Receptor mediated glycotargeting

The role of galactose, lactose, and galactose valency in the biorecognition of N-(2-hydroxypropyl)methacrylamide copolymers by human colon adenocarcinoma cells

PURPOSE

To examine the beta-galactoside and beta-lactoside binding capacity of three human colon-adenocarcinoma cell lines and their sugar specificity, using N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer conjugates of galactosamine, lactose, and triantennary galactose.

METHODS

Three types of HPMA copolymers containing the saccharide epitopes galactosamine (P-Gal), lactose (P-Lac), or triantennary galactose (P-TriGal) were synthesized. The relationship between the content of the saccharide moieties, the valency of the galactose residues, and their biorecognition by the cell lines (Colo-205, SW-480, and SW-620) was investigated using flow cytometry and confocal fluorescence microscopy analysis.

RESULTS

The binding of the glycoconjugates to the human colonadenocarcinoma cell lines was dependent on the type and the number of bound sugar residues per macromolecule. The higher the sugar contents in the HPMA copolymers, the higher the extent of binding. Although introduction of galactoside residues into the HPMA copolymer resulted in a significant increase in the binding of the copolymers to the cells, low biorecognition of the lactoside-containing HPMA copolymers by all cell lines used was observed. The trivalent galactoside-containing HPMA copolymers did not yield a notable glycoside cluster effect for the beta-galactoside-binding lectin expressed on human colon-adenocarcinoma cells. Among the various cell line little differences in the extent of binding of the glycopolymers to the cells were observed. The data on the internalization of HPMA copolymer conjugates obtained by confocal fluorescence microscopy correlated well with the flow cytometry analysis of their biorecognition by target cells.

CONCLUSIONS

The lectin-mediated endocytosis of the HPMA glycoconjugates in human colon cancer cell lines suggests their potential use as targeting tools of cytotoxic drugs to colon adenocarcinoma.

Synthesis of antisense oligonucleotides conjugated to a multivalent carbohydrate cluster for cellular targeting

Carrier-mediated delivery holds great promise for significantly improving the cellular uptake and therefore the therapeutic efficacy of antisense oligonucleotides in vivo. A multivalent carbohydrate recognition motif for the asialoglycoprotein receptor has been designed for tissue- and cell-specific delivery of antisense drugs to parenchymal liver cells. To combine low molecular weight with high receptor affinity, the synthetic ligand contains three galactosyl residues attached to a cholane scaffold via epsilon-aminocapramide linkers. Three-dimensional structural calculations indicate that this unique design provides proper spacing and orientation of the three galactosyl residues to accomplish high affinity binding to the receptor. Covalent conjugation of the bulky carbohydrate cluster to oligonucleotides has been achieved by solid-phase synthesis using low-loaded macroporous resins and optimized synthesis protocols.

Oligolysine-based saccharide clusters: synthesis and specificity

In search of specific and highly selective sugar clusters for cell receptors, such as membrane lectins, various disaccharides were coupled to small peptide cores through an amide bond. In a first step, the reducing disaccharides, i.e. lactose and three different dimannoses, were converted into glycosyl-pyroglutamyl-beta-alanine derivatives. The free carboxylic group of these conjugates was then coupled to the alpha and epsilon amino groups of the core peptide (Lys( n )-Ala-Cys-NH2) with n =1 to 5, with complete substitution leading to homogeneous glycoclusters. The thiol group of the cysteine residue was used to tag the glycosylated oligolysines upon reaction with fluorescein iodoacetamide. The affinity of these glycoclusters towards two plant lectins was assessed by surface plasmon resonance. The selectivity of their cell uptake was investigated by flow cytometry using two types of cells: a human hepatoma cell line (HepG2 cells) expressing the plasma membrane galactose-specific lectin, and monocyte-derived dendritic cells expressing the plasma membrane mannose-specific lectin. The glycoclusters containing four or five disaccharides were shown to bind plant lectins and cell surface membrane lectins with a narrow selectivity and with a high affinity.

