Milestone Pharmaceuticals, a Canadian company with a U.S. subsidiary in Charlotte, is back on track with a potential drug to treat rapid heart rates in certain cardiovascular disorders.The company in late March re-submitted a New Drug Application (NDA) to the U.S. Food and Drug Administration (FDA) seeking approval of etripamil for the management of paroxysmal supraventricular tachycardia (PSVT), a type of arrhythmia or abnormal heart rhythm.