Newly engineered prodrugs improve niclosamide (NIC) bioavailability for cancer therapy by directly addressing its main clinical limitations: extremely poor water solubility (~1.6 mg/L), a high melting point, and rapid systemic clearance, which otherwise prevent effective concentrations from reaching solid tumors.
These prodrugs overcome these barriers through several chemical and pharmacokinetic enhancements:
1. Significantly Increasing Water Solubility & Stability
Phenolic Functional Modification: Modifying niclosamide's 2-hydroxyl phenol group with strongly anionic or polar groups drastically increases aqueous solubility.
Gastric & Plasma Stability: For example, the O-sulfate ammonium salt prodrug SSL-0024 achieves a 2.2-fold increase in kinetic solubility and a 5-fold increase in thermodynamic solubility compared to native niclosamide6. It remains highly stable under gastric (pH 2.0) and physiological (pH 7.4) conditions, with 85.6% intact in human plasma, preventing premature degradation in circulation.
2. Dramatically Extending Systemic Exposure (AUC & Half-Life)
Substantial Bioavailability Gains: The water-soluble prodrug NIC-PS achieves an almost 10-fold increase in oral bioavailability (Area Under the Curve, AUC) compared to native oral niclosamide.
Sustained Release Profile: Unlike native niclosamide or earlier phosphate salt prodrugs that cause rapid plasma spikes followed by quick elimination11, compounds like SSL-0024 provide sustained release of active free niclosamide over 48 hours, resulting in 6.6-fold greater overall plasma exposure.
3. Targeted In Vivo Bioactivation & Safe Cleavage
Enzymatic Conversion: Prodrugs act as soluble precursors that undergo cleavage in vivo to release active free niclosamide.
Non-Toxic Byproducts: Endogenous aryl sulfatases in liver cells cleave SSL-0024 to release bioactive niclosamide alongside an ammonium sulfate counterion labeled as Generally Recognized As Safe (GRAS) by the FDA15. This avoids off-target toxicities (such as lung alveolar irritation) seen with older salt forms like niclosamide ethanolamine (NEN)1516.
4. Promoting Preferential Accumulation in Solid Tumors
Higher Tumor-to-Tissue Ratio: Prolonged systemic circulation enables niclosamide to accumulate preferentially within tumor tissues rather than healthy surrounding tissue.
In orthotopic hepatocellular carcinoma (HCC) patient-derived xenograft (PDX) models, NIC-PS achieves ~1.5-fold higher niclosamide concentrations in PDX tumors than in normal adjacent liver tissue17. Similarly, SSL-0024 achieves preferential tumor accumulation (~1.2-fold higher than normal liver at 48 hours) and achieves ~60% tumor growth inhibition at less than half (~45.8%) the molar equivalent dose of NEN18.
5. Complementary Nanohybrid Approaches
Beyond pure chemical prodrugs, nanoengineered formulations such as Penetrium (a niclosamide–magnesium oxide–HPMC nanodrug) use ionic interactions and localized pH buffering to increase oral bioavailability, protect the compound, and enable penetration through dense, fibrotic extracellular matrix (ECM) barriers.
References
(2026). Niclosamide and cancer: Research and clinical trials [Research overview report].
Rejinold, N. S., Kim, S., Jin, G.-W., & Choy, J.-H. (2025). Penetrium nanodrug remodels the tumor ECM to overcome pseudo-resistance and enhance anticancer therapeutic synergy [Preprint]. bioRxiv.
Schumacher, T. J., Iyer, A. V., Rumbley, J., Ronayne, C. T., & Mereddy, V. R. (2024). Repurposing mitochondrial-targeting anthelmintic agents with GLUT1 inhibitor BAY-876 for cancer therapy [Preprint]. bioRxiv.
Tan, M., Ye, W., Liu, Y., Chen, X., Huttad, L., Chua, M.-S., & So, S. (2024). Niclosamide prodrug enhances oral bioavailability and targets vasorin-TGFβ signaling in hepatocellular carcinoma [Preprint]. bioRxiv.
Tan, M., Schow, S., Liu, Y., Lum, R., Massoudi, D., Dhanasekaran, R., So, S., & Chua, M.-S. (2026). A novel niclosamide sulfate prodrug with enhanced bioavailability suppresses hepatocellular carcinoma via inhibition of multiple signaling pathways [Preprint]. bioRxiv.
I keep reading about this, but movement forward so the patients like us can get it.