Development and Evaluation of Ropivacaine Loaded Poly(Lactic-Co-Glycolic Acid) Microspheres with Low Burst Release

The local anesthetic drugs, especially ropivacaine, were considered favorable analgesia for postoperative management because of their effective local pain relief and low adverse effects. However, the short half-life and the resulting in bolus doses lead to the indistinctive improvement of these drugs in postoperative pain relief. Therefore, the ropivacaine microspheres with sustained release and low initial burst release were anticipated.

Three methods including oil in water (O/W), water in oil in water (W/O/W), and solid in oil in water (S/O/W) emulsion solvent evaporation method were used to optimize the ropivacaine loaded PLGA microspheres. The microspheres were evaluated both in vitro and in rats. The in vitro-in vivo correlation (IVIVC) was also investigated.

The microspheres prepared by O/W method showed more satisfactory properties and the microspheres used for evaluation were prepared by O/W method. The particle size, drug loading, encapsulation efficiency and burst release were 11.19±1.24 µm, 28.37±1.15%, 98.15±3.98%, and 10.96±5.37% for microspheres with PLGA of 12 kDa, and 6.64±0.61 μm, 19.62±0.89%, 92.74±4.21%, and 18.42±5.12% for microspheres with PLGA of 8 kDa, respectively. These microspheres were also injected into rats by subcutaneous, intramuscular and intraperitoneal route, respectively. It was indicated that the detectable concentration of ropivacaine could last for at least 20 days for both kinds of microspheres in spite of injection routes. The low burst releases at 1 d were also manifested in rats and they were 6.62%, 6.99%, 6.48% for the microspheres with PLGA of 12 kDa, and 4.72%, 4.33%, 4.48% for the microspheres with PLGA of 8 kDa by intraperitoneal, intramuscular and subcutaneous route, respectively. A linear relationship between the in vitro release and the in vivo adsorption of ropivacaine from microspheres was also established.

The ropivacaine microspheres with sustained release and low burst release were acquired, which indicated that the postoperative pain relief might last longer and the side effects might get lower. Therefore, the ropivacaine microspheres prepared in this paper have great potential for clinical use.