Shares of MannKind Corp. fell 7 percent on Friday after the FDA staff raised safety questions about the company’s inhalable insulin device, which could limit its use for certain patients. The Valencia company, which has devoted years to developing its Afrezza inhaler that delivers powdered insulin to diabetics through their lungs, faces a critical hurdle on April 1. That’s when an independent panel of experts will recommend whether the Food and Drug Administration should approve the device. The staff report, posted online just days before the panel meeting, looked at the drug’s potential risk of causing lung infections and bronchial spasms. The report tied use of Afrezza to a decline in lung function and bronchial spasms similar to asthma and coughing that caused some patients to stop treatment. Staff also questioned the accuracy of insulin doses in MannKind’s clinical studies. FDA staff are recommending that if the product is approved, a warning label would be needed to limit Afrezza’s use by smokers, asthma patients and those with kidney or liver impairment. That would reduce adoption of the device, which has cost the company hundreds of millions of dollars. Simos Simeonidis, an analyst at Cowen & Co. who covers MannKind, said in a note to investors that the staff report doesn’t paint a rosy picture. “We don’t see approval in Type 1 diabetes; the door possibly remains open in Type 2, but a positive vote and approval would be a surprise,” he wrote in a note to investors on Friday. MannKind’s first application to the FDA was rejected in 2011 because of changes to the inhaler design that the regulator said weren’t adequately studied. The agency asked for more clinical trials before granting another review. MannKind did not immediately return a call for comment. Shares MannKind closed down 37 cents to $4.83 on the Nasdaq.