FDA's new ‘plausible mechanism pathway’: Transforming regulatory approval for personalized gene therapies

The FDA’s groundbreaking ‘plausible mechanism pathway,’ announced in November 2025, represents a transformative approach to facilitate licensure of personalized therapies when randomized trials are not feasible.¹  This innovative regulatory approach is particularly valuable for treatments targeting rare genetic diseases. While initially appearing designed for biologics such as cell and gene therapies addressing rare, life-threatening genetic conditions, the FDA envisions broader future applications – potentially extending to traditional therapeutics and conditions that aren't necessarily ultra-rare or are caused by multiple distinct pathogenic mutations. 

To balance flexibility with practicality, this new regulatory pathway acknowledges the challenges in conducting traditional clinical trials for ultra-rare diseases while still maintaining scientific rigor through five key requirements: 

1. A clearly defined molecular basis for the disease 
2. A therapy that directly targets the underlying molecular alteration 
3. A well-understood natural history of the untreated disease 
4. Evidence that the biological target was successfully modified (e.g., through gene editing), when feasible 
5. Meaningful clinical improvement above expected variability 

This innovative approach could be transformative for patients with rare genetic conditions who previously had limited therapeutic options due to the impracticality of conventional drug development approaches. 

Biological plausibility and alternative evidence source to support regulatory approval 

For sponsors considering this pathway, establishing biological plausibility will be critical. In vitro, in vivo, or platform data can provide valuable evidence. Importantly, the FDA has indicated that nonclinical data, including studies in cells or in animal models, can be used for timely establishment of the therapy's mechanism and target engagement, or even for providing evidence of efficacy, as direct clinical confirmation may not be possible or appropriate in every case.  

In line with the FDA’s recent roadmap to reduce the use of animal testing, the agency has emphasized its openness  to “embrace nonanimal models where possible” as secondary evidence to support the new pathway, reinforcing its commitment to the use of new approaches for the development of biologics. 

Although safety and efficacy data from single-patient INDs have not historically served as the basis for licensure of biologics, the plausible mechanism pathway would allow anecdotal data for individualized therapies to be used as potential sources of evidence for product development and eventual marketing approval. FDA has indicated that confirmation of successful outcome (e.g., assessment of the magnitude of editing for the target gene and corresponding beneficial clinical effects) in a small number of consecutively treated patients may provide sufficient support to file a marketing application. After licensure, Platform Technology designation for the product could enable expansion to additional mutations or related conditions, with post-marketing commitments to gather data on long-term safety, potential off-target effects, and developmental outcomes for pediatric patients.² 

Next steps  

For sponsors that consider aligning their programs with the plausible mechanism pathway, Parexel's regulatory experts offer valuable support. Our team, with extensive experience in assessment of cell and gene therapies within CBER/FDA, can provide deep insights into how to apply this regulatory framework to guide biological product development. In recent years, we have supported cellular & gene therapy development programs in Phase III and BLA stage that have gone on to receive FDA approval and would be able to apply generalized lessons learned to your program.  

Please contact us, we are always available for a conversation. 

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References 

1. Prasad V, Makary MA. FDA’s New Plausible Mechanism Pathway. N Engl J Med 2025; 
   doi:10.1056/NEJMsb2512695 
2. FDA guidance on Platform Technology Designation Program for Drug Development, May 2024  

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