Low Molecular Weight Antagonists of the Myelin-Associated Glycoprotein: Synthesis, Docking, and Biological Evaluation

Abstract

The injured adult mammalian central nervous system is an inhibitory environment for axon regeneration due to specific inhibitors, among them the myelin-associated glycoprotein (MAG), a member of the Siglec family (sialic-acid binding immunoglobulin-like lectin). In earlier studies, we identified the lead structure 5, which shows a 250-fold improved in vitro affinity for MAG compared to the tetrasaccharide binding epitope of GQ1balpha (1), the best physiological MAG ligand described so far. By modifying the 2- and 5-position, the affinity of 5 could be further improved to the nanomolar range (-->19a). Docking studies to a homology model of MAG allowed the rationalization of the experimental binding properties. Finally, pharmacokinetic parameters (stability in the cerebrospinal fluid, logD and permeation through the BBB) indicate the drug-like properties of the high-affinity antagonist 19a.

Glycosphingolipid Functions

“…MAG and NGR1 signal transduction is mediated via activation of the RhoA GTPase pathway (Niederost et al 2002;Mimura et al 2006). Oligosaccharide analogs of these sialated glycoconjugates promote axon outgrowth from cerebellar neurons in vitro (Vyas et al 2005), and novel small molecule inhibitors based on these structures (Mesch et al 2010), provide improved potential targets for future paraplegia treatment.…”

From a Library of MAG Antagonists to Nanomolar CD22 Ligands

“…With a small library of sialosides from our MAG project, an initial screen regarding the optimal aglycone (R 1 group) was performed (Entries 1-6, Table 1). [28,29] The affinities were determined by surface plasmon resonance (SPR, see below for details). 2,3-Dichlorobenzyl (!9 c) led to the highest affinity and was therefore selected as aglycone for the new series of CD22 ligands.…”

“…The aim of the second screen-again based on available MAG antagonists-was the identification of the optimal modification at the 5-position (R 2 , Entries 7-11, Table 1). [29] As a result, o-nosyl (ortho-nitrophenylsulfonyl, !9 k) was identified to be the superior substituent, leading to a ligand with nanomolar affinity for CD22 and a modest selectivity regarding MAG (~12-fold).…”

“…Starting from sialyl donor 10, [31] test compound 13 b was obtained according to published procedures. [28,29] The o-nosyl derivatives were synthesized according to a different approach (Scheme 1). After cleavage of the N-acetate (!14), [32] the amine was allowed to react with o-nosyl chloride (!15) followed by glycosylation with 2,3-dichlorobenzyl alcohol to yield intermediate 16.…”

“…[15] Herein we present a series of CD22 ligands based on a screening hit identified from a small library of sialic acid derivatives, which were originally synthesized as antagonists of the myelin-associated glycoprotein (MAG, Siglec-4). [28,29] They were tested for pharmacodynamic (affinity and selectivity) and pharmacokinetic (lipophilicity, plasma protein binding, oral availability) properties.…”