**1. Introduction**

Cardiovascular diseases (CVDs) remain the leading cause of death worldwide. In 2016, 17.9 million people died from all causes of CVDs [1,2]. There were approximately 8.9 million deaths due to ischemic heart disease (IHD) worldwide, remaining the first cause of death; a less prevalent disease was ischemic stroke, with an incidence of 7.6 million globally [3–5].

Therapeutic advances in the last decades have reduced CVD mortality, with antithrombotic therapy being the cornerstone of medical treatment. Several antithrombotic drugs are currently used to either block platelet activation (Figure 1), prevent the activation of the coagulation cascade, or induce fibrinolysis once the clot is formed (Figure 2) [6–9]. Yet, although these antithrombotic agents have robustly demonstrated their effectiveness in preventing atherothrombotic events, they also carry an inherent risk of bleeding. Bleeding is associated with adverse cardiovascular outcomes and mortality; hence, there is a need to discover new targets and develop novel antithrombotic strategies to effectively inhibit thrombosis while preserving hemostasis.

In the following review, we will comment on the key limitations of the currently used antithrombotic regimes in ischemic heart disease and ischemic stroke and provide an in-depth and state-of-the-art overview of the emerging anticoagulant and antiplatelet agents in the pipeline with the potential to improve clinical outcomes.

**Citation:** Barriuso, I.; Worner, F.; Vilahur, G. Novel Antithrombotic Agents in Ischemic Cardiovascular Disease: Progress in the Search for the Optimal Treatment. *J. Cardiovasc. Dev. Dis.* **2022**, *9*, 397. https:// doi.org/10.3390/jcdd9110397

Academic Editor: Qingping Feng

Received: 3 October 2022 Accepted: 11 November 2022 Published: 16 November 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

**Figure 1.** Antiplatelet drugs currently used to treat ischemic heart disease and ischemic stroke. ENT: equilibrative nucleoside transporter; ASA: acetylsalicylic acid; TXA2: thromboxane A2; VWF: Von Willebrand Factor; PDR: phosphodiesterase; COX: cyclooxygenase. Figure created with BioRender.com.

#### **2. The Coagulation Cascade: Targeting the Intrinsic Coagulation Pathway**

Anticoagulants are the treatment of choice to prevent cardioembolic stroke in patients with atrial fibrillation [10,11]. During the last decade, the development of direct oral anticoagulants (DOACs; Figure 2) has brought many advantages as compared to vitamin K antagonists, including a predictable pharmacokinetic profile, rapid onset and offset of action, and fixed dosing with no need for laboratory monitoring or dietary discretion [12]. Conversely, different reversal agents have also been developed to block the effect of anticoagulants in case of need (Table 1).


**Table 1.** Anticoagulant reversal agents in clinical use and preclinical/clinical development.

Yet, important challenges still need to be addressed. As such, bleeding remains the most reported side effect of DOACs, and in certain sub-groups of patients, including patients with mechanical heart valves or triple-positive antiphospholipid disease syndrome, DOACs seem to be less effective than vitamin K antagonists and are not recommended [20–22].

Anticoagulants are also implemented in ischemic heart disease since patients who suffer an acute coronary event present an excess of thrombin generation that persists

beyond the acute presentation [23]. So far, several trials have demonstrated the ability of anticoagulants to protect against cardiovascular events. As such, the addition of warfarin [24], rivaroxaban [25,26], or ximelagatran [27] to a standard antiplatelet regime has shown to significantly reduce ischemic events, though at the expense of increased bleeding risk. Ximelagatran was, however, withdrawn from the market due to hepatotoxicity and the only anticoagulant recommended by the guidelines for long-term secondary prevention is rivaroxaban, which may be administered at low doses on top of aspirin at 1-year post-MI [25].

Altogether, these trials have evidenced the need to discover new targets that effectively block thrombin generation without displaying hemorrhagic side effects. In recent years, special attention has focused on the main components of the intrinsic coagulation pathway, particularly factor (F)XII, FXI, and FIX [28].

