Merck & Co has suffered a setback after regulators in the USA turned down the company's bid to market higher doses of its investigational insomnia drug suvorexant.

The company has received a complete response letter from the US Food and Drug Administration reflecting a recommendation from the agency's Peripheral and Central Nervous System Drugs Advisory Committee in May. Specifically, the FDA, which says the drug is safe at all doses, advised Merck that 10mg suvorexant should be the starting dose for most patients, and 15mg and 20mg could also be appropriate.

However the agency states that the safety data do not support the approval of suvorexant 30 mg and 40 mg. Also, the drug needs to be reviewed by the US.Drug Enforcement Administration to evaluate the compound as a controlled substance.

After reviewing the letter, Merck says additional trials of suvorexant 10mg will not be necessary but manufacturing studies will be required to advance that dosage form. The company gave no timelines as to how long those studies could take.

If finally approved, suvorexant would be the first in a new class of medicines, called orexin receptor antagonists. However, as Sanford Bernstein analyst Tim Anderson wrote in an investor note, the FDA's decision to force Merck "to go with lower doses, and simultaneously turning down the two higher doses because of safety issues, leaves suvorexant with even more mixed commercial prospects".

He added that "by virtue of rejecting the two higher doses, the FDA is essentially saying there is a safety issue." Mr Anderson  concluded by saying that that at a lower dosage range, the drug’s efficacy will be less, and this, coupled with safety concerns, "will make suvorexant a tougher sell in a market where there is plenty of choice and generic availability already".