FDA to Unveil Plausible Mechanism Guidance for Rare Disease Therapies on Feb. 23

The Draft Guidance

The Food and Drug Administration will release draft guidance on Monday, Feb. 23, that allows drug sponsors to win approval for individualized ultra-rare disease therapies based on biological mechanism and natural history data rather than traditional randomized controlled trials, according to two FDA officials familiar with the plan. The document, titled "Considerations for the Use of the Plausible Mechanism Framework to Develop Individualized Therapies that Target Specific Genetic Conditions with Known Biological Cause," will be published to coincide with Rare Disease Week and will open a 60-day comment period ending April 27, the officials said.

The new framework applies primarily to genome editing and RNA-based therapies such as antisense oligonucleotides, the officials said. It will permit sponsors to build cases for approval around a single adequate and well-controlled clinical investigation paired with confirmatory evidence, including mechanistic data, target engagement, pharmacodynamic biomarkers, and exposure-response relationships. The policy builds directly on a Feb. 18 article in the New England Journal of Medicine by FDA Commissioner Marty Makary and Center for Biologics Evaluation and Research Director Vinay Prasad titled "One Pivotal Trial, the New Default Option for FDA Approval," according to a hospital administrator briefed on the rule.

"The agency is preparing to tell sponsors that if the disease-causing abnormality is well understood and the therapy directly targets that root cause, a conventional randomized trial may not be necessary," one FDA official said. The official spoke on condition of anonymity because the guidance had not yet been announced. The second official said the document will retain existing statutory standards for substantial evidence of effectiveness and safety, but will shift the burden onto FDA review teams to justify requiring more than one trial.

The officials said the guidance will be announced at a formal launch event featuring HHS Secretary Robert F. Kennedy Jr., Makary, members of Congress, and rare disease advocates. The event is scheduled for the morning of Feb. 23 at FDA's White Oak campus in Silver Spring, Maryland. Draft copies have been circulated internally since Feb. 13, and senior leaders signed off on the final language on Feb. 18, the day the NEJM article appeared, according to the hospital administrator.

Industry Reaction and Scientific Backing

A lobbyist for device makers who has discussed the framework with FDA staff said the guidance is intended to answer years of complaints from rare-disease patient advocates and biotechnology companies that the agency's two-trial dogma has blocked therapies for conditions affecting only a handful of patients worldwide. The lobbyist said the draft is expected to include criteria requiring sponsors to identify the disease-causing abnormality, demonstrate that the therapy targets the root cause, rely on well-characterized natural history data, confirm target engagement, and show improvement in clinical outcomes or validated biomarkers.

The hospital administrator briefed on the rule said the framework could cut development costs and timelines for bespoke therapies, but warned that chemistry, manufacturing, and controls standards remain a significant hurdle. "For one-patient or ultra-small populations, making product at compliant scale is still expensive and technically demanding," the administrator said. The administrator added that FDA will likely require sponsors to share data across programs so that evidence generated for one patient can inform future approvals for similar genetic conditions.

Senator Bill Cassidy, chair of the Senate Health, Education, Labor and Pensions Committee, released a white paper on Feb. 23 titled "Patients and Families First: Building the FDA of the Future" that argues the current approval framework is ill-suited for personalized medicines, according to the administrator. The Senate Aging Committee has scheduled a hearing for Thursday, Feb. 26, titled "From Regulator to Roadblock: How FDA Bureaucracy Stifles Innovation," where the framework is expected to feature prominently.

The lobbyist said companies developing antisense oligonucleotides and CRISPR-based therapies have been watching for the guidance since November 2025, when Makary and Prasad first outlined the plausible mechanism pathway in a NEJM article and at a Duke-Margolis Institute for Health Policy convening. The lobbyist predicted the draft will draw strong comments from both sponsors eager for flexibility and critics worried about lowering evidentiary standards.

What Happens Next

FDA's announcement will mark the first formal policy document tied to Makary's broader push to make one pivotal trial the default standard for all novel drug approvals. The Feb. 18 NEJM article argued that advances in biology and trial design mean over-reliance on two trials no longer makes sense, and the draft guidance will operationalize that philosophy for the rarest diseases. The officials said the agency will not create a new statutory approval pathway, but will clarify how existing pathways can accommodate individualized therapies.

The 60-day comment period will close April 27, and the lobbyist predicted FDA will move quickly to finalize the guidance because of high-level attention from Kennedy and Makary. Several rare-disease stakeholders are expected to testify at the Feb. 26 Senate hearing, and the administration is likely to cite the framework as a centerpiece of its rare-disease agenda during President Donald Trump's address to Congress on March 3.

Investors and sponsors should watch for the precise language on confirmatory evidence requirements and post-marketing obligations, the FDA officials said. The officials cautioned that the draft guidance will not apply to all rare diseases, only to those where the biological mechanism is well characterized and the therapy directly addresses the underlying cause. "This is not a free pass," one official said. "It is a narrower route for a specific set of scientific circumstances."

The hospital administrator said the next 72 hours will bring intense scrutiny of the draft from Wall Street analysts and patient groups. Watch for statements from the National Organization for Rare Disorders and from sponsors with late-stage individualized therapy programs. The administrator also said FDA will likely issue a separate notice in the Federal Register establishing the official comment deadline, which will trigger a round of public briefings from biotechnology trade associations.