Shares of MannKind Corp. fell more than 18 percent Monday after the company disclosed a therapy it developed with United Therapeutics Corp. was rejected by the Food and Drug Administration.

MannKind in Westlake Village sells Afrezza, an inhalable insulin for diabetics. The collaboration with United Therapeutics focused on using a similar lung-based delivery system for a drug called Tyvaso DPI that treats pulmonary arterial hypertension.

“The FDA declined to approve the (new drug application) at this time, noting only one deficiency related to an open inspection issue at a third-party analytical testing center for treprostinil drug substance, the active ingredient of Tyvaso DPI,” the company said in a statement.
 “The complete response did not pertain to MannKind, and no issues were cited by the FDA as it relates to MannKind’s facility in Connecticut for manufacturing.”

In a statement, Michael Castaga, MannKind’s chief executive, said: “We continue to build pre-launch inventory of Tyvaso DPI and look forward to supporting United Therapeutics’ efforts in securing approval of Tyvaso DPI in the coming months.”

Shares of MannKind (MKND) closed Monday down 93 cents, or nearly 18.3 percent, to $4.16 on the Nasdaq on a day the market rose a fraction of a percent.