Pharmacological analysis of cyclooxygenase-1 in inflammation.
Journal: 1998/November - Proceedings of the National Academy of Sciences of the United States of America
ISSN: 0027-8424
PUBMED: 9789085
Abstract:
The enzymes cyclooxygenase-1 and cyclooxygenase-2 (COX-1 and COX-2) catalyze the conversion of arachidonic acid to prostaglandin (PG) H2, the precursor of PGs and thromboxane. These lipid mediators play important roles in inflammation and pain and in normal physiological functions. While there are abundant data indicating that the inducible isoform, COX-2, is important in inflammation and pain, the constitutively expressed isoform, COX-1, has also been suggested to play a role in inflammatory processes. To address the latter question pharmacologically, we used a highly selective COX-1 inhibitor, SC-560 (COX-1 IC50 = 0.009 microM; COX-2 IC50 = 6.3 microM). SC-560 inhibited COX-1-derived platelet thromboxane B2, gastric PGE2, and dermal PGE2 production, indicating that it was orally active, but did not inhibit COX-2-derived PGs in the lipopolysaccharide-induced rat air pouch. Therapeutic or prophylactic administration of SC-560 in the rat carrageenan footpad model did not affect acute inflammation or hyperalgesia at doses that markedly inhibited in vivo COX-1 activity. By contrast, celecoxib, a selective COX-2 inhibitor, was anti-inflammatory and analgesic in this model. Paradoxically, both SC-560 and celecoxib reduced paw PGs to equivalent levels. Increased levels of PGs were found in the cerebrospinal fluid after carrageenan injection and were markedly reduced by celecoxib, but were not affected by SC-560. These results suggest that, in addition to the role of peripherally produced PGs, there is a critical, centrally mediated neurological component to inflammatory pain that is mediated at least in part by COX-2.
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Proc Natl Acad Sci U S A 95(22): 13313-13318

Pharmacological analysis of cyclooxygenase-1 in inflammation

Searle Research and Development, 700 Chesterfield Parkway North, St. Louis, MO 63198
To whom reprint requests should be addressed. e-mail: moc.otnasnom@noskasI.C.reteP.
Communicated by Philip Needleman, Monsanto Company, St. Louis, MO
Communicated by Philip Needleman, Monsanto Company, St. Louis, MO
Received 1998 May 11; Accepted 1998 Aug 25.

Abstract

The enzymes cyclooxygenase-1 and cyclooxygenase-2 (COX-1 and COX-2) catalyze the conversion of arachidonic acid to prostaglandin (PG) H2, the precursor of PGs and thromboxane. These lipid mediators play important roles in inflammation and pain and in normal physiological functions. While there are abundant data indicating that the inducible isoform, COX-2, is important in inflammation and pain, the constitutively expressed isoform, COX-1, has also been suggested to play a role in inflammatory processes. To address the latter question pharmacologically, we used a highly selective COX-1 inhibitor, SC-560 (COX-1 IC50 = 0.009 μM; COX-2 IC50 = 6.3 μM). SC-560 inhibited COX-1-derived platelet thromboxane B2, gastric PGE2, and dermal PGE2 production, indicating that it was orally active, but did not inhibit COX-2-derived PGs in the lipopolysaccharide-induced rat air pouch. Therapeutic or prophylactic administration of SC-560 in the rat carrageenan footpad model did not affect acute inflammation or hyperalgesia at doses that markedly inhibited in vivo COX-1 activity. By contrast, celecoxib, a selective COX-2 inhibitor, was anti-inflammatory and analgesic in this model. Paradoxically, both SC-560 and celecoxib reduced paw PGs to equivalent levels. Increased levels of PGs were found in the cerebrospinal fluid after carrageenan injection and were markedly reduced by celecoxib, but were not affected by SC-560. These results suggest that, in addition to the role of peripherally produced PGs, there is a critical, centrally mediated neurological component to inflammatory pain that is mediated at least in part by COX-2.

