move, if successful, could change the dynamics of the marketplace, since it had been assumed that so-called CTLA4 combinations from AstraZeneca and Bristol-Myers would be approved before chemotherapy ones.

AstraZeneca’s closely watched Phase III study of its two drugs durvalumab and tremelimumab is due to report results in the first half of this year, while Bristol-Myers and Roche are also expected to have data during 2017.

Merck’s Keytruda alone is already approved as an initial, or first-line, treatment for advanced lung cancer in patients whose tumors have a high level of PD-L1 expression, the protein that the drug targets to help the immune system fight cancer.

“While Merck is unlikely to durably penetrate this entire population, especially with multiple competing regimens on the horizon, an approval in May would give them a significant first-mover advantage,” said Evercore analyst John Scotti.