Disc Medicine has reported that its experimental drug bitopertin met all primary and secondary endpoints in a phase 3 trial involving patients with erythropoietic protoporphyria (EPP), a rare inherited metabolic disorder that causes painful reactions to sunlight. The findings include a reduction in a disease-associated blood biomarker and an increase in the time patients could spend in light without pain. The company plans to resubmit the drug candidate for US regulatory approval by the end of 2026.
Phase 3 Trial Demonstrates Improvements in Biomarker and Sunlight Tolerance
The APOLLO trial evaluated bitopertin in 183 patients aged 12 and older. EPP causes protoporphyrin IX (PPIX) to accumulate in the blood, making patients highly sensitive to sunlight and potentially triggering painful skin reactions.
One of the trial’s co-primary endpoints measured whole-blood metal-free PPIX levels. Disc reported a 41.4% reduction from baseline relative to placebo, meeting this endpoint and demonstrating the drug candidate’s effect on the biomarker.
The other co-primary endpoint assessed the amount of time patients could spend in light without experiencing pain. During the sixth month of treatment, patients receiving bitopertin spent an average of 12.2 additional hours per month in light without pain compared with those receiving placebo.
These results address a concern raised by the US Food and Drug Administration (FDA) when it declined to approve bitopertin in February. At the time, regulators questioned whether reductions in PPIX levels would correspond to improvements in patients’ ability to tolerate sunlight.
Secondary Endpoints and Safety Findings
The study also met its secondary endpoints, which included a reduction in the rate of phototoxic reactions and an increase in the cumulative time patients spent in light without pain over the six-month treatment period.
Another measure showed an increase in the time between sunlight exposure and the onset of early symptoms. This analysis was limited to adults, according to the reported results.
Two exploratory measures of quality of life also showed significant improvements. Disc reported that bitopertin was generally well tolerated, with no treatment-emergent adverse events above grade 3.
Dizziness was the most commonly reported treatment-emergent adverse event among patients receiving bitopertin, affecting 50% of the group. The episodes were generally mild or moderate and temporary.
Commenting on the results, BMO Capital Markets analysts described the data as “overwhelmingly positive” in a note to investors.
Previous FDA Decision and Resubmission Plans
The FDA’s February decision followed concerns about whether the biomarker changes observed in earlier studies established a clear relationship between reduced PPIX levels and improved sunlight tolerance. The APOLLO results provide evidence from a late-stage trial measuring both outcomes.
Disc plans to resubmit its application for FDA approval by the end of 2026, with a regulatory decision anticipated by mid-2027.
Existing Treatment and Sales Estimates
Scenesse, developed by Clinuvel Pharmaceuticals, is the only FDA-approved treatment for EPP in the United States. The treatment is administered through an under-the-skin hormone implant. Avoiding sunlight, staying indoors, and wearing protective clothing remain ways to manage the condition.
Disc CEO John Quisel, Ph.D., said last month that a relatively small number of EPP patients receive Scenesse. The company aims to reach approximately 600 patients treated at the 10 to 15 leading treatment centers, with plans to expand its reach beyond that group.
BMO analysts estimate bitopertin’s peak sales could reach $1.2 billion. The consensus peak sales estimate, adjusted for the probability of approval, is $1 billion. Following the trial announcement, Disc Medicine’s shares rose sharply in premarket trading.
Disc Medicine Reports Positive Phase 3 Results for Bitopertin
Disc Medicine announced on October 19, 2026, that its experimental medicine bitopertin met all primary and secondary endpoints in the pivotal Phase 3 APOLLO clinical trial. The study evaluated the treatment in adults and adolescents with erythropoietic protoporphyria (EPP), including X-linked protoporphyria (XLP), rare inherited disorders that can cause severe pain and other reactions following sunlight exposure.
The results represent an important milestone for Disc Medicine as it prepares to resubmit a New Drug Application to the U.S. Food and Drug Administration (FDA). The company said it plans to resubmit the application by the end of 2026, with a potential FDA decision anticipated by mid-2027.
Although the Phase 3 findings are encouraging, bitopertin remains investigational and is not approved for use in any jurisdiction.
Disc Medicine plans to resubmit its application by the end of 2026 and anticipates a regulatory decision by mid-2027. The timing and outcome remain subject to FDA review, and approval is not guaranteed.
