1. ISSN 1040-4597
A Quarterly Clinical
Review
Volume VIII, Number 4–2001
Cyclooxygenases
Cyclooxygenases
Randy C. Mifflin, Ph.D. and
Don W. Powell, M.D.
Department of Internal Medicine
University of Texas Medical
Branch
Galveston, Texas
Key Concepts
• Cyclooxygenase (COX) enzymes
catalyze the rate limiting steps
in prostaglandin synthesis.
Prostaglandins play major
roles in diverse physiological
processes such as maintenance
of GI mucosal integrity and
pathological processes such as
inflammation and neoplasia.
• Two COX isoforms, COX-1 and
COX-2 exist in higher organisms.
They are highly similar in struc-
ture and enzymatic activity. The
main differences between the
two lie in their genetic regulation
and biological roles. COX-1
expression is constitutive in most
cell types and is thought to carry
out “housekeeping” roles in the
various tissues. In contrast,
COX-2 expression is induced in
Figure 1. Prostaglandin Biosynthesis.
response to inflammatory and
proliferative stimuli.
The Cyclooxygenase
• COX enzymatic activity is the
Reaction CONTENTS
target of widely used non-
steroidal anti-inflammatory Cyclooxygenase (COX) Cyclooxygenases ...................49
drugs NSAIDs). COX-2-selective Randy C. Mifflin, Ph.D. and
enzymes, also referrred to as
NSAIDs are currently being Don W. Powell, M.D.
prostaglandin H synthases or
developed in the hopes of limit-
prostaglandin endoperoxide Cyclooxygenases and GI
ing inflammation without adverse
Mucosal Protection ................56
synthases, catalyze the rate limit-
GI and renal effects.
Mark Feldman, M.D.
ing steps in prostaglandin (PG)
• The cellular source of inducible
and thromboxane (TX) synthesis Cyclooxygenase-2 and
COX-2 activity in acute and
Colon Carcinogenesis............60
(Fig. 1). Enzymatic COX
chronic GI inflammation and
Raymond N. DuBois, M.D. and
neoplasia is currently poorly substrates are 20 carbon polyun-
Moss Mann, M.D.
understood. saturated fatty acids, most often
49

2. for the Cox 2 isoform. In the last
Introduction profile will require post-market-
part of the article, the authors ing studies of the millions of
also review less well-known patients with inflammatory
areas of cyclooxygenase biology diseases who use the drugs.
such as the regulation of Cox In the last article, Drs. R.N.
gene expression in gastrointesti- DuBois and M. Mann review the
nal tissues. The latter is impor- concept that inhibition of
tant because an inhibition of cyclooxygenase enzymes has a
cyclooxygenase in the stomach therapeutic role in cancer risk
and intestine leads to gastroin- reduction in the gastrointestinal
testinal damage; side effects tract, especially for human
which limit the clinical effective- colorectal carcinoma. The
ness of cyclooxygenase authors review the role of Cox
inhibitors. inhibition in the prevention of
The second article by Dr. sporadic colorectal cancer and
Mark Feldman reviews the role their therapeutic effect in reduc-
Don W. Powell, M.D.
of cyclooxygenase in maintain- ing the size and number of
Issue Editor
ing the barrier function of the colonic adenomas in a genetic
gastrointestinal tract. Dr. Feld- form of colon cancer, familial
The development of clinically
man reviews the idea of the adenomatous polyposis (FAP).
effective inhibitors specific for
“cytoprotective effect” of These authors also review the
cyclooxygenase 2 (Cox 2) is a
prostaglandins in the gastroin- potential mechanisms for the
dramatic example of the power
testinal mucosa. He lists the chemopreventive effect of nons-
of modern biological research at
evidence for the concept that teroidal anti-inflammatory drugs
the turn of the 20th Century. In
cyclooxygenase 1 is the “house- (NSAIDs) in the prevention of
less than ten years after this
keeping” form of the prostaglan- colorectal cancer. Studies in
second isoform of cyclooxyge-
din-forming enzymes that main- animals suggest that the chemo-
nase was cloned, our understand-
tains structure and basic function preventive effects of NSAIDs
ing of the formation of prosta-
in tissues, whereas Cox 2 is the may occur through specific inhi-
glandins, their roles in normal
isoform that produces prosta- bition of Cox 2 enzymes. If this
biology and disease, and poten-
glandins during inflammatory is the case, then Cox 2 specific
tial ways to inhibit their forma-
states. The prostaglandins are the inhibitors might have a positive
tion have been revolutionized as
cause of pain, erythema, warmth chemopreventive effect without
a result of studies of the molecu-
and the edema of inflammation. the gastrointestinal side effects
lar biology and structure of these
These proposed differences in of Cox 1 inhibition. While it is
enzymes. This is a prime exam-
the two Cox isoforms have led to too early to be sure if this varia-
ple of the application of molecu-
the concept of “good” cyclooxy- tion of the Cox 2 therapeutic
lar biology and structural biol-
genase (Cox 1) and “bad” cyclo- hypothesis is correct, certainly it
ogy to drug development.
oxygenase (Cox 2). He reviews is an exciting idea that will result
The first article in this issue of
the Cox 2 therapeutic hypothe- in much basic and clinical inves-
the Regulatory Peptide Letter by
sis, which says that highly selec- tigation in the coming few years.
R.C. Mifflin and D.W. Powell
tive Cox 2 inhibitors should be The cloning of a second
summarizes the biology of the
efficacious in inflammatory isoform of the cyclooxygenase
two cyclooxygenase enzymes.
disease without causing the enzyme, as well as molecular
The authors briefly describe the
severe side effects, particularly and structural studies of both
biochemistry of prostaglandin
GI toxicity, which limits the proteins, have led to a new class
(PG) formation, the known func-
usefulness of this class of drugs. of therapeutic agents in less than
tions of the many members of
Dr. Feldman points out that a decade. While the ultimate
the PG family, and the various
although early reports of the promise and efficacy of these
distinct receptors through which
clinical trials of the specific Cox drugs remains to be determined
the PGs have their biologic
2 inhibitors now approved in the through their extensive use in
effects. They review the molecu-
United States, Celecoxib® and humans with disease, the
lar and structural organization of
Rofecoxib®, tend to support the creation of specific Cox 2
the Cox isoforms and how the
inhibitors represents an impor-
concept of a Cox 2 therapeutic
molecules that make up the
tant paradigm for the develop-
hypothesis, it is still early in the
enzyme determine its structure.
ment of pharmacologic agents
use of these drugs. Their ulti-
This structure has allowed indus-
s
in the 21st Century.
mate efficacy and side effect
try to create specific inhibitors
50

3. arachidonic acid (AA), released isotype switching to IgG1 and IgE
from phospholipid of cellular in B cells. Important roles for PGs
Editorial Board
membranes by a phospholipase in CNS function and development
A2. The first step involves addi- have also been identified. For
tion of O2 atoms to C-11 and C-15 example, increased PGE2 synthe- Chung Owyang, M.D.
to yield PGG2. Since this results sis by hypothalamic endothelial Editor-in-Chief
in cyclization of the fatty acid this cells is involved in the febrile Chief, Division of
is referrred to as the cyclooxy- response and PGs generated at Gastroenterology
genase activity. The 15-hydro- sensory nerve terminals cause Professor of Internal Medicine
peroxide group of PGG2 is then hyperalgesia. Increased PG University of Michigan
Medical Center
converted to an alcohol forming synthesis has also been correlated
Ann Arbor, MI
PGH2 by the peroxidase activity with seizure activity.
of the enzyme. PGH2 is subse- PGs have been implicated in a
quently converted to other PGs wide variety of disease processes. Haile T. Debas, M.D.
