Nanoparticles are submicron-sized colloidal particles. These colloidal systems are dispersions of one phase within another and can vary in size from 1nm to 1μm. They are widely used throughout scientific research in numerous applications in electronic, optical, biomedical and pharmaceutical fields. In the pharmaceutical field, nanoparticles are widely used as drug delivery systems. As a drug delivery system, nanoparticles can be prepared using different biodegradable materials such as natural or synthetic polymers, metals or lipids . In these nano-particulate drug delivery systems, the drug is either absorbed/ conjugated into its outer surface or encapsulated within its core . Nanoparticles are usually separated into two classes: nanospheres and nanocapsules. Nanospheres can be described as a polymeric solid particle that incorporates a drug into its system by dissolving, entrapping, binding, encapsulating, or absorbing it to its matrix . Likewise, nanocapsules are also composed of polymers but instead of trapping the drug within, the nanocapsule form a polymer membrane surrounding the drug.
Potential advantages of nanoparticles drug delivery systems include :
- High stability (e.g., longer shelf life)
- High carrier capacity
- Incorporation of both hydrophilic and hydrophobic substances
- Feasibility of drug delivery through various routes of administration (oral, inhalation, IV, etc.)
Although there are many advantages of using nanoparticles as a drug delivery system, the cons may be more concerning to researchers. The use of polymers and organic solvents gives rise to concerns about toxicity and biocompatibility. Therefore, the SLNs have been a hot topic of nanoparticle research in recent years. SLNs are similar to polymeric nanoparticles in their ability to encapsulate or bind to the drug involved, but are prepared with lipids that are biocompatible and without the use of toxic solvents. Consequently, the possibility of toxicity can be significantly reduced using SLNs.
Background into Solid Lipid Nanoparticles
SLNs were developed at the beginning of the 1990s as an alternative carrier system to emulsions, liposomes and polymeric nanoparticles . SLNs are an ideal drug delivery system because they do not require the use of organic solvents and their ability to encapsulate the drug within its lipid matrix allows for sustained drug release. As described by Uner et al , there are two types of drug incorporation models: the “solid solution” model and “core-shell” model. The drug is usually incorporated as a diluted solid solution into the lipid matrix. By the core-shell model, the drug is thought to be either incorporated into the shell of the nanoparticle or within the core of the SLNs. Drug incorporation within the core model occurs as the nanoemulsions are cooled, which leads to super-saturation of the drug. The drug then precipitates in the lipid melt before recrystallization. As the emulsion is cooled further, the lipid begins to recrystallize around the drug, forming a shell around the drug core. The opposite occurs by the process described as the shell model. In this model, the lipid recrystallization temperature is reached. As the dispersion is cooled further, the drug settles around the liquid outer shell of the SLNs. The drug release from lipid phase of nanoparticles is usually controlled by the nano-sized particles of prepared SLNs. Smaller particles in the nano-size ranges create a greater surface area and promote faster drug release. Slow drug release can be achieved when the drug is dispersed into the lipid matrix, which depends on the drug incorporation model. Finally, the crystallinization of the lipid carrier and high mobility of the drug can result in fast drug release. [5, 6].
SLNs Methods of Preparation
“High Pressure Homogenization” is a common method for the preparation of SLNs and can be divided into two types: hot homogenization and cold homogenization. In hot homogenization, lipids are melted followed by addition of drug. A surfactant solution is heated and added to the drug-lipid mixture at the same temperature. This formed pre-emulsion is homogenized, which results in the formation of a hot oil/water (O/W) nano-emulsion. The solution is allowed to cool at room temperature. In recent years, high pressure homogenization has been used in the preparation of SLNs for antitumor drugs such as Emodin (EMO)  and topical corticosteroid drugs such as Betamethasone valerate (BMV) . EMO SLNs were prepared using glyceryl monostearate (GMS) as a lipid. GMS is a commonly-used lipid and has been utilized in the preparation of many SLNs , more specifically in the topical corticosteroid drug BMV and the chemotherapy drug 5-flurouracil  (5-FLU). Poloxamer 188, a commonly used surfactant was utilized in the preparation of both EMO-SLNs and 5-FLU-SLNs, whereas Polysorbate 80 was used in the preparation of BMV-SLNs. Furthermore, a hot homogenization technique was used in the preparation of tocopherol (Vitamin E) SLNs which were incorporated into sunscreen . These Vitamin E-SLNs were prepared using tocopherol, acetyl palmitate (lipid) and Tego Care 450 as the surfactant. A gel formulation was formed with the addition of water, xanthum gum, and glycerol as the hydrating agent before adding it to the SLNs . By substituting the water phase with the aqueous SLNs and decreasing the lipid content, a cream was easily formulated. SLNs from cold homogenization technique are prepared by freezing the drug-loaded lipid with liquid nitrogen, grinding it to a micro particle, and then adding the resulting powder mixture to the aqueous surfactant mixture . This pre-emulsion is then homogenized at or below room temperature. Cold homogenization technique minimizes the loss of the lipid through melting, thereby reducing the loss of the drug in the aqueous phase . Also, by avoiding the use of any heat, temperature-induced drug degradation is less likely to occur.
