Preparation and application of minocycline polymer micelle thermosensitive gel in spinal cord injury

Abstract

Neuroprotection is one important approach for treatment of spinal cord injury (SCI). Minocycline (MC), a known neuroprotective agent has been utilized for SCI treatment, but its therapeutic effect is limited by instability and low bioavailability. Herein, we sought to incorporate MC into polyethylene glycol (PEG)- poly (lactate-co-glycolic acid) (PGLA) co-polymer to form micelles, and subsequently micellar thermosensitive MC-loaded hydrogel (MCPP-M-gel) for application in rat model of SCI. After incorporation of MC into micellar system (MCPP-M) via thin film hydration method, it was physically characterized using encapsulation rate, zeta potential, polydispersed index (PDI) and particle size. Later, the micelles were developed into MCPP-M-gel and characterized with appropriate physico-mechanical properties. In vitro release testing was evaluated via diffusion method, while cytotoxicity was performed using neural-crest-derived ectoderm mesenchymal stem cells (EMSCs). Regarding results, controlled and prolonged MC release from MCPP-M-gel for 72 h was observed with the hydrogel exhibiting no cytotoxicity to EMSCs at the studied dose. Afterward, MC, MCPP-M, MCPP-M-gel and blank micellar thermosensitive gel were injected into the injured site of SCI rats. Histopathological evaluation demonstrated that MCPP-M-gel improved histological assessments which depicted that MCPP-M-gel could promote neuronal regeneration at the injured site of the SC after 28 days. Immunofluorescence techniques exhibited that MCPP-M-gel increased expression of neuronal class III β-tubulin (Tuj1), myelin-basic protein (MBP), growth associated protein 43 (GAP43), neurofilament protein-200 (NF-200) and Nestin, as well as reduced glial-fibrillary acidic protein (GFAP) expression in damaged areas of SC. This experimental work provides foundation for subsequent investigation and application of MCPP-M-gel in SCI models or other disorders of neurons.