Lipid excipients are comprised of a large group of physically and chemically diverse substances, which provide the formulator with considerable latitude in formulation design. These excipients can readily be used to orally deliver drugs as solutions (liquid and semi-solid), suspensions, emulsions, microemulsions, SEDDS, or SMEDDS. In general, lipid based formulations have been found to yield greater and more uniform bioavailability than polyethylene glycol (PEG) solutions of the drug, which are water miscible and can precipitate upon aqueous dilution in the GIT. Two examples of this include Ro 15-0778 and seocalcitol, for which lipid based formulations provided superior bioavailability relative to simple water miscible solutions of the drug (Table 1). In other studies, Charman et al. (13) and Lin et al. (19) reported similar improvements in bioavailability, relative to conventional solid formulations, of the drugs WIN 54954 and L-365,260, respectively, when administered to dogs as either PEG 600 solutions or as SEDDS formulations. While both the SEDDS and PEG 600 solution formulations provided similar drug exposure, the SEDDS formulations provided more uniform plasma concentration profiles, suggesting more consistent solubilization of drug in the GIT.
Despite the considerable potential that lipid excipients offer, very few lipid-based formulations have reached the pharmaceutical marketplace. We believe this to be largely due to insufficient information regarding the relatively complex physical chemistry of lipids and concerns about formulated drug chemical and physical stability. In addition to these concerns, the interaction of a lipid-based formulation with the GI environment and its impact on drug absorption need to be considered. For instance, digestible lipids have been shown to be considerably more efficient enhancers of poorly soluble drug absorption, as compared to nondigestible lipids (e.g., liquid paraffin) and fatty acid chain length of the lipid can influence drug absorption, as well (1,20-23).
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