Repurposing niclosamide for the treatment of neurological disorders

Neurological disorders are still one of the major causes of death, and the vast need to find efficacious therapy is nowadays an essential goal of the scientific community. For Parkinson’s disease (PD), amyotrophic lateral sclerosis (ALS), spinal cord injury, and intracerebral hemorrhage, current pharmacological treatments are solely symptomatic, so there is a need to identify agents that can slow or stop neurodegeneration. Neurodegenerative diseases are caused by interactions between genetic, epigenetic, and environmental factors with consequent dysfunction of multiple cellular and molecular pathways. The multifactorial nature of the disorders could explain the modest results obtained by the treatments proposed so far. Moreover, the biochemical complexity of the pathological mechanisms highlights the need for multitarget therapies acting synergistically on different aspects of the diseases. Niclosamide, marketed as Yomesan for human use in 1962, is a Food and Drug Administration (FDA)-approved anti-helminthic drug used for over 50 years with considerable safety (Chen et al., 2018), and it is included in the World Health Organization’s list of essential medicines. Niclosamide is a member of the salicylanilide class of pharmacologic agents with an aryl β-hydroxy-carbonyl pharmacophore motif, usually present in many biological natural products. The pharmacophore motif confers to this small molecule its pleiotropic activities and the potential to interact with multiple biological targets. Its first documented action is to translocate protons across the mitochondrial membrane, resulting in mild mitochondrial uncoupling. This action is sufficient to kill tapeworms in the gastrointestinal tract but is generally well tolerated by human cells. In addition, niclosamide modulates Wnt/β-catenin, signal transducer and activator of transcription 3 (STAT3), mammalian target of rapamycin (mTOR), nuclear factor-kappa B (NF-κB), transmembrane protein 16 (TMEM16), and Notch signaling pathways (Chen et al., 2018). Since these molecules drive the transcription of multiple genes, it is possible that the broad biological activity displayed by the compound is the result of direct or indirect effects on these signaling pathways. Thanks to its pleiotropic actions, in recent years, niclosamide has been repurposed for several diseases. Preclinical validation proved that niclosamide has efficacy against solid cancers, rheumatoid arthritis, and fibrotic conditions, and it is currently in phase II–III clinical trial for metastatic colorectal cancer, prostate cancer, and coronavirus disease 2019 (COVID-19; Singh et al., 2022).

Drug repurposing, or drug repositioning, is a strategy for identifying new uses outside the scope of the original medical indication for approved drugs, providing them a new intellectual property. This strategy offers various advantages; in particular, the risk of failure is lower because the repurposed drug has already been found to be sufficiently safe, at least at established doses, in humans. The number of off-patent drugs is becoming much higher than new drugs, leading to new patents repurposing of previously developed compounds. The repurposing strategy has the potential to result in a less risky and more rapid return on investment, with lower average associated costs once failures have been accounted for. Drug repurposing is a desirable approach for neurological diseases for scientific and commercial reasons. It indeed enables the rapid assessment of drug efficacy in phase II trials owing to the established safety profiles of these drugs at previously validated doses and routes of administration.

The rationale for using niclosamide to treat neurological diseases has been frequently proposed in the literature (Massey and Robertson, 2018). It is supported by the notion that it is a small lipophilic molecule, highly predicted to cross the blood-brain barrier and to be not a substrate for glycoprotein p (www.drugbank.com), a significant obstacle for drug delivery to the central nervous system (CNS). The compound displays anti-inflammatory effects in the CNS, as demonstrated in primary microglia cells, where niclosamide was shown to reduce the pro-inflammatory markers NADPH oxidase 2, mTOR, and NF-κB, as well as reactivity-related parameters as cytoskeletal rearrangements, migration, and phagocytosis (Serrano et al., 2019), suggesting its potential use for dampening neuroinflammation.

