Niosomes as versatile nanocarriers in diabetes research

2.1 Structure, types and components

Niosomes are vesicular nanocarrier systems that are comprised of non-ionic surfactants developed as the best alternative to liposomes. These thermodynamically stable bi-layered structures are formed only when the surfactants and cholesterol are mixed in a proper proportion, along with temperature control above the gel liquid transition temperature. ^{20 , 21} This bi-layered structure contains a hollow space in the center, and their special geometry allows niosomes to encapsulate both hydrophilic and hydrophobic drugs respectively within their aqueous core and bilayer membranes. Figure 1 illustrates the bilayer structure, which clearly shows the two areas available for drug entrapment.

Figure 1:

The bilayer structure of a niosome.

In general, niosomes can be classified into three types: multilamellar vesicles (MLV), large unilamellar vesicles (L-ULV), and small unilamellar vesicles (S-ULV). Figure 2 illustrates the characteristics of each type. ^{22 , 23} In contrast to the lipids utilized in the formation of liposomes, niosomes exhibit greater physical and chemical stability in the presence of non-ionic surfactants. The presence of lipid components in liposomes often results in rapid rancidification and shorter shelf-lives. In contrast, niosomes exhibit improved drug entrapment/encapsulation efficiency (EE) with a lower cholesterol content, minimal costs and no storage requirements. Niosomes synthesized with non-ionic surfactants offer biodegradability, non-toxicity, non-immunogenicity, higher compatibility, stability and reduced toxicity; and were deemed superior compared to other systems. ¹⁷ In addition, the universality of niosomes makes them applicable for various drug delivery administrations. ²⁴

Figure 2:

The types of niosomes.

Niosomes were first utilized in the cosmetic industry as early as the 1970s; however, their benefits have since been extended to numerous pharmacological applications, including anticancer, antimicrobial, anti-inflammatory, and antioxidant treatments. These systems are currently the focus of clinical trials, particularly for dermal and transdermal applications. In addition, current research investigates the potential of niosomal oral formulations for blood glucose reduction, antihypertensive effects and analgesic drugs. ^{25 , 26} Niosomes are predominantly composed of non-ionic surfactants and cholesterol, with the addition of charged molecules to improve their properties. Non-ionic surfactants consisting of a hydrophobic tail and a polar head are the primary components. The hydrophilic heads of these surfactants face the aqueous environment, whereas the hydrophobic tails face inward. This folding generates a vesicle structure that is thermally stable. Importantly, niosomes do not possess a charge, which reduces tissue hemolysis and irritation. ²¹

2.2 Formulation components of niosomes

Several components were deemed critical in achieving the optimal formulation for niosome synthesis. These are mainly the non-ionic surfactants, cholesterol, charge-inducer molecules as well as hydration medium.

2.3 Techniques of synthesis

To date, various techniques have been reported in the niosome synthesis. Each of these procedures varies in terms of the preparation and the formulation that is produced. The principles of each technique are discussed in detail below.

1. Thin film hydration

   Thin film hydration is the most common technique applied in preparation of niosomes. This method involves the addition of an organic solvent with the non-ionic surfactants and cholesterol using a round bottom flask. This is followed by evaporation of these solvents to form a thin layer of film on the inner surface of the flask. This thin layer film is then rehydrated using aqueous solution (water or phosphate buffer), preferably at a higher temperature than the surfactant’s transition temperature. This process causes the layer to swell and form MLV vesicles containing the drugs. Niosomes containing zidovudine, paclitaxel, green tea extract and gallidermin are some common ones that have been synthesized using this technique. ^{28 , 29 , 30 , 31 , 32 , 33 , 34} This method is deemed simple and easy, leading to efficient control of the vesicle size and EE. ^{35 , 36} However, this technique faces challenges with the use of hazardous organic solvents that raise environmental concerns. Moreover, the scalability of this method is indeed limited due to the batch-wise nature of the process as well as the tediousness involved in controlling the uniformity of the thin film across large scales. ³⁷

1. Reverse phase evaporation

   This method involves the mixing of surfactant and cholesterol in an organic solvent, with a separate preparation of the drug in an aqueous solution. The aqueous phase will be then added to the organic phase; where it forms a two-phase system. The next step will involve homogenization of the systems followed by removal of the organic phase under negative pressure leading to synthesis of L-ULV. This technique has been utilized in niosomal preparation with isoniazid, ellagic acid, and bovine serum albumin. ^{38 , 39 , 40}

1. Sonication

   Sonication is carried out with the addition of a drug solution in buffer into a mixture of surfactant and cholesterol. Following this, the mixture will be probe sonicated at 60 °C to form MLV, followed by further ultrasonication that could yield ULV. ⁴¹

