Pharmac. Ther. Vol. 42, pp. 289-305, 1989 0163-7258/89 $0.00+ 0.50 Printed in Great Britain.All rights reserved Copyright© 1989PergamonPresspie Associate Editor: D. SHUGAR INHIBITORS OF VIRAL UNCOATING GuY D. DIANA,* DANIEL C. PEVEAR,* MICHAEL J. OTTO,'~ MARK A. MCKINLAY,* MICHAEL G. ROSSMANN,~ THOMAS SMITH~ a n d JOHN BADGER~ *Sterling-Winthrop Research Institute, Rensselaer, New York, U.S.A. ~fE.L DuPont de Nemours and Co., Glenolden, Pa., U.S.A. ~Department of Biological Sciences, Purdue University, West Lafayette, Indiana, U.S.A. 1. INTRODUCTION The discovery and development of antiviral agents has entered a new era. Whereas early discoveries of antiviral agents can be attributed to serendipity, increased understanding of virus replicative processes has allowed for the targeted design of specific inhibitors. One target, viral uncoating, is an essential step in viral replication which results in the release of the viral genome into either the cytoplasm (RNA viruses) or the nucleus (DNA viruses and RNA tumor viruses). As is the case with other viral-specific molecular targets, the uncoating process is an attractive target for therapy because inhibitors are likely to selectively inhibit virus replication without affecting host cell viability. 2. MECHANISMS OF UNCOATING Uncoating represents one of the early steps in the life cycle of a virus. Many enveloped and nonenveloped viruses use similar strategies to initiate infection, strategies which exploit the conditions of the normal endocytotic pathways of the cells (Dales, 1973). Much of our knowledge concerning the strategies used by viruses to introduce their genomic material into the cytosol has been gained from the use of agents which change the pH of intracellular vesicles, e.g. the lysosomotropic agents ammonium chloride, chloroquine, methylamine, tributylamine, rimantadine and amantadine, and weak ionophores such as monensin. The mechanism of entry of representative enveloped and nonenveloped viruses is briefly described here. 2.1. ENVELOPED VIRUSES The mechanism of entry, uncoating and replication of the alphavirus Semliki Forest virus (SFV) has been studied extensively and serves as a convenient example of how enveloped viruses initiate the infection process. Electron and fluorescence microscopy have shown that SFV undergoes multipoint attachment to specific receptors on the cell surface followed by eventual internalization via ciathrin-coated pits and vesicles (Helenius et al., 1980). The virus particles can be seen to rapidly accumulate in larger, electron-lucent prelysosomal vacuoles, and finally appear in secondary lysosomes (Helenius et al., 1980, 1982). These endosomal (prelysosomal) vesicles themselves are mildly acidic (pH 5-6) and, in the case of alphaviruses, may serve as the primary site of uncoating. Under conditions where SFV can be prevented from progressing from the endosomes to the secondary lysosomes (incubation temperature of 20°C), virion uncoating and infection are not inhibited (Marsh et al., 1983). 289