COX-2 inhibition and prevention of cancer

Best Practice & Research Clinical Gastroenterology Vol. 15, No. 5, pp. 821±833, 2001

doi:10.1053/bega.2001.0237, available online at http://www.idealibrary.com on

9 COX-2 inhibition and prevention of cancer Karl-Erik Giercksky

MD, PHD

Professor and Chairman Department of Surgical Oncology Department of Surgical Oncology, The Norwegian Radium Hospital and Institute of Cancer Research, The University of Oslo, Norway

The potential for cyclo-oxygenase inhibition in cancer prevention and treatment is founded on epidemiology (reduction of colorectal cancer in aspirin users), animal experiments and molecular genetics. Trials using the NSAID sulindac also reduced the number of polyps in patients with familial adenomatous polyposis, but the well-known gastrointestinal toxic e€ects of aspirin and NSAIDs have discouraged the exploitation of their antineoplastic potential. The advent of speci®c COX-2 inhibitors, which do not interfere with the cytoprotective constitutive COX-1 enzyme, and the demonstration of increased COX-2 expression in many common malignancies beside colorectal cancer, has opened up new therapeutic possibilities. Recently a non-cyclo-oxygenase e€ect of COX-2 inhibitors, which combines the PPARd and the APC tumour suppressor activity, was also demonstrated. The selective COX-2 inhibitor celecoxib has been approved by the FDA for adjuvant treatment of familial adenomatous polyposis, and a large number of prevention and treatment trials of colorectal and other common cancers (prostate and breast cancer) have been started. Key words: COX-2; cyclo-oxygenase; NSAIDs: colorectal cancer; PPARd; angiogenesis; apoptosis; sulindac; APC; FAP.

The willow tree has been a source of remedies against fever and in¯ammation since ancient times. Some 101 years ago, the German chemist Ho€man and the Bayer Company isolated and modi®ed an active compound from the willow tree, acetylsalicylic acid, and o€ered it for sale as Aspirin. In the second part of the previous century a number of di€erent chemical compounds with aspirin-like e€ects were synthesized and classi®ed as NSAIDs (non-steroidal anti-in¯ammatory drugs). The analgesic, anti-in¯ammatory, and antithrombotic e€ects of NSAIDs have been amply demonstrated and exploited in numerous ®elds of clinical medicine. Although NSAIDs usually lack a curative potential, and have well known serious side-e€ects, their symptomatic eciency in in¯ammatory bone and soft-tissue conditions has let the NSAIDs remain one of the major classes of drugs, both in sale and use. Their almost unrivalled popularity, and therefore economic potential, has led to an everincreasing number of NSAIDs being developed, each being advertised as more ecient and less likely to cause side-e€ects than previous products. Because aspirin and NSAIDs were generally looked upon as unable to cure or signi®cantly modify disease, the ®rst reports on the NSAID sulindac's ability to reduce the size and number of adenomatous polyps in patients with familial adenomatous polyposis (FAP) did not attract the usual attention of new cancer drugs. When cohort 1521±6918/01/050821‡13 $35.00/00

