Studies On Application Of Microemulsion Drug Delivery Systems In Improving Bioavailability Of Drugs With Poor Solubility Or Peamibility

Microemulsions are defined as thermodynamically stable, isotropically clear dispersions of immisible liquids, such as oil and water, stabilized by an interfacial film of surfactant molecular. In pharmaceutics, microemulsion can improve poorly soluble drugs solubility and enhance drugs absorption, therefore improve their oral bioavailability. In this disertation, simvastatin and alendronate were selected as poorly soluble and poor permeable model drugs, and simvastatin microemulsion and alendronate microemulsion were prepared to improve their bioavailability. Furthermore the potentials of microemulsion in improvement bioavailability of drugs categorized into classificationⅡandⅢof BCS（Biopharmaceutical classification systems） were discussed.As the first part of the investigation, various preformulation studies were carried out that necessary to understand the transport processes of simvastatin and alendronate across the intestinal epithelium along the gastrointestinal tract and define the rate-limiting step of absorption in vivo. These studies included the development of analysis method; solubility studies; partition studies; pKa and permeability on Caco-2 cell monolayers. In addition, solubility of simavastatin in various surfactants and oils was determined as it was relevant for formulation development studies.The aqueous solubility and apparent octanol-water partition coefficient of simvastatin were 2.43 ug·mL（-1） and 4.71, respectively, accessed in acetate buffer solution（pH5） in which maximum stability was maintained. Since simvastatin is devoid of any ionizable moiety, it is also expected to show any pH-independent solubility and partition behaviors. The uptake percent of simvastatin was 68.63 %, and independent of the drug concentration, which indicated that simvastatin was well uptaken by the cell by distribution mechanism. The transport of simvastatin across the monolayer was also investigated. Simvastatin transport rate was very fast（5.60×10-5 cm·s-1） and independent of drug concentration, of transport direction, of the presence of metabolic poison. These implied simvastatin was transported by transcellular route by passive diffusion. Overall, the results of preformulation studied indicated that simvastatin should be grouped into classⅡof BCS for its poor solubility and good permeability, therefore the solubility characteristic is expected to be a rate-limiting factor for the oral delivery of simvastatin.While Alendronate solubility was between 28.63～50.73 mg·mL（-1） in pH1～8 buffer solution and dose to solubility ratio was 0.0074, which showed that alendronate solubility was sufficient for its absorption. The octanol-water distribution efficient was only-3.21, make alendronate almost impossible to partition cell membrane. Furthermore, alendronate, with five pKa, was completely ionized and possessed negative charges at the physiological pH（6-8）, which implied that alendronate absorption would be hampered for the electronic charge-dependence of intestine absorption, which favors the transport of cations under normal physiological conditions.The intracellular uptake of alendronate was only about 0.014 % of total alendronate, independent of the drug concentration, which indicated alendronate was poorly uptaked by Caco-2 cell with distribution. The transport performance of alendronate was also investigated on Caco-2 nomolayers. Alendronate transport rate across Caco-2 monolayer was very slow(0.111×10^-7 cm·s（-1）) and independent of Transwell culture plate rotation, of transport direction, of temperature and of the presence of metabolic poison. These implied that alendronate was transported across Caco-2 monolayer by paracellular route. The drug transport was enhanced under"minimum calcium model", i.e. when extracellular Ca²⁺ concentration was reduced, or by the presence of EGTA in the incubation buffer, i.e. when free Ca²⁺ concentration was reduced and the intercellular spaces and the tight junctions were widened. An increase in the alendronate permeability coefficient was also observed when high concentration alendronate was used. These results showed that alendronate should be categorized into classⅢof BCS for its good solubility and poor permeability and it is the permeability that limited the oral absorption of alendronate.In the formulation development studies, the microemulsion region was defined and a series factors were evaluated for microemulsion formation by contrstruction of pseudo-ternary phase diagrams. The microemulsion region was reduced when the iron strength of aqueous phase was increase but not effected by pH. High temperature can reduce interfacial tension and viscosity of the systems, which favor the formation of microemulsion. Though microemulsion is therrnodynamically stable and forms spontaneously, the microemulsion formation is effected by the procedure of mixing ingredients and only the mixture of oils and surfactants forms microemulsion upon dilution with aqueous phase.For simvastatin microemulsion, the solubilization of microemulsion systems was incestigated and the formulation was optimized with D-optimal experimental design. Three ingredient variables（simulated gastrointestinal fluid, Cremophor EL and Labrafac CC/Lauroglycol） were included in the experimental design. Simvastatin apparent solubility in microemulsion was measured by HPLC after removal of excess drug by centrifugation. The reduced cubic polynomial equation successfully modeled the evolution of simvastatin apparent solubility. The analysis of response surface of the model revealed that simvastatin solubility reduced as the microemulsion concentration decreased. The presence of the oils played different acts on the drug solubility when the microemulsion concentration was different. simvastatin apparent solubility increased as oil increased in the microemulsion when the microemulsion concentration was low. However, simvastain apparent solubility decreased to a minimal value as oil increased in the microemulsion when the microemulsion concentration was high.The effects of three preparations（micellular solution, negative-charged microemulsion, and positive-charged microemulsion） on the transport of alendronate were assessed on the Caco-2 monolayers. The absorption enhancement of positive-charged microemulsion was the biggest among the three preparations. 188.63, 141.78 and 43.72-folder increase of alentronate transport were observed compared with alendronate sodoim solution when the microemulsions were diluted by 50, 250, and 500-folder, respectively. Furthermore, the absorption enhancement of the preparation was related with the decrease in TEER of the monolayers, which indicated that the preparations could widen the tight junctions to improve the drug absorption. The decrease in TEER of Caco-2 cell used formicellular solution, negative-charged microemulsion almost recovered proceeded with 48 h incubation after the experiment while only part of the decrease in TEER for positive-charged microemulsion recovered, which indicated the positive-charged microemulsion had toxicant effects on the monolayer.Finally, the bioavailability of simvastatin microemulsion and alendronate microemulsion were accessed with beagle dogs as model animals. A 1.9-folder increase in bioavailability was observed for simvastatin microemulsion compared with conventional tablet（Zocor）. The relative bioavailability of alendronate microemulsion was 294 % with commercially available alendronate tablets（Fosamax） as referent preparation. The data showed the potential of microemulsion drug delivery systems in improvement of bioavailability of classⅡandⅢof BCS.