Niclosamide repositioning for treating cancer: challenges and nano-based
drug delivery opportunities
Eduardo José Barbosa a, Raimar Löbenberg b, Gabriel Lima Barros de Araujo
a*, Nádia Araci Bou Chacra a
aDepartment of Pharmacy, Faculty of Pharmaceutical Sciences, University of São Paulo, São Paulo, Brazil
bFaculty of Pharmacy and Pharmaceutical Sciences, University of Alberta, Edmonton, Alberta, Canada
* Corresponding author
Email address: [email protected]
Drug repositioning may be defined as a process when new biological effects for known drugs are identified, leading to recommendations for new therapeutic applications. Niclosamide, present in the Model List of Essential Medicines, from the World Health Organization, has been used since the 1960s for tapeworm infection. Several preclinical studies have been shown its impressive anticancer effects, which led to clinical trials for colon and prostate cancer. Despite high expectations, proof of efficacy and safety are still required, which are associated with diverse biopharmaceutical challenges, such as the physicochemical properties of the drug and its oral absorption, and their relationship with clinical outcomes. Nanostructured systems are innovative drug delivery strategies, which may provide interesting pharmaceutical advantages for this candidate. The aim of this review is to discuss challenges involving niclosamide repositioning for cancer diseases, and the opportunities of therapeutic benefits from nanosctrutured system formulations containing this compound.
Keywords: Niclosamide, repositioning, physicochemical properties, cancer, clinical trials, nanostructured systems.
2.Mechanistic insights in the physicochemical properties of niclosamide 4
2.1Crystal stability and relationship with poor aqueous solubility 4
2.2Lipophilicity: lipid-associated oral absorption and drug-likeness considerations .. 7
3.The discovery of niclosamide anticancer activities 10
4.Repositioning clinical studies for colon and prostate cancer 11
5.Nanostructured systems: fundamentals, types, methods of production ……..
6.Niclosamide nano-based drug delivery formulations and prospective therapeutic benefits
The discovery of a new drug and its way to the market can be a risky process that may cost billions to pharmaceutical companies. The compound may fail due to its toxicity or lack of efficacy in clinical trials, besides the long time involved in the process until reaching the market, which is estimated between 10 – 17 years . In this context, to find new uses for existing drugs, generally referred to as drug repositioning, is a strategy that has been gaining attention. It can provide advantages compared to the traditional approach, for instance, the reduced risk of failure due to toxicity. In this case safety data and the pharmacokinetic profile of an approved drug are already known. It is also associated with lower costs and reduced timeline, as the drug has already been evaluated in early stages of clinical trials [1,2]. This process has an estimated cost of around $300 million and may last for about 6.5 years .
This strategy also provides an opportunity to develop innovative formulations, which may provide clinical benefits compared with those of older marketed drugs . In this case, nanostructured systems are especially interesting. The reduction in particle size to the nanoscale promotes changes in physicochemical and pharmacokinetic properties, among them increasing dissolution rate and bioavailability. This is particularly important for problematic
drugs with low solubility and absorption, allowing dose and toxicity reduction . Hence, as different alternatives of creation of the formulation are associated with the development of nanostructured systems , it also brings the perspective of patent protection for innovative products .
In this context, it is noteworthy to mention that there are different terms to express the concept of “drug repositioning”. In the study of Langedijk and colleagues, the authors analysed “repositioning” and other words used in the literature until 2013 . In this study, a great increase was observed in the number of publications using terms related to drug repositioning, after 2010. In addition, equivalent terms have been adopted, including “reprofiling” or “redirecting”, but “repurposing” has been the most common and interchangeable with “repositioning”. However, a common definition was not identified. Among them, terms referring to new therapeutic applications, or new uses for “old” or approved drugs were adopted. Hence, examples of successfully repositioned drugs include sildenafil, previously designed to treat coronary artery disease in the 1980s, but then during Phase I clinical trials, it was discovered that it could treat erectile dysfunction. After its failure in Phase II studies for treating angina, it was approved in 1998 for erectile dysfunction . Another example is thalidomide, initially designed to be used as a sedative, and then shown to treat morning sickness for pregnant women in the 1960s, until its withdrawal from the market due to birth defects. Then, several studies in the subsequent decades reported its anticancer effects, which eventually led to its approval for multiple myeloma in 2006 .
Niclosamide is a drug present in the Model List of Essential Medicines from the World Health Organization , used since the 1960s for tapeworm infection . For adults, the dose administered is 2 g orally, presenting efficacy and safety due to its local action in the gastrointestinal tract . Its mechanism of action is attributed to uncoupling of oxidative phosphorylation in the mitochondria, which interferes with the metabolism of these organisms [10,11]. Recently, new biological activities related to cancer diseases have been attributed to this molecule, which perhaps make this drug one of the most promising candidates for repositioning from new therapeutic indications [12,13].
Nonetheless, such expectations also must be accompanied by rational and realistic evaluation of its potential, since the repositioning process still requires proof of efficacy and safety of the compound. This implies different
challenges to be dealt with, such as the physicochemical properties of the drug, its oral absorption process and clinical outcomes. Therefore, this review aims to discuss challenges involving niclosamide repositioning for cancer diseases, in addition the prospect of therapeutic benefits that nano-based formulations may provide.
2.Mechanistic insights in the physicochemical properties of niclosamide
The development of innovative formulations aiming at drug repositioning requires knowledge of the physicochemical properties of the drug substance, since they are related to the complex drug absorption process, and its distribution in the tissues . These properties greatly influence decisions about the design and development of pharmaceutical products aiming at efficient drug delivery. Their mechanistic understanding provides a scientific basis in drug product performance [14,15].
2.1Crystal stability and relationship with poor aqueous solubility
Niclosamide is composed of yellow pale crystals, and it is practically insoluble in water, and moderately soluble in ethanol, chloroform and ether . This compound presents 327.12 g/mol molecular weight (M.W.) (Fig. 1), and three crystal forms are described in the literature: a hygroscopic anhydrous form, and two monohydrates, HA and HB, with melting points in the range 228- 230 °C (Table 1) [17,18]. The existence of different hydrated forms implies differences in physicochemical properties, such as water solubility and intrinsic dissolution . In aqueous medium, the anhydrous form rapidly tends to convert to HA, which in turn converts to the most stable, and the least water- soluble, HB [17,18]. Conventional analytical techniques, such as thermal analysis, infrared spectroscopy and X-ray diffraction, may be used to characterize these forms [17,18].
