In this issue of Blood, Shapiro et al report phase 2 results regarding daily concizumab prophylaxis in patients with hemophilia A or hemophilia B, thus providing proof-of-concept in preparation for the phase 3 clinical trial currently underway (NCT03741881).1 

(A-C) TFPI mechanism of action. (D) Anti-TFPI antibodies currently in clinical trials. AR, factor V acidic region; BR, factor V basic region.

(A-C) TFPI mechanism of action. (D) Anti-TFPI antibodies currently in clinical trials. AR, factor V acidic region; BR, factor V basic region.

Hemophilia A and B are severe inherited bleeding disorders that result from the deficiency or dysfunction of coagulation factors VIII (FVIII) and IX (FIX), respectively. Both FVIII and FIX are key components of the intrinsic tenase complex, which consists of FVIIIa, FIXa, FX, and a negatively–charged phospholipid surface, and which functions to activate FX, ultimately leading to a robust thrombin burst. Without FVIII or FIX as in hemophilia, FXa is produced by the tissue factor (TF)-FVIIa complex. Thus, inhibition of FXa and TF-FVIIa results in dampened clot formation. Studies have confirmed that increased production of FXa via the TF-FVIIa complex can improve thrombin generation in hemophilia.2  Thus, novel therapeutics seek to obstruct the natural anticoagulants that mediate the extrinsic coagulation pathway for therapeutic advantage. Tissue factor pathway inhibitor (TFPI) is a Kunitz-type serine protease that inhibits the extrinsic coagulation pathway (see figure).3  In humans, there are 2 main isoforms of TFPI, TFPIα and TFPIβ, which have identical K1 and K2 domains. The rate-limiting step in the inhibitory function of TFPI is the binding of FXa to the K2 domain. This initial interaction directly inhibits FXa and also facilitates the formation of a TFPI-FXa-TF-FVIIa quaternary complex and subsequent inhibition of TF-FVIIa by the K1 domain of TFPI. TFPIα has an additional K3 domain and a positively–charged C-terminal tail uniquely capable of binding partially cleaved forms of FV that have an exposed B domain acidic region. Thus, TFPIα, through the interaction of its C-terminal tail and FV, is distinctively capable of inhibiting early forms of prothrombinase, further diminishing the initiation of coagulation.4 

Several products targeting TFPI are currently under development for potential therapeutic use in patients with hemophilia, including 3 monoclonal antibodies: concizumab, PF06741086, and BAY-1093884. Concizumab is a monoclonal antibody targeting the K2 domain of TFPI and is the primary subject of this commentary.5  In the combined therapeutic portions of the phase 2 trials reported in this issue of Blood, Shapiro et al evaluated the efficacy of daily subcutaneous injections of concizumab as bleeding prophylaxis in 2 cohorts of adult patients over 24 weeks: patients with severe hemophilia A (HA) without inhibitors (Explorer 5 trial) and patients with HA or hemophilia B (HB) with a history of high-titer (≥5 BU/mL) inhibitors (Explorer 4 trial). Concizumab dosing was based on phase 2 pharmacokinetic and pharmacodynamic results to potentially achieve normal thrombin generation. The inhibitor patients were given a loading dose of concizumab and then 0.15 mg/kg once per day whereas the non-inhibitor patients did not receive a loading dose. Approximately 60% of HA patients continued to receive the 0.15 mg/kg dose; the remaining 40% required dose escalation to a maximum of 0.25 mg/kg once per day because of breakthrough bleeds. The majority (>87%) of patients with inhibitor HA or HB remained at a 0.15 mg/kg once per day dose. The data showed a decrease in annualized bleeding rate (ABR) in patients receiving concizumab with no concerning serious adverse events, including thromboembolic events and thrombotic microangiopathy. Curiously, the non-inhibitor patients had the highest ABR. This may be explained by the lack of a loading dose and the subsequent need for dose escalation. As expected, concizumab plasma concentrations were dose-dependently and directly associated with peak thrombin generation, D-dimer levels, and prothrombin fragment 1+2 levels and were inversely related to unbound TFPI. A total of 6 patients developed anti-drug antibodies, 3 of which were neutralizing on a single occasion, and none of which were clinically relevant.

Several concerns remain regarding targeting TFPI as a means of enhancing TF-FVIIa–mediated FXa and thrombin generation. The most pressing question regards the potential to swing the balance of coagulation toward an increased thrombotic risk as demonstrated by the fatal cerebral sinus thrombosis occurring in the Fitusiran trial.6  Low TFPI levels have been associated with increased risk of venous thromboembolism and atherosclerotic disease.7,8  Furthermore, complete TFPI deficiency seems to be incompatible with life because no cases have been reported, and animal models completely lacking TFPI result in embryonic lethality. By maintaining extraembryonic TFPI expression and selectively ablating the K1 domain, Castillo et al strategically developed a murine cell line with severe TFPI deficiency. These mice exhibited a prothrombotic phenotype with increased thrombin generation potential in vitro, renal fibrin deposits, and rare cases of brain ischemia.9  Despite interspecies differences in TFPI, this model will be useful in investigating the pathophysiology of TFPI, a topic of paramount importance because more anti-TFPI therapeutics are being evaluated in clinical trials. Although no thrombotic events or thrombotic microangiopathy were evident in the 2 trials reported by Shapiro et al, the dose-dependent elevation of D-dimer and prothrombin 1+2 fragments raise concerns regarding the potential thrombotic propensity of an iatrogenic TFPI deficiency state, acknowledging the baseline elevation of D-dimer in some cases before treatment with concizumab. One healthy volunteer in the Explorer 1 trial developed superficial thrombophlebitis.10  In addition, because of concern for less conventional clearance, reduction in fibrinogen, and elevation of D-dimers, more frequent injections of lower concizumab doses were chosen for the phase 2 trials. For the upcoming phase 3 trial, the authors plan to use a loading dose and to start at 0.25 mg/kg once per day. The sequential details in clot formation have not been fully elucidated, and elevated D-dimers may represent transient hypercoagulability with no thrombotic consequence. This particular concern requires careful monitoring.

Ultimately, the results of these trials will provide clinical proof-of-concept regarding anti-TFPI therapy in hemophilia patients and support the further evaluation of concizumab. If further studies confirm the safety and efficacy of anti-TFPI therapies, whether or not these agents can supplant emicizumab in the HA space remains to be determined. Perhaps more importantly, anti-TFPI therapy may have widespread applications in other rare bleeding disorders such as Glanzmann thrombasthenia and Bernard–Soulier syndrome and would provide a viable alternative to recombinant FVIIa prophylaxis for HB patients with inhibitors. As it stands, the care of patients with HA or HB has quickly shifted from conventional therapeutic protein replacement to the mitigation of bleeding via targeted FVIII mimetics, rebalancing natural anticoagulants, and viral vector–mediated gene therapy. It is clear that the next decade of hemophilia care will focus on normalizing the bleeding phenotype because the community standard has shifted from reducing life-threatening bleeding to a more normal life without bleeding events.

Conflict-of-interest disclosure: R.F.S. has received honoraria for participating in advisory boards from Genentech/Roche, Shire/Takeda, Bioverativ/Sanofi, Octapharma, Grifols, Biomarin, Uniqure, CSL Behring, and Novo Nordisk and has received research funding from Genentech, Shire/Takeda, Octapharma, Bioverativ, Grifols, and Kedrion. K.L.Z. has received research funding from the National Hemophilia Foundation-Shire Clinical Fellowship Program and from Pfizer.

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