Disc Medicine · Nasdaq: IRON · Bitopertin (erythropoietic protoporphyria)

Disc's 8-K attaches the letter itself. Its press release restates the FDA's two factors closely; it does not mention the letter's 'relatively modest' finding, its 'significant uncertainty' or its requirement for evidence based on clinical endpoint(s), and says Disc believes the issue is readily addressable. The clinical-endpoint requirement appears in Disc's investor presentation four days later.

The 8-K states: “A copy of the press release and a copy of the CRL are attached as Exhibit 99.1 and Exhibit 99.2, respectively” [8-K · Feb 13]. What follows compares the letter with the press release (Exhibit 99.1).

The letter says Trial 201 “demonstrated an effect on its primary endpoint” [FDA · Feb 13 · p. 2]; the press release says “the FDA agreed that AURORA and BEACON provided sufficient evidence that bitopertin significantly lowers whole blood metal-free PPIX” [8-K · Feb 13]. The letter adds that “The percent change in PPIX was relatively modest” [FDA · Feb 13 · p. 2]. The press release does not mention this. [8-K · searched]

On association with sunlight exposure, the press release follows the letter's wording: “the trials did not show evidence of association between percent change in PPIX and sunlight exposure-based endpoints” [8-K · Feb 13]. The letter says this “leaves significant uncertainty” [FDA · Feb 13 · p. 2]; the press release says “Disc believes the issue raised is readily addressable” [8-K · Feb 13]. It does not mention the letter's 'significant uncertainty'. [8-K · searched] The 10-K later says “the FDA determined that there is uncertainty” [10-K · Feb 26].

The letter says Disc “will need to provide evidence from an adequate and well-controlled trial(s)” [FDA · Feb 13 · p. 2] “based on clinical endpoint(s)” [FDA · Feb 13 · p. 2], with the option of “completing your ongoing trial and, if the trial is successful, submitting the results” [FDA · Feb 13 · p. 2]. The press release says “FDA indicated a need to see the results of the ongoing Phase 3 APOLLO study before making a decision” [8-K · Feb 13]. It does not mention clinical endpoints; [8-K · searched] Disc's Feb 17 presentation says Disc “will need to provide additional evidence demonstrating the efficacy of bitopertin for the treatment of protoporphyria based on clinical endpoint(s)” [8-K · Feb 17].

The letter asks for evidence based on clinical endpoint(s). APOLLO's co-primary endpoints, per the 10-K, are pain-free time in sunlight and PPIX change. In the earlier AURORA trial, the company reports, cumulative pain-free time in sunlight was not statistically significant versus placebo, while a post-hoc analysis of light tolerance was nominally significant. The letter says the trials showed no association between PPIX change and sunlight-exposure endpoints; the 10-K says greater PPIX reductions were associated with improvements in light tolerance measures.

The letter says Trials 201 and 202 “did not show evidence of association between percent change in whole blood metal-free PPIX and sunlight-exposure based endpoints” [FDA · Feb 13 · p. 2]. Disc's 10-K says of AURORA: “Greater PPIX reductions were associated with improvements in multiple light tolerance measures” [10-K · Feb 26].

On AURORA's sunlight measure, the 10-K says “the benefit in the placebo arm in the AURORA trial was greater than expected and the results were not statistically significant compared to placebo” [10-K · Feb 26]; a “post-hoc longitudinal analysis showed time-dependent improvement in light tolerance that was nominally significant compared to placebo” [10-K · Feb 26].

The letter asks for evidence “demonstrating the efficacy of bitopertin” [FDA · Feb 13 · p. 2] “based on clinical endpoint(s)” [FDA · Feb 13 · p. 2], and notes “You propose an ongoing trial, DISC-1459-301, as your confirmatory trial” [FDA letter, 8-K Exhibit 99.2 · Feb 13]. The 10-K lists APOLLO's “Co-primary endpoints of average monthly total time in sunlight without pain between 10:00 a.m. and 6:00 p.m. during the last month of the six-month treatment period and percent change from baseline in whole blood metal-free PPIX after six months of treatment” [10-K · Feb 26]; Disc's presentation says the PPIX endpoint was “Recommended as co-primary by FDA” [8-K · Feb 17].

After its Type A meeting, Disc reports that “the FDA indicated that results of the ongoing Phase 3 APOLLO trial, if successful, could serve as the basis for a response to the CRL” [10-Q · Jul 30].

The FDA accepted that bitopertin lowered PPIX, the proposed surrogate endpoint, but was not persuaded the change is reasonably likely to predict clinical benefit: it calls the reduction relatively modest and says the trials showed no association with sunlight-exposure endpoints. It asks for evidence based on clinical endpoint(s).

The letter says the FDA has “determined that we cannot approve this application in its present form” [FDA letter, 8-K Exhibit 99.2 · Feb 13]. Disc proposed PPIX “as a surrogate endpoint that is reasonably likely to predict clinical benefit” [FDA letter, 8-K Exhibit 99.2 · Feb 13].

On the first factor, the FDA says “we agree that Trial 201, one adequate and well-controlled trial, demonstrated an effect on its primary endpoint” [FDA · Feb 13 · p. 2].

On the second: “The percent change in PPIX was relatively modest (about 40% reduction from baseline to Day 121 for the higher 60 mg dose)” [FDA · Feb 13 · p. 2], and Trials 201 and 202 “did not show evidence of association between percent change in whole blood metal-free PPIX and sunlight-exposure based endpoints” [FDA · Feb 13 · p. 2]. This, the letter says, “leaves significant uncertainty that bitopertin will have the effect it purports or is represented to have” [FDA · Feb 13 · p. 2].

The letter says Disc “will need to provide evidence from an adequate and well-controlled trial(s) demonstrating the efficacy of bitopertin” [FDA · Feb 13 · p. 2] “based on clinical endpoint(s)” [FDA · Feb 13 · p. 2], and offers to discuss “completing your ongoing trial and, if the trial is successful, submitting the results to support a traditional approval” [FDA · Feb 13 · p. 2].

As of Disc's 10-Q for the quarter ended June 30, 2026, APOLLO is fully enrolled with topline data expected in Q4 2026; if successful, Disc plans to respond to the letter and anticipates an FDA decision by mid-2027.

The letter offers “completing your ongoing trial and, if the trial is successful, submitting the results to support a traditional approval” [FDA · Feb 13 · p. 2]. On Feb 13 Disc said “Upon completion of APOLLO, Disc would then file a response to the CRL and expect an updated FDA decision by mid-2027” [8-K · Feb 13].

After the Type A meeting, the 10-Q says “results of the ongoing Phase 3 APOLLO trial, if successful, could serve as the basis for a response to the CRL and could potentially support a traditional approval” [10-Q · Jul 30], and: “We completed enrollment in APOLLO (n=183) in the first quarter of 2026 and expect to report topline data in the fourth quarter of 2026” [10-Q · Jul 30].

Not in this record: APOLLO's results, the FDA's minutes of the Type A meeting, and Disc's filings after Sep 10, 2026. [not in record]