Discussion
The pooled analysis of all included studies included 214 851 patients and found that patients who took a PPI in addition to clopidogrel had the worst outcomes, including higher overall mortality, MI, ACS, CVA, stent thrombosis and the need for revascularisation procedures. These results are consistent with previous studies and meta-analyses.26 ,48 ,51 ,67 However, these data emerge mostly from nonrandomised observational studies, which are prone to selection bias and non-comparability between groups at baseline. Therefore, we conducted a separate analysis including data only from RCTs and PSM patients. In a population of 23 552 patients from eight studies, we found that all ischaemic end points evaluated were not increased in the clopidogrel-PPI group (figure 5). This analysis of RCTs and PSM patients highly suggests that PPIs are a marker of increased risk, rather than a direct cause of worse outcomes.
The contrast in outcomes between unadjusted and adjusted/randomised studies is supported by findings of increased CV risk among patients taking PPI regardless of simultaneous clopidogrel use. In a population of 31 704 patients who were not receiving clopidogrel, Charlot et al73 found that, compared with non-PPI users, patients on PPI had an increased risk of all-cause mortality (HR 1.58; p<0.01), CV mortality (HR 1.49; p<0.01), MI (HR 1.13; p=0.02) and CVA (HR 1.32; p<0.01). Furthermore, the magnitude of increased CV risk in the PPI group was similar between clopidogrel users and patients not receiving clopidogrel. An increased risk of ischaemic outcomes among patients taking a PPI has also been reported in concomitant use of placebo and ticagrelor.68 ,74
The mechanism of increased CV risk in patients receiving a PPI is most likely related to the difference in baseline characteristics between users and non-users of clopidogrel. In the study by Charlot et al,73 patients who received a PPI were on average 3 years older than the comparison group and also had a higher prevalence of diabetes with complications, chronic kidney injury and cerebrovascular disease at baseline. In Bhurke et al,30 patients taking clopidogrel had a higher Charlson comorbidity index at baseline, as well as a higher prevalence of heart failure. Similarly, the majority of unadjusted studies that reported an increased risk of CV events in PPI users had an unbalanced distribution of baseline characteristics, with sicker patients in the PPI group.31 ,52 ,63 ,67
Our study found a decreased incidence of GI bleeding among patients taking PPIs, a result that was confirmed in patients with similar baseline characteristics (RCT/PSM populations; figure 5F). Two different mechanisms may contribute as follows to the decreased incidence of GI bleeding with PPI use. The first is by direct inhibition of proton pumps with subsequent suppression of acid production, which has been shown to (1) prevent stress-ulcer related bleeding in critically ill patients;75 (2) decrease rebleeding in patients with a history of ulcer-related bleeding;6 and (3) decrease GI bleeding among patients on anticoagulants and dual anti-platelet therapy.76 Alternatively, the benefit in GI bleeding may be related to a PPI-mediated reduction in the antiplatelet effect of clopidogrel. Several pharmacokinetic studies have demonstrated a lower inhibition of platelet aggregation among patients taking a PPI in addition to clopidogrel, as compared to non-PPI clopidogrel users.14 ,15 ,66 Although our findings suggest that this mechanism is not clinically relevant in terms of adverse CV outcomes, platelet aggregation plays an important role in angiogenesis and the healing of peptic ulcers;77 therefore, a lesser degree of platelet inhibition certainly has the potential to decrease GI bleeding.
As illustrated in table 2, among patient with ACS, there was no increased risk of ischaemic CV end points with PPI use. Patients with ACS most likely have more comorbidities and a worse prognosis at baseline compared with elective patients, which can mitigate the differences in outcomes between PPI and non-PPI users. Inhibition of the CYP450 2C19 enzyme is heterogeneous within the class of PPIs. Omeprazole, esomeprazole and lansoprazole have been shown to be the strongest inhibitors,11 ,16 ,17 ,20 whereas some studies have suggested that pantoprazole and rabeprazole have no effect on the CYP450 2C19 enzyme.16 ,20 ,38 Our meta-analysis has demonstrated that the association between adverse outcomes and concomitant PPI-clopidogrel use persists in patients taking the low-risk PPIs rabeprazole or pantoprazole. Given that these medications are not expected to have a significant interaction with clopidogrel, this finding further supports the hypothesis that use of a PPI is not the cause of increased adverse outcomes, but rather a marker of increased baseline risk.
This study has limitations. Definitions of outcomes were not reported in a substantial part of the studies, which raises the concern for reporting bias. In addition, 36 of the 39 included studies were non-randomised and are inherently more susceptible to bias. The correction of possible baseline differences between groups led to a subanalysis of randomised and PSM studies; however, this analysis included only eight studies, which did not report on all the studied outcomes. Moreover, the absence of patient-level data, common in meta-analysis designs, prevented more detailed subgroup analyses, such as interaction between different generations of drug-eluting stents and the exact role of baseline characteristics on the clopidogrel-PPI interaction. Also, this systematic review was not registered prospectively, which would have allowed feedback about the protocol, further limiting the possibility of bias. Nevertheless, we believe we have conducted a transparent and reproducible protocol. Finally, given the high number of studies included and the differences in methods and outcome definitions among them, a substantial amount of heterogeneity was encountered. This has already been observed in previous meta-analyses, and therefore only a random-effects model was used. A prespecified definition of GI bleeding and stent thrombosis was also applied to minimise bias resulting from different outcome definitions.