Early antiviral agents were either too toxic to be used without harming the host, or were not potent enough to prevent viral replication. Acyclovir, introduced in the late 1970s and early 1980s was the first antiviral agent available that was nontoxic enough to be used in seriously ill patients. Since then the number of effective antiviral agents continues to grow. Currently used antiviral agents do not destroy viral particles; rather, they interfere with viral replication so that the virus cannot continue to reproduce to the level where normal cellular functions are disabled.
Viral nucleic acid is variable depending on the specific virus, and may be composed of DNA or RNA, single or double-stranded in a number of shapes – circular or linear, segmented or continuous. Despite the differences from human cell nucleic acids, viruses must be able to share the cell’s metabolic pathway and carry any missing pieces necessary to replicate successfully.
Viruses travel lightly carrying only enzymes essential for replication, but not available in the infected host cell. Viruses reproduce inside host cells (intracellularly) using the cell’s metabolic machinery. In order to prevent viral replication without affecting host cell metabolism, antiviral therapy is aimed at inhibiting viral enzymes rather than enzymes common to the host cell.
Viral replication involves a number of steps:
- cell attachment
- cell penetration
- uncoating of nucleic acid
- transcription and translation of regulatory proteins
- nucleic acid synthesis
- synthesis of structural proteins
- assembly of mature virions
- release from the cell
Any of the viral replication steps that are not also steps of the human nucleic acid replication process is a potential target for interference. Human nucleic acid is composed of a double-stranded DNA helix. When host cells replicate, new DNA is synthesized from the double-stranded parent DNA template in the cell’s nucleus. Messenger RNA (mRNA) is also transcribed from the DNA template and from the nucleus is transported to the cytoplasm were functional proteins are translated.