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.

Caplacizumab, a humanized anti-von Willebrand Factor (VWF) Nanobody®, is employed for immune-mediated Thrombotic Thrombocytopenic Purpura (iTTP) treatment. Its binding to the VWF A1 domain sterically inhibits VWF interaction with platelet glycoprotein Ib (GPIb), counteracting microthrombosis and accelerating the normalization of the platelet count. In caplacizumab-treated iTTP patients 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 VWF:GPIbM, VWF:GPIbR, and VWF:Ab. To evaluate the capacity of five VWF activity assays to measure caplacizumab in vivo inhibitory effect on VWF-GPIb interaction. 17 iTTP patients treated effectively with caplacizumab, as demonstrated by normal platelet counts, were assessed by VWF activity using five commercially available assays. Three patients were further evaluated with the Total-Thrombus-Analysis-System (T-TAS). 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, though 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. The VWF:GPIbM and VWF:RCo appear to be the most suitable assays for monitoring VWF activity in iTTP patients 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.