Concizumab As a Possible Treatment for Bleeding Disordersof Unknown Cause

Bleeding disorders of unknown cause (BDUC) are defined as a marked tendency to bleed in the presence of normal haemostasis. Its diagnosis is challenging because routine coagulation and platelet function tests do not detect the disorder. There are few studies on the management of bleeding in BDUC and most relate to the use of tranexamic acid (Baker et al, J Thromb Haemost. 2024;22:2059-2070).

We have previously shown that the thrombin generation(TG) test is useful in diagnosing some BDUC when contact phase activation is blocked to allow tissue factor (TF)-dependent coagulation (Acuña et al, Haemophilia, 30, S1, 2024: 169). We can speculate that drugs that act by enhancing this pathway may be useful in the treatment of these patients. Concizumab is a humanised monoclonal antibody in development as a prophylactic treatment for haemophilia of all subtypes that binds TF pathway inhibitor (TFPI) and prevents TFPI inhibition of FXa generated in the initiation phase of coagulation. We therefore aimed to investigate in vitro the possible beneficial effects of treating BDUC with concizumab.

This is a single-center, prospective, controlled study approved by our CEIM (PI-4514).

To diagnose a BDUC we performed blood count, PT, aPTT, TT, von Willebrand factor (vWF) antigen, vWF function, clotting factors VIII, IX and XI, platelet LTA, flow cytometry and T-Tas using the PL chip.

TGT was performed by calibrated automated thrombography (CAT) in citrated corn trypsin inhibitor (CTI, a contact activation phase inhibitor) platelet free plasma (PFP). A low level of tissue factor (1 pM TF) and 4 μM phospholipids was used as a trigger. TGT was performed without any drug and with concizumab 100, 400 and 4000 ng/ml and with rFVIIa (Novo Nordisk A/S) at a concentration of 20 nM, corresponding to a dose of 90 ug/kg.

Lagtime, peak and endogenous thrombin potential (ETP) were calculated. A healthy control group (n=40) was also included to determine ranges of normality.

TFPI activity and antigenic concentration were measured with ACTICHROME® TFPI and TFPI Invitrogen kits respectively.

We included 247 patients aged >18 years who from the Haematology Unit of the University Hospital La Paz between January 2021 and June 2024 because of spontaneous mucocutaneous bleeding or minor or major surgery. Abnormalities in platelet function and/or platelet count and/or coagulation factors or fibrinogen were detected in 187 of the patients studied and were excluded. The remaining 60 patients were considered BDUC and we performed TGT in citrated CTI PFP from these patients.

Thirteen of these patients had TG parameters within the normal.

Out of 47 patients, 3 showed no effect of concizumab and rFVIIa, whereas in the remaining 44 patients (14 men and 30 women, with BATH scores of 4.6±1.0 and 6.1±1.7, respectively), TG ETP and peak showed decreased values. In vitro addition of concizumab showed a concentration-dependent effect on these parameters: 100 ng/ml caused a slight increase in peak and ETP and 400 and 4000 ng/ml normalised them. We can assume that 400 ng/ml is sufficient to normaliseTG, since a higher concentration such as 4000 ng/ml does not cause a further increase in peak and ETP. The effect of concizumab 400 ng/ml was similar to that of rFVIIa (equivalent to 90 μg/kg).

Nineteen patients who recovered TG with concizumab had TFPI activity near the upper end of the normal range (0.80-1.23 Units/mL). The remaining 25 had higher TFPI activity than healthy controls (Units/mL, control: 1.17±0.17; BDUC: 1.33±0.33, p=0.02). No differences with controls were found in total TFPI (µg/mL, control: 38.4±8.9; BDUC: 35.0±14.9).

As TFPI acts on the TF-FVIIa complex, we assessed TG as a function of TF content of these samples using 4 µM PL as activator. Samples with higher TFPI activity had lower TG capacity, demonstrating the inhibitory effect of TFPI on TF. In support of this, we found an inverse relationship between TFPI activity and TG peak (Spearman r=-0.4509, p<0.05).

The cause of bleeding in BDUC patients is varied. In some cases, BDUC appears to be associated with a decrease in TF-dependent TG. In these cases, concizumab at concentrations similar to the trough plasma levels observed in haemophilia clinical trials (350-550 ng/ml, Matsushita T et al, N Engl J Med. 2023 Aug 31;389(9):783-794) and rFVIIa emerge as potential therapeutic alternatives for these patients.

Novo Nordisk Pharma S.A. provided concizumab and funded this study as well as ISCIII- European Union PI22/01489