c 2001 Harcourt Publishers Ltd. *

822 K.-E. Giercksky

studies in patients using `low-dosage' aspirin as propylaxis against cardiovascular problems indicated a signi®cant reduction in colorectal cancers, the ®eld of cancer prevention increased considerably. Because aspirin is cheap, and had already been more or less accepted as prophylaxis within the cardiovascular ®eld, its candidate role in cancer chemoprevention seemed to be more attractive than that of sulindac. This NSAID had not performed too well in the extremely competitive NSAID market, probably due to a combination of its complex metabolism (absorption and biliary excretion) and side-e€ects. Therefore, a life-long prophylaxis in otherwise healthy populations was not readily envisaged. For a drug to be useful in cancer chemoprevention, exept within small groups with very high incidence of cancer (e.g. familial adenomatous polyposis, FAP), it must, beside having a ®rmly documented anticancer e€ect, have few and only mild side-e€ects. This is not the case for either sulindac or any other NSAID. Aspirin in treatment dosages for rheumatic disease has the same unwanted side-e€ect pro®le, and even `low-dose' aspirin has been demonstrated to increase signi®cantly serious ulcer complications such as gastric perforation and bleeding.1 Because e€ect and side-e€ects were believed to due to the same global enzymatic inhibition, development of new NSAIDs without the classical side-e€ect pro®le did not have a fertile scienti®c platform until two di€erent genes encoding cyclo-oxygenase enzymes were cloned (Table 1) and the enzymes characterized in detail (Table 2).2 There is approximately 60% primary sequence identity between these two cyclo-oxygenase enzymes (COX-1 and COX-2), but one di€erence turned out to be crucial. COX-1 is expressed constitutively and is more or less part of the normal cell `housekeeping' while the other, COX-2, is induced locally as part of the in¯ammatory cascade and in the early development of tumours. The extremely rapid appearance of selective COX-2 inhibitors approved for clinical use, and their postulated safety pro®le compared to NSAIDs, have indeed altered the ®eld of cancer chemoprevention. INHIBITION OF A SPECIFIC CYCLO-OXYGENASE In 1971 Vane described the mode of action of aspirin-like drugs to be due to reduced production of prostaglandin by inhibition of the enzyme cyclo-oxygenase (also referred to as endoperoxide synthase).3 Prostaglandins are short-lived compounds acting as local hormones of continuous importance in normal cellular reactions, but they seem to increase in a number of pathological conditions, especially in¯ammation.4 Cyclooxygenases transform arachidonic acid to prostaglandins and thromboxanes. Prostaglandins and thromboxanes maintain physiological regulation in the stomach, platelets, kidneys and intestine, and the side-e€ects of aspirin and NSAIDs (gastropathy, renal insuciency, impaired vascular homeostasis) are due to reduction of the appropriate prostaglandins in these organs. The availability of substrate (i.e. arachidonic acid) was previously believed to regulate the synthesis of di€erent prostaglandins±until it was demonstrated that in¯ammatory cytokines are powerful inducers of prostaglandin production. This also proved the occurrence of locally induced prostaglandin production, thus departing from the dogma of constitutive enzyme activity regulated by disease or a trauma-released substrate. The evolutionary strategy of developing constitutive and inducible isoenzymes (and probably in that order) has been previously demonstrated for important biological messengers (e.g. nitric oxide) and it is unlikely that cyclo-oxygenase will be the last. X-ray crystallography clari®ed the structure of COX-1 and COX-2 and demonstrated, at the molecular level, how NSAIDs inhibit these enzymes. Regular NSAIDs

COX-2 inhibition and prevention of cancer 823 Table 1. Characteristics of the two cyclo-oxygenase genes.

Gene size mRNA size Localization Expression Inhibition

COX-1

COX-2

22 kb 2.7 kb Chromosome 9 Constitutive Instantaneous

8.3 kb 4.1 kb Chromosome 1 Inducible Time-dependent

Table 2. Characteristics of the two cyclo-oxygenase enzymes. COX-1

COX-2

Physiological

Homeostatic GI-cytoprotection; renal functions; macrophage di€erentiation

Reproduction Development Renal functions

Pathological

Reduced platelet functions; proliferation?

Tissue and wound repair In¯ammation Proliferation

inhibit both cyclo-oxygenases by binding to a polar arginine at position 120 and thus blocking the channel needed for enzyme activity. The COX-2 enzyme has a wide sidepocket guarded by valine at position 523 and is believed to be the binding site for speci®c inhibitors. In COX-1, access to this binding site at the side-pocket is blocked by the bulkier aminoacid isoleucine at the same position.5 Local control of symptoms and abnormal tissue growth, which does not disturb the constitutive and necessary general functions of prostaglandins, has been the aim for all NSAIDs. This has led to claims of `COX-2 speci®city' based on bewildering types of assays and experiments. For practical use these types of drugs can now be classi®ed into four main groups.6 Non-speci®c cyclo-oxygenase inhibitors demonstrate no meaningful biological or clinical di€erences in the inhibition of COX-1 versus COX-2 activity. The majority of NSAIDs should probably be included in this group. COX-2 preferential inhibitors have some analgesic or anti-in¯ammatory activity at doses that cause inhibition of COX-2, but less inhibition of COX-1. COX-2-speci®c inhibitors inhibit the COX-2 isoform but have no e€ect on the COX-1 enzyme over the entire range of therapeutic serum concentrations. Two such agents (celecoxib and rofecoxib) are now in clinical use, and both agents have been reported to have signi®cantly fewer serious gastrointestinal side-e€ects than the most widely used NSAIDs, and side-e€ects not signi®cantly di€erent from those of placebo.7,8 The previous observations that daily use of aspirin reduced the incidence of colorectal cancer, and sulindac's ability to reduce the number and growth of polyps in familial adenomatous polyposis (FAP), could now be exploited without the obvious risk of gastrointestinal and haemostatic problems because speci®c COX-2 inhibitors do not induce ulcers or interfere with thrombocyte activity.9 WHY SHOULD COX-2 INHIBITION BE USEFUL IN CANCER? The potential for cyclo-oxygenese inhibition in cancer prevention and treatment is founded on epidemiology (reduction of colorectal cancer in aspirin users), animal

