Repurposing Niclosamide Ethanolamine for Alveolar Echinococcosis Reveals a Disconnect Between In Vitro Efficacy and In Vivo Outcome

BackgroundEchinococcosis is a zoonotic disease caused by cestodes of the genus Echinococcus. Alveolar echinococcosis (AE), caused by E. multilocularis, primarily affects the liver and shows infiltrative, tumor-like growth of the metacestode stage. If untreated, AE is lethal. AE remains a neglected disease with current treatments based on albendazole or mebendazole that are parasitostatic, and not curative, underscoring the need for more effective therapies. Niclosamide is a chlorinated salicylanilide derivative with proven activities against intestinal helminths but is inactive against tissue-dwelling helminths due to poor absorption and limited bioavailability. In this study, we repurposed niclosamide ethanolamine (NEN), a formulation with improved systemic exposure, for the treatment of E. multilocularis infection in vitro and in vivo. Methodology/Principal FindingsWe assessed the in vitro efficacy of niclosamide and NEN against E. multilocularis metacestode vesicles (IC50<0.2 {micro}M) and primary parasite cells (IC50<0.3 {micro}M), with active concentrations largely corresponding to NEN levels reachable in the liver. Metabolic analysis suggested that NEN acts as a mitochondrial uncoupler. Electron microscopy showed that NEN-treatments induced profound structural damage in the metacestode vesicle tissue, but mitochondrial ultrastructure was not notably affected. In mice intraperitoneally infected with E. multilocularis, NEN was orally administered during 9 weeks either alone, or in combination with albendazole. Pharmacokinetic analyses showed that NEN reached blood level concentrations above 1 {micro}M. However, the parasite burden in NEN-treated mice was not significantly reduced. Conclusions/SignificanceAlthough niclosamide and NEN demonstrated potent activity against E. multilocularis in vitro, this efficacy did not translate in the mouse model. The lack of in vivo activity could be attributed to several factors such as infection model, limited drug uptake by the parasite in the animal, or the rapid metabolization of the compound. Future studies should explore novel niclosamide derivatives and formulations to enhance efficacy against AE in vivo. Author SummaryAlveolar echinococcosis (AE) is a severe disease caused by the larval stage of the fox tapeworm Echinococcus multilocularis. The parasite forms tumor-like lesions in the liver and can spread to other organs. The currently licensed drugs for the treatment of AE are not always effective, require long-term use, and can cause side effects that frequently require treatment interruption. Therefore, safer and more efficacious treatment options are urgently needed. Niclosamide is frequently applied for the treatment of adult tapeworm infections in the intestine, but its limited uptake and low biodistribution renders the compound unsuitable for systemic treatment. In this study, we tested a non-toxic salt formulation, niclosamide ethanolamine (NEN), exhibiting improved absorption. In vitro, NEN was highly effective against E. multilocularis metacestode vesicles. It induced profound structural alterations in metacestode vesicles and impaired the mitochondrial membrane potential, and thus interfering in energy production. However, NEN was not effective against AE in experimentally infected mice. Our results suggest that NEN treatment appears promising in vitro, but to translate to the in vivo situation, new formulations and delivery strategies should be developed to increase absorption, bioavailability and metabolic stability of the compound for an effective treatment for AE.