The FDA issued a rejection letter for Disc Medicine’s bitopertin, a candidate for a rare porphyria, citing uncertainties about the blood-based biomarker used as the trial’s efficacy endpoint. Disc had been one of the first companies to use the FDA commissioner’s new expedited review voucher program; regulators nevertheless concluded the biomarker’s linkage to clinical benefit was insufficient. The decision prompted an immediate market response, with Disc shares tumbling on the news. Reporting from STAT and other outlets noted the agency’s continuing skepticism toward biomarker-only approvals and framed the rejection as an early stress test for the new voucher pathway. Investors and rare-disease developers will parse the letter for guidance on evidentiary standards for surrogate biomarkers.