Clopidogrel appears to reduce the risk of colorectal cancer (CRC) as much as low-dose aspirin, based on a case-control study involving more than 15,000 cases.
Risk of CRC was reduced by 20%-30% when clopidogrel was given alone or in combination with aspirin, reported lead author Antonio Rodríguez-Miguel of Príncipe de Asturias University Hospital in Madrid and colleagues. This finding adds support to the hypothesis that low-dose aspirin is chemoprotective primarily because of its antiplatelet properties, they noted.
“The mechanism of action of low-dose aspirin to explain its protective effect is subject to debate,” the investigators wrote in Clinical Gastroenterology and Hepatology. “Although aspirin is a nonsteroidal anti-inﬂammatory drug (NSAID) and these drugs are known to prevent CRC through the inhibition of cyclooxygenase (COX)-2 in epithelial and stromal cells in the large bowel, at low doses (75-300 mg/d) aspirin has only transient effects on this isozyme, while permanently inactivating platelet COX-1 and suppressing thromboxane A2 production. The apparent lack of dose-dependence of the chemoprotective effect of aspirin, as well as the potential role of locally activated platelets in upregulating COX-2 expression in adjacent nucleated cells of the intestinal mucosa, have led [to] the postulation that low-dose aspirin could exert its chemoprotective effect via its antiplatelet action.”
Although previous studies have explored the chemoprotective potential of other antiplatelet agents, such as clopidogrel, the resultant body of evidence remains small. In 2017, for example, Avi Leader, MD, and colleagues reported that the chemoprotective effect of dual-antiplatelet therapy (DAPT) with clopidogrel and aspirin was superior to aspirin monotherapy, based on an additional 8% risk reduction. The present study aimed to build on such findings with evaluation of a Mediterranean cohort, which could reduce confounding lifestyle factors, owing to a lower rate of cardiovascular morbidity than other populations.
The nested, case-control study involved 15,491 cases of CRC and 60,000 controls who were randomly selected and frequency matched by sex, age, and year of indexing. Data were drawn from Base de datos para la Investigación Farmacoepidemiológica en Atención Primaria (BIFAP), a Spanish medical record database with more than 7 million patients. Records of patients involved in the present study were screened for prescription of three antiplatelet agents: low-dose aspirin, clopidogrel, and triflusal. Additional categorization identified current users, recent users, past users, and nonusers. The effects of clopidogrel and aspirin were evaluated separately, as monotherapies, and together, as DAPT.
Demographically, the mean age of the entire study population was 68.6 years, with a slight male predominance (59%). Median follow-up was similar between cases and controls, at approximately 3 years, ranging from about 1.5 to 6 years. Cases showed higher rates of gout, alcohol abuse, acute digestive diseases, and peripheral artery disease, whereas controls were more likely to have histories involving stroke, acute myocardial infarction, chronic digestive diseases, and constipation.
Controls were more likely to be current aspirin users than patients diagnosed with CRC (12.8% vs. 12.2%), giving an associated adjusted odds ratio (AOR) of 0.83. Risk reduction became statistically apparent after 180 days of aspirin usage, with an AOR of 0.79, and more prominent in the 1- to 3-year range, with an AOR of 0.73. This chemoprotective effect faded rapidly with discontinuation.
Current clopidogrel usage led to a comparable level of risk reduction, with an AOR of 0.80. It wasn’t until a year of continuous clopidogrel monotherapy that risk reduction became statistically significant, with an AOR of 0.65, which dropped to 0.57 between years 1 and 3.
Turning to a matched comparison of aspirin or clopidogrel monotherapy versus DAPT, the investigators found similar rates of chemoprotection. Current aspirin usage of any duration offered an adjusted risk reduction of 17%, compared with 25% for clopidogrel, and 29% for DAPT. Beyond 1 year of continuous and current usage, the superiority of DAPT was called into question, as clopidogrel monotherapy offered the greatest risk reduction, at 37%, compared with 22% for aspirin, and 22% for DAPT. Risk analyses involving triflusal lacked statistical significance.
“The results of the present study are compatible with a chemoprotective effect of clopidogrel against CRC, equivalent in magnitude to the one observed for low-dose aspirin,” the investigators wrote. “This finding indirectly supports the hypothesis that the chemoprotective effect of low-dose aspirin is mediated mostly through the permanent inactivation of platelet COX-1.”
The investigators pointed out that the chemoprotective effects of antiplatelet therapy begin to appear early in treatment, independently from lifestyle factors, but risk reduction depends on current usage. Although short-term usage of either aspirin or clopidogrel was associated with an increased risk of CRC, the investigators suggested that this was more likely a perceived risk rather than an actual one. “In our view, this observation could be explained in part by a detection bias, owing to an increased risk of GI bleeding induced by antiplatelet agents that could lead to a greater number of colonoscopies, and, as a result, an early cancer diagnosis,” they wrote.
The study was funded by the Fundación Instituto Teóﬁlo Hernando. Dr. García-Rodríguez disclosed a relationship with CEIFE, which has received funding from Bayer and AstraZeneca.
SOURCE: Rodríguez-Miguel et al. Clin Gastrenterol Hepatol. 2018 Dec 20.