1. Microfluidisation

   Microfluidic devices are seen to offer a significant advancement in the synthesis of niosomes. These devices incorporate the use of microchannels to precisely control the flow and mixing of reagents, which leads to uniform synthesis of niosomes with high reproducibility and improvised EE. ³⁷ In general, this method involves the dissolution of the drug and surfactants together, which are then pumped under pressure from a reservoir into an ice-packed interaction chamber. The solution is then passed through a cooling loop to ensure the heat is removed. Niosomes formed using this method are generally smaller in size with high uniformity. ^{17 , 33 , 39} This method is also seen to offer enhanced scalability compared to traditional techniques, that allow for continuous production of niosomes. In addition, the microfluidic devices tend to offer a more precise control in the mixing that minimizes organic solvent usage, and supports a more environmentally friendly process. Nevertheless, this technique is challenging as initial setup requires significant investment and specialized expertise that complicates their application. ³⁷

1. Ether/ethanolic injection

   This method involves dissolving both the surfactant and cholesterol in an organic solvent that is then injected into an aqueous phase. ³⁶ The reaction is maintained at above 60 °C. The rapid dilution of the ether or ethanolic solution leads to self-assembly of the surfactant molecules into niosomes. This technique is considered more rapid than the thin film hydration method, however, it was demonstrated that the control over vesicle size and EE may be less precise. The niosomes formed upon solvent evaporation usually vary in the range of 50–1,000 µm, which is usually for ULV. In terms of scalability improvisation, this technique was proposed to be combined with a microfluidic device that caters for precise control of flow rates and mixing, which generates a more uniformed and reproducible niosomes. ^{33 , 37 , 42}

1. Multiple extrusion

   This technique will involve a suspension of lipid-containing drug that will be passed through a porous device using a nozzle. The multiple extrusion technique is seen to synthesize niosomes of uniform size, in the range of 50–500 nm. ^{43 , 44}

1. Bubble

   The bubble method does not involve the addition of any organic solvent. Here, all of the components (cholesterol, surfactants and phosphate buffers) will be mixed together using three-necked flasks, with the temperature maintained using a water bath. The system is designed in such a way that each of the necks has its use where the first is used to place the thermometer, second neck used for nitrogen flow while the third one functions for water cooling reflux. The solution mixture will be dispersed at 70 °C, homogenized for 15 s, and nitrogen gas will be flowed immediately. This technique initiates formation of L-ULV, that are essentially required for size reduction to generate S-ULV. ³³

1. Transmembrane pH gradient

   In this method, both the surfactant and cholesterol are used in the same proportions, where they are dissolved in chloroform followed by removal of organic solvent under reduced pressure. This process generates formation of a thin lipid layer; which is then hydrated using citric acid or any other acid solution via vortexing. The resultant mixture is then subjected to a freeze–thaw cycle; followed by addition of aqueous drug solution and vortex-mixed. Disodium hydrogen phosphate solution is used for pH adjustment, and finally remote loading of the drug can be achieved. ^{17 , 39}

1. Heating

   The heating technique sees the hydration process of surfactant and cholesterol separately using a buffer solution. The cholesterol solution is then dissolved by heating at 120 °C for 1 h upon hydration. Next, the surfactant and other additives will be added at a lower temperature under continuous stirring that generates the vesicles. The synthesized niosomes will be kept at room temperature for 30 min, which is followed by storage at 4–5 °C, under nitrogen. ^{29 , 33}

1. Freeze and thaw

   This method successfully synthesizes frozen and thawed MLV. Briefly, here the niosomes are principally synthesized using thin film hydration via the freeze and thaw method, which sees the niosomal suspension subjected to five rounds of freezing cycles in liquid nitrogen followed by thawing in a water bath at 60 °C. However, this technique presents limitations in terms of lower EE and size shrinkage of the formed niosomes. ^{33 , 45 , 46}

1. Microfluidic hydrodynamic focusing

   This technique has the ability to synthesize niosomes that are better in size and distribution, compared to conventional methods. Here, niosomes of two miscible liquids will be formed via diffusive mixing with microfluidic hydrodynamic focusing. Niosomes synthesized via this technique were highlighted to vary with the surfactant’s chemical structure, device material and the conditions applied. Previous findings indicated smaller-sized niosomes were formed with higher flow rate which also decreases the diffusive mixing time. In contrast, the mixing times were seen to elevate with the use of a wider microchannel, which consequently generates large size niosomes. ⁴⁷

