United Therapeutics Corporation announced an update on the U.S. Food and Drug Administration (FDA) review of the new drug application (NDA) for Tyvaso DPI™, for the treatment of pulmonary arterial hypertension (PAH) and pulmonary hypertension associated with interstitial lung disease (PH-ILD), to improve exercise capacity. On October 15, 2021, the FDA issued a complete response declining to approve the NDA at this time. The FDA's letter noted only a single deficiency preventing approval of Tyvaso DPI, related to an open inspection issue at a third-party facility that performs analytical testing of treprostinil drug substance. In addition, the FDA did not cite any deficiencies or issues related to operations performed at the MannKind Corporation facility for manufacture, testing, and packaging of finished Tyvaso DPI, including its associated device. All other requests from the agency have been addressed. The draft labeling for Tyvaso DPI, as revised by FDA, includes the same indications as Tyvaso® (treprostinil) Inhalation Solution for PAH and PH-ILD, to improve exercise ability, and does not contain any contraindications or a boxed warning. The complete response also notes, but does not cite as a deficiency, that the FDA has not yet completed its review of a Citizen's Petition submitted to FDA in July 2021 concerning the safety of an excipient in Tyvaso DPI. TYVASO (treprostinil) is a prostacyclin mimetic indicated for the treatment of: Pulmonary arterial hypertension (PAH; WHO Group 1) to improve exercise ability. Studies establishing effectiveness predominately included patients with NYHA Functional Class III symptoms and etiologies of idiopathic or heritable PAH (56%) or PAH associated with connective tissue diseases (33%). The effects diminish over the minimum recommended dosing interval of 4 hours; treatment timing can be adjusted for planned activities. While there are long-term data on use of treprostinil by other routes of administration, nearly all controlled clinical experience with inhaled treprostinil has been on a background of bosentan (an endothelin receptor antagonist) or sildenafil (a phosphodiesterase type 5 inhibitor). The controlled clinical experience was limited to 12 weeks in duration. Pulmonary hypertension associated with interstitial lung disease (PH-ILD; WHO Group 3) to improve exercise ability. The study establishing effectiveness predominately included patients with etiologies of idiopathic interstitial pneumonia (IIP) (45%) inclusive of idiopathic pulmonary fibrosis (IPF), combined pulmonary fibrosis and emphysema (CPFE) (25%), and WHO Group 3 connective tissue disease (22%).