SOLID LIPID NANOPARTICLES

Solid Lipid Nanoparticles (SLNs) were initially described in 1990 when M.R. Gasco, R.H. Muller, and J.S. Lucks conducted the first lipid nanoparticle synthesis experiment in a lab in Germany. Solid lipid nanoparticles (SLN), ranging from 40 to 1000 nm, surpass conventional colloidal carriers like emulsions, liposomes, and polymeric particles. They stand out as a superior alternative, with the ability to dissolve or encapsulate active pharmaceutical ingredients (API) within lipids. They are made of high melting point lipid as a solid core coated by an aqueous surfactant. While the surfactant, which is typically made from triglycerides, glyceride mixes, or even waxes, functions as an emulsifier, the solid lipid serves as the dispersed phase. Lipid nanocarrier assembly hinges on robust hydrophilic-hydrophobic interactions and van der Waals forces, orchestrating a fusion of the benefits from polymeric nanoparticles, lipid emulsions, and liposomes. Noteworthy advantages of SLNs include excellent biocompatibility, minimal susceptibility to erosion, low toxicity, gradual absorption, resistance to mixing issues, and biodegradability. SLNs stand out as an optimal nanocarrier for a broad spectrum of drugs, excelling in delivering both water-soluble and lipid-soluble compounds. Additionally, it enhances the bioavailability and aqueous solubility of APIs, especially those identified as class II (high permeability and low solubility drugs) and class IV (low permeability and low solubility drugs) in the Biopharmaceutical Classification System (BCS) [9, 10, 13, 14]. Under the brand name Nanorepair Q10TM, the first cosmetic product composed of SLN was introduced to the market in October 2005 [9,10,12,11].

Structure of solid lipid nanoparticles:

The structure of Solid Lipid Nanoparticles (SLNs) is contingent on several factors, including formulation components, compound solubility, and production methodology. Three distinct SLN structures have been identified through transmission electron microscopy (TEM) and scanning electron microscopy (SEM) in various research studies which are illustrated in fig. 1. The homogeneous matrix model (Type I) involves cold or hot homogenization for highly lipophilic drugs. In this approach, the drug is dissolved in a lipid matrix, and nanoparticle formation results from mechanical breakage induced by high-pressure homogenization. In the second approach, the lipid is dissolved in a lipid matrix with a gradual temperature increase, leading to nanoparticle formation. The drug-enriched core model (Type II) generates nanoparticles when the drug concentration approaches its solubility limit in the molten lipid. In contrast to Type III, the drug precipitates first, forming the core, while the shell comprises lipids and a low drug concentration. Lastly, the drug-enriched shell model (Type III) involves hot homogenization, where lipid particles form a core during cooling. Simultaneously, the drug concentration rises in the remaining melted lipid until it reaches its solubility limit. Upon reaching this point, a crystallized outer shell is formed through the combination of the drug and melted lipid. While this model might be not optimal for prolonged drug release, the drug-enriched shell model (Type III) is compelling for enhancing drug penetration, particularly in topical applications, especially when coupled with the occlusive effects of SLNs [10].

Figure1: Types of solid lipid nanoparticles

NANOSTRUCTURED LIPID CARRIERS

Nanostructured lipid carriers (NLC), the second generation of lipid nanoparticles, exhibit sizes ranging from 10 to 500 nm [15]. These particles feature a matrix consisting of a combination of solid and liquid lipids, with ratios ranging from 70:30 to 99:1. Despite the inclusion of a liquid lipid (e.g., oil), the blend maintains a solid state at body temperature. The incorporation and immobilisation of drug molecules are improved by carefully controlling the amount of liquid lipids in the formulation. With this method, issues with Solid Lipid Nanoparticles are addressed, including the possibility of drug expulsion during storage and the partial drug loading capacity [18]. NLCs demonstrate minimal in vivo harm, easy breakdown, and versatility in accommodating both water-soluble and water-insoluble drugs. They are mainly utilized to improve the solubility and effectiveness of poorly soluble medications [17,18]. The introduction of oil disrupts the formation of pristine lipid crystals, creating imperfections that increase the uptake capacity for Active Pharmaceutical Ingredients (API). NLCs offer advantages over Solid Lipid Nanoparticles (SLNs), demonstrating a higher API loading capacity, initially observed with retinol, increasing from 1% to 5%. Moreover, NLCs reduce the risk of drug expulsion over time and maintain stable physical and chemical properties [8,16]. The drug-release pattern in NLCs is biphasic, meaning that the solid lipid component releases the API more slowly than the liquid phase, resulting in a fast release of the API from the NLC. This release pattern can be controlled by adjusting the ratios of liquid to solid lipid carriers within the NLC [16]. However, variations in key factors such as lipid selection, surfactants, other necessary excipients, and preparation techniques lead to differences in parameters like particle size, shape, phase transition, solubility, and drug bioavailability [18].

