Radiotherapy in Preclinical Models of Brain Metastases: A Review and Recommendations for Future Studies

Abstract

Brain metastases (BMs) frequently occur in primary tumors such as lung cancer, breast cancer, and melanoma, and are associated with notably short natural survival. In addition to surgical interventions, chemotherapy, targeted therapy, and immunotherapy, radiotherapy (RT) is a crucial treatment for BM and encompasses whole-brain radiotherapy (WBRT) and stereotactic radiosurgery (SRS). Validating the efficacy and safety of treatment regimens through preclinical models is imperative for successful translation to clinical application. This not only advances fundamental research but also forms the theoretical foundation for clinical study. This review, grounded in animal models of brain metastases (AM-BM), explores the theoretical underpinnings and practical applications of radiotherapy in combination with chemotherapy, targeted therapy, immunotherapy, and emerging technologies such as nanomaterials and oxygen-containing microbubbles. Initially, we provided a concise overview of the establishment of AM-BMs. Subsequently, we summarize key RT parameters (RT mode, dose, fraction, dose rate) and their corresponding effects in AM-BMs. Finally, we present a comprehensive analysis of the current research status and future directions for combination therapy based on RT. In summary, there is presently no standardized regimen for AM-BM treatment involving RT. Further research is essential to deepen our understanding of the relationships between various parameters and their respective effects.

Keywords: Animal models; Brain metastasis; Combined modality therapy; Dose fractionation, Radiation; Radiotherapy.

Figure 1

The Process of Establishing Brain Tropic Cells. Establishing brain-tropic cells (brain metastasis cells) requires in vivo and in vitro screening. The selection process involves modifying cancer cells with reporter genes such as luciferase or GFP, which allows changes to be easily visualized, assessed, and prepared, using IVIS or MRI. Additional rounds of selection are then carried out. The modified cells are then reintroduced into the mice, usually after a period of growth outside the body.

Figure 2

Precautions in Model Construction and Detection Indicators in the AM-BM. (A-D) Several key parameters significantly influence the tumor formation rate during AM-BM model development. (A) Species including C57BL/6 mice, SCID mice, BALB/c nude mice, and rats are frequently utilized in AM-BM studies. (B) Various cell lines, such as lung cancer (H2030-BrM, PC-9-BrM3), breast cancer (BT474-BrM3, 4T1Br4), and melanoma cell lines (B16-F10, B78), are commonly employed for AM-BM establishment. (C) The quantity of injected cells is a critical determinant of successful model construction and the optimal time window for treatment. (D) Current BM modeling methods encompass intracerebral injection (ICB), intracardiac injection (ICD), internal carotid artery injection (ICA), tail vein injection (IV), and spontaneous or induced models (SP). (E-G) Common parameters assessed in in-vivo studies include: (E) Tumor lesion, tumor number, and tumor volume. (F) Survival. (G) Tumor biomarkers, such as the expression of Ki67, γH2AX, and so on.

Figure 3

Animal Species, Irradiation Methods Selection, and Effects of RT on AM-BM. (A) Animal species currently employed in RT studies of AM-BM encompass mice, chicken embryos, monkeys, rats, dogs, and rabbits. (B) Different irradiation methods are utilized based on the modeling approach: SRS or WBRT is commonly applied for local inoculation modeling, WBRT for intracardiac and internal carotid artery injections, and PCI for tail vein injection. (C-D) The prognostic factors influencing survival were as follows: (C) RT-induced side effects on brain tissue, such as radiation edema, necrosis, neurotoxicity, and hippocampal damage. (D) Factors such as radiation resistance genes (TopBP1 and Claspin), secretion of S100A9, and the overexpression of RAGE limit the survival benefits of RT. (E) In vivo studies reveal differential responses of the blood-brain barrier/blood-tumor barrier in various mouse strains to RT. Notably, doses of 3 Gy/1F, 12 Gy/3F, 15 Gy/1F, 15.5 Gy/1F, and or 20 Gy/2F did not significantly alter the permeability of the blood-brain barrier/blood-tumor barrier in BALB/c nude mice. However, doses of 15.5 Gy/1F and 30 Gy/5F can induce changes in the blood-brain barrier/blood-tumor barrier permeability in C57BL/6 mice. The time window during which RT induces BBB/BTB opening in AM-BMs has not been determined.

Figure 4

Changes in Molecular Pathway Induced by RT with or without Additional Treatments in Animal Models of Brain Metastases from Lung Cancer, Melanoma, and Breast Cancer. (A-C) Lung Cancer BM. (A) AZD-3759 enhances NSCLC radiosensitivity by inhibiting EGFR and JAK1; (B) AZD7762 promotes NSCLC radiosensitivity by suppressing CHK1; (C) RT inhibits phosphorylation of JAK2 and STAT3, inducing apoptosis. (D-F) Melanoma BM. (D) Thymoquinone (TQ) increases radiation-induced apoptosis by inhibiting JAK2 phosphorylation; (E) Riluzole enhances radiosensitivity of melanoma BM cells by inhabiting GRM1; (F) 5-ALA increases melanoma BM radiosensitivity by increasing the porphyrin content. (G-K) Breast Cancer BM. (G) L-arginine mediates radiosensitivity through NO-dependent inhibition of GAPDH and PARP activation; (H) Metformin enhances tumor suppression when used as an adjuvant in RT; (I) LRRC31 importation via nanomaterials inhibits DNA repair and radiosensitivity in breast cancer BM; (J) Vorinostat, a histone deacetylase inhibitor, increases radiation sensitivity by inhibiting HDAC; (K) BM secreting S100A9, which binds to the RT-induced RAGE receptor, activates NF-κB-mediated RT resistance.

Figure 5

Combination Treatment with RT in Current AM-BM Research. Current research on AM-BMs explores diverse combination treatments with RT, including immunotherapy, novel drug applications, targeted therapy, surgery, chemotherapy, nanomaterial applications, ultrasound (mediating oxygen-containing microbubble rupture), magnetic field therapy, and electric field therapy. These comprehensive approaches signify the multifaceted strategies being investigated to optimize the efficacy of RT in AM-BM.