A high level of cyclooxygenase-2 inhibitor selectivity is associated with a reduced interference of platelet cyclooxygenase-1 inactivation by aspirin

Abstract

Both nonsteroidal anti-inflammatory drugs, such as ibuprofen, and the prototypical selective cyclooxygenase (Cox)-2 inhibitors DuP-697 and NS-398 block the inhibition of Cox-1 by aspirin in vitro. However, clinical studies have shown that the Cox-2 selective drugs (or coxibs) rofecoxib and etoricoxib, at therapeutic doses, do not interfere with the antiplatelet effect of aspirin, in contrast to ibuprofen. Here, we have evaluated the relative potential of ibuprofen and various coxibs to interfere with the inactivation of Cox-1 by aspirin by using purified enzyme and calcium ionophoreactivated human platelets. The irreversible inactivation of Cox-1 by aspirin can be antagonized by ibuprofen and coxibs, albeit with widely different potencies. The rank order of potencies for this process (ibuprofen > celecoxib > valdecoxib > rofecoxib > etoricoxib) parallels that obtained for the inhibition of Cox-1-mediated thromboxane B 2 production by calcium ionophore-stimulated platelets. The antagonism of aspirin therefore likely involves a competition at the enzyme active site. The EC 50 value for the antagonism against 10 M aspirin for each drug is Ϸ10-to 40-fold lower than the corresponding IC 50 value for inhibition of platelet Cox-1 activity, consistent with the much weaker initial binding of aspirin to Cox-1 as compared with arachidonic acid. These results show that a low affinity for Cox-1 and a high degree of Cox-2 selectivity confers a low potential to block aspirin inhibition of platelet Cox-1, consistent with the results of clinical studies. The two cyclooxygenase (Cox) isozymes (or prostaglandin H synthases), which share Ϸ60% sequence identity, perform the first committed steps in the prostaglandin pathway by catalyzing the oxygenation of arachidonic acid to PGG 2 (Cox activity) and the reduction of PGG 2 to PGH 2 (peroxidase activity) (1-3). Cox-1 is constitutively expressed in most cell types, including platelets, whereas Cox-2 is absent from most healthy tissues but is induced by proinflammatory or proliferative stimuli. Cox-1 plays a role in the production of prostaglandins involved in protection of the gastric mucosal layer and thromboxanes (TX) in platelets. Cox-2 generally mediates elevations of prostaglandins associated with inflammation, pain, and pyresis (2). Nonsteroidal antiinflammatory drugs (NSAIDs) such as aspirin and ibuprofen are generally nonselective inhibitors of Coxs. This lack of selectivity has been linked to their propensity to cause gastrointestinal side effects. The new Cox-2 selective inhibitors, or coxibs, show the same antiinflammatory, analgesic, and antipyretic effects as nonselective NSAIDs but have reduced side-effect profiles (4).The use of low-dose aspirin (50-325 mg͞day) is currently indicated for cardiovascular prophylaxis (5). The mechanism of this cardioprotective effect is because of the irreversible inactivation of platelet Cox-1, resulting in a reduced production of the proaggregatory TXA 2 . As platelets are anuclear, the function of the platelets is inhibited for the rema...

Coxibs interfere with the action of aspirin by binding tightly to one monomer of cyclooxygenase-1

“…Previous studies have shown that low concentrations of coxibs that did not affect COX-1 activity attenuated the inhibition of COX-1 by aspirin in various cells and tissue extracts (11)(12)(13)(14). Celecoxib is the most potent coxib in interfering with the effect of aspirin on COX-1 in human platelets (14), and so we focused on the effect of celecoxib on aspirin inhibition of COX-1.…”

“…Celecoxib is the most potent coxib in interfering with the effect of aspirin on COX-1 in human platelets (14), and so we focused on the effect of celecoxib on aspirin inhibition of COX-1. From the IC 50 value for ovCOX-1 (Fig.…”

“…Coxibs, at concentrations that do not inhibit COX-1 catalytic activity, have been reported to interfere with the abilities of nsNSAIDs including aspirin to inhibit COX-1 in vitro (11)(12)(13)(14). This suggests that COX-2 inhibitors are able to bind to COX-1 and somehow compete with nsNSAID actions on COX-1 without affecting AA oxygenation.…”

“…However, rofecoxib is the COX-2 inhibitor that is least effective in attenuating aspirin inhibition of COX-1 in vitro (14). Currently, the most widely used coxib is celecoxib.…”

Antinociceptive Effects of Alpinia katsumadai via Cyclooxygenase-2 Inhibition

“…The cyclooxygenase-2 activity was determined by a spectrophotometric assay conducted using recombinant ovine COX-2 enzymes (Cayman Chemicals, Ann Arbor, MI) (Ouellet et al, 2001). Briefly, 50 nM COX-2 in 100 mM Tris buffer (pH 8.0) and 0.5 mM heme were added to the appropriate wells.…”