Structure of nanostructure lipid carrier:

The positioning of drug components within Nanostructured Lipid Carriers (NLCs) and changes in lipid composition contribute to microstructure disorganization. Utilizing highly purified lipids induces void formation, allowing for increased accommodation of drugs or bioactive molecules in NLCs [19,15,16]. There are three types of NLCs which are identified and shown in fig. 2. NLC type I or Imperfect crystal, NLC type II or multiple types, NLC type III or amorphous type.

Figure 2: Types of nanostructured lipid carriers

Imperfect type NLC (Imperfectly structured solid matrix):

An imperfect kind of crystal NLC is typically composed of a crystalline matrix that is extremely disoriented and has a lot of voids and spaces. These spaces allow extra drug molecules to be arranged in amorphous clusters. These crystal order flaws are caused by combining liquid lipids at a lower concentration than solid lipids. When disparate lipids are mixed spatially, the crystal lattice becomes defective enhances the capacity for drug payload, allows for drug ejection during the crystallisation, and provides the lowest possible entrapment efficiency in the defective NLC [15,17,18,19].

Amorphous type (structure less solid amorphous matrix):

These results from the cooling of particles, preventing crystallization and maintaining an amorphous state. This characteristic minimizes drug expulsion, as there is no transition between polymorphic forms. [16,19]

Multiple types (multiple oils in fat in water (O/F/W) carriers):

Multiple-type Nanostructured Lipid Carriers (NLCs) incorporate oil, lipid, and water components. Leveraging the higher solubility of lipophilic drugs in liquid lipids, a high content of liquid lipids is used in the development of these NLCs. The introduction of oil in low concentrations ensures effective dispersion within the lipid matrix. Beyond the oil's solubility limit, phase separation occurs, forming small nano-compartments of oil within the solid matrix. Employing a Type III model through a cooling process in hot homogenization offers advantages for lipophilic drugs, including high drug entrapment efficiency, controlled drug release, and minimized drug leakage. [16,17,19]

Components of nanostructured lipid carriers:

LIPIDS

Lipids are physiologically acceptable, biodegradable, and generally recognized as safe (GRAS) lipids in their inner cores, influencing the stability, drug loading capacity, and sustained release behaviour of the formulation. The selection of appropriate lipids is crucial, with the solubility or partition coefficient of the bioactive in the lipid being a key criterion. The type and structure of lipids significantly impact various characteristics of nanocarriers [15]. A preferred ratio of 70:30 to 99.9:0.1 for combining solid and liquid lipids is recommended, although adjustments may be made based on formulation characteristics [17]. Higher liquid lipid content in NLCs is associated with accelerated drug release, and stabilization can be achieved using either a single surfactant or a combination of multiple surfactants. The selection and concentration of surfactants play crucial roles in NLC design, with higher concentrations linked to smaller particle sizes [17]. To address lipid crystallinity and polymorphism issues, a binary mixture of two spatially distinct solid lipid matrices, specifically a solid lipid and a liquid lipid (or oil), is employed in the preparation of lipid nanoparticle dispersions, now referred to as nanostructured lipid carriers. The extent of lipid crystallization significantly impacts drug entrapment, loading, size, charge, and overall effectiveness of Nanostructured Lipid Carriers (NLCs) [18].

Solid lipids:

Solid lipids must possess chemical stability, biodegradability, and freedom from harmful substances. The solid lipids that are employed are crystalline at ambient temperature, but they melt above 80°C and higher. Testing the drug's solubility in lipids is the first step in selecting lipids for formulation since it directly affects drug entrapment and loading efficiency [16]. Stearic acid, glyceryl palmitostearate, glyceryl behenate, and glyceryl monostearate (GMS) are the solid lipids that are most commonly used in the production of NLC. In particular, they are transporters of surface-active compounds [17]. Stearic acid has been reported to be reasonably less harmful and biocompatible with human tissues and bodily fluids, while GMS is widely used and is non-poisonous and non-irritating due to its minimum 40% monoacylglycerol saturated fatty acid content. Owing to many flaws in the crystalline cross-section, the glyceryl behenate exhibits high entrapment efficiency and promising stability [17]

Liquid lipids:

Liquid lipids in Nanostructured Lipid Carriers (NLCs) are derived from digestible oils or oily components, prioritizing biocompatibility, cost-effectiveness, and non-irritation. All lipids used must have regulatory approval as Generally Recognized as Safe (GRAS). The quantity of liquid lipids significantly influences NLC particle size and release rate, reducing drug escape, and increasing drug entrapment, resulting in a smaller size, greater surface area, and enhanced cumulative drug release. It was observed decrease in particle size with increased liquid lipid is attributed to reduced viscosity and heightened molecular mobility [16].

