The Liquid-solid Phase Transition Process Of Solvent Removal Precipitation-based In Situ Forming Implants And Its Effect On Drug Release Mechansim

Objective:Solvent removal precipitation-based in situ forming implant（SRP-ISFI）is a injectable and controlled release system that comprised of a water insolublebiodegradable polymer dissolved or swollen in a water miscible,physiologicallycompatible solvent.After injection,the organic solvent contained in SRP-ISFI released and exchanged with the water in physiological environment,and the semi-solid orsolid polymeric depot formed through the mechanism of non-solvent-induced phase separation.Many parameters have been shown to affect the drug release profile of injectable implants including solvent polarity,polymer concentration,drug loading,polymer degradation rate and drug permeability coefficient.Unlike traditional polymer implants,SRP-ISFI has a more complex drug release mechanism due to the existence of liquid-solid phase transition process,and the phase transition process is of greatsignificance to the whole drug release process.There are many complex physical processes during the phase transition of SRP-ISFI,including solvent exchange,polymer precipitation and crystallization,drug release or embedded in polymer.The initial burst release of the drug is a common problem in SRP-ISFI,which can result in tissue irritation and even systemic toxicity.The delay between administration anddepot formation has been regarded as the chief cause of burst release.The process of phase transition is significant to the forming and drug controlled release of SRP-ISFI.Although the importance of phase transition have been confirmed,the relationshipbetween crystalline structure of polymer depot,formation regulation of crystallization and mechanism of drug release are still unclear.In this study,we focus on the phase transition process of SRP-ISFI,and the organic solvent exchange process,polymer crystallization behavior and drug burst release process were taken as the breakthrough points.The main research purposes including three aspects:1.By reveal the essential mechanism of phase transition process,the drug release mechanism of SRP-ISFI are deeply analyzed,and the important factors of drug release,especially burst release during the process of phase transition are explored.2.By using Poly（lactic acid）fragment contained ABA triblock polymer to prepare SRP-ISFI,clarify the close interactive relationship between the process ofcrystal forming and drug releasing,confirm the effect of different crystalline states of polymers on drug release mechanism in SRP-ISFI.3.Finally,the SRP-ISFI with better drug release performance was developed by controlling the crystallization behavior of polymer in the phase transition stage.Method:The research was mainly divided into two parts:1.Osthole-loaded SRP-ISFIs were prepared by dissolving polylactide（PLA）,poly（lactide-co-glycolide）（PLGA）,polycaprolactone（PCL）,or poly（trimethylene carbonate）（PTMC）in different organic solvents,including N-methyl-2-pyrrolidone（NMP）,dimethyl sulfoxide（DMSO）,and triacetin（TA）.Drug release,polymerdegradation,solvent removal rate,depot microstructure and the in vivo release ofOsthole was investigated.By comparing the effects of different polymers and organic solvents,the drug release mechanism of SRP-ISFI was analyzed,and the important factors of drug release process,especially drug burst release behavior during phase transition process were explored.2.Using trimethylene carbonate and lactide with different rotations as monomers,Polylactide fragment contained ABA triblock polymer,PLA-b-PTMC-b-PLA withdifferent monomer ratios and molecular weights were synthesized and characterized.DSC,IR,X-ray diffraction and atomic force microscope were used to investigate the crystallization behavior of PLA fragment in the phase transition stage and the effect of different PLA optical rotation.We confirmed multiple crystalline structures,including crystal of stereocomplex-PLA,Sc-PLA,can be detected in the depots after phasetransition the process.ATR-FTIR was used to track the crystallization behavior of the polymer in the phase transition stage and investigate the effect of polymercrystallization behavior on drug release during the phase transition process.The SRP-ISFI with self-assembly of sc-PLA was prepared.The drug controlled release property,polymer degradation,pharmacokinetic and biocompatibility of the sc-PLA contained SRP-ISFI were investigated in vitro and in vivo.The feasibility of sc-PLA contained SRP-ISFI as drug carrier was demonstrated.Results:1.In this study,a series of SRP-ISFIs with different components were prepared by using bulk degradation polymers PLGA,PLA,PCL and surface erosion polymerPTMC as materials,combined with different organic solvents,including NMP,DMSO and TA.The in vitro drug release and polymer degradation results