In this large, randomized, placebo-controlled trial, celecoxib was associated with a decrease in the three-year cumulative incidence of adenoma among patients with wild-type, variant CYP2C9*2 (≥ one R144C) or CYP2C9*3 (≥ one I359L allele) genotypes of CYP2C9, the principal enzyme responsible for celecoxib metabolism from the active to inactive state. However, compared with the lower dose, the additional benefit of the higher dose was restricted to those with the CYP2C9*3 genotype. Although statistical power was limited by the small number of events, it also appeared that among patients with variant genotypes, the increased incidence of cardiovascular and thrombotic events was primarily observed with high-dose celecoxib but not low-dose. In contrast, among patients with wild-type genotypes, both doses of celecoxib appeared associated with risk. There did not appear to be a significant independent main effect of genotype on risk of adenoma or cardiovascular events.
Our results regarding a differential effect of celecoxib on adenoma recurrence according to CYP2C9 genotype are consistent with our present understanding of the role of CYP2C9 in phase 1 drug metabolism and the functional alterations associated with variant genotypes.5–13
The CYP2C9 enzyme metabolizes celecoxib from its active state to an inactive form; the CYP2C9*3 genotype is associated with significantly greater impairment of this metabolism than either the CYP2C9*2 or wild-type genotype. Thus, high-dose celecoxib may overwhelm a diminished capacity to inactivate drug among individuals with CYP2C9*3 genotypes. This may lead to substantially increased accumulation of active celecoxib, in turn potentiating greater anti-cancer efficacy. In contrast, the metabolic capacity of patients with wild-type or CYP2C9*2 genotypes may be sufficient to inactivate high-dose and low-dose celecoxib with equivalent efficiency. Thus, for individuals with these genotypes, high and low doses of celecoxib may yield comparable levels of bioavailability, resulting in no clinically apparent difference in therapeutic effectiveness.
To our knowledge, our study is the first to examine the relationship between CYP2C9 genetic variants and randomly assigned celecoxib use with clinical outcomes. In support of our findings, previous work demonstrated the relevance of CYP2C9 genetic variation with other drug substrates and endpoints. Several studies have shown that CYP2C9 genetic variants are associated with decreased elimination of warfarin, resulting in an increased risk of overanticoagulation and complications from bleeding.11, 21–23
Previous case-control studies have also demonstrated that compared with wild-type, individuals with CYP2C9 variant genotypes have a lower risk of colorectal cancer associated with ibuprofen,24
as well as an increased risk of gastroduodenal bleeding related to NSAIDs.17
Interestingly, although there did not appear to be an overall independent effect of CYP2C9 genotype on risk of adenoma, patients in our study with the CYP2C9*2 genotype appeared to have a somewhat higher risk of adenoma in the subgroup using low-dose aspirin. Previous studies have suggested that CYP2C9 variant genotype may be independently associated with adenoma risk through differential metabolism of carcinogens and endogenous prostanoids or induction of COX-2.13, 16, 25–27
This finding may also be explained by an effect of the *3 variant allele in CYP2C8, which is in strong linkage with the CYP2C9 *2 allele.28
In support of this hypothesis, we did not observe any increase risk of adenoma related to the CYP2C9*3 variant, which is not strongly linked to the *3 allele of CYP2C8. The *3 allele of CYP2C8 has been associated with impaired metabolism of arachidonic acid, which influences adenoma risk as well as vascular tone, potentially also explaining the higher prevalence of baseline hypertension among patients in this trial with CYP2C9*2 genotypes.29
Given the importance of the APC trial in identifying unexpected toxicity associated with celecoxib,4
we also examined the influence of CYP2C9 genetic variation on risk of adverse events. For both renal and hypertensive events, there did not appear to be a consistent differential effect of celecoxib according to genotype. We also did not find a relationship between variant genotype and gastrointestinal bleeding associated with celecoxib. This finding contrasts with a small case-control study which did observe an association between CYP2C9 variant genotype and short-term risk of NSAID-related gastroduodenal bleeding.17
However, most patients in this case-control study were exposed to COX-2 non-selective NSAIDs (e.g. diclofenac) which may be associated with a greater risk of bleeding than celecoxib. In fact, in the main analysis of the APC trial, an overall association between celecoxib and gastrointestinal bleeding was not observed.2
Interestingly, among individuals with either CYP2C9*2 or CYP2C9*3 genotypes, the excess number of cardiovascular and thrombotic events associated with celecoxib was restricted to those randomized to high-dose treatment. Overall, these data highlight the potential that individuals with impaired metabolism may be at particularly high risk of dose-related cardiovascular toxicity. Although it is unclear why low-dose celecoxib was not strongly related to cardiovascular and thrombotic events among individuals with variant genotypes, it is important to note that other randomized trials of celecoxib also did not demonstrate an increased risk of cardiovascular events associated with low-dose (200 mg twice daily or 400 mg once daily).3, 30–32
Nonetheless, because we had a limited number of events, our findings should be viewed as exploratory and support the importance of examining CYP2C9 genotype within ongoing clinical trials focused on adverse events associated with celecoxib.32
Our study had several strengths. First, celecoxib treatment was randomly assigned, dosing was strictly defined, treatment duration was long-term, and use of other NSAIDs (except low-dose aspirin) was not permitted.2
Thus, it is unlikely that our findings are related to differential dosing, heterogeneity in treatment duration, or exposure to other CYP2C9-metabolized NSAIDs. Second, patients were randomized to two different doses of celecoxib, permitting a detailed assessment of the effect of genotype on dose. Other studies examining single doses of celecoxib or exposure irrespective of dose would be unable to uncover a specific effect of genotype on dose-related outcomes. Third, as all patients were enrolled in a randomized trial, treatment and colonoscopic surveillance for outcomes, as well as reporting of adverse events, were uniform and standardized.
