Merck has hit another wall in its effort to gain FDA approval for its muscle relaxant reversal agent, sugammadex, which is marketed in more than 50 other countries as Bridion.

The most recent in a series of setbacks came this week when the FDA said it had concerns about the operational aspects of a study it requested to test the potential for the drug to trigger hypersensitivity in patients. No other details have been made available, though Merck announced that it has received a "Complete Response Letter" from the agency and will respond very soon.

The FDA first rejected the drug in 2008 over concerns about allergic reactions and increased risks of bleeding when used with blood-thinners. In March of this year, after Merck submitted additional study data, the FDA again refused to approve the drug, asking for another 3 months to complete its review.

Then in July, the agency canceled an advisory committee review, saying it needed more time to assess the results of one of the hypersensitivity studies it had requested.

The drug reverses the muscle-relaxing effects of rocuronium and vecuronium.

Jim Burger