(PGD2, PGE2, PGF2 , PGI2) or The huge annual market for non- Dean
thromboxanes (TXA2 ) by specific steroidal antiinflammatory drugs School of Medicine
cellular synthases (Fig. 1). (NSAIDs) which inhibit COX University of California
activity is a testament to the role San Francisco, CA
Functions of of PGs in acute inflammation and
chronic inflammatory diseases
Prostaglandins Robert T. Jensen, M.D.
such as asthma, rheumatoid/osteo Chief, Digestive Diseases
PGs play critical roles in arthritis, and inflammatory bowel Branch
normal physiological processes. disease (IBD). Epidemiological National Institutes of
Platelet-derived TXA2 is an and animal studies indicate that Diabetes, Digestive and
important mediator of platelet inhibition of PG synthesis is Kidney Diseases
aggregation and thus hemostasis. efficacious in the prevention of National Institutes of Health
During periods of stress, PGs of Bethesda, MD
coronary artery thrombosis,
the E and I series are important Alzheimer’s disease, and gastro-
regulators of renal blood flow. intestinal and breast cancer. The Gabriel M. Makhlouf, M.D., Ph.D.
PGs likewise are important in properties of PGs that contribute Professor of Medicine
modulating many aspects of to disease progression include Director of Gastroenterology
reproductive biology including their thrombotic activity, ability to Research
ovulation, fertilization, fetal modulate cellular apoptosis and Division of Gastroenterology
development, and parturition. The other cell cycle parameters, Medical College of Virginia
opposing actions of different PG Richmond, VA
angiogenic activity, and other
classes help to maintain bronchial functions yet to be identified.
tone. The processes of bone In the accompanying article, Don W. Powell, M.D.
formation and resorption are also Dr. Dubois will cover the role of Edward Randall and Edward
subject to regulation by PGs. COX enzymes and PGs in the Randall Jr. Professor and Chair
Macrophage differentiation is development of colorectal cancer Department of Internal Medicine
likewise modulated by PGs. As and discusses their role in neo- Professor of Physiology and
discussed in Dr. Feldman’s article plasia in more detail. Biophysics
to follow, PGs are vital to the The University of Texas
maintenance of mucosal integrity Medical Branch
PG Receptors
in the GI tract and also play a role Galveston, TX
The effects of PGs upon cells
in the regulation of motility and
are realized when each binds its
secretion. PGs also affect immune James C. Thompson, M.D.
specific membrane-bound recep-
function in a number of ways. Ashbel Smith Professor
tor (Table 1). These constitute a
Through its ability to differen- of Surgery
homologous family of G protein-
tially inhibit cytokine synthesis Department of Surgery
coupled receptors containing
by TH1 cells, PGE2 can shift the The University of Texas
seven transmembrane domains.
balance of an immune response in Medical Branch
Differential responses to specific
favor of TH2 cells. PGE2 also Galveston, TX
PGs are determined by the type of
synergizes with IL-4 to activate
51

4. Table 1. Membrane-Based Eicosanoid Receptors.
(Adapted from E.J. Goetzl, S. An, and W.L. Smith FASEB J. 9: 1051–1058, 1995.)
Receptor Tissue Transductional
Eicosanoid Name Distribution* Signal Effects
Ca++
PGE2 EP1 Smooth Muscle, Fibroblasts SM Contraction
Mobilizatiion Proliferation
PGE2 EP2 Smooth Muscle, Epithelial Cells, cAMP Increase SM Relaxation, Stimulate Intestinal
Mast Cells, Neurons, Fibroblasts Secretion, Sensation, Inhibit Mast
Cell Degranulation
PGE2 EP3 Smooth Muscle, Adipocytes, cAMP Decrease SM Contraction, Inhibit Lipolysis,
Ca++ Mobilization
Neurons, Epithelial Cells. Neurotransmitter Release, Stimulate
Kidney Renal H20 Reabsorption
PGE2 EP4 Fibroblasts, Myofibroblasts, cAMP Increase SM relaxation, Induction of
Smooth Muscle Stellate Morphology in Myofibroblasts
PGD DP Platelets, Smooth Muscle, cAMP Increase SM Relaxation, Inhibition of
Neurons Neurotransmitter Release,
Platelet Aggregation
PGI IP Platelets, Smooth Muscle, cAMP Increase SM Relaxation, Inhibition of Platelet
Neurons Aggregation, Stimulate Intestinal
Secretion
Ca++ Mobilization
PGF2 FP Kidney, Myofibroblasts, SM Contraction, Myofibroblast
Astrocytes, Smooth Muscle Contraction
Ca++ Mobilization
TXA2 TP Platelets, Smooth Muscle SM Contraction, Platelet Aggregation,
Glomerular Filtration, Intestinal
Secretion
* not comprehensive
G protein (G s, G i, G q, G 12) associated receptors (PPARs). heme-containing proteins with a
coupled to each receptor. At least These receptors are members of molecular weight of roughly 71
four distinct PGE2 receptors exist the nuclear hormone receptor KDa. They share 63% identity at
which couple to different signal- superfamily of ligand activated the amino acid level. COX-2
ing pathways. As a general rule, transcription factors which target contains an 18 amino acid inser-
G s-coupled receptors result in to the nucleus upon ligand bind- tion in its carboxyl terminal
increased levels of intracellular ing. Thus certain PGs are able to region while COX-1 contains an 8
cyclic adenosine monophosphate directly modulate transcription of amino acid insertion at the amino
(cAMP); G i-coupled receptors specific genes via interaction with terminus of the mature protein.
result in inhibition of cAMP PPARs. Both proteins are glycosylated;
generation; and G q-coupled three conserved N-linked glyco-
COX-1 and COX-2
receptors result in intracellular sylation sites exist in both
Ca2+ mobilization. Signaling via enzymes and COX-2 contains an
Two distinct COX enzymes
G 12-coupled receptors is not additional site within the 18
exist. COX-1 was first purified
completely undersood. Further amino acid insertion.
and characterized in the 1970s
diversity in the response to each The mature proteins contain
and the gene was isolated in 1988.
