(Bloomberg) -- Merck & Co. said U.S. regulators declined to approve the company’s application for its experimental chronic-cough drug in its current form, a setback as the drugmaker seeks new potential blockbusters. 

The U.S. Food and Drug Administration sent Merck a complete response letter requesting additional information regarding the measurement of efficacy of the medication, gefapixant, according to a statement on Monday from the Kenilworth, New Jersey-based company. Merck said the agency’s decision wasn’t related to the safety of the drug.

At the same time, regulators in Japan opted to clear the treatment, Merck said. The drug will be sold in that country under the brand name Lyfnua.

Merck has been studying gefapixant for treatment of refractory or unexplained chronic cough, and if approved in the U.S. it would be the first drug of its type to reach the market. But the treatment had mixed results in a late-stage study, with a 45 milligram dose meeting efficacy targets, while a smaller 15 milligram dose fell short. Eliapixant, a rival drug from Bayer and Evotech had positive results in late-stage trials.

Though the U.S. agency’s decision wasn’t a surprise, according to Bloomberg Intelligence analyst Sam Fazeli, Japan’s approval “will reassure some that the FDA setback may not be the end of the story for the product in the U.S.,” he wrote in a note.

Shares of Merck were down 2.3% at 10:34 a.m. in New York, amid a decline in the wider U.S. stock market.

Merck has faced investor pressure to diversify beyond the cancer drug Keytruda, which is its most profitable medication and will face cheaper competition starting in 2028. The company has previously described gefapixant as a key part of its pipeline, though it omitted it from a slide outlining promising drug candidates at the JPMorgan Healthcare Conference this month.

“Gefapixant we think could be an important product,” Frank Clyburn, then Merck’s president of human health, said at an event with analysts in December. Clyburn, who is leaving Merck to take over as chief executive of International Flavors & Fragrances Inc. in mid-February, said cough was a “very significant marketplace” but declined to provide specific projections.

Merck says it is reviewing the letter and will meet with the FDA to discuss next steps.

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