Design and synthesis of glycodendrimers

Multivalent neoglycoconjugates with well-defined structures have considerable potential as inhibitors of cell surface protein-carbohydrate interactions and as tools for studying such recognition processes in vitro. In this review, we outline strategies and synthetic methods for making one such class of neoglycoconjugates based on dendrimers--the so-called glycodendrimers. Glycodendrimers can be classified as: (i) carbohydrate-coated; (ii) carbohydrate-centered; and (iii) fully carbohydrate-based. Approaches to their construction have included both the modification of commercially available dendrimers and de novo dendrimer synthesis. Examples from the authors' and other laboratories are drawn upon to illustrate design considerations and the application of dendritic synthetic principles--including divergent and convergent syntheses--for making glycodendrimers. Key coupling reactions for the synthesis of glycodendrimers include: amide and thiourea formation; glycosylation; photoaddition to allyl ethers; and reductive amination. The advantages and disadvantages of using protected and unprotected saccharide building blocks and potential applications for glycodendrimers in both biotechnology and materials science are also discussed.

Synthesis of galactosyl compounds for targeted gene delivery

Cell-specific DNA delivery offers a great potential for targeted gene therapy. Toward this end, we have synthesized a series of compounds carrying galactose residues as a targeting ligand for asialoglycoprotein receptors of hepatocytes and primary amine groups as a functional domain for DNA binding. Biological activity of these galactosyl compounds in DNA delivery was evaluated in HepG2 and BL-6 cells and compared with respect to the number of galactose residues as well as primary amine groups in each molecule. Transfection experiments using a firefly luciferase gene as a reporter revealed that compounds with multivalent binding properties were more active in DNA delivery. An optimal transfection activity in HepG2 cells requires seven primary amine groups and a minimum of two galactose residues in each molecule. The transfection activity of compounds carrying multi-galactose residues can be inhibited by asialofetuin, a natural substrate for asialoglycoprotein receptors of hepatocytes, suggesting that gene transfer by these galactosyl compounds is asialoglycoprotein receptor-mediated. These results provide direct evidence in support of our new strategy for the use of small and synthetic compounds for cell specific and targeted gene delivery.

Enhanced biorecognition and internalization of HPMA copolymers containing multiple or multivalent carbohydrate side-chains by human hepatocarcinoma cells

N-(2-hydroxypropyl)methacrylamide (HPMA) copolymers containing pendant saccharide moieties (galactosamine, lactose, and triantennary galactose) were synthesized. The relationship between the content of saccharide moieties and three-dimensional arrangement of galactose residues and their biorecognition and internalization by human hepatocarcinoma HepG2 cells was investigated. The results obtained clearly indicated preferential binding of the trivalent galactose and the lactose-containing copolymers to these cells. The higher the saccharide moieties content in HPMA copolymers, the higher the levels of binding. The biorecognition of the glycosylated HPMA copolymers by HepG2 cells was inhibited by free lactose. The data on the internalization and subcellular trafficking of HPMA copolymer conjugates obtained by confocal fluorescence microscopy correlated well with the flow cytometric analysis of their biorecognition by target cells. Structural features of the glycosides responsible for the specific recognition of the HPMA copolymers have been identified. The results underline the potential of glycosylated HPMA copolymers for delivery of pharmaceutical agents to hepatocarcinoma cells.

Determination of the upper size limit for uptake and processing of ligands by the asialoglycoprotein receptor on hepatocytes in vitro and in vivo

