The FDA rejected Disc Medicine’s bitopertin for a rare porphyria disorder, citing uncertainties about the surrogate biomarker that underpinned the company’s efficacy claims. The decision came even after the candidate received expedited handling through a newly launched priority review program, and the agency’s letter prompted a steep sell‑off in Disc’s shares. Separately, PTC Therapeutics withdrew its U.S. application for Translarna (ataluren) for nonsense‑mutation Duchenne muscular dystrophy after the FDA signaled the existing data package was unlikely to meet the agency’s threshold for substantial evidence of effectiveness. Both developments underscore regulatory friction over surrogate endpoints and biomarker acceptance in rare‑disease submissions.