824 K.-E. Giercksky

experiments and molecular genetics.10 Trials using the NSAID sulindac also reduced the number of polyps in patients with familial adenomatous polyposis.11 As stated by Prescott et al, the epidemiological evidence is considered robust because it contains so many separate studies of di€erent design and populations, but one randomized trial failed to confer any bene®t.10,12 Common to most epidemiological studies is the ®nding that a bene®t from aspirin or NSAIDs is evident only after prolonged use and that there is a dose±response relationship.13 Because chronic in¯ammation is a recognized risk factor for epithelial carcinogenesis14,15 the ®rst simpli®ed explanation was that the abnormal growth of tumour tissue led to an in¯ammatory reaction from the surrounding and invading cells necessary for further growth and inhibitable by NSAIDs. This has now been re®ned and further elaborated by molecular studies. The epidemiological results placed colorectal cancer and polyps, and their animal equivalents, as the major research objects, but recently similar molecular mechanisms have been reported to occur in a large number of other common tumours.16±21 So far it is not clear whether the development of colorectal cancers is connected to COX-2 in a more crucial way than other tumours, but certain characteristics of such tumours, beside epidemiological data, have kept them in the centre of attention. In the western part of the world colorectal cancer is common and is probably the second leading cause of cancer deaths. Due to widespread availability of endoscopy, diagnosis and tissue specimens for research are easily obtained. Suspect or pre-malignant lesions can be observed and their development studied, and nature has even provided a genetic disorder (FAP) which demonstrates the progression from polyp to cancer. Genetically and chemically induced experimental models of polyps and colorectal cancer in animals have provided further possibilities for pre-clinical testing of di€erent interventions. The development from adenoma to cancer (the adenoma±cancer sequence) has been amply demonstrated and elucidated, even at the molecular level, and seems to be the regular event for most or all colorectal cancers.22±26 A number of endoscopic screening projects involving either ¯exible sigmoidoscopy or colonoscopy are now being done to ®nd, characterize and remove polyps and cancers. If inhibition of COX-2 prevents an important step in the adenoma±carcinoma sequence its clinical value could be tested in such screening programmes. Increased amounts of prostaglandins can be found in adenomas and colorectal cancers27 and this is associated with increased expression of COX-2, but not COX-1, in 70±80% of such tumours.28±30 Consistent with this is the decreased intestinal tumorigenesis in mice with inactivation of the COX-2 gene. The APC-mouse is an animal model of human FAP, and the number of intestinal polyps was reduced by 87% in COX-2 null mice and by 66% when one copy of the COX-2 gene was disrupted.26 Taken together, such experimental data suggest that induction of COX-2 is an early event in the development of tumours and that mutation of a `gate-keeper' gene, such as the APC gene, results in the induction of COX-2. Increased transcription seems to be the most common result, but increased mRNA may also occur via translational regulation.31 COX-2-RELATED INHIBITION OF NEOPLASTIC CHANGES The development of tumours is a complex, multistage process involving numerous pathways. Di€erences in growth patterns between an adenocarcinoma of the colon and an adenoma are often obvious to the naked eye and impossible to overlook with a microscope. Molecular methods have given us insight into a number of aspects of

COX-2 inhibition and prevention of cancer 825

disturbed growth regulation following mutations in tumour cells, leading to the view that if inhibition of a single factor should be rewarding it must occur either very early in the development of tumours, before compensating factors have developed, or attack one growth-limiting factor of the disseminated tumour that cannot be circumvented.

