The results of this study demonstrate a significant association between treatment of CCBs and aspirin responsiveness for patients with stable CAD. In these patients, the concomitant use of CCBs had a weakening effect on the pharmacologic potency of aspirin. Our study did not find an association between the type of CCB used and HAPR.
Our results are consistent with similar studies performed by Gremmel et al. which demonstrated that CCBs are associated with a decreased platelet response to clopidogrel [18,19]. In these studies, tests were performed via flow cytometry and the VerifyNow system to conclude that the addition of CCBs to clopidogrel leads to a reduced antiplatelet response compared to clopidogrel alone. Potential mechanisms include the inhibition of cytochrome 3A4 and the inhibition of P-glycoprotein (P-gp) by CCBs [18,19]. The first mechanism is based on clopidogrel as a prodrug that requires the activity of cytochrome 3A4 for its active form. If cytochrome 3A4 is inhibited by CCBs, clopidogrel cannot be appropriately metabolized and its effect is diminished [18–22]. The latter mechanism considers that several CCBs inhibit the effect of P-gp, which is a necessary protein for clopidogrel’s intestinal absorption [19,23].
Our hypothesis is based on similar explanations for how CCBs attenuate aspirin’s effect, as evidenced by the increased rate of HAPR. The P-gp theory could be one of the mechanisms that accounts for our results. While some studies have concluded that aspirin is not a substrate for P-gp , others have found that by P-gp causes efflux of aspirin  and is therefore associated with P-gp-based absorption in the gut. Hence, the concomitant use of CCBs with aspirin may lead to decreased bioavailability of the anti-thrombotic drug. In our study, the P-gp inhibiting CCBs that were used included Nifedipine and Diltiazem. The association we found with increased HAPR was with use of CCBs overall, irrespective of the type of CCB; we estimate therefore that the P-gp theory accounts for only part of this association.
Another mechanism that could explain the increased HAPR with usage of CCBs is based on the Etingen study that demonstrated how CCBs enhance cholesterol ester (CE) hydrolysis . The liver is the site for both CE and aspirin hydrolysis, with CE hydrolyzed into cholesterol and aspirin hydrolyzed into salicylic acid [27,28]. When aspirin is hydrolyzed into salicylic acid, its antiplatelet activity is transformed to anti-inflammatory activity . It is therefore also possible that CCBs upregulate the hydrolysis of aspirin in a similar fashion to the hydrolysis of cholesterol esters. The increased formation of salicylic acid depletes the aspirin load and leads to an increase in HAPR.
Finally, one of the mechanisms of action of aspirin is through the inhibition of prostaglandin (PG) H2 synthase (cyclooxygenase), preventing the generation of prostaglandin G2 and prostaglandin H2, and ultimately TXA2 . For these events to occur, PGH2 synthase requires arachidonic acid liberation from phospholipids. However, arachidonic acid is released from phospholipids only once calcium levels rise within the cell. If calcium levels fail to rise due to the presence of calcium-blocking agents, aspirin may have a diminished ability to exert its anti-platelet effects [4,7,30]. In this scenario, CCBs hamper the antiplatelet effects of aspirin by limiting its potential to prevent TXA2 formation. It is therefore presumable that dihydropyridine and non-dihydropyridine should share similar effectivities on HAPR due to arachidonic acid presence in both smooth and cardiac muscle . Dihydropyridines primarily act on smooth muscle whereas non-dihydropyridines primarily act on cardiac muscle; their influences would each cause an independent decrease in TXA2 formation.
In our study, the non-dihydropyridine CCB (Diltiazem) did not show an independent association with HAPR as compared to the dihydropyridine CCBs (Amlodipine, Lercanidipine, and Nifedipine). Similarly, both P-gp inhibiting (Nifedipine and Diltiazem) and non-P-gp inhibiting CCBs (Amlodipine and Lercanidipine) were used. However, we did not identify an association between the type of CCB and HAPR. Future studies, which compare larger number of patients receiving CCBs, may re-assess such associations between the type of CCBs and aspirin response.
While observational, our results are the first to examine the association between CCBs and HAPR, and may suggest a mechanism for the lack of prognostic benefit in patients with ischemic heart disease treated with CCBs [13–15]. Understanding this mechanism may allow for a more personalized assessment of how to maximize cardio-protection for patients with cardiovascular risk factors. Further research is needed in order to determine the translational impact of this observation, and to guide the medical management of patients with stable CAD treated with antiplatelet therapy.