Abstract: It was shown that NSAIDs, such as aspirin or Celebrex, are effective cancer preventive agents when taken regularly. However, the long-term use of NSAIDs, the cyclooxygenase (COX) inhibitors, may have significant adverse effects - primarily on the gastrointestinal (inhibiting COX-1) and cardiovascular (inhibiting COX-2) systems. Genetic analysis of enzymes (including COX) involved in the prostaglandin synthesis should reveal and predict a person's benefits vs. toxicity resulting from the NSAID treatment.
Non-steroidal anti-inflammatory drugs (NSAIDs) include compounds such as aspirin, indomethacin, piroxicam, sulindac, and ibuprofen. These are widely used to treat the signs and symptoms of inflammation, as well as rheumatic diseases. NSAIDs primarily inhibit the activity of the cyclooxygenase (COX) enzymes, which are key players in prostaglandin synthesis. Prostaglandins are multipotent signalling molecules involved in a wide range of physiological processes, including, especially, inflammation.
Inflammatory processes can be caused by both infectious and non-infectious processes, with the latter being chronic tissue injury or irritation, for example. Inflammation results in the generation of reactive oxygen species, which can damage molecules including DNA. The release of prostaglandins and other signalling molecules also elicits cellular proliferation and the growth of blood vessels. Together, these effects facilitate carcinogenesis, perhaps in a synergistic manner. Indeed, it is now clear that inflammation is a risk factor for cancer. Many cancers arise from predisposing inflammatory conditions, both those associated with microorganisms like hepatitis B (hepatitis B virus) or chronic gastritis (H. pylori) and other conditions in which microorganisms are not known to be part of the process, such as Barrett esophagus and ulcerative colitis (an inflammatory disease of the colon). Not surprisingly, aspirin and other NSAIDs have emerged as potent substances for cancer prevention (for review see Ulrich et al., 2006).
The finding that NSAIDs can prevent carcinogenesis dates back to the early 1980s, when it was reported that indomethacin could inhibit tumor growth in rats. Subsequently, numerous epidemiologic and clinical studies have confirmed the potency of aspirin and other NSAIDs in cancer chemoprevention (Sandler et al., 2003; Baron et al., 2003). These results are most compelling for cancers of the gastrointestinal tract, including the colon and esophagus, but are also seen to some extent for other cancer types. Most recently, large-scale randomized clinical trials have shown that aspirin or celecoxib (a specific COX-2 inhibitor, explained below) can successfully reduce the risk of recurrence of colorectal polyps among patients who had a first polyp removed during colonoscopy. These polyps are established precursors of colorectal cancer. The reductions in risk were substantial, up to a halving in risk for advanced lesions among those who regularly took aspirin or celecoxib, and unequivocally illustrate that NSAIDs can inhibit the progression towards cancer.
How Do NSAIDs Work on a Molecular Level?
The main mechanism of action of NSAIDs is the inhibition of COX enzymes (more specifically, the inhibition of the COX activity of prostaglandin H synthase enzymes) (see Figure 1) (Ulrich et al., 2006; Gupta and DuBois, 2001). Two closely related forms of COX exist: COX-1, which is expressed on a constitutive basis in many tissues, and COX-2, which can be induced and plays a role in many inflammatory and proliferative processes. Both COX enzymes synthesize prostaglandin H, a precursor of several prostaglandins and other eicosanoids that are produced by multiple specific synthases. The COX-enzymes are the bottleneck in this process. Prostaglandin E2 appears to be most relevant for carcinogenesis; this signalling molecule has been shown to cause tumors in mouse models of colon carcinogenesis and is connected to several other signalling pathways that are relevant to cancer formation. Cancer cells appear to utilize several mechanisms to increase the amount of pro-carcinogenic prostaglandins: for example, upregulation of COX-2 results in higher production of prostaglandins, while, conversely, the level of an enzyme that can degrade prostaglandins, prostaglandin dehydrogenase, is frequently reduced.
Two Sides of a Coin - NSAID Use Can Cause Toxicity
Unfortunately, the long-term use of NSAIDs can have harmful side effects. For example, up to a quarter of patients using aspirin and other NSAIDs long-term can experience serious gastrointestinal problems, including bleeding. Many of these side effects are thought to be related to the inhibition of COX-1, because COX-1 is relevant for the production of thromboxane A2, an eicosanoid important for platelet aggregation (see Figure 1).
To reduce the unwanted effects of NSAIDs, several companies have during the past decades actively developed NSAIDs that inhibit more specifically the COX-2 enzyme, which was assumed to be the major player in inflammation (Gupta and DuBois, 2001). Those agents are called COXibs (e.g., rofecoxib = Vioxx; celecoxib = Celebrex) and they are indeed better tolerated in relation to gastric effects. However, several COXibs were recently found to carry another serious risk, cardiovascular toxicity. Although the probability for a myocardial infarction or other cardiovascular event is small, this is clearly a risk that many healthy persons would not want to trade for reducing their cancer risk. However, for a patient with an inherited mutation in the DNA mismatch repair machinery, who has a high risk for colorectal cancer as part of a familial cancer syndrome, the benefits of NSAIDs may easily outweigh the risks. For the more general population, however, an effective chemopreventive agent needs to have minimal toxicity (Ulrich et al., 2006).
Tailoring NSAID Use With Genetic Knowledge
How can one utilize the power of NSAIDs for chemoprevention while avoiding adverse side effects?
