"Plausible Mechanism" Framework for Ultra-Rare Individualized Therapies

Summary

On February 23, 2026, the FDA released draft guidance titled “Considerations for the use of the Plausible Mechanism Framework to Develop Individualized Therapies.” The framework aimed to lower the evidentiary barrier for bespoke treatments for ultra-rare diseases with a known biological cause — cases where randomized trials are infeasible because patient populations are vanishingly small. Commissioner Marty Makary later credited it as one of four key reforms from Prasad’s CBER tenure.

The draft guidance was opened for 60 days of public comment. Trade publications including Applied Clinical Trials covered the release as a significant attempt to create a workable regulatory pathway for individualized, often one-patient-at-a-time genetic therapies.

The reform illustrated the dual character of Prasad’s tenure: simultaneously tightening evidence demands for large-market products (vaccines, oncology drugs) while loosening them where randomized trials were considered impractical, such as ultra-rare diseases with clear mechanistic rationales.

Timeline

• Feb 23, 2026 — FDA releases the “Plausible Mechanism” draft guidance for individualized therapies for ultra-rare diseases; 60-day public comment period opens.
• March 2026 — Makary’s farewell statement credits Prasad with the framework as one of four key reforms of his tenure.
• April 30, 2026 — Prasad departs the FDA; the draft guidance’s finalization is left to his successors.