indicated that,drug release from SRP-ISFI could be separated into three steps:the initial burst release,then a long period of constant release mainly caused by matrix diffusion and finally the last stage of release due to the erosion of the polymer.Both the drug burst release andpolymer depot microstructure were closely related to the removal rate of organic solvent.PCL with NMP and DMSO as organic solvents had the highest drug bursteffect,and more than 88%of Ost had been released in the first day.Regarding the less water-miscible solvent,TA,the drug burst release was significantly decreased.SEM results showed that the polymer matrix properties,such as density,moisture and pore size were also closely related to the solvent release rate.Fast solvents exchange were more likely to develop loose and porous structures.Both of drug diffusion and organic solvent exchange were related to the polymer permeability.As to small molecularsincluding drugs and solvents,polymer with higher permeability often displayed faster diffusion rates.The solvent exchange process was not only affected by the watersolubility of the solvent,also be estimated by polymer permeability andpolymer-solvent affinity,which could be predicted by Hildebrand and Hansen solubility parameters.Polymer degradation could be accelerated due to theautocatalytic effect of TA.The Osthole release of SRP-ISFI prepared by PLGA and TA was investigated in vitro and in vivo.After subcutaneous injection,the Osthole plasma concentration presented a double peak curve.The first peak concentration was due to the burst release of Ost,and the second peak concentration was caused by the breakup of the depots after polymer degradation.2.In order to investigate the crystallization behavior during polymer precipitation in the phase transition stage,and the relationship among thecrystallization and permeability of polymer and drug release,PLLA-b-PTMC-b-PLLA and PDLA-b-PTMC-b-PDLA were synthesized and characterized by NMR and FTIR.SRP-ISFIs were prepared with blockpolymers which contained different opticalrotation PLA segments.The results confirmed multiple crystalline structures,including crystal of stereocomplex-PLA,Sc-PLA,can be detected in the depots after the process of phase transition.SRP-ISFI named sc PLA-PTMC-sc PLA were prepared by mixed PLLA-b-PTMC-b-PLLA and PDLA-b-PTMC-b-PDLA equally.After phase transition process,the XRD patterns of solid implants formed characteristic peaks on 12゜,21゜and 24゜which belong to Sc-PLA crystal.The results of DSC and FTIR also confirmed the formation of Sc-PLA crystal in the polymer precipitation.The results of atomicforce microscopy showed that the microphase separation of sc PLA-PTMC-sc PLA were layered structure.ATR-FTIR was used to track the polyer precipitation and crystallization process in SRP-ISFIs.The results showed that the infrared spectra at1131,1188,1339 and 1750 cm^-1 changed obviously due to polymer crystallization during the phase transition.The results of drug release in vitro indicated that thedifferent crystal forms of PLA segments resulted in different drug release rates in SRP-ISFI.SRP-ISFI with Sc-PLA crystal self-assembly property was prepared by sc PLA-PTMC-sc PLA which monomer ratio was TMC:LA=60:40.The Sc-PLA contained SRP-ISFI had excellent drug release performance,and could maintainosthole release in 90 days in vivo.The degradation behavior of sc PLA-PTMC-sc PLA was surface dissolution type and disintegration of the carrier was also avoided,so the Osthole plasma concentration presented as Unimodal distribution.Thebiocompatibility of the SRP-ISFI was greatly improved because the degradation ofsc PLA-PTMC-sc PLA didn’t produce so many acidic products.These results indicated the SRP-ISFI with Sc-PLA crystal self-assembly property could be considered as an excellent drug controlled release system.Conclusion:In phase transition stage,the organic solvent and drug molecules diffused together with synergy and lead to drug burst release.And,the permeability of solid implant formed by polymer precipitation and crystallization affects the steady-state diffusion rate of the drug.Therefore,changing the permeability of the polymer in the phase transition stage can enhance the sustained-release effect of SRP-ISFI.In this paper,ABA blockpolymers PLA-b-PTMC-b-PLA were synthesized.By using thedifferent optical rotation of PLA segments and the polymorph of PLA,it was confirmed multiple crystalline structures can be formed in the process of phasetransition.By mixed PLLA-b-PTMC-b-PLLA and PDLA-b-PTMC-b-PDLA equally,the prepared SRP-ISFI could self-assemble to form Sc-PLA crystal during phasetransition process.SRP-ISFI with Sc-PLA crystal self-assembly property has been proved to be an excellent drug controlled release system,and its drug releaseperformance,polymer degradation performance and biocompatibility are greatly improved.