EMULSIFIER

Surface active agents, or emulsifiers, play a crucial role in stabilizing the liquid dispersion of Nanostructured Lipid Carriers (NLCs) by adsorbing at the interface, reducing tension between the lipid and aqueous phases. The choice and concentration of surfactant significantly impact the physical stability, toxicity, quality, and efficacy of NLCs, as well as drug permeability and dissolution. Factors such as the route of administration, impact on particle size, hydrophilic-lipophilic balance (HLB) value, and lipid modification influence surfactant selection. Combining ionic and non-ionic surfactants enhances stability, improves viscosity, and effectively prevents particle aggregation [19]. When formulating NLCs, a critical consideration is the required HLB (rHLB) for the chosen surfactant. Achieving a small particle size and a stable nanosystem is facilitated by selecting surfactants with an appropriate HLB. NLCs with more hydrophilic surfactants and an HLB value exceeding 18 exhibits faster release patterns. Studies suggest that low molecular weight surfactants redistribute more quickly than their high molecular weight counterparts [16]. Commonly incorporated hydrophilic emulsifiers include polysorbates (Tween) and Pluronic F68 (poloxamer 188), while lipophilic emulsifiers such as lecithin and Span 80 are frequently used. The presence of polyethylene glycol (PEG) in the nanoparticle shell delays the circulation period of medication by preventing uptake by the reticuloendothelial framework [17]. Consideration of surfactant toxicity is essential, as not all surfactants are suitable for manufacturing all types of SLNs and NLCs. Surfactants are ranked in decreasing order of toxicity as follows: amphoteric, cationic, anionic, and non-ionic [14].

POLYMORPHISM AND CRYSTALLINITY OF LIPIDS

Polymorphism arises when molecules adopt distinct lattice arrangements, resulting in more than one crystalline form. Variations in polymorphic forms stem from differences in the packing of hydrocarbon chains. Colloidal lipid systems, particularly the inner layer lipids, display intricate crystalline behaviours during both cooling and storage. The use of complex lipids, like long fatty acid chains, is the best option to improve long-term stability and increase the number of medications that can be encapsulated to prevent drug shedding caused by polymorphic transitions, a blend of long- and short-chain fatty acids to attain the intended particle size and stability. Cryoprotectants can be used to overcome the effects of polymorphism and crystallinity of lipids on drug incorporation, drug loading efficiency, and release properties in SLNs [14]

Table 1. COMPARISON BETWEEN SLN and NLC

PRODUCTION METHODS TO OBTAIN SLNs and NLCs

The methods employed in the creation of SLNs and NLCs have a significant impact on their characteristics like particle size, drug loading capacity, drug release characteristics, and drug stability.  For the production of finely dispersed SLNs and NLCs, various techniques are employed. The most popular techniques include high-pressure homogenization (both hot and cold), ultra-sonication or high-speed homogenization, double emulsion, micro emulsification, solvent injection, solvent emulsification (evaporation or diffusion), spray drying, microfluidics, microwave-assisted synthesis, ultrasound-assisted synthesis. These methods are schematized in fig. 3 and 4 and are briefly described in the following sections [10].

Figure 3: Production methods to obtain SLNs and NLCs

High-Pressure Homogenization (HPH):

This approach is widely favoured for its solvent-free preparation, making it a reliable and robust method for large-scale production of Solid Lipid Nanoparticles (SLNs) and Nanostructured Lipid Carriers (NLCs). It yields highly stable particles without the need for the addition of organic solvents. Two variants of high-pressure homogenization exist—hot HPH and cold HPH. In both, the initial step involves dissolving the drug in a solid lipid melted at a temperature approximately 5–10 ̊C above its melting point [10]. In (Hot HPH) method, the drug–lipid melt at the same temperature as the surfactant solution is introduced into surfactant solution. Following this, high-shear mixing homogenously disperses the substances, forming a pre-emulsion. The hot pre-emulsion, maintained at the same temperature, undergoes high-pressure homogenization to achieve nanoscale particle size, typically requiring 3-5 cycles at 500-800 bars. The final step involves cooling the oil-in-water (o/w) nanoemulsion to room temperature, facilitating lipid re-crystallization and the formation of the solid lipid nanoparticle matrix. Generally, elevated temperatures result in smaller particle sizes due to decreased inner phase viscosity. However, higher temperatures also accelerate drug and carrier degradation [10].

