It looks like Merck & Co will have to wait a little longer for US approval of its sugammadex, after US regulators said they need more time to assess the results of a recently completed inspection of a clinical trial site. 

The drug giant has confirmed that the US Food and Drug Administration called off a meeting of its Anesthetic and Analgesic Drug Products Advisory Committee scheduled this week to discuss approval of the drug for reversing the effects of anaesthesia after surgery.

The clinical site in question was one of four that carried out a hypersensitivity study previously requested by the agency, and Merck said it is now engaged in discussions with the regulator to identify the steps necessary for a complete review.

The move is yet another setback in the drug's path to market. The agency originally rejected sugammadex in August 2008 - despite a unanimous recommendation by its advisory committee - requesting additional data related to allergic reactions and bleeding events.

Sugammadex, which was approved in Europe in 2008 as Bridion and is marketed in more than 40 countries around the globe, is designed to inactivate the muscle relaxants rocuronium or vecuronium to reverse anaesthesia.

Merck bagged the drug when it snapped up Schering-Plough for $41 billion in 2009. Currently, sugammadex pulls in global sales of around $260 million in 2012, but with US approval could fast approach $1 billion by 2020, according to analysts at JPMorgan.