1. Dehydration–rehydration

   The vesicles will be first prepared via thin film hydration, which is then followed by freezing in liquid nitrogen and overnight lyophilization. The powdered niosome will be hydrated using phosphate buffer saline (pH 7.4) at 60 °C. Researchers have presented better EE with niosome vesicles of this technique; compared to those formed via thin film hydration or freeze–thaw method. ^{33 , 45}

1. Supercritical carbon dioxide fluid

   This technique involves the use of non-inflammable, non-toxic and volatile solvents; with the niosome vesicles formed in the usual range within 100–440 nm. ⁴⁴

2.4 Advantages and limitations of niosomes

Niosomes stand out against other delivery systems, with several advantages. Generally, the niosome formulation can be modified by manipulating various parameters. The presence of functional group on their hydrophilic head allows for easy alteration of the niosomal surface. Moreover, since the niosomes does not possess any charge, they have less toxicity and higher biocompatibility. Interestingly, niosomes exhibit enhanced chemical stability and longer shelf life, with better osmotic activity, especially in comparison to liposomes. ^{21 , 27 , 48 , 49}

Despite their highlighted benefits, niosome vesicles also face distinct challenges. The main limitation is on the stability issue with respect to their aqueous nature as there is high possibility for the drugs to be hydrolyzed. Entrapped drug also indicates chances of leakage from the entrapment site. At the top of the list, the possibility of aggregate formation in the niosomal suspension is regarded as a major cause of concern. ^{21 , 43} The detailed-out advantages and limitations are illustrated in Figure 3 below. ²¹

Figure 3:

Advantages and limitations of niosomes.

3.1 Techniques

The synthesized niosomes must be characterized in order to understand their properties. It is hypothesized that their different synthesis techniques, compositions and ratio of selected surfactants, the nature of the drug itself, and other synthesis conditions cause them to exhibit distinct properties. Characterization of niosomes involves various techniques to evaluate their physical and chemical properties. Some common methods for niosome characterization include particle size and size distribution analysis, morphological examination, EE, zeta potential measurement, membrane rigidity, lamellae number, drug release studies, in-vitro and in-vivo studies as well as stability assessment.

1. Particle size, morphology and size distribution

   In general, the size and morphology of niosomes are typically determined using a variety of techniques. ^{50 , 51} Particle size and size distribution of niosomes can be determined using techniques such as dynamic light scattering (DLS), laser diffraction or transmission electron microscopy (TEM) where it provides information on the average size and polydispersity index (PDI) of niosomes. The morphology and shape of niosomes can be observed using microscopy techniques such as light/optical microscope, TEM or scanning electron microscopy (SEM). Additionally, these techniques help to visualize the vesicular structure and confirm the presence of MLV or ULV. A distinct point is that the particle size measured using TEM is much smaller than the DLS as a result of the different measurement principles used by each technique. ⁵²

1. Entrapment/encapsulation efficiency (EE)

   The EE of niosomes refers to the number of drug/active compounds that are entrapped within the vesicles. The EE can be determined via subtraction of the unloaded drug amount from the total quantity. ³⁹ Gel chromatography, centrifugation, filtration or exhaustive dialysis are some of the techniques that have been applied to measure the unloaded drug. ^{53 , 54 , 55 , 56 , 57 , 58 , 59 , 60} The EE % is determined using the following equation: ³³

   E E   % = Quantity   of   drug − loaded   in   niosome Total   drug   in   suspension × 100 %

1. Zeta potential and charge

   The presence of charge on niosomes relates to the presence of electrostatic repulsion that stabilizes them by preventing aggregation and fusion. ⁴² This charge is referred to as zeta potential, and it can be measured using a zeta potential analyzer, mastersizer, DLS, microelectrophoresis, pH-sensitive fluorophores or high-performance capillary electrophoresis. ⁶¹

1. Membrane rigidity

   The fluorescence probe can be used to measure membrane rigidity in relation to temperature. ³⁴ In the case of the membrane’s micro-viscosity, it was stated that the fluorescence polarization can also be used to identify the structural packing of the niosomal membrane. ^{44 , 62}

1. Lamellae number

   The determination of lamellae number can be performed using various techniques such as atomic force microscopy (AFM), nuclear magnetic resonance (NMR), small-angle X-ray spectroscopy and electron microscopy. ^{34 , 44 , 63} By combining the technique of small-angle X-ray scattering and in situ energy-dispersive X-ray diffraction, the bilayer thickness could be analyzed. ^{44 , 64 , 65 , 66 , 67 , 68 , 69}

1. Drug release

   The drug release studies for niosomal formulations are usually conducted using the dialysis membrane technique. In this method, niosomes are placed in a dialysis bag that is immersed in a dissolution medium-filled container. This assembly is placed on a magnetic stirrer under a controlled temperature of 37 °C. The drug release concentration will then be calculated based on the collection of samples from the receptor compartment at specified time intervals. ^{34 , 39 , 70} Several published papers have demonstrated the application of this method with various niosomal formulations, such as with the temozolomide, curcumin cationic PEGylated and diltiazem. ^{71 , 72 , 73} In addition to the dialysis technique, other studies have described various in-vitro release protocols. ^{50 , 51} These studies provide insights into the drug release profile and mechanisms over time.

