Beyond Base Camp: Promise and Pitfalls of PI3K/mTOR Inhibition in Pediatric High- Grade Gliomas

High-grade gliomas (HGGs), including diffuse midline glioma (DMG), represent the most aggressive and deadly pediatric brain cancers. Despite recent advances in understanding their molecular underpinnings, these tumors remain universally fatal. A hallmark feature of pediatric HGGs is the frequent presence of mutations and amplifications in components of the phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) signaling pathway. These alterations drive unchecked tumor growth, confer resistance to standard therapies, and contribute to the dismal survival outcomes observed in affected children.

While the PI3K/mTOR axis has been recognized as a critical dependency in DMG and other pediatric HGGs, clinical translation of pathway inhibitors has been limited by several major barriers. Most notably, the blood-brain barrier (BBB) restricts the delivery of conventional PI3K and mTOR inhibitors, many of which lack sufficient central nervous system (CNS) penetration. Furthermore, even when delivered to the tumor site, these agents often encounter rapid adaptive resistance through activation of compensatory pathways, reducing their therapeutic benefit. Treatment-related toxicities, including hyperglycemia, rash, and mucositis, further limit tolerability and patient adherence.Emerging brain-penetrant PI3K/mTOR inhibitors represent a new generation of targeted therapies with the potential to overcome these pharmacological limitations. However, increasing drug exposure does not necessarily equate to improved outcomes, particularly when used in combination with immunotherapies or other targeted agents. Achieving optimal therapeutic efficacy while minimizing systemic toxicity remains a central challenge, requiring careful consideration of drug dosing, timing, and combination strategies tailored to each individual patient.

This review explores the current landscape of PI3K/mTOR targeting in DMG, highlighting both the therapeutic promise and inherent challenges. We discuss known resistance mechanisms, the need for better CNS-optimized compounds, and the importance of individualized treatment strategies. Finally, we propose a roadmap for future research, emphasizing rational drug combinations, refined patient stratification, and the development of next-generation therapies aimed at improving outcomes for children with these devastating malignancies.