basil rigas and khosrow kashfi division of cancer...

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Cancer prevention: A new era beyond COX-2

Basil Rigas and Khosrow Kashfi

Division of Cancer Prevention, Department of Medicine, SUNY at Stony Brook, Stony Brook, NY

11794-5200 (BR); Department of Physiology and Pharmacology, City University of New York

Medical School, New York, NY 10031 (KK)

JPET Fast Forward. Published on April 1, 2005 as DOI:10.1124/jpet.104.080564

Copyright 2005 by the American Society for Pharmacology and Experimental Therapeutics.

This article has not been copyedited and formatted. The final version may differ from this version.JPET Fast Forward. Published on April 1, 2005 as DOI: 10.1124/jpet.104.080564

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Running Title: Cancer prevention beyond COX-2

Corresponding authors:

Basil Rigas, MD

Division of Cancer Prevention, Department of Medicine

SUNY at Stony Brook

Stony Brook, NY 11794-8160

Tel: (631) 632-9035; Fax: (631) 632-1992; e-mail: [email protected]

Number of:

Text pages: 32

Tables: 1

Figures: 2

References: 65

Words in the Abstract: 238

Words in Text: 5,685

Abbreviations: NSAIDs, nonsteroidal anti-inflammatory drugs; COX, cyclooxygenase; LOX,

lipoxygenase; PG, prostaglandin; FAP, familial adenomatous polyposis


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ABSTRACT

The seminal epidemiological observation that nonsteroidal anti-inflammatory drugs (NSAIDs)

prevent colon and possibly other cancers has spurred novel approaches to cancer prevention.

The known inhibitory effect of NSAIDs on the eicosanoid pathway prompted studies focusing on

cyclooxygenase (COX) and its products. The increased PGE2 levels and the overexpression of

COX-2 in colon and many other cancers provided the rationale for clinical trials with COX-2

inhibitors for cancer prevention or treatment. Their efficacy in the prevention of sporadic colon

and other cancers remains unknown; one COX-2 inhibitor has been withdrawn due to side

effects and there are concerns about whether this effect is class-specific. There is evidence to

suggest that COX-2 may not be the only or the ideal eicosanoid pathway target for cancer

prevention. Six sets of observations support this notion: the relatively late induction of COX-2

during carcinogenesis; the finding that NSAIDs may not require inhibition of COX-2 for their

effect; the modest effect of coxibs in cancer prevention; that currently available coxibs have

multiple non-COX-2 effects which may account for at least some of their efficacy; the possibility

that concurrent inhibition of COX-2 in non-neoplastic cells may be harmful; and the possibility

that COX-2 inhibition may modulate alternative eicosanoid pathways in a way promoting

carcinogenesis. Given the limitations of COX-2 specific inhibitors and the biological evidence

mentioned above, we suggest that targets other than COX-2 should be pursued as alternative

or complementary approaches to cancer prevention.


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Cancer, a major medical challenge during the last 100 years, has touched the life of

almost every American family and absorbs an inordinate amount of health care resources. In

the US cancer has moved to the first place among the causes of death, surpassing

cardiovascular disease. That cancer prevention is better than treatment is almost axiomatic in

medicine. In contrast to cardiovascular and infectious diseases, whose prevention has had a

substantial impact on their associated morbidity and mortality, gains in cancer prevention have

been limited. The main reasons for this can be found in two areas: identification of informative

biomarkers and development of safe and effective chemopreventive agents. In this

commentary, we present a brief overview of current efforts to develop effective

chemopreventive drugs focused on cyclooxygenase (COX) inhibition and, taking a rather

iconoclastic approach, we suggest that a search for alternative targets for drug development

may be in order.

The clear message of NSAIDs for cancer prevention and its limitations

The year 1988 marked a watershed point for cancer prevention; it was then that the first

epidemiological demonstration that nonsteroidal anti-inflammatory drugs (NSAIDs) prevent

human colon cancer established the feasibility of human cancer chemoprevention (Kune et al.,

1988). That long-term use of NSAIDs prevents an array of cancers, is now firmly established.

This conclusion is based on two sets of data: a) epidemiological studies, which document an

association between NSAID use and cancer risk; and b) interventional clinical trials

demonstrating that the administration of NSAIDs can actually prevent cancer. The

epidemiological studies reported to date (Thun et al., 2002), describing collectively results on >1

million subjects, have pointed out the protective effect of NSAIDs against colon (Fig. 1),

esophageal, gastric, breast (estrogen receptor positive) and perhaps pancreatic and ovarian

cancers. For colon cancer, two randomized interventional studies using polyp recurrence as a


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general end point demonstrated the preventive effect of aspirin (Baron et al., 2003; Sandler et

al., 2003). The relative risks following administration of aspirin ranged between 0.59 and 0.96,

depending on the specific end point and aspirin dose. Several aspects of this effect appear

unclear at this point, but the overall significance of these landmark studies proving the general

concept of chemoprevention by NSAIDs cannot be overstated.