PG is achieved through alternate three distinct domains. The first is
The discovery and cloning of the
splicing of PG receptor mRNAs a conformation which is highly
second COX isoenzyme, COX-2,
generating different carboxyl similar to that of epidermal
in 1991 initiated a revolution in
termini. growth factor (EGF) and is termed
our understanding of PGs and
Recent evidence also indicates the EGF-like domain. The func-
their functions in normal physiol-
that certain PGs, such as PGJ2 and tion of this domain in COX
ogy and disease.
its derivatives, are also potent enzymes is poorly understood but
The two enzymes are highly
ligands for a class of receptors is thought to facilitate recruitment
similar in structure and enzymatic
termed peroxisome proliferator- and interaction with other cellular
activity. Both are homodimeric
52

5. proteins. The second domain
contains a series of amphipathic
helices which comprise the
membrane attachment site. COX
enzymes are unlike other integral
membrane proteins in that they are
not anchored via transmembrane
domains. Instead, they associate
with the endoplasmic reticulum
(ER) membrane via hydrophobic
interactions and are thus mono-
topic membrane proteins. It is
interesting that while both
enzymes are associated with the
luminal face of the ER, COX-2 is
also enriched in the perinuclear
region. The association of COX-2
with the nucleus raises questions
about a direct role of COX-2-
derived PGs on gene expression
Figure 2. A: Diagrammatic representation of the orientation of COX-1 and COX-2 in
via association with PPARs. The
the ER membrane. Shown are a COX-1 and a COX-2 homodimer demonstrating
third domain is a large globular the association with the lumenal ER surface via the amphipathic helices. In the
region which contains the cyclo- central portion of each monomer is shown a cutout section demonstrating NSAID
oxygenase and peroxidase active binding to the active site. In the case of COX, the active site is narrower allowing
access only to NSAIDs with smaller side chains. COX-2 specific NSAIDs contain
sites. The COX active site lies in
larger side chains not accommodated by the COX-1 pocket. B: Diagrammatic
a narrow hydrophobic channel representation of the molecular structure of flurbiprofen, a nonselective NSAID, and
framed by the membrane attach- celecoxib, a COX-2 specific NSAID.
ment helices which allows arachi-
differences in this channel that Mechanisms of
donic acid cleaved by PLA2 direct
also play a role in determining COX Inhibition
access from the ER membrane
substrate and inhibitor specificity
without having to transit a hydro-
Based upon their inhibitory
include histidine (COX-1) to argi-
philic environment (Fig. 2). The
mechanisms COX inhibitors
nine (COX-2) at position 513,
amino acids involved in substrate
can be grouped into four classes
and serine (COX-1) to alanine
binding and catalysis are by and
(Table 2). All but the first class
(COX-2) at position 516.
large identical between the two
are reversible inhibitors in that
Another interesting difference
enzymes. Two important differ-
once the drug is removed, COX
between COX-1 and COX-2 is
ences are found at residues 434
activity is restored, albeit at
that each enzyme apparently
and 523 (COX-1 numbering)
different rates depending upon the
utilizes a distinct source of cellu-
where isoleucine occupies each
compound. The first class, which
lar arachidonate as substrate
position in COX-1 and valine is
includes aspirin and recently
resulting in a functional compart-
present in each position in COX-2.
developed COX-2-specific
mentalization of COX-1 versus
These amino acids are part of the
aspirin-like molecules irreversibly
COX-2 activity. For example,
substrate binding channel and one
inactivate COX activity by acety-
aggregation of IgE receptors on
consequence of these substitutions
lating an active site serine.
mast cells results in a biphasic
is that COX-2 has a wider channel.
Aspirin is considered COX-1
release of PGD2. The first phase
This channel difference is the
selective since doses 10 to 100
is mediated by COX-1 utilizing
basis behind the broader substrate
fold higher than those required for
arachidonate released by a form
specificity of COX-2 and, as
COX-1 are necessary to acetylate
of phospholipase A2 called secre-
discussed below, the basis behind
the COX-2 active site. Although
tory PLA2 while the second phase
the design of drugs specifically
aspirin-acetylated COX-1 retains
is mediated by COX-2 utilizing
targeted to inhibit COX-2. In
no enzymatic activity, due to its
arachidonate released by a differ-
fact, changing isoleucine 523 in
larger substrate binding channel,
ent phospholipase A2 termed
COX-1 to valine renders it sensi-
acetylated-COX-2 retains its
cytosolic PLA2.
tive to some COX-2-selective
peroxidase activity, and is effec-
inhibitors. Other amino acid
53

6. sible for generation of PGs which
Table 2. Four Modes of COX Inhibition by NSAIDs. mediate homeostatic or “house-
keeping” functions such as main-
Mode of
tenance of vascular tone and
Inhibition Selectivity Examples Comments
mucosal integrity in the GI tract.
Covalent COX-1 Aspirin Acetylation of active
The human COX-1 promoter
Modification COX-2 (APHS) APHS* site serine
region resembles that of other
Compete with AA† for
Reversible, COX-1 and 2 Ibuprofen
housekeeping genes in that it
Competitive Mefenamate active site.
lacks a TATA box and is generally
Inhibition
not subject to transcriptional
Slow, Time- COX-1 and 2 Indomethacin Salt bridge formation
induction. However, COX-1
dependent Flurbiprofen with Arg. 120
expression is subject to develop-
Inhibition
mental and inducible regulation
under certain circumstances. For
Time-dependent COX-2 Celecoxib Larger side groups to
COX-2 Inhibition Rofecoxib occupy extra side example, stem cell factor (SCF)
SC58125 pocket in COX-2 treatment of immature murine
bone marrow derived mast cells
• o-(acetoxyphenyl)hept-2-ynyl sulfide
† arachidonic acid
results in a 6–8 fold induction of
COX-1 mRNA and protein levels.
tively converted into a lipoxyge- inhibitor. These agents exhibit a Differentiation inducing stimuli
nase enzyme capable of generat- slow, time-dependent inhibition (eg. transforming growth factor- ,
ing 15-R-hydroxyeicosatetraenoic of both COX isoforms. The phorbol esters) have also been
acid (HETE). Recently, the aspirin delayed kinetics of inhibition by shown to result in transient 1.5 to
derivative, o-(acetoxyphenyl)- this class probably reflects the 3-fold induction of COX-1 expres-
hept-2-ynyl sulfide (APHS) has time necessary for formation of a sion in monocytes and macro-
been developed which exhibits salt bridge between the carboxy- phages. Estrogen-induced expres-
increased selectivity and potency late of the drug and arginine 120 sion of COX-1 is responsible for
toward COX-2. APHS, like (COX-1 numbering). the increase in PGI2 synthesis in
aspirin, irreversibly inactivates The fourth class of COX perinatal pulmonary vascular beds
the cyclooxygenase activity of inhibitors selectively inhibit COX- and is partly responsible for the
COX-2. APHS-modified COX-2 2. These include recently-devel- pulmonary vasodilation seen
retains the ability to generate oped drugs such as celecoxib and during this period.
