Effect of a Potent Cyclooxygenase Inhibitor, 5-Ethyl-4-Methoxy-2- Phenylquinoline (Ktc-5), on Human Platelets

Because the metabolites of arachidonic acid participate in many physiopathological responses, including inflammation and platelet aggregation, cyclooxygenase inhibitors are important in the treatment of associated diseases. A biologically active compound, 5-ethyl-4-methoxy-2- phenylquinoline (KTC-5), selectively and concentration dependently inhibited aggregation of platelets from man and ATP release caused by arachidonic acid (200 mum) and collagen (10 mug mL(-1)) without affecting the aggregation caused by thrombin (0.1 U mL(-1)) and U46619 (2 mum). The IC 50 value (drug concentration inhibiting maximum response by 50%) of KTC -5 for aggregation induced by arachidonic acid and collagen was 0.11 +/- 0 .04 mum and 0.20 +/- 0.03 mum, respectively. This inhibitory effect of KTC -5 was reversible and time dependent. KTC-5 specifically inhibited intracellular calcium mobilization initiated by arachidonic acid or collagen without affecting that caused by thrombin or U46619 in human platelets. Furthermore, KTC-5 inhibited thromboxane B-2 and prostaglandin D-2 formation provoked by arachidonic acid. The IC50 value of KTC-5 for arachidonic- acid-induced thromboxane B-2 formation was 0.07 +/- 0.02 mum . Based on these observations, the data indicated that KTC-5 potently inhibited human platelet aggregation and ATP release mainly via the inhibition of the cyclooxygenase-1 activity. Moreover, KTC-5 inhibited lipopolysaccharide- induced prostaglandin E-2 formation in RAW264.7 cells in the presence of external arachidonic acid with an IC50 value of 0.17 +/ - 0.06 mum. Immunoblot analysis showed that KTC-5 did not affect the cyclooxygenase-2 expression in the presence of lipopolysaccharide on RAW 264.7 cells. This result indicated that KTC-5 affects the activity of cyclooxygenase-2. According to these data, we concluded that KTC-5 is a cyclooxygenase inhibitor for both subtypes