What these studies have failed to provide is the details of the big picture: which one of

the nearly 30 NSAIDs is the most effective, what is the optimal dose and what is the ideal

schedule of administration? Even if such information existed, however, it would probably be of

limited or no practical value. The reason is that, although we have unassailable proof of

principle for chemoprevention, current NSAIDs cannot overcome two prohibitive limitations

concerning their safety and efficacy. For example, for colon cancer, the one most thoroughly

studied, NSAIDs can prevent at best 50% of the cases (Thun et al., 2002). Although no precise

numbers are available for the incidence of the side effects of NSAIDs, it seems that among

patients using NSAIDs, up to 4% per year suffer serious gastrointestinal complications

(Bjorkman, 1999). The issue of safety is clearly underscored by the report that in 1998 in the US

the number of deaths from NASAID-induced gastrointestinal complications was virtually equal to

the number of deaths from AIDS (Singh and Triadafilopoulos, 1999).

These considerations should be viewed against the fundamental distinction between

chemotherapy and chemoprevention. In chemotherapy both patient and physician are prepared

(and justified) to accept substantial treatment-related toxicity to save the patient’s life from a fully

developed cancer. In contrast, chemoprevention agents will be often prescribed to a healthy

subject for a cancer he/she may never develop. Thus, safety and efficacy assume a different

value and criteria for them ought to be more demanding than those applied to chemotherapy.

The ideal chemopreventive agent ought to have an efficacy approaching 100% and a safety

profile that is nearly perfect. Such perfection is, however, probably unattainable and the

challenge may be simply to define what would constitute an acceptable deviation from the ideal


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compound. Agents with imperfect performance will be judged on the basis of their individual

characteristics and against competing agents. Aspirin, for example, is widely employed for the

prevention of cardiovascular thrombotic events, although its efficacy is far from perfect (e.g. it is

only 28% for primary prevention) and the side effects even of the low dose of 81 mg daily are

not insignificant [reviewed in (Kimmey, 2004)].

Efficacy, safety, cost and ease of administration will likely be the main criteria for

candidate chemoprevention agents. A fifth criterion that may be employed in the future is the

degree of risk of developing a given cancer. It is quite possible that genetic testing will be able

to quantify risk as well as the likelihood for a given subject to respond to an agent, thus making

possible individualized cancer prevention. Whether articulated or not, considerations of safety

end efficacy have spurred the search for a “better NSAID”, with coxibs, selective inhibitors of

COX-2, being the most notable outcome.

COX-2 as a target for cancer prevention: Pros and cons

The development of coxibs has been based on the notion that inhibition of COX-2, the

induced isoform of COX, will diminish the proinflammatory activities of COX, whereas sparing

COX-1, the constitutive isoform of COX, will diminish the gastrointestinal and perhaps other side

effects of nonsteroidal anti-inflammatory drugs (Simmons et al., 2004). The observation that

COX-2 is overexpressed in cancer led to the idea that coxibs could serve as excellent

chemoprevention agents. The pros and cons for this concept are discussed below.

The pros

The best known biochemical effect of NSAIDs is inhibition (suicide or competitive) of

COX, an important enzyme in the eicosanoid cascade that ultimately leads to prostaglandins

(PGs) and related compounds (Simmons et al., 2004). This provided an immediate and


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plausible explanation of the epidemiological data on NSAIDs and cancer prevention. To

substantiate this conjecture, we determined the levels of various eicosanoids in human colon

cancers showing that the most striking abnormality was elevated PGE2 levels compared to

uninvolved mucosa, confirming earlier findings by Bennett et al, who had used somewhat crude

analytical methodologies (Bennett et al., 1987; Rigas et al., 1993). Subsequently, DuBois and

colleagues demonstrated overexpression of COX-2 in 45% of colon adenomas and 85% of

colon carcinomas (Eberhart et al., 1994). Overexpression of COX-2 has been demonstrated for

several human cancers, besides colorectal adenocarcinomas , including gastric, breast, lung,

esophageal and hepatocellular carcinomas (Turini and DuBois, 2002). Given the potentially

enormous significance of this finding, a recent study assessed the possibility that single

nucleotide COX-2 polymorphisms may be associated with disease or with individual responses

to drug therapies. Lin et al have reported reduced an association between the COX-2 V511A

polymorphism and reduced susceptibility to colon cancer in African Americans (Lin et al., 2002).

Sequencing of the COX-2 gene of 72 individuals identified rare polymorphisms, including the

V511A polymorphism which, however, was not functionally important (Fritsche et al., 2001).

Neither hypothesis, that polymorphism in the COX-2 gene could contribute to individual

susceptibility to cancer nor that COX-2 polymorphisms may have an impact on individual

response to NSAIDs, appears to be supported. Polymorphisms in other genes of the eicosanoid

pathway may account for inter-individual differences in cancer susceptibility or response to

NSAIDs.

