FXII inhibition: multipronged benefits

In this issue of Blood, Silasi et al¹  identified that inhibition of activated factor XII (FXIIa) using an antibody (5C12) reduces the activation of coagulation and the kallikrein-kininogen pathways, induced by heat-inactivated Staphylococcus aureus (HI-SA). Furthermore, by inhibiting FXII function, there was decreased activation of complement and inflammatory cytokines, resulting in preserved organ function and survival of baboons subject to a challenge with HI-SA. These findings suggest potential benefit of prophylactic treatment of patients at increased risk of pathological coagulation and inflammatory responses using an inhibitor of FXIIa. Such indications include prevention of sepsis or its mitigation in severe infections in which FXIIa activity is involved in its pathogenesis.

For many years, the role of FXII in coagulation was not understood, until Renne et al²  discovered that FXII plays an essential role in the propagation phase of pathological clot formation while not being required for normal hemostasis. This paradigm shift of a separation between thrombosis and hemostasis led to the concept of anticoagulation using agents with minimal bleeding side effects (ie, the development of inhibitors of FXIIa and FXIa) as next-generation anticoagulants. FXII-deficient individuals do not exhibit a bleeding phenotype and are asymptomatic, whereas FXI deficiency has a mild bleeding phenotype upon a challenge such as trauma or surgery (with the level of bleeding not correlating with FXI levels). Inhibitors of FXIIa and FXIa have shown excellent anticoagulant efficacy with lower bleeding risk than anticoagulants on the market in preclinical models. Less explored is the role FXIIa and FXIa inhibition on the inflammatory pathways. FXII activity leads to increased response of the kallikrein-kinin pathway, resulting in bradykinin release and inflammation.^3,4  In addition, FXIIa is reported to activate the complement pathway.⁵  Lupu et al⁶  have demonstrated previously that targeting FXI results in decreased coagulation and inflammatory responses using a sepsis baboon model treated with HI-SA. This current manuscript demonstrates that baboons challenged with HI-SA, which were treated an anti-FXII antibody (5C12, which inhibits FXII function), exhibited mild and transient clinical symptoms, recovering fast and reaching the 7-day end point without symptoms. The vehicle-only group did not survive the 7-day study period. The 5C12 FXII inhibitor treatment of the challenged baboons led to reduced reciprocal activation of FXII and prekallikrein, resulting in less high molecular weight kininogen cleavage (indicative of bradykinin release) and downstream activation of coagulation, compared with challenged animals treated with vehicle only. HI-SA challenge in the baboons caused release of tPA and increased levels of plasminogen activator inhibitor-1, as is common in disseminated intravascular coagulation. Elevated markers of fibrinolysis were observed in plasma samples from HI-SA–challenged animals administered vehicle only, but these markers were dramatically reduced in challenged baboons treated with 5C12. HI-SA challenge to the baboons (with vehicle only) led to an early increase in red blood cell and hematocrit, potentially indicative of increased capillary permeability induced by bradykinin generated after high molecular weight kininogen cleavage. This increase was not observed in challenged animals treated with 5C12. Furthermore, striking accumulation of neutrophils was shown in the lungs, evidenced by staining for neutrophil elastase and was accompanied with a marked increase of myeloperoxidase released into the plasma in vehicle only, HI-SA challenged animals, but not those treated with 5C12. This indicates decreased activation of neutrophils associated with inhibition of FXII, which prevented lung pathology at the 7-day end point. HI-SA–challenged baboons with vehicle only had progressive multiorgan dysfunction with organ failure in all animals. These baboons demonstrated signs of early cardiorespiratory distress with a drop in mean systemic arterial pressure, tachycardia, tachypnea, and transient increase of body temperature. Furthermore, these animals gradually developed liver, pancreas, and kidney dysfunction or damage, indicated by elevated serum blood urea nitrogen, creatinine, ALT, and pancreatic amylase. After an initial increase in blood glucose at 2 hours, animals became hypoglycemic within 4 to 8 hours postchallenge, indicative of metabolic disturbance. In contrast, 5C12 treatment prevented the drop in blood pressure and rise in body temperature. The treated animals were protected from acute organ damage, and hypoglycemia was less severe. In addition, untreated SI-HA–challenged baboons had highly elevated levels of circulating nucleosomes (a marker of cell death). Inhibition of FXIIa significantly reduced plasma nucleosomes, consistent with organ preservation.

Baboons subjected to HI-SA without 5C12 demonstrated increased complement activation, evidenced by elevated circulating C3b and soluble terminal complement complex C5b-9. Inhibition of FXII with 5C12 resulted in markedly reduced C3b and sC5b-9. HI-SA challenge in baboons with vehicle only provoked systemic inflammation evidenced by high levels of plasma cytokines, including tumor necrosis factor; interleukin-1β (IL-1β), IL-6, IL-8, IL-10; and granulocyte-macrophage colony stimulating factor. This cytokine storm was reduced when the HI-SA–challenged animals were treated with 5C12.

These exciting data show promise for reduced inflammation and coagulation with prophylactic treatment using an FXIIa inhibitor. Although the 2 studies are not directly comparable, the effects of FXII inhibition in the same animal model showed equivalent positive effects on reducing activation of coagulation but with apparent stronger effects on reducing inflammatory biomarkers than prophylactic FXI inhibition⁶  (although other factors such as dosage/potency would need to be taken into consideration as potential limitations of such a direct comparison).

To summarize, inhibition of FXIIa activity in HI-SA–treated baboons prevented the activation of kallikrein-kininogen and coagulation systems, decreased complement activation and inflammatory cytokines, and preserved organ function, leading to survival of the baboons against the HI-SA challenge (see figure). The Silasi et al article demonstrates potential of FXII inhibition beyond its antithrombotic properties but also anti-inflammatory benefits that may apply to many clinical indications beyond the model described. Interestingly, another company (CSL Behring) has also developed an antibody targeting FXIIa (garadacimab or CSL312), which has shown promise in the treatment of patients with hereditary angioedema.⁷  A phase 2, multicenter, double-blind, randomized, placebo-controlled study to evaluate CSL312 in coronavirus disease 2019 has just been completed.⁸  It will be interesting to see the data from this study. The future is very exciting with the potential of employing FXIIa inhibitors prophylactically, for both anticoagulant and anti-inflammatory efficacy with minimal bleeding side effects.

Conflict-of-interest disclosure: The author is a founder of LUNAC Therapeutics, which develops small molecule FXIIa inhibitors. ▪