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Scorpion Venom Peptides: Chlorotoxin and Cancer Imaging

16 min read|March 25, 2026

Venom Peptides

36 amino acids

Chlorotoxin, a small peptide from deathstalker scorpion venom, binds glioma cells with enough selectivity to guide a surgeon's hand during brain tumor removal.

El-Qassas et al., Molecular Cancer Therapeutics, 2024

El-Qassas et al., Molecular Cancer Therapeutics, 2024

Molecular illustration of chlorotoxin peptide binding to a glioma cell surfaceView as image

A 36-amino-acid peptide from the venom of the deathstalker scorpion (Leiurus quinquestriatus) has traveled from neurotoxicology lab bench to operating room. Chlorotoxin (CTX) was first isolated in 1993 as a chloride channel blocker. Five years later, researchers discovered it binds glioma cells with striking selectivity, ignoring healthy brain tissue almost entirely. That property turned a venom component into a cancer-targeting tool.[1] Today, chlorotoxin-based imaging agents are in clinical trials, chlorotoxin-directed CAR T cells are treating patients with recurrent glioblastoma, and the peptide's binding profile extends to melanoma, breast cancer, and lung tumors. This article covers the full arc of the research, from scorpion gland to surgical suite. For a broader view of how animal venoms produce peptide drug candidates, see Venom Peptides: How Deadly Toxins Become Life-Saving Drugs.

Key Takeaways

  • Chlorotoxin binds glioma cells through at least three molecular targets: MMP-2, Annexin A2, and glioma-specific chloride channels (ClC-3), a redundancy that makes antigen escape difficult
  • In a Phase 1 trial of 17 glioma patients, tozuleristide (BLZ-100), a chlorotoxin-ICG conjugate, detected tumors with 82% sensitivity and 89% specificity at doses up to 30 mg with no dose-limiting toxicity
  • Chlorotoxin-directed CAR T cells produced tumor regression in orthotopic xenograft models and showed stable disease in 3 of 4 patients in a Phase 1 clinical trial at City of Hope
  • A first-in-human skin cancer study found BLZ-100 fluorescence in 4 of 5 basal cell carcinomas and 4 of 4 melanomas, demonstrating binding beyond brain tumors
  • Scorpion venom contains over 400 identified peptides across multiple species, with at least six showing direct antitumor activity in preclinical models
  • The peptide Smp43 from Scorpio maurus palmatus venom killed A549 lung cancer cells through mitochondrial disruption, representing a separate anticancer mechanism from chlorotoxin's targeting approach

What is chlorotoxin and where does it come from?

Chlorotoxin is a 36-amino-acid peptide stabilized by four disulfide bridges, isolated from the venom of the deathstalker scorpion (Leiurus quinquestriatus). The peptide was originally characterized as a blocker of small-conductance chloride channels in epithelial cells. Its compact, disulfide-rich structure gives it unusual stability for a peptide of its size, making it resistant to degradation in biological fluids.

The deathstalker is one of approximately 2,500 scorpion species, and scorpion venoms collectively contain hundreds of bioactive peptides with diverse pharmacological properties.[2] Chlorotoxin stands apart because its primary research value shifted from neurotoxicology to oncology after Soroceanu and colleagues at the University of Alabama at Birmingham demonstrated that CTX binds specifically to glioma cells while sparing normal brain tissue.[1] A 2024 comprehensive review by El-Qassas et al. traced this discovery through its subsequent clinical development. A follow-up study in 2002 confirmed that chlorotoxin binds not only to gliomas but to tumors of neuroectodermal origin more broadly, including medulloblastoma, neuroblastoma, melanoma, and small cell lung carcinoma.

How chlorotoxin targets tumor cells

Chlorotoxin's tumor selectivity depends on at least three molecular interactions, each of which is overexpressed on tumor cells relative to normal tissue.

Matrix metalloproteinase-2 (MMP-2). Deshane and colleagues showed in 2003 that chlorotoxin's anti-invasive effect on glioma cells is mediated primarily through MMP-2. When CTX binds, it inhibits MMP-2 enzymatic activity and reduces the surface expression of MMP-2, blocking the protease activity that glioma cells use to invade surrounding brain tissue. MMP-2 is overexpressed in the vast majority of high-grade gliomas but present at low levels in normal brain.