Fig. 1. Molecular structure of niclosamide (M.W. 327.12 g/mol).
Table 1. Physicochemical properties of niclosamide.
Crystal form properties
Melting point d
Water solubility e
Anhydrous – 229 13.32
6.89 a 7.25 b
3.91 a 4.45 c
HB 173 230 0.61
a: In silico prediction by Chemicalize.com; b: determined at 25 ºC by capillary electrophoresis ; c: determined at 22 ºC in octanol/water partition ; d: Determined by differential scanning calorimetry (DSC) [17,18]; e: Measured at 25ºC .
According to the Chemicalize.com prediction, the molecule is a weak acid, with higher proportion of anionic species in blood pH (7.4) (Fig. 2). According to the Biopharmaceutical Classification System (BCS), solubility and permeability are two important factors associated with drug absorption by oral route: since the drug is in a solid state, it has to be dissolved in gastrointestinal fluids to be absorbed by intestinal tract into the blood stream [21,22]. Considering the dose of 2 g used for tapeworm infection, niclosamide is classified as a low solubility drug by BCS. In this case, its oral absorption is not an issue when the objective is the local action in the gastrointestinal tract, but obviously becomes a challenge when it is aimed at efficient systemic drug exposure.
Fig. 2. pKa and logD in silico predictions by Chemicalize.com. At pH 6.89, distribution is 50% for neutral and anionic species, and at this pH LogD is 3.6.
Neutral molecules are able to permeate biological membranes, a phenomenon energetically unfavourable to charged species . Then, oral absorption of niclosamide becomes a “double-edged sword”: the compound is a weak acid; therefore it is neutral at low pHs (Fig. 2), which restricts its solubility in the gastric region. On the other hand, as the molecule enters the small intestine, although an increase of pH theoretically favours its solubilization, permeation through enterocyte membranes becomes less favoured with higher proportion of charged species. In addition, no significant difference in solubility was observed in acid medium (0.1 N HCl) and in buffered medium (6.8 pH) , which further increases the challenge for the absorption. In this context, based on its molecular and crystalline structure, and also considering the work of Bergström and colleagues , a hypothesis may be derived to explain the fundamentals involved in niclosamide’s poor solubility in water.
In their study, the relationship between physicochemical properties and aqueous solubility were evaluated using statistical tools for a set of 15 poor soluble drug substances . According to the authors, “brick dust” and “grease balls” are terms generally used to refer to poor soluble compounds: the former refers to those composed of a stable crystalline structure, marked by strong intermolecular bonds that prevent interaction with water, whereas the latter refers to high lipophilic compounds that do not interact with this solvent . In brief, solid-state properties (such as melting point, enthalpy and entropy of melting) showed low correlation, while properties such as lipophilicity and
nonpolar surface area showed a correlation to aqueous solubility, indicating that these drugs would be “grease balls”, with solvation limited solubility .
For niclosamide, the existence of N–H···O intramolecular interaction favours a near planar conformation for the structure, whereas O–H···O (from hydroxyl to carbonyl group) align the molecules along an axis, being connected and stabilized by interactions between Cl···NO2 groups and between aromatic rings, promoting the formation of layered molecular chains [26,27]. The presence of hydrogen-bonding sites (NO2, OH, carbonyl groups), and two Cl atoms contribute to solvation by water, and the anhydrous crystal lattice allows the entry of water or other molecules in the cavities of the structure, forming hydrates or solvates [26,27].
Solvation and disruption of the crystal packing are among the processes related to solubilization of crystalline compounds, which requires energy to occur . Besides, the presence of water molecules may provide stability to a crystal lattice, comparing to the anhydrous form . Thus, a niclosamide crystalline structure might restrict interactions with water and breaking of the crystal, which is reflected in the low solubility of the anhydrous form (13.32 µg/mL). In addition, formation of stabilized hydrates might limit even more niclosamide interactions with water, reflecting the lower aqueous solubility for HA and HB monohydrates (0.95 and 0.61 µg/mL, respectively). Therefore, niclosamide water solubility could also be associated with a solvation limited phenomenon. However, since it presents a stable crystalline structure (M.P. 228-230ºC), its profile could be more similar to tolfenamic acid. As referred to by Bergström and colleagues, specifically for this also small molecule (261.1 g/mol), the combination of lipophilicity and stability of the crystal (M.P. 212 ºC) was likely related to its low solubility .
2.2Lipophilicity: lipid-associated oral absorption and drug-likeness considerations
If niclosamide has poor aqueous solubility, on the other hand, a high lipophilicity profile is indicated by the in silico and experimental logP values (3.91 and 4.5, respectively). These values show that niclosamide solubility is thousands of times higher in the organic phase than in aqueous phase, reflecting a tendency to dissolve in lipids, in crossing biological membranes, and
binding to proteins [29,30]. The LogD parameter also refers to lipophilicity, but the pH variable is included in the calculation. Hence, the value of 3.6 at pH 6.9 (Fig. 2) indicates a decreasing trend in its partition in the lipid phase due to a higher proportion of ionized species at this pH. The high lipophilicity also might favour the development of lipid-based formulations . However, not all poor water-soluble compounds may present good solubility in lipids, since the high stability may also be an obstacle to solubilization in these components . Therefore, care should be taken with the development of the formulation, such as a rational approach for the selection of the excipients, and use of design of experiments (DOE) .
A possible mechanism by which lipid formulations might be associated with niclosamide oral absorption was studied by Alskär and colleagues, when lipolysis-triggered supersaturation and precipitation of eight compounds were evaluated, and correlated with their physicochemical properties . Three types of formulations were prepared: one containing long-chain lipids (soybean oil and Maisine® 35-1), a second containing medium-chain lipids (Capmul® MCM EP and Captex® 355), and a third containing only surfactant and cosolvent (Cremophor® EL and Carbitol™). After in vitro lipolysis tests, using intestinal simulated fluid as medium, solubility and solid characterization were performed. In brief, niclosamide significantly precipitated as a crystalline hydrate after addition of pancreatic extract and, despite the decrease in solubility due to the precipitation, the compound remained in a supersaturated state during the tests .
