Product name :  Aciclovir  or  Acyclovir

Chemicial name :   Acycloguanosine

Chemical data :  

Formula                   C₈H₁₁N₅O₃
Mol. mass               225.21 g/mol

Pharmacokinetic data : 

Bioavailability         10–20% (oral)
Protein binding      9–33%
Metabolism            Viral thymidine kinase
Half life                   2.2–20 hours

Systematic (IUPAC) name :

2-amino-9-((2-hydroxyethoxy)methyl)-1H-purin-6(9H)-one

Pharmacology Mechanism of action :

One of the most commonly-used antiviral drugs, it is primarily used for the treatment of herpes simplex virus infections, as well as in the treatment of herpes zoster (shingles).And it is extremely selective and low in cytotoxicity.

Aciclovir differs from previous nucleoside analogues in that it contains only a partial nucleoside structure: the sugar ring is replaced by an open-chain structure. It is selectively converted into acyclo-guanosine monophosphate (acyclo-GMP) by viral thymidine kinase, which is far more effective (3000 times) in phosphorylation than cellular thymidine kinase. Subsequently, the monophosphate form is further phosphorylated into the active triphosphate form, acyclo-guanosine triphosphate (acyclo-GTP), by cellular kinases. Acyclo-GTP is a very potent inhibitor of viral DNA polymerase; it has approximately 100 times greater affinity for viral than cellular polymerase. As a substrate, acyclo-GTP is incorporated into viral DNA, resulting in chain termination. It has also been shown that viral enzymes cannot remove acyclo-GTP from the chain, which results in inhibition of further activity of DNA polymerase. Acyclo-GTP is fairly rapidly metabolised within the cell, possibly by cellular phosphatases.

In sum, aciclovir can be considered a prodrug: it is administered in an inactive (or less active) form and is metabolised into a more active species after administration.

Indications :

Aciclovir is indicated for the treatment of HSV and VZV infections, including:

1,Genital herpes simplex (treatment and prophylaxis)
2,Herpes simplex labialis (cold sores)
3,Herpes zo,ster (shingles)
4Acute chickenpox in immunocompromised patients
5,Herpes simplex encephalitis
6,Acute mucocutaneous HSV infections in immunocompromised patients
7,Herpes simplex keratitis (ocular herpes)
8,Herpes simplex blepharitis (not to be mistaken with ocular herpes)
9,Bell's Palsy

Pharmacokinetics :

Aciclovir is poorly water soluble and has poor oral bioavailability (10–20%), hence intravenous administration is necessary if high concentrations are required. When orally administered, peak plasma concentration occurs after 1–2 hours. Aciclovir has a high distribution rate, only 30% is protein-bound in plasma. The elimination half-life of aciclovir is approximately 3 hours. It is renally excreted, partly by glomerular filtration and partly by tubular secretion.

The poor oral bioavailability may also be improved by administering Valaciclovir, which has an oral bioavailability of about 55%. Valaciclovir is then converted to Aciclovir by esterases via hepatic first-pass metabolism.

Toxicity :

Since acyclovir can also be incorporated into cellular DNA, it is a chromosome mutagen, therefore, its use should be avoided during pregnancy.However it has not been shown to cause any teratogenic or carcinogenic effects.The acute toxicity (LD50) of aciclovir when given orally is greater than 1 g/kg, due to its low oral bioavailability