Annexin A2. Kesavan and colleagues identified Annexin A2 as a second binding partner for chlorotoxin. This phospholipid-binding protein is upregulated on the surface of multiple cancer cell types and plays roles in cell migration and angiogenesis. The dual binding to MMP-2 and Annexin A2 helps explain why chlorotoxin targets such a broad range of tumor types.

Glioma-specific chloride channels (ClC-3). The original discovery of chlorotoxin's glioma affinity centered on a chloride channel specifically expressed on glioma cell membranes. Upon CTX binding, this channel complex is internalized into lipid rafts, a process that contributes to the inhibition of glioma cell migration and invasion. This internalization is also what makes chlorotoxin useful as a delivery vehicle: once bound, it gets pulled inside the cell.

This triple-target profile is unusual for a small peptide. Wang et al. (2020) demonstrated that effective targeting by chlorotoxin-directed CAR T cells required cell surface expression of MMP-2, confirming its central role in the binding mechanism.[3] The redundancy across three targets makes it harder for tumors to escape chlorotoxin binding through loss of a single antigen, a persistent problem with other targeted therapies.

Tumor paint: from concept to clinical trial

The idea of conjugating chlorotoxin to a fluorescent molecule for intraoperative imaging originated at the Fred Hutchinson Cancer Research Center. In 2007, Veiseh and colleagues created "Tumor Paint," a bioconjugate of chlorotoxin and the near-infrared fluorescent dye Cy5.5 (CTX:Cy5.5). In mouse models, this probe successfully delineated malignant glioma, medulloblastoma, prostate cancer, intestinal cancer, and sarcoma from adjacent normal tissue. It detected metastatic cancer foci as small as a few hundred cells in lymph channels.

The clinical-grade version of this concept is tozuleristide (BLZ-100), which conjugates chlorotoxin to indocyanine green (ICG) instead of Cy5.5. ICG is already FDA-approved for other imaging applications, which simplified the regulatory path. When injected intravenously, BLZ-100 accumulates in tumor tissue and fluoresces under near-infrared light, allowing surgeons to see tumor margins that are invisible to the naked eye.

Phase 1 glioma trial results

Butte et al. (2019) conducted a Phase 1 safety, pharmacokinetics, and fluorescence imaging study of tozuleristide in 17 adults with newly diagnosed or recurrent gliomas. Key findings:

  • Doses ranged from 3 mg to 30 mg intravenously
  • No dose-limiting toxicity at any dose level
  • No adverse events considered related to tozuleristide
  • Serum half-life of approximately 30 minutes at doses above 9 mg, while fluorescence persisted in tumor tissue for over 24 hours
  • Fluorescence signal was detected in both high-grade and low-grade glial tumors
  • High-grade tumors showed greater fluorescence intensity than lower-grade tumors
  • Sensitivity of 82% and specificity of 89% for tumor localization
  • Signal intensity increased with higher doses in high-grade tumors

The long tumor retention time relative to the short serum half-life is pharmacologically favorable: the drug clears from the blood quickly but stays in the tumor, creating a widening contrast window for the surgeon.

Skin cancer trial

In a separate first-in-human study, Fidel et al. (2021) administered BLZ-100 intravenously to 21 adults with skin cancer at doses of 1 to 18 mg.[1] At intermediate dose levels (3 to 12 mg), 4 of 5 basal cell carcinomas and 4 of 4 melanomas showed positive BLZ-100 fluorescence. No serious adverse events, deaths, or discontinuations occurred. No maximum tolerated dose was identified. This trial confirmed that chlorotoxin's binding extends well beyond glioma to other tumor types, consistent with its multi-target mechanism.