As a comparison, according to the authors, the compound showed poor saturation solubility in the medium with enzymes (30 µg/ml). However, during lipolysis, the values related to the aqueous phase dropped from 250 to around 80 µg/ml after 60 minutes of test, using the medium-chain rich lipid-based formulation as drug carrier . Obviously that solubility values may vary according to conditions of the test, but this range is clearly higher than a reported value for the most water-soluble anhydrate form (13.32 µg/ml) . Therefore, although the possibility of solid formation and the subsequent dissolution-rate dependence for absorption, lipid-based formulations might improve niclosamide bioavailability due to the ability to maintain some degree of supersaturation in the gastrointestinal fluids . This could be optimized if maintained as long as possible during the passage of the drug throughout the
intestine . Using multivariate analysis, Alskär and colleagues observed that compounds with low molecular weight (< 350 g/mol) and high melting point (> 200 ºC), among them niclosamide and tolfenamic acid, tended to precipitate in crystalline forms, also indicating some similarity between these two molecules related to the solid state .
During the digestion of a formulation, the lipids can be absorbed or partitioned into micelles or mixed micelles, composed of bile salts and/or lipidic excipients [33,34]. Then, the drug released is also subjected to partition into the micellar species formed [33,34]. Thus, supersaturation might not be the only phenomenon related to a possible enhancement of niclosamide oral absorption. For instance, lipid particles can adhere to the membrane of the enterocytes, releasing the drug within the cells, which can be optimized by reducing the particle size, providing a higher surface area for adhesion . Additionally, inhibition of drug efflux transporters by surfactants and other excipients, such as Tween® 80 and Cremophor® EL, were already proposed as a possible mechanism, increasing the permeability and oral absorption of drugs [35-37]. Therefore, these examples reflect the need for a better understanding of the phenomena that may be associated with oral absorption of niclosamide and other drugs.
Regarding the lipophilicity of drugs, increasing logP leads to an increase in the volume of distribution of the compound, and also in central nervous system (CNS) penetration . Nonetheless, a high logP value (> 5) is not desirable for drug candidates because it may be reflected in poor aqueous solubility and absorption, and likelihood of in vivo toxicity, which could compromise approval in clinical trials . Thus, because niclosamide is a drug candidate, the possibility of systemic action raises the question about its drug-like features, that is, if its physicochemical properties are related to likelihood of successful approval .
Based on molecular descriptors and physicochemical properties, different rules of thumb or guidelines have been proposed to guide the design and development of drug candidates [38,39]. One of the first, and most known, was the work of Lipinski, which proposed a cLogP (calculated logP) ≤ 5 for good oral candidates, based on a statistic evaluation of compounds that reached, at least, Phase II clinical trials [38,39]. Another example was the work of Gleeson, who used a set of compounds from GlaxoSmithKline (GSK) to evaluate the influence
of molecular descriptors in ADMET properties (absorption, distribution, metabolism, excretion, toxicity). The author suggested that molecules with both M.W. < 400 and clogP < 4 have a better chance of presenting desirable ADMET properties . Lastly, Waring proposed an optimum range between 1 and 3 and for lipophilicity, since high hydrophilic compounds also present undesirable properties, such as high renal clearance and low permeability and tissue distribution .
Hence, considering cLogP value (3.91), niclosamide would not be so distant from a desirable value for lipophilicity, further regarding LogD 3.6 at pH 6.9. However, obviously the experimental value (4.45), and its water solubility place this candidate in a “not ideal” drug-likeness region, compared to other compounds. This means that high doses might be required to achieve therapeutic blood levels. Hence, it justifies diverse pharmaceutical strategies, such as nanostructured systems, to improve its aqueous solubility and oral absorption.
3.The discovery of niclosamide anticancer activities
A study by MacDonald and colleagues, in 2006, perhaps may be the first that indicated a possible repositioning process of niclosamide for cancer diseases, when unexpected results were revealed for this compound . In this study, protein-protein interactions in HEK 293 cells were evaluated by protein-fragment complementation assays (PCA). The objective was to observe hidden or not expected biological activities from 107 compounds, of different therapeutic classes. Using this approach, drug substances that could present unexpected antiproliferative activities were identified. The action of niclosamide was confirmed in five different cancer cell lines (PC3, A549, MiaPaCa, LOVO and U87MG), with a mean IC50 of 0.6 µM .
Then, several preclinical studies associated its anticancer activity with the inhibition of the signaling pathways Wnt/β-catenin [43-49], STAT3 [50-60], Notch [61-64], NFκB [65-68] and mTOR [69-72], which are cellular mechanisms related not only to different cell functions, but also to pathological conditions when deregulated. The effect in multiple signaling pathways contributes to its action against different types of cancer cells, among them colon, prostate, ovarian, breast and lung [12,13].
4.Repositioning clinical studies for colon and prostate cancer
From 2009 to subsequent years, preclinical studies that evaluated niclosamide against colon and prostate cancer cells provided encouraging results. For colon cancer, its activity was related to inhibition of Wnt signaling, which is an important mechanism for stem cells, associated with self-renew and homeostasis of tissues in adults, but also related to several diseases when aberrantly active [73,74]. Niclosamide then showed its potential for use against colon cancer, including metastatic conditions [43,44,46].
For prostate cancer, promising results were reported by studies that evaluated niclosamide combined with drugs approved by the Food and Drug Administration (FDA) agency. Abiraterone (Zytiga®) is an androgen synthesis inhibitor approved in 2011, while enzalutamide (Xtandi®) is an androgen receptor (AR) antagonist and inhibitor approved in 2012 [75-77]. The main strategy for treating metastatic prostate cancer is castration by surgical or chemical intervention, which reduces androgen levels and, consequently, tumour growth [78,79]. However, in most cases, cellular adaptations involving AR signaling still promote disease progression [78,79]. The use of abiraterone or enzalutamide provides clinical benefits, but therapy resistance may arise and can be associated with AR variants [78-80]. Hence, inhibition of the STAT3 pathway by niclosamide not only provided anticancer effects, but also helped overcome resistance to these drugs [55-57].
Taken together, these results supported and prompted a series of clinical trials that began in 2015 (Table 2), with the first results being published in 2018 (Table 3).
