Merck & Co has been dealt a blow after US authorities knocked back its anaesthesia reversal drug sugammadex for the second time.

The US Food and Drug Administration (FDA) has sent the firm, which is known as MSD outside of the US and Canada, a Complete Response Letter citing concerns about operational aspects of a hypersensitivity study it previously requested. 

Back in 2008, the agency turned down the drug for reversing the neuromuscular blockade of anaesthesia after surgery, asking for additional data relating to allergic reactions and bleeding events.

The decision came as a complete surprise to Schering-Plough (which merged with Merck in 2009), as just months before an FDA advisory committee had unanimously backed the drug's approval.

Sugammadex was also approved in Europe, where it is marketed as Bridion, days before the FDA's rejection.

So news that approval of its jab, which is designed to reverse the effects of deep muscle relaxation induced by the agents rocuronium and vecuronium, has hit yet another snag will no doubt be hugely disappointing to the firm.

Nevertheless, Merck is remaining upbeat, insisting that it "expects to be able to determine a path forward in the very near future".

If approved, sugammadex would be the first in a new class of medicines, known as selective relaxant binding agents, to be used in the US. The drug is already marketed in more than 50 countries around the globe.