REGENXBIO · Nasdaq: RGNX · RGX-121 (clemidsogene lanparvovec)

The 8-K restates the FDA's three main concerns almost word for word, and the paths forward. It does not mention the proposed threshold, or any of the letter's three further comments, including the brain tumor case.

The letter says the data “are insufficient to provide substantial evidence of effectiveness” [FDA · Feb 7 · p. 2]; the 8-K says “the FDA did not agree the data set provided substantial evidence of effectiveness” [8-K · Feb 10], and that the company “believed it had addressed the points raised in the CRL” [8-K · Feb 10].

The 8-K's list of the FDA's concerns follows the letter closely: “the study eligibility criteria to adequately define a population with neuronopathic disease” [8-K · Feb 10], the external control, and “the appropriateness of CSF HS D2S6 as a surrogate endpoint reasonably likely to predict clinical benefit” [8-K · Feb 10]. The letter's version reads “the appropriateness of CSF HS D2S6 with the proposed performance threshold” [FDA · Feb 7 · p. 2]. The 8-K does not mention the threshold. [8-K · searched]

Nor does it mention the letter's further comments: confounding by enzyme replacement therapy or immunosuppression, [8-K · searched] missing data, [8-K · searched] or the brain tumor case and a possible safety postmarketing requirement. [8-K · searched] On the paths forward, the 8-K says they “would be challenging in an ultra-rare disease population” [8-K · Feb 10].

REGENXBIO disclosed the tumor case and the clinical hold on Jan 28, 2026, before the letter. The letter raised it as a safety comment; the 8-K about the letter does not mention it; later filings do.

On Jan 28, 2026 the company announced that “The FDA also placed a clinical hold on RGX-121” [8-K · Jan 28], after a tumor in which “Preliminary genetic analysis of the resected tumor detected an AAV vector genome integration event” [8-K · Jan 28]; it said “The causality has not been established” [8-K · Jan 28].

The FDA's letter of Feb 7 raised the case as a comment, saying “This adverse event may lead to a requirement for a safety PMR for BLA 125840 in the future” [FDA · Feb 7 · p. 7]. The 8-K of Feb 10 about the letter does not mention it. [8-K · searched]

The 10-K says the FDA “will require additional information regarding safety assessment related to malignancy risk” [10-K · Mar 5], and on Aug 24 the company reported a new clinical hold after “asymptomatic spine MRI findings in five participants” [8-K · Aug 24].

The FDA concluded the single study's data could not show effectiveness, citing three deficiencies: the patient population, the external control and the biomarker threshold.

The letter says the Part 2 data “are insufficient to provide substantial evidence of effectiveness to support approval of clemidsogene lanparvovec” [FDA · Feb 7 · p. 2].

It lists three deficiencies: eligibility, since “Patients were only required to meet one and not all four eligibility criteria” [FDA · Feb 7 · p. 2]; the external control, citing a “lack of comparability between the Part 2 patients and the external control MPS II patients” [FDA · Feb 7 · p. 4]; and the biomarker: “You did not provide sufficient evidence supporting the proposed threshold” [FDA · Feb 7 · p. 5].

It adds comments that it says “do not preclude approval” [FDA · Feb 7 · p. 6]: possible confounding by enzyme replacement therapy and immunosuppression, “missing data for several key covariates” [FDA · Feb 7 · p. 6], and a “malignant brain tumor following treatment with a gene therapy product (RGX-111, under development for MPS I)” [FDA · Feb 7 · p. 7].

As paths forward, the company “may choose to perform a new clinical study or enroll additional patients” [FDA · Feb 7 · p. 5].

As of the company's Aug 24, 2026 8-K, RGX-121 is on clinical hold and the company does not expect to resubmit in the near term; the FDA's safety questions remain open.

After the letter the company said it “plans to request a Type A meeting to discuss the CRL, as well as the planned BLA resubmission” [8-K · Feb 10]. The 10-K says the FDA “will require additional information regarding safety assessment related to malignancy risk, longer-term follow up data and cross-program safety assessment” [10-K · Mar 5].

On Aug 24, 2026: “REGENXBIO does not expect to resubmit the RGX-121 Biologics License Application (BLA) in the near term” [8-K · Aug 24]. Not in this record: REGENXBIO's filings after Aug 24, 2026. [not in record]