Chlorotoxin

Glioblastoma multiforme (GBM) is the most common malignant primary brain neoplasm having a mean survival of <24months. Scorpion toxins are considered promising cancer drug candidates, primarily due to the discovery of hlorotoxin, derived from the venom of the Israeli yellow scorpion. This intriguing short peptide of only 36 amino-acids length and tight configuration, possess the ability to bind to GBM cells in a grade-related manner with ~100% of GBM cells staining positive and no cross reactivity to normal brain. Chlorotoxin has an anti-angiogenic effect as well. Molecular targets for Chlorotoxin include voltage gated chloride channels (GCC), calcium-dependent phospholipid-binding protein Annexin-2, and the inducible extracellular enzyme Matrix Metalloproteinase-2 (MMP-2). Of all its targets, MMP-2 seems to bear the most anti-neoplastic potential. Chlorotoxin is a promising tumortargeting peptide. Its small size and compact shape are convenient for intracranial delivery. We present a short discussion on Chlorotoxin. The structure, biological activity, molecular targets and possible clinical role of Chlorotoxin are discussed. Chlorotoxin can be utilized as a targeting domain as well, attaching different effector functions to it. Clinical applications in GBM therapy, intraoperative imaging, nano-probes and nano-vectors based technology; targeted chemotherapy and immunotherapy are discussed as well. Chlorotoxin is likely to play a significant role in effective GBM immunotherapy in the future.

Chlorotoxin (CTX) is a neurotoxin comprising 36 amino acids and is isolated from the venom ofLeiurus quinquestriatus, a scorpion of the Buthidae family. A characteristic feature of the peptide is the four disulfide bonds that give it a tight tertiary structure and a single tyrosine residue that can be iodinated. Originally, CTX was described as a calcium channel blocker (1). Later, selective and specific binding of CTX to glioma cells was demonstrated by immunochemical techniques, and radiolabeled CTX was shown to bind only to tumor cells in a mouse xenograft glioma tumor model (1). On the basis ofin vitrostudies, CTX was discovered to bind only to malignant (glioma) and not normal (glial) cells, indicating that the toxin had a binding specificity for embryologically related tumors, particularly those of neuroectodermal origin (2). Using a recombinant form of the toxin, it was shown that CTX binds to and reduces the activity of a matrix metalloproteinase (MMP) that is associated with and indirectly regulates functioning of the chloride channels on cell membranes (3, 4). Deshane et al. showed that isoform 2 of the MMP (MMP2) was the specific cellular receptor for CTX and that the ligand did not bind to the other MMPs also expressed on the glioma cells (4). In otherin vitrostudies it was shown that CTX reduced the migration ability of glioma cells through tight extracellular spaces in the brain tissue by inhibition of the MMP2, because this prevented the cells from shrinking and releasing from the extracellular matrix (5).The MMP2 enzyme activity was observed to be significantly elevated in malignant glioma cells compared to low-grade glioma and normal brain tissues, and the upregulation correlated with the malignant progression of human gliomasin vivo(6). In the same study, which used immunohistochemical techniques, MMP2 was shown to be localized only in the cells and vasculature of malignant glioma tumors. These observations explained the specificity and selectivity of CTX binding to the malignant glioma tumors observed by the various investigators. Currently, resection of the tumor is the only effective treatment for glioma, but the tumor often reappears at the same spot or close to it because not all neoplastic cells can be completely removed during surgery.Because of its specificity and selectivity for glioma cells, a synthetic version of CTX, TM-601, was generated either by production inEscherchia colior by chemical synthesis (2, 4, 7). The peptide has been radiolabeled with radioactive iodine (131I) to obtain131I-TM601 and used for preclinical and clinical imaging and radiotherapy of malignant glioma (8, 9). The iodinated peptide is currently beingevaluated in clinical trialsin the United States for the treatment of solid tumors or recurrent high-grade gliomas in humans (10).

This review reports the recent advances in chlorotoxin (CTX)-targeted nanoparticles (NPs) for imaging and therapy of glioma. CTX has been identified as a targeting ligand to specifically bind to glioma. Through different conjugation approaches, CTX can be conjugated onto iron oxide NPs, quantum dots, and rare-earth upconversion NPs for targeted magnetic resonance and fluorescence imaging of glioma. Likewise, CTX-conjugated NPs can also be used as a carrier system to load anticancer drugs or therapeutic genes for targeted chemotherapy or gene therapy of glioma, respectively. Some of the key developments in this area of research will be introduced in detail. Challenges and future perspectives in the development of CTX-conjugated NPs will be discussed.