Figure 4: Spray drying (A) and microfluidics (B) methods to obtain SLN and NLC

The cold HPH method uses liquid nitrogen or dry ice to quickly cool the drug-lipid melt. The drug must be evenly dispersed throughout the lipid matrix due to the quick cooling. Subsequently, the resulting solid is ground into micron-sized particles (using a mortar or ball mill), and then it is dissolved in a cooled aqueous surfactant solution (pre-emulsion). Next, at room temperature or lower, the pre-emulsion undergoes high-pressure homogenization, which breaks the microparticles into nanoparticles. Cold-homogenized samples generally show bigger particle sizes and a wider size distribution than hot-homogenized samples [10].

Ultra-Sonication or High-speed Homogenization:

The lipid phase and the aqueous phase containing the surfactant are mixed using ultrasonication and high-speed homogenization. This approach often yields high polydispersity nanoparticles, which can be reduced by a probe-based sonicator. Although a less diffused distribution is obtained, cross-contamination from the probe metal remains a possibility [10].

Double Emulsion:

The preparation of warm (w/o/w) double microemulsions involves a two-step process. Firstly, a primary emulsion (w/o) is created by dissolving a hydrophilic drug molecule in an aqueous solvent (inner aqueous phase). This primary emulsion is dispersed in a melted lipid phase containing a surfactant and co-surfactant, forming the oil phase. In the second step, an aqueous solution containing a hydrophilic emulsifier is introduced to the primary emulsion, leading to the formation of a double emulsion (w/o/w) upon stirring. Despite its ability to generate nanoparticles, this method is cautioned for certain administration routes due to the resulting high polydispersity [10].

Microemulsion:

Similar to HPH, it begins with melting the solid lipids at a temperature that is 5 to 10 ̊C higher than its/their melting temperatures, then dissolves the drug in the melted lipids. The drug-lipid melt is then combined with an aqueous surfactant solution that is heated same as the melted lipids temperature. This mixture is continuously stirred until a transparent microemulsion is produced. The resulting microemulsion is gently stirred in cold water to disperse the microparticles, which then break into nanoparticles that crystallise to create the SLN or NLC. This approach yields diluted nanoparticles, hence lyophilization or ultrafiltration must be used to concentrate the preparation at the end of the process. The requirement for a high concentration of surfactants is the primary drawback [10].

Solvent Injection:

This method involves dissolving lipids in pharmacologically acceptable water-miscible organic solvents, including ethanol, acetone, or isopropanol, and then injecting the mixture into an aqueous phase while continuously mixing it to precipitate the lipids. After that, the dispersion is filtered to get rid of extra fat. Lipid droplets form at the injection site when an emulsifier is added to the aqueous phase, stabilising the particle until solvent diffusion takes place [10].

Solvent Emulsification-Evaporation:

This procedure involves dissolving the drug and lipophilic substance in an organic solvent before emulsifying them in an aqueous solution while stirring. While stirring, the organic solvent evaporates, causing the lipid to precipitate in the aqueous phase and form nanoparticles with a mean size of 25 nm. By using high-pressure homogenization, the solution was emulsified in an aqueous phase. Evaporation at low pressure (40–60 mbar) was used to extract the organic solvent from the emulsion. The size of the particles is directly related to the lipid concentration because there is no thermal stress with this approach, it is appropriate for medications that are thermolabile. Drugs that can interact with the organic solvent, however, cannot be treated with this approach. [10,12].

Solvent Emulsification-Diffusion:

Particles can be obtained with typical sizes ranging from 30 to 100 nm. This method prevents the solvent from diffusing from the droplets into the aqueous phase by using a partially water-miscible organic solvent that contains saturated water to reach thermodynamic equilibrium. In a water bath set at 50ˊ C, the drug and lipid are dissolved in the organic phase. They are then added to an acidic aqueous solution that contains a surfactant to alter the zeta potential and form an o/w emulsion that is easily separated by centrifugation. To produce the particles, more water is added, which promotes precipitation of the nanoparticles and solvent diffusion into the continuous phase [10,12].

SECONDARY PRODUCTION STEPS

Lyophilisation:

The process of lyophilization holds great promise for enhancing long-term chemical and physical stability. For a product containing hydrolysable pharmaceuticals or one that was appropriate for oral administration, lyophilization is necessary to achieve long-term stability. Oswald ripening and hydrolytic reactions would be avoided by transition into the solid state. Due to the existence of aggregates in between the nanoparticles, all of the lipid matrices employed in the product's freeze-drying process create larger nanoparticles with a wider size dispersion. The aggregation of SLNs and NLCs is facilitated by the conditions of the freeze-drying process and the elimination of water. The agglomeration of nanoparticles during the freeze-drying process can be prevented with a sufficient dose of cryoprotectant [12,43].

