The US Food and Drug Administration has rejected Merck's cladribine, the first drug manufactured to treat multiple-sclerosis.
In a complete response letter, the FDA recommended that the German company provide an improved understanding of safety risks and the overall benefit-risk profile, either through additional analyses or additional studies.
The EU, which would have been the drug's largest market, also rejected the pill in January, prompting analysts to remove revenues for cladribine from Merck estimates.
Merck has said that it intends to request an end-of-review meeting with the FDA to clarify next steps and to identify whether data from completed and ongoing clinical studies can address the agency's questions.
Fereydoun Firouz, president and CEO of Merck's US subsidiary EMD Serono, said, "Our commitment to transform the way people living with multiple sclerosis approach their therapy options remains steadfast.
"We look forward to working with the FDA to address the safety issues in its letter and will continue to move toward identifying a potential path that provides patients and physicians the opportunity to have access to cladribine tablets in the treatment of multiple sclerosis."
Cladribine is a small molecule that may interfere with the behaviour and the proliferation of certain white blood cells, particularly lymphocytes, which are thought to be involved in the pathological process of multiple sclerosis.