Breast cancer pathology

Parrish-Novak et al. (2019) demonstrated that tozuleristide fluorescence was readily observed in invasive and in situ breast carcinoma specimens. Invasive carcinomas showed bright, focal fluorescence, while ductal carcinoma in situ produced a less intense, more diffuse pattern. This suggests potential applications in intraoperative margin assessment during breast-conserving surgery, where ensuring complete tumor removal during the initial operation reduces reoperation rates. For context on how targeted peptides are used in peptide-coated nanoparticles for cancer, chlorotoxin's approach represents a complementary strategy of direct conjugation rather than nanoparticle loading.

Chlorotoxin-directed CAR T cells

Beyond imaging, chlorotoxin's tumor-binding specificity has been engineered into a cellular therapy. Wang et al. (2020) at City of Hope developed chimeric antigen receptor (CAR) T cells using chlorotoxin as the targeting domain (CLTX-CAR T cells).[3]

Standard CAR T cells for glioblastoma target single antigens like EGFRvIII or IL-13Rα2. The problem is tumor heterogeneity: not every glioblastoma cell expresses these antigens, so some tumor cells escape. Chlorotoxin binds a much broader proportion of glioblastoma cells because its multi-target mechanism (MMP-2, Annexin A2, chloride channels) covers more of the cell population.

In preclinical models, CLTX-CAR T cells:

  • Killed glioblastoma cells that lacked expression of other GBM-associated antigens (EGFRvIII, HER2, IL-13Rα2)
  • Caused tumor regression in orthotopic xenograft models
  • Showed no observable off-target effector activity against normal cells
  • Required MMP-2 surface expression for effective targeting

A Phase 1 trial (NCT04214392) subsequently evaluated intracavity and intratumoral delivery of CLTX-CAR T cells in patients with MMP-2-expressing recurrent glioblastoma. Interim results from four patients showed no dose-limiting toxicities, and three of four participants (75%) achieved stable disease as their best response. While "stable disease" is not tumor regression, it is a meaningful outcome for recurrent glioblastoma, where median survival after recurrence is typically 6 to 9 months. This is one of the first uses of a venom-derived peptide as the targeting domain of a CAR T cell, a design approach that expands the repertoire of targetable tumor antigens.

TM-601 and radioimmunotherapy

Before the imaging and CAR T applications, chlorotoxin's first clinical incarnation was TM-601, a synthetic version of the peptide conjugated to iodine-131 (131I-TM-601). Mamelak et al. (2006) demonstrated that when injected directly into the tumor cavity after surgical resection, 131I-TM-601 was retained selectively in tumor tissue and delivered localized radiation to residual glioma cells. The compound completed Phase I/II trials and received FDA clearance to proceed to a Phase III trial in patients with newly diagnosed glioma. 131I-TM-601 also functions as a SPECT imaging agent, allowing clinicians to visualize the extent of the primary tumor while simultaneously delivering therapy, a property now called "theranostics." For more on this dual-purpose approach, see Theranostic Peptides: Diagnose and Treat Cancer with the Same Molecule.

Beyond chlorotoxin: other scorpion venom anticancer peptides

Chlorotoxin is the most clinically advanced scorpion venom peptide, but it is not alone. Scorpion venoms contain hundreds of bioactive peptides, and multiple research groups have identified additional compounds with antitumor properties.[1]

Smp43 from Scorpio maurus palmatus venom displayed potent antitumor activity against A549 non-small-cell lung cancer cells in work by Deng et al. (2023).[4] Unlike chlorotoxin, which works through receptor-mediated targeting, Smp43 is a cationic antimicrobial peptide that kills cancer cells through direct membrane disruption and mitochondrial pathway activation. This represents a fundamentally different anticancer mechanism from the same venom source.

Leptulipin-p28 fusion protein. Khalid et al. (2024) designed a computational fusion of the scorpion venom peptide Leptulipin with the cell-penetrating peptide p28, targeting HER2-positive breast cancer cells.[5] Molecular dynamics simulations showed stable binding to the HER2 extracellular domain. This work remains computational, but it illustrates how scorpion peptide scaffolds can be engineered for targets beyond the brain.

