Viral and host-cell protein kinases: enticing antiviral targets and relevance of nucleoside, and viral thymidine, kinases

Abstract

Numerous targets are known for development of antiviral agents, and some significant successes have been achieved with nucleoside analogues. These are "activated" by phosphorylation by viral and/or host-cell nucleoside kinases, the final target being principally the viral polymerase. With latency of herpes viruses, the viral thymidine kinase may be the ultimate target. Less attention has been devoted to viral protein kinases as antiviral targets, largely because 5 years ago, these the study of such enzymes was considered "still in its infancy." In the interim, identification of viral and host-cell protein kinases involved in viral gene expression, and viral replication, has made impressive advances. In conjunction with current progress in development of specific inhibitors of cellular protein kinases, and the differences in sequence motifs between these and the viral enzymes, the latter are indeed attractive targets, as are also some host-cell protein kinases. Examples include, amongst others, the essential protein kinases of vaccinia virus; the nonsegmented negative-strand RNA viruses, all essentially dependent on host-cell kinases, e.g., protein kinase CK-II (casein kinase-II), for which good inhibitors, such as halogenated benzimidazoles and benzotriazoles, are known; herpes viruses, with emphasis on human cytomegalovirus, the UL97 gene of which codes for a protein kinase that, like viral thymidine kinases, "activates," by phosphorylation, a nonpeptide antiviral acyclonucleoside ganciclovir, an analogue of the antiherpes aciclovir. The latter, in turn, is active against animal cytomegaloviruses following phosphorylation by the products of their UL97 gene homologues. Attention is also directed to the antiviral activity of the cyclic phosphate of ganciclovir, a structural analogue of the second messenger cyclic GMP.