During the past years, genetics has emerged as a tool that may help us distinguish between those who will benefit from NSAID use and those who are more likely to experience serious side effects (Ulrich et al., 2006). Individuals differ based on both inherited genetic factors and health behaviors/environmental influences. With the completion of the Human Genome Project, we are now better equipped to understand the role of genetics in cancer prevention and drug response (i.e., pharmacogenetics). As discussed above, NSAIDs most prominently target one molecular pathway, prostaglandin synthesis. It is now known that small, but common, genetic differences between individuals (called polymorphisms) exist in the enzymes of this pathway. For example, some individuals in a population have a genetic variation that results in a reduced ability to induce the COX-2 (-765G>C) enzyme, and thus these persons have a reduced response to pro-inflammatory stimuli. Similarly, certain enzymes are needed for the processing of NSAIDs in the body. Several polymorphisms in these enzymes have been identified that determine the “internal dose,” i.e., what is the achieved level and the half-life (see Figure 2; Fries et al., 2006).
Our research group has actively investigated the relationship between these polymorphisms in prostaglandin synthesis and NSAID metabolism in relation to colorectal carcinogenesis. Our intriguing initial results show that variants (i.e., the less common genetic form in the population) in the COX-2 enzyme can reduce the risk of colorectal polyps. At the same time, those who carry the variant allele do not appear to benefit further from NSAID use (Ulrich et al., 2005). Thus, it appears that the suppression of COX-2 - either genetically or with NSAIDs - reduces risk of colorectal polyps. We have observed similar results for COX-1 and prostaglandin I synthase, and for polymorphisms in the enzymes that metabolize NSAIDs (Ulrich et al., 2004; Poole et al., 2006; Bigler et al., 2001). Finally, genetic polymorphisms are likely to determine to what extent an individual produces prostaglandins that are more related to adverse side effects.
Knowledge of these genetic components will help us to tailor therapy and prevention. It is important to note that most of these variants are not rare in the population. For example, the form of COX-2 with reduced expression occurs in about every 25th person, while about every 6th person carries a variant form of COX-1 that has been shown to modify NSAID-associated risks for colorectal adenoma. At the same time, the biologic processes are a complex interplay of many proteins. We need to understand the entirety of the genetics of prostaglandin synthesis and other NSAID-related processes to obtain the most meaningful results.
NSAID Pharmacogenetics - Where Are We Heading Next?
In addition to the epidemiologic studies of NSAID-gene interactions that we and others are undertaking, it is now important to evaluate polymorphisms for their pharmacokinetic characteristics. Further, it will be immensely useful to characterize and establish the impact of these polymorphisms on drug efficacy in the cancer prevention trials (with aspirin and celecoxib), which were discussed above. Our goal is to develop a tailored strategy for cancer chemoprevention by NSAIDs: In the future, physicians may consider not only a person’s family history of cancer and current medications and diseases (e.g., high blood pressure), but also what is known about their genetically-determined capacity to process NSAIDs and the particular variants of the genes in the prostaglandin pathway that they carry (Ulrich et al., 2006). Many of the tools are in hand to allow us to work toward this goal and new high-throughput genotyping methodologies will further facilitate this process. In turn, NSAID pharmacogenetics may become a model that allows the development of other tailored therapies and preventive strategies.
References and Further Readings
Baron JA, Cole BF, Sandler RS, Haile RW, Ahnen D, Bresalier R, McKeown-Eyssen G, Summers RW, Rothstein R, Burke CA, et al. A randomized trial of aspirin to prevent colorectal adenomas. New England Journal of Medicine 348:891-899, 2003.
Bigler J, Whitton J, Lampe JW, Fosdick L, Bostick RM, Potter JD. CYP2C9 and UGT1A6 genotypes modulate the protective effect of aspirin on colon adenoma risk. Cancer Research 61:3566-3569, 2001.
Fries S, Grosser T, Price TS, Lawson JA, Kapoor S, DeMarco S, Pletcher MT, Wiltshire T, FitzGerald GA. Marked interindividual variability in the response to selective inhibitors of cyclooxygenase-2. Gastroenterology 130:55-64, 2006.
Gupta RA, DuBois RN. Colorectal cancer prevention and treatment by inhibition of cyclooxygenase-2. Nature Reviews Cancer 1:11-21, 2001.
Poole E, Bigler J, Whitton J, Potter J, Sibert J, Ulrich C. Prostacyclin synthase and arachidonate 5-lipoxygenase polymorphisms and risk of colorectal polyps. Cancer Epidemiology, Biomarkers and Prevention 15:502-508, 2006.
Sandler RS, Halabi S, Baron JA, Budinger S, Paskett E, Keresztes R, Petrelli N, Pipas JM, Karp DD, Loprinzi CL, et al. A randomized trial of aspirin to prevent colorectal adenomas in patients with previous colorectal cancer. New England Journal of Medicine 348:883-890, 2003.
Ulrich CM, Bigler J, Sparks R, Whitton J, Sibert JG, Goode EL, Yasui Y, Potter JD. Polymorphisms in PTGS1 (COX-1) and risk of colorectal polyps. Cancer Epidemiology, Biomarkers and Prevention 13:889-893, 2004.
Ulrich CM, Whitton J, Yu JH, Sibert J, Sparks R, Potter JD, Bigler J. PTGS2 (COX-2) -765G>C promoter variant reduces risk of colorectal adenoma among nonusers of non-steroidal anti-inflammatory drugs. Cancer Epidemiology, Biomarkers and Prevention 14:616-619, 2005.
Ulrich CM, Bigler J, Potter JD. Non-steroidal anti-inflammatory drugs for cancer prevention: promise, perils, and pharmacogenetics. Nature Reviews Cancer 6:130-140, 2006.
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