Valencia biotech firm MannKind Corp.'s stock fell 11% Thursday after reports that a former senior manager said he had uncovered potentially serious problems with clinical trials of the company's experimental insulin inhaler.

The Food and Drug Administration is reviewing the Afrezza inhaler and is expected to make a decision Dec. 29 on whether to approve it.

The former MannKind manager, John Arditi, filed a lawsuit against the company in New Jersey Superior Court, saying he was wrongfully fired after internal audits he conducted in November 2009 uncovered "potential fraud and scientific misconduct" involving Afrezza clinical trial data. The lawsuit, which was filed in September, was first reported on TheStreet.com.

Arditi, who worked in a MannKind facility in New Jersey, said in the lawsuit that he urged his superiors at the firm to report his findings to the FDA but that the company refused because "if the FDA was notified of these concerns, it might delay approval" of the inhaler.

MannKind addressed the lawsuit in its most recent quarterly earnings report, stating that the company had completed an internal investigation into Arditi's claims and had hired an outside firm to conduct an independent investigation.

"Neither investigation found any basis for his claims," MannKind said. "The company believes that the allegations in the complaint are without merit and intends to defend against them vigorously."

Matthew J. Pfeffer, MannKind's corporate vice president, said the company was working on a legal response to the lawsuit. MannKind has until Dec. 3 to file its response in court, Pfeffer said.

MannKind has yet to share with the FDA the findings of its internal investigation or of the independent investigation, he said.