#### *2.1. Targeting Factor XII*

FXII has been associated with thrombosis, hereditary angioedema, and (neuro) inflammation. On the other hand, FXII deficiency (i.e., Hageman factor deficiency) is a rare genetic blood disorder that is entirely asymptomatic, showing prolonged active partial thromboplastin times (aPTT) as the only alteration on coagulation tests [29]. FXII circulates in plasma in a zymogen form, and its activation is brought about by the interaction with negatively charged molecules that induce a conformational change in zymogen FXII leading to activated protease FXIIa followed by activation of the enzyme precursors FXI and FIX (Figure 2) [30]. Hence, FXII inhibitors are expected to be particularly efficient in patients whose blood is exposed to non-physiological surfaces of medical devices such as vascular catheters, hemodialysis circuit tubes and membranes, and mechanical valves or stents, that expose negative charge molecules [31]. Alternatively, contact system proteins FXII, high-molecular-weight kininogen (HK), and plasma kallikrein (PK) may assemble on cell surface proteoglycans of various cardiovascular cells. Contact with surface-exposed moieties and plasma-borne soluble contact activators induces FXII activation, which initiates the intrinsic coagulation pathway and activates PK leading to the release of the proinflammatory mediator bradykinin (BK) by PK-mediated cleavage of HK. FXII inhibitors are also being evaluated as a potential treatment for hereditary angioedema, a BK-mediated life-threatening inherited swelling disorder where Serpin C1 esterase inhibitor (a major plasma inhibitor of FXII and and PK) is dysfunctional or deficient [16].

**Figure 2.** Diagram of the coagulation and fibrinolytic pathways and the different anticoagulants and fibrinolytic agents used in the clinical setting. LMWH: low molecular weight heparin. \* Indirect inhibition of factors II, VII, IX and X.

Multiple prototypes have been discovered within the last years, including monoclonal antibodies, small interfering RNAs, antisense oligonucleotides, and serine protease inhibitors which are currently being tested at a preclinical level (details are provided in Table 2) [32]. However, each one of these strategies confers different pharmacological properties, which may limit their indications. Antibodies and approaches to silence gene expression require parenteral delivery by subcutaneous or intravenous injection, whereas small molecules can be delivered orally or parenterally (Table 2). On the other hand, small interfering RNAs and antisense oligonucleotides have a slow onset of action requiring about four weeks to achieve therapeutic levels. Although they are not optimal for use in acute settings, their effect extends over time which may enable once-monthly administration. On the other hand, however, they may also require the development of antidotes. In contrast, antibodies and serine protease inhibitors have a rapid onset of action and an expected half-live of <24 h, thereby limiting the need to develop reversal strategies [33].

**Table 2.** Factor XIIa inhibitors currently under development. This table includes the emerging FXIIa inhibitors and details the studies conducted so far to assess their efficacy and safety.


aPTT: activated partial thromboplastin time; ECMO: extracorporeal membrane oxygenation.

Garadacimab, a monoclonal antibody, has been the sole FXIIa inhibitor to reach phase III clinical trials (NCT04656418) in patients with hereditary angioedema, showing promising preliminary data after a 6-month follow-up. Another phase III trial (NCT04739059) is ongoing to evaluate its benefits in a longer term (32 months). Based on its proven safety profile and the outcome of both trials, garadacimab may be considered a promising strategy for other indications, including CVDs.

#### *2.2. Targeting Factor XI*

Factor XI congenital deficiency has been proven to protect against arterial and venous thrombosis reducing the incidence of deep-vein thrombosis, ischemic stroke, myocardial infarction, and vascular graft occlusion [48–51]. Most importantly, FXI-deficient patients do not generally exhibit spontaneous bleeding, and the bleeding associated with injury or surgery tends to be mild [52,53]. These observations have supported the development of multiple FXI inhibitors, most of which have reached Phase II testing. Table 3 details the studies conducted so far as per FXI inhibitors.

**Table 3.** Factor XIa inhibitors currently under development. This table includes the emerging FXIa inhibitors and details the studies conducted so far to assess their efficacy and safety. VTE: venous thromboembolism; AF: atrial fibrilation; aPTT: activated partial thromboplastin time; MI: myocardial infarction.