1. In-vitro and in-vivo studies

   In-vitro studies such as cell culture experiments can be conducted to assess the cellular uptake, cytotoxicity and therapeutic efficacy of niosomes. In-vivo studies with animal models can provide insights into the biodistribution, pharmacokinetics, and therapeutic outcomes of niosome formulations. In-vivo studies for niosomes is dependent on the delivery route, drug concentration, effect and time presence of the drug in tissues. ^{33 , 42} Animal models are usually employed to examine the tissue distribution of a drug, where the animals will be sacrificed and the tissues are collected, washed, homogenized, centrifuged and the resulting supernatant will be analyzed for the drug content. ³⁴

1. Stability studies

   Stability studies are crucial to evaluate the physical and chemical stability of niosomes under various conditions. Parameters such as size, zeta potential, drug content, and vesicle integrity can be monitored over a specified period. There is known possibility of drug leakage from the niosomes upon storage which is correlated with aggregation and fusion formation. ³³ The stability studies of niosomes are performed under different temperatures, humidity and light conditions for two months. The effect of storage is evaluated periodically in terms of its size, shape and EE. ⁷⁴ Examples of these are reported in the literature for green tea extracts, ginkgo biloba and cefdinir niosomes. ^{32 , 41 , 75} In addition, the stability aspects under gastrointestinal conditions were also evaluated with different enzymes (e.g. pepsin, trypsin, chymotrypsin) that showed the ability of niosomes to prevent the drug degradation. ³⁰

3.2 Limitations of current/existing techniques and potential improvements

Regardless of the available technologies to date, it is still crucial to note that these characterization techniques still face several limitations, that in turn impact their efficacy and application. These limitations and suggested potential improvements are discussed in detail below.

1. Inadequate size and distribution analysis

   One of the primary challenges revolves around the insufficient details in measuring their size and distribution. The conventional technique of DLS generates average size distributions; however, it often fails in capturing the structural heterogeneity and PDI aspects. ⁷⁶ The size aspect is critical as they are strongly associated with their drug release patterns and permeability characteristics. Optimally, PDI below 0.3 is desired, since higher ranges may lead to inconsistent drug release. In order to tackle the inadequate information pertaining to the size and distribution analyses, it was proposed that the integration of advanced techniques such as nanoparticle tracking analysis could generate precise measurements of niosomal formulation size distribution. Moreover, this analysis could also differentiate particles as per size, and consequently allows for real-time monitoring of niosomal dynamics. Overall, these data were believed to complement information obtained from the DLS technique, hence offering a more comprehensive information of the formulation characteristics. ⁷⁷

1. Limited understanding of morphological properties

   Morphological assessment is conducted with electron microscopy techniques, such as TEM. TEM micrographs provide insights into the morphology and structure of niosomes, however they may not effectively evaluate the dynamic characteristics of these vesicles under different conditions. ⁷⁸ The morphological properties inclusive of the shapes, form and lipid bilayer organizations are critical to understanding the drug release mechanisms, however they are often inadequately addressed in current techniques, observed under different conditions. As a counteractive measure, the approach of applying combined imaging techniques such as AFM with EM could further enhance the understanding of their morphology. AFM provides the topographical details, and EM offers insights into their structural integrity. Overall, this multi-faceted approach could generate a more holistic view of niosomal features that are deemed critical for optimizing the formulations. ⁷⁸

1. Unknown rheological properties

   Based on available literature, it was often noticed that the rheological properties of niosomal formulations are not analyzed extensively, which in turn is believed to impact their applicability in different formulations. The existing procedures generally focus on viscosity aspects without indulging in elasticity and viscosity measurements. ⁷⁹ The rheology aspects are deemed vital especially in gel formulation for transdermal applications since the flow behavior needs to complement the desired delivery profile. Furthermore, existing studies usually apply a limited range of temperatures in studying these properties, while neglecting the potential variability exposed under different physiological conditions. Additionally, it should also be emphasized to simulate conditions similar to human body dynamics in evaluating performance of niosomal formulations. Hence, future approaches should look into thorough rheological assessment as an improvisation measure incorporating a wider temperature range and varied shear rates. This is seen to be beneficial as it can provide deeper insights into the mechanical properties of the formulations. Furthermore, implementing oscillatory rheometry could also uncover the viscoelastic properties, thus enabling better predictions of niosome properties in relation to stress, which is crucial for applications under dynamic environments. ⁷⁹