15-R-HETE. The development of SC58125 which incoporate In contrast COX-2 expression
APHS as a COX-2-selective agent sulphonamide or sulphone groups is undetectable in most normal
was based upon structural data of in place of carboxylic acid. They tissues. Important exceptions to
the COX-2 substrate channel and also contain larger side groups this rule are the brain and renal
the observation that effective which penetrate the larger binding cortex where constitutive COX-2
COX-2-selective inhibitors have pocket of COX-2, but their size expression occurs. COX-2 expres-
sulfur-containing side chains in prevents them from entering the sion in many cell types is highly
place of a carboxylic acidic group. smaller pocket of COX-1. These induced in response to proinflam-
APHS represents a parent com- compounds are effective time- matory stimuli such as IL-1,
pound which is certain to be dependent inhibitors of COX-2; TNF , and bacterial lipopolysac-
followed by more effective, irre- the time dependence is thought to charide (LPS). Nucleotide
versible, COX-2 specific NSAIDs. reflect the time required for opti- sequence analysis of the human
The second class of inhibitors mal insertion of the inhibitor into COX-2 gene promoter reveals the
consists of reversible, competi- the deeper pocket of COX-2 presence of potential binding sites
tive inhibitors of both enzymes. (Table 2). for a variety of transcription
These compounds compete with factors activated by inflammatory
Regulation of COX
arachidonic acid for binding to and proliferative stimuli. These
the cyclooxygenase active site. include NF-1, AP-2, STATs,
Gene Expression
Ibuprofen and mefenamate are NFkB, NFIL6/cEBP, CREB/ATF,
COX-1 expression is constant
examples of this class of inhibitor. and E-box-binding proteins.
(constitutive) in most tissues and
Indomethacin and flurbiprofen Signaling pathways which play a
cell types. COX-1 is, therefore,
exemplify the third class of COX role in COX-2 induction include
considered as the isoform respon-
54

7. generation of cAMP, activation of elevations in COX-2 expression prevention of certain GI cancers
protein kinase C isoforms, gener- occur in response to acute or (see accompanying article by
ation of inositol trisphosphates, chronic mucosal inflammation Dr. Dubois). However, recent
generation of ceramide, activation and ulceration. studies indicate that COX-2-
of mitogen activated protein The cell types in which COX-2 derived PGs play a beneficial role
kinases (MAPKs) such as c-Jun expression increases in response in the healing of gastric and
N-terminal kinase (JNK), P38 to mucosal injury and inflamma- intestinal ulcers, and thus COX-2
kinase, and extracellular signal tion have not been clearly identi- inhibition in patients with already
regulated kinases (ERKs), as well fied and defined. Epithelial cell existing GI lesions could be
as Janus-associated kinases COX-2 expression has been detrimental. Dr. Feldman in the
(JAKs). COX-2 gene expression demonstrated following invasion accompanying article discusses
is also subject to negative regula- by bacteria, in patients suffering the COX-2 therapeutic hypothesis
tion. The anti-inflammatory from IBD, and at the latter stages in more detail.
cytokines IL-4, IL-10, and IL-13, of carcinogenesis. However, stud- In summary, great strides have
and corticosteroids inhibit COX-2 ies from several laboratories show recently been made on the struc-
expression. COX-2 expression is that the vast majority of intestinal ture and genetic regulation of the
also regulated at post-transcrip- PG production in inflammatory two COX isoforms. Significant
tional levels by various mecha- conditions occurs in the lamina advances have also been made in
nisms including mRNA splicing, propria and submucosa. In a rat defining the role played by each
message stability, and translation. model of colitis, increased levels in normal biological processes
The 3 untranslated region of the of COX-2 mRNA are seen and the and disease. However, many
COX-2 mRNA contains multiple bulk of immunoreactive COX-2 is unanswered questions still remain
copies of the pentanucleotide localized to cells of the lamina regarding the cellular sources of
motif AUUUA which confers propria in regions occupied by inducible PG synthesis in GI
message instability upon a subepithelial myofibroblasts, inflammation and cancer and the
number of cytokine and proto- mast cells, neutrophils, and therapeutic value of recently
oncogene mRNAs. Such motifs smooth muscle cells, and in the developed COX-2 selective
represent potential targets by muscularis of the colon. Like- NSAIDs.
which agents such as IL-1 stabi- wise, recent studies using rat
lize, and corticosteroids destabi- models of NSAID-induced gastric
lize the COX-2 message, thus ulceration localized COX-2
promoting elevated or decreased expression to the lamina propria
levels of enzymatic activity, of regenerative regions. Interest-
respectively. ingly, in a murine model of famil-
ial adenomatous polyposis coli,
COX Expression Oshima et. al. localized COX-2
transcription in early adenomas,
in GI Tissues
not in epithelial cells, but to a
Numerous studies have docu- location directly subjacent to the
mented expression of COX-1 epithelial cells in the area occu-
throughout the length of the GI pied by intestinal subepithelial
tract. COX-1 immunoreactivity myofibroblasts.
has been demonstrated in crypt The notion that COX-1 repre-
epithelial cells, endothelial cells sents the “good COX” and COX-
of blood vessels, lamina propria 2 represents the “bad COX” is
mast cells, macrophages, lympho- probably an oversimplification.
cytes, fibroblasts, and smooth Prolonged COX-1 inhibition
muscle cells to name a few. Using certainly can produce adverse
sensitive detection methods, GI side effects (ulcers) while
COX-2 mRNA can be found in recently developed COX-2-
normal stomach and intestinal specific inhibitors (see below)
tissue and occasional COX-2 result in fewer ulcers. Further-
immunoreactive inflammatory more, COX-2 inhibition may
cells are seen. However, dramatic prove to be beneficial for chemo-
55

8. Suggested Reading Cyclooxygenases leukotrienes via an alternate
5-lipoxygenase (5-LOX) pathway.
and GI Mucosal
Goetzl, E.J., An, S., Smith, W.L. The enzyme prostaglandin H
Specificity of expression and effects
Protection synthase (COX) actually performs
of eicosanoid mediators in normal
two sequential reactions: a cyclo-
physiology and human diseases.
FASEB. J. 9, 1051–1058, 1995. oxygenase reaction, which
Mark Feldman, M.D.
An excellent review which covers the
converts arachidonic acid to
Professor and Vice Chairman
biology of other eicosanoid media-
Department of Internal Medicine PGG2, followed by a peroxidase
tors, in addition to PGs, as well as
University of Texas Southwestern reaction, which converts PGG2 to
their receptors.
