The FDA has denied the approval of a lung disease medication developed by United Therapeutics and Mannkind, citing an inspection issue at a third-party facility.

The medicine, Tyvaso, is being used to treat pulmonary arterial hypertension (PAH) and PAH associated with interstitial lung disease, according to a preliminary label released by the FDA on Monday.

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The FDA did not mention any operational issues at MannKind's  Tyvaso device manufacturing and testing facilities in its comprehensive response letter.

Mannkind's stock was down in premarket trading.