Spray Drying:

As an alternative to lyophilization techniques in the manufacturing of NLCs, the spray drying method is more frequently employed for lipid phases with high melting points. This process causes the particles to partially melt and increase in kinetic energy, which leads to repeated particle collisions. This process also causes particle agglomeration due to exposure to high temperatures and shear stress. Solid lipid with a melting point of more than 70 ̊C should be used at a concentration of 1% w/v in an aqueous trehalose solution to maximise the yield. Upon drying, the carbohydrates create a thin protective coating around the particles, lessening the destabilising effects of shear and heat. Spray drying is more cost-effective and efficient than other methods, but it is not a commonly used process for producing NLCs because of the risk of particle aggregation and degradation, potential structural changes to the lipid core [20].

NEW FORMULATION METHODOLOGIES

Using thermal heating, also known as conductive heating, is a common component of the previously listed methods [22]. In most cases, conductive heating is used to heat formulation ingredients using an external heat source, such as an oil or water bath. Because this kind of heating is dependent on the thermal conductivity of each material, the mixture's temperature will always be uneven and the exterior of the container will always be warmer than its interior. As a result, there is very little energy transfer efficiency, which results in goods with irregular qualities [23].

Microwave-Assisted Synthesis (MAS):

As a result of the heating process being dielectric—that is, dependent on the dielectric properties of materials originally proposed the creation of solid lipid nanoparticles by microwave-assisted synthesis as a way to get around the aforementioned obstacle. With this method, the temperature within the formulation container rises quickly as a result of the microwaves coupling with particles within. Ionic conductance and/or dipole polarisation will happen instantly due to overheating, as there is no reliance on the reaction vessel's thermal conductivity. In this way obtaining more homogeneous and better-characteristic formulations [24,26].

Advantages of Microwave Heating over Conventional Heating:

Electric heaters or bath systems with adjustable temperatures are examples of external heat sources that are commonly used in conventional heating. Thus, thermal energy is collected by the barrier separating the reaction vessel from the heat source and exchanged with the reaction solvent. As a result, the reactants start interacting and eventually combine to form the reaction's product. The chemistry of microwave-assisted synthesis indicates that it provides the opportunity for continuous heating of the reagents throughout the reaction medium, transferring heat rapidly and uniformly where thermal energy intervenes from the materials' nuclei to the exterior, producing an effective method of homogenous production [25]. Nonetheless, a few factors will determine the heating's dimensions: the strength of the microwave field, the material's volume, the reaction duration, the material's dielectric characteristics, and the shape of the reaction vessel [24,27]. Compared to traditional heating, MAS can accelerate the reaction rate by a factor of 10 to 1000 which is illustrated in fig. 5. For instance, with microwave heating, reactions that typically take place in ten hours could instead take thirty-five minutes [28].

Figure 5: Conventional technique (A) and microwave-assisted synthesis (B)

Ultrasound-Assisted Synthesis (UAS):

Apart from the MAS, the UAS is notable in terms of the new methods for the elaboration of SLN. This method is predicated on the emission of sound waves at various intensities and frequencies that are higher than the range of human hearing (>16 kHz) [29]. Due to their inverse relationship with frequency, these intensities can be categorised as low or high. Since low-intensity ultrasound has very low powers (<1 W/cm2) and frequencies between 1 and 10 MHz, it does not result in physicochemical changes that include destruction. However, the frequencies and powers of high-intensity ultrasound range from 16 to 100 kHz and 10 to 1000 W/cm2, respectively. It is frequently employed to modify a material's characteristics to promote a restructuring, such as when making emulsions, depolymerizing, deflocculating, and reducing particle size [29,30].

CHARACTERIZATION

Characterizing nanocarriers is necessary before using them in therapeutic environments. Due to their small size and the diverse nature of the system in comparison to other colloidal carriers, SLNs and NLCs are challenging to characterize. The main attributes of the SLN and NLC are particle size, shape, polydispersity index (PI), zeta potential, % drug entrapment efficiency, drug crystallinity, stability [31].