Table 2. Niclosamide repositioning clinical studies for cancer diseases. For this search, the keyword “niclosamide” was used at the ClinicalTrials.gov website.
Type of cancer
Duke University USA (NCT02687009)
Niclosamide (PO QD), during 7 days, prior
to surgical resection of primary tumour.
Dose limiting toxicity, niclosamide blood levels
University of California, Davis
Niclosamide (PO BID) and enzalutamide (PO QD) on weeks 1-4. Treatment repeats
every 4 weeks in the absence of disease
progression or unacceptable toxicity.
Dose limiting toxicity, adverse events, OS, PFS,
PSA response, time to
University of California, Davis
Niclosamide (PO BID), prednisone (PO BID), and abiraterone acetate (PO QD). Treatment
repeats every 4 weeks in the absence of disease progression or unacceptable toxicity.
Dose limiting toxicity, OS, PFS, PSA response,
Niclosamide (PO QD)
until disease progression or toxicity.
OS, PFS, TP,
disease control rate,
University of Washington
Niclosamide (PO TID) and
enzalutamide (PO) for 28 days, until disease
progression or unacceptable toxicity.
Dose limiting toxicity, niclosamide blood levels,
PO: by mouth, QD: once a day, BID: twice a day, TID: three-times-daily, OS: overall survival, PFS: progression-free survival, PSA: prostate-specific antigen, TP: time to progression.
Table 3. Treatments and first results from clinical trials for niclosamide repositioning. The study from the University of Washington was completed in 2017 and published in 2018 . University of California, Davis  and Charité University  published preliminary results in 2018.
Study (niclosamide treatment)
Niclosamide Blood levels (ng/mL) Target Achieved
University of Washington (500 – 1000 mg/PO TD)
No PSA reduction
University of California, Davis
(1600 mg/PO TD)
Niclosamide, abiraterone, prednisone
100-162 ** Nausea, diarrhea
PSA reduction for at least four patients
Charité University (2000 mg/PO QD)
No drug related toxicity reported
Possibility of longer
time until disease
* Corresponds to Cmax values. ** Corresponds to trough levels. PO: by mouth, QD: once a day, BID: twice a day, TID: three-times-daily; PSA:
The first study completed was from the University of Washington (NCT02532114), in which relevant findings were published by Schweizer and colleagues . In this study, niclosamide was administered in soft gelatin capsules of 500 mg, for patients with castration-resistant prostate cancer (CRPC), previously treated with abiraterone. Two dose levels were assessed: 500 and 1000 mg, three-times-daily (TID) each. According to the authors, due to the poor oral bioavailability typically reported for niclosamide, the objective was to administer higher doses of the compound than those used for tapeworm infection (2 g/day).
Briefly, combined treatment was well tolerated with the 500 mg dose regimen, but the two patients treated with 1000 mg dose level presented adverse events, among them nausea, colitis and diarrhea, which were to some extent already predicted, as this dose level was clearly higher than those normally used for helminthic infection . As previously mentioned, for adults the treatment is 2 g in a single dose, and in the case of Hymenolepis nana infection, it is followed by 1 g daily for 6 days . Particularly, for one of the patients, symptoms began on day 26, suffering from diarrhea lasting > 72 hours, whereas for the other, abdominal pain and diarrhea started on day 8, leading to hospitalization and medical care. Thus, the 500 mg dose was
considered the maximum tolerated dose . Complementary studies showed that even after administrating high doses of niclosamide no clinical evidence of activity is observable . This fact can be assigned to the inability to overcome the poor bioavailability and consistently achieve the plasma concentrations necessary to inhibit tumour growth, based on CRPC models [55-57]. However, despite the clinical failure of the compound, the authors recognized some limitations of the study, such as the small number of patients and non-evaluated drug-drug interaction and the need for the development of alternatives with improved oral bioavailability . In this context, as long as niclosamide shows striking anticancer activity, a better understanding of the potential for repositioning can be obtained from the ongoing clinical trials.
What reinforces this argument is that different findings were described in preliminary results from the University of California, Davis (NCT02807805) (Table 3), when niclosamide was evaluated in combination with abiraterone and prednisone . In a preliminary phase, CRPC patients were evaluated, with dose escalation of niclosamide from 400 mg PO BID to 1,600 mg PO TID (Table 3). According to results, the 1,600 mg PO TID regimen was well tolerated in five patients, considering the recommended dose for Phase II trial, which is, intriguingly, even higher than those assessed in the previous study from the University of Washington, with nausea and diarrhea being reported.
Although the niclosamide blood levels are in a range comparable to the failed study from the University of Washington (Table 3), the trial from the University of California used “trough level” as a pharmacokinetic parameter, which refers to the lowest blood concentrations during therapeutic drug monitoring, obtained at the end of a dose interval . Thus, these Universities adopted different strategies to address the relationship between the anticancer activity and niclosamide blood levels. The University of California trial emphasized that therapeutic blood levels are achievable. Hence, this might indicate that higher concentrations can be obtained, that would be necessary for some anticancer effect. An indication for this is that, in this study, PSA reduction was reported for two patients (<0.01 ng/mL). In addition, two other patients presented reduction of ≥ 50% in PSA response . In summary, the authors considered that the combination of niclosamide with abiraterone and prednisone presented promising preliminary safety and efficacy results .
For colon cancer, initial results from the Cherite University trial (NCT02519582) (Table 3) were described in the study of Burock and colleagues (2018) . Patients with metastasized colorectal cancer received 2 g of niclosamide in tablets (Yomesan®), once a day, until observation of disease progression or toxicity. In brief, no toxicity was reported. In addition, Cmax values were clearly higher than values from the University of Washington trial (Table 3), which further indicates that higher niclosamide blood levels might be achieved. A patient with the highest median plasma level (598 ng/mL) showed stable disease at 4 months and, according to the authors, preliminary results indicated that those with higher plasma concentrations might present longer time until disease progression, justifying more investigation .