In terms of mechanism, we showed that PGE2 increased colon cancer cell proliferation

(Qiao et al., 1995), while DuBois’ group showed that it suppressed apoptosis (Sheng et al.,

1998). The role of eicosanoids in carcinogenesis has been further expanded by studies

demonstrating that in certain cases, lipoxygenase (LOX) products may also play a role in

carcinogenesis (Shureiqi and Lippman, 2001) (Fig 2). It was, therefore, reasonable to conclude


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that inhibition of COX-2 would arrest carcinogenesis and thus would a) prevent cancer

development, and b) regress cancer once developed.

A series of detailed cell culture, animal studies and published clinical trials on the use of

coxibs in patients with familial adenomatous polyposis (FAP) and in patients with cancer,

culminated in one COX-2 inhibitor, celecoxib, receiving FDA approval for cancer prevention in

patients with FAP.

The supportive animal studies Two sets of animal studies support the role

of COX-2 in carcinogenesis a) studies using genetically modified animals, and b) studies using

pharmacological inhibitors of COX-2. They are summarized below.

Although COX enzymes may not be acting simply as oncogenes in tumor development

[reviewed in (Simmons et al., 2004)], several studies have indicated that they may be required

for tumorigenesis. Oshima et al showed that deletion of COX-2 decreased significantly the

number of intestinal tumors in Apc 716 mice (Oshima et al., 1996). Deletion of COX-1, however,

also attenuated tumor formation in the same mice (Chulada et al., 2000). A stronger case was

made by the studies of Hla’s group, who showed that multiparous but not virgin female

transgenic mice that overexpress the human COX-2 gene in the mammary glands exhibited a

greatly exaggerated incidence of focal mammary gland hyperplasia, dysplasia, and

transformation into metastatic tumors (Liu et al., 2001). The clear implication from these data is

that enhanced COX-2 expression is sufficient to induce mammary gland tumorigenesis. Studies

have also addressed whether COX-2 is critical for skin carcinogenesis. Overexpression of COX-

2 in basal epidermal cells of transgenic mice (using the keratin 5 promoter), which leads to high

levels of epidermal PGs, is insufficient for tumor induction but sensitizes the tissue to genotoxic

carcinogens (Muller-Decker et al., 2002). In contrast, transgenic mice that overexpress COX-2

(under control of the keratin 14 promoter), in the epidermis and some other epithelia, developed

tumors at a much lower frequency than did their littermate controls; tumors were induced by an


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initiation/promotion protocol (Bol et al., 2002). The latter results raised questions regarding the

role of COX-2 in skin tumor development.

There have been numerous animal studies using specific COX-2 inhibitors providing

support for the concept that COX-2 inhibition both prevents and regresses tumors arising from a

variety of tissues, including colon, lung, breast, pancreas, skin and others [reviewed in

(Dannenberg and Subbaramaiah, 2003)]. Perhaps the most dramatic result was reported by

Reddy’s group who studied the effect of celecoxib on colon carcinogenesis in the azoxymethane

rat model. Remarkably, dietary administration of celecoxib inhibited both the incidence and

multiplicity of colon tumors by about 93% and 97%, respectively. It also suppressed the overall

colon tumor burden by more than 87%.

Taken together these studies (along with many more not reviewed here) provide a

spectrum of evidence for the role of COX-2 in carcinogenesis, ranging from the compelling

(breast carcinogenesis) to the rather weak and controversial (skin carcinogenesis). Overall, the

evidence from animal studies suggests strongly that COX enzymes, particularly COX-2,

participate in a significant way in carcinogenesis.

The cons

There are six sets of observations that challenge the intellectual beauty and utter

simplicity of the notion that COX-2 is central to the pathogenesis of several cancers, and that its

inhibition would prevent them and regress those already established. They include a) the

relatively late induction of COX-2 during carcinogenesis; b) the finding that NSAIDs, which

prevent cancer, may not require inhibition of COX-2 for their effect; c) the modest effect of

coxibs in cancer prevention; d) the fact that currently available coxibs have multiple non-COX-2

effects which may account for at least some of their efficacy; e) the possibility that concurrent

inhibition of COX-2 in non-neoplastic cells may be harmful; and f) the possibility that COX-2


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inhibition may modulate alternative eicosanoid pathways in a manner that promotes

carcinogenesis. Below, we discuss briefly these points as well as the potential role of the large

family of eicosanoids in carcinogenesis.

Pattern of COX-2 expression The pattern of COX-2 expression during colon

carcinogenesis does not entirely fit the model that COX-2 is central to carcinogenesis. There is

no COX-2 expression in human aberrant crypt foci, the earliest recognizable premalignant

lesion in the colon (Nobuoka et al., 2004). For COX-2 to be the ideal chemoprevention target, its

expression in aberrant crypt foci would be highly advantageous, if not a requirement, as

chemoprevention would be easiest at this stage of lower complexity. Moreover, the expression

of COX-2 commences only at the adenoma stage and in less than half of them, increasing as

the transition to cancer progresses and becoming positive in 85% of them (Eberhart et al.,

1994).