The asialoglycoprotein receptor (ASGPr) on hepatocytes plays a role in the clearance of desialylated proteins from the serum. Although its sugar preference (N-acetylgalactosamine (GalNAc) >> galactose) and the effects of ligand valency (tetraantennary > triantennary >> diantennary >> monoantennary) and sugar spacing (20 A 10 A 4 A) are well documented, the effect of particle size on recognition and uptake of ligands by the receptor is poorly defined. In the present study, we assessed the maximum ligand size that still allows effective processing by the ASGPr of mouse hepatocytes in vivo and in vitro. Here too, we synthesized a novel glycolipid, which possesses a highly hydrophobic steroid moiety for stable incorporation into liposomes, and a triantennary GalNAc(3)-terminated cluster glycoside with a high nanomolar affinity (2 nm) for the ASGPr. Incorporation of the glycolipid into small (30 nm) [(3)H]cholesteryl oleate-labeled long circulating liposomes (1-50%, w/w) caused a concentration-dependent increase in particle clearance that was liver-specific (reaching 85 +/- 7% of the injected dose at 30 min after injection) and mediated by the ASGPr on hepatocytes, as shown by competition studies with asialoorosomucoid in vivo. By using glycolipid-laden liposomes of various sizes between 30 and 90 nm, it was demonstrated that particles with a diameter of >70 nm could no longer be recognized and processed by the ASGPr in vivo. This threshold size for effective uptake was not related to the physical barrier raised by the fenestrated sinusoidal endothelium, which shields hepatocytes from the circulation, because similar results were obtained by studying the uptake of liposomes on isolated mouse hepatocytes in vitro. From these data we conclude that in addition to the species, valency, and orientation of sugar residues, size is also an important determinant for effective recognition and processing of substrates by the ASGPr. Therefore, these data have important implications for the design of ASGPr-specific carriers that are aimed at hepatocyte-directed delivery of drugs and genes.

Glycopeptide synthesis and the effects of glycosylation on protein structure and activity

Despite the omnipresence of protein glycosylation in nature, little is known about how the attachment of carbohydrates affects peptide and protein activity. One reason is the lack of a straightforward method to access biologically relevant glycopeptides and glycoproteins. The isolation of homogeneous glycopeptides from natural sources is complicated by the heterogeneity of naturally occuring glycoproteins. It is chemical and chemoenzymatic synthesis that is meeting the challenge to solve this availability problem, thus playing a key role for the advancement of glycobiology. The current art of glycopeptide synthesis, albeit far from being routine, has reached a level of maturity that allows for the access to homogeneous and pure material for biological and medicinal research. Even the ambitious goal of the total synthesis of an entire glycoprotein is within reach. It is demonstrated that with the help of synthetic glycopeptides the effects of glycosylation on protein structure and function can be studied in molecular detail. For example, in immunology, synthetic (tumour-specific) glycopeptides can be used as immunogens to elicit a tumour-cell-specific immune response. Again, synthetic glycopeptides are an invaluable tool to determine the fine specificity of the immune response that can be mediated by both carbohydrate-specific B and T cells. Furthermore, selected examples for the use of synthetic glycopeptides as ligands of carbohydrate-binding proteins and as enzyme substrates or inhibitors are presented.

Pharmacokinetics and biodisposition of fluorescein-labeled arabinogalactan in rats

Fluorescein-labeled arabinogalactan (FA) was prepared by the reaction with FITC in methyl sulphoxide according to the method of deBelder and Granath. A systemic kinetic analysis of FA in rats was carried out by using a specific high-performance size-exclusion chromatography. Intravenously administered FA was rapidly eliminated from the blood circulation followed by an appreciable distribution to the liver and kidney. FA was accumulated in these organs over a long period whereas negligible levels of FA were detected in the other organs. A marked dose-dependency was seen in the hepatic uptake of FA which was markedly reduced by coinjected asialofetuin whereas the renal uptake of FA was not altered. Measurement of the hepatocellular localization demonstrated the overwhelming distribution of FA in the parenchymal liver cell fraction. Furthermore, the microscopic examination revealed FA that was effectively endocytosed by the parenchymal liver cells. These results suggested that FA which is bound to the asialoglycoprotein receptor with a high affinity is subsequently internalized to the hepatocyte via receptor-mediated endocytosis. FA was partially activated by periodate oxidation in order to acquire aldehyde groups to which guest molecules can be bound. A 12.5% oxidized arabinogalactan keeping a hepatocellular targetability showed a good conjugating reactivity to guest molecules via Schiff-base formation or by reductive amination. It was suggested that arabinogalactan can serve as a potential carrier for the delivery of enzymes and drugs to the parenchymal liver cells via the asialoglycoprotein receptor.