Apoptosis The development of normal tissue into a malignant tumour with metastatic potential is a complex process requiring a change in the balance between proliferation and programmed cell death (apoptosis). Decreased apoptosis has been demonstrated in both adenomas and adenocarcinomas of the colon, and several studies have indicated that the predominant e€ect of aspirin and sulindac is to induce apoptosis. Overexpression of COX-2 increases the level of anti-apoptotic Bcl-2 protein and cell survival, but can be reversed by NSAID treatment.30,32

Angiogenesis Tumour growth beyond the small size that can be fed by di€usion depends on a corresponding growth of vascular structures. The crucial importance of tumour angiogenesis has been demonstrated by Folkman.33 Clinical tumours seem to secure and provide their own blood supply by secreting vascular growth factors. This has led to an extensive search for inhibitors of tumour-produced inducers of angiogenesis. Overexpression of COX-2 in colonic cancer cells has, in a number of in vitro studies, been demonstrated to enhance three of the important aspects of angiogenesis: (i) production of vascular growth factors, (ii) migration of endothelial cells through the collagen matrix, and (iii) formation of tubular networks34, and, together with increased apoptosis, has been suggested to be the two main features of their antineoplastic e€ect. Reduction in tumour angiogenesis by selective COX-2 inhibitors has been demonstrated experimentally.30,35

Metastasis The ability of malignant cells to break loose from their own tissue, enter into neighbouring organs or settle in new and distant places via blood or lymphatic vessels is not regarded as an early event in the development of tumours. This state of widely disseminated tumour disease, which usually marks the end of the process and of our curative capabilities, is dependent on the invasiveness of the tumour or of certain clone(s) of tumour cells. Central to invasiveness is the ability to digest biological membranes by di€erent matrix-metalloproteinases and to reduce the intercellular anchorage provided by, among other interactions, cadherin±catenin interactions. In vitro cellular studies have demonstrated a possible role for COX-2 in such events. Human colon cancer cells permanently transfected with a COX-2 expression vector had increased prostaglandin production, enhanced invasive properties and increased mRNA expression of metalloproteinases compared with normal cells.36 These properties, which are believed to be closely connected to the metastatic process, could be reversed by sulindac.

826 K.-E. Giercksky

Immunosuppression A relationship between the development of tumours and the status of the immune system has been observed for a long time. The mechanisms are extremely complex and far from being understood in detail, but immunosuppression results in a signi®cantly increased appearance of tumours and is dependent on both duration and type of immunosuppression. Human organ transplantation has provided important insight into this problem. Two aspects of tumour development following organ transplantation and iatrogenic immunosuppression have become evident. The majority of transplanted patients will develop tumours if they live long enough, but the majority of tumours they are likely to develop are not the same as in the untransplanted population, and are often related to skin and lymphatic tissue. One of the more common malignant tumours, breast cancer, is actually found less frequently in transplanted patients than in the control population. Aspirin and NSAIDs have been found to attenuate tumourmediated immunosuppression due to inhibition of prostaglandin (PGE2) production. PGE2 is a well known modulator of lymphokines and T and B cell production, and it also regulates the cytotoxic activity of natural killer cells.30,37,38 Chronic in¯ammation The most serious form of symptomatic chronic in¯ammation in gastroenterology is ulcerative colitis. The increased incidence of colon cancer is well documented, and colitis persisting for more than 35 years carries an absolute risk of colon cancer between 20 and 35%.10,15,39 The fact that ulcerative colitis does not seem to increase the risk of other types of cancer has been regarded as support for the ability of local chronic in¯ammation to induce cancer.10,39 COX-2 is highly expressed, as would be expected in such in¯amed and often infected tissue. No clinical trial of speci®c COX-2 inhibitors in ulcerative colitis has been reported, but for many reasons it will be important to study the e€ect of speci®c COX-2 inhibitors in such patients. If COX-2 inhibition prevents development of cancer the strategy of long-term treatment of ulcerative colitis would change. The general reduction in local inducible antiin¯ammatory responses, which is the hallmark of COX-2 inhibition, could also work in another direction and facilitate the invasion of microbes and provoke sepsis. Because patients with ulcerative colitis asking for symptomatic relief for arthritis are common, this problem should be given priority. The few animal studies in experimental models of ulcerative colitis have not demonstrated bene®cial e€ects40, but clinical deterioration of ulcerative colitis following the use of NSAIDs has been observed. The use of speci®c COX-2 inhibitors in such patients should, therefore, be carefully controlled until further data are available. GENERAL CONSIDERATIONS Inhibition of COX-2 has been shown to in¯uence almost all the known di€erences in growth patterns between normal and cancer cells ± such as angiogenesis, apoptosis and metastatic potential; even the ability to increase the e€ect of radiation therapy has been reported.41,42 This is, of course, of considerable interest, but it should not be forgotten that the great majority of these studies have been carried out using in vitro or animal models. This does not reduce their scienti®c value, but the past has provided us with a large number of cancer treatment strategies which worked admirably in an

