Carrier potential of glycoproteins

Endogenous lectins as targets for drug delivery

To minimize side effects of drugs it would be ideal to target them exclusively to those cell types which require treatment. As a means to this end prototypical cellular recognition systems pique our interest to devise biomimetic strategies. Since oligosaccharides of glycoconjugates outmatch other information-carrying biomolecules (proteins, nucleic acids) in theoretical storage capacity by far, work on the sugar code can spark off development of effective targeting devices. Conjugation of custom-made glycan epitopes to proteins or biocompatible non-immunogenic polymeric scaffolds produces neoglycoconjugates with purpose-adaptable properties. In the interplay with endogenous receptors such as lectins, suitable oligosaccharides such as histo-blood group trisaccharides as parts of neoglycoconjugates have already proven their practical applications in histopathology. Elucidation of the structure of cell lectins with currently five main families aids to tailor ligand characteristics rationally. They include the types of functional groups and their topological presentation to optimize the bimolecular binding as well as the optimal spatial clustering and spacer characteristics to exploit cooperativity. Indeed, the potent trivalent cluster glycosides designed for the C-type asialoglycoprotein receptors furnish an instructive example how to turn the theoretical guideline on ligand modification into nM-affinity. By placing emphasis on tissue lectins as targets of neoglycoconjugate-mediated drug delivery, the long-term perspective is opened to likewise test members of these families themselves for routing of therapeutic payloads, aiming at cell addressins. This review illustrates the conceivable potential which work on the sugar code with custom-made neoglycoconjugates and tissue lectins can have in store for drug delivery.

in vivo targeting of colloidal carriers by novel graft copolymers

One of the major obstacles to the targeted delivery of colloidal carriers (nanocapsules) is the body's own defence mechanism in capturing foreign particles by the reticuloendothelial system (RES). This means that following intravenous administration, practically all nanometer size particles are captured by the RES (mainly the liver). This paper draws attention to a recent initiative on the design of 'macromolecular homing devices' which seem to disguise nanoparticles from the RES, and hence are of potential interest for the targeted delivery of nanocapsular carriers. The idea is based on a graft copolymer model embodying a link site for attachment (binding) to the carrier, a floating pad for maintaining the particles afloat in the blood stream, an affinity ligand for site-specific delivery and a structural tune for balancing the overall structure of the homing device. A general synthetic scheme for the preparation of such graft copolymers is given, and preliminary biological evaluations relating to the floating pad concept are discussed.

[Tumor lectinology--status and perspectives of clinical application]

A detailed knowledge of the mechanisms of molecular recognition is a prerequisite to rationally improved diagnostic and therapeutic procedures in diseases. In addition to sequences of amino acids, carbohydrate structures apparently store biological information that is thought to be relevant for physiologically important processes. Such ligands, namely the carbohydrate part of cellular glycoconjugates, can be recognized by specific endogenous binding proteins like lectins. If their presence can be reliably ascertained and correlated to the clinical course of the disease, e.g. in oncology, lectinology may help to define a yet undisclosed role for this class of proteins in tumor progression and spread.

Targeting of neoglycoprotein-drug conjugates to cultured human embryonal carcinoma cells

Fluorescent neoglycoproteins were used to screen for the presence and sugar specificities of cell surface lectins in two human embryonal carcinoma cell lines. Efficient labeling correlated with extent of lectin-mediated uptake of neoglycoproteins, as measured by inhibition of DNA synthesis by drug-neoglycoprotein conjugates. These conjugates contain covalently linked carbohydrate moieties on the carriers to render them accessible to the membrane lectins, most effectively galactosides and alpha-glucosides. They furthermore contain chemically linked cytotoxic drugs (etoposide, cis-diamminedichloroplatinum II and methotrexate) which are intracellularly released after lysosomal breakdown of the carrier, as indicated by the effect of leupeptin. Sugars can confer a greater than 10-fold increase in cytotoxic capacity to the nonglycosylated carrier-drug conjugate, nearly reaching the level of toxicity of the freely diffusible drug. Two different neoglycoproteins, reacting with independently targeted membrane lectins, were shown to be useful in a model for combination chemotherapy. These results therefore suggest potential usefulness of custom-made glycosylated carriers in the targeting of therapeutic agents to human embryonal carcinoma cells.