Facile synthesis of stable and lectin-recognizable DNA-carbohydrate conjugates via diazo coupling

Stable and lectin-recognizable DNA-carbohydrate conjugates were prepared by diazo coupling of lactose and cellobiose derivatives to fragmented salmon testes DNA. The diazo coupling is suggested to take place selectively to guanine bases since the amount of lactose moiety introduced was directly proportional to the G content of various DNAs with different G contents. According to the CD spectra, the conjugates bearing carbohydrate less than 25% content kept a typical B-type conformation similar to native DNA. The conjugates possessed higher melting temperature and stronger nuclease resistance both to exo- and endonucleases than native DNA. Gel shift assay and fluorescence binding assay showed that the DNA-lactose conjugates were specifically bound to galactose-specific lectin RCA(120) with strong binding affinity (Ka = 10(4)-10(5) M(-1)) due to glycoside cluster effect. This facile method will be a useful protocol of molecular design for cell-targeted gene therapy.

Synthesis of homogeneous glycopeptides and their utility as DNA condensing agents

Two glycopeptides were synthesized by attaching purified glycosylamines (N-glycans) to a 20 amino acid peptide. Triantennary and Man9 Boc-tyrosinamide N-glycans were treated with trifluoroacetic acid to remove the Boc group and expose a tyrosinamide amine. The amine group was coupled with iodoacetic acid to produce N-iodoacetyl-oligosaccharides. These were reacted with the sulfhydryl group of a cysteine-containing peptide (CWK18), resulting in the formation of glycopeptides in good yield that were characterized by 1H NMR and ESIMS. Both glycopeptides were able to bind to plasmid DNA and form DNA condensates of approximately 110 nm mean diameter with zeta potential of +31 mV. The resulting homogeneous glycopeptide DNA condensates will be valuable as receptor-mediated gene-delivery agents.

Synthesis of a lipophilic prodrug of 9-(2-phosphonylmethoxyethyl)adenine (PMEA) and its incorporation into a hepatocyte-specific lipidic carrier

PURPOSE

9-(2-Phosphonylmethoxyethyl)adenine (PMEA), a potent inhibitor of Hepatitis B virus replication, is in vivo hardly taken up by parenchymal liver cells (the site of infection). Our aim is to examine whether lactosylated reconstituted HDL (LacNeoHDL), a lipidic particle that is specifically internalized by parenchymal liver cells, is a suitable carrier for the selective delivery of PMEA to this cell type.

METHODS

To incorporate PMEA into LacNeoHDL, we synthesized a lipophilic prodrug (PMEA-LO) by coupling PMEA via an acid-labile phosphonamidate bond to lithocholic acid-3alpha-oleate.

RESULTS

The yield of the synthesis was 52% ([3H]PMEA-LO: 24%). [3H]PMEA-LO readily incorporated into LacNeoHDL (13 molecules/particle) without affecting the size and net negative charge of the carrier. Further, incubation studies at lysosomal pH showed [3H]PMEA was completely released from the carrier whereas, at neutral pH or in plasma, appreciable release was not observed.

CONCLUSIONS

The conjugation of PMEA with lithocholic acid-3alpha-oleate results in a lipophilic prodrug that readily associates with Lac-NeoHDL. The association of the prodrug does not affect the physicochemical properties of the particle, and PMEA is released from the carrier at lysosomal pH. These findings indicate that by using the prodrug approach, LacNeoHDL is a suitable carrier to deliver PMEA to parenchymal liver cells.