This pattern of COX-2 overexpression is subject to an alternative interpretation, namely

that COX-2 expression is the result of and not a dominant contributor to carcinogenesis. The

strongest argument against this interpretation are the animal studies demonstrating that

disruption of either the COX-1 or COX-2 gene reduces the development of tumors in mice.

Interestingly, Chulada et al proposed that since COX-1 (as well as COX-2) plays a key role in

intestinal tumorigenesis it may also be a chemotherapeutic target for NSAIDs (Chulada et al.,

2000). A counterargument to this is the recently reported study in which targeted

overexpression of human microsomal PGE synthase-1 (mPGES) in the alveolar type II cells of

transgenic mice, accompanied by highly elevated PGE2 production (12.2 fold over control), was

not sufficient to induce lung tumors (Blaine et al., 2004).

An interesting idea has been advanced by Takeda et al who demonstrated that most

COX-2-expressing cells in the polyps of Apc∆716 mice are stromal fibroblasts in which COX-1,

COX-2 and mPGES colocalized. COX-2 was induced only in polyps >1 mm in diameter, but

COX-1 was found in polyps of any size. Based on these data, they proposed that COX-1


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expression in the stromal cells secures the basal level of PGE2 that can support polyp growth to

~1 mm, and that simultaneous inductions of COX-2 and mPGES support the polyp expansion

beyond ~1 mm by boosting the stromal PGE2 production (Takeda et al., 2003).

COX-independent effects of NSAIDs At variance with the notion that COX-2 has

a central role in carcinogenesis is the concept, originally proposed by us, that NSAIDs do not

require the presence of COX-2 to prevent cancer (Hanif et al., 1996). This was based on the

finding that in vitro NSAIDs display effects compatible with cancer prevention such as inhibition

of cell proliferation, induction of apoptosis, inhibition of angiogenesis and many others, in the

absence of COX-1 or -2. This initial observation is now firmly established by many studies

pointing to an array of molecular targets affected by NSAIDs (Shiff and Rigas, 1999). It is

unclear which one(s), if any, of them is the pathway(s) mediating the chemopreventive effect of

NSAIDs. Quite astonishing in this regard has been our recent observation that a modified

aspirin inhibited cell proliferation, induced apoptosis, inhibited NF-κB activation and Wnt

signaling while inducing COX-2 expression without affecting COX-1 expression (Williams et al.,

2003). Of note, such COX-2 was catalytically competent as evidenced by the proportional

increase in PGE2 production.

A reservation concerning the non-COX effects of NSAIDs is that they occur only at

“industrial strength” concentrations of NSAIDs (Marx, 2001). This is not, however, entirely

accurate or necessarily relevant. For example, some NSAIDs, have IC50s for the inhibition of

colon cancer cell growth in the µM range, with probably sulindac sulfide, the active metabolite of

sulindac, having the lowest (about 175 µM) and ASA the highest (2.5 mM); the IC50 for sulindac

approaches 1 mM (Shiff et al., 1995). The corresponding IC50 for some COX-2 inhibitors

appears to be around 30 µM (Grosch et al., 2001). Thus there is a clear discrepancy between

cell culture results and pharmacological efficacy. These observations prompt a reminder of the

limitations of the cell culture system, whose findings should be viewed for what they are: a mere

indication of what may happen in the complexity of the human organism. For example, as


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discussed below, sulindac is more effective in the prevention of colon cancer in patients with

FAP than coxibs, a complete reversal of the conclusions that might have been reached from the

cell culture data. It should also be kept in mind that cell culture systems are by their very nature

time sensitive. Most studies using cell culture systems have to be terminated by 72 to 96 hours

and thus high concentrations of an agent are at times necessary to detect an effect. In contrast

in in vivo chemoprevention studies comparatively lower doses of an agent are administered for

much longer periods of time and the effect of the agent may thus become apparent. The “low

dose - long duration” or “high dose - short duration” balance needs to be considered so that

potentially useful agents are not abandoned unjustifiably.

The limited clinical efficacy of coxibs When the COX-2 inhibitor celecoxib

was used in FAP, suppression of the neoplastic process was modest. After six months, FAP

patients receiving 400 mg of celecoxib twice a day had a 28.0% reduction in the mean number

of colorectal polyps (P=0.003 for the comparison with placebo) and a 30.7% reduction in the

polyp burden (the sum of polyp diameters) (P=0.001), as compared with reductions of 4.5% and

