Doctor of Philosophy in Chemistry (PhD)
Immunomodulation through modification of cell surfaces using polymers
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No abstract available.
The full abstract for this thesis is available in the body of the thesis, and will be available when the embargo expires.
With the advent of antibiotic resistance and crisis, it is crucial to find substitutes to conventional antibiotics. Antimicrobial peptides (AMPs) are considered to be viable alternatives, because they are broad spectrum and bacteria develop little or no resistance towards AMPs. Interestingly, only few AMPs are used as therapeutics, due to problems such as host toxicity, protease cleavage and short half-life. Therefore, there is a need to improve the efficacy of AMPs by the use of D-peptides and/or delivery vehicles. The introduction of the thesis describes the diversity and various mechanisms of action (MOA) of AMPs. The issues and ways to improve the efficacy of AMPs, which forms the foundation of this thesis, are also discussed.Recently, hyperbranched polyglycerol (HPG) has gained attention due to its excellent biocompatibility, multifunctionality and long blood circulation time. The body of the thesis describes a methodology to covalently attach aurein 2.2 and its mutants to HPG and study the influence of the molecular weight on the antimicrobial activity. A peptide array was used to design tryptophan and arginine mutants of aurein 2.2. Mutant peptide 77 had significantly superior antimicrobial and antibiofilm activity compared to aurein 2.2 but was more toxic. We found that HPG can be used as a general scaffold to alleviate the toxicity of the peptides. The conjugates/peptides were tested in an in vivo mice skin infection (abscess) model. Surprisingly, peptide 73 and aurein 2.2 has similar efficacy in vivo indicating both the antimicrobial activity and toxicity, i.e. therapeutic index, are important. The conjugates (HPG-73c) were not active in mice abscess model, whereas 73c and D-73 encapsulated in micelles composed of DSPE-PEG2000 had excellent activity suggesting the release of the peptide from the delivery vehicle is necessary for in vivo activity. Without encapsulation D-73 was too toxic. A bacterial expression system was used to produce isotopically (¹⁵N) labeled aurein 2.2 and its interaction with whole bacterial cells was examined by nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM) confirming the MOA. Finally, the results presented will be discussed in the broad context of designing AMPs for therapeutics and understanding their MOA.
Heparins exert anticoagulation by potentiating anti-factor (F)Xa and anti-thrombin activity of antithrombin (AT), whereas oral anticoagulants (DOACs) directly target FXa or thrombin. FXa and thrombin are the key proteases required for blood clotting. Anticoagulants are therefore used for the prophylaxis and treatment of thrombosis and during surgeries. However, anticoagulation associated haemorrhage is a concern. The only approved antidote for unfractionated heparin (UFH), protamine do have limitations including cardiovascular complications. No approved antidotes are available for low molecular weight heparins, fondaparinux and direct FXa inhibitors. Therefore, there is a need for antidotes that are nontoxic and efficient. In this thesis, we reveal the mechanism of action, hemocompatibility and efficiency of three antidote molecules that are under development. UHRA: UHRA is a synthetic antidote developed by the Kizhakkedathu laboratory at the UBC. Thermodynamics based on isothermal titration calorimetry (ITC) and fluorescence studies revealed the molecular design of UHRA, the importance of steric shield produced by PEG brush, the selectivity of UHRA against heparins and its mechanism of action. Clotting studies confirm the antidote activity of UHRA. Studies also show that UHRA even in the absence of heparins, do not interact with fibrinogen, alter fibrin polymerization or abrogate blood clotting. Unlike protamine, UHRA does not incorporate into blood clots, form clots with normal morphology, and lysis profile. Studies in mice reveal that UHRA reverses UFH anticoagulant activity without the lung injury as seen with protamine. Studies confirm the superiority of UHRA compared to protamine.Andexanet Alfa (AnXa) and PER977: AnXa is a truncated FXa recombinant protein developed by Portola Pharmaceuticals and PER977 is a small cationic molecule developed by Perosphere Pharmaceuticals. ITC confirms high-affinity binding of AnXa to heparin/AT complex and to DOACs studied. PER977 shows weak binding to heparins and no binding to DOACs tested. Both antidotes do not influence fibrin polymerization, fibrin and blood clot architecture even in the absence of anticoagulants. Electron micrographs of blood clots containing edoxaban treated with AnXa or PER977 reveal restoration of impaired fibrin formation. However, in clotting assays, PER977 failed to show antidote activity, whereas AnXa neutralized the anticoagulation activity of all tested anticoagulants.
Traumatic brain injury (TBI) has been proven as an established risk factor of Alzheimer’s disease (AD). Historically, progesterone (Pro) has been found to promote recovery from moderate TBI. However, the utility of this drug as a TBI treatment is severely hampered by its near total insolubility in water due to its hydrophobicity, which contributes to an inability to rapidly administer the drug after injury. The present work describes the synthesis, characterization, development and in vitro evaluation of nanoparticulate formulations of Pro for treatment of TBI. The nanoparticles developed for Pro consist of a library of hyperbranched polyglycerols (HPGs), which were hydrophobically modified with alkyl chains (C₆,₈,₁₀,₁₂,₁₄,₁₈) to enable loading the hydrophobic drug, and were further modified with MPEG chains to increase the solubility and stability of the formulations. Hydrophobically derivatized HPGs (HPG-Cn-MPEG), also known as dHPG(Cn), were characterized by GPC and NMR methods. Pro encapsulation by and release from the drug-binding pocket was determined through a reverse-phase UPLC method. Combination of binding, release and kinetic studies of the dHPG(Cn)/Pro library presented a relatively high number of drug molecules encapsulated, slow release and stable formulations. In vitro assays, including blood biocompatibility, cytotoxicity and cellular uptake, were performed on dHPG(Cn)/Pro. Blood biocompatibility studies demonstrated that the polymer-drug formulations do not cause significant changes in blood coagulation time (APTT assay), nor have they significant effects on red blood cell aggregation, lysis or platelet aggregation. There was no platelet activation observed in this study. Study of viability of human cortical microvascular endothelial cells and human astrocytoma cells in the presence of dHPG(Cn)/Pro demonstrated no toxicity. Studies on the same cells presented significant uptake with relatively even distribution of the formulation inside the cells. Further investigations indicated no degradation pathway for dHPG(Cn) over short periods of time (~ 8 h). Overall, the in vitro studies suggest that dHPG(Cn) are compatible and harmless to cells, suitable for carrying hydrophobic drugs and molecules, such as Pro, to the target tissues.
