Evaluating the Clinical Relevance of Thrombin Generation Potential during Emicizumab Therapy
Authors: Van Der Zwet Konrad , Roest Mark , Huskens Dana , Van Beers Eduard J. , Fischer Kathelijn , Van Vulpen Lize Fie Ditteke , Schutgens Roger EG , Urbanus Rolf T. 
Published: December 2024

Background: Althoughemicizumab has proven to be effective in reducing the annualized (joint) bleeding rate (A(J)BR), both spontaneous breakthrough bleeds and thromboses have been reported in hemophilia A patients receiving emicizumab therapy. This underscores the need for a test to monitor hemostatic activity in patients on steady-state emicizumab therapy. Thrombin generation (TG) can be used to assess plasma coagulation potential and might be used for monitoring hemostatic activity during emicizumab therapy.

Aim: To explore the association between TG parameters and clinical efficacy, as measured by both emicizumab plasma concentrations and bleeding phenotype, in patients with hemophilia A on steady-state emicizumab prophylaxis.

Methods: In this single-center cohort study samples were collected prospectively during the steady state of emicizumab therapy at 3 and 12 months. TG was measured in platelet-poor plasma with calibrated automated thrombinography, initiated with both tissue factor (TF, 1 pM) and activated factor XI (FXIa, 200 pM). Data were analyzed with Thrombinoscope CAT software.

Emicizumab concentrations were measured using a validated liquid-chromatography tandem mass spectrometry method. Bleeding data were collected from the electronic patient records and/or electronic patient diary, only treated bleeds were recorded.

Patient characteristics were summarized as numbers (%) and medians with IQR (P25-P75). Mean

A(J)BR were estimated by negative binomial regression modelling. The association between TG parameters (endogenous thrombin potential (ETP) and peak height) and emicizumab plasma concentrations was analyzed using Pearson's correlation. The Wilcoxon sum rank test was used to compare TG parameters between bleeders and non-bleeders. Changes over time for TG parameters and emicizumab concentrations were analyzed using multilevel regression analysis.

Results: 49 patients were included in the study, longitudinal samples were available from 36 patients (84%). The median age was 26 (IQR: 15 -48) years, including 16 children. The majority of patients had severe hemophilia A (92%, n=45) and no FVIII-inhibitors (96%, n=46).

Thrombin generation kinetics differed substantially between TF and FXIa initiated TG, with much sharper and higher peaks for FXIa than for TF. At 12 months of emicizumab therapy, the ETP of TF-initiated TG was 76% relative to normal pooled plasma (NPP) (median 757 [IQR 637 - 883] nM*min) and the ETP of FXIa-initiated TG was 85% of NPP (median 887 [IQR 831 - 990] nM*min). Peak height was 56% relative to NPP for TF (median 51 [IQR 30 - 67] nM) and 62% relative to NPP for FXIa (median 186 [IQR 159-213] nM).

The median emicizumab concentration was 66.8 (IQR 56.7 - 77.6) µg/ml one year after initiation of emicizumab therapy. Although we found a moderate correlation between ETP and emicizumab concentration for TF-initiated TG (r 0.47), ETP did not correlate with emicizumab concentration for FXIa-initiated TG (r 0.15). We observed weak to moderate correlations between peak height and emicizumab concentration for both TF (r 0.41) and FXIa (r 0.40).

For 36 patients with longitudinal samples, emicizumab concentration levels at 3 months were similar to levels at 12 months (mean difference 1.3 µg/ml, p= 0.587), as were thrombin generation parameters obtained with TF (mean difference in ETP 63 nM*min, p=0.285; mean difference in peak height 3 nM, p=0.622). However, ETP and peak height obtained with FXIa declined over time (Mean difference in EPT 106nM*min, p&lt;0.001 and mean difference in peak height 12 nM, p=0.071)

The mean ABR was 0.6 (95CI 0.4 - 0.9) and AJBR was 0.3 (95CI 0.2 - 0.5). Most bleeds were traumatic (97%, n=30) with only one bleeding episode categorized as spontaneous. Emicizumab concentrations were similar in patients with (n=19) and without (n=30) treated bleeding episodes (65.9 vs 69.4 µg/ml, p=0.682), as were TG parameters for both TF and FXIa-initiated assays.

Conclusion: Despite identifying weak to moderate correlations between TG parameters and emicizumab concentrations, our findings suggest that both TF and FXIa TG parameters do not relate with bleeding phenotype. While this raises questions about the clinical relevance of TG, only one spontaneous bleed was observed in the current cohort. Furthermore, TG parameters initiated with TF were more stable over time compared to those initiated with FXIa.