Several limitations of this study deserve comment. First, we had a limited number of patients with the CYP2C9*3 genotype although our study is consistent with the prevalence of this variant in other cohorts not specifically selected according to genotype.11, 16, 33
Second, as we have previously mentioned, we had a limited number of adverse events for analysis, especially among patients with variant genotypes. Nonetheless, our study represents the largest cohort of patients who have been randomized to long-term treatment with celecoxib for whom toxicity data are available. Third, although two copies of the I359L allele (CYP2C9*3 homozygotes) may be associated with greater impairment of drug metabolism than one copy (CYP2C9*3 heterozygotes),9, 12, 34
we did not have enough patients with two copies of I359L alleles to analyze outcomes according to gene dosage. Fourth, we restricted our analysis of CYP2C9 variants to CYP2C9*2 and CYP2C9*3 polymorphisms, which are the most prevalent polymorphisms among non-Hispanic whites.5, 20, 35, 36
However, because non-Hispanic Whites composed more than 90% of our study population, it is unlikely that misclassification of genotypes which are more prevalent among non-Whites would influence our results. Importantly, our results were also not materially altered when we restricted our analyses to non-Hispanic White patients (data not shown). Given potential differences in the prevalence of genetic variants as well as their associated metabolizer phenotypes in non-White populations, further studies are warranted to examine CYP2C9 variants and celecoxib use in other populations.
At baseline, there were differences in low-dose aspirin use according to genotype. Unlike many non-aspirin NSAIDs, aspirin is not primarily metabolized by CYP2C9.37–40
Thus, it is unlikely that this association reflects genotype-specific differences in tolerance to aspirin therapy. Nonetheless, because low-dose aspirin therapy was included as a stratification variable, we were able to carefully adjust for use of low-dose aspirin in all of our multivariate models to minimize any potential confounding.
Our study has specific clinical implications. Although celecoxib effectively prevents colorectal neoplasia, the associated cardiovascular risk does not permit routine use of the drug for population-based chemoprevention. Despite this, efforts to use genetic information to personalize and optimize chemoprevention have been identified as a high priority and may be warranted for specific patients.41
For example, high dose celecoxib at 400 mg twice daily is currently approved by the Food and Drug Administration for adjunctive treatment of patients with familial adenomatous polyposis. Celecoxib treatment also has been proposed for specific individuals who are at high risk for colorectal cancer, low risk for cardiovascular events, and unable to tolerate routine endoscopic surveillance. Finally, celecoxib treatment is continuing in studies examining its potential role in multi-agent chemoprevention or as an adjunctive treatment in patients with established cancers.42, 43
Our data suggest that for the vast majority of individuals without CYP2C9*3 genotypes, high-dose celecoxib does not confer any additional chemopreventive benefit. Thus, consideration of celecoxib for any chemopreventive strategy should be based on the potential risks and benefits associated with low-dose.
Finally, our study provides proof-of-principle that genetic determinants can influence an individual’s pharmacokinetic response to celecoxib with a significant, clinically apparent impact on outcome.41
Because celecoxib is widely used for other indications, such as treatment of arthritis,44
determining the influence of CYP2C9 genotype on responsiveness to therapy as well as susceptibility to toxicity within other patient populations remains critically important.
In summary, this study observed a pharmacogenetic association between CYP2C9*3 variant genotype and risk of adenoma according to celecoxib dosing within a large, randomized, placebo-controlled trial. Although the number of patients with variant genotypes was relatively small, limiting the statistical power, there was also a potential relationship between CYP2C9 variant genotypes and risk of cardiovascular and thrombotic events according to celecoxib exposure. These data support the potential importance of genetic variability in determining susceptibility to the benefits and hazards of celecoxib. Further research is needed to examine the routine use of genetic information in tailoring treatment with celecoxib across a range of doses, indications for use, and patient populations.