Medical School
PGH2. In the GI mucosa, PGH2
Kalgutkar, A.S., Crews, B.C., Rowlin- Dallas, Texas
is then converted to various
son, S.W., Garner, C., Seibert, K.,
Marnett, L.J. Aspirin-like molecules prostaglandins, including PGE2,
Key Concepts
that covalently inactivate cyclooxyge-
PGF 2 , and PGI2 (prostacyclin)
nase-2. Science 280, 1268–1270,
• Cyclooxygenases, particularly and, to a lesser extent, to PGD2.
1998.
cyclooxygenase-1 (COX-1), are
Platelets, on the other hand,
An original research report describ-
important in protecting the
ing the development of APHS, an irre- convert PGH2 to thromboxane A2.
gastrointestinal mucosa by
versible COX-2 selective inhibitor.
Leukocytes convert arachidonic
catalyzing synthesis of mucosa-
protective prostaglandins.
Kurumbail, R.G., Stevens, A.M., acid to PGs (such as PGE2 ) via
Gierse, J.K., McDonald, J.J., Stege- • Inhibitors of COX-1, such as the COX pathway and to leuko-
man, R.A., Pak, J.Y., Gildehaus, D., aspirin and non-steroidal anti-
trienes via the 5-LOX pathway.
Miyashiro, J.M., Penning, T.D., Seib- inflammatory drugs (NSAIDs),
ert, K., Isakson, P.C., Stallings, W.C. The chemistry of COX is
reduce endogenous PG synthe-
Structural basis for selective inhibition sis and increase the incidence of described in more detail by
of cyclooxygenase-2 by anti-inflam- gastrointestinal ulceration. Drs. Mifflin and Powell in the
matory agents. Nature 384, 644–648,
• Highly selective inhibitors of accompanying article.
1996.
COX-2 are anti-inflammatory and
An original research report describ- Acetylsalicylic acid (aspirin)
analgesic, with reduced GI toxic-
ing the crystal structure of COX-2
irreversibly blocks COX activity
ity. However, the safety and cost-
complexed with various NSAIDs.
effectiveness of these agents is by acetylating a serine residue
under ongoing evaluation.
Reuter, B.K., Asfaha, S., Buret, A., near the active site of the enzyme.
Sharkey, K.A., Wallace, J.L. Exacer-
Non-salicylate nonsteroidal anti-
bation of inflammation-associated
Introduction inflammatory drugs (NSAIDs)
colonic injury in rat through inhibition
of cyclooxygenase-2. J. Clin. Invest. such as indomethacin, naproxen,
Prostaglandin H synthase,
98, 2076–2085, 1996.
and ibuprofen reversibly inhibit
An original research report which
more commonly referred to as COX activity by binding at sites
localizes COX-2 expression to lamina
cyclooxygenase, or COX, is the different than the aspirin site.
propria cells in a rat model of colitis.
In addition, this report demonstrates rate-limiting enzyme for cellular Acetylation of the key serine
an adverse effect of COX-2 inhibition
moiety of COX by aspirin
synthesis of prostaglandins (PGs)
on the course and severity of disease
prevents arachidonic acid from
and thromboxane A2 (TxA2).
in this model.
reaching the active (catalytic) site
Arachidonic acid, the precursor
Smith, W.L. & DeWitt, D.L. Prostaglan-
of COX. Because the platelet is
din endoperoxide H synthases-1 and of endogenous PGs and TxA2, is
-2. Adv. Immun. 62, 167–215,1996. not nucleated, it cannot generate
a polyunsaturated fatty acid
An excellent review which includes a new enzyme after its COX has
(C20:4) that is a component of
discussion of the mechanisms of
been irreversibly acetylated and
COX catalysis.
phospholipid in cell membranes inactivated by aspirin. Thus,
Vane, J.R., Bakhle, Y.S., Botting, R.M. throughout the body. Under an thromboxane A2 production from
Cyclooxygenases 1 and 2. Annu. Rev.
appropriate stimulus, arachidonic arachidonic acid is curtailed for
Pharmacol. Toxicol. 38, 97–120,
acid is released from the cell
1998. the life of the platelet (7 to 10
An excellent review covering many
membrane by the action of the days). Nucleated cells, such as GI
aspects of COX structure, inhibition,
epithelial cells, can produce COX
enzyme phospholipase A2.
regulation and role in physiology and
s mRNA and new COX protein,
disease. Arachidonic acid is then converted
permitting gradual restoration of
to either prostanoids (PGs, TxA2)
COX-catalyzed PG synthesis
via this prostaglandin H synthase
once the aspirin has been excreted
(COX) pathway and/or to
or metabolized to salicylate. It is
56

9. remarkable that aspirin remains misoprostol in preventing human A2 production, which leads to
in the bloodstream for around gastric ulcers caused by NSAIDs. platelet aggregation and vasocon-
three hours after oral dosing, with The observation that NSAIDs also striction, aiding the hemostatic
presumably transient suppression cause ulcers in the jejunum and process. Likewise, constitutive
of mucosal PG synthesis, yet ileum implies that gastric acid is expression of COX-1 in the
once-a-day, low dose aspirin less important than prostaglandin gastrointestinal (GI) tract mucosa
therapy results in significant depletion in the pathogenesis of is responsible for production of
gastric mucosal damage, as NSAID ulcers. An exception PGs such as PGE2 which, through
described below. appears to be NSAID-induced a variety of mechanisms
duodenal ulcers in humans which (enhanced bicarbonate and mucus
Cyclooxygenase and GI are readily prevented with hista- secretion, increased mucosal
blood flow, increased cell prolif-
mine-2 receptor antagonists or
Mucosal Cytoprotection
eration, and perhaps others),
misoprostol, indicating that gastric
PGs such as PGE2, produced protect the mucosa against ulcera-
acid may play a role in the patho-
from arachidonic acid in the tion. NSAIDs that interfere with
genesis of this type of ulcer.
normal GI tract mucosa by the the action of COX-1 reduce
Profound inhibition of gastric
actions of cyclooxygenase, play a constitutive prostanoid synthesis
acid secretion by proton pump
critical role in protecting both the in these tissues and, as a conse-
inhibitors in humans virtually
mucosa of stomach and small quence, have the potential to
eliminates duodenal ulcers caused
intestine against injury. The interfere with the normal physio-
by NSAIDs and also reduces, but
evidence that endogenous GI logic processes that prostanoids
does not eliminate, NSAID-
prostaglandins are “cytoprotec- mediate. Thus, side effects of
induced gastric ulcers.
tive” is two-fold. NSAIDs that inhibit COX-1
A second observation which
First, when GI mucosal synthe- include excessive bleeding
indicates that endogenous GI PGs
sis of PGs is blocked by NSAIDs through impaired platelet-medi-
are cytoprotective is that lethal GI
that inhibit COX, ulcers frequently ated hemostasis, and GI ulcer
ulcers develop in experimental
develop in the stomach and/or formation through impaired GI
animals in whom PGs have been
small intestine of both humans and mucosal “cytoprotection.” A
depleted by specific PG antibod-
experimental animals. Further- combination of these two toxici-
ies. GI ulcers in rabbits or dogs
more, when humans and experi- ties may result in life-threatening
can be produced by active or
mental animals given NSAIDs are bleeding ulcers. Unfortunately,
passive immunization against
aspirin and traditional NSAIDs
co-treated with PGE analogs, there PGE2, PGF2 , PGD2, or 6-keto
inhibit COX-1 at customary doses
is remarkable protection against PGF1 . These animal experi-
and thus may cause life-threaten-
GI ulcers. In animals, low doses of ments indicate that deficiency of
ing ulcer complications.