In summary, niclosamide showed consistent anticancer activity in preclinical studies, which justified initiating clinical trials for its repositioning. Initial clinical results might show the importance of the blood levels to present some effect against colon and prostate cancer. However, high doses were administered to achieve target concentrations, as predicted by its physicochemical properties. This raises the question, in the case of approval, as to which doses will be therapeutically applied. Although it is well tolerated when used for tapeworm infection, generally a single dose is administered in these cases. Thus, the impact of its administration during longer periods for patients with cancer diseases has still not been clarified. In this case, nausea, diarrhea and gastrointestinal irritation will likely be commonly observed adverse effects. Therefore, taken together, these considerations further justify the need for nano-based drug delivery systems.
5.Nanostructured systems: fundamentals, types, and methods of production.
According to FDA guidelines for the industry, materials in the nanoscale range can exhibit different physical or chemical properties, or biological effects, differing from their larger counterparts . This guidance defines these systems as 1) those that have, at least, one of their dimensions in the range from 1 to 100 nm; or 2) if the dimensions are in the range from 100 to 1,000 nm, different properties or effects are attributed to their size . Another feature
commonly described in the literature is that they have a higher surface to volume ratio . This means that the reduction of size provides a higher surface area for the system, compared to the bulk material, which also means that a higher proportion of atoms or molecules are present on the surface of the particle . Hence, at the nanoscale, altered properties, such as optical, electrical, magnetic and thermal, can emerge, which can be explored for pharmaceutical or biomedical applications, for instance as therapeutic or diagnostic purposes [86-88].
One example is related to drug substances in their pure solid state, when the reduction of particle size provides physicochemical improvements supported by the Noyes-Whitney equation (1) :
According to this model, dC/dt is the dissolution rate, S is the surface area, D is the diffusion coefficient, h is the diffusion layer thickness, V is the volume of dissolution, Cs is the saturation solubility, and C is the concentration at time t . Then, an increase of the surface area enhances dissolution rate. Another commonly reported effect is that particle size also affects the diffusion layer thickness and saturation solubility [89,90]. Thus, increased saturation solubility and dissolution rate are two important features of solid particles at the nanoscale, which is a valuable strategy to improve absorption and bioavailability of poor water-soluble drugs.
Other advantages are usually attributed to nanomaterials. One example is that a higher surface area contributes to adhesiveness of the particles to biological membranes, which can improve their uptake by cells . In addition, incorporation of drugs into nanocarriers can reduce degradation and toxicity . Drug targeting, in turn, consists of coating the surface of nanocarriers with ligands that bind to specific receptors, generally overexpressed by target cells, which is useful for cancer diseases . This strategy, therefore, can provide specificity for treatment, with the prospect of reducing damage to normal tissues .
However, if nanoparticles provide interesting advantages for pharmaceutical products, on the other hand one of the biggest challenges related to their development is the stability of the system. The higher surface to volume ratio also means that the higher proportion of chemical species on the surface are not surrounded by their counterparts (comparing to the inner species of the particle), being in contact with the external medium . Therefore, these systems present high total surface energy, being thermodynamically unstable, and phenomena like agglomeration and Ostwald ripening reflect the tendency of the particles to aggregate, increasing the particle size . Thus, the use of polymers (to promote steric hindrance) and/or surfactants (to provide electrostatic repulsion or reduction of interfacial tension) are strategies usually adopted to preserve the stability of the system .
Different strategies have been adopted to develop nanostructured systems. Using nanocrystals is the simplest approach. As previously mentioned, this consists of reducing the particle size of the micronized compound to the nanoscale, being stabilized in water by the presence of polymers or surfactants . Among the advantages related to them is the simplicity of the formulation, and the fact that established techniques for large- scale production are used for their preparation, which supports their approval for clinical use [90,96]. In fact, different products are already on the market . One marked feature is related to nanocrystals: the formulation does not require incorporation of the drug into a matrix system, providing then 100% drug loading to the nanoparticle .
The other types may be defined as matrix systems or nanostructured carriers. These structures are classified differently, such as lipid-based, polymeric-based, carbon or magnetic based . Briefly, lipid-based formulations include nanoemulsions, composed of mixture of two immiscible liquids, with submicron droplets of a liquid dispersed in another liquid, stabilized by the presence of surfactants . The two types of nanoparticles that are solid at ambient temperature are: solid-lipid nanoparticles, composed of a solid lipid matrix containing the drug; and the nanostructured lipid carriers, composed of a mixture of solid and liquid lipids [98,99]. Polymeric nanoparticles include the use of natural or synthetic polymers to encapsulate drugs in vesicular reservoirs
or in solid polymeric matrix , whereas polymeric micelles are composed of amphiphilic polymers (two or more in the case of mixed micelles) which self- assemble into nanocarriers . Still considering polymers, the application of a strong electric field is used to produce nanofibers, which can also act as drug carriers . Examples of carbon derived nanomaterials include nanotubes, graphene and fullerenes, which can also be functionalized with ligands . Magnetic nanostructures are composed of metal or metal oxides, which are directed to the targeted tissue by an external magnetic field .
Two types of methods of production are used to prepare nanostructured systems: bottom-up and top-down. A third method is a combination of these two [105,106]. In bottom-up techniques, the drug are dissolved in a solvent, and the production of nanoparticles are carried out by its precipitation, by adding an antisolvent. The advantages include the low cost and use of low energy for preparation. On the other hand, drawbacks include the use of toxic solvents, and the challenge of large-scale production [105,106]. In contrast, top-down techniques start from the micronized drug, and application of mechanical forces reduces the particle size to the nanoscale. Therefore, these are high energy techniques [105,106]. The disadvantages include the use of high energy equipment, and possibility of alteration of the crystal form of the starting compound. Among the advantages, established techniques (media milling and high pressure homogenization) have good reproducibility, with the prospect of industrial production. The combination of these techniques is generally done by preparing nanoparticles through a bottom-up technique, and then reducing the size by a top-down method [105,106].
6.Niclosamide nano-based drug delivery formulations and prospective therapeutic benefits
Table 4 presents research findings on nano-based formulations containing niclosamide. From a total of 17 studies, distribution is: 3 included nanocrystals [116, 122, 123]; 6 included polymeric nanoparticles [108, 110, 117, 119, 120, 123]; 3, lipid nanoparticles [107, 111, 121]; 3, nanofibers [112, 114, 118]; 2, micelles [109, 115]; and 1, carbon nanoparticles . In these studies, niclosamide also presented in vitro and in vivo anticancer activity when present
in formulations. However, promising improvements in pharmacokinetic parameters were described in some of these studies. In addition, considering
niclosamide’s diverse anticancer activity, the possibility of synergy with other compounds and drug targeting were also explored.
able 4: Nanostructured systems containing niclosamide.
