Carrier-Mediated Antiviral Therapy

Drug-delivery systems that appear to be suitable for antiviral compounds can be grouped in three major categories (Table 1). Accordingly, when a drug is encapsulated in a carrier or attached to macromolecules, endocytosis is the only mode of entry. Two types of endocytosis, phagocytosis and pinocytosis, constitute the physiologic basis of the delivery systems. In phagocytosis, particulate materials are transported in large intracellular vesicles. The drug is encapsulated in an insoluble carrier engulfed by phagocytic cells and is released after enzymatic breakdown of the carrier. In pinocytosis, soluble materials are transported in small vesicles. The linkage between drug and soluble carrier is stable in the plasma but is susceptible to hydrolysis by lysosomal enzymes. The third is an in-between type. The substance, the drug or the biological is encapsulated in a polymeric or liposomal carrier that, because of its location or size, is not taken up by the cells. Because of diffusion of the substance and bioerosion of the carrier, the biological or the drug is constantly released from the carrier into the circulation and eventually enters the cell via active or passive transport.