Vortosiran in Atrial Fibrillation

Atrial fibrillation (AF) is among the most prevalent cardiovascular conditions, affecting approximately 2 4% of the adult population worldwide. AF represents the most common sustained supraventricular arrhythmia and is associated with substantial morbidity, mortality, and health-care burden. Patients with AF are at an elevated risk of thromboembolic complications, particularly ischemic stroke and systemic embolism, while also facing an increased risk of bleeding associated with long-term antithrombotic therapy administered to prevent these complications. The principal aim of management in AF is to prevent stroke while minimizing haemorrhagic risk through careful risk stratification for both thromboembolic risk and bleeding risk, and informed and individualised treatment selections.

According to current guideline recommendations, oral anticoagulation is recommended for stroke prevention and may be guided by CHA2DS2 -VASc clinical risk assessment tool. Nonvitamin K antagonist oral anticoagulants (NOACs, also referred to as direct oral anticoagulants, or DOACs) are preferred over vitamin K antagonists (VKAs) in most cases due to their superior safety profile and lower incidence of intracranial bleeding. Bleeding risk can be assessed using validated tools such as HAS-BLED, and modifiable risk factors should be addressed to reduce overall treatment-related complications.

Despite a superior efficacy-safety profile of NOACs relative to VKAs, patients treated with NOACs remain at residual risk for stroke, systemic embolism, and cardiovascular mortality. Furthermore, although NOACs are associated with reduced rates of intracranial haemorrhage compared with VKAs, they still confer a considerable risk of major bleeding and clinically relevant non-major bleeding events. Consequently, the clinical need for strategies that further optimize the balance between thromboembolic prevention and bleeding risk in patients with AF remains.

Recent advances have positioned coagulation factor XI (FXI) inhibition as a promising strategy in anti-thrombotic drug development. FXI is a crucial element of the intrinsic coagulation pathway, and genetic FXI deficiency (haemophilia C) is typically associated with only mild bleeding even in complete deficiency, suggesting the potential for improved safety compared to conventional anti-thrombotic drugs by disengaging physiologic haemostasis from pathologic, intraluminal thrombosis. FXI inhibition reliably prolongs activated partial thromboplastin time (APTT) in vitro, and epidemiological studies demonstrate that inherited FXI deficiency confers protection against venous thromboembolism (VTE) and ischemic stroke. Conversely, elevated circulating activated FXI (FXIa) is linked to higher risks of ischemic stroke, VTE, and acute coronary syndrome (ACS). Contemporary research utilizing coagulometric assays has confirmed FXIa and tissue factor activation in patients with stable coronary artery disease, ACS of less than 12 hours' duration, and heart failure, reinforcing FXI's role as a pathogenic factor in these conditions.

From the patient perspective, the limitations of current DOAC-based strategies extend beyond rare catastrophic haemorrhage to include a spectrum of patient-relevant bleeding and treatment burdens that erode quality of life and adherence. Easy bruising, epistaxis, gingival bleeding, menorrhagia, and minor gastrointestinal bleeding are often labelled nuisance or non-major events in clinical trials, but patients describe these as anxiety-provoking, stigmatizing, and disruptive to daily activities. Such events, together with polypharmacy, twice-daily dosing for some agents, drug drug interactions, and the need for uninterrupted long-term therapy, frequently lead to missed doses, dose reductions, or unsupervised discontinuation of DOACs, which in turn compromises stroke protection at the population level. In parallel, important patient subgroups including those with advanced chronic kidney disease or dialysis, advanced liver disease, prior intracranial haemorrhage, or frailty have been underrepresented in pivotal DOAC trials and often withheld treatment or remain untreated because of bleeding concerns. By targeting FXI, vortosiran has the potential to dissociate pathological thrombosis from physiological haemostasis, and to deliver sustained antithrombotic protection with a lower risk bleeding than currently available therapies and an adherence profile that is better aligned with patients' priorities and lived experience.

Clinical trials targeting FXI inhibition in the context of cardiovascular disease, including AF, are ongoing or recently completed. Several Phase 2 and 3 trials are investigating FXI/FXIa inhibitors by use of different modalities such as monoclonal antibodies and small molecules across multiple high-risk groups, evaluating efficacy and bleeding outcomes against standard anticoagulants for stroke prevention in AF (SPAF). These studies collectively support the notion that FXI/FXIa may be a favourable anticoagulation target, capable of reducing thrombotic events with a lower side effect profile, especially bleeding, compared to traditional agents. However, while several trials confirm safety advantages, data on efficacy remain to be further demonstrated, necessitating continued research focus. Vortosiran is an N-acetyl galactosamine (GalNAc) conjugated small interfering ribonucleic acid (siRNA), enabling highly specific and highly durable enrichment of the siRNA in hepatocytes, offering a modality specifically inhibiting the production of FXI. Vortosiran is under development to achieve a durable antithrombotic effect by reducing the levels of FXI.

The rationale for the current trial is based on data from multiple Phase 2 and 3 studies of FXI/FXIa inhibitors that have assessed FXI clotting activity and the effect on venous thrombosis inhibition, and is currently being tested for stroke prevention in atrial fibrillation patients. Based on the collective observations for these studies, that FXI clotting activity inhibition translates into a lower risk of thrombosis with a lower risk of bleeding compared to standard of care, the ongoing Phase 3 studies, and that vortosiran treatment can achieve identical or even higher degrees of FXI clotting activity inhibition, there is a rationale that vortosiran could potentially improve benefit-risk balance for stroke prevention in patients with atrial fibrillation compared to standard of care with FXa inhibitors.