Early indications for the unique role factor XI may play in the coagulation cascade were observed in humans with inherited deficiencies. A study of those with normal, mild, and moderate–severe deficiencies in factor XI concluded that the moderate–severe cohort had significantly lower cardiovascular and VTE events [6]. Additionally, in those that had factor XI levels above the 90th percentile, there were significantly more VTE events than in the control group [7]. Analyses that stratified the study population into quartiles of factor XI levels and examined VTE rates showed a dose-dependent relationship between higher factor XI level quartiles and lower VTE incidence.

These findings prompted animal testing of factor Xia inhibition. A study conducted in the early 2000s sought to understand rates of venous thromboembolism in mice with knockout factor XI versus wild type [24]. After inducing vena cava thrombosis with iron chloride (FeCl3), the study concluded that mice without factor XI did not form thrombosis at lower doses of FeCl3, and this protective effect exceeded that of heparin, clopidogrel, and argatroban. Additionally, the study found bleeding risk was similar in knockout mice compared with wild type, providing a murine-tested confirmation for a hypothesized physiologic phenomenon.

Arterial thrombi formation in factor XI-deficient mice has also been studied. By inducing carotid artery damage with FeCl3, researchers found factor XI-deficient mice had significantly lower clotting rates than even those with factor VII deficiency, suggesting the intrinsic pathway plays a more critical role in clot formation than previously considered [25]. A second study, based on a similar carotid artery injury model, aimed to find the difference between factor XI and IX in murine models [26]. The study determined that while both factors were protective against clot formation, factor IX significantly prolonged bleeding time (5.8-fold increase) above the wild-type baseline. These biological studies corroborated the proposed effects of factor XI inhibition, which prompted further research into pharmacologic factor XI inhibitors.

Investigators introduced rabbit models to milvexian and studied the effect on thrombus formation in rabbit arteriovenous (AV) shunts [27]. Significant decreases in thrombus weight and dose-dependent increases in PTT were observed. Additional work examining the hemostatic changes observed with milvexian in rabbit arterial models found that milvexian in combination with aspirin did not increase bleeding time even when compared to monotherapy, with either aspirin or milvexian, alone [28].

Phase I trials with milvexian began in 2015 (Table 1). The first trial examined the safety, tolerability, and pharmacokinetics of both single and multiple doses of the drug [29]. This randomized, parallel assignment, double-blinded trial included 104 participants. Participants were assigned to either the single ascending dose (SAD) or multiple ascending dose (MAD) arms with adverse effects monitored for a maximum of 21 days. The drug was generally well tolerated with the most common adverse event being headache and only minor bleeding events reported [8]. The bleeding events included one participant each with epistaxis, gingival bleeding, and petechia. None of the events were considered severe enough to discontinue treatment. The study concluded that SADs up to 500 mg and MADs of up to 200 mg twice daily (BID) were well tolerated. The half-life of the drug ranged from 8 to 14 h in SAD panels and 11–18 h in MAD panels. Little to no renal excretion was observed.

A second phase I trial began in 2021 to examine the relative oral bioavailability and food effect of both single and multiple doses of milvexian in healthy adults using various modes of drug delivery [30]. This randomized, crossover assignment, open-label trial included 114 participants and concluded enrollment and follow up in 2022. Results of the investigation have yet to be published.

Two phase II randomized controlled trials (RCTs) designed to evaluate milvexian have been completed. The first, AXIOMATIC-TKR, tested efficacy and adverse outcomes of the drug when compared to subcutaneous enoxaparin [31] in a multicenter international trial that enrolled 1242 participants aged 50 or older with elective unilateral total knee replacement. Participants were randomly assigned in a 1:1:1:1:1:1:2 approach to milvexian 25 mg BID, 50 mg BID, 100 mg BID, 200 mg BID, 50 mg daily, 200 mg daily, or enoxaparin 40 mg daily, respectively. After 252 patients had been enrolled, the independent safety committee recommended discontinuation of the milvexian 25 mg daily dose based on prespecified criteria for low efficacy.

The primary outcome was a composite of postoperative deep-vein thrombosis assessed by unilateral venography 10–14 days after knee replacement. The criteria for proof of efficacy were defined as either a dose–response trend with the twice-daily milvexian dosing or an incidence of VTE significantly lower than 30%, a conservative estimate of postoperative VTE rates in those undergoing TKR. Secondary outcomes included all-cause mortality, nonfatal proximal or distal deep-vein thrombosis, and nonfatal pulmonary embolism. The principal safety outcome was bleeding of any severity.

There was a significant dose-dependent relationship observed between increasing milvexian dosage and decreases in primary outcome events, with VTE rates of 21%, 11%, 9%, and 8% in the 50 mg, 100 mg, 200 mg, and 400 mg doses, respectively. VTE rates in the twice-daily milvexian arm were 12%; therefore, both primary outcome efficacy criteria were met. Additionally, there was no statistically significant increase in bleeding rates between milvexian and enoxaparin or with increasing doses of milvexian. Pharmacodynamic measures were as expected; milvexian increased the aPTT ratio in a dose-dependent manner, and neither of the study drugs increased the PT ratio. Therefore, the study concluded oral milvexian reduced the proportion of postoperative thromboembolism rates without a concurrent increase in bleeding risk when compared with enoxaparin in those undergoing knee arthroplasty.