SLNs can also be formulated by the use of various emulsification techniques. Microemulsions are prepared using a lipid, surfactant and/or co-surfactant, and water. Microemulsions are prepared at approximately the melting temperature of the involved lipid . At these temperatures, the surfactant mixture is added with the lipid. The mixture is stirred and a cold (2-3°C) aqueous medium is added, ensuring that the small particulates within the emulsion are formed due to precipitation and not induced from the mixing process. The simplicity in preparing microemulsions makes this technique ideal for manufacturing SLNs in a larger scale. The microemulsion procedure could be modified with the addition of multiple lipids and surfactants. In various formulations prepared by Li et al, tetrandrine (TET), used to treat hypertension and inflammation, was formulated along into SLNs using the melt emulsion technique . Similar to the microemulsion technique, melt emulsification incorporates both the lipid and surfactant solution, slowly adding it to an aqueous phase. Instead of TET being added to a single lipid/ surfactant melted solution, Li et al prepared various SLNs formulations and tested their effectiveness using three lipids (GMS, Precirol ATO 5 (PA), and stearic acid (SA)) and a combination of three surfactants (Lipoid E80, Pluronic F68, and sodium deoxycholate (SDC)). Lastly, the “multiple emulsion” technique has been used in preparing SLNs. In the research carried out by Du et al, 5-FLU-SLNs were formulated with the lipid/ surfactant mixture containing GMS and soya lecithin dissolved in chloroform, slowly incorporating the aqueous phase containing 5-FLU . This initial phase formed a primary emulsion. A secondary emulsion was formed when the primary emulsion was added to another surfactant mixture containing Poloxamer 188 and 5% glucose. In both experiments, a homogenous mixture was obtained with a rapid stirring rate and/or was sonicated using an ultrasonic probe.
Characterization of SLNs
SLNs are characterized by their particle size and zeta potentials . Particle size of SLNs is usually determined using photon correlation spectroscopy (PCS) and laser diffraction (LD) . PCS is widely used in measuring nanoparticles about 5μm or smaller. This instrument measures the fluctuations in the intensity of light caused by the particles moving in solution . LD is the ideal tool for larger microparticles. LD measures the diffraction angle on the particle radius , therefore a smaller particle can have more intense scattering at high angles compared to larger ones. Zeta potential measures the particle charges within the emulsion. The greater the zeta potential (either positive or negative), the greater the repelling electric charges are between the particles. It is observed that a high zeta potential will result in less aggregation within the emulsion. Other methods used to characterize drug and lipid behavior in SLNs include X-Ray Diffraction (XRD) and Differential Scanning Calorimetry (DSC). X-Ray Diffraction (XRD) can provide information about the crystallization of drugs and lipids in SLNs. Various polymorphic transitions of lipids can be studied using XRD. DSC gives a qualitative and quantitative insight by determining endothermic and exothermic transitions of SLNs .
SLNs in Cosmetics and Topical Drugs
SLNs in Cosmetics and Topical Drugs Particularly beneficial SLNs features in dermal applications include :
- Excellent tolerability: SLNs are composed of physiological and biodegradable lipids that exhibit low toxicity and low cytotoxicity.
- Excellent transdermal delivery: the small size of SLNs ensures close contact with the stratum corneum, increasing the amount of drug penetrating through the skin.
- Hydration Benefits: SLNs provide occlusive properties that result in increased skin hydration.
There are a few drawbacks that have to be considered before using SLNs for physiologically-tolerated formulations. SLNs used in cosmetic or dermal applications have higher water content, about 70-99% . High water content can lead to various stability issues within the final topical formulation. Therefore, in some topical applications, nanostructured lipid carriers (NLCs) are preferred. NLCs possess a greater lipid content (40- 50% w/w) that considerably reduces water content in the final formulation . This principle is successfully used in preparing stable SLN formulations containing the corticosteroid betamethasone-17-valerate (BMV). BMV was found to be more soluble in a melted lipid containing a high concentration of monoglycerides . Jensen et al correlated this observation to the fact that monoglycerides possess surfactant properties that contribute to the dissolving of the BMV, thereby increasing its solubility. BMV NLCs showed a decrease in drug release with increasing lipophilicity of the corticosteroid. Therefore, SLNs with corticosteroids mostly resemble a drug enriched shell model possibly combined with a solid suspension .
