** Shares of drug developer Milestone Pharmaceuticals MIST.O fall 62.7% to 84 cents premarket
** US FDA declined to approve MIST's nasal spray, Cardamyst, to treat a type of heart condition, company says
** FDA raised no concerns about clinical safety or efficacy, but highlighted two key chemistry, manufacturing and controls issues - MIST
** Issues include needing additional information on nitrosamine impurities, cancer-causing compounds formed during manufacturing and an inspection of a testing facility
** Shares up 25% in the last 12 months, as of last close
(Reporting by Kamal Choudhury in Bengaluru)
((Kamal.Choudhury@thomsonreuters.com;))