COX-2 inhibition and prevention of cancer 827

experimental situation but failed in clinical tests. The major practical di€erence between experimental and clinical cancer treatment situations is that when a clinical cancer is detected it has already developed the intracellular machinery necessary to circumvent, neutralize or expel most or all of the inhibitors or stimulators we are able to expose it to without killing the patients. Experimentally induced or transferred cancers or cell lines have often been selected by other criteria and do not re¯ect all of the abilities of clinical cancers to break through natural host defences. Aspirin, classical NSAIDs or even speci®c COX-2 inhibitors can be demonstrated to interfere with di€erent cascade reactions of in¯ammatory cartilage diseases, but they do not heal or even prevent the ongoing destruction of cartilage. Thus, in their classical indications, NSAIDs are today regarded as drugs that relieve symptoms but not as diseasemodifying drugs. Aspirin and NSAIDs became available before we had elaborate methods to test their modes of action, but it can be easily imagined how promising their curative potential for in¯ammatory diseases would have been (and the following disappointment) if they had been introduced today after elaborate in vitro experiments, demonstrating an e€ect on almost any in¯ammatory reaction. If the previous track record of NSAIDs is anything to go by, a certain cautiousness seems appropriate. On the other hand, it could also be that aspirin and NSAIDs were aimed at the wrong target from the outset. Ho€man, the discoverer of aspirin, was deliberately trying to ®nd a remedy for his father's arthritis, and when Vane demonstrated aspirin's vascular potential, angiogenesis was not a central issue of cancer research. If all the experimental and the few clinical facts about the possible role of aspirin, NSAIDs and speci®c COX-2 inhibitors were evaluated together, there would be little disagreement about prevention of tumours being more promising than treatment of disseminated tumours. The well-known incidence and seriousness of the side-e€ects of aspirin and NSAIDs have also worked against large-scale testing of their ability to prevent the development of cancer. Any type of drug used by healthy populations as chemoprevention for cancer should have few, and only mild, side-e€ects. In two large doubleblind studies, speci®c COX-2 inhibitors have already demonstrated a signi®cantly lower incidence of gastrointestinal toxicity7,8, but the long-term (years instead of months) toxicity will be even more important before the decision to expose large populations of healthy persons to chronic medications can be made. Until these issues are clari®ed, chemoprevention with COX-2 inhibitors should be restricted to populations with a known increased risk of developing cancer and, as far as possible, should be done within controlled trials.

SIDE-EFFECTS OF SELECTIVE COX-2 INHIBITORS Even if the adverse e€ect of celecoxib on kidney functions is less than that provoked by NSAIDs, the di€erence is small and of dubious clinical signi®cance. The same rules of cautiousness used with NSAIDs should be applied to selective COX-2 inhibitors. The lack of activity on platelets should not be regarded as a problem. The so-called `extra bene®t' of reduced platelet activity of using NSAIDs is indeed a variable one within this class of drugs. If there are indications for anti-platelet therapy this can be easily instituted in a more controlled way with speci®c medications. COX-2 has important roles in implantation and the development of the embryo, and COX-2 inhibitors should not be given during pregnancy or to fertile women without birth control. Delayed healing of