Conjugation of plasmid DNAs with lactose via diazocoupling enhances resistance to restriction enzymes and acquires binding affinity to galactose-specific lectin

Oligosaccharide-plasmid DNA conjugates were synthesized simply and effectively via the diazocoupling method. Plasmids (pUC19, pTRI-beta-actin, and pEGFP-C1) were treated with an N-beta-lactoside-substituted diazonium salt to yield diazocoupling products with degree of substitutions of 2.5-3.1 mol% of overall nucleobases. The lactose-pUC19 conjugate was found to resist restriction enzymes more strongly than the nonconjugated plasmid DNA and to acquire a strong binding affinity to galactose-specific lectin RCA(120). The diazocoupling modification of pTRI-beta-actin plasmid DNA little influenced in vitro transcription with T7 RNA polymerase. When lactose-pEGFP-C1 conjugate was transfected to baby hamster kidney (BHK) cells by means of cationic lipids, transduced gene was expressed in BHK cells similarly with the nonconjugated pEGFP-C1. The modification of plasmid DNA with carbohydrate enhanced the resistance to restriction enzymes and developed a strong binding affinity to galactose-specific lectin.

Preparation and isolation of neoglycoconjugates using biotin-streptavidin complexes

Glycoproteins commercially available in multi-gram quantities, were used to prepare milligram amounts of neoglycoproteins. The glycoproteins bromelain and bovine gamma-globulin were proteolyzed to obtain glycopeptides or converted to a mixture of glycans through hydrazinolysis. The glycan mixture was structurally simplified by carbohydrate remodeling using exoglycosidases. Glycopeptides were biotinylated using N-hydroxysuccinimide activated-long chain biotin while glycoprotein-derived glycans were first reductively aminated with ammonium bicarbonate and then biotinylated. The resulting biotinylated carbohydrates were structurally characterized and then bound to streptavidin to afford neoglycoproteins. The peptidoglycan component of raw, unbleached heparin (an intermediate in the manufacture of heparin) was similarly biotinylated and bound to streptavidin to obtain milligram amounts of a heparin neoproteoglycan. The neoglycoconjugates prepared contain well defined glycan chains at specific locations on the streptavidin core and should be useful for the study of protein-carbohydrate interactions and affinity separations.

A simple synthesis of 8-(methoxycarbonyl)octyl 3,6-di-O-(alpha-D-mannopyranosyl)-alpha-D-mannopyranoside and derivatives and their use in the preparation of neoglycoconjugates

8-(Methoxycarbonyl)octyl 3,6-di-O-(alpha-D-mannopyranosyl)-alpha-D-mannopyranoside (10) was synthesized in 54% yield by regioselective diglycosylation of unprotected mannoside 4, employing the trichloroacetimidate donor 1, followed by debenzoylation. Derivatives of compounds 4 and 10 were used to prepare conjugates containing fluorochromes for the study of carbohydrate-lectin interactions, as well as conjugates with phospholipids for the preparation of liposomes.

Comparative affinity of synthetic multi-antennary galactosyl derivatives for the Gal/GalNAc receptor of rat hepatocytes and peritoneal macrophages

The binding affinity of synthetic bi- and triantennary galactose ligands (Kichler, A. and Schuber, F. (1995) Glycoconj. Chem., 12, 275 281) has been determined for the Gal/GalNAc receptors of rat hepatocytes and macrophages. The highest affinities were observed with the triantennary structures, in agreement with the clustering effect known to occur with more complex oligosaccharide structures. However, these ligands present very similar affinities for the receptors of both cell types and thus lack the necessary selectivity for specific hepatocyte targeting.

Lectin-mediated drug targeting: quantification of binding and internalization of Wheat germ agglutinin and Solanum tuberosum lectin using Caco-2 and HT-29 cells

