Effect of Two Weeks of Low-Dose Apixaban on Platelet Reactivity in Patients With Stable Coronary Artery Disease on Dual Antiplatelet Therapy (Aspirin and Clopidogrel) (ACER)

Background Coronary artery disease (CAD) is a leading cause of death worldwide. It develops through a combination of atherosclerosis, inflammation, metabolic abnormalities, dysfunction of the blood vessel lining (endothelium), and blood clot formation (thrombosis). Dual antiplatelet therapy (DAPT), most often aspirin plus clopidogrel, is a cornerstone of treatment to prevent further heart attacks and strokes after an acute coronary syndrome or percutaneous coronary intervention (stent placement).

Despite DAPT, a substantial proportion of patients continue to have high on-treatment platelet reactivity (HPR), meaning their platelets remain more active than intended. HPR is associated with a higher risk of recurrent atherothrombotic events such as heart attack, stent thrombosis, and stroke. The response to clopidogrel in particular varies widely between individuals, and the U.S. Food and Drug Administration (FDA) has added a warning to the clopidogrel label about reduced effectiveness in some patients. This has prompted research into strategies that achieve more effective platelet inhibition without substantially increasing bleeding. Current options include switching to newer P2Y12 inhibitors such as prasugrel or ticagrelor, but these are not always available, affordable, or suitable for every patient.

Rationale Apixaban is an oral anticoagulant that directly and reversibly blocks factor Xa, a key enzyme in the clotting cascade. It was approved by the FDA in 2012 for stroke prevention in non-valvular atrial fibrillation and later for the treatment and prevention of venous thromboembolism. In the ARISTOTLE trial, apixaban was superior to warfarin in reducing stroke or systemic embolism, major bleeding, and death from any cause in patients with atrial fibrillation. In the AUGUSTUS trial, patients with atrial fibrillation and a recent acute coronary syndrome or stent procedure who received apixaban with a P2Y12 inhibitor had significantly less bleeding and fewer hospitalizations than those receiving a vitamin K antagonist or triple therapy.

Coagulation and platelet activation are closely linked. By blocking factor Xa, apixaban reduces the generation of thrombin, which is one of the most potent activators of platelets, acting mainly through protease-activated receptor-1 (PAR-1). Laboratory and preclinical studies also suggest that factor Xa inhibition may reduce endothelial inflammation, oxidative stress, and platelet reactivity. These mechanisms are directly relevant to CAD. However, no prospective clinical study has examined whether low-dose apixaban affects platelet function in patients with stable CAD who are on long-term DAPT. This pilot study is intended to provide initial evidence on that question and to generate hypotheses for larger studies.

The study hypothesis is that adding low-dose apixaban to aspirin and clopidogrel will reduce platelet reactivity, with an expected reduction of approximately 5% from each participant's baseline value. The null hypothesis is that there will be no significant difference in platelet reactivity before and after apixaban.

Study Design This is a prospective, single-center, single-arm, open-label pilot study using a before-and-after design. There is no control group; each participant serves as their own control. Participants and investigators will know that apixaban is being taken. There is no washout period, and participants continue their usual aspirin and clopidogrel throughout. Apixaban is an FDA-approved medication; the study assesses platelet function before and after a short course of it rather than testing an experimental drug.

After screening and written informed consent, the study team will record each participant's demographic and body measurement data, medical and procedural history, and current medicines on a case report form. Baseline platelet function testing is performed before the first dose of apixaban. Participants then take apixaban 2.5 mg twice daily for 14 days (28 doses), after which platelet function testing and safety blood work are repeated. A study physician conducts a follow-up telephone interview at Day 30 to check for adverse events; no additional testing is done at that contact. Participants are given a printed report of their platelet function results.

Platelet Function Testing Blood samples are drawn from a vein in the arm using a 21-gauge needle. The first portion of blood drawn is discarded to avoid artificial platelet activation, and a 5 mL citrated sample is then collected for testing. Platelet function is measured with the VerifyNow point-of-care system (Werfen), a rapid whole-blood assay, using the P2Y12 assay (reported as P2Y12 reaction units [PRU] and percentage inhibition of platelet aggregation) and the Aspirin assay (reported as aspirin reaction units [ARU]). All assays are performed according to the manufacturer's instructions. Blood is sampled on two occasions only: at baseline and at the end of the 14-day treatment period.

Safety Monitoring The main risks are those of blood sampling (bruising, vessel spasm, local infection, and rarely nerve injury) and of apixaban, chiefly bleeding. Participants will be screened for bleeding risk before enrollment and monitored closely for signs and symptoms of bleeding during the study. Bleeding events will be classified using Bleeding Academic Research Consortium (BARC) definitions. Any serious adverse event will be reported to The University of the West Indies Campus Research Ethics Committee within 24 hours, and participants will receive care according to the policies of the participating institution's cardiology department. A Data Safety Monitoring Board is not required given the small sample size, short treatment duration, and use of an approved medication at a low dose.

Sample Size and Statistical Analysis The sample size of 16 participants was calculated to detect a 5% reduction in PRU, from an expected mean of 217 PRU on clopidogrel to 207 PRU with apixaban, using a two-sided type I error rate of 5%. Continuous variables will be summarized as mean with standard deviation, and categorical variables as frequencies and percentages. Paired t-tests will compare each participant's measurements before and after apixaban. Categorical variables will be compared using Fisher's exact test or Pearson's chi-square test as appropriate. A p-value below 0.05 will be considered statistically significant, and no adjustment will be made for multiple comparisons. Missing data will not be imputed. Analyses will be performed using SPSS software. An adverse event analysis will be performed if any serious adverse events occur.

Data Management Case report forms will be stored in locked cabinets at the Department of Clinical Medical Sciences, The University of the West Indies, St. Augustine. Data will be anonymized and entered into a password-protected, encrypted database. The Principal Investigator will review each case report form and the study database to minimize errors and reconcile conflicting data.

Ethics The study is conducted in accordance with the Declaration of Helsinki and International Council for Harmonisation Good Clinical Practice guidelines. Participation is voluntary, there is no payment or other incentive for taking part, and participants may withdraw at any time without any effect on their usual care.

Expected Significance This study will provide initial data on whether low-dose apixaban reduces platelet reactivity in patients with stable CAD on DAPT, including those whose platelets respond poorly to standard therapy. The findings may help inform future antithrombotic strategies, support the design of larger trials, and indicate whether apixaban could serve as an add-on option alongside the antiplatelet medicines currently available in Trinidad and Tobago.