4.9%, respectively, in the placebo group (Steinbach et al., 2000). Rofecoxib had a statistically

significant but marginal effect on the number of polyps in FAP patients (6.8% reduction from

baseline values) (Higuchi et al., 2003). Another study assessed the effect of celecoxib on

duodenal polyps in FAP patients (Phillips et al., 2002). There was a trend to improvement in

duodenal disease when absolute adenoma numbers were assessed but this was not statistically

significant. However, when taking a global overview of the state of the duodenum, overall,

patients taking celecoxib 400 mg twice daily for 6 months had a 14.5% reduction in involved

areas compared with a 1.4% for placebo (p=0.436). However, patients with clinically significant

disease at baseline (greater than 5% covered by polyps) showed a 31% reduction in involved

areas with celecoxib 400 mg twice daily compared with 8% on placebo (p=0.049)

These results could be interpreted in one of two ways: either COX-2 is not central to the

neoplastic process or celecoxib is not a sufficiently strong COX-2 inhibitor. However, all


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available clinical and preclinical evidence, suggests that it is a strong COX-2 inhibitor [reviewed

in (Dannenberg and Subbaramaiah, 2003)]. Of interest, the dose required to inhibit COX-2

activity by 80% also causes a 60% inhibition in COX-1 activity (Warner et al., 1999).

Sulindac, an NSAID, had a pronounced effect on colorectal polyps in FAP patients. A

statistically significant decrease in the mean number of polyps and their mean diameter

occurred in patients treated with sulindac (150 mg orally twice a day), as compared with those

given placebo. When treatment was stopped at nine months, compared to baseline values, the

number of polyps had decreased by 56% and the diameter of the polyps by 65% (Giardiello et

al., 1993). Furthermore, Winde et al performed a pilot study using rectally applied sulindac in

colectomized FAP patients (Winde et al., 1995). All patients responded to therapy within 6 to 24

weeks. Sixty and 87% of patients achieved complete adenoma reversion after 12 months at 53

and 67 mg of sulindac per day per patient on average, respectively. Reversion was evident

compared with the control group. Tissue PGE2 levels were greatly reduced. It has also been

shown that long-term use of sulindac is effective in reducing polyp number and preventing

recurrence of higher-grade adenomas in the retained rectal segment of most FAP patients

(Winde et al., 1997; Cruz-Correa et al., 2002). Although the coxib and sulindac studies are not

directly comparable, given the different duration of drug administration, it appears that sulindac

may have a stronger effect than coxibs, at least compared to rofecoxib. These findings suggest

that inhibition of targets other than or in addition to COX-2, may contribute to sulindac’s stronger

chemopreventive effect in the colon.

Another disappointing result, although not directly related to chemoprevention, was the

recently reported failure of celecoxib combined with trastuzumab to have an effect on patients

with HER2/neu-overexpressing, trastuzumab-refractory metastatic breast cancer (Dang et al.,

2004). Preclinical studies had demonstrated a link between overexpression of HER-2/neu and

COX-2 activity. Similarly, in a phase II study of metastatic colon cancer, rofecoxib in


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combination with chemotherapy showed increased toxicity and no efficacy (Becerra et al.,

2003).

COX-2-independent effects of coxibs There have been extensive data

establishing that coxibs have several COX-2 independent activities which may account, at least

for part of their cancer preventive properties (Table 2). Relevant to this argument are studies

showing that, for example, celecoxib inhibits the growth of various cancer cell lines (Maier et al.,

2004) including hematopoietic cell lines (Waskewich et al., 2002) that are COX-2 deficient.

Moreover, celecoxib inhibited the growth of COX-2 deficient colon cancer xenografts in nude

mice, providing strong evidence for its COX-2 independent antitumor effect (Grosch et al.,

2001). Quite recent data suggest a disturbing COX-2 independent effect of selective COX-2

inhibitors. While in COX-2 positive pancreatic cancer, a selective COX-2 inhibitor reduced tumor

growth and angiogenesis, the opposite effect was observed in COX-2 negative pancreatic

cancer. That is to say, the selective COX-2 inhibitor increased angiogenesis and tumor growth

(Eibl et al., 2005).

These findings allow an alternative interpretation of the existing data, one that is at

variance with the idea that COX-2 is a preeminent, if not altogether the target for cancer

prevention. Thus the chemopreventive effect of COX-2 specific inhibitors may be due to their

effect on these targets and not on COX-2. If this reasoning is correct (and it may not be), it may

reflect the classic limitation encountered when evaluating biology using pharmacological

inhibitors: additional effects by the inhibitor can never be ruled out, making conclusions always

subject to re-interpretation.

A few years ago we proposed a model that attempted to resolve the apparent

contradiction between data suggesting that both COX-2 and non-COX-2 inhibition were

preventing cancer (Rigas and Shiff, 2000). Analysis of the then-available data led us to suggest

that inhibition of either the COX-2 pathway alone or of a non-COX-2 pathway(s) alone could

prevent cancer. We are still missing some of the information needed to validate or reject this


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model, mainly results of trials evaluating the efficacy of COX-2 inhibitors and their effects on

relevant molecular targets. The limited efficacy of COX-2 inhibitors in FAP and the in vitro

evidence that COX-2 inhibitors act also beyond COX-2 inhibition suggest that the first part of our

conclusion, i.e., that COX-2 inhibition alone is sufficient to arrest carcinogenesis, may be wrong.

