Microemulsions oral drug delivery systems

MICROEMULSIONS AS DRUG DELIVERY SYSTEMS 9.7.1 Oral Delivery... 

In many cases in drug development, the solubility of some leads is extremely low. Fast dissolution rate of many drug delivery systems, for example, particle size reduction, may not be translated into good Gl absorption. The oral absorption of these molecules is usually limited by solubility (VWIImann et al., 2004). In the case of solubility limited absorption, creating supersaturation in the Gl Luids for this type of insoluble drugs is very critical as supersaturation may provide great improvement of oral absorption (Tanno et al., 2004 Shanker, 2005). The techniques to create the so-called supersaturation in the Gl Luids may include microemulsions, emulsions, liposomes, complexations, polymeric micelles, and conventional micelles, which can be found in some chapters in the book. 

Microemulsions, and Lipid-Based Drug Delivery Systems for Drug Solubilization and Delivery—Part II Oral Applications... 

Microemulsions are thermodynamically stable isotropically clear dispersions composed of a polar solvent, oil, and a surfactant(s). Labrafil and Gelucire 44/14 are all-in-one self-emulsifying surfactants which are in many oral products throughout the world. Microemulsions have much potential for drug-delivery since very hydrophobic molecules can be solubilized and formulated for oral administration (Tenjarla, 1999). All of the commercial products are actually nonaqueous microemulsions, also known as microemulsion preconcentrates or self-emulsifying drug delivery systems (SEDDS), since the polar solvent is not water. Upon contact with aqueous media, such as gastrointestinal fluids, a SEDDS formulation spontaneously forms a fine dispersion or aqueous microemulsion. 

Ghosh, P.K., Majithiya, R.J., Umrethia, M.L. andMurthy, R.S.R. (2006) Design and development of microemulsion drug delivery system of acyclovir for improvement of oral bioavailability. AAPS Pharmscitech, 7, 77. 

There are a few marketed drug delivery systems containing microemulsions that are related to the lipid systems we describe here. Sandimmune Neoral (Novartis, Switzerland) [17], which delivers the lipophilic peptide cyclosporin A, is based on medium-chain triglycerides obtained from coconut oil, a semisynthetic emulsifier (ricinoleate), and propylene glycol. The absorption of the drug taken orally as a microemulsion increases dramatically compared to that of an aqueous suspension [17]. Another marketed example, in which Cs/Cio triglycerides from coconut oil are used, is a soft gelatin capsule of vitamin D3 [18]. 

In this chapter we will provide a brief overview of the early approaches to bioavailability enhancement by use of simple lipid-based delivery systems (lipid solutions, emulsions etc), and then describe recent progress in the application of self-emulsifying- and microemulsion-based formulations. The effects of lipids on the oral bioavailability of co-administered poorly water-soluble drugs may also be classified from a mechanistic (and to a degree, historical) perspective as physicochemically mediated effects (solubility, dissolution, surface area) and biochemically mediated effects (metabolism, transport related events), and these will be approached separately. It is readily apparent, however, that in many cases physicochemically and biochemically mediated mechanisms will operate side by side. In some instances, bioavailability may also be enhanced by the stimulation of intestinal lymphatic transport, and these studies will be addressed in a separate section. 

In addition to their usefulness in the enhancement of oral bioavailability of lipophilic drugs, microemulsion formulations have found considerable application as potential delivery systems for peptides whose delivery is often limited by poor GI permeability. W/O microemulsions provide a convenient means of delivery of both permeability-enhancing lipids and water-soluble peptides. The GI permeability-enhancing effects of lipids and their use in the delivery of highly water-soluble compounds are reviewed elsewhere [18, 56, 59],... 

Micelle Formulations A micelle system can be either water-based or oil-based. The use of a micelle formulation for poorly water-soluble drugs for systemic delivery has been well recognized. In recent years, the effective development of self-emulsifying microemulsions or mixed micelle-based lipid formulations products, such as Sandimmun Neoral (cyclosporin), Norvir (ritonavir), and Fortovase (saquinavir), has substantially increased interest in the application of lipid-based micelle formulation to improve oral delivery of poorly water-soluble drugs as well as protein and peptide drugs.51... 

In recent years there has been an increased interest in the utility of lipid-based delivery systems to enhance oral bioavailability (4). It is generally known that membrane permeability is directly correlated to a drug s water-lipid partition coefficient however, the systemic availability of highly lipophilic drugs is impeded by their low aqueous solubility. In an effort to improve this solubility-limited bio-availabiliy,formulators have turned to the use of lipid excipients to solubilize the compounds before oral administration. Several formulations are currently on the market, for example, Sandimmun/Neoral (cyclosporin microemulsion), Norvir (ritonavir), and Fortovase (saquinavir)... 

R.P. Scherersol system Liquid formulations for softgels, which incorporates the drug in a microemulsion preconcentrate or microemulsion form. Oral delivery of proteins and peptides enhanced oral bioavailability with reduced inter- and intraindividual variability in pharmacokinetics of certain drugs. Sandimmune Neoral (cyclosporine). 

Another type of biocompatible microemulsion, formed by water, triglycerides, and monoglycerides [64] has been studied, e.g., by Engstrom [65] (Fig. 8). It was found that this system formed a rather extensive Lz phase at 40°C. Based on X-ray diffraction, this phase was proposed to have a lipid bilayer structure even at high oil content. Considering the biocompatibility and the ease of biodegradation of these components, this type of microemulsion is particularly attractive for oral delivery of drugs, where toxicity otherwise limits the applicability of microemulsions. 

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