CARBOHYDRATE-MEDIATED LIPOSOMAL TARGETING AND DRUG-DELIVERY

The surface of the cell is rich in carbohydrate moieties attached to b oth membrane glycolipids and glycoproteins. These oligosaccharides con stitute potential recognition sites for carbohydrate-mediated interact ions between cells and drug carriers bearing suitable site-directing m olecules. Liposomes constitute a potentially valuable type of drug car rier. The ability to incorporate various site-binding molecules into t he liposomal surface leads to a wide range of delivery systems based o n carbohydrate-mediated interactions. In this review the types of reco gnition site on the surface of mammalian cells are considered with reg ard to both their chemical composition and the physical constraints wh ich might effect their recognition by drug carriers with site-directin g groups. Of particular current interest are the cell surface carbohyd rate-binding proteins (lectins) which have yet to be fully exploited a s recognition sites for carrier systems. Potential liposomal drug carr iers have been considered under a range of headings: glycolipid-bearin g, glycoprotein-bearing, virus spike glycoprotein-bearing (the so-call ed 'virosomes'), antibody-bearing immunoliposomes, lipopolysaccharide, polysaccharide-bearing and lectin-bearing. Examples of the applicatio ns of these various types of liposomes and their targeting to recognit ion sites on cells are reviewed. It is clear that while these systems could be of considerable value for the targeting of drugs, a great dea l more work has been reported on the preparation, control and targetin g of liposomal systems involving carbohydrate-mediated interactions th an has been reported on their actual use for the delivery of specific drugs to cells. Carbohydrate-mediated drug delivery using liposomes is a very sophisticated approach which will not be easy to exploit witho ut very considerable investment. At present, much of the technology re quired to exploit carbohydrate-mediated interactions in drug delivery is available and future progress will depend on a determination to use this technology to develop drug-carrying liposomal systems. The diver sity of potential systems is large and the choice of liposomal system for targeting a drug to a particular cell type will have to be determi ned by balancing specificity of the liposomal carrier against ease of production.