Desferrioxamine (Desferal®, DFO), deferiprone (Ferriprox®, L1) and desferasirox (Exjade®, ICL-670) are clinically approved iron chelators used to treat transfusion associated iron overload, a common condition in patients with severe hemoglobin disorders like β-thalassemia, sickle-cell disease and the myelodysplastic syndromes. The poor pharmacokinetics and inefficacy of iron chelators necessitate administration of almost maximum tolerated doses to achieve adequate iron removal. This causes toxicity ranging from neurological dysfunction in DFO users, agranulocytosis and neutropenia in L1 users, and severe kidney toxicity in ICL-670 treated patients. This also hinders the use of iron chelators during gestation. Thus, developing iron chelators with improved long-term efficacy and reduced toxicity is essential. All currently approved iron chelators are of low molecular weight (MW) (
Repeated transfusion of red blood cells (RBCs) is the only treatment modality currently available for certain blood related genetic disorders such as thalassemia and sickle cell anemia. Due to chronic transfusion of RBCs in these patients, clinical problems surrounding alloimmunization develops in approximately 30% of patients. The pathology arises from adverse immune reactions to minor antigens that are either not routinely typed for, or cannot be readily matched. Hence, the development of donor RBCs that reduces the risk of alloimmunization would be highly beneficial. An innovative approach to address this problem involves the use of polymers to mask the immunogenic blood group antigens on RBC membranes. Given potential applications of polymer grafted RBCs, non-toxic and non-immunogenic materials are desired. In this research, we have investigated the covalent attachment of hyperbranched polyglycerols (HPG), a highly biocompatible polymer, to red blood cell surfaces. The aim is not only to shield immunogenic blood group antigens, but also to prevent the degradation of biomaterial modified cells by the immune system, particularly by the proteolytic convertases of the complement system. We investigated the mechanism of complement activation on HPG modified cells, and the influence of various polymer properties, including: grafting concentration, molecular weight, and degree of HPG functionalization in an effort to optimize the grafting process on cells. Traditional assays using antibody sensitized sheep erythrocytes and rabbit erythrocytes were used to assess the overall complement activation. Complement activation products C4a, C3a, Bb, and SC5b – 9 were quantified by ELISAs to determine the specific pathway of complement activation by HPG modified RBCs. Flow cytometry was also performed to demonstrate the effectiveness of antigen protection by the different graft properties.HPGs with a molecular weight greater than 28 KDa at grafting concentrations greater than 1.0 mM, as well as a high degree of HPG functionalization result in the activation of complement via the alternative pathway. No activation was observed when these threshold levels were not exceeded. These insights may have an impact on devising key strategies in developing novel therapeutics, especially in the fields of both transfusion and transplantation medicine.
Use of synthetic materials in medical applications is one of the most common practices in modern medicine. Yet occurrence of surface-induced thrombus formation on these materials, especially those associated with cardiovascular applications, generates a need for surface modifications. Limiting thrombus formation on a biomaterial surface represents the ultimate success for blood contacting devices. One interesting approach is to enhance fibrinolysis before the blood clot becomes stabilized. Herein, two synthetic polymers, poly-N- [(2, 2-dimethyl-1, 2-dioxolane) methyl] acrylamide (PDMDOMA) and poly- (N-isopropylacrylamide) (PNIPAm), were tested for this particular antithrombotic property. Surface-grafted PNIPAm samples, brush-PNIPAm and star-PNIPAm, were also tested for the biological activity.We evaluated the influence of these synthetic polymers on blood hemostasis by studying the fibrin polymerization process, the three-dimensional clot structure, and the mechanical properties of blood clot such as its clot strength, clot elasticity and clot fibrinolysis. Both linear PDMDOMA and PNIPAm altered the normal fibrin polymerization by changing the rate of protofibril aggregation and resulting in a 5-fold increase in the overall turbidity. Fibrin clots formed in presence of these synthetic polymers exhibited thinner fibers with less branching and resulted in a more porous and heterogeneous clot structure in scanning electron micrographs. The structural changes in these clots led to significant difference to their mechanical properties. Lower clot strength and clot elasticity were recorded from the thromboelastography study. More interestingly, enhanced clot lysis was measured by thromboelastography when whole blood was clotted in presence of PDMDOMA or PNIPAm. Further evidence of the altered clot structure and clot cross-linking was obtained from the significant decrease in D-dimer levels measured from degraded plasma clot. Similar results were obtained when star-form of PNIPAm was used but not for brush-form PNIPAm.The antithrombotic activity of soluble PDMDOMA and PNIPAm could potentially lead to the development of novel antithrombotic agents that could enhance endogenous fibrinolytic activity by modulating the fibrin clot structure. In the exploratory analysis of surface grafted PNIPAm (brush-PINPAm), brush-PNIPAm showed that the biological activity of attached chains is quite different from soluble polymers and several parameters need to be optimized to generate an antithrombotic coating for biomaterials.