Introduction

Peripheral artery disease (PAD) is a widespread age‑dependent disease with increasing prevalence, which in recent years has reached 30% among the 80‑year olds. Along with coronary artery disease (CAD), it remains one of the leading causes of morbidity and death.^1,2 Despite the use of recommended pharmacotherapy, the annual rate of cardiovascular events and mortality remains high in the long‑term observation.^3,4 The results of studies with antiplatelet drugs, which are the most effective therapies decreasing the risk for recurrent cardiovascular events including death in both acute and stable forms of atherothrombotic disease,^5-7 indicate that patients with PAD are still at a high risk of adverse cardiovascular events. In a subanalysis of the PLATO (Platelet Inhibition and Patient Outcomes) trial conducted by Sumaya et al,⁸ in patients after acute coronary syndrome (ACS) treated with double antiplatelet therapy consisting of acetylsalicylic acid (ASA) and ticagrelor, prolonged clot lysis time (CLT) and maximum turbidity reflecting clot density and impaired fibrinolysis were associated with adverse outcomes in 1‑year follow‑up, increasing the risk of cardiovascular death with each 50% increase in maximum turbidity and lysis time. The residual risk associated with thrombin activation has given rise to the COMPASS (Rivaroxaban for the Prevention of Major Cardiovascular Events in Coronary or Peripheral Artery Disease) trial,⁹ where a combination of ASA and factor Xa inhibitor rivaroxaban 2.5 mg twice daily was tested in comparison with ASA alone or rivaroxaban 5 mg twice daily alone. This trial documented benefits from the combined therapy vs monotherapies. This dual pathway inhibition (DPI) reduced the risk of cardiovascular death, stroke or myocardial infarction (MI) by 24% in patients with chronic CAD and PAD.⁹ The VOYAGER PAD study¹⁰ (Efficacy and Safety of Rivaroxaban in Reducing the Risk of Major Thrombotic Vascular Events in Subjects With Symptomatic Peripheral Artery Disease Undergoing Peripheral Revascularization Procedures of the Lower Extremities) confirmed the effectiveness of DPI in PAD patients after lower‑extremity revascularization, as compared with ASA alone in terms of the primary efficacy composite end point of acute limb ischemic events, major amputation, MI, stroke, and cardiovascular mortality.

The mechanisms underlying marked clinical benefits from ASA plus low‑dose rivaroxaban as compared with ASA- or rivaroxaban‑based monotherapies are elusive. Aspirin and rivaroxaban (similarly to most anticoagulant agents) have been reported to improve plasma fibrin clot properties. Aspirin, via protein acetylation, increases clot permeability by forming looser fibrin networks, and rivaroxaban, by reducing thrombin generation, accelerates lysis.¹¹ However, such effects are dose‑dependent and undergo various modifications depending on the atherosclerosis burden and comorbidities, such as diabetes, chronic kidney disease as well as some medications, particularly statins.^11-13 It is known that PAD, similarly to coronary artery disease, is characterized by prothrombotic fibrin clot properties.¹² To our knowledge, there have been no studies exploring whether the prothrombotic fibrin clot phenotype is favorably altered in PAD patients when rivaroxaban is added to ASA.

The aim of this preliminary study was to evaluate the effect of 6‑month use of low‑dose rivaroxaban added to ASA on coagulation parameters, thrombin generation, and fibrin clot characteristics in stable PAD patients.

Patients and methods

We studied 16 outpatients with documented stable PAD who were on 75 mg/d ASA. They were recruited at the Center for Coagulation Disorders, St. John Paul II Hospital, Kraków, Poland, from June 2018 to August 2019. The exclusion criteria were a history of ACS or ischemic cerebrovascular episode, prior venous thromboembolism, limb amputation, recent surgery or invasive procedure, acute infection, known cancer, end‑stage renal failure, severe liver injury, and current oral anticoagulation therapy or the use of clopidogrel. All eligible patients fulfilled the inclusion criteria used in the COMPASS trial⁹ and did not have a high bleeding risk as defined in this trial.

The study protocol conformed to the ethical guidelines of the 1975 Declaration of Helsinki and was approved by the Bioethics Committee at the Jagiellonian University Medical College (KBET 1072.6120.136.2018).

The decision to start low‑dose rivaroxaban was based on the clinical situation and suboptimal effects of prior therapy. All patients were assessed twice, namely at baseline while on ASA alone and after at least 6 months of treatment with ASA 75 mg once a day and vascular‑dose (low‑dose) rivaroxaban 2.5 mg twice daily, as recommended.¹⁴ Baseline demographic and clinical data were collected using a questionnaire. Ankle‑brachial index was assessed.

Results

Discussion

To our knowledge, this preliminary study is the first to assess how the addition of low‑dose rivaroxaban to aspirin affects blood coagulation, fibrin clot density, and fibrinolysis in PAD patients. We showed the reduction of thrombin generation and the subsequent D‑dimer concentrations on combined antithrombotic therapy, which is in line with the anticoagulant effect of factor Xa inhibitors. We did not observe any significant changes in fibrin clot network density and its lysability, along with 2 key fibrinolysis inhibitors, though K_s was numerically higher and CLT shorter on the combined therapy than at baseline. However, we identified a subset of PAD patients in whom improved fibrinolysis and looser fibrin networks were observed after 6 months of the rivaroxaban + aspirin treatment. It appears that the “worse,” that is, more prothrombotic fibrin clot characteristics in plasma‑based assays were while on ASA alone, the more probable was the combined therapy‑induced improvement in the clot phenotype. Larger studies are needed to investigate this concept.

Rivaroxaban administered at a daily dose of 20 mg or 15 mg favorably alters plasma fibrin clot properties, including increased K_s and shorter CLT,¹⁷ therefore, at the nadir concentration just before the next dose of 2.5 mg, it is likely that no significant changes could be detected despite a slight reduction in thrombin generation known to improve fibrin clot characteristics.¹⁷ It might be speculated that thrombin generation lowering reflected by ETP was not sufficient to alter the fibrin clot structure assessed using clot permeation, though it is well known that decreased thrombin formation renders fibrin clot looser and more susceptible to lysis.¹⁸ Most likely, larger changes in thrombin generation are necessary to affect fibrin properties in the assays applied in this study. Another factor that might have contributed to the absence of any association of rivaroxaban administration with fibrin clot properties could be statin‑induced LDL‑C lowering and other related potentially antithrombotic changes (eg, neutrophil extracellular traps), which have been reported to correlate with improved plasma fibrin clot phenotype.^19,20 In such a situation, further improvement related to low‑dose rivaroxaban could be hard to achieve in PAD patients. It is unclear whether the effect of rivaroxaban addition is more potent in PAD patients with higher cholesterol concentrations or enhanced inflammation.

Our hypothesis‑generating study has several limitations. The study group was small, and we cannot rule out that in a larger patient population the changes in fibrin properties could be significant. We did not assess rivaroxaban concentration at the time of analysis and no measurements were performed at the peak of anticoagulant effects, namely about 4 hours since the drug intake. Assessment of other biomarkers, for example, plasma thrombin or platelet markers, along with microscopic imaging of plasma clots were beyond the scope of the current research. Patient compliance was not assessed; however, the patients declared regular drug intake.