The U.S. Food and Drug Administration has issued a Complete Response Letter to PTC Therapeutics, declining to approve the company’s New Drug Application for vatiquinone as a treatment for Friedreich’s ataxia. The agency said the filing did not provide substantial evidence of efficacy and called for an additional adequate, well-controlled study before the application can be resubmitted. PTC Therapeutics said it is "disappointed" with the decision and plans to meet with the regulator to discuss next steps. Friedreich’s ataxia is a rare, progressive neuromuscular disorder that typically manifests in childhood and has limited treatment options. Investors reacted quickly; PTC shares fell more than 5% in pre-market trading after disclosure of the FDA’s letter, trimming intraday losses to about the same level once regular trading opened.