Study Of Poly(L-Lactide) Based Drug Delivery System By Supercritical Fluid Technology

Poly(L-lactide) (PLLA) was widely studied in the drug delivery system due toits biodegradability and biocompatibility; Supercritical fluids have revealed greatpotential in particle engineering, especially the supercritical anti-solvent process hasbeen wildly explored in particle design. In this study, the drug carries based onPLLA was prepared using solution-enhanced dispersion by supercritical CO₂ (SEDS),and its application in the delivery of small molecular drug and protein drugs wasinvestigated.Firstly, the PLLA microspheres were prepared by SEDS process. The effects ofmixing acetone into PLLA DCM solution, temperature, pressure, PLLAconcentration, flow rate of organic solution and MW of PLLA on the properties ofPLLA microspheres were studied. The results indicated that the PLLA microspheresprepared by SEDS process were smaller than that by SAS process due to the smallersize of initial drops and more intense transfer rate in SEDS process. Mixing ofacetone into DCM generated a higher saturation ratio of PLLA solution for theanti-solvent effect of acetone, while the other operating parameters were constant.PLLA microspheres with smaller mean particles size and narrower particles sizedistribution were resulted. PLLA concentration and flow rate of organic solutionplayed important roles on the properties of products, the mean particle size was decreased with the decreasing of PLLA concentration or flow rate of organicsolution, and the agglomeration of products was increased with the increasing ofPLLA concentration or flow rate of organic solution, while the temperature andpressure had minor effects on the properties of products. However, the mean particlesize was decreased slightly with the increasing of CO₂ density, and theagglomeration of products was increased with the increasing of temperature. ThePLLA microspheres with MW of 10 KDa had the biggest particle size. Withincreasing of the PLLA MW, the particle size of product decreased first and thenincreased, and the product with MW of 200 KDa shifted to be irregular in shape andgave rise to partially bridged particles and agglomerates.Secondly, the 5-Fu-PLLA microspheres were developed by SEDS process. Themicronization of 5-Fu was successfully performed in this process, and then themicronized 5-Fu was utilized in producing 5-Fu-PLLA microspheres. Themorphology, particle size and its distribution, drug loading, encapsulation efficiency,drug release profile and DCM residue of 5-Fu-PLLA microspheres were investigated.The results show that the particle size of 5-Fu after micronization was less than 500nm, after stirring for 3 hours, its concentration in ethanol at 25℃was increased to6.43 mg.ml^-1, however, the saturated concentration of 5-Fu milled by agate mortarand the original 5-Fu were 3.22 mg·ml^-1 and 2.32 mg·ml^-1, respectively. The5-Fu-PLLA microspheres with the MW of 50 KDa and 100 KDa synthesized in theSEDS process both exhibited a rather spherical shape with smooth surface and anarrow particle size distribution. Their mean particle size were 793 nm and 980 nm,respectively, and their drug loading and encapsulation efficiency were 2.4%, 3.05%and 15.8%, 17.8%, respectively. The DCM residue in 5-Fu-PLLA microsphereswithout further treatment was 46 ppm, which meets the requirement ofPharmacopeia of People's Republic of China (2005) (max. 600 ppm). The drugrelease from 5-Fu-PLLA microspheres was a first-order process, which wascontrolled by drug diffusion, and smaller PLLA MW and particle size both increasedthe drug release. The results indicated that the SEDS process could be used toco-precipitate 5-Fu and PLLA as composite microspheres. It also showed that the use of toxic solvents can be greatly reduced when SEDS process was used forpreparation of microspheres from commonly used polymers.Thirdly, the macromolecular drug-PLLA composite microspheres wereprepared in a SEDS process. Lysozyme was used as macromolecular drug model,and the lysozyme-PLLA microspheres were prepared. The effect of drug loading,PLLA MW, PEG content and PEG MW on the properties of lysozyme-PLLAmicrospheres were studied. Then the Hb-SiO₂-PLLA microspheres with a core/shellstructure were also prepared by SEDS process, and its morphology, particle size,drug loading and drug release profile were characterized. The results indicated thatthe particle size of lysozyme-PLLA microspheres decreased and the drug releaseincreased with the increasing of drug loading, PEG content or PEG MW, while theparticle size of lysozyme-PLLA microspheres increased first and then decreased withthe increasing of PLLA MW, and its drug release was controlled by both particle sizeand PLLA MW. The results of the FTIR spectra and CD spectra showed that therewas no significant change in the secondary structure of lysozyme after SEDS process,while the activity test indicated that its activity was increased 51.2% after SEDSprocess. The resulted Hb-SiO₂-PLLA microspheres have a narrow particle sizedistribution with a mean particle size of 897 nm and a drug loading of 7.12%, andthe drug released mainly in a zero-order kinetics, which exhibited a good controlledrelease efficiency.Fourthly, the magnetic Fe₃O₄-PLLA microspheres were developed in theprocess of SEDS, and their morphology, particle size, magnetic mass content,surface atom distribution and magnetic properties were characterized. Theindomethacin was used as drug model. The Indo-Fe₃O₄-PLLA compositemicrospheres were prepared in SEDS process, and their morphology, particle sizeand drug release profiles were also investigated. It was found that there was nosignificant difference in morphology and particle size among the Fe₃O₄-PLLAmicrospheres prepared in different ratio of Fe₃O₄ and PLLA, and their mean particlesize ranged from 714 to 803 nm. TG-DSC test showed that the Fe₃O₄ content inFe₃O₄-PLLA microspheres (before SEDS process, Fe₃O₄/PLLA=1/2) was 40.64%. The XPS test indicated that most of the Fe₃O₄ were encapsulated by PLLA, and mostof the Fe₃O₄-PLLA microspheres had a core/shell structure. The Fe₃O₄-PLLAmicrospheres had a good magnetic property, and its saturation magnetizationincreased with the increasing of Fe₃O₄ content. However, most of the indomethacinprecipitated on the surface of Fe₃O₄-PLLA microspheres or loosely contacted withFe₃O₄-PLLA microspheres. After removal of the unencapsulated and looselysurfacebound indomethacin, the Indo-Fe₃O₄-PLLA microspheres had a slightretardant efficiency in drug release.Finally, the in vitro degradation of PLLA microspheres was investigated byincubating PLLA microspheres in 37℃pH 6.8 PBS, and the of PLLAmicrospheres and Fe₃O₄-PLLA microspheres was measured by MTT assay. It wasfound that after being incubated for 10 weeks, there were some tiny holes on thesurface of PLLA microspheres, showing that the PLLA MW was decreased. Thelarger was the MW, the bigger was the decreasing scope of MW. The MTT assaydemonstrated that the PLLA microspheres and Fe₃O₄-PLLA microspheres had nocytotoxicity, and also were biocompatible.