DIFFERENTIAL INHIBITION OF SUPEROXIDE, HYDROXYL AND PEROXYL RADICALS BY NIMESULIDE AND ITS MAIN METABOLITE 4-HYDROXYNIMESULIDE

The superoxide and hydroxyl radical scavenging activities of nimesulide (CAS 51803-78-2) and its main metabolite 4-hydroxynimesulide (CAS 109032-22-6) were investigated by Election Spin Resonance (ESR) spectroscopy, with the spin trapping technique. Hydroxynimesulide is a good scavenger of both O-2 degrees(-) (IC50 = 40 mu mol/l) and HO degrees (IC50 = 54.8 mu mol/l) radicals, and its high reactivity towards HO degrees was confirmed by the rate constant for reaction with HO degrees (K = 8.9 x 10(10) mol(-1) I s(-1)) determined by competition kinetic studies with 5,5-dimethyl-1-pyrroline-N-oxide. Nimesulide, which has been shown by ESR to be inactive as a superoxide quencher, has a rate constant of reaction with HO degrees slightly greater than that of its metabolite (3.3 x 10(11) mol(-1) l s(-1)). In the HO degrees-induced peroxidation of phosphatidylcholine (PC) liposomes, both compounds act as potent preventive antioxidants, but the HO degrees entrapping capacity of the parent drug was again greater than that of hydroxynimesulide (IC50 2.12 vs 3.84 mu mol/l). The metabolite is also a potent scavenger of the peroxyl radical (ROO degrees) in the propagation phase of PC peroxidation (marker conjugated dienes), with an IC50 = 2.67 mu mol/l: at 5 mu mol/l it induces a lag time in the decomposition of PC hydroperoxides (PC-OOH) into aldehydic products of 40 h longer than in the controls (markers: conjugated dienes and total carbonyl functions). In PC liposomes, in the presence of preformed PC-OOH, the metabolite prevents PC peroxidation stimulated by 5 mu mol/l Fe2+ via the Fenton reaction (marker: conjugated dienes), at the micromolar level (IC50 = 17 mu mol/l) through an anti-free radical activity and a free iron chelating mechanism. Hydroxynimesulide in fact interacts with Fe2+ ions giving rise to a strong complex, with a stability constant (log K) estimated to be around 8/9. In addition, hydroxynimesulide efficiently protects ex vivo synovial fluid lipids from the oxidative stress induced by Fe2+/ascorbate, already at 10 mu mol/l (marker: thiobarbituric acid reactive substances). These results provide evidence for strong antioxidant and iron-chelating properties of 4-hydroxynimesulide, which can act synergistically with the specific HO degrees scavenging activity of the parent drug in preventing and limiting in vivo the free radical-mediated tissue damage in both acute and chronic inflammatory situations.