Type of nanoparticle
(method of preparation)
PDI Z.P. (mV)
Stearylamine, polysorbate 80,
112.18 ± 1.73 0.417 ± 0.026 +23.8 ± 2.7
82.21 ± 0.62
~ 18 *
100 ± 25
~ 2 (MCF-7, MDA-MB231)
tumour growth from MCF-7 cells in mice
Self-assembly polypeptidic micelles
Reduced tumour volume from HCT116 cells in mice
Chitosan, polysorbate 80,
glutaraldehyde, sodium sulfate, sodium metabisulfite
7.5 (MCF-7), 8.75 (A549)
polysorbate 80, PEG400
204.2 ± 3.2
0.328 ± 0.02 –33.16 ± 2
89.1 ± 0.03
11.08-fold increase in bioavailability in rabbits,
compared to marketed drug
1.24 (A549), 1.21 (MCF-7)
Pristine carbon nanoparticles
Agave nectar, cucurbituril
88 ± 5
–18 ± 5
21 ± 2 (MCF-7)
Reduction of 50%
in tumour size in mice,
comparing to control
Nanofibers crosslinked with magnetic nanoparticle
( electrostatic spinning )
bPEI, PEO, Fe3O4, folic acid,
655 ± 76
Pluronic® , biotin
31.8 ± 1.7
–3.37 ± 1.08
91.9 ± 1.9
< 0.9 (A549)
105 ± 21
3.59 (CP70), 3.38 (SKOV-3)
Reduced tumour growth from CP70 and SKOV-3
cells in mice
5 (A549), 2.6 (MCF-7)
(electrostatic spinning )
bPEI, PEO, glutaraldehyde
+6 to +12
3.129 (A549), 2.147 (U87MG)
1 ± 0.5 (MDA-MB231),
90 ± 10
30 ± 5 (MCF-7), 20 ± 5 (SKBR-3),
5 ± 1 (BT549)
12 ± 2 (MDA-MB231), 5 ± 1 (MCF-7),
1 ± 0.5 (C32)
a: PEG; b: without PEG (melt dispersion / HPH)
PEGM, oleic acid, MCT, egg lecithin, soybean lecithin,
polysorbate 80, glycerin.
a: 162.2 ± 3.8 b: 307.8 ± 5.2
a: –21.8 ± 0.24 b: –20.8 ± 0.45
a: 9.12 *** b: 9.06 ***
~5-fold increase in
bioavailability, compared to
drug solution, in rats
Nanocrystals (wet milling)
Polysorbate 80, poloxamer 188,
PVP, SDS, TPGS
Delayed tissue distribution, compared to drug solution,
a: 2.44 (CP70),
a: nanocrystals b: polymeric (electrospray)
b: PLGA ****
a: 105 ± 21 b1: 662 ± 121 b2: 584 ± 110
5.52 (SKOV-3); b1: 1.37 (CP70),
7.81 (SKOV-3); b2: 5.55 (CP70), 7.45 (SKOV-3)
PDI: Polydispersity index; Z.P.: zeta potential; E.E.: encapsulation efficiency; PEGCE: Polyethylene glycol cetyl ether; PS-b-PAA: poly(styrene)-block-poly(acrylic) acid; bPEI: branched poly(ethylenimine); MCF-7, MDA-MB231, SKBR-3 and BT549: breast cancer cells; HCT116: colon cancer cells; A549: lung cancer cells; PEG400: Polyethylene glycol 400; AUC: area under the curve; PEO: poly(ethylene oxide); PBS: phosphate-buffered saline; PVA: poly(vinyl alcohol); CP70 and SKOV-3: ovarian cancer cells; U87MG: brain cancer cells (glioma); C32: skin cancer cells (melanoma); HPH: High Pressure Homogenisation; PEGM: Polyethylene glycol monooleate; MCT: medium-chain triglyceride; PVP: polyvinylpyrrolidone; SDS: sodium dodecyl sulfate; TPGS: Tocopheryl polyethylene glycol succinate; EC9076: esophageal cancer cells (carcinoma); PLGA: poly lactic-co-glycolic acid. *: Measured as cytotoxic concentration (CTC50); **: Percentage of L132 and KB cell viability were compared with formulations, not as a function of the concentrations. ***: Refers to drug loading. ****: b1 refers to single and b2 refers to dual-capillary electrospray system.
Examples of pharmacokinetic improvements were reported by Zhang and colleagues when nanoemulsions were designed for evaluating niclosamide pharmacokinetic properties in rats . In this study, two types of nanoemulsions were prepared: with and without poly(ethylene glycol) monooleate (PEGM), in order to assess the ability of this polymer to provide “stealth” properties, that is, to avoid lipolysis by digestive enzymes. Among the results, no significant differences were observed in the lyposis test, considering the presence of PEG, which the authors associated with the insufficient molecular mass of the polymer (~1200) in to promote “stealth” properties.
Regarding pharmacokinetic evaluation, niclosamide was administered by oral gavage in Sprague-Dawley rats (250 ± 20 g), at the dose of 20 mg/kg. Then, Cmax values for nanoemulsions were 0.726 and 0.432 µg/mL, for with and without PEGM, respectively, whereas 0.195 µg/mL for the suspension . Despite differences in particle size between the nanoformulations, similar bioavailability values were observed: ~2.5 (AUC0-t µg.h/mL) for nanoemulsions, whereas ~0.5 for the suspension, resulting in a 5-fold increase. Hence, considering the possible mechanisms related to niclosamide oral absorption, these results support the prospect of increasing bioavailability and reducing the dose by lipid-based matrix systems.
The authors still note that while blood levels in rats are not directly related to humans, a low dose (20 mg/kg) was administered to the animals, which for them could be an indication that target blood levels in humans could be easily achieved. In this context, the pharmacokinetic of niclosamide had also been evaluated in a preclinical study from Duke University . In this study, niclosamide was mixed into a polyethylene glycol system (90% polyethylene glycol-300, 10% 1-methyl-2-pyrrolidone) for oral administration of 200 mg/kg, using NOD/SCID mice (23-25 g) as in in vivo model. Thus, Cmax was 0.893 µg/mL (tmax = 15 minutes), whereas blood levels ranged from ~0.04 to ~0.08 µg/mL (0.5 to 12h). According to the authors, high doses were administered to the animals, which for them could be associated with complex absorption behavior of the compound in different regions of the gastrointestinal tract .
