The FDA panel of outside experts will discuss the data and hear from researchers, patients and public health advocates at the July 13-14 meeting before offering their advice, which could range from leaving the drug on the market and collecting more data to calling for its withdrawal.
But at least one expert witness for the plaintiffs, a Canadian cardiologist, said what he saw was so critical that he sought to testify before the FDA advisers this week. He did not indicate the nature of the documents.
In a letter to the FDA, McGill University's Allan Sniderman said he has "access to data that has not previously been available to any similar group" but is barred by a court protective order from discussing the documents he has seen.
"These data are, in my opinion, essential to review," he wrote last month.
Lawyers for patients who say they have been harmed by GlaxoSmithKline Plc's Avandia say critical data is still under wraps even as a U.S. advisory panel prepares to weigh the fate of the controversial diabetes drug.
The lawyers, who represent thousands of patients or family members suing the British drugmaker in multi-district federal litigation, say they have uncovered critical documents.
They are pushing the court to allow their release to both the U.S. Food and Drug Administration and the Senate Finance Committee, which has investigated Glaxo's handling of Avandia.
An FDA advisory panel is already wading through hundreds of pages of conflicting data and studies, and is due to meet for two days starting Tuesday on whether to recommend that Avandia be pulled off the market due to heart risks.
Avandia has been under fire since 2007 over concerns that it raises the risk of heart attack, stroke and death more than Takeda Pharmaceutical Co's rival diabetes pill Actos.
It is unclear what the court-sealed documents contain, but it raises questions about whether the FDA will be acting on Avandia without the benefit of all available information.