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Disc Medicine (IRON) is surging today after announcing that its pivotal Phase 3 APOLLO trial of bitopertin met both co-primary and all secondary endpoints in patients with erythropoietic protoporphyria (EPP). IRON described the results as a potential breakthrough, demonstrating reductions in toxic protoporphyrin IX (PPIX) alongside improvements in light tolerance, fewer phototoxic reactions, and better patient-reported outcomes.

• Bitopertin is an investigational oral GlyT1 inhibitor designed to reduce PPIX accumulation, which causes severe sensitivity to sunlight in EPP. The placebo-controlled APOLLO trial enrolled 183 adults and adolescents aged 12 and older with EPP or X-linked protoporphyria (XLP). Currently, there is only one FDA-approved treatment for EPP.
• Bitopertin achieved statistical significance on both co-primary endpoints, reducing whole-blood metal-free PPIX by 41.4% versus placebo and increasing average monthly pain-free time in light by 12.2 hours during month six. All secondary endpoints were also met, including fewer phototoxic reactions and greater cumulative pain-free light exposure.
• The drug also demonstrated statistically significant improvements in patient-reported outcomes, supporting its potential quality-of-life benefits. Bitopertin was generally well tolerated, although dizziness occurred in 50% of treated patients, with no treatment-emergent adverse events above Grade 3.
• Notably, the FDA issued a Complete Response Letter in February, declining accelerated approval after prior studies failed to establish a sufficient association between PPIX reductions and improvements in sunlight exposure. Following discussions with the FDA, IRON indicated that successful APOLLO results could support traditional approval. IRON plans to resubmit its NDA by year-end 2026, with an FDA decision anticipated by mid-2027.

Briefing.com Analyst Insight

Overall, these results represent a significant development for IRON, particularly following the FDA's rejection earlier this year. The combination of substantial PPIX reductions and statistically significant improvements in light tolerance directly addresses a central concern raised by regulators regarding the drug's clinical benefits. While FDA approval remains uncertain, the comprehensive Phase 3 results strengthen bitopertin's regulatory prospects and bring IRON closer to potentially commercializing a treatment for a rare disease with limited available options.