For the potential use of Wheat germ agglutinin (WGA) and Solanum tuberosum lectin (STL) as auxiliary excipients for targeting drugs to colonocytes, the number of Caco-2 and HT-29-bound lectins was determined by fluorimetry using fluorescein-labelled derivatives of the N-acetylglucosamine-specific lectins. After 1 h of incubation, the WGA-binding capacity of 5 x 10(4) Caco-2 cells was 26.9 +/- 0.5 pmol at 4 degrees C and 27.2 +/- 1.0 pmol at 37 degrees C respectively. In comparison, 19.5 +/- 2.9 pmol (37 degrees C) and 16.7 +/- 0.9 pmol (4 degrees C) WGA were bound within 1 h to 5 x 10(4) HT-29 cells referring to about 80% of the total amount of WGA bound within 4 h of incubation. In contrast, binding of STL to the colon carcinoma cell lines was independent of incubation times and temperatures tested exhibiting a binding rate of 8.4 +/- 0.6 pmol (HT-29) and 9.9 +/- 0.8 pmol (Caco-2) STL/5 x 10(4) cells. As determined by flow cytometry, non-specific binding is lower than 1.0% (WGA) and 3.4% (STL). Uptake and intracellular accumulation of the lectins were investigated by confocal laser scanning microscopy at 4 degrees C and 37 degrees C respectively. A decrease of initially membrane-bound lectins concurrent with increasing cytoplasmic enrichment by time was observed by digital cell image analysis. Due to specific and numerically sufficient adhesion as well as internalization, WGA and STL are anticipated as targeting tools in lectin-mediated drug delivery systems.

Novel glycosynthons for glycoconjugate preparation: oligosaccharylpyroglutamylanilide derivatives

The reducing sugar of an oligosaccharide reacting with the alpha-amino group of an amino acid is converted to an N-oligosaccharylamino acid which can then be stabilized by N-acylation. Oligosaccharides in solution in N,N-dimethylformamide reacted with alpha-glutamyl-p-nitroanilide at 50 degrees C for a few hours, leading to an N-oligosaccharylglutamyl-p-nitroanilide. Then, the gamma-carboxylic group of the glutamyl moiety, activated by adding (benzotriazol-1-yloxy)tris(dimethylamino)phosphonium hexafluorophosphate (BOP), reacted with the substituted alpha-amino group of the glutamyl residue, leading to an N-oligosaccharylpyroglutamyl-p-nitroanilide within 0.5 h. Such a one-pot two-step reaction was shown to be very efficient in the case of a disaccharide such as lactose, or pentasaccharides such as lacto-N-fucopentaoses, Lewis(a) or Lewis(x). The glycosynthons were characterized by chromatography (HPAEC and HPLC); their molecular mass was determined by electrospray ionization mass spectrometry, and the glycosylamides were shown to have a beta-anomeric configuration on the basis of their proton NMR. The N-oligosaccharylpyroglutamyl-p-nitroanilides are quite stable at room temperature over a large pH range. They are easily converted to N-oligosaccharylpyroglutamyl-p-isothiocyanatoanilides which can be used to prepare glycoconjugates such as cationic glycosylated polylysines suitable for specifically delivering genes or oligonucleotides in a sugar-dependent manner.

Glycotargeting: the preparation of glyco-amino acids and derivatives from unprotected reducing sugars

Lectins are present on the surface of many cells. Many lectins actively recycle from membrane to endosomes and efficiently take up glycoconjugates in a sugar-dependent manner. On this basis, glycoconjugates, specially those obtained by chemical means, are good candidates as carriers of drugs, oligonucleotides or genes. In this paper, we present a panel of methods suitable to transform unprotected reducing oligosaccharides into glycosynthons designed to be easily linked to therapeutic agents. All the glycosynthons presented here are glycosylamines or derivatives, mainly glyco-amino acids or glycopeptides. Glycosylamines are easy to obtain, but they are very labile in slightly acidic or neutral medium; they must be stabilized, by acylation for instance. The coupling efficiency of a reducing sugar with ammonia as well as an alkylamine or an arylamine is higher at high temperature, however, because of the Amadori rearrangement, special conditions have to be selected to prepare the expected glycosylamine derivative with a high yield. Glycosylamines are easily acylated by N-protected amino acids, or by halogeno acids which can then be transformed into amino acids. Alternatively, unprotected reducing oligosaccharides may very efficiently be transformed into N-glycosyl-amino acids and then protected by N-acylation. With a glutamyl derivative having both the alpha-amino and the gamma-carboxylic groups free, the coupling and the acylation, which is intramolecular, are roughly quantitative. N-oligosaccharyl-amino acid derivatives are interesting glycosynthons, because their sugar moiety bears the specificity towards membrane lectins while the amino acid part has the capacity to easily substitute a therapeutic agent.

