The U.S. Food and Drug Administration has issued a response letter to Indianapolis-based Eli Lilly and Co. (NYSE: LLY) saying it is unable to approve the new drug application for a once-daily treatment for rheumatoid arthritis. The FDA says additional clinical results are needed before the drug can be approved.

Lilly developed the drug, known as baricitinib, in collaboration with Delaware-based Incyte Corp. (Nasdaq: INCY). In the response letter, the FDA says it needs more data to determine the most appropriate doses of the drug and to further characterize safety concerns.

"We are disappointed with this action. We remain confident in the benefit/risk of baricitinib as a new treatment option for adults with moderate-to-severe RA," said Christi Shaw, president of Lilly Bio-Medicines. "We will continue to work with the FDA to determine a path forward and ultimately bring baricitinib to patients in the U.S."

Lilly says a timeframe for resubmitting the new drug application will be determined based on further discussions with the FDA. 

The companies submitted the NDA in January 2016. Three months later, Lilly said the drug was returning positive results in late-stage testing, which were published in the New England Journal of Medicine