PB1277 - Thrombin generation and clot formation and lysis properties in hemophilia A with and without emicizumab

Background: Emicizumab is a non-factor therapy for persons with hemophilia A (PwHA) and exhibits factor VIII (FVIII)-mimetic activity. The FVIII-equivalent activity of emicizumab, however, remains unclear.

Aims: Here, we investigated how global assays of hemostasis could be used to quantify the hemostatic effect of emicizumab compared to FVIII levels in PwHA.

Methods: Citrate plasma was sampled from PwHA of all severities and therapeutic regimens. We measured thrombin generation using a commercially available assay (TGA, Technothrombin, Technoclone, Vienna, Austria) and plasma clot formation and lysis properties (CLA) with a turbidimetric assay. FVIII activity was measured by chromogenic substrate assay (CSA, Biophen, Hyphen Biomed, Neuville-sur-Oise, France). Emicizumab concentration were derived from measurements with a modified one-stage clotting assay (r² Diagnostics , South Bend, IN, USA). We regressed FVIII and emicizumab levels on parameters from both global assays using ordinary-least-squares. Predictive performances of those parameters were assessed by R2, correcting for optimism by bootstrapping.

Results: In total, we included 221 samples from 96 patients with hemophilia A (39 severe, 17 moderate, 40 mild). Twenty-six samples had been collected from seven subjects who were on emicizumab at various stages of their loading or maintenance dosing regimens and had not administered FVIII recently. Overall, we found substantial amount of variation in FVIII levels that remained unexplained by global assay parameters with peak thrombin (R2=0.39; 95%CI 0.27, 0.53) and amplitude (R2=0.21; 95%CI 0.14, 0.33) performing best out of the TGA and CLA parameters, respectively. With lower variation over the measured ranges, the explained variability of emicizumab concentrations by global assay parameters was more modest (Table 1). Figure 1 depicts the relationship of parameters of global assays with FVIII and emicizumab, respectively.

Conclusion(s): In summary, the predictive performance of single parameters of both TGA and CLA was modest for FVIII and over the range of emicizumab concentrations measured in this study.