Efficient Liver Targeting by Polyvalent Display of a Compact Ligand for the Asialoglycoprotein Receptor

A compact and stable bicyclic bridged
ketal was developed as a
ligand for the asialoglycoprotein receptor (ASGPR). This compound
showed excellent ligand efficiency, and the molecular details of binding
were revealed by the first X-ray crystal structures of ligand-bound
ASGPR. This analogue was used to make potent di- and trivalent binders
of ASGPR. Extensive characterization of the function of these compounds
showed rapid ASGPR-dependent cellular uptake in vitro and high levels
of liver/plasma selectivity in vivo. Assessment of the biodistribution
in rodents of a prototypical Alexa647-labeled trivalent conjugate
showed selective hepatocyte targeting with no detectable distribution
in nonparenchymal cells. This molecule also exhibited increased ASGPR-directed
hepatocellular uptake and prolonged retention compared to a similar
GalNAc derived trimer conjugate. Selective release in the liver of
a passively permeable small-molecule cargo was achieved by retro-Diels–Alder
cleavage of an oxanorbornadiene linkage, presumably upon encountering
intracellular thiol. Therefore, the multicomponent construct described
here represents a highly efficient delivery vehicle to hepatocytes.