Reduce The Initial Burst Release Of Thermosensitive Hydrogel Loaded Bupivacaine Hydrochloride By Liposomal Encapsulation

Bupivacaine hydrochloride（Bup）is a water-soluble local anesthetic with rapid onset and short half-life and is the most cardiotoxic local anesthetic,which needs to be limited in its burst release and systemic distribution.Thermosensitive hydrogel is particularly prone to initial abrupt release when loaded with bupivacaine hydrochloride,especially during sol-gel phase transitions.Based on this,this paper introduces liposomes（Lip）as an intermediate carrier to reduce the initial burst release of bupivacaine hydrochloride and enhance the long-term sustained release of bupivacaine hydrochloride thermosensitive hydrogel（Bup/Gel）with the assistance of the impeding effect of liposomal phospholipid bilayers.Firstly,the in vitro analytical method for bupivacaine hydrochloride was established.The optimal liquid phase conditions for the determination of bupivacaine hydrochloride by high performance liquid chromatography（HPLC）were determined,under which the method demonstrated good specificity,recovery,and precision.Later,bupivacaine hydrochloride liposome（Bup/Lip）was prepared by using the ammonium sulfate gradient method.The dialysis method was used to measure the drug encapsulation efficiency（EE）,and the encapsulation efficiency was then used as an index to screen and improve the prescription and preparation process.The particle size Zeta potential,drug loading and encapsulation efficiency of the optimal prescription Bup/Lip were evaluated.The results showed that the drug encapsulation efficiency and drug loading capacity（DL）of the optimal prescription were 73.50%and 4.83%,respectively,and the liposome particle size was 166.42 nm,the polydispersity index（PDI）was less than 0.2,and the Zeta potential was-7.02 m V.The stability experiments showed that Bup/Lip could be stored stably at 4°C for at least 7 days.FTIR analysis showed no intermolecular interaction between bupivacaine hydrochloride and liposome,which implies that bupivacaine hydrochloride is present as a free drug within the hydrophilic cavity of liposomes.Then,bupivacaine hydrochloride liposome thermosensitive hydrogel（Bup/Lip-Gel）was prepared by cold method,chitosan（CS）and sodiumβ-glycerophosphate（β-GP）were used as the temperature-sensitive hydrogel matrix to encapsulate Bup/Lip.The prescription and preparation process were screened and optimized using the phase transition time as the index,and the stability,morphology,rheology,and in vitro drug release of Bup/Lip-Gel obtained from the optimal prescription and process were evaluated.The results showed that the phase transition of Bup/Lip-Gel prepared by the optimal prescription could occur within 70 seconds.The stability experiments showed that the Bup/Lip-Gel could be stored stably at 4°C for at least 7 days,and 3%（w/v）sucrose was used as the lyophilization protectant to meet the requirements of the lyophilization process of Bup/Lip-Gel.Bup/Lip-Gel was revealed to be a rough,porous network structure by scanning electron microscopy（SEM）,and Bup/Lip was adsorbed in the network pores.Bup/Lip-Gel’s good injectability was verified by the injectability test.Rheological studies showed that the internal structure of Bup/Lip-Gel has long-term stability.Swelling experiments showed a significant decrease in the swelling ratio（Sr）of Bup/Lip-Gel,but the sol-gel fraction experiments showed that Bup/Lip did not affect the degree of crosslinking of the polymer in the thermosensitive hydrogel,while the higher Sol fraction（Sf）and lower hemolysis ratio（Hr）suggested that Bup/Lip-Gel may have good biocompatibility.FTIR analysis showed no intermolecular interactions between Bup/Lip and Gel.In vitro release experiments showed that the initial burst release of Bup/Lip-Gel was significantly lower compared to Bup/Gel,and the cumulative release of Bup/Lip-Gel was significantly lower than that of both Bup/Lip and Bup/Gel within 72 h,indicating that it has good long-term sustained release ability in vitro.Finally,Bup/Lip-Gel was also evaluated for its in vivo release behavior.An in vivo HPLC method for the analysis of bupivacaine hydrochloride was established using carbamazepine as the internal standard,and the plasma drug concentrations in the Bup solution group,Bup/Gel group,Bup/Lip group and Bup/Lip-Gel group were investigated at various time points in rats by subcutaneous injection administration.The results showed that the peak concentration(C_max)of Bup/Lip-Gel was significantly lower,the mean retention time(MRT_（0-t）)was significantly longer,and the area under the drug-time curve(AUC_（0-t）)was increased,indicating that the initial burst release of Bup/Lip-Gel was significantly lower compared to Bup/Gel,which improved the slow-release performance and enhanced the bioavailability of the thermosensitive hydrogel.In this paper,Bup/Lip-Gel was prepared using Bup as a model drug and Lip as an intermediate carrier.Compared with Bup/Gel,the drug in Bup/Lip-Gel showed lower initial burst release and slower release in vitro and in vivo,indicating that Bup/Lip-Gel is an effective local drug delivery system with potential for long-term analgesic treatment.