Scorpion venom heat-resistant peptide (SVHRP) from Buthus martensii Karsch has shown neuroprotective effects rather than direct antitumor activity. Wu et al. (2021) demonstrated that SVHRP attenuates microglia activation and neuroinflammation through inhibition of the NF-κB signaling pathway.[6] Cheng et al. (2025) extended this work, showing that a synthetic version of SVHRP alleviated DSS-induced colitis in mice, suggesting anti-inflammatory applications beyond the nervous system.[7]

A 2025 proteomic analysis by Arcos et al. cataloged the full peptide and protein composition of multiple scorpion venoms, identifying novel compounds with anticancer potential through cell cycle arrest, apoptosis induction, angiogenesis inhibition, and metastasis suppression.[8] The field is moving from single-peptide studies to systematic venom profiling, which should accelerate the identification of new drug candidates. For how similar approaches have worked with other venoms, see Bee Venom Peptides: Melittin and Apamin in Research and Cone Snail Peptides: Hundreds of Drug Candidates from a Snail.

Drug delivery applications

Chlorotoxin's cell-internalizing behavior has made it attractive as a targeting ligand for drug delivery systems. When conjugated to nanoparticles, liposomes, or other carriers, CTX directs these payloads preferentially to tumor cells.[9]

Applications under investigation include:

  • CTX-conjugated iron oxide nanoparticles for combined MRI contrast and drug delivery
  • CTX-labeled liposomes loaded with doxorubicin for targeted chemotherapy to glioma cells
  • CTX-directed gene delivery vectors for delivering therapeutic genes specifically to brain tumors

A 2025 review by Mazurs et al. placed chlorotoxin in the broader context of venom peptides used as drug delivery targeting agents, alongside RGD peptides, bombesin, and substance P analogs.[9] Chlorotoxin's advantage over these alternatives is the breadth of its tumor-type coverage and the depth of its clinical validation.

The limitation of these approaches is that most remain preclinical. The transition from nanoparticle-in-a-dish to injectable therapeutic requires solving manufacturing, stability, and pharmacokinetic challenges that are independent of the targeting peptide itself. Related peptide-nanoparticle strategies are covered in Peptide-Coated Nanoparticles for Cancer: Guided Missiles at the Molecular Level.

What remains unknown

The chlorotoxin story has real clinical data, which puts it ahead of most venom-derived peptide candidates. But several questions remain unresolved.

Which molecular target matters most? Chlorotoxin binds MMP-2, Annexin A2, and chloride channels. The relative contribution of each interaction to tumor selectivity is still debated. Wang et al. (2020) showed that MMP-2 expression is required for CLTX-CAR T cell activity, but whether that means MMP-2 is the primary receptor or merely a necessary co-receptor is unclear.[3]

Long-term imaging performance. The Phase 1 tozuleristide trial was designed for safety, not efficacy. The 82% sensitivity figure is encouraging but comes from 17 patients. Larger trials with controlled comparisons to 5-ALA (the current standard for fluorescence-guided glioma surgery) are needed to establish whether chlorotoxin-based imaging changes surgical outcomes.

Normal tissue binding. Chlorotoxin's selectivity for tumor over normal tissue is high but not absolute. The tozuleristide trial noted fluorescence in some non-tumor tissue. One case report documented unexpected binding to cerebral vascular malformations, raising questions about specificity in patients with co-existing vascular pathology.

Scalability of scorpion venom peptides. Chlorotoxin can be produced synthetically, which solves the supply problem. But the broader scorpion venom pharmacopeia, containing hundreds of potentially useful peptides, presents a bottleneck: most have not been synthesized at scale, and venom harvesting from live scorpions yields microgram quantities.[10]

The Bottom Line

Chlorotoxin has progressed from a venom-derived chloride channel blocker to a clinical-stage cancer imaging agent and CAR T cell targeting domain over three decades of research. Phase 1 trial data for tozuleristide (BLZ-100) show safety and tumor detection in glioma, skin cancer, and breast cancer. The peptide's multi-target binding mechanism (MMP-2, Annexin A2, ClC-3) provides broad tumor coverage, but larger efficacy trials and head-to-head comparisons with existing imaging agents are still needed to determine clinical adoption.

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