Evaluation of different platelet-dependent von Willebrand factor activity assays to assess the in vivo inhibitory effect of caplacizumab on the von Willebrand factor–platelet interaction

Background: Caplacizumab, a humanized anti–von Willebrand factor (VWF) Nanobody, is used for immune-mediated thrombotic thrombocytopenic purpura (iTTP) treatment. Its binding to the VWF A1 domain inhibits VWF interaction with platelet glycoprotein (GP)Ib, counteracting microthrombosis and accelerating the normalization of the platelet count. In caplacizumab-treated patients with iTTP with bleeding episodes, measuring platelet-dependent VWF activity (VWF activity) is crucial for monitoring treatment with VWF concentrates. This activity, traditionally assessed by the VWF-ristocetin cofactor assay (VWF:RCo), nowadays could be evaluated with alternative methods as gain-of-function mutant GPIb binding assay (VWF:GPIbM), ristocetin-triggered GPIb binding assay (VWF:GPIbR), and monoclonal antibody binding-based VWF activity (VWF:Ab). Objectives: This study aimed to evaluate the capacity of 5 VWF activity assays to measure caplacizumab in vivo inhibitory effect on VWF-GPIb interaction. Methods: In total, 17 patients with iTTP treated effectively with caplacizumab, as demonstrated by normal platelet counts, were assessed by VWF activity using 5 commercially available assays. Three patients were further evaluated with the total thrombus analysis system (T-TAS). Results: Patients treated with caplacizumab showed no VWF activity using VWF:GPIbM and VWF:RCo assays. The VWF:Ab assay produced the highest values, while both VWF:GPIbR assays showed reduced VWF activity levels, although not to the extent of VWF:GPIbM and VWF:RCo. In patients analyzed by T-TAS no capillary occlusion was observed, indicating complete inhibition of the VWF-GPIb interaction by caplacizumab. Conclusion: The VWF:GPIbM and VWF:RCo appear to be the most suitable assays for monitoring VWF activity in patients with iTTP undergoing treatment with caplacizumab, as confirmed by T-TAS analysis. The VWF:GPIbR assays may be useful, but have a limited ability to detect caplacizumab's inhibition. The VWF:Ab assay is unsuitable for this purpose.