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Synthesis and Functional Characterization of Novel Sialyl LewisX Mimic-Decorated Liposomes for E‑selectin-Mediated Targeting to Inflamed Endothelial Cells
Sialyl
LewisX (sLeX) is a natural ligand of E-selectin that is overexpressed
by inflamed and tumor endothelium. Although sLeX is a potential ligand
for drug targeting, synthesis of the tetrasaccharide is complicated
with many reaction steps. In this study, structurally simplified novel
sLeX analogues were designed and linked with 1,2-distearoyl-<i>sn</i>-glycero-3-phosphoethanolamine-polyethylene glycol-2000
(DSPE-PEG) for E-selectin-mediated liposomal delivery. The sLeX structural
simplification strategies include (1) replacement of the Gal-GlcNAc
disaccharide unit with lactose to reduce many initial steps and (2)
substitution of neuraminic acid with a negatively charged group, i.e.,
3′-sulfo, 3′-carboxymethyl (3′-CM), or 3′-(1-carboxy)Âethyl
(3′-CE). While all the liposomes developed were similar in
particle size and charge, the 3′-CE sLeX mimic liposome demonstrated
the highest uptake in inflammatory cytokine-treated human umbilical
vein endothelial cells (HUVECs), being even more potent than native
sLeX-decorated liposomes. Inhibition studies using antiselectin antibodies
revealed that their uptake was mediated primarily by overexpressed
E-selectin on inflamed HUVECs. Molecular dynamics simulations were
performed to gain mechanistic insight into the E-selectin binding
differences among native and mimic sLeX. The terminally branched methyl
group of the 3′-CE sLeX mimic oriented and faced the bulk hydrophilic
solution during E-selectin binding. Since this state is entropically
unfavorable, the 3′-CE sLeX mimic molecule might be pushed
toward the binding pocket of E-selectin by a hydrophobic effect, leading
to a higher probability of hydrogen-bond formation than native sLeX
and the 3′-CM sLeX mimic. This corresponded with the fact that
the 3′-CE sLeX mimic liposome exhibited much greater uptake
than the 3′-CM sLeX mimic liposome