Phagocytosis particulate carriers

The process of phagocytosis is of particular relevance when particulate delivery systems, such as microspheres, liposomes and other advanced delivery systems (described in Chapter 5), are used. Such particulate carriers are susceptible to MPS clearance. Sequestration by the MPS is useful in some cases, for example in the treatment of certain microbial diseases. However, if the drag is to be delivered to sites other than the MPS, it is highly undesirable. Therefore considerable research effort is being directed towards methods of avoiding MPS uptake of drag delivery systems. Strategies to both exploit and avoid MPS uptake are described in detail in Chapter 5 (see Section 5.1.4). 

Soluble macromolecules of both natural and synthetic origins have been used as drug carriers. When compared with the particulate carriers, soluble macromolecules (i) encounter fewer barriers to their movement around the body and can enter into many organs by transport across capillary endothehum or in the liver by passage through the fenestration connecting the sinusoidal lumen to the space of Disse (ii) penetrate the cells by pinocytosis, which is a phenomenon universal to aU cells and which, unlike phagocytosis, does not require an external stimulus and (Hi) can be found in the blood many hours after their introduction (particulate carriers, in contrast, are rapidly cleared from the blood by the RES). The fate of soluble macromolecules in animals and humans, with special reference to the transfer of polymers from one body compartment to another, has been reviewed by Drobnik and Rypacek (67). 

Ahsan F, Rivas IP, Khan MA, et al. Targeting to macrophages role of physicochemical properties of particulate carriers-liposomes and microspheres on the phagocytosis by macrophages. J Control Release 2002 79 29-40. 

One approach where the characteristics of the liposomal carrier system are well matched to the intended therapeutic application is the delivery of drugs to the MPS. Because of their particulate nature, the major route of clearance of liposomes, when administered in vivo by a variety of routes, is phagocytosis by MPS cells, especially macrophages in liver and spleen. Obviously, this "natural" fate of liposomes in vivo is an advantage if one attempts to treat diseases... 

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