Background and objective Ropivacaine hydrochloride is a commonly used local anesthetic in clinics. However, local injection or continuous infusion of ropivacaine has been associated with several disadvantages. Accordingly, it is important to develop a new controlled release system for local administration of ropivacaine to achieve a prolong anesthetic effect, improve efficacy, and minimize the side effects.

Methods We developed injectable hydroxypropyl chitin thermo-sensitive hydrogel (HPCH) combined with hyaluronan (HA), which was used to synthesize a ropivacaine (R)-loaded controlled release system. We then conducted drug release test and cytotoxicity assay in vitro. Importantly, we examined the analgesic effects and biocompatibility of this system in vivo by injecting different concentrations of R-HPCH-HA (7.5, 15, 22.5 mg/mL), ropivacaine hydrochloride (R_HCL, 7.5 mg/mL), or saline (all in 0.5 mL) near the sciatic nerve in rats.

Results R-HPCH-HA induced concentration-dependent thermal-sensory blockade and motor blockade in vivo. In hot plate test, R-HPCH-HA (22.5 mg/mL) induced a significant longer thermal-sensory blockade (17.7±0.7 hours), as compared with R_HCL (7.5 mg/mL, 5.7±0.8 hours, n=6/group, p<0.05). It also produced a more prolonged motor blockade (6.8±0.8 hours) than R_HCL (3.5±0.8 hours, p<0.05). R-HPCH-HA caused less cytotoxicity than R_HCL, as indicated by the higher cell viability in vitro (n=8/group).

Conclusion Our findings in a sciatic nerve block model demonstrated that the injectable, ropivacaine-loaded controlled release system effectively prolonged the local analgesic effect in rats without notable side effects.