Sulphated polysaccharides derived from dextran: biomaterials for vascular therapy

A new macromolecular paramagnetic MR contrast agent binds to activated human platelets

A new functionalized macromolecular magnetic resonance (MR) contrast agent has been developed from a carboxymethyldextran-Gd(DOTA) devoid of biospecificity. The functionalized contrast agent was synthesized in order to mimic PSGL-1, the main ligand of P-selectin, a glycoprotein mainly expressed on the surface of activated platelets. The starting compound, CM1, was first carboxymethylated by monochloroacetic acid leading to a series of 10 derivatives varying in their carboxymethyl content. CM8 derivative, with a degree of substitution in carboxymethyl of 0.84, was chosen for subsequent fluorolabeling and sulfation to give CM8FS. CM8FS has an average number molecular weight of 27 000 +/- 500 g/mol, a hydrodynamic radius of 5.7 +/- 0.2 nm and a high relaxivity (r(1) = 11.2/mM (Gd)/s at 60 MHz). Flow cytometry experiments on whole human blood or on isolated platelets evidenced in vitro a preferential binding of CM8FS on TRAP-activated human platelets. Interestingly, CM8FS did not bind to other blood cells or to resting platelets. Pellets of TRAP-activated human platelets have also been imaged in tubes with a 1.5 T MR imager. A MR signal was observed for activated platelets incubated with CM8FS. Altogether, these in vitro results evidenced the recognition of activated human platelets by a fluorescent paramagnetic contrast agent grafted with carboxyl and sulfate groups. This biomimetic approach associated with the versatile macromolecular platform appears promising for the development of new contrast agents for molecular imaging of activated platelets in cardiovascular diseases such as atherosclerosis and aneurysms.

Influence of functional groups on the in vitro anticoagulant activity of chitosan sulfate

A new method for the chemical modification of chitosan sulfate was used to prepare N-propanoyl-, N-hexanoyl- and N,O-quaternary substituted chitosan sulfate. Structural analysis by elemental analysis, FTIR, 13C NMR, and 1H NMR spectroscopy, and gel-permeation chromatography showed that these methods could conveniently be used for the introduction of functional groups. The influences of the acyl or quaternary groups on the anticoagulant activity of the polysaccharides were studied with respect to activated partial thromboplastin time (APTT) thrombin time (TT), and prothrombin time (PT). The propanoyl and hexanoyl groups increased the APTT activity, and the propanoyl groups also increased the TT anticoagulant activity slightly, while the N,O-quaternary chitosan sulfate showed only a slight TT coagulant activity.

Heparin-like polymers modulate proinflammatory cytokine production by lipopolysaccharide-stimulated human monocytes

The search for heparin-like materials remains an intensive field of research. In this context, we studied the immunomodulatory properties of semisynthetic dextran derivatives and naturally occurring sulfated polysaccharides present in brown seaweed (fucans). In this study, we investigated the functional potencies of fucan and dextran derivatives by analyzing their effects on the release of proinflammatory cytokines by resting or lipopolysaccharide (LPS)-stimulated human monocytes and their interactions on monocyte surfaces. The results showed that fucan, dextran derivatives, and heparin differentially (1) triggered interleukin-1alpha, tumor necrosis factor alpha, interleukin-6, and interleukin-8 production by monocytes in a dose-dependent manner, (2) modulated cytokine production by LPS-stimulated monocytes, and (3) specifically inhibited the binding of biotinylated LPS to monocyte membranes. Taken together, these data indicated that fucan and dextran derivatives displayed interesting immunomodulatory effects on human blood cells that could be relevant as new drugs or biomaterial coatings. Indeed, such polysaccharides, by regulating monocyte activation, could contribute to the improved biocompatibility of implants.

New antibody purification procedure using a thermally responsive poly(N-isopropylacrylamide)-dextran derivative conjugate

Through their specificity and affinity, antibodies are useful tools in research and medicine. In this study, we investigated a new type of chromatographic method using a thermosensitive polymer for the purification of antibodies against a dextran derivative (DD), as a model. The thermally reversible soluble-insoluble poly(N-isopropylacrylamide)-dextran derivative conjugate, named poly(NIPAAm)-DD, has been synthesized by conjugating amino-terminated poly(N-isopropylacrylamide) to a DD via ethyl-3-(3-dimethylaminopropyl)-carbodiimide. On one hand, this report describes the two steps of poly(NIPAAm)-DD conjugation and characterization. On the other hand, the poly(NIPAAm)-DD conjugate was used as a tool to purify polyclonal antibodies in serum samples from rabbits subcutaneously immunized with the derivatized dextran. Antibodies were purified and quantified by immunoenzymatic assays. Our results indicate that antibodies recognized both DD and poly(NIPAAm)-DD. In contrast, they did not bind to native poly(NIPAAm) or poly(NIPAAm) conjugated with another anionic dextran. We conclude that the conjugation of a polysaccharide to poly(NIPAAm) leads to an original and efficient chromatographic method to purify antibodies. Moreover, this novel method of purification is rapid, sensitive, inexpensive and could be used to purify various types of antibodies.

Anticoagulant properties of dextranmethylcarboxylate benzylamide sulfate (DMCBSu); a new generation of bioactive functionalized dextran

Dextranmethylcarboxylate benzylamide sulfate (DMCBSu), a functionalized dextran, exhibits anticoagulant properties. Its synthesis involves three steps: a carboxymethylation with monochloroacetic acid in alkaline water-iso-propanol, a benzylamidification of some of the methylcarboxylate groups with benzylamine in the presence of a water soluble carbodiimide and a partial sulfation of the remaining hydroxyl groups with SO3-pyridine in dimethylformamide. This procedure yields reproducibly DMCBSu with degrees of substitution in methylcarboxylate (MC), benzylamide (B) and sulfate (Su) groups, respectively, up to 1.61, 0.35 and 1.5, each obtained in one step. For a degree of substitution of methylcarboxylate ca. 1, the presence of sulfate groups is absolutely necessary to confer anticoagulant activities to the samples. In addition, the anticoagulant ability is higher for derivatives bearing benzylamide groups. The anticoagulant ability of DMCBSu increases with the degree of sulfation, reaching 20% of heparin activity for a degree of substitution of Su groups about 1.3.

Toward new biomaterials

Polymers are widely used for a large range of medical devices used as biomaterials on a temporary, intermittent, and long-term basis. It is now well accepted that the initial rapid adsorption of proteins to polymeric surfaces affects the performance of these biomaterials. However, protein adsorption to a polymer surface can be modulated by an appropriate design of the interface. Extensive study has shown that these interactions can be minimized by coating with a highly hydrated layer (hydrogel), by grafting on the surface different biomolecules, or by creating domains with chemical functions (charges, hydrophilic groups). Our laboratory has investigated the latter approach over the past 2 decades, in particular the synthesis and the biological activities of polymers to improve the biocompatibility of blood-contacting devices. These soluble and insoluble polymers were obtained by chemical substitution of macromolecular chains with suitable groups able to develop specific interactions with biological components. Applied to compatibility with the blood and the immune systems, this concept has been extended to interactions of polymeric biomaterials with eukaryotic and prokaryotic cells. The design of new biomaterials with low bacterial attachment is thus under intensive study. After a brief overview of current trends in the surface modifications of biocompatible materials, we will describe how biospecific polymers can be obtained and review our recent results on the inhibition of bacterial adhesion using one type of functionalized polymer obtained by random substitution. This strategy, applied to existing or new materials, seems promising for the limitation of biomaterial-associated infections.