828 K.-E. Giercksky

experimental gastric ulcers by inhibition of COX-2 has been reported.43±45 The clinical signi®cance of this is not clear, but any ulcers appearing during treatment or prophylaxis with speci®c COX-2 inhibitors should be treated appropriately. Whether a longer time of acid reduction is needed is not known. The demonstration of a delayed healing time during COX-2 inhibition and the increased level of COX-2 in normally healing ulcers has led to the suggestion that COX-2 is also an inducer of a wound healing factor in an anti-in¯ammatory reaction.46 This putative healing factor has been suggested as `COX3', but could also be a splice variant of COX-1.31,46 Delayed and disturbed healing or local host defence mechanisms may have implications beyond the gastrointestinal tract for populations using chronic COX-2 inhibition. If this is due to disturbed local defence mechanisms, meticulous surveillance of any increase of infectious problems should be an important part of all long-term COX-2 inhibitor trials. It is indeed unlikely that such a widely used and, for in¯ammation±central±mechanism as local COX-2 induction could be abolished over long periods of our lives without any loss of host defence capabilities. The hope is that the bene®ts of cancer prevention are of a greater magnitude than any problem(s) that are likely to appear.

IS INHIBITION OF PROSTAGLANDIN PRODUCTION THE ONLY EFFECT OF COX-INHIBITORS? Since Vane demonstrated the e€ect of aspirin on prostaglandin production, the focus of interest has been the inhibition of cyclo-oxygenase, but some experimental data have been dicult to ®t into this frame. Inhibitors of cyclo-oxygenase have been reported to have e€ects on cells without cyclo-oxygenase enzymes, and a chemical derivate of sulindac without NSAID e€ect still has the capability to inhibit colon cancer cell lines and to reduce the number of adenomatous polyps in the colon.47±49 Cancer cells devoid of cyclo-oxygenase activity are growth-inhibited as e€ectively as cells producing cyclo-oxygenase, and COX-1 and COX-2 null mouse embryo ®broblast cells remain sensitive to the antineoplastic e€ect of NASIDs.50 The COX-2 protein is elevated in the neoplastic epithelial cells of human tumours while COX-2 expression in the mouse intestinal cancer models is located mainly in the stromal cells.51 Both types of tumour are inhibited by NSAIDs. Due to recent studies on genetic abnormalities during the adenoma±carcinoma sequence and induction of nuclear hormone receptors, these previously divergent observations can be combined with the classical cyclo-oxygenase dogma to yield new insights into pathogenesis and treatment possibilities for colorectal cancer.52 The nuclear receptor(s) involved belong to the PPARd subfamily of the peroxisome-proliferator-activated receptor family and are an important link between the adenomatous polyposis (APC) tumour suppressor pathway and eicosanoids.53 The protein encoded by the APC gene, b-catenin, also moves into the cell nucleus and becomes part of a DNA-binding complex capable of promoting cell proliferation (Figure 1). This complex also in¯uences the expression of PPARd, which has been found in abundance in colorectal cancer cells. Through an unknown mechanism, NSAIDs prevent the activation of genes through PPARd by disrupting the ability of this receptor to bind DNA, and they induce apoptosis in colon cancer cells.51±53 As pointed out by He et al, this may make it possible to develop new drugs that speci®cally target PPARd and which are likely therefore to be a more ecacious and less toxic means of preventing colorectal cancer.51

COX-2 inhibition and prevention of cancer 829

APC tumour suppressor gene

Known ligands for PPARd: Eicosanoids, Fatty acids

b-catenin

TCF-4

DNA binding and induction of PPARd RXR PPARd

PPARd ligand

DNA binding and transcription of growth and proliferation-related genes

Iloprost, Carbaprostacyclin Cyclo-oxygenasedependent inhibition Cyclo-oxygenase-independent inhibition

Figure 1. The two modes of NSAID-inhibited proliferation. Mutation in the APC gene is presumed to be the central event in the adenoma±carcinoma sequence, leading to an increased amount of free b-catenin. bCatenin will enter the nucleus, forming a complex with TCF-4 (T-cell factor) protein. The TCF-4 and bcatenin complex binds DNA and activates proliferation but also expression of the nuclear receptor PPARd. Together with another nuclear receptor (retinoid X receptor), and after binding a ligand, target genes are activated. COX-2 inhibitors reduce both the availability of ligands (cyclo-oxygenase-dependent) and the DNA binding of the complex (cyclo-oxygenase-independent).51±53

