A mechanistic model of in vitro plasma activation to evaluate therapeutic kallikrein-kinin system inhibitors

Background: The kallikrein-kinin system (KKS) is a complex biochemical pathway that plays a crucial role in regulating several physiological processes, including inflammation, coagulation, and blood pressure. Dysregulation of the KKS has been associated with several pathological conditions such as hereditary angioedema (HAE), hypertension, and stroke. Developing an accurate quantitative model of the KKS may provide a better understanding of its role in health and disease and facilitate the rapid and targeted development of effective therapies for KKS-related disorders. Objectives: Here, we present a novel, detailed mechanistic model of the plasma KKS, elucidating the processes of Factor XII (FXII) activation, the kallikrein feedback loop, cleavage of high molecular weight kininogen leading to bradykinin (BK) production, and the impact of inhibitors. Methods: The model incorporates both surface and solution-phase reactions of all proteins in the KKS, describing how binding site concentration affects the rate of surface reactions. The model was calibrated and validated using a variety of published and in-house experimental datasets, which encompass a range of dextran sulphate (DXS) concentrations to initiate contact activation and various KKS inhibitors to block bradykinin production. Results: Our mathematical model showed that a trace amount of activated FXII is required for subsequent FXII activation. The model also reveals a bell-shaped curve relationship between the activation of the KKS and the number of DXS surface binding sites. Simulations of BK generation in healthy and HAE plasma demonstrated the impact of C1 esterase inhibitor (C1inh) deficiency via increased peak BK levels and accelerated formation in HAE plasma. The efficacy of KKS inhibitors, such as CSL312, ecallantide, and C1inh, was also evaluated, with CSL312 showing the most potent inhibition of BK generation. Conclusions: The present model represents a valuable framework for studying the intricate interactions within the plasma KKS and provides a better understanding of the mechanism of action of various KKS-targeted therapies. Author summary: The plasma kallikrein-kinin system (KKS) is a biological pathway that impacts inflammation, blood clotting, and blood pressure. This system is regulated by endogenous inhibitors; however, in diseases such as hereditary angioedema, overactivation of the plasma KKS occurs due to insufficient inhibition. The KKS operates via a series of biochemical reactions within the blood and on the surfaces of blood vessels. Each step activates an enzyme essential for the subsequent step, ultimately leading to the formation of a pro-inflammatory peptide called bradykinin. The system is initiated by the activation of Factor XII (FXII) upon contact with negatively charged surfaces. FXII activation can also occur in vitro by the addition of dextran sulphate (DXS). In this study, we describe an innovative mechanistic model of the plasma KKS. This model integrates both surface and volume reactions within the KKS, illustrating how variations in DXS concentration can influence reaction rates in a bell-shaped manner. The model was validated using experimental data, covering a range of DXS concentrations and clinical KKS inhibitors. We demonstrate that, in contact activation-initiated systems, targeting FXII inhibition is a more effective treatment approach than inhibiting downstream enzymes for the prevention of bradykinin generation.