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Aim: To evaluate the analgesic effects of 2 celecoxib derivatives and their inhibitory effects on cyclooxygenase (COX). Methods: Four antinociceptive assays were used: the acetic acid-induced writhing test, hot plate test, hot tail-flick test and formalin test. Three doses were used in the analgesic assays and ED50 values were calculated. For the selectivity assay, macrophages were incubated with test compounds at various concentrations and then stimulated with calcimycin or lipopolysaccharide (LPS). The amounts of 6-keto-prostaglandin F1α (6-keto-PGF1α)and prostaglandin E2 (PGE2) in the supeatant were examined by radioimmunoassay (RIA). The selectivity of the test compounds was expressed as the IC50,COX-1/IC50,COX-2 value. Results: Celecoxib and its 2 derivatives had a significant analgesic effect. The ED50 values of celecoxib, PC-406 and PC-407 were 94.2, 67.9, and 63.3mg/kg, respectively, for the acetic acid-induced writhing test; 104.7, 89.1, and 30.0mg/kg, respectively, for the hot tail-flick response test; 60.7, 56.7, and 86.2 mg/kg,respectively, for the hot plate response test; 67.1,55.8, and 68.8 mg/kg, respectively,for the formalin-induced response. That is, the ED50 of PC-406 was the lowest for the formalin and hot plate tests, which focus on changes above the spinal cord level; however, the ED50 of PC-407 was lowest for the tail-flick and writhing tests,which focus on changes at the spinal cord level. Celecoxib and PC-407 inhibited COX-1 with IC50 values of 39.8 and 27.5 nmol/L, respectively. PC-406 inhibited COX-1 with an IC50 value of more than 1000 nmol/L. The IC50 values for the effect of celecoxib, PC-406 and PC-407 on COX-2 were 4.8, 8.9, and 1.9 nmol/L respectively.The IC50,COX-1/IC50,COX-2 ratios for celecoxib and PC-407 were 8.3 and 14.4, respectively. For PC-406, the ratio was greater than 112.2. Conclusion: Derivatives of celecoxib via substitution with an isopropyl or naphthyl group at the 5 position in the pyrazole ring still have analgesic effects and the ability to selectively inhibit COX-2. Substitution with a naphthyl group may have more effect on the peripheral pain pathway, whereas substitution with an isopropyl group may have more effect on the central pain pathway. This phenomenon occurs partly because substitution with an isopropyl group is more beneficial for COX-2 selectivity than is substitution with a naphthyl group.