Studies On Polymeric Microspheres Prepared With Emulsion-solvent Evaporation Under Reduced Pressures

Emulsion-solvent evaporation technique is always applied in the process ofmicrosphere preparation for its simplicity in operation. The preparation procedurewith this method is often conducted under atmospheric pressure. Not only the processis time-consuming, but does the drug leakage always happen in the microspheresolidification process which could result in lower encapsulation efficiency and seriousburst effect.In order to shorten the preparation time and improve the incorporation efficiencyby increasing the solidification rate of microspheres, the reduced-pressure solventevaporation method was applied as a modification of the existed preparationtechnique. Firstly, Azithromycin was applied as a kind of model drug to investigatethe reduced pressure conditions on the properties of the microspheres. Both the X-raydiffraction spectra and DSC thermographs demonstrated that the crystallinity of thepolymers could be decreased obviously by increasing the solvent evaporation rate; theSEM images showed that solvent removal at atmosphere gave microspheres of porousand rough surfaces, but smooth surfaces appeared in the RSE microspheres; theincorporation efficiency as well as the burst release (cumulative release in the first24hours) of the Azithromycin-loaded microspheres prepared under atmospheric pressurewas (39.94±1.18)%and (23.96±2.01)%respectively, yet the encapsulation efficiencyof the microspheres fabricated under reduced pressure was high up to (57.19±3.81)%and the burst release was (4.12±0.15)%. The final reduced pressure condition was thatthe preparation pressure was385mmHg and the temperature was25℃.Double emulsion-solvent evaporation method was applied here to produceLysozyme-loaded microspheres after the reduced pressure condition was determined.Firstly, the determination methods of the drug content and bioactivity were founded.MicroBCA method was used to determine the amount of the drug in the in vitrorelease studies. In this method, the result of the recovery test was (99.02±1.73)%, andthe RSD was below2%. Besides, the mean RSD of the precision tests was below3%.All these results indicated that the microBCA method could meet the measurementrequirement. Turbidimetry-spectrophotometric method was applied to determine the bioactivity of the Lysozyme. The standard curve of the enzyme activity presented agood linear relationship when the drug concentration ranged from0.01to0.02mg/ml.Using the drug loading, encapsulation efficiency, burst release and the drugactivity as evaluating indexes, single factor experiments were executed to investigatethe preparation technique of the Lysozyme-loaded microspheres. The optimalformulation was determined by orthogonal design L9(34) which was based on theresults of one-way ANOVA and t test between groups. The best prescription was: theultrasonic power400W, the weight ratio of PELGA and PLGA2:1, the concentrationof the polymers10%(w/v) and the shearing speed of the sencond emulsification4000rpm. Three batches of microspheres were prepared with the best prescription.The average diameter of the microspheres was28.53μm, the drug loading andincorporation efficiency was11.97%and91.90%respectively. The obtainedmicrospheres provided sustained release of Lysozyme over a30-day period and the invitro drug release profiles could be fit well by Ritger-Peppas equation. Besides, theinitial burst could be decreased greatly from32.75%to3.11%when the microspheresproduced with the best prescription were embedded in the16%(w/v) F127gelsubstration. And the in vitro release profiles were in accordance with Higuchiequation.The encapsulation efficiency of biodegradable sustained-release microspheresprepared by reduced-pressure solvent evaporation method could be enhancedobviously. Besides, the initial burst and drug release rate from microspheres could bedecreased greatly.