In chemoresistant cancer cells—such as 5-Fluorouracil (5-FU)-resistant colorectal cancer cells—fenbendazole induces ferroptosis through a p53-independent pathway closely linked to metabolic disruption:
Bypassing p53 Dysfunction: In wild-type cancer cells, fenbendazole typically activates p53-mediated apoptosis. However, in 5-FU-resistant cancer cells that have lost normal p53 function, fenbendazole bypasses this pathway to induce cell death, promoting p53-independent, ferroptosis-augmented apoptosis.
Interference with Glycolysis & Energy Metabolism: Chemoresistant cancer cells often rely heavily on accelerated glycolysis to survive. Fenbendazole disrupts this survival mechanism by inhibiting the GLUT1 glucose transporter and suppressing hexokinase II activity, thereby reducing glucose uptake and depriving resistant cells of glycolytic energy.
Selective Oxidative Stress & Cell Death: By combining metabolic starvation with increased intracellular oxidative stress, fenbendazole triggers iron-dependent lipid peroxidation characteristic of ferroptosis alongside programmed apoptosis.
These multi-targeted mechanisms enable fenbendazole to overcome cross-resistance in tumor lines resistant to conventional agents such as 5-FU, paclitaxel, and docetaxel.
Fenbendazole Pharmacokinetics
The pharmacokinetics (PK) of fenbendazole (FBZ) are characterized by low oral bioavailability, rapid hepatic biotransformation, and specific physical formulation challenges:
1. Absorption & Solubility Limitations
Poor Aqueous Solubility: Pure fenbendazole exhibits extremely low water solubility (0.3 μg/mL) and poor membrane permeability, hindering its ability to reach necessary systemic therapeutic levels following standard oral administration.
Dissolution Kinetics: Pure fenbendazole releases only about 5% of the drug within 15 minutes and 81% within 1 hour.
Rodent Pharmacokinetic Baseline: In male rats administered an oral dose of 10 mg/kg, fenbendazole achieved a peak plasma concentration (Cmax) of 0.32 μg/mL at a Tmax of 0.25 hours, a elimination half-life (T1/2) of 8.23 hours, and an area under the curve (AUClast) of 0.73 μg⋅h/mL. After reaching peak concentrations, plasma levels rapidly fell below 0.1 μg/mL.
2. Hepatic Biotransformation & Active Metabolites
Fenbendazole undergoes extensive presystemic and systemic liver metabolism via specific cytochrome P450 (CYP) and flavin-containing monooxygenase (FMO) pathways:
Sulfoxidation to Oxfendazole: FMO and CYP3A4 rapidly oxidize fenbendazole into its sulfoxide metabolite, oxfendazole (fenbendazole sulfoxide). Oxfendazole is the primary metabolite responsible for systemic biological activity and has a significantly higher aqueous solubility (44.12 μg/mL) and bioavailability than parent fenbendazole. Through systemic circulation, oxfendazole can be reversibly reduced back to fenbendazole.
Hydroxylation: CYP2J2 and CYP2C19 metabolize fenbendazole into hydroxyfenbendazole.
Sulfone Formation: Further systemic oxidation converts fenbendazole into fenbendazole sulfone, which is often the predominant circulating metabolite in plasma.
3. Excretion Routes
Following hepatic processing, fenbendazole and its metabolites are eliminated primarily via feces and urine.
In livestock studies (cattle), 36% of an oral dose was recovered unchanged or metabolized in the feces, with negligible urine recovery. When administered intravenously, over 50% of the dose was excreted as hydroxyfenbendazole.
4. Formulation Strategies to Increase Bioavailability
To overcome fenbendazole’s pharmacokinetic constraints for systemic oncology applications, several drug delivery systems have been developed:
Methyl-β-Cyclodextrin Inclusion Complexes: Complexing FBZ with methyl-β-cyclodextrin at a 1:1 ratio increases its water solubility to 20.21 mg/mL (a ~60,000-fold increase), boosting 15-minute drug release from 5% to 75%.
CYP Enzyme Inhibition (DMSO / DNTC): Formulations using DMSO or DNTC mix (DMSO, NMP, Tween-80, Cremophor) inhibit CYP2C19 and CYP3A4 enzymes, slowing hepatic clearance and prolonging circulating drug levels.
Polymeric Micelles & Nanoparticles: Encapsulation in Soluplus® polymeric micelles or PLGA nanoparticles lowers total clearance (CL) and volume of distribution (Vd), achieving higher plasma Cmax and sustained release (e.g., 50.4% released at 72 hours).
Cocrystals: Fenbendazole-salicylic acid cocrystals achieve a solubility of 1.052 mg/mL and 100% drug release in under 1 hour.
Dietary Interactions: Co-administering oral fenbendazole alongside food/fatty meals or vitamin supplements significantly increases gastrointestinal absorption and systemic exposure.
Reference
Nguyen, J., Nguyen, T. Q., Han, B., & Hoang, B. X. (2024). Oral fenbendazole for cancer therapy in humans and animals. Anticancer Research, 44(9), 3725–3735.