Development Of Novel Silk Nano-microspheres For Controlled Drug Release Applications

Many chronic diseases are caused by ecological environment deteriorating,which may increase patients' pain and raise social costs of the treatments.Although various therapeutic drugs have been discovered,many of them encountered problems during development,such as poor solubility,poor stability,high toxicity,poor pharmacokinetic and non-targeted tissue selectivity,etc.One strategy to tackle these problems is to embed or encapsulate drug molecules in a polymeric carrier,through which the original biological activity of the drug can be maintained for a long period of time,the release of the drug can be prolonged,the efficacy and safety of drugs can be enhanced,and the patient compliance can be improved due to reduced dosing frequency.Despite these advantages,polymeric nano/microsphere carriers still face some problems that have hindered their broad and rapid applications,including the uncontrollable degradation of the polymer material,inflammatory response of some degradation products,complex steps and uses of organic solvents during material processing,and suboptimal drug loading and release properties.Hence,it is highly desirable to develop new nano/microsphere systems with distinguished material features to solve these problems.Silk fibroin(termed silk thereafter)is a naturally derived polymeric material with unique properties the meet the requirements for controlled drug delivery.First of all,diverse silk materials can be fabricated via all-aqueous processing without using chemical cross-linkers and organic solvents.Secondly,proteolytic degradation of silk can be tuned by changing the content of crystalline ?-sheet structure,and the degradation products are natural amino acids and peptides,thus conferring silk no or low inflammatory responses after implantation in vivo.Finally,the hierarchical and amphophilic nano-structures formed in the silk materials are beneficial for the embedding and stabilization of bioactive molecules with different molecular weights and solubilities,and the release rates can be controlled by tuning the amounts and distribution of the nano-structures in the material matrices.Polyethylene glycol(PEG)is a FDA-approved synthetic polymer that has been widely used as excipients to formulate drugs.Owing to its high hydrophilicity and flexibility,low immunogenicity and great biocompatibility,PEGs with different low molecular weights can be used to dissolve hydrophobic compounds,encapsulate drug powders and modify peptides,antibodies and enzymes,etc.,in order to improve drug solubility,prolong drug circulating half-life,reduce nonspecific intake,and increase targeting efficiencies.In this study,a novel silk nano/microsphere system was developed by using PEG as an emulsifier and solidifier,and silk nano-microspheres can be formed via an one-step emulsification-sedimentation process in solution containing PEG.The presence of residual hydrophilic PEG in silk nano/microspheres regulated the hydrophilic/hydrophobic environment that would be mainly hydrophobic otherwise.Drug loading and release properties of the system was further studied and the mechanism underlying controlled release was elucidated.Firstly,it was found the molecule weight and concentration of PEG,the silk concentration and the temperatures used for incubation of silk/PEG mixture could influence the size,surface morphology and porosity of the silk nano-microspheres.Further study found that control of solution pH during preparation could change the ?-sheet content of silk nano/microspheres,resulting in the improvement of the stability and degradability of spheres.In addition,regulation of the ?-sheet content of spheres via solution pH adjustment also improved the loading and release profiles of the model drugs(adriamycin and curcumin)in silk spheres.In another related study,utilization of PEG-silk spheres to improve the bioavailability of curcumin was studied.In vitro studies showed that the amount of curcumin loaded in the silk spheres influenced silk ?-sheet content in the spheres and thus the curcumin release profiles.Compere to pH 7.0,silk microsphere fabricated at pH 3.6 improved the encapsulation efficiency of curcumin up to 27%,and prolonged adriamycin and curcumin release in vitro.In vivo pharmacokinetic studies after oral administration of curcumin-silk spheres in rat showed that the amount and duration of curcumin absorbed into the blood from the silk sphere samples were greater than from the plain curcumin powder(control),and smaller spheres(229.3 nm)showed higher bioavailability(3 times)as compared to the larger ones(831.4 ?m).Finally,quantum dots(QDs)-loaded silk spheres were prepared by both polyvinyl alcohol(PVA)phase separation and PEG emulsification-precipitation methods,with an aim to obtain highly stable and less toxic silk-based fluorescent probes by taking the advantages of all-aqueous and mild preparation conditions of silk.It turned out that compared to the PVA method,the preparation of PEG-silk spheres required fewer steps and less time and could incorporate more silk and QDs in the spheres(> 96%).Further characterization showed that the QDs-silk microspheres(30?m)prepared by PEG method had high stability(> 26 h in vitro and > 24 h in vivo)as well as reduced cytotoxicity as compared to QDs alone.In summary,a green,safe,simple,highly reproducible and scalable method for fabricating silk nano/microspheres have been developed in the study.Curcumin,a water insoluble drug,was successfully embedded in silk nano/microspheres with different sizes,and the improved bioavailability and prolonged blood exposure time of the curcumin-silk spheres were demonstrated in vivo.Furthermore,control of drug loading and release as well as stabilization of fluorescent QDs were achieved by tuning silk ?-sheet contents in the spheres.The novel silk nano/microsphere system developed in this study will be useful for a variety of applications in the future,such as disease diagnosis and treatments,tissue engineering,nutrition supplementation and daily cosmetics,etc.