FDA’s plausible mechanism pathway to advance personalized therapies

The US Food and Drug Administration (FDA) has introduced the “plausible mechanism pathway,” a new regulatory path designed to support the development of personalized therapies for diseases with clearly defined biological causes, particularly where randomized trials are not feasible.

Writing in the New England Journal of Medicine, Vinay Prasad and Martin A. Makary of the FDA outline the limitations of existing regulatory mechanisms and describe how this new pathway aims to more closely match oversight with advances in molecular and genetic medicine. As they note, it will apply only to conditions “for which the biologic cause is known,” restricting eligibility to diseases with a specific molecular or cellular abnormality and excluding those defined solely by clinical symptoms or uncertain genomic associations.

The pathway is based on five key principles:

1. a clearly defined molecular or cellular abnormality
2. a therapy that directly targets the causal defect
3. a well-characterized natural history to understand expected disease progression
4. evidence that the target has been successfully modified, using animal, non-animal, or biopsy data where appropriate
5. clear, sustained clinical improvement

Clinical data must be strong enough “to exclude regression to the mean,” the authors emphasize, noting that in some cases patients may serve as their own control. Therapies acting far downstream of the causal defect, such as broadly acting corticosteroids, would not qualify.

Prasad and Makary highlight the case of “baby KJ” as an example of how the pathway may function. The newborn developed severe hyperammonemia within the first 48 hours of life and was diagnosed with carbamoyl-phosphate synthetase 1 (CPS1) deficiency. Genetic sequencing confirmed biallelic CPS1 mutations, providing a precise molecular target. Under a single-patient expanded-access investigational new drug (IND) authorization, approved within a week, clinicians administered three doses of a lipid nanoparticle therapy containing a guide RNA and an mRNA-encoded adenine base editor designed to correct the defect. According to the FDA, the infant later showed improved protein tolerance and a reduced need for nitrogen-scavenging therapy, accompanied by broader clinical improvement.

Natural history data played a significant role in interpreting these findings. CPS1 deficiency and related urea cycle disorders typically involve “progressive neurological damage” caused by recurring hyperammonemic episodes during infancy. This expected trajectory helped assess whether the patient’s clinical course diverged from typical progression.

Target engagement was supported by mouse studies demonstrating “successful editing in 42% of liver cells,” although direct biopsy in the patient was not clinically appropriate. The FDA states that it will “embrace nonanimal models where possible” and may, in certain cases, consider data from “the first-in-class subject dosed” when direct confirmation is not feasible.

The pathway also outlines how products may progress toward market approval. As described in the FDA article,

“Once a manufacturer has demonstrated success with several consecutive patients with different bespoke therapies, the FDA will move toward granting marketing authorization for the product.”

Sponsors may then rely on platform data to support approvals for additional mutations or related conditions through either the accelerated or standard approval route.

Post-marketing requirements will focus on confirming long-term efficacy, assessing off-target edits, and monitoring safety and developmental outcomes. The FDA notes that expectations will depend on disease biology and feasibility; for example, assessing off-target activity in the nervous system may not always be possible.

While the pathway will prioritize rare and severe childhood disorders, it will also apply to certain common diseases with substantial unmet need. The agency notes that a single condition involving numerous pathogenic variants may require multiple targeted therapies, making this approach particularly relevant.

Prasad and Makary conclude:  “The FDA will work as a partner and guide in ushering these therapies to market, and our regulatory strategies will evolve to match the pace of scientific advances,” underscoring the agency’s commitment to fostering innovation while maintaining appropriate safeguards.