Developing Novel Lipidated Glycotherapeutics
Authors:   , Stewart Arran 
Published: December 2023

<p><strong>This thesis is comprised of two distinct sections.</strong></p><p>The first section, Chapters 2 to 4, focuses on developing novel dendritic heparan sulfate (HS) mimetics for potential use as anti-cancer and anti-viral agents. This includes the design, synthesis, and purification of sulfated mimetics and their bioisosteric analogues.</p><p>HS is a naturally occurring glycosaminoglycan (GAG) widely present in mammalian tissues. Its complex structure and negative charge allow it to interact with a plethora of different proteins, playing many roles in physiological processes. Additionally, HS-protein interactions are implicated in several diseases, including viral infections, neurodegenerative disorders, and different types of cancer. These conditions have been associated with altered HS and HS binding protein levels and disrupted HS biosynthesis. Therefore, HS-protein interactions have emerged as viable therapeutic targets.</p><p>Research into HS-protein interactions has led to the design of compounds that mimic the structure of HS, showing promising results for treating various diseases, such as cancer and Alzheimer's disease. However, these compounds require complex and costly syntheses. In response, the Ferrier Research Institute has designed and synthesised simpler tetramer dendritic mimetics. These novel compounds not only exhibit biological activity comparable to established HS mimetics, but their production involves fewer steps and incurs lower costs. Consequently, trimer analogues were synthesised in a previous investigation, assessing the minimal complexity required for an effective dendrimer. This study also introduced an element of lipophilicity, a characteristic found in other mimetics but absent in the tetramer, potentially enabling their integration into the cell membrane.</p><p>This thesis will detail the synthesis of a sulfated dimer analogue and evaluates its antiviral efficacy relative to trimer and tetramer counterparts, aiming to establish the simplest dendrimer configuration that maintains significant biological activity. Furthermore, the work investigates the synthesis of a diacyl dimer to quantify the effect of increased lipophilicity on biological activity, informed by prior findings that suggest lipophilic attributes can enhance the biological performance of HS mimetics. Another key focus of this thesis is the investigation of the influence of bioisosteres on biological activity. While the importance of the negative charge imparted by the sulfate group in cancer and viral entry is well-established, it has yet to be determined whether this negative charge must specifically originate from a sulfate group. Therefore, bioisosteres such as phosphates and phosphorothioates, which maintain the required tetrahedral geometry while providing a negative charge, may serve as effective alternatives. The synthesis and purification of such bioisosteric analogues are also described and comparisons to their sulfated counterparts based on the available data are made.</p><p>Preliminary antiviral screening has been conducted against pathogens known to utilise HS for cell entry, including Herpes Simplex Virus (HSV), Cytomegalovirus (CMV), Vaccinia Virus (VACV), and Human Papillomavirus (HPV). The findings demonstrate that sulfated analogues exhibit superior potency and lower cytotoxicity across all assays. Furthermore, an observed trend suggests that an increase in lipophilicity correlates with heightened cytotoxicity. In the case of HPV, all compounds exhibited high activity and low toxicity, though the phosphates were still marginally less effective than the sulfates.</p><p>In Chapter 5, the thesis elaborates on the development of a novel native chemical ligation (NCL) method utilising a 1,2-dithiolan-4-yl carboxylate ester. The formulation of this method was driven by the need to overcome the intrinsic challenges associated with the creation of a reactive C-terminal thioester inherent in conventional NCL processes. The curated 1,2-dithiolan-4-yl carboxylate ester surfaced as a potential alternative, anticipated to maintain stability up until the selective induction of the NCL reaction, commonly facilitated by reducing agents such as tris(2-carboxyethyl)phosphine (TCEP). Notably, the refined methodology resulted in the successful execution of NCL on several peptide sequences, underscoring the versatility and applicability of the devised approach. However, complications arose when applying the methodology to the synthesis of an α-galactosylceramide (α-GalCer) based vaccine against Human Papillomavirus (HPV), a notable oncovirus. The research journey was marked by significant complexities and unforeseen challenges. Despite exhaustive efforts, the synthesis of the vaccine precursor remained unattainable, representing a significant and central challenge in this thesis.</p><p>Although this thesis establishes a novel native NCL methodology, the instability of the immediate 1,2-dithiolan-4-yl carboxylate ester precursor suggests that its practical application is currently limited. Future research may focus on developing a more stable synthetic route for this precursor.</p>