Particle Morphology and Surface Charge:

A crucial feature of lipid nanocarriers is their particle size and range of size distribution, which has a substantial impact on the drug release profile, mucoadhesion, stability, efficiency, encapsulation, and cell uptake of the nanocarrier. Particles smaller than 500 nm, having a narrow size distribution range, exhibit greater stability and a reduced propensity to aggregate while being stored. For instance, a drug's sustained delivery is possible when the particle size is larger than 300 nm, in this instance, fast action is demonstrated when the size range is between 50 and 300 nm. To create safe, stable, and effective nanocarriers, homogenous (monodisperse) populations of nanocarriers of a particular size must be prepared. The size distribution is indicated by the polydispersity index (PDI) [14,31].

Laser diffraction (LD) and photon correlation spectroscopy also referred to as dynamic light scattering (DLS) are frequently used to measure the size of lipid nanocarriers. Laser diffraction is used to detect particle size for the range of 100 nm to 180 μm, while DLS is primarily used to determine the micro-particle size and the intensity of scattered light resulting from the random motion of particle size range within 0.1µm to 2500 µm [21]. Using transmission electron microscopy (TEM) and scanning electron microscopy (SEM) to examine the surface morphology of drug-loaded NLCs [20]. The size of the charge on the particle surface in an aqueous dispersion is determined by the zeta potential. This enables the prediction of the formulations' long-term physical stability, pH, ionic strength, solvent type, and ions in the surrounding aqueous phase can all affect the zeta potential (ZP) [14]. Minimum zeta potentials of more than -60.0 mV and more than 30.0 mV are needed, respectively for adequate physical stability in SLNs and NLCs [31]. Particle aggregation is reduced and electrostatic repulsion is increased with increasing surface charge [20].

Degree of Crystallinity:

The efficacy of drug release from NLCs is intricately tied to the crystal lattice structure and lipid constituents' conditions. The incorporation of pharmaceuticals benefits from additional crystal lattice flaws, theoretically improving encapsulation. Differential scanning calorimetry (DSC) is employed to evaluate lipid component states, leveraging variable melting enthalpies and points across different lipids. Enhancing medication encapsulation and chemical stability in NLCs involves utilizing solid lipids with multiple lattice defects [20]. DSC identifies temperature-related issues and can detect even the slightest thermal activity based on device sensitivity. Further characterization through techniques like thermogravimetry (TGA) and X-ray diffraction (XRD) is necessary to discern thermal shifts, polymorphic changes, melting, hydrate water loss, and breakdown. XRD produces interference effects from X-rays scattered by a crystal's atoms, revealing crystal structure and polymorphic forms. TGA allows the examination of melting point, crystallinity, polymorphism, and endothermic/exothermic properties by heating samples under controlled atmospheres like nitrogen, oxygen, and argon [14].

Load Capacity and Entrapment Efficiency:

The term "drug-loaded" refers to the relationship between the total weight of the particles and the active ingredient API loading capacity; meanwhile, "drug entrapment efficiency" refers to the proportion of the total drug present in the dispersion that is trapped in the carrier. It is crucial to ascertain the encapsulation efficiency since the amount of medicine enclosed in the nanocarriers affects the release kinetics. Following the removal of the free API, EE is calculated using ultracentrifugation, through dialysis, ultrafiltration, and gel filtration. The drug's quantity is calculated using a standard analytical method, such as high-performance liquid chromatography (HPLC) or UV spectrophotometry [14]. Drug-loaded percentage in EE Formulation characteristics including the type and concentration of SL and LL have an impact on lipid nanocarriers. In addition, the type of the medicine may have an impact on EE per cent; for example,

amount of theoretical API in the formulation (mg)

Stability:

According to International Council on Harmonization (ICH) guidelines, the stability of lipid nanocarriers can be assessed by monitoring changes in parameters such as zeta potential, shape, size, polydispersity index, drug content, entrapment efficiency, drug release profile, and viscosity during storage at different temperatures. Long-term stability is predominantly influenced by external factors like temperature, light, and mechanical stress [23]. Storage at 20°C did not induce aggregation or drug release in lipid nanocarriers, but rapid particle size growth occurred at 50°C. Generally, a zeta potential higher than -60 mV is crucial for maintaining physical stability, and storage at 4°C provides a more favourable environment. For SLNs, spray drying and lyophilization are employed to enhance or extend stability, especially for preparations intended for intravenous delivery. The addition of cryoprotectants and improvements in redispersion can reduce SLN aggregation by preventing contact between discrete lipid nanoparticles, promoting the vitreous state of the frozen sample, and lessening water's osmotic activity and crystallization [12,14].