Avidin targeting of intraperitoneal tumor xenografts

BACKGROUND

Lectins (proteins that bind specific sugar molecules on glycoproteins and glycolipids) are expressed at various levels on the surface of tumor cells. Conjugation of cytotoxic agents to glycoproteins recognized by lectins could be useful in the treatment of tumors. Avidin (a highly glycosylated, positively charged protein found in egg white) contains terminal N-acetylglucosamine and mannose residues that bind to some lectins. In this study, we tested the ability of avidin, labeled through conjugation to radioactive biotin (a B vitamin), to target intraperitoneal tumors.

METHODS

Biotin was radioactively labeled with 111In. Four tumor models (one ovarian, one lung, and two colon) were established in nude mice by intraperitoneal injection of cultured cancer cells. The following two approaches were used in the intraperitoneal administration of avidin: 1) radioactive biotin-avidin conjugates were injected and 2) avidin was injected 1-24 hours before the injection of radioactive biotin (avidin pretargeting; avidin-biotin conjugates formed in vivo). The distribution of injected radioactivity in the tissues of treated animals was assessed.

RESULTS

Radiolabeled avidin localized highly and rapidly in the tumors. More than 50% of the administered dose of avidin-biotin conjugate accumulated per gram of tumor tissue 2 hours after injection; high tumor uptake of radioactivity was observed up to 24 hours after conjugate injection. In contrast, accumulation of radioactivity in normal tissues was low, yielding high tumor to nontumor ratios. With avidin pretargeting, accumulation of radioactivity in the liver, kidney, and spleen was reduced to a greater extent than that in the tumor, and tumor to nontumor ratios were increased.

CONCLUSIONS

Avidin may be a promising vehicle for the delivery of radioisotopes, drugs, toxins, or therapeutic genes to intraperitoneal tumors.

Strength in numbers: non-natural polyvalent carbohydrate derivatives

Many processes mediated by protein-carbohydrate interactions involve multivalent low-affinity binding, which is inherently difficult to study. New structural templates for the generation of multivalent carbohydrate displays have recently been developed, and tailored multivalent saccharide derivatives can now be used to study and modulate a wide variety of biological recognition events.

Tissue targeting of multivalent Le(x)-terminated N-linked oligosaccharides in mice

The target site for N-linked biantennary and triantennary oligosaccharides containing multiple terminal Le(x) determinants was analyzed in mice. N-linked oligosaccharides containing a single tert-butoxycarbonyl-tyrosine attached to the reducing end were used as synthons for human milk alpha-3/4-fucosyltransferase to prepare multivalent Le(x) (Gal beta 1-4[Fuc alpha 1-3]GlcNAc) terminated tyrosinamide oligosaccharides. The oligosaccharides were radioiodinated and examined for their pharmacokinetics and biodistribution in mice. The liver was the major target site in mice at 30 min, which accumulated 18% of the dose for Le(x) biantennary compared with 6% for a nonfucosylated Gal biantennary. By comparison, Le(x)- and Gal-terminated triantennary accumulated in the liver with a targeting efficiency of 66 and 59%, respectively. The liver targeting of Le(x)-biantennary was partially blocked by co-administration with either galactose or L-fucose whereas Le(x) triantennary targeting was only reduced by co-administration with galactose. In contrast to these results in mice, in vivo experiments performed in rats established that both Le(x) and Gal terminated biantennary target the liver with nearly identical efficiency (6-7%). It is concluded that the asialoglycoprotein receptor in mice preferentially recognize Le(x) biantennary over Gal biantennary, whereas little or no differentiation exists in rats. Thereby, the mouse asialoglycoprotein receptor apparently possesses additional binding pockets that accommodate a fucose residue when presented as Le(x).