CLINICAL USE OF SPECIFIC COX-2 INHIBITORS IN CANCER TREATMENT AND PROPHYLAXIS In 1999 celecoxib received FDA approval for FAP as an adjuvant to other regimens. The approval was based on previous experience with sulindac and a small double-blind trial with placebo where celecoxib reduced the number of polyps by 28% compared to 5% for placebo.54,55 So far a number of questions remain unanswered and only meticulous control of these patients over time will allow us to tell whether the number, size and invasiveness of the remaining polyps will also be reduced and whether, in the end, these changes will prevent, or at least delay, their development into cancer. The experience from FAP and the demonstration of increased COX-2 levels in a large number of common types of cancer, and even in some pre-malignant changes, has led to a large number of ongoing clinical trials. Pre-malignant lesions with a possible in¯ammatory pathogenesis, such as Barrett's oesophagus and super®cial bladder cancer, are, as expected, primary targets. Trials of breast and prostate cancers are ongoing, as is prophylaxis against colorectal cancer after endoscopic removal of polyps in patients without FAP undergoing regular endoscopic control. The time-consuming problems of evaluating new anticancer drug strategies are even more prominent for COX-2 inhibitors than for more classical chemotherapeutics because, so far, the main focus is on early changes and prevention±which, again, will require considerable time for a proper evaluation of ecacy. However, the previous epidemiological indirect evidence of cyclo-oxygenase inhibition, the FAP experience, and the exciting, newly discovered, interference of selective COX-2 inhibitors within the common pathway of PPARd and APC tumour suppressor activity, are promising indicators of possible clinical e€ects. SUMMARY The analgesic, anti-in¯ammatory and antithrombotic e€ects of aspirin and non-steroidal anti-in¯ammatory drugs (NSAIDs) have been amply demonstrated and exploited in

830 K.-E. Giercksky

numerous ®elds of clinical medicine, and their activity has proved to be due to inhibition of the cyclo-oxygenase enzyme. Their well-known±and sometimes serious±toxic e€ects on the gastrointestinal tract have been related to the cytoprotective e€ects of locally produced prostaglandins by cyclo-oxygenase. The ®ndings were of two di€erent genes producing two variant cyclo-oxygenase enzymes, COX-1 being constitutive and protective and COX-2 being induced in in¯ammation and in neoplasia. Speci®c COX-2 inhibitors have been developed and have led to an increased interest in selective COX-2 inhibition. The potential for cyclo-oxygenase inhibitors in cancer prevention and therapy is evident from epidemiological data of long-term aspirin users, the ability of the NSAID sulindac to reduce the number of polyps in FAP, animal experiments and recent advances in the molecular pathology of colorectal cancer. FDA has now approved the selective COX-2 inhibitor celecoxib for adjuvant use in FAP after a randomized trial demonstrated a dose-dependent and signi®cant reduction in the numbers and volume of polyps. Interestingly, increased expression of COX-2 has been found in cases of early cancers (e.g. breast and prostate) and in pre-malignant changes (Barrett's oesophagus) outside the colorectal area. Due to the signi®cantly reduced side-e€ect pro®le of this new class of drugs and the experimental ®ndings of COX-2 inhibitor activity on important aspects of neoplasia such as angiogenesis and apoptosis, a large number of cancer prevention and treatment trials are now in progress.

Practice points . the anti-in¯ammatory e€ects and at least some of the antiproliferative e€ects of aspirin and NSAIDs are due to COX-2 inhibition . gastrointestinal side-e€ects are due mainly to COX-1 inhibition . selective COX-2 inhibitors seem not to induce ulcers or mucosal bleeding and have no platelet activity . COX-2 inhibitors also have e€ects on nuclear proliferation and growth signals independent of cyclo-oxygenase activity . a signi®cant dose-dependent reduction in the number and volume of polyps has led to FDA approval of the speci®c COX-2 inhibitor celecoxib for FAP

Research agenda . is COX-2 inhibition able to halt the adenoma±carcinoma sequence in colorectal polyps? . what is the long-term e€ect of continuous COX-2 inhibition with regard to normal host defence mechanisms and infectious problems? . is speci®c COX-2 inhibition a treatment alternative after macroscopic radical removal of disseminated or locally advanced cancer? . will the cost-bene®t of selective COX-2 inhibition prove to be of value as a general prevention of common cancers in otherwise healthy persons or only in speci®ed risk groups? . will further development of the activity against the non-cyclo-oxygenasedependent PPARd±APC pathway lead to new antineoplastic agents with even fewer side-e€ects?

COX-2 inhibition and prevention of cancer 831

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