Synthesis and evaluation of glycomimetic antagonists for the lectins DC-SIGN and FimH

Lectins are carbohydrate-binding proteins that are widely spread in nature and crucially involved in a multitude of biological processes. This thesis addresses the design of glycomimetic antagonists for the human lectin DC-SIGN (chapter 2) and the bacterial lectin FimH (chapter 3), which are both involved in infectious diseases.
Dendritic cell-specific ICAM-3-grabbing non-integrin (DC-SIGN) is a C-type lectin expressed on immature dendritic cells (DCs) prevalent in mucosal tissue. Besides its function as an adhesion molecule enabling the migration of DCs and binding to T cells, DC-SIGN is one of the major pathogen recognition receptors on DCs. In general, pathogen binding leads to phagocytosis, DC maturation, and migration to the lymph nodes, where antigenic fragments are presented to resting T cells which finally initiate a specific immune response. However, a variety of pathogens, such as viruses (e.g. HIV-1), bacteria (e.g. Mycobacterium tuberculosis), and parasites (e.g. Schistosoma mansoni), exploit this initial interaction with DC-SIGN to evade the immune system and, instead, efficiently infect the host. With its Ca2+-dependent carbohydrate recognition domain (CRD), DC-SIGN binds oligomannosides or fucose-containing Lewis antigens such as Lewisx (Lex = Gal(1-4)[Fuc(1-3)]GlcNAc) present on the surface of microbial cells or on viral envelop proteins. Blocking the first interaction between the microorganisms and DC-SIGN by suitable antagonists is therefore a promising therapeutic approach towards the prevention of infectious diseases.
The first part of this thesis addresses the development of fucose-based glycomimetic antagonists for DC-SIGN. To this end, the interaction of Lewis-type structures with DC-SIGN was elucidated. STD NMR experiments were conducted to determine the binding epitopes of Lewis trisaccharides bearing different aglycones. This study revealed a switch of the binding mode upon introduction of aromatic aglycones as a result of an additional hydrophobic interaction (chapter 2.2).
A series of trisaccharide mimics of Lex was synthesized to elucidate the role of D-Gal and D-GlcNAc in Lewis-type structures for binding to DC-SIGN. For this purpose, the central D-GlcNAc was replaced with (1R,2R)-cyclohexane-1,2-diol based moieties and the D-Gal moiety was replaced with various deoxy analogues. Affinity data including thermodynamic binding parameters indicate that, first, D-Gal is not crucial for binding and, second, mimicking of one sugar moiety enhances binding affinity (chapters 2.3.1 and 2.3.2).
Based on the preliminary results, further glycomimetics were developed to enable the interaction with the hydrophobic area in the binding site and tested for their potential as DC-SIGN antagonists (chapter 2.3.3).
FimH is a bacterial, mannose-specific lectin expressed on the tip of filamentous surface organelles of uropathogenic Escherichia coli. The CRD of FimH interacts with glycoconjugates such as uroplakin Ia present on urothelial cells. This bacterial adhesion is the initial and most crucial step in the establishment of urinary tract infections (UTIs), since it prevents the bacteria from being washed out by the bulk flow of urine. UTIs are among the most common infectious diseases affecting millions of people every year. The treatment with antibiotics encounters increasing bacterial resistance and demands for alternative strategies to prevent and treat UTIs. The development of anti-adhesive agents that are able to inhibit the crucial interaction of FimH with the urothelial cells presents a promising, alternative therapeutic approach.
Intestinal absorption and renal clearance are key issues for orally dosed FimH antagonists to reach the therapeutic target in the human bladder. Besides high affinity and selectivity for the target, a potent FimH antagonist thus must exhibit favourable pharmacokinetic (PK) properties. The second part of this thesis covers three studies that aim at improving these characteristics in mannose-based FimH antagonists.
The first study was directed towards the replacement of a conserved water molecule within the CRD of FimH. For this purpose, an appropriately modified D-mannoside was synthesized and biologically evaluated. The unexpected loss in affinity towards FimH could be explained by detailed molecular dynamics studies (chapter 3.2.1).
A Topliss-based structure-activity relationship study was conducted for the investigation of biphenyl mannosides as FimH antagonists. The pi-pi stacking of the aromatic aglycone with Tyr48 at the rim of the binding site was elucidated and a group of high-affinity antagonists with promising physico-pharmacological properties was identified (chapter 3.2.2).
One of these compounds was further investigated as part of a bioisosteres study for its potential as orally available FimH antagonist. In addition to the optimal in vitro PK/PD profile, this antagonist showed an excellent PK profile in vivo (chapter 3.2.3).