Pediatric osteosarcoma represents the most common primary malignant bone tumor in children, presenting significant challenges in pediatric oncology despite decades of treatment advances. The disease typically originates in the metaphyseal regions of long bones, with the distal femur accounting for 43% of cases, followed by the proximal tibia at 23% and humerus at 10%. The highly aggressive nature of osteosarcoma, combined with its propensity for rapid progression and poor prognosis, has driven researchers to explore novel therapeutic approaches beyond traditional chemotherapy regimens.
Immune Microenvironment Complexity
The tumor microenvironment in pediatric osteosarcoma comprises a complex ecosystem of immune cells, osteoblasts, endothelial cells, stromal cells, extracellular matrix, and signaling molecules. Predominantly, tumor-associated macrophages with an M2 phenotype are present, alongside T lymphocytes, myeloid cells, and dendritic cells. Research has shown that elevated levels of infiltrating macrophages and CD8+ T cells correlate with reduced metastasis and improved survival in osteosarcoma patients.
Conversely, increased infiltration of antigen-presenting cells, such as dendritic cells, correlates with poorer clinical outcomes. The osteosarcoma microenvironment often impairs natural killer cell function through various immunosuppressive mechanisms, including the secretion of inhibitory cytokines and the expression of immune checkpoint molecules.
Osteosarcoma cells employ multiple strategies to evade immune surveillance. They release PD-L1 to suppress immune responses and indoleamine 2,3-dioxygenase, which inhibits neoantigen generation by dendritic cells. Additionally, signaling pathways like VEGF, IL-10A, TGF-β, and STAT3 modulate the immunosuppressive microenvironment by affecting suppressive cells, macrophages, and stromal fibroblasts.
Promising Immunotherapeutic Approaches
Mifamurtide Success
The most compelling evidence for immune modulation in osteosarcoma comes from mifamurtide (liposomal muramyl tripeptide phosphatidylethanolamine), which has been approved for use with standard treatment regimens in non-metastatic osteosarcoma. Clinical trials demonstrated that mifamurtide raised the 6-year survival rate from 70% with chemotherapy alone to 78%. The drug activates macrophages and monocytes, leveraging the immune system's cancer-fighting capabilities while also inducing a shift in macrophage phenotype from M1/M2, promoting a balance between pro-inflammatory and immunoregulatory functions.
CAR-T Cell Therapy Developments
CAR-T therapy represents a cutting-edge immunotherapeutic approach that has revolutionized treatment of certain hematological malignancies and is now being explored in solid tumors, including pediatric osteosarcoma. This therapy involves genetic modification of a patient's T cells to express receptors that specifically target tumor-associated antigens. Potential targets for CAR-T therapy include HER2, GD2, and B7-H3, which are overexpressed on osteosarcoma cells.
Preclinical studies have demonstrated the efficacy of CAR-T cells in recognizing and eliminating osteosarcoma cells both in vitro and in vivo. Clinical trials are currently underway to evaluate the safety and efficacy of CAR-T cell therapy in pediatric osteosarcoma patients, with ongoing research focused on enhancing the persistence and infiltration of CAR-T cells within the tumor microenvironment.
Antibody Therapies
Several antibody therapies are gaining ground in osteosarcoma treatment. Glembatumumab-vedotin targets osteoactive substances overexpressed on osteosarcoma cells, showing cytotoxic effects in both osteosarcoma and breast cancer. Trastuzumab, initially developed for HER2+ breast cancer, targets HER2 tyrosine kinase activity critical for cell proliferation, which is also present in osteosarcoma cells.
Research with nivolumab in humanized mouse models revealed that while primary tumor volume and growth rate matched control groups, the rate of lung metastases was significantly reduced, underscoring the potential of targeting the tumor microenvironment with immunotherapy.
Combination Strategies and Novel Approaches
Recent strategies include liposomal and aerosolized drug formulations such as sustained-release lipid inhalation targeting cisplatin and aerosolized granulocyte-monocyte colony-stimulating factor alongside NK cell infusions and aerosol IL-2. These approaches enhance local chemotherapy efficacy for lung metastases in metastatic osteosarcoma.
IL-2 activates lymphocytes into lymphokine-activated killer cells, which are effective against multidrug-resistant cells and target lung metastasis sites. Additionally, denosumab, targeting the receptor activation of nuclear factor kappa-β ligand, reduces fracture risks in tumor metastases and showed 99% inhibition in giant bone cell tumor progression in Phase II studies.
Prognostic Biomarker Advances
The immunological characteristics of the osteosarcoma tumor microenvironment provide valuable prognostic information. Tumor-associated macrophages and T cells play key roles in determining cancer prognosis and the efficacy of immunotherapies. Inflammation markers such as the Systemic Immune-Inflammation Index are potent predictors of tumor prognosis, offering new directions for predicting survival at different time points.
A higher Immune-Score correlates with better survival due to greater immune infiltration, whereas higher Tumor-Purity correlates with lower survival rates. On the genetic front, researchers have identified 69 differentially expressed genes related to metastasis and immune infiltration, with GATA3, LPAR5, EVI2B, RIAM and CFH demonstrating prognostic potential.
Challenges and Future Directions
Despite these advances, challenges remain in managing the drug's impact on standard chemotherapy effectiveness and deciphering the mechanisms behind resistance to immunotherapy. The tumor microenvironment is dominated by immunosuppressive M2 or non-functional M0 macrophages, hindering effective immune responses.
Patients with CD8+ T cell infiltration may derive greater survival benefits from immunotherapies. Thus, inducing the transition of M0 or M2 macrophages to the pro-inflammatory M1 phenotype represents a promising strategy to enhance anti-tumor immune responses in pediatric osteosarcoma.
Future research should prioritize optimizing specific immunotherapeutic strategies, such as enhancing the efficacy of immune checkpoint inhibitors and macrophage-targeted therapies, developing personalized treatment plans tailored to individual patients' immune profiles and genetic backgrounds, and designing robust clinical trials to evaluate the safety and effectiveness of these novel approaches.
