Pediatric Brain Tumors: Challenges and New Strategies
Overview of Pediatric Brain Tumors
Pediatric brain tumors continue to pose significant treatment challenges, leading to the highest mortality rates among childhood cancers. Researchers at Stanford University have identified a novel approach involving the antibody-based blockade of CD47, a mechanism that allows cancer cells to evade immune detection, as a promising strategy for treating malignant pediatric brain tumors.
Types of Pediatric Brain Tumors
Malignant gliomas, which arise in the brain or spine, present considerable treatment difficulties, particularly in children, resulting in poor outcomes. In contrast, pediatric embryonal tumors such as medulloblastoma (MB), atypical teratoid rhabdoid tumor (ATRT), and primitive neuroectodermal tumor (PNET) develop from embryonic cells and tend to show better survival rates post-treatment. However, standard treatment protocols for brain cancers in children under three years old are complicated due to the adverse long-term effects of radiation on brain development. While surgery and chemotherapy are available options, they often yield limited effectiveness.
The Role of CD47 in Cancer
Recent studies have demonstrated that CD47 is frequently expressed on various cancer cells, including those in brain tumors, enabling them to evade the immune system. CD47 is found on the surface of cancer cells and binds to SIRP-α, a protein on macrophages, which are immune cells responsible for destroying foreign cells. This interaction sends a “don’t-eat-me” signal to macrophages, allowing cancer cells to avoid phagocytosis. Normal cells, lacking CD47 expression, are more susceptible to being engulfed by macrophages.
Targeting CD47 in Pediatric Brain Tumors
The immune resistance and escape mechanisms facilitated by CD47 in malignant brain tumors present a viable target for therapeutic intervention. A study published in Science Translational Medicine explored whether disrupting CD47 signaling could serve as an effective treatment for malignant pediatric brain tumors. Researchers utilized a humanized antibody against CD47, known as Hu5F9-G4, designed to closely resemble naturally occurring human antibodies.
Study Design and Findings
Testing the Efficacy of Hu5F9-G4
The study involved five types of malignant pediatric tumors: medulloblastoma (MB) group 3, pediatric ATRT, PNET, epidermal growth factor receptor-amplified pediatric glioblastoma multiforme (pGBM), and diffuse intrinsic pontine glioma (DIPG). The researchers established a xenograft model by implanting fluorescently labeled primary cells from patients or modified cancer cell lines directly into the brains of mice lacking immune cells, except for phagocytic macrophages.
After injecting the tumor cells, the engraftment was confirmed using bioluminescence imaging (BLI), and the mice were divided into treatment and control groups. Hu5F9-G4 was administered systemically via the abdominal cavity.
Results of the Preclinical Study
The results were encouraging. In mice with MB group 3, Hu5F9-G4 treatment led to a significant reduction in tumor size after 14 days. The treatment group exhibited a 100% survival rate, whereas the control group succumbed within 40 days. Additionally, the treatment group had enhanced macrophage recruitment in the brain, and the incidence of spinal metastasis decreased. These findings indicate that the antibody effectively penetrated the blood-brain barrier and provoked a response in primary tumors. There was also a noted reduction in CD15+ cancer stem cells, suggesting that the blockade could hinder cancer cell proliferation at the stem cell level.
Similar positive outcomes were observed in ATRT, PNET, pGBM, and DIPG xenograft models, with increased macrophage recruitment and significant tumor size reduction leading to improved overall survival.
Impact on Immune-Competent Models
To assess the effects of CD47 blockade in an immune-competent environment, the researchers employed a glioblastoma xenograft model in non-immunocompromised mice. The treatment group exhibited a slower tumor growth rate compared to controls, with median survival extending from 21 days in the control group to up to 38 days with a dose-dependent response. Significant tumor burden reduction and macrophage infiltration at tumor sites were observed, with no detectable damage to normal brain tissue.
Further Investigations and Future Directions
Exploring Bystander Effects
Following these findings, the researchers investigated whether the antibody blockade affected other brain cells. They utilized labeled neural progenitor cells (NPCs) that differentiate into various brain cell types. In this experiment, labeled NPCs were engrafted in mice, and unlabeled MB cells were introduced. Systemic treatment with Hu5F9-G4 demonstrated no adverse effects on NPC growth, and treated animals showed prolonged survival compared to controls.
Conclusion and Implications
These results suggest that Hu5F9-G4 could be a strong candidate for treating various malignant pediatric brain cancers. However, the study’s limitation lies in its predominance in immune-compromised settings, indicating that further research is necessary to comprehend the mechanisms of immune evasion and to enhance “eat-me” signals for cancer cells. Nevertheless, this study opens the door for developing anti-CD47 therapies for malignant pediatric brain tumors, potentially in conjunction with existing treatments like surgery and chemotherapy.
Written By: Joan Zape, PhD(c)
Related Topics of Interest
– Researchers develop novel system for brain tumor drug development
– Can a new technique stop bleeding during brain surgery?
– What is the difference between an MRI and a CT scan?