A major complication with enzyme replacement therapy of Factor VIII (FVIII) in Hemophilia A (HA) is the development of anti-drug antibodies. Recently, we have shown that FVIII administration in the presence of heterogeneous phosphatidylserine (PS) nanoparticles derived from a natural source induces tolerance to FVIII, suggesting that PS converts an immunogen to a tolerogen. However, the specific structural features responsible for the immune-regulatory properties of PS is unclear. Identifying a specific PS species that is responsible is critical in order to further develop and optimize this nanoparticle. Further, clinical development of this lipid-based strategy requires optimization of the lipid particle that is homogeneous and synthetic. Here, we investigate the ability of mono-acylated Lyso-PS to induce hypo-responsiveness towards FVIII in HA mice. Administration of both PS and Lyso-PS FVIII significantly reduced anti-FVIII antibody responses despite rechallenge with FVIII. Additionally, the Lyso-PS-mediated effect was shown to be antigen-specific as mice responded normally against a rechallenge with an unrelated antigen, ovalbumin. Furthermore, the hypo-responsiveness observed with Lyso-PS may involve interactions with a specific PS receptor, TIM-4, along with increasing regulatory T-cells. These data indicate that using Lyso-PS allows for a more homogenous formulation in order to induce tolerance towards therapeutic proteins.

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Phosphatidylserine (PS) exposure during apoptosis leads to silent clearance of cells without adverse immune reactions to self-proteins. Given the biological functions of PS in cellular cleanup and global immunosuppression, we hypothesized that administration of PS-protein complexes would reduce immunogenicity. Here, we report that exposing Pompe disease mice to acid alpha glucosidase (rhGAA) with PS or immunosuppressant dexamethasone resulted in lower anti-rhGAA antibodies than in animals receiving rhGAA alone. However, upon rechallenge with rhGAA, only PS-rhGAA pre-exposed mice displayed a durable hyporesponsiveness even after PS administration was ceased. Thus, pre-exposure of antigens administered together with PS were not silently cleared, but the immune system acquired memory about the antigen that averted mounting of a response during rechallenge. In hemophilia A mice, PS hyporesponsiveness toward Factor VIII was reversed by administration of function-blocking antibody against the PS receptor T-cell immunoglobulin and mucin 4, implicating this receptor in PS's effect. Moreover, pre-exposure of myelin oligodendrocyte glycoprotein peptide with PS delayed the onset and reduced the severity of experimental autoimmune encephalomyelitis. These observations suggest that PS's function in apoptosis is not limited to silent antigen clearance without immune responses toward self-proteins but shows that PS reduces immune response during rechallenge to several antigens that also involves initiation of antigen tolerance.

Factor XII (FXII) is a serine protease involved in multiple cascades, including the kallikrein-kinin system. It may play a role in diseases in which the downstream cascades are dysregulated, such as hereditary angioedema. Garadacimab (CSL312) is a first-in-class, fully human, monoclonal antibody targeting activated FXII (FXIIa).We describe how translational pharmacokinetic (PK) and pharmacodynamic (PD) modeling enabled dose selection for the phase I, first-in-human trial of garadacimab.The PK/PD data used for modeling were derived from preclinical PK/PD and safety studies. Garadacimab plasma concentrations rose with increasing dose, and clear dose-related PD effects were observed (e.g., a mechanism-based prolongation of activated partial thromboplastin time). The PK/PD profile from cynomolgus monkeys was used to generate minimal physiologically-based pharmacokinetic (mPBPK) models with target-mediated drug disposition (TMDD) for data prediction in cynomolgus monkeys. These models were later adapted for prediction of human data to establish dose selection. Based on the final mPBPK model with TMDD and assuming a weight of 70 kg for an adult human, a minimal inhibition (<10%) of FXIIa with a starting dose of 0.1 mg/kg garadacimab and a near maximal inhibition (>95%) at 10 mg/kg garadacimab were predicted. The phase I study is complete, and data on exposure profiles and inhibition of FXIIa-mediated kallikrein activity observed in the trial support and validate these simulations. This emphasizes the utility and relevance of translational modeling and simulation in drug development. Study Highlights WHAT IS THE CURRENT KNOWLEDGE ON THE TOPIC?Models based on physiology and mechanisms of action can be highly useful in translating pharmacokinetic/pharmacodynamic (PK/PD) data from animal studies to expectations in humans.

Phosphatidylserine (PS) is a naturally occurring anionic phospholipid that is primarily located in the inner leaflet of eukaryotic cell membranes. The role of PS during apoptosis is one of the most studied biological functions of PS. Externalization of PS during apoptosis mediates an "eat me" signal for phagocytic uptake, leading to clearance of apoptotic cells and thus maintain selftolerance by immunological ignorance. However, an emerging view is that PS exposure-mediated cellular uptake is not an immunologically silent event, but rather promoting an active tolerance towards self and foreign proteins. This biological property of PS has been exploited by parasites and viruses in order to evade immune surveillance of the host immune system. Further, this novel immune regulatory property of PS that results in tolerance induction can be harnessed for clinical applications, such as to treat autoimmune conditions and to reduce immunogenicity of therapeutic proteins. This review attempts to provide an overview of the biological functions of PS in the immune response and its potential therapeutic applications.