1. Lack of comprehensive stability assessment

   The stability assessment of niosomal formulations often employs simplified methodologies, without incorporating potential impacts of various environmental factors. ⁸⁰ As an example, some reported studies were noticed to exclude evaluation of longer-term stability of niosomes, under various manipulated conditions (pH, ionic strength, temperature) which are predicted to possibly affect and regulate the niosome vesicles structural integrity and drug retention capabilities. ⁷⁸ The potential improvement in stability studies was proposed to incorporate accelerated aging approaches that could stimulate long-term storage conditions. Under such environments and extended durations, it was predicted to obtain more reliable insights into the niosomal formulation performance and long-term stability which is indeed crucial for clinical applications. ⁸⁰

1. Ineffective assessment of in-vivo performance

   The transition from in-vitro to in-vivo studies still remains a significant challenge, since the majority of techniques do not necessarily provide adequate information to minimize gaps of laboratory scale and practical therapeutic outcomes. Moreover, animal models also lack relevance to human pharmacokinetics and pharmacodynamics, which is believed to potentially provide misleading efficacy results. ⁸¹ Thus, more thorough clinical investigations were necessary in expansion of the formulation applicability in real-world scenarios. To counteract these challenges, it was proposed to apply computational modelling as a measure of in-vivo predictions. The incorporation of computational modelling and simulation techniques could better manage the gap between in-vitro findings and in-vivo efficacy. Moreover, advanced simulations can also replicate human physiological conditions more accurately, hence minimizing study dependency on animal models which further strengthens the outcome translations towards clinical investigations. ⁸²

4.1 Oral administration

Oral delivery is identified as the most common route pertaining to its advantages, that include ease of use, patient preference, cost-effectiveness, lack of sterile precaution, as well as suitability for repeated and prolonged usage. ^{83 , 84} Nevertheless, this route of administration encounters limitations in terms of drugs degradation caused by first-pass metabolism, highly acidic environment, and mucosal enzymes prior to their entrance into the systemic circulation. ⁸⁵ Niosome-loaded substances generally withstand these conditions better than classic oral formulations, with respect to their ability to cross the intestinal membrane via paracellular permeation, M-cell mediated absorption and endocytosis. ⁸⁶

Moreover, certain formulations have also been improvised and innovated with surface modification to further enhance their therapeutic benefits. One such example is the coenzyme Q10 (CoQ10) which is entrapped within niosomes coated with chitosan and polyethylene glycol (PEG). The outcome disclosed that this modification increased the hydroxyl radical scavenging capacity of CoQ10. In addition, PEG-coated niosomes indicated better sustained release effects examined via dialysis for 24 h. However, with chitosan coating a different scenario was observed with arising adverse effects that disrupt the bilayer structure and causes CoQ10 leakage. Regardless, chitosan coating allowed increased gastrointestinal residence time and bioavailability of poorly water-soluble drugs. ^{87 , 88}

A recent study by AbuElfadl and colleagues ⁸⁹ showed that drug loaded onto a modified surface niosome presented an increase of 37 % in bioavailability effects. Similarly, another study that investigated niosome/chitosan-shell hybrid nanocarriers with cordycepin showed that this formulation prevents gastrointestinal degradation and also improved their ability to target mucosal surfaces. ⁹⁰ In addition, proniosomes which are a dehydrated form of niosomes require soaking prior to usage. In their dried forms, proniosomes present multiple benefits against normal niosomes in terms of aggregation, fusion, caking, transportation and distribution. ^{91 , 92} This was presented in a study that showed tramadol HCl-loaded niosomes via proniosomes technology further prolonged their pharmacological effects. ⁹³

Niosomal formulation administration via the oral route is deemed advantageous for the ability to encapsulate multiple solubility drugs, along with improved stability, bioavailability, and controlled deliveries at the targeted site. Moreover, niosomes may significantly prolong the drug’s half-life, hence possibly minimize the required frequency of drug administration. Nevertheless, surface modification of niosome formulation remains to be explored more widely towards achieving more enhanced drug deliveries to desired locations. ⁹⁴

