Anouk Donners

158 Chapter 9 Background People with haemophilia A (PwHA) have a deficiency of coagulation factor VIII (FVIII) [1]. These individuals present with recurrent bleeding, predominantly into major joints, eventually resulting in chronic arthropathy [2]. The first effective treatment of bleeding became available with the discovery of cryoprecipitate in the 1960 [3] followed by the development of plasma-derived concentrates of FVIII (pdFVIII) in the 1970s, enabling home therapy [4]. Subsequently, the first recombinant FVIII (rFVIII) concentrates were introduced in the 1990s, eliminating the virus-related problems associated with pdFVIII [5, 6]. In recent decades, rFVIII products with extended plasma half-lives have been bioengineered, non-factor replacement products (e.g., emicizumab) were introduced [4, 5], and gene therapy was recently regulatory approved [7]. Figure 1. Process of measuring (blue) and monitoring (yellow) in a clinical setting. Measuring and monitoring in a clinical setting (Figure 1) are important, as described in the general introduction. Measuring involves assigning a numerical or categorical value to a certain parameter, whereas monitoring involves clinically interpreting these (potentially changing) values to make clinical decisions regarding diagnosis or therapy. The process of measuring and monitoring FVIII activity is key in the care management of people with haemophilia A [8]. This FVIII activity is measured using a clotting assay and expressed as a percentage compared with the FVIII activity in the pooled human plasma of people without a FVIII deficiency. This biomarker directly correlates with the likelihood of bleeding and is used for diagnosis, clinical support and dose optimisation (see Figure 2 of general introduction) [9]. On the other hand, measuring and monitoring emicizumab is not yet standard care. For instance, emicizumab was regulatory approved with a bodyweight-based dose regimen without requiring dose adjustments based on laboratory monitoring [10]. In the pre-approval clinical trials, the emicizumab concentration was measured for pharmacokinetic purposes with an enzyme-linked immunosorbent assay (ELISA) [11-14]. In current clinical practice, this concentration is infrequently measured (see Figure 2 of general introduction) using a modified, calibrated clotting assay [15]. The liquid chromatography (LC) coupled with tandem-mass spectrometry (MS/MS) analysis is a relatively novel technique for quantifying protein concentrations in human plasma. Prior to this thesis, the potential value of concentration measurements with LC-MS/MS analysis for monitoring PwHA in clinical practice was unclear. The knowledge gaps regarding this process concerned the relationship between the FVIII concentration