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.