PGE analogs that are cytoprotec- even a single endogenous
COX-2, unlike COX-1, is
tive against NSAID-induced ulcers prostaglandin promotes the
present in undectable or very low
do not decrease gastric acid secre- development of ulcers in the
amounts in most tissues, except
tion. In humans, the PGE analog stomach and intestine.
kidney and brain. However,
most studied, misoprostol, does
COX-2 production can be
reduce gastric acid secretion in COX-1 and COX-2 increased dramatically in cells by
doses that prevent NSAID ulcers.
inducers of this enzyme, particu-
In 1991, it was discovered that
However, the protective effect of
larly bacterial lipopolysaccharides
there are two different isoforms of
misoprostol on gastric ulcer
(endotoxin) and certain cytokines
COX, COX-1 and COX-2. Differ-
formation in NSAID users is most
and growth factors. At inflamma-
ences between COX-1 and COX-2
likely a consequence of replace-
tory sites, cytokines such as tumor
are described in the accompany-
ment of PGE and not simply a
necrosis factor alpha (TNF ) and
ing article by Drs. Mifflin and
consequence of acid secretion
interleukin-1 (IL-1) can induce
Powell. Briefly, COX-1 is present
inhibition. This is because hista-
synthesis of large quantities of
in virtually all tissues, is
mine-2 receptor antagonists such
COX-2, which enhances local
expressed at a fairly constant
as ranitidine or famotidine, which
synthesis of PGs by inflammatory
level, and plays a physiological
are equipotent or even slightly
cells. Thus, NSAIDs that interfere
role in several tissues (“house-
more potent than misoprostol in
with the activity of induced
keeper”). For example, COX-1 in
inhibiting gastric acid secretion in
COX-2 have the potential to
platelets catalyze thromboxane
humans, are not as effective as
57

10. technically a selective COX-2
inhibitor with no GI toxicity.
The COX-2
Therapeutic Hypothesis
The current strategy for clinical
research in this field has been to
(a) develop COX-2 specific or
highly selective drugs, (b) test
them for efficacy in disease states
(e.g., osteoarthritis or rheumatoid
arthritis) that have been benefited
by conventional, non-selective,
COX-inhibiting NSAIDs, and
then (c) determine whether the
incidence of GI toxicity with the
COX-2 selective drug is less than
Figure 1. Correlation of gastric IC50 with COX-1 IC50 in blood for 25 different NSAIDs
the comparison (non-selective)
and anti-inflammatory/analgesic compounds. ASA = acetysalicylic acid (aspirin);
drug. The COX-2 hypothesis is
6-MNA = 6-methoxy napthalene acetic acid, the active metabolite of nabumetone
(Relafen®). From Cryer and Feldman (see references). Published with permission that the COX-2 selective
from The American Journal of Medicine, Excerpta Medica, Inc., New York, NY. inhibitors will maintain clinical
efficacy without GI toxicity.
reduce local PG production at example, the expression of COX-2
Unfortunately, there is no
sites of inflammation, thus mRNA is induced at the edges of
uniform way of defining or deter-
ameliorating the inflammatory gastric ulcers in rodents, but not
mining the COX-2 selectivity of
response. If, however, the same in the adjacent non-ulcerated
an NSAID. In the past few years,
NSAID also blocks COX-1 (i.e., mucosa. This interesting observa-
many investigators around the
the NSAID is not specific for tion suggests that PGs generated
world (including my own labora-
COX-2 at the dose employed), by newly generated COX-2
tory) have utilized fairly standard
side effects from COX-1 inhibi- protein may play an important
whole blood assays to address this
tion discussed earlier are possible. role in the ulcer healing process,
issue. A venous blood sample is
Even highly COX-2 selective and that interference with this
obtained and then exposed to the
NSAIDs such as diclofenac may pathway by a selective COX-2
drug in question as it is allowed to
reduce COX-1 activity at clini- inhibitor may impair healing.
clot. The ability of increasing
cally prescribed doses and cause Thus, a peptic ulcer caused by the
concentrations of the drug to
COX-1 related toxicity. What is bacterium Helicobacter pylori or
reduce serum thromboxane B2
desired is a COX-2 specific agent by a COX-1 inhibitor (such as
(TxB2 ) generation during clotting
that has little or no COX-1 effect low-dose aspirin) conceivably
in a test tube may be used as a
at clinically prescribed doses. could heal less rapidly if a specific
COX-1 assay because virtually all
Such an agent could prove to be COX-2 inhibitor is also being
of the TxB2 found in serum
anti-inflammatory, anti-prolifera- used. COX-2 induced by growth
during clotting is derived from
tive, analgesic, and/or antipyretic, factors may also play a role in
constitutive COX-1 in platelets.
yet free of GI toxicity. However, a tumor growth. Thus, COX-2
The drug concentration which
COX-2 specific drug may have inhibitors could be useful as
reduces serum TxB2 by 50% is
a unique toxicity that is not anti-tumor agents.
referred to as the COX-1 IC50.
presently apparent. For example, Acetaminophen (Tylenol,
Our laboratory has recently
COX-2 may play a role in renal others) is a fairly potent inhibitor
shown that there is a correlation
development, in the renin-angio- of bacterial lipopolysaccharide-
between the ability of a drug to
tensin system, in ovulation, and in induced COX-2 activity in human
inhibit COX-1 in whole blood and
uterine function during pregnancy. white blood cells, with an IC50
its ability to reduce gastric
similar to the IC50 for aspirin
COX-2 induction by tissue
mucosal PGE2 synthesis (Fig. 1).
