Studies On The PLGA Coated Doxycycline Hydrochloride Hydroxyapatite Microspheres

Hydroxyapatite (HAP) is one of the main inorganic constituents of mammalian bones and tooth enamel. Now they have gained additional prominence in clinical application due to their excellent biocompatibility, osteoconductive, and ostheophilic nature. The low-cost, porous hydroxyapatite microsphere, which consisted of nano-sized primary grains with high specific surface area, is becoming increasingly important as a promising vehicle for drug delivery in pharmaceutical field. In this paper, a water-soluble small molecular, doxycycline hydrochloride (Doxy·HCl), was selected as the model active drug; HAP microspheres were taken as the carrier for drug delivery; and poly (lactic-co-glycolic acid) was used as the coating material. The Doxy·HCl loaded HAP microspheres were obtained by adsorption-lyophilization, which were then coated with PLGA via S/O/W emulsion-solvent evaporation method. The influence factors of above procedures were systematically investigated in order to provide some theoretical and applied information for the delivery of small molecular drug sustained release delivery system.Ultraviolet spectrophotometry and high performance liquid chromatography with ultraviolet detector were applied for the determination of the content of Doxy·HCl in samples. Both of the analytical methods were proved to be specific, stable, precise and accurate.Various characters of Doxy·HCl such as the partition coefficient, apparent solubility and stability in various solvents, and the effects of stabilizers were determined. The morphology and the water absorbing capacity of HAP microspheres were also studied initially.The effects of initial Doxy·HCl concentration, pH, kinds of buffer solution, various solvents, and ionic strength on the adsorption/desorption behaviors were investigated at room temperature, respectively. And the mechanism of the interaction between Doxy·HCl and HAP microspheres was discussed.2mg/ml Doxy·HCl solution was selected as the optimum concentration for the preparation of Doxy·HCl loaded HAP microspheres. The results of desorption experiments showed that although the volume of desorption medium influenced the release amount of Doxy·HCl from HAP microspheres directly, which could provide sustained release characters under certain physiological conditions, such as periodontal pocket, it is still necessary for the further processes, such as immersing or coating with polymers, to obtain proper sustained release patterns. The S/O/W emulsion-solvent evaporation method was utilized to prepare PLGA coated Doxy·HCl HAP microspheres. The typical procedures were described as follows:PLGA dissolved in 2mL DCM was mixed with Doxy·HCl HAP microspheres by vortex to get the S/O suspensions. This S/O suspensions were then poured into polyvinyl alcohol (PVA) simulated body fluid (SBF) solution with magnetic stirrer for the S/O/W emulsion, which then underwent mechanical agitation gently to remove the organic solvent. The resulting solidified microspheres were collected by filtration after washing 3 times with 50mL purified water and dried under vacuum at room temperature. The influences of PVA, PLGA and oil-water volume ratio etc. were studied. And it was found that the PLGA coated Doxy·HCl HAP microspheres with mean particle size of about 20μm and the entrapment efficiency of 80% could be prepared through adjusting formulation and procedure.The influence parameters of drug release from PLGA coated Doxy·HCl HAP microspheres were investigated in the in vitro drug release experiments via immediate drug release method with purified water as the release medium. The results showed that PLGA coated Doxy·HCl HAP microspheres indicated the low burst release and the in vitro release data were fitted the Higuchi function and the first-release function. And the release of Doxy·HCl from PLGA coated microspheres was controlled by the diffusion, in which the thickness of PLGA layer and the properties of PLGA were determining factors. The formulation with PLGA/HAP=1/4 was chosen for the preparation of PLGA coated Doxy·HCl HAP microspheres used in in vivo test.The pharmacokinetics study in mice demonstrated that compared with Doxy·HCl loaded HAP microspheres, the PLGA coated microspheres performed remarkable changes in release profiles and indicated obvious sustained release trend. And the release profiles in vivo and in vitro exhibited good correlation.