In vitro drug release:

In assessing the in vivo performance of lipid nanocarriers, an in vitro analysis of drug release provides valuable insights [32]. Guidelines governing the release of the encapsulated API from lipid nanocarriers consider factors such as particle size, homogeneous dispersion, lipid crystallinity, and API mobility. Drug release mechanisms include dissociation from the outer layer, diffusion through the polymer matrix, membrane-controlled diffusion, erosion of the nanoparticle matrix, or a combination of diffusion and erosion processes. Typically, the release profile of APIs trapped in NLCs exhibits a biphasic pattern, characterized by an initial burst effect followed by sustained release over hours or days [14]. Dialysis bags are commonly used to measure the total amount of released medication from lipid nanocarrier dispersions. Samples are taken at intervals, centrifuged, and analysed for drug content using appropriate techniques. An alternative method involves Franz diffusion cells, where a cellulose membrane separates the release buffer in the receptor compartment from the drug-loaded Nanostructured Lipid Carriers in the donor compartment. Regular analysis of the release medium determines the drug release profile using techniques like High-Performance Liquid Chromatography (HPLC) or UV spectrophotometry [12,20].

ROUTES OF ADMINISTRATION

Oral delivery:

NLCs represent an effective approach for enhancing the oral delivery of poorly water-soluble medications with low bioavailability. Their remarkable dispersivity, leading to a large specific surface area, makes them susceptible to enzymatic assault by intestinal lipases. The oral administration of NLCs offers additional advantages, including increased drug loading, improved drug inclusion, enhanced patient compliance, high particle concentration, and a cream-like carrier consistency. Factors contributing to NLC absorption from the gastrointestinal (GI) tract involve direct uptake, heightened surfactant permeability, reduced degradation, and clearance. Furthermore, NLCs can adhere to the intestinal wall, prolonging residence time and consequently improving absorption rates. Poloxamer, a key component, facilitates NLC paracellular transit by deforming intestinal epithelial cell membranes and opening tight junctions. Additionally, poloxamer 407 not only enhances NLC transport across the intestinal mucosa but also inhibits the p-glycoprotein efflux pump. Recent findings suggest that oleic acid may impact the enzyme activity of CYP3A [19].

Topical delivery:

When delivering drugs to cutaneous locations, the topical route has been extensively utilised with lipid-based nanoparticles by forming a large concentration gradient on the skin, NPs can increase the apparent solubility of medications that are captured and aid in drug penetration. To increase penetration and prolonged release, NPs are applied topically to a variety of drug categories [19]. For instance, the topical treatment of psoriasis with acitretin NLC-loaded gel has demonstrated improvement in therapeutic response and reduction in local side effects. The produced NLCs had a spherical form, and the release analysis revealed a biphasic drug release pattern, with a constant drug release phase coming after an initial sustained release phase that lasted up to 10 hours [33].

Parenteral delivery:

Over the past 20 years, nanoparticles (NPs) have shown great promise for better parenteral delivery of hydrophobic agents. They have been viewed as a viable substitute for liposomes and emulsions because of their improved properties, which include ease of manufacturing, high drug loading, and increased flexibility in modifying drug release profiles. In addition, because NPs are aqueous and the excipients are biocompatible, they can be used to deliver drugs intravenously with passive targeting and simple elimination. With regards to drug distribution within the body, it has been observed that SLN causes an increase in drug concentrations in the brain, spleen, and lungs, whereas the traditional version of the medication causes distribution into the kidneys and liver. Additionally, the irritating impact brought on by microparticles is lessened when the medicine is injected as SLNs [19,21].

Pulmonary delivery:

Inhalation drug delivery shows promise in treating various pulmonary disorders, offering advantages over traditional dosage forms such as non-invasiveness, avoidance of systemic toxicity and first-pass metabolism, reduced dosing frequency, and site specificity with direct drug delivery to the lung epithelium. Nebulization of SLNs containing medications for conditions like tuberculosis, asthma, and cancer has proven effective in enhancing drug bioavailability and reducing the required dosage, particularly for lung-targeted action. For instance, paclitaxel-loaded SLNs administered as inhalers demonstrated enhanced efficacy in treating lung tumours [21]. Although inhalation delivery of anti-cancer drugs using nanoparticles and liposomes has been explored infrequently, challenges such as nebulization instability, biodegradability, drug leakage, and unfavourable side effects persist. However, the use of a combination of liquid and solid lipids has successfully encapsulated the lipophilic COX-2 inhibitor celecoxib in NLC [19].

