| Microemulsion is a good drug carrier, which can increase the solubility of insoluble drugs, and promote transdermal absorption. In recent years, microemulsion is widely used in the solubilization of insoluble drugs and transdermal delivery system. This paper studies characteristics and laws of microemulsion solubilization to different drugs, and investigated transdermal absorption of drug-loading microemulsions. We hope that through these studies more insoluble drugs could be made of microemulsion.Pseudo-ternary phase diagram of oleic acid/Tween-80/propylene glycol/water and Labrafil M 1944 CS/soybean lecithin/isopropyl alcohol/water two system was drawn by titration; Designing orthogonal test screened and prepared five blank microemulsions, and then investigated their structure type, physical stability, particle size and distribution and so on; The HPLC analysis methods of selegiline hydrochloride(SLJL), sibutramine hydrochloride(XBQM) and finasteride(FNXA) were established; The solubilities of drugs in microemulsions and in components of microemulsions were determined. Finally, To obtain transdermal absorption parameters of drug-loading microemulsion, experiment in vitro was studied.Obtained blank microemulsion A, B, C, D were O/W type, and microemulsion E was W/O type, their average particle diameters were respectively 11.6nm, 16.9nm, 52.3nm, 49nm and 43.4nm; 25℃for three months physical stability of microemulsions was good, but microemulsions have be muddy in 4℃and back to the 25℃clarified again.The solubilities of SLJL, XBQM and FNXA in microemulsion A, D and E were largest, respectively 2898.6 mg/mL, 121.92 mg/mL, 8.66 mg/mL, and were 4.84 times, 5.4 times and 207 times in water. Better water-soluble drugs, greater the solubility in the microemulsions; As mutual constraints between different solubilities of internal-external phase and concentration difference, water insoluble drugs were more easily solubilized in W/O type microemulsion. The solubility of drugs in the microemulsion is not fully equal to the sum of drugs solubilities in the water phase, oil phase and mixed surfactant, and it was also related with drug solubility, the composition of microemulsion and the type of microemulsion.Microemulsion can significantly increase the transdermal absorption of drugs, especially water insoluble drugs. For promotion of SLJL, XBQM and FNXA percutaneous absorption rate, the best microemulsions were respectively B, D, A, and the penetration rate (Jss) were separately 1318.1μg·cm-2·h-1, 410.67μg·cm-2·h-1, 27.28μg·cm-2·h-1, and permeability coefficient (Kp) were 16.48 cm·h-1, 5.13 cm·h-1, 34.10 cm·h-1, and Kp were 2.19 times, 7.03 times, 310 times the control. Better water-soluble drugs, greater Jss of drug-loading microemulsion; Larger solubility of drug in the microemulsion, smaller JSS; When drugs logP was 2～3, the Kp of drug-loading microemulsion was greater and the transdermal absorption of drugs was easy; Higher melting point of drugs, greater Jss of drug-loading microemulsions.Microemulsion can increase drug solubility, and significantly promote the transdermal absorption of drugs. But the microemulsion prescriptions were determined by physical and chemical properties of drugs and different application purposes, if solubilization was the purpose, optimal microemulsions of selegiline hydrochloride, sibutramine hydrochloride and finasteride were separately microemulsion A , D and E, but if the goal is to promote transdermal absorption of drugs, optimal microemulsions were microemulsion B, D and A.
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