4.2 Transdermal delivery

Previous research has demonstrated improved drug penetration into the epidermis and dermis layers through topical applications of niosomes. These mechanisms occur in a series of events, where firstly the drug-loaded niosomes will be absorbed on the skin surface creating a high degree of thermodynamic activity at both the vesicle and stratum corneum. These reactions facilitate the drug penetration via the stratum corneum. Next, niosomal formulations tend to disrupt the densely packed lipids and in turn, enhance the drug permeability via the stratum corneum conformational modification. This is followed by improved transdermal penetration with the assistance of non-ionic surfactants that alter the intercellular lipids of the stratum corneum and elevates the overall membrane fluidity. Lastly, the niosomes tends to induce a hydrating effect, that is followed by loosening of the tight cellular arrangement of the stratum corneum. ^{94 , 95}

The topical or transdermal route has shown usage of stable dosages with minimal systemic side effects as drug release tends to be localized at the application site. ⁹⁶ This administration route is remarkable in the sense that it is non-invasive and possesses high bioavailability since it eliminates food–drug interactions and prevents issues of first-pass metabolism and enzyme breakdown in the gastrointestinal tract. ⁹⁷ Despite of this superiority, the drug absorption might be hindered by the protective nature of the transdermal barriers. Furthermore, the liquid nature of niosomal formulations requires addition of other components for their suitability as topical application. Also, the drug used is preferably of lower molecular weight with high hydrophobicity which limits the potential application of this delivery route. ^{70 , 97}

4.3 Ocular delivery

The eye is a crucial organ that possesses a complex structure with numerous barriers and defense mechanisms which lead to challenges with drug deliveries. Conventional therapies for ocular diseases mainly include topical eye drops and ointments, with the cornea as the primary pathway. However, the efficiency of this route is dependent on the molecular weight, lipophilicity and diffusive nature of the drugs. In addition, they encounter issues with low bioavailability and retention time permeability. Under certain circumstances, drugs tend to be washed away from the ocular surface into the nasolacrimal ducts or lacrimal glands owing to insufficient time to settle in the conjunctiva. ^{98 , 99}

Niosome-loaded drugs were shown to counteract these complications, where they are able to effectively come into contact with the corneal surface and then release the drugs. Past findings have demonstrated excellent capacity of various niosomal formulations’ effectiveness in ocular drug delivery. Niosome-based treatment indicated enhanced chemical stability, bioavailability, and drug permeability while lowering the adverse effects. Moreover, the utilization of niosomes have shown capacity to resist drainage in response to reflex tearing and blinking, hence leading to better retention and spreading ability on the eye surface, compared to other carriers. Despite these benefits, however, it is still vital to explore more aspects of their suitability as ophthalmic medications, due to other reported issues of instability, drug leakage and decreased EE. Also, the cytotoxicity investigations are essential in order to verify the safety of these formulations’ usage in the eyes. ⁹⁴

4.4 Respiratory/pulmonary delivery

Drug delivery via the respiratory system has shown both local and systemic effects. This route is widely used for respiratory diseases, diabetes as well as some other systemic diseases. This route also has several limitations, with regards to low effectiveness of inhalation systems, low amount of drug delivered, as well as drug instability. ¹⁰⁰ The suitability of niosomal formulations was proven with a study on lung cancer that showed suitable optimized condition with an aerosol output of 96.22 %. ¹⁰¹ Pulmonary drug absorption generally benefits from an extensive contact area that is created by the presence of dense capillary network within the alveoli, which enhances the interaction between air and blood. The niosomal formulation is seen to assist in improving the permeation via pulmonary administration through the airway mucus and these vesicles were also found to significantly increase drug uptake by the human lung fibroblasts. The niosomal drug delivery through this route extends its residence time within the lung, preventing the possibility of systemic side effects. Nevertheless, certain limitations still remain with respect to drug irritation, toxicity, in-vivo stability, unguaranteed drug transport to action site, as well issues of drug retention and clearance. ⁹⁴

4.5 Nasal administration

The nasal administration route presents a unique and increasingly explored avenue, offering several advantages against the oral or intravenous route. The accessibility, large surface area of the nasal mucosa and rich vascularization channels their potential in both local and systemic drug delivery. Yet, the nature of the complex anatomy of the nasal cavity along with presence of mucus and enzymatic activity poses serious challenges to achieve effective drug delivery. Niosomes are considered a useful alternative to overcome these challenges. ^{102 , 103}

In general, the nasal route is deemed beneficial for drug deliveries for several reasons, including non-invasiveness, extensive vascular network catering for rapid drug absorption, bypasses the first-pass hepatic metabolism thus preventing effects on the drug’s bioavailability, and large surface area of the nasal cavity which allows efficient drug absorption. ^{81 , 102 , 104} However, certain limitations exist with regard to their complex anatomy which influences drug deposition and absorption, presence of mucus layer that hinders drug penetration, reduced efficacy of drugs as a result of enzymatic activity, as well as tendency for rapid removal of drugs due to clearance mechanisms prior to their absorption. ^{102 , 103 , 104 , 105}

