Time-Dependent Inhibition of Prostaglandins Synthesis by Nonsteroidal Antiinflammatory Drugs; Time-Dependent Alteration of Inhibitory Effect and Characteristics of its Active Site

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Abstract
The characteristics of the time-dependency of nonsteroidal antiinflammatory drugs (NSAID) and their active sites in inhibiting cyclooxygenase activity were investigated using bovine seminal vesicle microsomal fraction. The Hill''s coefficients of the inhibition curve of most NSAID were .apprx. 1.0, except for some drugs which had Hill''s coefficients that were obviously > 1.0. The values for indomethacin, diclofenac and flurbiprofen were .apprx. 1.5, 1.7 and 2.0, respectively. A time-dependent cyclooxygenase inhibiting effect was observed only with the NSAID in the group which had Hill''s coefficients that were significantly > 1.0. A time-dependent increase in inhibiting activity of these drugs was observed by the change in the inhibition curve. Time-dependently, the Hill''s coefficient of the curve of the NSAID approached 1.0 from more than 1.0. With this change, the shift of the curve to the left was also observed time-dependently. These changes in the curve were no longer seen when the Hill''s coefficient dropped to 1.0. These time-dependent effects were protected by salicylic acid, which did not have any effect on cyclooxygenase activity in vitro, in a dose-dependent manner. The 2 classes of NSAID, time-dependent and non-time-dependent, are clearly differentiated by the slope of the inhibition curve of NSAID. The degree of time-dependence of the NSAID depends on the Hill''s coefficient of the inhibition curve and the value is important for the potency of the cyclooxygenase inhibition, in addition to the affinity for the catalytic site of the enzyme.