The FDA rejected Disc Medicine’s experimental porphyria therapy bitopertin, citing uncertainties about the blood-based biomarker used as the efficacy endpoint. The agency said the surrogate marker’s correlation with clinical benefit remained unclear, blocking a pathway that had been accelerated under a newly launched commissioner-level priority review program. Disc, which had been an early beneficiary of the administration’s voucher initiative, saw its shares plunge after the decision. The rejection underscores regulatory skepticism about biomarker-driven approvals and raises questions about the practical value of the new expedited-review vouchers. Clinicians and investors will watch for whether Disc can supply additional clinical outcome data or pursue another regulatory strategy.