British Journal ofRheumatology 1996;35(snppL l):l-3
INTRODUCTION: MECHANISM OF ACTION OF NSAIDS
J.R.VANE
The William Harvey Research Institute, St Bartholemew's Hospital Medical College, London
IN 1971 we proposed that the mechanism of action of
the aspirin-like drugs was through their inhibition of
prostaglandin biosynthesis [1]. Since then, there has
been intense interest in the interactions between this
diverse group of inhibitors and the enzyme known as
cyclooxygenase (COX or PGH2 synthase). We now
know that it exists in two isofonns, COX-1 and COX-2
(see later). Over the last two decades, several new drugs
have reached the market based on COX-1 enzyme
screens. Picot et al [2] determined the three-dimensional
structure of COX-1, providing a new understanding for
the actions of COX inhibitors.
COX first oxidizes arachidonic acid to prostaglandin
G2 (PGG2) through a cyclooxygenase process and then
peroxidizes PGG2 to PGH2 (Fig. 1). This bifunctional
enzyme comprises three independent folding units: an
epidermal growth factor-like domain, a membrane
binding motif and an enzymatic domain. The sites for
peroxidase and cyclooxygenase activity are adjacent but
spatially distinct. The conformation of the membranebinding motif strongly suggests that the enzyme
integrates into only a single leaflet of the lipid bilayer
and is thus a monotopic membrane protein. Three of
the helices of the structure form the entrance to the
cyclooxygenase channel and their insertion into the
membrane could allow arachidonic acid to gain access
to the active site from the interior of the bilayer.
The cyclooxygenase active site is a long, hydrophobic
channel and Picot et al [2] present arguments that some
of the aspirin-like drugs such as flurbiprofen inhibit
COX-1 by excluding arachidonate from the upper
portion of the channel. Tyr385 and Ser530 are at the
apex of the long active site. Aspirin irreversibly inhibits
COX-1 by acetylation of the Ser530, thereby excluding
access for arachidonic acid [3]. The S (-)-stereoisomer
of flurbiprofen interacts, via its carboxylate, with
Argl20, thereby placing the second phenyl ring within
Van der Waals contact of Tyr385. There may be a
number of other sub-sites for drug binding in the
narrow channel.
The constitutive isoform of COX, COX-1, has clear
physiological functions. Its activation leads, for
instance, to the production of prostacycUn which, when
released by the endothelium, is antithrombogenic [4],
and by the gastric mucosa is cytoprotective [5]. The
inducible isoform, COX-2, was discovered some 5 years
ago and is induced in a number of cells by proCorrespondence to: Professor Sir John Vane, The William Harvey
Research Institute, Charter House Square, London EC1M 6BQ.
inflammatory stimuli [6]. Its existence was first
suspected when Needleman and his group reported that
bacterial lipopolysaccharide increased the synthesis of
prostaglandins in human monocytes in vitro [7] and in
mouse peritoneal macrophages in vivo [8]. This increase
was inhibited by dexamethasone and associated with de
novo synthesis of new COX protein. A year or so later,
COX-2 was identified as a distinct isoform encoded by a
different gene from COX-1 [9-12]. Since COX-2 is
induced by inflammatory stimuli and by cytokines in
migratory and other cells, it is attractive to suggest that
the anti-inflammatory actions of NSAIDs are due to
the inhibition of COX-2 whereas the unwanted sideeffects such as irritation of the stomach lining and toxic
effects on the kidney are due to inhibition of the
constitutive enzyme, COX-1.
Over the years, the theory that inhibition of
prostaglandin formation accounts for the therapeutic
activity and the side-effects of the aspirin-like drugs has
been challenged, notably by Weissmann [13]. His
arguments were partly based on comparing the actions
of salicylate and aspirin, which are said to be equally
effective against arthritis in the clinic [14], whereas in
the original observations on COX, aspirin was 10 times
stronger than salicylate as an inhibitor. As Weissmann's
comparisons were based on COX-1, this apparent
contradiction may now be explained by the existence of
the two isofonns of COX, for salicylate and aspirin are
both weak inhibitors of COX-2, although the
mechanism of action of salicylate may be compounded
by a suppression of the induction of COX [15].
