Shares of inhaled insulin drug developer MannKind Corp. lost more than half of their value Wednesday after Pfizer warned of increased instances of lung cancer among users of its competing drug Exubera.

Shares in MannKind plummeted 60 percent, or $3.50 per share, to close at $2.35.

Valencia-based MannKind’s Technosphere is the only remaining inhaled insulin product still in development after two other companies dropped their programs. MannKind contends its inhaler, which is in late-stage clinical trials, is more effective and safer than competitors.

Pfizer stopped marketing Exubera in October due to poor sales after gaining U.S. Food and Drug Administration approval in 2006. After reviewing clinical trial data and following up with clinical participants, Pfizer said Tuesday that it found six instances of lung cancer out of 4,740 Exubera-treated patients as compared with one case out of 4,292 patients not given the drug. The six patients with cancer either had been or were smokers.

The company, while noting there were too few cases to conclude that Exubera had triggered the cancer, is changing the label on its remaining inventory to note the risk.