Preparation And Evaluation Of Injectable Sustained-release Microspheres Of Rivastigmine

Objective: The preparation of rivastigmine tartrate-loaded injectable sustained release microspheres and physicochemical evaluation in vitro as well as in vivo pharmacokinetic performance.Methods: (1) The in vitro determination method of rivastigmine was established by HPLC, and the stability of drug in release media was investigated. (2) Microspheres containing rivastigmine were manufactured by O/W, W1/O/W2, O1/O2 emulsification-solvent evaporation techniques and spray drying. In terms of loading capacity and release profile in vitro, the optimized method and formulation for sustained-release over one week and one month were obtained respectively. The size distribution and morphology of microspheres were characterized under optical microscopy, and the drug state in microspheres was analyzed by X-ray diffraction (X-RD). (3) The stability study was conducted to investigate the effects of high humidity and strong light on loading capacity and morphology of microspheres. And the impact of 60Co irradiation on various characteristics of microspheres was examined. (4) Gas chromatography was employed to determine the residual amount of dichloromethane and acentonitrile in rivastigmine-containing microspheres. (5) The concentration of rivastigmine in plasma was determined by LS-MS/MS, and the preliminary pharmacokinetic study of microspheres sustained release over one week was carried out in rats after subcutaneous administration.Results: (1) The in vitro determination method of rivastigmine was stable and reliable, and rivastigmine showed good stability in release test media. (2) Microspheres manufactured by O/W, W1/O/W2 emulsification-solvent evaporation method and spray drying showed relative high release rate of rivastigmine in the period of preliminary 24 hours. When rivastigmine was encapsulated into microspheres by O1/O2 emulsification-solvent evaporation method and PLGA 502H was employed (called formulation A), microspheres sustained release over one-week period in vitro were obtained within size range of 80-150μm. Microspheres prepared by the blend of PLGA 752H and PLA 202S at the ratio of 1:3 (called formulation B) showed sustained release profile in one-month period in vitro, with size distribution of 50-100μm. The results of X-ray diffraction indicated that rivastigmine entrapped in the microspheres existed in molecular or amorphous state. (3) Microspheres prepared by formulation A were not stable in high humidity, whereas microspheres manufactured by formulation B showed good stability both in high humidity and strong light condition. And 60Co irradiation did not significant change the characteristics of microspheres including morphology, loading capacity and in vitro release. (4) Organic solvent residual amount in microspheres prepared by formulation A did not exceed relevant limitation. However, the residual amount of acentonitrile in microspheres prepared by formulation B was within the limitation, and the residual amount of dichloromethane exceed the limitation. (5) The results of preliminary pharmacokinetic study of microspheres in rats suggested that rivastigmine plasma concentration could maintain stable for several days with certain extent of burst release.Conclusion: The preparation methods of rivastigmine tartrate-loaded injectable microspheres for sustained release in predetermined time period are successfully established, and the pharmaceutical characterization and pharmacokinetic evaluation were carried out.