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The ability of metamizol to inhibit cyclooxygenase-1 and cyclooxygenase-2 activities has been evaluated using different cyclooxygenase sources. Metamizol inhibited purified cyclooxygenase-1 and cyclooxygenase-2 with an IC of about 150 mgrml. A similar IC 50 50value for cyclooxygenase-2 was obtained in lipopolysaccharide-activated broken murine macrophages. Consistent with these findings, molecular models of the complexes between cyclooxygenase-1 or cyclooxygenase-2 with 4-methylaminoantipyrine, the major active derivative of metamizol, suggested a common binding mode to both isoforms. In intact cells, however, the inhibition profiles were Ž . markedly different. The IC values of metamizol for cyclooxygenase-1 in intact bovine aortic endothelial cells BAEC cells and human 50 platelets were 1730 " 150 mgrml and 486 " 56 mgrml, respectively. Inhibition of cyclooxygenase-2 activity in murine macrophages and primary human leukocytes activated by lipopolysaccharide yielded IC values of 12 " 1.8 mgrml and 21 " 2.9 mgrml, 50 respectively. These data indicate that the IC values obtained with purified enzymes or disrupted cells cannot always be extrapolated to 50 Ž . the cyclooxygenase inhibitory activity of nonsteroidal antiinflammatory drugs NSAIDs in intact cells. The data presented here also indicate that cyclooxygenase-2 inhibition could play an important role in the pharmacological effects of metamizol. q 1999 Elsevier Science B.V. All rights reserved.Ž .

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