Disc Medicine aligns with FDA on bitopertin CRL response path

Disc Medicine has completed a Type A meeting with the US Food and Drug Administration to discuss its path forward following a Complete Response Letter (CRL) to the New Drug Application for bitopertin, its oral GlyT1 inhibitor, in erythropoietic protoporphyria (EPP). The Watertown, Massachusetts-based company said both parties agreed that positive results from the ongoing Phase 3 APOLLO study could serve as the basis for a CRL response and potentially support traditional approval.

The development is a meaningful procedural step for Disc. A Type A meeting is typically reserved for urgent matters — including post-CRL discussions — and the FDA's agreement that a single Phase 3 study could anchor the resubmission reduces the clinical workload that might otherwise have been required. Disc now expects to submit its CRL response by end of 2026, with an FDA decision expected by mid-2027.

The APOLLO study

APOLLO is a double-blind, placebo-controlled trial enrolling patients aged 12 and above with EPP and X-linked protoporphyria (XLP) across sites in the United States, Canada, Europe, and Australia. The study carries two co-primary endpoints: average monthly total time in sunlight without pain during the final month of a six-month treatment period, and percentage change from baseline in whole-blood metal-free protoporphyrin IX (PPIX) after six months. Topline data are expected in the fourth quarter of 2026.

John Quisel, President and Chief Executive Officer of Disc Medicine, said the FDA meeting provided "valuable clarity on the path forward for the bitopertin NDA" and that the company remains committed to advancing bitopertin as a potential treatment for patients with EPP.

Bitopertin works by inhibiting glycine transporter 1, a membrane protein on developing red blood cells that supplies glycine for heme biosynthesis. By reducing glycine availability, the drug is designed to lower PPIX accumulation — the underlying driver of photosensitivity in EPP. Disc licenced global rights to the compound from Roche in May 2021 and has since run multiple clinical programmes, including the Phase 2 BEACON and AURORA trials and an open-label extension study, HELIOS.

Regulatory and competitive context

EPP is a rare, inherited metabolic disorder affecting fewer than one in 75,000 people in most studied populations, in which accumulation of PPIX causes severe, sometimes debilitating phototoxic pain upon light exposure. There are currently very limited approved treatment options; afamelanotide (Scenesse, Clinuvel Pharmaceuticals) is approved in Europe and the United States for adult EPP patients but acts via a different mechanism — melanocyte stimulation rather than heme pathway modulation. Bitopertin, if approved, would be positioned by Disc as potentially the first disease-modifying therapy in the erythropoietic porphyrias, though that claim remains contingent on APOLLO's outcome.

The CRL that Disc received suggests the FDA previously identified deficiencies in the original NDA package — the specific grounds have not been publicly disclosed. The APOLLO study's co-primary endpoints, one functional (sunlight exposure time) and one biomarker-based (PPIX reduction), appear designed to address the kind of efficacy-evidence concerns that commonly underlie such letters in rare-disease submissions. How regulators weight patient-reported functional outcomes against mechanistic biomarkers in rare-disease settings has been an area of ongoing policy discussion at both the FDA and EMA, making the eventual review a potentially instructive precedent for the broader rare-haematology space.

Disc trades on NASDAQ under the ticker IRON. The company's portfolio also includes programmes targeting iron homeostasis pathways, and it has positioned itself as a specialist in red blood cell biology more broadly. The mid-2027 potential decision window will be a defining moment for the company's near-term commercial prospects.