Sulfated oligo- and polysaccharides (Krusat and Streckert, 1997 and Kwilas et al., 2009) including muparfostat (this report) target mainly PCI-32765 nmr the RSV attachment protein G. Indeed, analysis of the viral variants resistant to muparfostat revealed a G protein mutation, N191T, occurring in the heparin-binding domain (Feldman et al., 1999) responsible for interaction of this protein with GAGs. Interestingly, in HSV muparfostat targeted proteins that, like RSV G protein, contain the mucin-like region, and the resistant variants

of HSV-1 expressed attachment protein gC with the entire mucin-like segment deleted (Ekblad et al., 2007) while HSV-2 produced no envelope glycoprotein gG (Adamiak et al., 2007). In contrast to muparfostat, RSV variants resistant to PG545 exhibited only a weak resistance to this drug. Nonetheless, these weakly resistant variants comprised two amino acid substitutions F168S and P180S in the central region of the G protein that includes the cysteine noose. Thus, analysis of RSV variants resistant to muparfostat and PG545 indicates that both these compounds target the G protein. However, in comparison with muparfostat,

PG545 reduced the virus attachment to cells less extensively while demonstrating a more pronounced inhibitory effect on infection of cells by virus that was adsorbed to cells at 4 °C prior to the addition of PG545. Collectively, poor resistance of RSV to PG545 and moderate reduction of the virus binding to cells by this compound suggest that CDK inhibitor in addition to the G protein PG545 may target other components of the viral envelope. Indeed, an expected affinity of cholestanol very component of PG545 for lipid

membranes suggests that this compound could be inserted into the viral lipid envelope thus creating a coat of artificial sulfo-glycolipids/sterols, a structure that could prevent fusion of viral and cellular membranes and thereby neutralize the virus. Lack of PG545 activity against influenza A virus, a pathogen that does not require GAGs for initial binding to cells, suggests that the sulfated oligosaccharide component of PG545 can be responsible for specific affinity of this compound for the GAG-binding viruses, an event followed by hydrophobic interaction of cholestanol with viral lipids. Thus, it is likely that PG545 may target more than one viral component to exhibit anti-RSV activity. Mutations detected by us in the G protein were not found in the published sequences of clinical isolates of RSV. It is noteworthy that another cholestanol- tetrasaccharide conjugate 14 failed to generate resistance in HSV-2 (Ekblad et al., 2010). Kimura et al. (2004) generated NMSO3 variants of RSV Long strain which, following 15 and 33 passages in HEp-2 cells, achieved 4.8- and 9.3-fold resistance to this drug.