Abstract

Nonsteroidal anti-inflammatory drugs (NSAIDs) are widely used to treat inflammation and pain (1). The cardinal signs of inflammation, including edema, hyperalgesia, and erythema, develop as an acute response to a local inflammatory insult. These symptoms result from the action of inflammatory agents such as bradykinin, histamine, neurokinins, complement, and nitric oxide, which can originate locally or from cells that infiltrate the site of insult (2, 3). Elevated levels of prostaglandins (PGs) are also produced during inflammation (4) and enhance or prolong signals produced by pro-inflammatory agents, but alone do not cause inflammation (510). NSAIDs reduce or prevent the production of PGs by direct inhibition of the cyclooxygenase (COX) enzymes (11, 12). The observation that NSAIDs inhibit COX activity attests to the contribution of PGs to inflammation.

The COX enzymes catalyze the bis-oxygenation of free arachidonic acid to PGH2, the committed step in PG formation. PGH2 is converted into the other PGs or thromboxane (Tx)A2 by specific synthases (13). There are two COX enzymes, referred to as COX-1 and COX-2, which catalyze identical reactions (1416). COX-1 is thought to produce PGs important for homeostasis and certain physiological functions and is expressed constitutively in most tissues and cells (17), although it can be induced in some cell lines under certain conditions (18). A second, inducible, form of COX was hypothesized to exist on the basis of the finding of a glucocorticoid-regulated increase in COX activity observed in vitro and in vivo in response to inflammatory stimuli (19, 20). The isolation of a distinct gene and enzyme for COX-2 confirmed this hypothesis and led to the supposition that selective inhibition of inducible COX-2 would be anti-inflammatory, while preserving the physiological functions of COX-1 derived PGs. This hypothesis was corroborated by the discovery and synthesis of anti-inflammatory compounds that selectively and potently inhibit COX-2 but not COX-1 (2128). In contrast, NSAIDs inhibit both forms of COX at approximately equivalent concentrations (21, 29). Selective inhibition of COX-2 only partially reduces the level of PGs at sites of either acute or chronic inflammation, in comparison to NSAIDs, which reduce PGs to undetectable levels (23, 27, 30). Therefore, COX-1 may contribute significantly to the total pool of PG at a site of inflammation. To date, the contribution of COX-1 to inflammation has not been clearly established.

One approach to understanding the role of COX isoforms in inflammation is to use gene disruption technology in mice (knockouts). Using this approach, Langenbach et al. (31) reported that swelling elicited by application of arachidonic acid to the ear was diminished in COX-1-deficient mice compared with wild-type mice, suggesting a potential role for COX-1 in some forms of acute inflammation. Whether the results of this acute application of substrate represent events that occur in response to inflammatory stimuli is uncertain. To better understand the role of COX-1 in both inflammation and normal physiology, the anti-inflammatory and analgesic activities of SC-560 (COX-1 IC50 = 0.009 μM; COX-2 IC50 = 6.3 μM), a selective and potent inhibitor of COX-1, have been characterized. SC-560 provides a pharmacological tool to analyze the role of COX-1-derived PGs in inflammation and pain. The experiments reported herein using well established models of inflammation indicate that COX-1 may not play a significant role in acute inflammatory pain.

Compounds were administered orally 1 hr before sample collection. Whole blood collected from animals (n = 4) was stimulated with calcium ionophore A23187. Plasma was prepared and TxB2 was quantified by ELISA. Data are reported as the mean ± SEM.

Acknowledgments

We thank Dr. Joseph Portanova for his excellent critique.

Acknowledgments

ABBREVIATIONS

COXcyclooxygenase
PGprostaglandin
NSAIDnonsteroidal anti-inflammatory drug
LPSlipopolysaccharide
CNScentral nervous system
CSFcerebrospinal fluid
ABBREVIATIONS

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