(between 10 and 15 µM). Unlike
growth factors (e.g., epidermal
This correlation is not surprising,
aspirin, acetaminophen has no
growth factor or basic fibro-
since the majority of COX in the
inhibitory effect on gastric COX
blast growth factor) may be criti-
normal gastric mucosa is the
activity. Thus, acetaminophen is
cal in normal wound healing. For
58

11. COX-1 isoform. We have also Table 1. Examples of NSAIDs that have little or no COX-2 Selectivity or are
found that the IC50 for gastric COX-2 Selective in Human Whole Blood Assays
COX in human gastric mucosa for
NSAIDs such as aspirin corre- Little or no COX-2 Selectivity COX-2 Selective
sponds very closely with the IC50
Naproxen 6-MNA* Diclofenac Meloxicam
in vivo. Thus, effects of new COX
Indomethacin Aspirin Nimesulide Celecoxib
inhibitors on the stomach can be
Ibuprofen Ketoprofen NS-398 Rofecoxib
predicted to some extent by an
COX-1 assay of whole blood. * 6-methoxy naphthalene acetic acid, the active metabolite of nabumetone
In addition to measuring the (Relafen®).
whole blood COX-1 IC50 for a
given NSAID, the ability of
Table 2. Therapeutic Serum Concentrations After Usual Dosing and Human
increasing concentrations of the Gastric Mucosal IC50 of Two COX-2 Selective NSAIDs Listed in Table 1.
same drug to reduce PGE2 synthe-
sis when a blood sample is NSAID Therapeutic Gastric Ratio*
exposed to bacterial lipopolysac- Concentration (µM) IC50 (µM)
charide (endotoxin) can also be
Nimesulide 14.6 1.50 10
determined. Virtually all of the
Diclofenac 6.1 0.23 27
PGE2 produced in blood under
these experimental conditions is * Ratio of therapeutic concentration to gastric IC50. From reference by Cryer and
derived from COX-2 that has been Feldman.
induced by endotoxin in leuko-
cytes, particularly blood mono- concentrations than they inhibit Journal of Rheumatology and
cytes. Once the COX-2 IC50 is COX-1 activity in whole blood, Immunology in 1994. One such
determined, the ratio of IC50’s for nimesulide and diclofenac study compared nimesulide to
the drug in question is then calcu- concentrations at customary doses naproxen for 2 weeks in 200
lated (COX-2/COX-1). A ratio are still well above the IC50 for patients with tendonitis and bursi-
close to one indicates little or no gastric COX. The second reason tis. Adverse GI events occurred in
COX selectivity. Some examples to question the ultimate GI safety 16% with 200 mg/day nimesulide
of nonselective NSAIDs are given of COX-2 selective inhibitors is and 22% of patients on 1100
in Table 1. An NSAID with a low that two of these agents, meloxi- mg/day of naproxen, an insignifi-
COX-2/COX-1 ratio is COX-2 cam (whole blood IC50 ratio of cant difference.
selective; some examples are 0.09) and nimesulide (whole Two new COX-2 selective
listed in Table 1. Some of the new blood IC50 ratio of 0.02–0.06), agents have recently been
COX-2 selective inhibitors have have already been evaluated in approved in the United States.
COX-2/COX-1 ratios in human Celecoxib (Celebrex®) was
several clinical trials, and the
whole blood assays of 0.1 or less. anticipated GI safety has not been approved in 1998 for osteo-
Examples include rofecoxib, cele- realized. Several studies of arthritis and rheumatoid arthritis.
coxib, DuP-697, flosulide, Rofecoxib (Vioxx®) was
meloxicam were published in a
flurbinitroxybutylester, L-745,337, supplement to British Journal of approved in 1999 for osteo-
meloxicam, nimesulide, NS-398, Rheumatology in 1996; interested arthritis and for the short-term
and SC-58125. However, there are readers are referred to this supple- management of acute pain in
two reasons to question the idea ment for more details. The largest adults, including menstrual pain.
that COX-2 selective drugs will of these studies compared meloxi- Very little data is published as yet
not be toxic to the GI tract. First, cam to naproxen (a non-selective on these compounds, but early
some currently marketed NSAIDs COX inhibitor) in 370 patients reports indicate that both drugs
that are at least 10-fold COX-2 with rheumatoid arthritis who (a) cause slightly, but not signifi-
selective (COX-2/COX-1 IC50 were treated for 6 months. cantly, more upper GI ulcers
ratio < 0.1) are still able to inhibit Adverse GI effects occurred in detected by endoscopy than
COX-1 in the blood and in the 35.5% of patients on 750 mg/day placebo; (b) cause much fewer
stomach at clinically prescribed naproxen and in 26.6% of patients ulcers detected by endoscopy than
doses and concentrations on 7.5 mg/day of meloxicam, an equally-effective doses of tradi-
(Table 2). Thus, even though insignificant difference. Several tional NSAIDs; (c) cause signifi-
nimesulide and diclofenac reduce studies of nimesulide were cantly fewer clinically important
COX-2 activity at much lower published in the European ulcer events, including GI bleeds,
59

12. than traditional NSAID compara- Cyclooxygenase-2 number of groups have initiated
tors. Serious ulcer events such as research efforts focused on eluci-
and Colon
bleeding have been reported with dating the molecular basis of the
Carcinogenesis
both celecoxib and rofecoxib, anti-neoplastic effects of aspirin
although cause and effect have and other NSAIDs. Many of these
not been established. Careful efforts suggest that inhibition of
Raymond N. DuBois, M.D., Ph.D.
post-marketing surveillance stud- the enzyme cyclooxygenase-2
and Moss Mann, M.D.
ies will be needed to determine (COX-2) by NSAIDs plays some
Departments of Medicine
whether celecoxib and rofecoxib role in cancer risk reduction.
and Cell Biology
Vanderbilt University
prove the COX-2 hypothesis. Hopefully, continued study of
Medical Center the role of the cyclooxygenase
Veteran Affairs Medical Center
Suggested Reading enzymes in colorectal carcino-
Nashville, TN
genesis will determine whether
Cryer, B. & Feldman, M. Cyclooxyge- COX-2 selective inhibitors can be
nase-1 and cyclooxygenase-2 selec- Key Concepts used in future cancer prevention
tivity of widely used NSAIDs and
regimens.
• Colorectal cancer is the second
other anti-inflammatory or analgesic
leading cause of cancer death in
drugs: Studies in whole blood and
the U.S.
gastric mucosa of healthy humans. Evidence for the role of
Am. J. Med. 104:413-421, 1998. • Regular NSAID use appears to
COX in colon cancer
This study compares whole blood reduce the risk of developing
prevention
COX-1 and COX-2 IC50’s, and gastric colorectal cancer.
IC50’s for 25 different NSAIDs and
• There are data suggesting that
Risk reduction in human
anti-inflammatory or analgesic
NSAIDs reduce the risk of
compounds, and develops evidence sporadic colorectal carcinoma
developing colorectal cancer
that gastric COX is mainly COX-1.
by inhibiting cyclooxygenase-2 Of the several observational
(COX-2).