In fact, it would not be altogether surprising if it is COX-2 that is the “collateral target” in cancer

prevention.

Non-neoplastic COX-2 expression The expression of COX-2 is not restricted to

tumor cells. While COX-2 is undetectable in most tissues in the absence of stimulation, it is

induced in a limited repertoire of cells, notably in monocytes, macrophages, neutrophils, and

endothelial cells (Simmons et al., 2004). Among the stimulants of COX-2 induction are bacterial

lipopolysaccharides (LPS), growth factors, cytokines, and phorbol esters. Relevant to the

cardiovascular side effects of coxibs may be the observations that COX-2, although detectable

at only low levels in vascular endothelium, may be expressed in response to normal blood flow

(Topper et al., 1996). A study in healthy humans after administration of celecoxib or ibuprofen

suggests that COX-2 is a major source of systemic prostacyclin biosynthesis (McAdam et al.,

1999). High levels of COX-2 are detected in activated and proliferating vascular tissues, such as

angiogenic microvessels and atherosclerotic tissues. Atheromatous lesions contain both COX-1

and COX-2, colocalizing mainly with macrophages of the shoulder region and lipid core

periphery, whereas smooth muscle cells show lower levels (Schonbeck et al., 1999).

There is an interesting corollary to these observations, which may impact the design of

pharmacological strategies aimed at this extensive cascade of enzyme-catalyzed reactions.

Eicosanoids often form “opposing pairs” whose balance determines the final result. A classic

pair consists of thromboxane A2 and prostacyclin, which have opposite effects on platelets and

vascular tone (Simmons et al., 2004). Shifting the balance of such a pair could have either

beneficial (e.g., prevention of cardiovascular events by low dose aspirin) or catastrophic effects.

The latter possibility, discussed below, may account for the cardiovascular side effects of


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coxibs. This may be particularly relevant to chemoprevention, in which a chemopreventive agent

against cancer will be administered on a long-term basis to older subjects, i.e., those likely to

have atheromatous lesions.

COX-2 inhibition may modulate alternative eicosanoid pathways The concept

of the role of COX-2 in cancer should be viewed against the accumulating appreciation of the

role of the eicosanoid pathways in carcinogenesis. An intricate system of talented enzymes,

including phospholipases, cytochrome P450, COX, lipoxygenases (LOX) and the so-called

“terminal enzymes”, i.e., those converting endoperoxides to end products, generates an array of

biologically active eicosanoids from polyunsaturated fatty acids such as arachidonic and linoleic

acids (Fig. 2). At times these eicosanoids have antithetic functions. Besides products of COX

isoforms, LOX products may be important in carcinogenesis. Some LOX products have pro-

tumorigenic activities while others are anti-tumorigenic (Shureiqi and Lippman, 2001). Some of

the “terminal enzymes” may have their own distinctive role in cancer. For example, increased

pulmonary production of PGI2 (prostacyclin) by lung-specific overexpression of prostacyclin

synthase, which operates downstream of COX, decreases lung tumor incidence and multiplicity

in both chemically induced murine lung cancer models and in a tobacco smoke exposure model

(Keith et al., 2002; Keith et al., 2004). Such findings suggest the possibility that COX inhibition

may not be always desirable for cancer control.

Inhibition of COX may shift its substrate fatty acid to a non-COX pathway and generate a

pro-carcinogenic end product. There is little evidence for substrate channeling from the COX

active site to the peroxidase active site of the COX monomer. Functional coupling between

COXs and phospholipase A2s has, however, been described (Murakami et al., 2002) as has

been colocalization of COX isoforms with other enzymes of the eicosanoid cascade [e.g., (Liou

et al., 2001)]. Thus a case can be considered where inhibition of COX-2 could shift arachidonic

acid to the LOX pathway thereby suppressing apoptosis -- not a desirable effect in cancer

prevention. A recent human study suggested that oral celecoxib increased LTB4 production in


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the lung microenvironment, under physiologic conditions, although the functional significance of

this effect was uncertain (Mao et al., 2004).

These seemingly subtle issues emphasize the complexity of the system and suggest the

need for caution in devising chemopreventive strategies. Viewed more creatively, this situation

may also represent a pharmacological opportunity; indeed, multi-pathway inhibition may be the

next wave in chemoprevention. Such considerations have led to the development of dual

inhibitors of both COX and 5-LOX. Licofelone, a compound blocking both a LOX and COX is

already in clinical trials.