The beneficial anti-inflammatory effect of niclosamide was proved in models of intracerebral hemorrhage, an event characterized by a strong neuroinflammatory component. In this model, the drug reduced the expression of pro-inflammatory cytokines in microglia in vitro and ameliorated brain edema, decreased neuroinflammation, and improved neurological deficits in mice. The authors demonstrated that niclosamide, being a mild mitochondrial uncoupler, conferred protection by enhancing AMPK activation. This pathway promotes anti-inflammatory responses in microglia, suggesting a potential role of the drug in treating this condition (Pan et al., 2020). Since inflammation in the brain persists several days after intracerebral hemorrhage, we envisage that niclosamide could be useful to control the evolving secondary injury in people with intracerebral hemorrhage.

Emerging evidence indicates the potential niclosamide efficacy with chronic, uncurable neurodegenerative conditions. In models of PD, it has been recently reported that niclosamide exerts neurotrophic actions by increasing neurite length in SH-SY5Y cells and primary dopaminergic neurons. Moreover, it prevents neurodegeneration induced by the neurotoxins MPP⁺ and 6-hydroxydopamine and by overexpression of α-synuclein (Goulding et al., 2021). In primary neurons, niclosamide is capable of activating the PTEN-induced kinase 1 (PINK1), through the reversible impairment of the mitochondrial membrane potential (Barini et al., 2018). Existing therapies for PD are symptomatic, essentially consisting of dopaminergic drugs, to compensate for the lack of dopamine release. Given the limited efficacy of these treatments, the possibility to counteract PINK-related mitophagy is currently proposed as an effective strategy to protect neurons from degeneration. In this scenario, niclosamide could represent an alternative or secondary drug whose effects in PD warrant further investigation in in vivo models of the disease.

The first study identifying a potential beneficial role of niclosamide in ALS and spinal cord injury, both characterized by robust neuroinflammation and glial scar formation, was published in 2014 by Natarajan and co-authors. They demonstrated that STAT3 inhibitors, such as niclosamide, enhanced motor neuron differentiation in human neural stem cells as shown by homeobox gene Hb9 increase. Interestingly, the enhanced motor neuron differentiation was accompanied by a decrease in the glial fibrillary acidic protein-positive astrocytes, suggesting that the modulation of STAT3 signaling could support the control of the excessive gliogenic environment and enhance neural repair (Natarajan et al., 2014). Besides analgesics, the current treatments for spinal cord injury (corticosteroids and NSAIDs) are limited to anti-inflammatory actions and in the long run, can have serious side effects. The combined action of niclosamide on two key pathological mechanisms of the disease such as motor neuron degeneration and glial scar formation suggests the compound as a potential alternative candidate to the drugs proposed so far.

Niclosamide showed beneficial effects in human-induced pluripotent stem cell-derived neurons expressing TAR DNA-binding protein 43 (TDP-43), an ALS-linked gene. Acting on the STAT3 pathway it prevented TDP-43 cytoplasmic mislocalization, TDP-43 insolubility, and attenuated morphological changes under stressors agents. Furthermore, niclosamide activated mitophagy via the PINK1-parkin-ubiquitin pathway, suggesting that niclosamide could be effective in ALS for its role in contrasting microglia over-reactivity (Serrano et al., 2019), and for its capability to interfere with pathological hallmarks of the disease, as the formation of the widespread TDP-43 aggregates (Kato and Sakamoto, 2021).