Limitations of the study include a low number of clinically relevant non-major bleeding (CRNM) and major bleeding events for detection of differences in bleeding rates between milvexian doses and enoxaparin, an open-label design allowing study participants to know which drug they were assigned, and potential investigator bias in ascertaining bleeding rates. However, the dose of milvexian was blinded to study participants, and a blinded event committee reviewed all potential endpoint events.

The second phase II trial, AXIOMATIC-SSP, specifically examined safety and efficacy of milvexian versus placebo in those with acute ischemic stroke or high-risk TIA [32,33,34]. This multicenter, international, double-blinded trial enrolled 2366 participants with acute ischemic stroke or TIA and randomly assigned to milvexian 25 mg daily, 25 mg BID, 50 mg BID, 100 mg BID, 200 mg BID, or matching placebo with a 1:1:1:1:1:2 ratio, respectively. Inclusion criteria for stroke and TIA characterization were based on a combination of acute neurological deficits, neuroimaging findings, and National Institutes of Health Stroke Scale (NIHSS) and ABCD [2] scores [35, 36]. All participants were to be treated with low-dose aspirin daily and clopidogrel loading dose following maintenance for 21 days and then aspirin alone for 90 days.

The primary outcome was symptomatic ischemic stroke and covert brain infarction characterized by comparison of magnetic resonance imaging (MRI) at 90 days. Secondary endpoints were the individual components and composite of symptomatic ischemic stroke, myocardial infarction (MI), and all-cause mortality, volume and number of new infarcts detected on MRI, and pharmacokinetic and pharmacodynamic measures. The principal safety outcome was major bleeding. The nature of the bleeding was characterized by both Bleeding Academic Research Consortium (BARC) classification and International Society on Thrombosis and Haemostasis and PLATelet inhibition and patient Outcomes (PLATO) criteria [37,38,39].

There was no difference in event proportion for the primary outcome between any milvexian dose and matching placebo. The primary efficacy outcome occurred in 17% of participants in the placebo group versus 18%, 14%, 15%, and 16% in the 25 mg, 50 mg, 100 mg, and 200 mg twice-daily groups, respectively. There was no significant dose-dependent response between milvexian and the primary efficacy outcome. Major bleeding rates across all trial arms were 1–2%. There was no significant dose-dependent response between milvexian and major bleeding rates. There were fewer symptomatic ischemic strokes at all milvexian doses except for 200 mg twice daily when compared to placebo. The study concluded that in those with ischemic stroke or TIA, oral milvexian did not reduce the composite outcome of brain infarction or ischemic stroke or increase bleeding risk at 90 days when compared with placebo on a background of dual antiplatelet therapy.

The results of the phase I and II studies have provided a strong foundation for the LIBREXIA phase III program. The phase III program will include three separate, event-driven trials with common leadership and operations in acute stroke, acute coronary syndrome (ACS), and atrial fibrillation (AF).

LIBREXIA STROKE (NCT05702034), a randomized, parallel assignment, double-blinded study, has an enrollment target of 15,000 participants and an estimated completion date in 2026 [40] (p. 3). Patients with an acute ischemic stroke or high-risk TIA will be randomly assigned to oral milvexian 25 mg twice daily in addition to their standard-of-care antiplatelet therapy or placebo. The primary outcome is time to first occurrence of ischemic stroke. Key secondary outcomes include a composite of cardiovascular death (CVD), myocardial infarction (MI), or ischemic stroke; and a composite of CVD, MI, ischemic stroke, major adverse limb events, symptomatic pulmonary embolism, or deep-vein thrombosis.

LIBREXIA ACS (NCT05754957), a randomized, parallel assignment, double-blinded study, has an enrollment target of 16,000 participants and an estimated completion date in 2026 [41] (p. 3). Patients within 7 days of an ACS, either with or without percutaneous intervention, will be enrolled. Participants will be randomly assigned to oral milvexian 25 mg twice daily in addition to their standard-of-care antiplatelet therapy or placebo. The primary outcome is time to a composite of CVD, MI, or ischemic stroke. Key secondary outcomes include time to a composite of CVD, MI, ischemic stroke, major adverse limb events, or symptomatic venous thromboembolism; a composite of all-cause mortality, MI, or ischemic stroke; CVD; and all-cause mortality.

LIBREXIA AF (NCT05757869), a randomized, parallel assignment, double-blinded study, has an enrollment target of 15,500 participants and an estimated completion date in 2027 [42] (p. 3). Patients with non-valvular AF and appropriate for anticoagulant therapy will be enrolled. The participants will be randomly assigned oral milvexian 100 mg twice daily or apixaban (2.5 or 5 mg) twice daily. The trial is designed as a non-inferiority trial, and the primary outcome is occurrence of stroke or non-CNS systemic embolism. Key secondary outcomes include major bleeding; major or clinically relevant non-major bleeding; CVD, MI, stroke, or non-CNS embolism; CVD; all-cause death, MI, stroke, or non-CNS embolism; and CVD, MI, stroke, acute limb ischemia, or urgent hospitalization for vascular cause.