It is apparent from these considerations that the discovery of COX-2 overexpression in

various cancers has provided a strong stimulus in the last decade to unravel many pathways

likely related to cancer pathogenesis and sharpen the focus on cancer prevention as a realistic

option. However, the central concept of a dominant role of COX-2 in cancer prevention may

have significant limitations that necessitate its reappraisal. In terms of practical applications, its

Achilles’ heel may be the fact that COX-2 expression is not cancer-restricted, and thus long-

term inhibition of COX-2 for cancer control may be associated with limiting or even

unacceptable side effects. It is conceivable that cancer prevention achieved by current coxibs is

to some extent due to their non-COX-2 effects, while their cardiovascular toxicity is due to their

COX-2 effects. This notion could provide the basis for an alternative look at the wealth of

available data.

The withdrawal of rofecoxib and its implications for cancer prevention

In some patients, NSAIDs inhibit growth of colorectal adenomas. Hence, the APPROVe

(Adenomatous Polyp Prevention on Vioxx) study was launched to evaluate the efficacy of


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rofecoxib in this group of patients. Two thousand six hundred subjects with previously removed

colorectal polyps were randomly assigned to receive rofecoxib or placebo. The data showed

that 3.5% of rofecoxib recipients and 1.9% of placebo recipients suffered myocardial infarctions

or strokes during the trial. This prompted the termination of this and all related trials and the

permanent withdrawal of rofecoxib. This has been a setback to the use of COX-2 inhibitors to

prevent colon and perhaps other cancers. At this time, it is still unclear whether or not the COX-

2 inhibitor celecoxib shares this side effect and is still marketed. Although the CLASS

(Celecoxib Long Term Arthritis Safety Study) trial showed this agent to have fewer

gastrointestinal side effects and no increase in cardiovascular risk at 6 months, the 12 month

data suggest that celecoxib did not differ from the traditional NSAIDs in its GI effects and,

importantly, a retrospective analysis of the data suggests signs of increased cardiovascular risk

(Silverstein et al., 2000). This has prompted concerns for a “class (side) effect” (Fitzgerald,

2004). The mechanism of the increased cardiovascular risk is not yet clear. It has been

suggested that since COX-2 is the principal enzyme involved in the production of PGI2

(prostacyclin), its inhibition by COX-2 inhibitors could increase cardiovascular risk by tipping the

balance toward platelet aggregation and vasoconstriction (Fitzgerald, 2004). Of note, an elegant

recent study showed that COX-2 derived prostacyclin confers atheroprotection on female mice,

suggesting that chronic treatment of patients with selective inhibitors of COX-2 could undermine

protection from cardiovascular disease in premenopausal females (Egan et al., 2004).

Two important considerations merit mention here. First, strictly speaking, the COX-2

specific inhibitors may be only COX-2 preferential inhibitors (Vane and Warner, 2000). Second,

individual variations between COX-2 inhibitors may be due to their varied effect on molecular

targets outside COX-2. Parenthetically, this may account for the presumed differential behavior

of celecoxib, compared to, for example, rofecoxib. Indeed, as mentioned earlier, these agents

do have COX-2 independent activities that may account for their cancer preventive properties.


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Targets beyond COX-2: The rationale and the promise

That NSAIDs and COX-2 specific inhibitors alike modulate targets other than COX-2 is

now beyond doubt and should be factored in when analyzing their effects on cancer prevention.

The data discussed above make it unlikely that inhibition of COX-2 alone could prevent cancer;

even COX-2 specific inhibitors may require the contribution of their non-COX-2 effects for their

(probably suboptimal) effect on cancer prevention. Combined with the recent withdrawal of one

COX-2 inhibitor, these data make a strong case to evaluate for drug development targets

beyond COX-2. Of note, Niitsu and his group in Sapporo have actively pursued the idea that

modulating targets other than COX-2 can prevent cancer. In a series of elegant studies, they

have shown that the phase II enzyme glutathione-S-transferase P1-1 may be an appropriate

target for chemoprevention (Niitsu et al., 2004; Nobuoka et al., 2004).

Whenever compounds, such as the various NSAIDs, modulate multiple pathways, it is

crucial to assess the mechanistic relevance of each pathway. This is more so in the case of

cancer that represents a process rather than an abrupt transition from normalcy to malignancy.

In this context, and examined from the viewpoint of the drug, it is important to resolve the issues

of dominance versus cooperation versus redundancy of pathways. It is conceivable that only

one of the pathways affected by the drug may actually mediate the effect under study

(dominance), the remaining being either minor accessories or simple downstream effects. In

case of cooperation, concerted modulation of more than one pathway is needed to achieve the

result, while in case of redundancy each of several pathways could autonomously arrest the

process. Redundancy is of particular interest when dealing with cancer, where several sub-

clones, each with different biochemical profiles, can develop “under pressure”, mainly from the

host or from treatment. An agent displaying mechanistic redundancy could affect multiple sub-

clones and thus is better equipped to arrest or abort the carcinogenic process.


