Abstract: BACKGROUND: The mechanism of intervertebral disc degeneration is complicated. The activation of inflammatory pathways, the proliferation of inflammatory factors, and the excessive accumulation of reactive oxygen species can all accelerate the process of intervertebral disc degeneration. Curcumin is a natural drug extracted from traditional Chinese medicine curcuma, which has pharmacological effects such as inhibiting inflammation, antioxidant stress and so on. 
OBJECTIVE: To study the curative effect of local injection of curcumin loaded sustained-release microspheres on intervertebral disc degeneration in rats.
METHODS: (1) Sustained release microspheres loaded with curcumin nanoparticles were prepared by microfluidic technology. The drug release and degradation were evaluated in vitro. (2) Sustained-release microspheres and unloaded microspheres were co-cultured with nucleus pulposus cells in vitro. Cells cultured alone were used as controls. The three groups of cells were cultured in normal environment and oxidative stress environment (adding H2O2). The activity and proliferation of nucleus pulposus cells were evaluated by CCK-8 assay and live/dead staining. The expression levels of related factors were analyzed by qRT-PCR. (3) A total of 30 SD rats were randomly divided into normal group, degeneration group, unloaded microsphere group, curcumin group and sustained-release microsphere group (n=6 per group). The models of intervertebral disc degeneration were established by percutaneous puncture in the 7-8 and 8-9 segments of the caudal vertebra of the rats. The corresponding solutions were injected into the degenerated intervertebral discs in the unloaded microsphere group, the curcumin group, and the sustained-release microsphere group. After 4 weeks, the changes of intervertebral disc tissue structure were observed by imaging and histology. 
RESULTS AND CONCLUSION: (1) The sustained-release microspheres could slowly and continuously release curcumin, and the total amount of curcumin released was (84.11±2.71)% at 28 days. In PBS containing collagenase, the mass of the sustained-release microspheres gradually decreased and disappeared at the 5th week. (2) The results of CCK-8 assay and live/dead staining indicated that the sustained-release microspheres did not reduce the proliferation ability of nucleus pulposus cells under normal conditions and could maintain the proliferation activity of nucleus pulposus cells under oxidative stress compared with unloaded microspheres. qRT-PCR experiments indicated that compared with the normal environment, the oxidative stress environment activated the overexpression of mRNA of various inflammatory factors in nucleus pulposus cells. Under oxidative stress, the mRNA expression levels of tumor necrosis factor-α, interleukin-1β, interleukin-6 and matrix metalloproteinase 3 in the sustained-release microsphere group were decreased and mRNA expression of type II collagen and proteoglycan was increased compared with the unloaded microsphere group and the control group. (3) The results of animal in vivo experiments showed that the imaging and histological evaluation of the unloaded microsphere group was not significantly different from that of the degeneration group after 4 weeks. The intervertebral space height and MRI score of the curcumin group were improved to a certain extent compared with the degeneration group and the unloaded microsphere group at 1 week, but the imaging and histological evaluations at 4 weeks were not as good as those of the sustained-release microsphere group. (4) In conclusion, curcumin sustained-release microspheres could inhibit the apoptosis of nucleus pulposus cells induced by H2O2 and retard the progression of intervertebral disc degeneration. Its mechanism is associated with inhibiting the nuclear factor κB pathway and reducing the level of oxidative stress. 

Key words: microfluidic technology, nanoparticle, microsphere, freeze-drying, curcumin, intervertebral disc degeneration, gelatin methacryloyl