Pharmacokinetic-pharmacodynamic Modeling of the Antinociceptive Effect of Diclofenac in the Rat

The relationship between the pharmacokinetics and the antinociceptive effect of diclofenac was evaluated using the pain-induced functional impairment model in the rat. Male Wistar rats were injected with uric acid in the knee joint of the right hind limb, which induced its dysfunction. Once the dysfunction was complete, animals received a p.o. dose of 0.56, 1, 1.8, 3.2, 5.6 or 10 mg/kg of sodium diclofenac, and the antinociceptive effect and drug blood concentration were simultaneously evaluated at selected times for a period of 6 h. Diclofenac produced a dose-dependent antinociceptive effect, measured as a recovery of the functionality of the injured limb. However, the onset of the antinociceptive effect was delayed with respect to blood concentrations. Moreover, the effect lasted longer than expected from pharmacokinetic data. Therefore, when functionality index was plotted against diclofenac blood concentration, an anticlockwise hysteresis loop was observed for all doses. Hysteresis collapse was achieved using the effect-compartment model, and the plot of functionality index against diclofenac concentration in the effect-compartment data was well fitted by the sigmoidal Emax model. Our data suggest slow equilibrium kinetics between diclofenac concentration in blood and at its site of action, which leads to a delayed onset of the antinociceptive effect as well as a longer duration of the response resulting from drug accumulation in synovial fluid.