Persubstituted cyclodextrin-based glycoclusters as inhibitors of protein-carbohydrate recognition using purified plant and mammalian lectins and wild-type and lectin-gene-transfected tumor cells as targets.
Multivalent glycoclusters have the potential to become pharmaceuticals by virtue of their target specificity toward clinically relevant sugar receptors. Their application can also provide fundamental insights into the impact of two spatial factors on binding, i.e., topologies of ligand (branching mode, cluster presentation) and carbohydrate recognition domains in lectins. Persubstituted macrocycles derived from nucleophilic substitution of iodide from heptakis 6-deoxy-6-iodo-beta-cyclodextrin by the unprotected sodium thiolate of 3-(3-thioacetyl propionamido)propyl glycosides (galactose, lactose and N-acetyllactosamine) were prepared. The produced glycoclusters were first tested as competitive inhibitors in solid-phase assays. A plant toxin from mistletoe and an immunoglobulin G fraction from human serum were markedly susceptible. A nearly 400-fold increase in inhibitory potency of each galactose moiety in the heptavalent form relative to free lactose (217-fold relative to free galactose) was detected. Thus, these glycoclusters can efficiently interfere, for example, with xenoantigen-dependent hyperacute rejection. Among the tested galectins selected from this family of adhesion- and growth-regulatory endogenous lectins, the substituted beta-cyclodextrins acted as sensors to delineate topological differences between the two dimeric prototype proteins. The relatively strong reactivity with chimera-type galectin-3, a mediator of tumor metastasis, disclosed selectivity for glycocluster binding among galectins. Equally important, the geometry of ligand display (maxiclusters, bi- or triantennary N-glycans) made its mark on the inhibitory potency. To further determine the sensitivity of a distinct galectin presented on the cell surface and not in solution, we established a stably transfected tumor cell clone. We detected a significant response to presence of the multivalent inhibitor. This type of chemical scaffold with favorable pharmacologic properties might thus be exploited for the design of galectin- and ligand-type-selective glycoclusters.