Lecomte, J. et al. Treatment of
studies of the effects of exposure
tendonitis and bursitis: a comparison • Selective COX-2 inhibitors show
to NSAIDs (usually aspirin) and
of nimesulide and naproxen sodium in promise as chemopreventive
the subsequent development of
a double-blind parallel trial. Eur. J. agents with fewer gastrointesti-
Rheumatol. Inflamm. 14:29-32, 1994. colorectal cancer, all but one have
nal side effects; however, their
A 2-week trial showing no significant safety record is not yet proven. demonstrated a protective effect
difference in adverse GI events
of NSAIDs. The studies were
between the former COX-2 selective
performed in a variety of settings
agent and the latter non-selective
Introduction
agent. in the US and Australia, utilizing
both colorectal cancer occurrence
Colorectal cancer is a major
Patrignani, P., Panara, M.R., Sciulli,
M.G., Santini, G., Renda, G., Patrono, and mortality as outcomes. In
cause of illness and death in the
C. Differential inhibition of human the studies, exposure to NSAIDs
United States and other parts of
prostaglandin endoperoxide synth-
was measured by interview or
the world. In 1998 there were over
ease-1 and -2 by nonsteroidal anti-
computerized pharmacy records.
130,000 new cases of colorectal
inflammatory drugs. J. Physiol. Phar-
macol. 4:623-631, 1997. In the Nurses Health Study, a
cancer and about 55,000 deaths
A report of whole blood assays for protective effect was seen only
from the disease (Table 1). Ameri-
COX-1 and COX-2 activity of a wide
after 10–15 years of aspirin use.
cans have a 1 in 20-lifetime risk of
variety of NSAIDs, including meloxi-
Similar studies have revealed a
developing colorectal cancer, and
cam, nimesulide, and NS-398.
protective effect of NSAIDs in
approximately one in ten has a
Wojtulewski, J.A. et al. A six-month
relation to adenomatous polyp
family member who develops this
double-blind trial to compare the effi-
detection. Additionally, a small
cacy and safety of meloxicam 7.5 mg disease. Epidemiologic research
daily and naproxen 750 mg daily in number of observational studies
indicates that there is a 40–50%
patients with rheumatoid arthritis. Br
have shown a significant risk
reduction in mortality from colo-
J. Rheum. 35(S 1):22-28, 1996.
reduction with use of non-aspirin
rectal cancer in persons who take
A trial of 370 patients showing similar
NSAIDs.
adverse GI events with meloxicam, a aspirin or other non-steroidal anti-
COX-2 selective agent, and naproxen, The effect of aspirin use on the
inflammatory drugs (NSAIDs) on
a non-selective agent. Only 2 ulcers
development of colorectal cancer
a regular basis. Clearly, an effect
s
occurred, both with naproxen.
has been assessed in a randomized
of this magnitude could have a
clinical trial that had a principal
significant impact on health care,
goal of evaluating aspirin for the
both in terms of lives saved and
prevention of myocardial infarc-
health care dollars recovered. A
60

13. tion. A secondary analysis of this Table 1. U.S. Colorectal cancer statistics (estimated) by gender for 1998.
study of 22,071 male physicians
randomized to placebo or aspirin Men Women Total
325 mg every other day demon-
Estimated # of new 64,600 67,000 131,600
strated no protective effect against
crc cases
the development of colorectal
% of all new ca cases* 10% 111% 10%
cancer. It is possible that certain
characteristics of the study group Rank vs other 3rd (behind lung 3rd (behind lung 2nd overall
ca types* and prostate) and breast)
(such as diet, exercise regimen,
age, and gender) or the relatively Estimated # deaths 27,900 28,600 56,500
from crc
low dose of aspirin could have
obscured a protective effect. % of all ca deaths 9% 11% 10%
Unfortunately, the low Rank vs other causes 3rd (behind lung 3rd (behind lung 2nd overall
frequency of colorectal cancer of ca death and prostate) and breast)
makes a large scale randomized
Abbreviations: crc= colorectal cancer; ca= cancer; vs = versus.
clinical trial financially and
temporally difficult. More defini- * Excluding non-melanoma skin cancers and non-bladder carcinoma-in-situ.
tive recommendations concerning
Data from Landis, S.H., et al., Cancer Statistics, 1998. CA Cancer J. Clin. 48:6–29.
aspirin use likely will be based on
the results of an ongoing random-
lar activities, but differing in
that regular use of the NSAID
ized clinical trial of aspirin use
expression characteristics and
sulindac led to regression of rectal
which utilizes adenomatous polyp
inhibition profiles by NSAIDs.
adenomas in four patients with
incidence as an intermediate
COX-1 mRNA and protein are
FAP, and this phenomenon was
endpoint. This multi-center study
expressed constitutively in many
confirmed in several other case
tests the effect of aspirin at one of
tissues. A second, inducible
reports. This observation was then
two doses versus placebo on the
isoform of cyclooxygenase,
confirmed in randomized, placebo
development of adenomatous
referred to as cyclooxygenase-2
controlled, double-blinded,
polyps among patients who have
(COX-2) was independently
crossover studies of sulindac use
undergone prior colonoscopy
cloned by two groups. COX-2
in FAP patients. These studies,
with polypectomy. Hopefully,
expression is induced by a
collectively, indicate that sulindac
data from that study will help
number of extracellular and intra-
has a significant effect on polyp
determine the degree of benefit
cellular stimuli. The formation of
regression in FAP patients.
and optimal dose of aspirin in a
COX-2 protein parallels the
chemopreventive regimen.
Potential mechanisms increase in prostaglandin produc-
NSAID use and reduction of tion following stimulation with
for chemoprevention of
adenoma size and number in mitogens or tumor promoters in a
intestinal tumors by
FAP patients wide variety of cell types.
aspirin and other NSAIDs
Does dysregulation of COX-2
Familial adenomatous polypo-
Inhibition of cyclooxygenases expression coincide with develop-
sis (FAP) is an autosomal domi-
ment of gastrointestinal malig-
The anti-inflammatory proper-
nant inherited disease with vari-
nancy? We have previously
ties of NSAIDs are most likely
able phenotypic expression that is
reported increased COX-2 expres-
due to their inhibition of cyclo-
associated with an increased risk
sion in human colorectal adenocar-
oxygenase enzymes. These
of colorectal cancer at a young
cinomas when compared to normal
enzymes catalyze key steps in the
age. FAP is responsible for only
adjacent colonic mucosa; these
conversion of arachidonic acid to
1% of colorectal carcinomas
findings have been confirmed by
endoperoxide (PGH2 ), which is a
detected in the general popula-
other investigators using different
substrate for a variety of prosta-
tion. The genetic mutation
techniques and patient populations.
glandin synthases which catalyze
responsible for this disease
Additionally, COX-2 mRNA and
the formation of prostaglandins
resides in the adenomatous poly-
protein levels are increased in
and other eicosanoids (see accom-
posis coli (APC) gene. Somatic
intestinal tumors that develop in
panying article by Drs. Mifflin
mutations in the APC gene have
rodents following carcinogen treat-
and Powell). Two isoforms of
been reported also in up to 50% of
ment and in adenomas taken from
cyclooxygenase have been identi-
spontaneous colorectal cancer.
multiple intestinal neoplasia (Min)
fied to date, each possessing simi-
Wadell and Loughry first reported
61