Relation between plasma concentration of indomethacin and its effect on prostaglandin synthesis and platelet aggregation in man.

The dose and plasma levels of indomethacin correlated with inhibition of prostaglandin synthesis as measured both by urinary excretion of the major metabolite of prostaglandin E2 (PGE-M) and by the release of prostaglandin E2 from thrombin-stimulated platelets. Considerable intersubject variability was observed in the suppression of PGE-M excretion. In some patients 37.5 mg indomethacin daily, usually considered subtherapeutic, caused suppression. Maximal suppression (greater than 90%) occurred in some after a daily dose of 75 mg, whereas 150 mg was required to achieve this level of inhibition in others. Suppression of the excretion of PGE-M by 60% occurred when the end of the dosage interval plasma levels of indomethacin were in the range 0.05 to 0.3 microgram/ml, which implies that a somewhat higher average steady-state concentration during the dosage interval was required to achieve this effect. A similar degree of inhibition of the release of PGE2 on thrombin-stimulated platelets was associated with the same range of plasma levels. Upon discontinuation of the drug, the levels of indomethacin in plasma decreased exponentially; inhibition of the release of PGE2 from platelets by indomethacin declined linearly with time and in parallel with the logarithm of the diminishing plasma levels.