Lipid-based nanoformulations were also designed and prepared in Rehman et al. . In this study, solid lipid nanoparticles were prepared by micro-emulsion technique, using stearic acid (SA), polysorbate 80 and
polyethylene glycol (PEG). Four variables were evaluated: concentrations of SA, polysorbate-80 and PEG, and stirring time. Then, optimized formulations were prepared varying concentrations of the drug. The objective was to evaluate the phamacokinetic profile of the compound by oral route in rabbits (2 ±0.3 kg), comparing with marketed drug (Mesan®). In brief, Cmax values for the nanoparticles and marketed drug were 3.97 and 1.84 µg/mL, respectively,
whereas reported bioavailability (AUC0-t µg.h/mL) were 16.74 for
nanoformulation, and 1.51 for Mesan® .
One of the justifications for this study was that the solid lipid nanoparticles could improve absorption of niclosamide to the lymphatic system, reducing the first-pass effect by the liver. This possibility is attributed to lipid formulations in general, and it is based on the way that lipids from the diet are digested, and then absorbed in the intestine . This phenomenon is mediated by the synthesis and secretion of chylomicrons by enterocytes, which secrete the lipoproteins to the lymphatic capillaries. The hypothesis for the absorption and transport of drugs by lymphatics is that, after the digestion of the lipid components of the formulations (a combination of lipids, surfactants and or co- solvents), drug and lipids (fatty acids and monoglycerides) are uptaken by the enterocytes [124-126]. Then, long-chain triglycerides are synthetized in the cell and usually incorporated into chylomicrons, along with the drug and proteins. Thus, after secretion by the enterocytes, the size of these large lipoproteins limits their entry into blood vessels, which favours the entry into lymphatic capillaries. Hence, it is believed that drug and chylomicrons follow the unidirectional flow of the lymphatic vessels, reaching the systemic blood circulation by subclavian vein [124-126]. Therefore, despite a possible crystalline solid precipitation, lipid-based formulations might enhance niclosamide oral absorption not only due to phenomena such as supersaturation in intestinal fluids, but also due its partition in the lipid phase. In this case, the possibility of solubilization in triglycerides and incorporation into chylomicrons could favour its absorption toward the lymphatic system. This could be a strategy to achieve therapeutic benefits, as increasing bioavailability, reduction of the first pass-effect by the liver and drug-drug interaction.
Still considering lipid-based formulations, a pharmaceutical advantage that might be achieved could be related to the crystal transformations of
niclosamide. This phenomenon might be critical to nanocrystals, since the lack of a matrix allowed the nanosized crystalline structure to be in contact with water, being subject to hydration. As previously discussed, this implies changes in physicochemical properties, such as saturation solubility and intrinsic dissolution. Considering the in silico and experimental logP values (3.89 and 4.45, respectively), niclosamide has a much higher tendency to dissolve in lipid phase than in aqueous phase. Hence, incorporation and solubilization of this drug substance in a lipidic matrix system could diminish the influence of niclosamide crystal transformations (anhydrous to monohydrate HA, and then to HB) in stabilizing the nanoparticles. On the other hand, possible disadvantages include lower drug loading, compared with nanocrystals, and the required compatibility among the excipients. In the case of solid lipid nanoparticles, polymorphic changes of the solid lipid may be a critical factor in stabilizing the system .
Considering nanocrystals, this strategy was presented in the work of Lin and co-workers, when niclosamide nanoparticles were prepared by electrospray technique to evaluate its anticancer activity against ovarian cancer cells . The nanosuspension contained 1% polyvinyl alcohol (PVA) in a phosphate- buffered saline solution, and formulation test presented an average particle diameter of 105 nm. Among the results, the nanosuspension was able to suppress the metabolism and the in vitro growth of CP70 and SKOV3 cells, with IC50 (µM) of 3.59 and 3.38, respectively. Besides, using NOD/SCID mice, and after administration of 100 mg/kg of nanoniclosamide by oral gavage, reduced tumour growth was observed in the animals .
In this study, they also evaluated the pharmacokinetic profile of niclosamide. Using Sprague-Dawley rats, nanocrystals dispersed in PBS solution (0.46 mg/mL) were administered by oral gavage (5 mg/kg) or intravenous injection (2 mg/kg). In brief, the estimated bioavailability by oral route was 25%. According to the authors, despite the better results compared to a reported 10% value, they emphasized the need for improvement, considering its potential for ovarian cancer treatment . Nonetheless, if this difference was maintained in humans, target blood levels could be more feasible to achieve, allowing the prospect of reducing the dose. Thus, strategies based on
nanocrystals might work as if they “brought” the compound closer to a desirable drug-likeness region.
Still considering their study, a second peak was observed in the plasma concentration profile by oral and IV route, which for them could be associated with an enterohepatic circulation phenomenon . In the study of Duke University, the authors associated the oral absorption of the compound in rats with complex behavior in different regions of the gastrointestinal tract, but which was less likely to be related to enterohepatic circulation . Lastly, in the study of Zhang and colleagues, a similar pattern, with a second peak, is also observed in the pharmacokinetic profiles of the nanoemulsions, although a hypothesis for this phenomenon was not discussed .
Drugs that reach the bloodstream are eliminated mainly by two pathways: by the kidneys or the liver. In the latter case, compounds may be removed in an unchanged form or as metabolites, and some of them may be excreted via bile . Hence, enterohepatic circulation is the elimination of drugs or other substances from the bile to the small intestine, which are available for reabsorption, being subject to new elimination by the liver or reaching systemic circulation [127,128]. Examples of cases already observed in humans include the drugs diltiazem, irinotecan, nevirapine, meloxicam and piroxican [127,128]. Molecules that undergo this phenomenon usually show multiple peaks in plasma concentration versus time profiles. However, obviously drug metabolism in animals is not directly related to humans, and it is difficult to attribute a single cause for multiple peaking in pharmacokinetic profiles .
