Investigation of formulation variables and physiological processing on the behaviour of lipid-based formulations for poorly water-soluble drugs

The absorption and oral bioavailability of poorly water-soluble drugs is often limited by poor aqueous solubility and slow dissolution in the gastrointestinal (GI) tract. Lipid-based formulations are a popular formulation approach to enhance oral bioavailability for drugs where water solubility is the primary limitation to absorption. The research undertaken in this thesis examines the use of different types and masses of lipids to improve drug solubilisation and absorption, and investigates the contribution of gastric processing to the improvements in oral bioavailability typically seen after co-administration of poorly water-soluble drugs with lipids and lipid-based formulations. A simple in vitro lipid digestion model was used to assess the effect of lipid type and mass on the solubilisation of three model lipophilic drugs (danazol, cinnarizine and halofantrine). Digestion of medium chain triacylglyceride (MCT) formulations yielded improved drug solubilisation (and resulted in drug supersaturation) at high lipid mass (250 mg). In contrast, for long chain triacylglyceride (LCT) formulations, drug concentrations in the aqueous phase of the digests were higher after digestion of the smallest lipid masses, regardless of drug lipophilicity. In all cases, digestion of the LCT formulations was incomplete, resulting in a residual oil phase. At low masses of LCT lipid (50 mg), digestion was more complete, resulting in increased drug transfer into the aqueous phase. For the more lipophilic drugs, partitioning into the residual oil phase increased. Drug lipophilicity, the choice and quantity of lipid, and the need for complete digestion of the formulation were therefore important indicators of the performance of the in vitro lipid digestion assay. Cinnarizine (CZ) was subsequently chosen as a model poorly water-soluble drug to exemplify the effects of lipid load on drug exposure in in vivo studies and to compare in vitro and in vivo performance. In vivo bioavailability studies were undertaken at fixed and varied lipid:CZ ratios and after administration with LCT and MCT. In all cases, the bioavailability of CZ was higher after administration of LCT rather than MCT formulations, regardless of lipid mass. At a fixed lipid:CZ ratio, increasing the quantity of formulation did not affect oral bioavailability, and linear pharmacokinetics were observed. When the lipid:CZ ratio was increased, CZ absorption increased at lipid doses from 50 mg to 250 mg, but did not increase further beyond 250 mg. The data suggest that the type and mass of lipid co-administered are important, but that in most cases, LCT formulations outperform the equivalent MCT formulation. The same lipids were also given by intraduodenal administration as both a lipid solution and as a dispersed lipid formulation, to assess the contribution of gastric processing to oral bioavailability. CZ bioavailability was reduced when either formulation was administered intraduodenally and similar trends were evident for MCT and LCT. The data suggest that gastric and intestinal processing contribute to improved CZ absorption. Finally, aspiration of GI content after formulation administration revealed that the digestion of MCT was more prevalent in the stomach than LCT. Gastric processing may explain the improvements in bioavailability when MCT formulations (both solution and dispersion) were administered orally when compared to intraduodenally. Surprisingly, LCT formulations were seemingly less dependent on gastric processing. In summary, the research described in this thesis highlights the potential utility (and drawbacks) of in vitro lipid digestion models to predict in vivo absorption, and further shows that the mass and type of lipid, and processing in both the stomach and the intestine are important determinants of oral bioavailability from lipid-based formulations.