In recent years it has become more and more evident that the development of new drugs alone is not sufficient to ensure progress in drug therapy. Exciting experimental data obtained in vitro are very often followed by disappointing results in vivo. Main reasons for the therapy failure include:
• Insufficient drug concentration due to poor absorption, rapid metabolism and elimination (e.g. peptides, proteins). Drug distribution to other tissues combined with high drug toxicity (e.g. cancer drugs)
• Poor drug solubility which excludes iv injection of aqueous drug solution
• High fluctuation of plasma levels due to unpredictable bioavailability after peroral administration, including the influence of food on plasma levels (e.g. cyclosporine)
A promising strategy to overcome these problems involves the development of suitable drug carrier systems. The in vivo fate of the drug is no longer mainly determined by the properties of the drug, but by the carrier system, which should permit a controlled and localized release of the active drug according to the specific needs of the therapy. (1) In this article, we will review micro/nanoemulsions and solid lipid nanoparticles, which are lipid carrier systems.
Microemulsions: Microemulsions are clear, thermodynamically stable, isotropic liquid mixtures of oil, water and surfactant, frequently in combination with a cosurfactant. The aqueous phase may contain salt(s) and/or other ingredients, and the "oil" may actually be a complex mixture of different hydrocarbons and olefins. (2) There are three kinds of microemulsions: oil dispersed in water (o/w), water dispersed in oil (w/o), and bicontinuous. The presence of o/w droplets is likely to be a characteristic of microemulsions where the amount of oil is low. In contrast, the existence of w/o droplets is a characteristic of microemulsions where the water fraction is low. Droplet diameter varies in the range of 10–140 nm. Microemulsions have been utilized in several disciplines like fuels, detergents, agrochemicals food, cosmetics, and pharmaceutics. (3)
Nanoemulsions: Nanoemulsions are kinetically stable liquid-in-liquid dispersions with droplet sizes ranging from 20-500 nm. Their small size leads to useful properties such as high surface area per unit volume, robust stability, optically transparent appearance, and tunable rheology. A typical nanoemulsion contains oil, water, and an emulsifier. The addition of an emulsifier is critical for the creation of small-sized droplets as it decreases the interfacial tension i.e., the surface energy per unit area, between the oil and water phases of the emulsion. The range of nanoemulsion applications spans diverse fields including drug delivery, where O/W nanoemulsions have been used to deliver hydrophobic drugs; the food industry, and in the cosmetic industry where nanoemulsions have been tested for skin hydration and ease of application. Nanoemulsions have been used in most forms of drug delivery, namely topical, ocular, intravenous, internasal, and oral delivery. These applications leverage the lipophilic nature of nanoemulsions to solvate water-insoluble drugs, and tunable charge and rheology of nanoemulsions to formulate aqueous solutions that can be easily delivered to a patient. Researchers have also shown that many problems faced in current methods of pharmaceutical crystallization processes can be avoided with nanoemulsions. High energy and low energy methods are used for preparing O/W nanoemulsions. High energy such as high-pressure homogenization (HPH) and ultrasonication break macroemulsion drops into smaller droplets. Low energy methods start with W/O macroemulsions and break coarse emulsions into smaller droplets as they pass through a state of low interfacial tension during phase inversion. (4)
Nanoemulsions and microemulsions have a larger surface area per unit volume than do macroemulsions because of their size. In addition, due to strong kinetic stability, nanoemulsions are less sensitive to physical and chemical changes. (4)
Solid lipid nanoparticles: Solid lipid nanoparticles (SLNs) are nanoparticles made of solid lipids with a photon correlation spectroscopy (PCS) mean diameter approximately between 50 and 1000 nm. (2) SLNs are introduced as a carrier system for in effectively water dissolvable medication and corrective dynamic medication. SLNs have attracted increasing attention in recent years. High-pressure homogenization (HPH) has emerged as a reliable and powerful technique for the preparation of SLN. (1) SLNs offer interesting properties, for example, little size, huge surface zone, high medication stacking and the communication of stages at the interface and are appealing for their potential to enhance the execution of pharmaceuticals. (5) Proposed advantages of SLNs include the possibility of controlled drug release and drug targeting, increased drug stability, high drug payload, incorporation of lipophilic and hydrophilic drugs feasible, no biotoxicity of the carrier, avoidance of organic solvents, no problems with respect to large scale production and sterilization. (1)
(1) Mehnert, Wolfgang, and Karsten Mäder. "Solid lipid nanoparticles: production, characterization and applications." Advanced drug delivery reviews 64 (2012): 83-101.
(2) Battaglia, Luigi & Gallarate, Marina & Panciani, Pier Paolo & Ugazio, Elena & Sapino, S. & Peira, Elena & Chirio, Daniela. (2015). Techniques for the Preparation of Solid Lipid Nano and Microparticles. 10.5772/58405.
(3) S. Nafisi, H.I. Maibach. ” Nanotechnology in Cosmetics” Cosmetic Science and Technology (2017) 337-369
(4) Gupta, Ankur, et al. "Nanoemulsions: formation, properties and applications." Soft matter 12.11 (2016): 2826-2841.
(5) Lingayat, Vishal J., Nilesh S. Zarekar, and Rajan S. Shendge. "Solid lipid nanoparticles: a review." Nanoscience and Nanotechnology Research 2 (2017): 67-72.