Niosomal formulations hold significant potential due to their ability to deliver drugs directly to nasal mucosa, making them an excellent choice for treating conditions of nasal allergies or sinusitis. ¹⁰⁵ Furthermore, the possibility of targeted drug delivery using ligand-conjugated niosomes is seen to further enhance the therapeutic efficacy of this approach, and hence minimizes off-target effects. ¹⁰³ A study with diltiazem drug-loaded niosomes, comparing their oral and nasal administration route presented better bioavailability and less elimination via nasal delivery. Despite this, certain limitations were still present with regard to short residence time, airflow obstruction as well as sensitivity of nasal mucosa which tends to impact the drug permeation and systemic bioavailability. ⁷²

4.6 Intravenous administration

The intravenous route of administration is favourable in the sense that the drug is able to enter directly into the systemic circulation. Moreover, this delivery route presents enhanced drug stability with prolonged sustained duration in the blood. Additionally, it was also shown that it is possible to achieve drug delivery to the desired site of action. In a study by He and co-researchers, niosomes were subjected for surface modification with addition of PEG. They reported the capability to oppose the uptake from mononuclear phagocytic system, improving the circulation time, stability as well as bioavailability. ^{21 , 106}

5.1 Niosomes in antidiabetic applications

DM is a complicated metabolic disorder, and owing to its complex pathophysiology, its treatment is often troublesome. Traditional anti-diabetic medications, such as insulins, biguanides, meglitinides, sulfonylureas and thiazolidinediones, have been linked to gastrointestinal issues, hepatic and renal impairment, and other side effects. Consequently, there is a growing interest in utilizing naturally-derived sources with minimal side effects and therapeutic benefits. Hence, the usage of naturally-derived sources with significant therapeutic benefits and minimal complications have attracted significant attention. ¹¹³

Natural compounds have demonstrated potential for preventing, treating and managing metabolic disorders such as diabetes. Despite the evidence provided over recent decades regarding the immense health benefits of phytochemicals, their efficient delivery stands as a conundrum especially in terms of their stability and bioavailability issues. ^{114 , 115} To overcome these limitations, researchers have resorted to nanoencapsulation techniques for transporting bioactive compounds to the intended site. Nano-based drug delivery systems have gained attention in recent times as a promising alternative for remedying these challenges in therapeutic management of DM. ⁹⁴

Nanoformulations have shown substantial improvement compared to conventional strategies, along with their capability of targeted distribution that allows for integrative approaches. Niosomes, as highlighted previously are one significant drug delivery system that could be administered via various routes. Generally, the niosome can be identified as a drug depot since it has the potential to transfer the drugs to the desired site, via a controlled or sustained mode. A diverse range of drugs are capable of being accommodated within the niosome structure due to their hydrophilic and lipophilic properties. Moreover, this characteristic further strengthens their ability to pass through the stomach and withstand the acidic conditions, which makes them a suitable drug delivery mode for antidiabetic applications. ¹¹⁶

Although the use of niosomes in the treatment of DM is relatively new, quite a number of researchers have demonstrated their promising aspects. Table 1 covers the available in-vitro and in-vivo studies on niosome-based encapsulation strategies focused towards the prevention and treatment of DM. Based on the reported findings, it was observed that the niosome formulations consistently exhibit greater efficacy in comparison to the free extracts. The different type of plant extracts and active compounds which were explored for their potential antidiabetic activity are highlighted in Table 1. These include Tradescantia pallida leaf, Brucea javanica, Fumaria officinalis, 4-hydroxyisoleucine, stevia, gymnemic acid, plumbagin, carnosine, embelin, metformin hydrochloride and glipizide. ^{116 , 117 , 118 , 119 , 120 , 121 , 122 , 123 , 124 , 125 , 126}

The particle sizes reported among these studies were generally small, in the range of 80–300 nm. However, a study by Alam and colleagues generated larger particle size of embelin-loaded niosomes, in the range of 500–734 nm. Despite their large size, better efficacy than the free form was noticed, along with comparable hypoglycemic activity to the standard drug. ¹²³ Similarly, a recent study with PEGylation of niosomes encapsulating 4-hydroxyisoleucine also synthesized larger sized vesicles (460–580 nm), compared to the non-PEGylated ones. ¹²⁵