Another example of an SLN topical formulation is that of lutein, a carotenoid that is found naturally in human skin and plays a protective role in skin cancer by filtering blue light and scavenging the free radicals . Lutein plays a role in maintaining skin health by reducing UV-induced erythema and inflammation . Mitri et al prepared lutein SLN formulations that followed a drug-enriched core model . Another antioxidant used in derma-pharmaceutical and cosmetic products is tocopherol acetate (Vitamin E) . Wissing et al formulated tocopherol SLNs formulations that were able to reduce skin damage caused by UV radiation . UV blockers usually work in two ways: by the absorption and reflection/scattering of UV radiation. Tocopherol acetate is a weak UV absorber . When incorporated into a carrier system like an SLN that has a UV blocking effect, tocopherol’s overall UV-blocking effect was also found to increase .
SLNs and Cancer Drugs
SLNs are being currently used in numerous studies as drug delivery systems for cancer drugs [7, 10, 13, 17]. Due to their small size and structure, SLNs create the ideal drug delivery system for allowing penetration into cancer tumors. There are a few notable examples of anticancer drugs in recent drug delivery research using SLNs. In one study by Du et al, 5-Fluorouracil (5-FLU), a commonly used antimetabolite anticancer treatment for lung carcinoma, was incorporated into an SLN complex to create a more efficient drug delivery system. Like many problems with anticancer drugs, 5- fluorouracil was limited in its use due to its high toxicity, short half-life, and low bioavailability . SLN formulations allowed 5-FLU to be encapsulated within its matrix, protecting it from chemical degradation, increasing drug bioavailability and decreasing the dosing frequency . Studies of 5-FLU were done on rats, comparing intravenous injections of 5-FLU-SLN suspensions and a placebo mixture of 5-FLU/ 5% (w/v) glucose solution control group. Du et al observed an increase in bioavailability of 5-FLU after injecting the FLU-SLNs into the rats10. Overall, the use of SLNs as a drug delivery system with hydrophilic drugs such as 5-FLU allowed greater drug localization and sustained drug release.
With hydrophobic anti-tumor drugs like Tetrandrine (TET) and Emodin (EMO), a common problem in their clinical application is their poor oral bioavailability due to their low aqueous solubility[7, 13]. TET was poorly soluble in water but was highly soluble in fatty acids. Therefore, Li et al formulated TET SLNs using three lipids in a 3:1:1 ratio (Precirol: GMA: Stearic Acid). By doing so, the drug was encapsulated within the SLNs, resulting in high drug entrapment efficiency. Similarly, EMO remained stable in SLNs containing stearic acid as lipid and Poloxamer and Tween 80 as surfactants. EMO-SLNs resembled a drug-enriched core model and were found to have a sustained drug release over a period of 72 hours . Temozolomide (TMZ) is another cytotoxic drug specified for brain tumors that has been incorporated into an SLN complex. Although TMZ has shown significant effects in the treatment of intracalvarium spongioblastoma and malignant tumors, a topic of concern is its side effects. Some side effects from usage of TMZ include cardiomyopathy and acute toxicity, such as oral ulcerations and bone marrow depression. Huang et al concluded from their analysis on test rabbits that TMZ-SLNs were efficiently targeting the brain and crossing the blood brain barrier by phagocytic uptake of the brain blood vessels endothelial cells . Also, TMZ-SLNs were seen to have slower rates of clearance from the body. With TMZ-SLNs, the researchers observed a change in the pharmacokinetics of TMZ when incorporated into SLNs, showing a reduction in toxicity when compared with the TMZ solution (TMZ-SOL) currently used in chemotherapy .
SLNs and Parkinson’s Treatment
To reduce the deficiency of dopamine (DA), agonist drugs such as Bromocriptine (BK) are given to Parkinson’s patients. BK is said to exhibit a slow onset of action and prolonged half-life, which may contribute to the lower dyskinesia (involuntary movement) in patients . Therefore, Esposito’s et al prepared BK-SLNs that prolonged the uptake of BK and increased its half-life, effectively reducing dyskinesia in patients. BK-SLNs were prepared using a combination of two lipids (of tricarpin, monostearin, tristearin, tribehenin) in various concentrations. Poloxamer 188 was used as a surfactant. These BKSLNs were found to have long-term stability and prolonged drug release . The SLNs were able to cross the blood brain barrier, showing great potential in brain targeting for Parkinson’s treatment.
Dr. Harsh Chauhan is an Assistant Professor at Creighton University, Omaha, NE. Dr. Chauhan received his Ph.D. in Pharmaceutics from MCPHS, Boston, MA. His research emphasizes on amorphous systems and solid lipid nanoparticles. He worked as visiting scientist at Vertex pharmaceuticals and his professional affiliations include AAPS, ACS and AACP.
Dr. Aimee Limpach is working as an Associate Professor at Creighton University. She completed her Ph.D. from UNMC and worked as a postdoctoral fellow in the Cardiovascular Research Center at Harvard Medical School from 2000-2003. She works in cellular biology and human anatomy with an emphasis in developmental anatomy.
Anne Grana attended Mount St. Mary’s College in Los Angeles, graduating with a Bachelor of Science degree in Chemistry in 2006. Currently she is pursuing a Master’s degree in Pharmaceutical Sciences at Creighton Univeresity in Omaha, Nebraska .
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