Functions of Lipids for Enhancement of Oral Bioavailability of Poorly Water-Soluble Drugs 717 Tab. 5. Influence of the digestibility of the lipid vehicle on the rate and extent of drug absorption. Compound Triamterene [36] Penclomedine [37] Cyclosporine [38] BA…bioavailability. Study Design Oral BA study in rats BA in rats after intraduodenal administration Intraduodenal administration to rats Lipids Peanut oil, oleic acid, triolein SCT / MCT / LCT, mineral oil Lipid micro emulsion Observation and comments BA from OA > PO > TO ≥ aqueous suspension. With 20 μl volumes, PO ≅ aqueous suspension BA from MCT > LCT > mineral oil > SCT > aqueous suspension; BA a function of digestibility of vehicle Absorption decreased in bile duct ligated and pancreatomised rats The dependency of cyclosporine absorption on bile secretion has also been evaluated after administration of a coarse lipid emulsion to liver transplant patients with a T-tube inserted in their common bile duct [38]. The absorption of cyclosporine was assessed before and after clamping the T-tube (i.e. with or without bile diversion) and the relative bioavailability of cyclosporine was 3–4 fold higher in the patients without bile diversion. Novel Lipid Formulations The carrier systems that have been most extensively studied to control the release of the incorporated substances are: • Liposomes • Polymeric nano- and microparticles • Solid lipid nanoparticles (SLNs) • Self-emulsifying drug delivery systems (SEDDS) Liposomes are spherical particles composed of one or more concentric phospholipid bilayers. This kind of structure makes it possible to incorporate lipophilic drugs into lipid bilayers as well as hydrophilic drugs into the aqueous compartment. Drug release from liposomes, stability, and pharmacokinetic profiles depend on composition, size, surface charge, and drug solubility. Liposome formulations of many different drugs show a significant increase in therapeutic activity compared with non-liposomal formulations. Liposomes are mostly biocompatible and biodegradable, but also face some disadvantages including low stability, low encapsulation efficiency, and difficult scale-up [38–42]. Polymeric nano- and microparticles are general terms that include spheres as well as capsules. Hydrophilic and lipophilic drugs can be incorporated or entrapped into these polymeric carriers, but reaching therapeutic levels is one the most crucial issues. These Sci Pharm. 2011; 79: 705–727