Comment from Kami Traasdahl

The brain tumor community appreciates recent statements from leadership at the U.S. Food and Drug Administration indicating that the “Plausible Mechanism Framework” is intended to apply more broadly than individualized gene or RNA therapies. We support this direction. However, the current draft guidance does not sufficiently codify this broader intent and may be interpreted too narrowly in implementation. To ensure appropriate and consistent application, the final guidance should explicitly state that the Plausible Mechanism Framework applies to diseases characterized by: Well-understood biological or molecular drivers; Small or functionally small patient populations; and Situations where randomized controlled trials are not feasible or ethical. The guidance should further clarify that this framework applies to molecularly defined subsets of disease, including cancers that are not rare in aggregate but are subdivided into small, biologically distinct populations. This clarification is essential in neuro-oncology. While gliomas are not rare as a general category, the field has transitioned to integrated molecular classification, as reflected in the WHO 2016 and 2021 CNS updates. Under this framework, entities such as diffuse midline glioma (DMG), H3 G34-mutant diffuse hemispheric glioma, high-grade astrocytoma with piloid features (HGAP), and other molecularly defined subgroups constitute small, distinct populations with poor outcomes and well-characterized molecular drivers. The final guidance should explicitly recognize that such populations are appropriately treated as functionally rare diseases for purposes of applying the Plausible Mechanism Framework. The guidance should also explicitly include within scope: Personalized cancer vaccines; Cell-based therapies, including autologous and engineered immune cell therapies; and Other individualized or highly targeted therapeutic approaches guided by validated molecular or immunologic biomarkers. These modalities are inherently mechanism-based and are often developed for narrowly defined patient populations where large randomized trials are not feasible. Absent explicit inclusion, there is a significant risk that these therapies will be excluded in practice despite fitting the scientific intent of the framework. In addition, the final guidance should state that: Demonstration of a well-understood disease mechanism, combined with a substantial and durable clinical effect, may be sufficient to support regulatory decision-making; and Such evidence may be derived from appropriately designed small studies, including single-arm trials, externally controlled studies, and real-world data, where randomized trials are impractical. Finally, the guidance should clarify that tumor heterogeneity and molecular stratification strengthen—rather than weaken—the rationale for applying mechanism-based regulatory approaches. Without these explicit clarifications, there is a risk that patients with high unmet need—particularly those with molecularly defined cancers such as high-grade gliomas—will be inadvertently excluded from a framework intended to accelerate access to effective therapies. We urge the FDA to ensure that the final guidance fully reflects contemporary scientific understanding and is implemented in a manner that is inclusive of these patient populations and therapeutic approaches