Study On Sustained-release Systems Of Protein Drugs With High Molecular Weight

Frequent injection and long period administration of protein drugs greatly reduce patients’ compliance.As this kind of drugs is widely used in scientific and clinical research,it is urgent to improve their administration form.Currently,high-efficiency non-injectable dosage forms and sustained-release systems are the two proved medicinal alternatives to frequent injections.Among them,sustained-release systems already have multiple marketed products on peptides,while protein drugs,especially those with high molecular weight(exceeding 50,000),have not been successful.One of the requirements for protein drugs remaining their conformation is to avoid organic solvents.In this way,Prof.Steve Schwenderman of the University of Michigan proposed one idea that it was possible to suspend pre-prepared PLGA porous microspheres in protein solution,after proteins being absorbed into the porous structure of microspheres,most of the pores would be closed when heating to the phase-transition temperature of the polymer.However,this method also has some disadvantages.Firstly,there is no thermodynamic driving force for proteins going inside through the pore structure,which would lead to low encapsulation efficiency.Secondly,the chose temperature for pores sealing is too high to deactivate the proteins.Another popular sustained-release method which does not need organic solvents is the in situ thermosensitive gelling system.While there is a big problem that pre-loaded proteins could be extruded from the polymer structure instantaneously during the period of gelling,this is because polymer is likely to collapse when the hydrogen bonds absorb water and later desorb in high temperature,causing serious initial burst release.After analyzing the above technical challenges and based on the previous research in the laboratory,my work focused on improving the preparation method of porous microspheres and in situ thermosensitive hydrogel,and also made an attempt to encapsulate protein drugs for protection.In brief,PLGA porous microspheres pre-loaded with dextran particles were prepared by invented phase-guided method,followed by being suspended in the aqueous solution of PEG of high molecular weight with protein drugs.Because of the neat driving force,proteins could be propelled into the porous microspheres mildly and combined with dextran phase by preferential partition.In order to reduce pores sealing temperature,small molecule PEG(Mw 200 Da,400Da)which could make PLGA swell were used to suspend the harvested microspheres absorbed with proteins,and then the molecular chain of PLGA would move to close pores at a reduced phase transition temperature of the polymer,thereby greatly increasing the encapsulation efficiency.Experiments with model drugs have shown that porous microspheres prepared by phase-guided method could achieve microencapsulation successfully and remain proteins’ natural conformation at the same time.Regarding in situ thermosensitive gelling systems,protein-DEX particles were prepared by freeze-induced phase separation method and two aqueous emulsion method separately.In this way,because of the support of the polysaccharide particle skeleton,hydrophobic collapse of the polymer can be effectively avoided.The protein-DEX particles were then suspended in PLGA-PEG-PLGA aqueous solution at room temperature.When heating to the phase transition temperature of the polymer,the system would turn to gel,obtaining the protein-DEX particles composite PLGA-PEG-PLGA thermosensitive hydrogel eventually.Experiments with model drugs have shown that this method can greatly improve the initial burst release problem of in situ hydrogels and effectively prolong the release cycle to some extent.