Glycotherapy: new advances inspire a reemergence of glycans in medicine

Abstract

The beginning of the 20(th) century marked the dawn of modern medicine with glycan-based therapies at the forefront. However, glycans quickly became overshadowed as DNA- and protein-focused treatments became readily accessible. The recent development of new tools and techniques to study and produce structurally defined carbohydrates has spurred renewed interest in the therapeutic applications of glycans. This review focuses on advances within the past decade that are bringing glycan-based treatments back to the forefront of medicine and the technologies that are driving these efforts. These include the use of glycans themselves as therapeutic molecules as well as engineering protein and cell surface glycans to suit clinical applications. Glycan therapeutics offer a rich and promising frontier for developments in the academic, biopharmaceutical, and medical fields.

Figure 1. Timeline of Glycans in Medicine

The first half of the 20^th century was marked with major breakthroughs in glycan-based treatments. However, further progress was dampened by a lack of structural understanding that was not available until the past 60 years. A large number of discoveries in the 1980s elucidated the molecular and mechanistic details of glycan-mediated biological events and provided the impetus to expand the use of glycans in therapeutic endeavors.

Figure 2. Structures of Glycan-Based Small Molecule Drugs Currently on the Market or in the Pipeline

All compounds are natural products or based on a carbohydrate scaffold. Their uses range from the prevention of bacterial (neomycin) and viral infection (oseltamivir, zanamivir) to the treatment of glycan-based diseases (miglitol, diabetes; miglustat, Gaucher), sickle cell crisis (GMI-1070), and as an anticoagulant (fondaparinux).

Figure 3. Polyvalent Glycan Structures Designed for Increased Avidity to Target Lectins

(A) STARFISH, a Shiga-like toxin inhibitor. (B) Sulfated lactose PEO star dendrimer to inhibit selectins. (C) High mannose dendrimers, which inhibit DC-SIGN mediated HIV infection. (D) PolyBAIT, which inhibits Shiga-like toxin and increases its clearance. (E) CS-E glycopolymer mimic for controlling neuronal healing. (F) SET-LRP glycopolymers with controlled assembly for DC-SIGN binding. (G) Qβ glycodendronan particle that inhibits DC-SIGN-dependent Ebola infection. (H) Iron oxide sLe^x nanoparticels for selectin imaging. (I) Lipid nanoparticles for delivery to Sialoadhesin cells.

Figure 4. Synthetic Carbohydrate-Based Vaccines

Representative vaccines against microbial pathogen- (A–D), HIV- (E), and cancer-associated (F) glycan epitopes.

Figure 5. Effect of Glycan Structure on IgG Antibody Effector and Immune Function

Without glycosylation, IgG does not bind Fc receptors or activate complement, whereas the addition of different sialic acids can elicit anti-inflammatory or immunogenic effects.

Figure 6. Synthetic and Enzymatic Methods for the Production of Homogeneous Glycoproteins

A glycoprotein with an initial monosaccharide can be generated by total synthesis, enzymatic removal of a heterogeneously glycosylated precursor, or attachment by a chemoselective reaction. This intermediate can then be further elaborated by enzymatic methods. Additionally, the full-length, fully glycosylated protein can be synthesized de novo or the complete glycan installed by a chemoselective reaction. X and Y represent functional groups that undergo chemoselective reactions.

Figure 7. Modified Monosaccharides that Can Be Incorporated via Metabolic Oligosaccharide Engineering Display a Range of Functional Groups on the Cell Surface

Four modified monosaccharides have been incorporated into vertebrate glycans (Neu5Ac, GalNAc, GlcNAc, and fucose), which contain handles for chemoselective reactions or photoaffinity tags.

Figure 8. Cell Surface Glycoengineering

Glycosylation can be modified by exogenous enzymes (A) or materials (B and C) to alter glycosylation status for various functions. Cells with increased surface sLe^x bind to the selectins, which increase rolling along the endothelium or increase the efficiency for bone marrow homing (A and B). Additionally, glycocalyx engineering with cell surface-integrating sialic acid polymers can be used to protect cells from NK cell-mediated cytotoxicity (C).