Therefore, it is still not clarified if niclosamide is subject to enterohepatic circulation. If it is another challenge for its oral absorption, bioavailability improvements provided by nanostructured systems might be achieved by two different strategies. Nanocrystals may promote a higher amount of dissolved drug that reaches the portal circulation, that might provide therapeutic concentrations more easily. The other strategy is the drug delivery based on lipid formulations. This could contribute to niclosamide absorption not only due to supersaturation, but also due to the transport by the lymphatic system, reducing the first-pass effect by the liver. These considerations are summarized in Fig. 3.
Fig. 3. Summary of nanocrystals and lipid-based strategies for niclosamide. Nanocrystals may improve the oral absorption due to an increase in dissolution rate and saturation solubility, and the enhanced surface area that favours adhesion to cell membranes. It does not prevent metabolization by the liver, but provides a higher amount of drug that reaches systemic circulation. Reduction of particle size of lipid-based formulations may improve adhesion to cell membranes and, after digestion, supersaturation provides a higher amount of solubilized drug available for absorption. This strategy is also associated with the hypothesis of targeting the lymphatic system, which is based on the incorporation of drugs into chylomicrons that, after secretion by enterocytes, enter lymphatic capillaries and follow the unidirectional flow of the vessels, reaching systemic circulation by the subclavian vein. M.W.: molecular weight; FA: fatty acids; MG: monoglycerides; TG: triglycerides. M.W., pKa and logP values obtained by Chemicalize.com.
A particular feature of niclosamide that may be explored, and which does not involve oral absorption necessarily, is its diversity for anticancer activity. As previously shown, this compound presents actions in different signaling pathways. Hence, this might provide synergistic effects with other compounds, which was observed not only with abiraterone and enzalutamide, but also with erlotinib, a tyrosine kinase inhibitor used for lung cancer [51,52]. The therapeutic benefits include overcoming of drug resistance and reduction of dose [129,130]. Besides, this may reinforce the argument for the repositioning
process, further considering that a combination therapy is a common approach for serious diseases like cancer [130,131].
This idea was evaluated in the study of Misra and colleagues, when polymeric nanoparticles were prepared to encapsulate niclosamide, a STAT3 blocker, and amonafide, a topoisomerase-II inhibitor, using them against triple negative breast cancer (TNBC) cells . The strategy was to prepare
“hyperstar polymers” (HSP) as nanocarriers, when hyperbranched macro- initiators were polymerized to produce a core-shell structure to contain the two compounds. The spherical nanostructure was composed of protonable tertiary amine groups in the shell (for water dispersion), and acid-degradable acetal groups in the core (for drug release under acid conditions). Then, the nanoparticles were compared with the compounds alone, and with a conventional formulation containing the two drugs substances, in MTT assays. Among the results, the combination were able to produce synergistic effect against TNBC cells (IC50) (~5 µM for BT549, and ~1 µM, for MDA-MB231) compared to MCF-7 and SKBR-3 cells (~30 and ~20 µM, respectively) .
Later, Misra and colleagues also adopted the strategy of drug targeting for breast cancer stem cells, which could be also associated with synergy . The concept of cancer stem cells (CSC), briefly, is based on evidence that, from a population of cancer cells, a small percentage present stemness features, which can self-renew or differentiate into rapidly proliferating ones [132,133]. Thus, conventional chemotherapy would have an effect against the majority of the cells, but the remaining unaffected CSCs would be responsible for repopulation, therapy resistance and metastasis formation [133,134]. In the study, polyethylene glycol cetyl ether (PEGCE) and poly(styrene)-block- poly(acrylic) acid (PS-b-PAA) were used to prepare polymeric nanoparticles containing niclosamide, being marked with a CD44-targeting peptide. Among the results, from a population around 10% of CSC MCF-7 cells (CD44+), maximum in vitro reduction was observed with targeted nanoparticles: around 60%, comparing with 20-30% using niclosamide alone or encapsulated in non- targeted structures. In addition, the reduction of tumour growth in xenograft mice, the targeted nanostructures reduced the CD44+/CD24- CSC population in vivo and downregulated stemness marker genes. Mechanistically, the authors
attributed the anticancer effects to inhibition of STAT3 phosphorylation by niclosamide .
Different strategies against CSCs include inhibition of cellular mechanisms (Wnt, Hedgehog, Notch, NFκB) . Niclosamide may block three of these pathways (Wnt, Notch, NFκB) [12,13]. Besides, Misra and colleagues attributed its action by inhibition of STAT3 signaling. Hence, since this compound may present activity against CSCs, which can be optimized with nanostructured systems, treatments including the combination of this molecule with conventional drugs could provide better results considering recurrence, therapy resistance and metastasis.
The challenges involving niclosamide repositioning for cancer diseases begin with its physicochemical properties, since its stable crystalline structure and its lipophilicity restricts solubility in water, which implies undesirable drug- like features that affect oral absorption. In fact, this was reflected by the high daily doses administered in preliminary clinical studies, sometimes much higher than the usually used for tapeworm infection. This raises concerns of whether the dose will be different if applied therapeutically, and what would be the impact of administering it for long periods. Initial results indicated that the blood levels might be critical for some effect against prostate and colon cancer. Thus, it also raises the question of whether the activity will be confirmed in ongoing clinical trials. An unsuccessful performance does not exclude this candidate, since niclosamide already showed effects against different types of cancer cells, but further highlights the challenges involving its repositioning. The drawbacks of the physicochemical properties bring the opportunity for the development of nano-based formulations, which were not evaluated in the clinical studies. Nanocrystals are the simplest strategy, as the increase in saturation solubility and dissolution rate might improve oral absorption and provide therapeutic blood levels. Despite the possibility of precipitation, drug supersaturation by lipid-based formulations might also improve absorption, and are associated with the hypothesis of targeting the lymphatic system, preventing metabolization by the liver as well. Niclosamide diverse activity may be explored by the
combination with other compounds, in order to achieve synergistic effects. Approaches that are more sophisticated may include the use of ligands for drug targeting. In this case, treatments including a combination with conventional chemotherapy against CSCs might provide therapeutic benefits related to drug resistance and recurrence.
“This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001″. Jim Hesson of Academic English Solutions edited the manuscript (https://www.academicenglishsolutions.com).
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