What the Results Mean for Rare Disease Drug Development
The APOLLO findings highlight the importance of evaluating both biological markers and outcomes that matter directly to patients. For rare diseases, clinical trials must often work with relatively small patient populations while demonstrating convincing evidence of safety and effectiveness.
By measuring PPIX levels alongside pain-free light exposure and phototoxic reactions, the trial evaluated multiple dimensions of the disease.
For Disc Medicine, successful regulatory review could establish bitopertin as an important addition to its hematology-focused development portfolio. For the wider biotechnology sector, the results demonstrate how a targeted approach to disease biology may create new opportunities for conditions with limited treatment options.
However, clinical trial success is only one stage of drug development. Regulatory assessment, manufacturing readiness, access, and long-term safety monitoring will remain important considerations.
Conclusion
Disc Medicine’s positive Phase 3 APOLLO results mark a significant development for bitopertin and the treatment landscape for erythropoietic protoporphyria. The reported reductions in PPIX, increased pain-free light exposure, and improvements across secondary endpoints provide encouraging evidence of potential clinical benefit.
The next major milestone will be the planned FDA application resubmission. Until regulators complete their review, bitopertin remains an investigational therapy. Its eventual impact will depend on the full evidence supporting its safety, effectiveness, and appropriate use.
Understanding Erythropoietic Protoporphyria
Erythropoietic protoporphyria is a rare genetic disorder associated with abnormalities in heme production. Heme is an essential component of hemoglobin, the protein in red blood cells that carries oxygen throughout the body.
In people with EPP, a substance called protoporphyrin IX (PPIX) can accumulate in the blood and other tissues. When exposed to sunlight, this substance can trigger intense pain, burning sensations, swelling, and other phototoxic reactions.
For affected individuals, ordinary outdoor activities can become difficult. Some patients limit their time outside, wear protective clothing, and carefully plan activities to avoid sunlight exposure.
The condition can also affect quality of life and, in some cases, contribute to complications involving the liver and biliary system. These challenges create a significant need for treatments that address the underlying biology of the disease rather than relying solely on sunlight avoidance and symptom management.
How Bitopertin Works
Bitopertin is an oral investigational medicine developed by Disc Medicine to modulate heme biosynthesis. It inhibits glycine transporter 1, or GlyT1, which helps supply glycine for the production of heme in developing red blood cells.
By altering this pathway, bitopertin is designed to reduce the production and accumulation of PPIX. The goal is to address an important biological mechanism contributing to the symptoms of EPP.
This approach differs from strategies that primarily help patients manage their response to sunlight. If its clinical benefits are confirmed and the medicine receives regulatory approval, bitopertin could offer another treatment option for people living with this rare disorder.
Key Findings From the Phase 3 APOLLO Trial
The APOLLO study enrolled 183 participants aged 12 and older across the United States, Canada, Europe, and Australia. Participants were randomly assigned to receive either 60 mg of oral bitopertin once daily or a placebo for six months.
Disc Medicine reported positive results across both co-primary endpoints.Disc Medicine is advancing the development of innovative treatments for rare diseases with its investigational medicine bitopertin, which recently achieved positive results in the Phase 3 APOLLO clinical trial for erythropoietic protoporphyria (EPP). This rare inherited disorder can cause severe pain, burning sensations, and skin reactions following sunlight exposure.
Disc Medicine reported that bitopertin met all primary and secondary endpoints, demonstrating improvements in biological markers and patients’ ability to tolerate light. The treatment works by targeting glycine transporter 1 (GlyT1), a mechanism designed to reduce the accumulation of protoporphyrin IX, a substance associated with EPP symptoms. These findings represent an important milestone for Disc Medicine as it works to address the medical needs of patients who have limited treatment options.
The company plans to resubmit its New Drug Application to the U.S. Food and Drug Administration, with a potential regulatory decision anticipated in 2027. If approved, bitopertin could strengthen Disc Medicine’s position in rare disease drug development and offer a new approach to managing sunlight sensitivity. However, further regulatory review is required, and the medicine remains investigational until approval is granted. The progress also highlights how Disc Medicine is using scientific research and targeted drug development to explore new treatment opportunities for patients living with challenging genetic conditions.