Ophthalmic delivery:

The formulation of ophthalmic drugs is enhanced by the mucoadhesive properties of nanoparticles, leading to prolonged pre-corneal retention, increased penetration, and improved bioavailability in the aqueous humour. This is achieved by incorporating permeation enhancers like Gelucires 44/14 (a solid lipid type) and Transcutol IP into the formulations. These enhancers, along with stearyl amine, contribute to optimizing the ocular drug delivery of NLCs, resulting in higher bioavailability compared to conventional eye drops. As per the findings of the in vivo distribution study, thiolate NLC has the potential to extend the pre-corneal residence duration and introduce elevated quantities of cyclosporine into the ocular surface and anterior chamber tissues of the eye [19,21].

APPLICATIONS

Chemotherapy:

Cancer, characterized by the abnormal growth of tissues, poses challenges in current treatments due to the toxicity of anti-cancer drugs to both tumours and normal cells during chemotherapy. Recent studies highlight the potential of nanoparticles in enhancing drug efficacy and stability while minimizing undesirable side effects. In cancer biology, nanotechnology offers innovative therapeutic strategies, particularly in parenteral drug delivery. Intravenous administration of tamoxifen citrate-loaded SLNs in rats demonstrated favourable pharmacokinetic parameters, suggesting prolonged blood circulation. Topotecan-loaded NLCs exhibited stabilization and prolonged release, addressing challenges associated with SLNs. Hyaluronic acid-coated NLCs containing paclitaxel demonstrated increased circulation time, enhanced drug accumulation in tumours, and higher anti-tumour efficacy with fewer side effects compared to Taxol. [9,19].

Brain delivery:

Targeting the brain not only elevates drug concentration in the cerebrospinal fluid but also diminishes dosing frequency and side effects. This route offers key advantages, such as bypassing first-pass metabolism and ensuring a rapid onset of action compared to oral administration. Second-generation NLCs emerge as a significant strategy for drug delivery without requiring modification to the drug molecule. Their bioacceptability, swift brain uptake, ability to cross the blood-brain barrier (BBB), and biodegradability contribute to their effectiveness [19].

Cosmetics:

These nanocarrier dispersions are available in various cosmetic forms, including gel, cream, lotion, and ointment, providing a wide range of beneficial effects in cosmeceuticals. These effects include improved skin bioavailability of active ingredients, UV protection, film formation, controlled occlusion, penetration enhancement, epidermal targeting, and enhanced physical and chemical stability, as well as in vivo skin hydration. Studies have shown that using glyceryl behenate SLNs, compared to traditional formulations, results in better localization of vitamin A in the higher layers of the skin. Furthermore, the development of a topical gel containing valdecoxib-NLC demonstrated quicker and more effective relief for inflammation. Tests for an epicutaneous patch, which is non-irritating to the skin, revealed that NLCs containing Cutanova Cream Nanorepair Q10 performed better in terms of skin hydration compared to a traditional oil-in-water cream with the same composition [34-36].

Gene delivery and gene therapy:

Efficient and safe gene therapy, particularly in mammalian cell gene transfer, remains a significant challenge. Gene delivery systems are broadly categorized as viral vectors with high transfection efficiencies and non-viral vectors known for low immunogenicity. Colloidal particulate delivery systems, including cationic liposomes, SLNs, nanoemulsions, micelles, and certain polymers like polyethyleneimine (PEI), emerge as promising candidates for efficient non-viral gene delivery. Cationic liposomes and PEI have been extensively studied, forming complexes with DNA for delivery through endosomes. Their polycationic NLC loaded with triolein demonstrated enhanced transfection efficiency, establishing its effectiveness as a non-viral gene transfer vector [37,38].

Nutraceuticals:

Bioactive substances called nutraceuticals have therapeutic or health benefits, such as illness prevention and treatment. Carotenoids are a very significant class of natural pigments due to their diverse range of structures, several roles, and extensive distribution across plant tissues. Using natural oils and a flexible high-shear homogenization method, carotene-LNC with strong antibacterial and highly antioxidant properties was effectively synthesised [39].

CONCLUSION

In recent years, both Solid Lipid Nanoparticles (SLNs) and Nanostructured Lipid Carriers (NLCs) have emerged as promising drug delivery systems, demonstrating efficacy across various administration routes for therapeutic purposes. NLCs, preferred for their superior stability and drug-loading capacity, offer flexibility and controlled release profiles, impacting fields like medicine, gene editing, food, and cosmetics. Extensive research underscores the low toxicity of these lipid-based systems at therapeutic doses, emphasizing their biocompatibility and ability to accommodate diverse drugs. With targeted delivery and improved permeation, ongoing developments aim to overcome existing challenges, paving the way for third-generation lipid nanocarriers. This highlights the potential of lipid-based nano-drug delivery systems as valuable tools for efficient drug delivery in diverse therapeutic applications.

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