The valuable effects of niclosamide in ALS models were also demonstrated in patients’ primary cells, in particular in fibroblasts bearing the C9orf72 hexanucleotide repeat expansion, responsible for most ALS familial cases, and in those with no recognized ALS mutations. In these cells, niclosamide reduced inflammatory, autophagic, and profibrotic mechanisms and interfered with different pathogenic pathways and markers associated with the disease, such as mTOR, sequestosome 1 (SQSTM1/p62), STAT3, α-smooth muscle actin, and NF-κB. Significantly, the in vivo treatment with niclosamide of a transgenic ALS model linked to fused in sarcoma (FUS) gene, provided positive results at different levels. It reduced the expression of the neuroinflammatory and fibrotic markers S100A4, α-smooth muscle actin, and platelet-derived growth factor receptor β in the spinal cord. It diminished gliosis in central and peripheral nervous tissues, along with axonal impairment. In addition, niclosamide treatment displayed beneficial effects on muscle atrophy by promoting muscle regeneration and reducing fibrosis. These results highlight that the drug could represent a promising pharmacological tool for ALS, targeting multiple systems involved in the disease (Milani et al., 2021).

As for other neurodegenerative diseases, the finding of an effective therapy still represents an unmet need in ALS research. Unfortunately, the existing drugs to counteract ALS, i.e., riluzole and edaravone, acting respectively on glutamatergic neurotransmission and oxidative stress, display only mild effects on disease progression. In this context, it might be worth investigating the combined effects of one of these drugs with niclosamide since they target different mechanisms and therefore could provide synergistic results.

Besides the actions on the CNS, the beneficial neuroprotective effects of niclosamide have been demonstrated in peripheral neuropathies, as neuropathic pain induced by oxaliplatin during chemotherapy. In neuron-like cells, niclosamide downregulated the production of H₂O₂ mediated by oxaliplatin, thereby preventing cell death. In a mouse model of neuropathy, niclosamide prevented tactile hypoesthesia, thermal hyperalgesia, and abrogated membrane hyperexcitability. It also improved intraepidermal nerve fiber density reduction and demyelination. Niclosamide decreased oxaliplatin-induced interleukin-6, tumor necrosis factor-α, and advanced oxidation protein products, overall reducing oxidative stress and neuroinflammation (Cerles et al., 2017). Consistently, in the Drosophila model of peripheral neuropathy induced by chemotherapy, niclosamide significantly ameliorated thermal hyperalgesia by decreasing mitochondrial oxygen reactive species and aberrant mitophagy in neurons in a PINK1-dependent manner, suggesting its potential use for the treatment of chemotherapy-induced peripheral neuropathy (Jang et al., 2022).

In conclusion, the widely recognized niclosamide safety, tolerance, and pharmacokinetic profiles at validated dosages render the drug repurposing approach advantageous in terms of costs, time, and effort concerning novel drug development useful to treat different aspects of central and peripheral neurological diseases (Figure 1).

Figure 1.

Niclosamide as a repurposing strategy in neurological diseases.

Schematic illustration of in vitro and in vivo actions of niclosamide suggesting its potential use in amyotrophic lateral sclerosis (ALS), intracerebral hemorrhage (ICH), neuropathic pain (NP), Parkinson’s disease (PD), and spinal cord injury (SCI). GFAP: Glial fibrillary acidic protein; FUS: fused in sarcoma; ROS: reactive oxygen species; TDP-43: TAR DNA-binding protein 43.

Indeed, since it is now clear that a possible treatment for neurological conditions has to be multifunctional and multitarget, the use of a pleiotropic compound such as niclosamide, capable of affecting NF-κB, STAT3, autophagy-related mechanisms, fibrosis and neuroinflammation meets these requirements. Remarkably, it could be exploited in patients with familial or sporadic forms of nervous system pathology, as an alternative or in cotreatment with currently used drugs. Notwithstanding the promising data generated in preclinical models, proof of efficacy trials and safety data are still required for each potential application in accordance with the dosages, bioavailability, and ways of administration required to obtain satisfactory clinical outcomes. Keeping in mind these caveats, the repurposing of an “old” drug like niclosamide for a “new” therapeutic purpose, i.e., neurological disorders, appears to be a viable strategy to slow down the disease progression and deserves further investigations by the scientific community.

This work was supported by “ReNicALS” grant from AriSLA – Fondazione Italiana di ricerca per la SLA to SA.