A recent study attempted to investigate the comparative effect of a plant extract, B. javanica against its niosome-loaded counterpart. Niosomes were encapsulated with this extract using modified ethanol injection technique with combination of glycolipids, Tweens surfactants and cholesterol. The different Tweens (T20, T40, T60, T80) were utilized, and the final optimized formulation were selected with T20 favoring their characteristics of particle size, stability, morphological properties and in-vitro release. The in-vitro release profile indicated that the niosomal formulation presented a significantly prolonged release; hence demonstrating their potential in sustained drug delivery applications. The outcome of this research suggested further investigation via in-vivo and clinical trials to look into their pharmacokinetic properties and clinical efficacies whereby this would be beneficial in paving the way for integration into conventional treatment approaches. ¹²⁴

Another plant extract, T. pallida leaf extract was investigated for its nanoniosome formulation via both in-vitro and in-vivo studies. Here, a combination of Span 60 and cholesterol was utilized, with several formulations to determine the most optimal condition. The vesicle size obtained was around 160 nm, PDI of 0.132 and 89.12 % EE. In-vitro studies presented a drug release profile up to 12 h, and additionally the antidiabetic potential via α-amylase assay proved better inhibition with the niosomal formulation compared to pure extract and acarbose, with 89.8 %. In-vivo investigations also presented similar findings, where the niosomal formulation was a better antidiabetic agent tested against alloxan-induced diabetic mice models. ^{118 , 127} .

Similarly, as presented in Table 1, it was evident that the niosomal formulation of the other extracts or active compounds presented positive outcomes with better antidiabetic activity compared to their free-form. Henceforth, the compiled findings suggest that the approach of innovative niosome-based formulations of the natural sources (extracts/active compounds) were deemed suitable and highly efficient in improvisation efforts of their regarded potential. Although these sources are capable of projecting their antidiabetic agent characteristics, the modifications with niosomal formulation further improve these aspects enabling them to further impart their promising applications in the prevention and treatment approaches of DM.

5.2 Future trends in niosome antidiabetic therapies

Future prospects in niosomal based antidiabetic therapies see ongoing research in terms of the formulation advancements towards achieving enhanced drug delivery. These include choice of surfactants, lipid compositions, ligand-based conjugation, stimuli-responsive systems, and also a focus on combination therapies.

The choice of surfactants is essential as they impact the properties of the niosomes (size, stability, EE). In addition, the utilization of biodegradable and biocompatible surfactants (alkyl polyglycosides, polysorbates) is beneficial as it has the potential to minimize toxic effects and in turn enhances the safety profile of niosomal formulations. Apart from this, the incorporation of other compounds (cholesterol, lipids) within the niosome bilayer provides superiority in improving the membrane stability, kinetic control of the drug release along with enhancement of the drug entrapment. Overall, the exploration of various combinations of different compositions is crucial in order to tailor-make their properties in antidiabetic drug entrapment towards maximizing the desired therapeutic effects. ¹²⁸

Another approach involves ligand-based conjugation; with the incorporation of peptides, antibodies or aptamers to the niosome surface which functions to facilitate specific targeting of cells or tissues. This strategy focuses on improvement of the drug efficacy by optimizing the drug concentration at the target site while minimizing off-target effects and lowering the systemic toxicity. An example of this involves the conjugation of niosomes with ligands that bind specifically to pancreatic β-cells, which allows for improvement of glycemic control through enhanced delivery of antidiabetic agents to these cells. Regardless, various ligand and conjugation techniques are being investigated to achieve optimized drug delivery. ¹²⁹

The development of stimuli-responsive niosomes represents a significant paradigm shift in drug delivery. These systems are designed in such a way that the drugs are released in response to specific stimuli (e.g. pH, temperature, glucose concentration, etc.) allowing precise control of drug release along with maximizing therapeutic efficacy and minimizing side effects. For example, glucose-responsive niosome systems tend to release insulin in effect to increased blood glucose levels, thus leading to a more physiological control of blood sugar levels. Similarly, in a system with pH-sensitive agents, they could be utilized to release the antidiabetic agents within small intestines that are slightly acidic thus leading to enhanced absorption and consequently reduced the need for injections. The development of novel stimuli-responsive materials and their incorporation within niosomal formulations are continuously explored and remain as one of the key areas of research. ^{130 , 131 , 132}

Apart from this, combination-based therapies are another tactic that is also presented as a promising future development. This include the potential of niosomes to be applied for simultaneous administration of multiple antidiabetic drugs which are believed to instigate synergistic effects, resulting in improved glycemic control and hence reduce the need for individual therapies. Nevertheless, the development of optimized formulations is indeed vital to ensure the stability and control-release measure of several drugs, while preventing drug interaction with maximal therapeutic benefits. In addition, it was also suggested that combinatory therapeutic approaches hold significant promise for treating the multi-faceted nature of diabetes. The unique advantages of niosomes, combined with ongoing research and development efforts, suggest a promising future for this innovative technology in the fight against diabetes. ¹²⁸