Shares of MannKind Corp.
MNKD,
-19.16%

tumbled 18.3% in trading on Monday after the company said the Food and Drug Administration would not approve a pulmonary arterial hypertension treatment it is developing with United Therapeutics Corp.
UTHR,
-2.10%
.
United’s stock was down 2.6%. MannKind said in a news release that the FDA had issued a complete response letter to United Therapeutics, citing a problem with a third-party testing center for the substance used to make Tyvaso DPI. MannKind said it looks “forward to supporting United Therapeutics’ efforts in securing approval of Tyvaso DPI in the coming months.” MannKind’s stock is up 32.7% for the year, while the broader S&P 500
SPX,
+0.23%

has gained 19.0%.

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