Injectable In Situ Hydrogel-microsphere Composite Drug Delivery System For Long-term Analgesia

Acute postoperative pain is a common and challenging problem,usually occurring within 48 h after surgery.It can affect the quality of life and functional recovery,resulting in increased postoperative morbidity and delay in hospital discharge.Relief can be obtained by the administration of antipyretic analgesics.However,these medications,especially opioids,are usually accompanied with severe side effects,such as nausea,vomit,and respiratory inhibition.Recently,the induction of peripheral nerve blockade(PNB)using local anesthetics has attracted considerable attention because of their ability to relieve pain with few complications.However,owing to the low molecular weight of anesthetics,the duration of the analgesia induced by a single injection is usually only a few hours,which does not meet the requirements for clinical applications.Continuous infusion of anesthetics through a catheter can provide long-term analgesia,but it is frequently accompanied by complications,such as catheter abscission,epidural hematoma,and nerve damage.Sustained-release formulations have been investigated to extend the duration of analgesia induced by local anesthetics.Recently,the poly(lactic-co-glycolic acid)microspheres(PLGA MSs)have gained in popularity due to their excellent biodegradability,biocompatibility,and stable physicochemical properties during manufacturing and storage.PLGA MSs have been successfully loaded with lidocaine,bupivacaine,ropivacaine,and other local anesthetics,achieving nerve blockade durations of 6-24 h.However,the poor in situ property of MS and the initial burst release of drugs can reduce blockade duration and result in local and systemic toxicity.Recent years,thermosensitive polymer hydrogel has gain wide attention as local drug delivery system.At low temperatures(e.g.,4 °C),drugs can be mixed into a polymer solution for convenient injection.As the temperature reaches body temperature,the micelles automatically transform into a viscous gel,forming an in situ depot.Thus,the gelation system can achieve sustained drug release in situ.Hydrogel has been applied in drug delivery,anti-adhesion and tissue engineering.To solve the problems of local anesthetics in clinical application of and improve its analgesia effect,PLGA microspheres with excellent particle size and thermosensitive PLGA-PEG-PLGA hydrogel were prepared,and Gel-MS composite drug delivery system was prepared by mixing at 4 °C.The drug delivery system exhibited continuous slow release of BUP in situ without obvious burst release achieved longer analgesia duration than that of MS or Gel.Furthermore,after the addition of DEX,the analgesic effect of the Gel-MS system was further enhanced,and the drug content in the circulation system and the major organs decreased significantly.Specifically,the experiment mainly includes the following aspects:1)Preparation of BUP base and BUP loaded PLGA microspheresIn this study,bupivacaine free base(BUP)was prepared by alkali precipitation method.Subsequently,BUP-loaded PLGA microsphere(MS/BUP)was prepared by oil-in-water method.The method is simple and MSs exhibited uniform particle size with high loading capacity.The mean diameter of the MSs was about 15?m with smooth surface when observed by SEM.2)BUP loaded Gel-MS system for long-term analgesia.In this study,an in situ-forming injectable hydrogel-microsphere(Gel-MS)system consisted of PLGA-PEG-PLGA hydrogel(Gel)and MS/BUP was prepared for precision-guided long-acting analgesia and a series of in vitro and in vivo characterizations were carried out.The results showed that the release of BUP from the Gel-MS system was regulated by both the inner MS and the outer hydrogel matrix,demonstrating sustained BUP release in vitro without an initial burst release.More importantly,incorporation of the Gel immobilized the MS and hindered the diffusion of MS from the injection site because of its in situ property.In vivo,a single injection of Gel-MS/BUP allowed rats to maintain nerve blockade significantly longer than treatment with MS/BUP or BUP-loaded hydrogel(Gel-BUP).Histopathological results demonstrated the excellent biodegradability and biocompatibility of the Gel-MS system.3)Gel-(DEX-MS/BUP)system with sequentially release of DEX and BUP for long-term synergetic analgesia.Gel-MS composite that loads local anesthetic bupivacaine(BUP)and ?2 adrenergic receptor agonist DEX,termed as Gel-(DEX-MS/BUP)were developed for prolonged analgesia with only a single injection.In this composite,DEX was released preferentially,which binds to ?2 receptor and contracts the blood vessels,followed by inhibiting the absorption of BUP into the blood.The sustained release of BUP then blocked the peripheric nerve conduction and lead to a long-acting analgesia.In conclusion,Gel-MS/BUP composites prepared in this experiment exhibited great temperature sensitivity and could be injected to the sciatic nerve as liquid at room temperature.In addition,it changed to gel status in the stimulation of body temperature,showing good in situ property and provided sustained drug release,and further improved the analgesic effect of local anesthetics.In addition,the Gel-MS system exhibited good biocompatibility and biodegradable.Furthermore,the addition of a2-adrenergic receptor agonist DEX to Gel-MS system could further improved the analgesic effect of the sustained-release system.