Interactions of PGH synthase isozymes-1 and -2 with NSAIDs

There are two isozymes of prostaglandin endoperoxide (PGH) synthase (cyclooxygenase) called PGH synthase-1 and -2 or COX I and II. Both isozymes catalyze the same two reactions: oxygenation of arachidonate to yield PGG2 and reduction of PGG2 to PGH2. PGH synthase-1 is expressed constitutively and is found in most tissues. PGH synthase-2 is undetectable in most cells but can be induced in fibroblasts, endothelial cells, ovarian follicles, and macrophages by various mitogens, cytokines, and tumor promoters. PGH synthase-1 (PGHS-1) has been presumed to be the site of action of nonsteroidal antiinflammatory drugs (NSAIDs). However, the discovery of the second isozyme, PGH synthase-2 (PGHS-2), and its association with inflammation has suggested that this latter enzyme may be the therapeutic target of NSAIDs functioning in their antiinflammatory capacities. We have cloned cDNAs for murine PGHS-1 and PGHS-2, expressed these enzymes in cos-1 cells, and compared the relative sensitivities of the two isozymes to some common NSAIDs. Indomethacin, piroxicam, and sulindac sulfide were found to preferentially inhibit PGHS-1. Ibuprofen and meclofenamate inhibit both enzymes with comparable potencies. 6-Methoxy-2-naphthylacetic acid, the active metabolite of Relafen, inhibits murine PGHS-2 preferentially. Aspirin irreversibly inhibits PGHS-1, preventing this isozyme from forming PGH2 or any other oxygenated product; in contrast, aspirin treatment of PGHS-2 causes this enzyme to form 15-hydroxy-5c,8c,11c,13t-eicosatetraenoic acid (15-HETE) instead of PGH2. Our results indicate mouse PGHS-1 and PGHS-2 are pharmacologically distinct. Thus, it should be possible to develop agents highly selective for each PGHS isozyme. PGHS-2 is not expressed in stomach but is increased by inflammatory cytokines in cells such as macrophages. Thus, a selective inhibitor of PGHS-2 could be an antiinflammatory agent but without being ulcerogenic.