Synergistic Interaction Between Justicia spicigera Extract and Analgesics on the Formalin Test in Rats

Background: Combining antinociceptive drugs with different mechanisms of action can reduce the doses and the adverse effects, with a possible increase in the antinociceptive effect. This work evaluated the antinociceptive effect of the combination of an ethanol extract of Justicia spicigera (JSE) with naproxen (NPX) or tramadol (TML) using the formalin test in rats. Methods: Rats received JSE (30-200 mg/kg p.o.), NPX (50-300 mg/kg p.o.), or TML (5-50 mg/kg p.o.) 60 min before paw administration with formalin (5%). Different proportions of the combination between NPX and JSE, as well as TML and JSE, were used in the formalin test to obtain the dose-response curve of each drug and the experimental effective dose 50 (ED₅₀). The levels of IL-1β and COX2 were assessed using a Western blot analysis as a possible mechanism of action for the combination of JSE and analgesics. A pharmacokinetic study was conducted to evaluate the effect of JSE on the pharmacokinetic parameters of NPX. Results: The ED₅₀ values for the proportions NPX:JSE were 107.09 mg/kg (1:1), 102.44 mg/kg (3:1), and 73.82 mg/kg (1:3). The ED₅₀ values for the proportions TML:JSE were 66 mg/kg (1:1), 29.5 mg/kg (1:3), and 78 mg/kg (3:1). The combination NPX:JSE (1:3) showed the best synergistic interaction index (0.501). The pharmacokinetic study revealed that there were no significant changes in the pharmacokinetic parameters of NPX administered individually and the combination NPX:JSE. Conclusions: In this preclinical study, the combination NPX:JSE showed antinociceptive effects by decreasing the levels of COX2 and IL-1β without affecting NPX's pharmacokinetics.