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Use of chlorotoxin for targeting of primary brain tumors
L Soroceanu1, Y Gillespie, M B Khazaeli
1Department of Neurobiology, University of Alabama at Birmingham, 35294, USA.
Abstract:
Gliomas are primary brain tumors that arise from differentiated glial cells through a poorly understood malignant transformation. Although glioma cells retain some genetic and antigenic features common to glial cells, they show a remarkable degree of antigenic heterogeneity and variable mutations in their genome. Glioma cells have recently been shown to express a glioma-specific chloride ion channel (GCC) that is sensitive to chlorotoxin (CTX), a small peptide purified from Leiurus quinquestriatus scorpion venom [N. Ullrich et al, Neuroreport, 7: 1020-1024, 1996; and N. Ullrich and H. Sontheimer, Am. J. Physiol. (Cell Physiol.), 270: C1511-C1521, 1996]. Using native and recombinant 125I-labeled CTX, we show that toxin binding to glioma cells is specific and involves high affinity [dissociation constant (Kd)=4.2 nM] and low affinity (Kd=660 nml) binding sites. In radioreceptor assays, 125I-labeled CTX binds to a protein with Mr=72,000, presumably GCC or a receptor that modulates GCC activity. In vivo targeting and biodistribution experiments were obtained using 125I- and (131)I-labeled CTX injected into severe combined immunodeficient mice bearing xenografted gliomas. CTX selectively accumulated in the brain of tumor-bearing mice with calculated brain: muscle ratios of 36.4% of injected dose/g (ID/g), as compared to 12.4% ID/g in control animals. In the tumor-bearing severe combined immunodeficient mice, the vast majority of the brain-associated radioactivity was localized within the tumor (tumor:muscle ratio, 39.13% ID/g; contralateral brain:muscle ratio, 6.68%ID/g). Moreover, (131)I-labeled CTX distribution, visualized through in vivo imaging by gamma ray camera scans, demonstrates specific and persistent intratumoral localization of the radioactive ligand. Immunohistochemical studies using biotinylated and fluorescently tagged CTX show highly selective staining of glioma cells in vitro, in situ, and in sections of patient biopsies. Comparison tissues including normal human brain, kidney, and colon were consistently negative for CTX immunostaining. These data suggest that CTX and CTX-conjugated molecules may serve as glioma-specific markers with diagnostic and therapeutic potential.
Insights
Chlorotoxin (CTX) selectively targets glioma-specific chloride ion channels (GCC) on brain tumors. This study shows CTX accumulates in gliomas in vivo, suggesting its potential as a diagnostic and therapeutic agent for glioma.
Area of Science:
- Neuroscience
- Oncology
- Biochemistry
Background:
- Gliomas are primary brain tumors with poorly understood malignant transformation.
- Glioma cells exhibit antigenic heterogeneity and genomic mutations.
- Glioma cells express a glioma-specific chloride ion channel (GCC) sensitive to chlorotoxin (CTX).
Purpose of the Study:
- To investigate the specificity and potential of chlorotoxin (CTX) as a glioma-targeting agent.
- To evaluate CTX binding affinity and identify its target protein.
- To assess CTX's in vivo biodistribution and diagnostic/therapeutic potential for gliomas.
Main Methods:
- Radioligand binding assays using 125I-labeled CTX to determine binding affinity and identify target proteins.
- In vivo biodistribution studies in mice bearing xenografted gliomas using 125I- and 131I-labeled CTX.
- In vivo imaging and immunohistochemical analysis of CTX distribution and specificity in tumor tissues and patient biopsies.
Main Results:
- CTX demonstrated specific high-affinity (Kd=4.2 nM) and low-affinity (Kd=660 nM) binding to glioma cells.
- 125I-labeled CTX bound to a 72,000 Mr protein, likely GCC or a related receptor.
- In vivo, CTX selectively accumulated in gliomas in tumor-bearing mice, with significant tumor-to-brain ratios.
- Immunohistochemistry confirmed highly selective CTX staining of glioma cells in vitro, in situ, and in patient samples, with no staining in normal tissues.
Conclusions:
- CTX exhibits specific binding to glioma cells via the GCC.
- CTX demonstrates selective in vivo accumulation in gliomas, indicating potential for targeted delivery.
- These findings support the use of CTX and its conjugates as glioma-specific diagnostic markers and therapeutic agents.