Acyclovir was the first anti-viral drug ever to reach the market; it was discovered and marketed by The Wellcome Foundation (now GlaxoSmithKline). The original formulation was a topical ointment called Zovirax which is highly effective in treating cold sores caused by Herpes simplex virus (HSV). It has since been licensed to treat Varicella zoster infections (chicken pox and shingles). Acyclovir is also available in capsules, tablets, suspensions and for injection.

The discovery of acyclovir was a breakthrough in science because up until that point it was thought that viral infections could never be treated. Antibiotics usually work against bacteria by killing them without harming the host because bacteria live and replicate independently of the host cells. Viruses, however, have to use the replication mechanisms provided by the (infected) cells. It was generally believed that anything that prevented virus replication would therefore be lethal to its host.

It was discovered that Herpes viruses require the enzmes thymidine kinase, (TK), and DNA Polymerase in order to begin replication and these are not present in the host. Small quantities are therefore carried within the protein coat of the virus so that it can start replicating straight away upon infection. Acyclovir works because it is phosphorylated by the viral thymidine kinase into a nucleoside analogue. The DNA Polymerase then 'thinks' that it is the real thing and incorporates it into the growing DNA chain, rendering it useless.

In order to be effective anti-viral drugs should be taken as soon as the first symptoms appear, be it the mild tingling that precedes a cold sore or at the very first sign of a rash with chicken pox because this is when the virus is replicating fastest.

The main problem with acyclovir is that it is isn't very readily absorbed when taken orally (capsules and tablets). This means that quite high doses have to be given fairly frequently, and although the drug is extremely well tolerated, it is expensive and inconvenient. More recent additions to the anti-viral family include the drugs valacyclovir (marketed as Valtrex) and famcyclovir (Famvir) because much higher serum levels can be achieved and they are quickly converted to acyclovir at the site of action.

The discovery of acyclovir opened the flood-gates for more research into anti-viral agents. HIV and AIDS had only just been discovered, it was vital for the drugs companies to be first to come up with the cure for this potentially deadly epidemic - AZT was the first of these.

Refs: Thanks to dwardu for his help in clarifying certain points
BlueDragon cos she used to work there