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This line of reasoning underscores the need to explore targets beyond COX-2,

especially in view of emerging limitations of the COX-2 approach. Developing a rational

approach to cancer prevention would likely require a strategy capitalizing on two facts: a)

NSAIDs do prevent cancer in humans, and b) NSAIDs act on multiple molecular targets, of

which COX-2 is but one. With this as a point of departure, it is clear that the study of the non-

COX-2 targets is likely to point to opportunities for cancer prevention hitherto unappreciated.

Inherent in this approach is the need to resolve issues of mechanistic dominance, cooperation

and redundancy and then make informed choices for drug development. This represents the

promise of “molecular targets beyond COX-2”.

Concluding remarks

The preceding analysis makes three conclusions clear. First, conventional NSAIDs

prevent colon and other cancers most likely by modulating several molecular targets; their

safety and efficacy limitations, however, preclude their application to humans for this purpose.

Second, focusing exclusively on COX-2 inhibition may not be an effective strategy for cancer

prevention. Third, exploring targets beyond COX-2 is likely to yield a productive approach to

cancer prevention.

The most pressing need in cancer chemoprevention is to identify agents or combinations

of agents that combine high efficacy with minimal toxicity. Although this may represent a difficult

goal, the enormity of the “cancer burden” to our society should compel us to strive for it. The

development of successful chemoprevention agents seems to require a strategy that not only

generates mechanistic insights, but also advances them rapidly to their clinical evaluation. The

work on COX-2 to date constitutes an excellent example of an effort based on this premise. As

the results of trials using coxibs for sporadic colon cancer prevention are not available, the


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ultimate jury on COX-2 inhibitors is still out. The balance of their risk/benefit relationship will

need to be assessed. In case these studies do not provide compelling results or COX-2

inhibitors are limited by side effects, we believe that expanding our focus to targets beyond

COX-2 will provide alternative or complementary approaches, the latter representing

mechanism-driven drug combinations.

In addition to its apparent rationality, this approach rests on the solid fact that traditional

NSAIDs prevent several cancers in humans. Thus it is a reasonable inference that if the

molecular targets mediating their effect are identified, it will be feasible to enhance this effect

with new agents that will meet the general criteria of a successful chemopreventive agent. A

new era may have already begun.


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Grant Support: This work was supported by the NIH Grants CA92423 and CA34527 and PSC-

CUNY Grant 65201-00 34


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FIGURE LEGENDS

Fig. 1. Epidemiological cohort studies of the association between NSAID use and colorectal

cancer or adenomatous polyps. Circles: relative risk estimates; lines: 95% confidence

intervals. Adapted from (Thun et al., 2002).

Fig 2. Overview of the eicosanoid pathway. Arachidonic acid, the substrate of three major

biosynthetic pathways, is derived from diet, is released from membrane phospholipids

through a series of reactions requiring phospholipases, or is synthesized from linolenic

acid. The COX pathway produces various eicosanoids and thromboxane; the LOX

pathways produce leukotrienes and hydroxyeicosatetraenoic acids; and the cytochrome

P450 pathways produce EET and dihydroxyacids.

Abbreviations: Phospholipases A2, C, and D, PLA2; PLC; PLD; Prostaglandins, PGE2,

PGF2α, PGD2; Prostacyclin, PGI2; Thromboxane A2, TxA2; Leukotrienes, LTA4, LTB4,

LTC4, LTD4, LTE4; 13-S-hyroxyoctadecadienoic acid, 13-S-HODE; Epoxyeicosatrienoic

acid, EET.

T-shaped arrows denote inhibition. Broken arrow: putative pathway.


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Table 1

Selected COX-2 independent effects of coxibs relevant to cancer

Effect Reference

Inhibition of cell proliferation (Badawi et al., 2004)

Induction of apoptosis (Ding et al., 2005)

Induction of cell cycle block (Maier et al., 2004)

Survivin expression (Teh et al., 2004)

Cytochrome c release (Sun et al., 2002)

NAG-1 induction (Yamaguchi et al., 2004)

Carbonic anhydrase inhibition (Weber et al., 2004)


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0.5 1 1.5

Relative Risk�

Thun, 1991, men

Thun, 1991, women

Gridley, 1993

Pinczowski, 1994

Schreinemachers, 1994

Gioovannucci, 1994

Giovannucci, 1995

Kauppi, 1996

Smalley, 1999

Collet, 1999

Langman, 2000

Garcia-Rodriguez, 2001

Fig 1

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Arachidonic Acid

PLA2PLCPLD

Activation

Stimuli

EET Dihydroxy

acids

Cyt P450epoxygenase

COX-1Constitutive

COX-2Inducible

PGE2, PGF2α, PGI2, TxA2

Homeostasis

PGE2, PGI2Inflammation

NSAIDs Coxibs

5-LOX

LTA4

LTB4

Adhesion

LTC4, D4 ,E4

Vasoconstriction

15-LOX-1

Linoleic Acid

13-S-HODE

Proliferation

COX-3Brain

PGD2

Fever, Pain

Paracetamol

?

ZileutonCaffeic acid

Membrane Phospholipids

Licofelone

Steroids

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