Veterinary antiparasitics are at the forefront of “double repositioning”—a specialized drug-repurposing strategy in which existing pharmaceuticals are repurposed across species (from veterinary to human medicine) and disease indications (from parasitic infections to oncology) at the same time.
This approach addresses the severe economic and time constraints of conventional de novo anticancer drug discovery, which typically takes 10 to 15 years, costs $1 to $2.5 billion, and sees only about 5% of candidate molecules that enter Phase I clinical trials reach final market approval.
1. Major Classes of Repurposed Veterinary Antiparasitics
There are two primary chemical classes of veterinary antiparasitics being actively investigated for human cancer treatment:
Benzimidazole (BZ) Carbamates: Heterocyclic aromatic compounds widely used to treat internal helminths in animals and humans. Prominent veterinary and human examples include fenbendazole (FZ), flubendazole (FLU), mebendazole (MZ), albendazole (ABZ), and ricobendazole (RBZ).
Halogenated Salicylanilides (HS): Synthetic compounds containing a salicylic acid ring attached to an anilide ring. Key representatives include niclosamide (Nic), closantel, and rafoxanide (RFX).
2. Primary Molecular Mechanisms of Action
Benzimidazole Carbamates (Microtubule & Metabolic Disruption)
Tubulin Depolymerization: The primary molecular target of benzimidazole carbamates is β-tubulin. By selectively binding tubulin and inhibiting microtubule polymerization, benzimidazole carbamates disrupt mitotic spindle assembly, cell division, organelle transport, and cell motility.
Glucose Depletion: By blocking microtubule elongation, benzimidazole carbamates suppress cellular glucose uptake by downregulating GLUT transporters and key glycolytic enzymes such as hexokinase, exhausting energy reserves and inducing cell death.
Cell Cycle Arrest & Apoptosis: Benzimidazole carbamates induce G2/M phase cell cycle arrest, marked by upregulation of cyclin-dependent kinase inhibitors p21 and p27 alongside c-Myc downregulation. Intrinsic apoptosis is executed via cytochrome-C release, caspase-3 activation, and PARP cleavage.
Halogenated Salicylanilides (Mitochondrial & Pathway Uncoupling)
Mitochondrial Uncoupling: Halogenated Salicylanilide compounds act primarily as uncouplers of oxidative phosphorylation, suppressing enzymes like succinate dehydrogenase and fumarate reductase to halt cellular ATP production.
Multitarget Signal Transduction Blockade: Agents like niclosamide and rafoxanide downregulate key oncogenic cascades driving tumor proliferation and metastasis, including Wnt/β-catenin, STAT3, Hedgehog, mTOR, and NF-κB pathways.
Immunogenic Cell Death (ICD): Rafoxanide has been shown to act as a potent inducer of ICD in colorectal cancer cells by promoting ecto-calreticulin exposure and releasing damage-associated molecular patterns (DAMPs) such as ATP and HMGB1.
3. Overcoming Chemoresistance: Fenbendazole in Resistant Colorectal Cancer
Veterinary antiparasitics demonstrate unique utility in overcoming chemotherapy resistance:
p53-Independent Apoptosis: In 5-fluorouracil-resistant colorectal cancer cells (SNU-C5/5-FUR), fenbendazole bypasses mutant or inactivated p53, inducing apoptosis without requiring p53 expression or activation.
Ferroptosis Augmentation: In drug-resistant cells, fenbendazole-induced cell death is strongly augmented by ferroptosis (an iron-dependent cell death pathway) via concurrent downregulation of GPX4 and SLC7A11 transporters, alongside marked HMGB1 release.
4. Translational Challenges & Novel Delivery Systems
Despite strong preclinical efficacy, translating veterinary antiparasitics to human oncology presents distinct pharmacological hurdles:
Pharmacokinetic Limitations: Most benzimidazole carbamates and halogenated salicylanilide compounds suffer from poor water solubility, low gastrointestinal absorption, and rapid first-pass hepatic metabolism, limiting effective systemic exposure.
Advanced Delivery Formulations: To overcome absorption barriers, researchers are developing targeted Antibody-Drug Conjugates (ADCs)—attaching benzimidazole carbamates to tumor-specific antibodies via cleavable linkers for localized endosomal/lysosomal release inside cancer cells—as well as lipid nanoparticles and individualized serum drug monitoring.
Self-Administration & Trial Confounding: Widespread media reporting has led some patients to self-administer veterinary antiparasitics (such as fenbendazole protocols) alongside conventional therapies, which can confound clinical trial outcomes if physicians do not monitor them.
Double Repositioning: Veterinary Antiparasitic to Human Anticancer
Sultana, T.; Jan, U.; Lee, J.I. Double Repositioning: Veterinary Antiparasitic to Human Anticancer. Int. J. Mol. Sci. 2022, 23, 4315. https://doi.org/10.3390/ijms23084315