Paracetamol also posed a problem for the original
theory, for in therapeutic doses it has weak antiinflammatory activity but is a stronger analgesic and
antipyretic [16]. In 1972 we showed that COX
preparations from the brain were more sensitive to
paracetamol than those from the spleen and suggested
that there may be different isoforms of COX [17].
Perhaps in the light of recent discoveries, there might
also be a COX-3 on which paracetamol has a preferential action?
The importance of the discovery of the inducible
COX-2 is highlighted by the differences in pharmacology of the two enzymes [18]. Aspirin, indomethacin
and ibuprofen are much less active against COX-2 than
against COX-1 [19]. Indeed, the two strongest inhibitors
of COX-1 are aspirin and indomethacin, the two
NSAIDs which cause the most damage to the stomach
[20]. The spectrum of activities of some 10 standard
NSAIDs against the two enzymes ranges from high
O 1996 British Society for Rheumatology
STUDIES ON MELOXICAM (MOBIC)
Physiological
Stimulus
Inhibition by
NSAJDs
u
Inflammatory
Stimulus
Inhibition by
NSAIDs
\
COX-1
Constitutive
Macrophages/Other Celts
COX-2
Induced
\
PGi
platetots
2
endotheflum
stomach mucosa
etc
PQEj
Proteases
Other
Inflammatory
mediators
PQs
etc
Physiological
Functions
Inflammation
SIDE EFFECTS
OF NSAIDs
ANTMNFLAMMATORY
EFFECTS OF NSAIDs
Flo. 1.—Relationship between the pathways leading to the generation of prostaglandias by COX-1 or COX-2. Activation of COX-1 promotes
release of eicosanoids involved in physiological processes (eg. thromboxane A3, prostacyclin or prostaglandin E2). Inhibition by NSAIDs of COX-1
results in side-effects (eg. gastrointestinal irritation). However, the protective effect of aspirin in heart attacks and strokes is also due to inhibition of
COX-1 in platelets. Inhibition of COX-2 reduces inflammation. Most currently available NSAIDs are more potent inhibitors of COX-1 than COX-2.
NSAIDs that preferentially inhibit COX-2 reduce inflammation with less inhibition of the production of physiologically-active eicosanoids, so
potentially reducing the risk of side-effects.
selectivity towards COX-1 (150-fold for aspirin)
through to equiactivity on both [21]. Meloxicam is a
new potent NSAID with the best activity ratio for
COX-2/COX-1 [22]; it also has minimal damaging
effects on the gastrointestinal tract [23]. We have found
a COX-2/COX-1 activity ratio of 0.2 for meloxicam in
guinea-pig peritoneal macrophages and 0.8 in
macrophages of the mouse [24].
The range of activities of NSAIDs against COX-1
compared with COX-2 nicely explains the variations in
the side-effects of NSAIDs at their anti-inflammatory
doses. Drugs which have the highest potency against
COX-2 and a better COX-2/COX-1 activity ratio will
have potent anti-inflammatory activity with fewer
side-effects in the stomach and kidney. Bateman [25] has
published a comparison of epidemiological data on the
side-effects of NSAIDs. Piroxicam and indomethacin
were found to produce the highest gastrointestinal
toxicity. These drugs have a much higher potency
against COX-1 than against COX-2 [26]. Thus, when
epidemiological results are compared with COX-2/
COX-1 ratios, there is a clear relationship between
gastrointestinal side-effects and COX-2/COX-1 ratios.
Clinical data on meloxicam, as presented in this volume,
show that it has a good gastrointestinal safety profile, as
would be predicted from its selective inhibition of
COX-2 relative to COX-1.
All the results so far published (and many yet to be
published from the drug industry) support the
hypothesis that the unwanted side-effects of NSAIDs
are due to their inhibition of COX-1 whilst their
anti-inflammatory (therapeutic) effects are due to
inhibition of COX-2. Other roles for COX-2 will surely
be found in the next few years, for prostaglandin
formation is under strong control in organs such as the
uterus. The identification of selective inhibitors of
COX-1 and COX-2 will not only provide an opportunity
to test the new hypothesis but also lead to advances in
the therapy of inflammation. As selective inhibitors of
COX-2 come to the market, so the present clutch of
NSAIDs will become obsolete. Arthritic patients will
surely benefit before the year 2000 from the important
discovery of more selective COX-2 inhibitors.
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VANE: INTRODUCTION
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