(Mode of Action of Drugs)

PharmacodynamicsPharmacodynamicsPharmacodynamicsPharmacodynamicsPharmacodynamicsPharmacodynamicsPharmacodynamicsPharmacodynamicsPharmacodynamicsPharmacodynamics

Chapter

1.4

Chapter

9.6 Antiviral Agents

Viruses have no cell wall and made up of
nucleic acid core enclosed in a protein coat
which consists of identical subunits.
Viruses are of two types, DNA
(deoxyribonucleic acid) viruses and RNA
(ribonucleic acid) viruses. DNA viruses are
herpes simplex, small pox, hepatitis B,
varicellazoster etc. and RNA viruses are
rabies, measles, dengue, rubella, yellow
fever, poliomyelitis and HIV etc.

In viral infections, replication of viruses
are at peak, at or before the manifestation
of clinical symptoms. So, the treatment
generally depends either on early initiation
of therapy or prevention of infection i.e.
chemoprophylaxis.

The various antiviral agents are
classified as under (the doses for specific
infections is given in text) in table 9.6.1.

ANTI-HERPES AGENTS

IDOXURIDINE

It is chemically related to thymidine and
acts by competing with it in the
synthesis of DNA and ultimately
preventing the utilization of thymidine.

It prevents the replication of DNA

viruses and its clinical use is limited to

herpes simplex keratitis.

Toxicity includes alopecia, leucopenia,

thrombocytopenia and liver damage.

It is used in herpes simplex keratocon-

junctivitis in 0.1 to 0.5% solution/eye oint-

ment applied one to two hourly.

ACYCLOVIR

Acyclovir is a synthetic purine

nucleoside analogue with in vitro and in

vivo inhibitory activity against human

herpes viruses.

Acyclovir triphosphate interferes with

the viral DNA polymerase and inhibits

viral DNA replication with resultant chain

termination following its incorporation into

the viral DNA.

Acyclovir is only partially (20%)

absorbed from the gut.

Most of the drug is excreted un-

changed by the kidney by tubular secre-

tion and glomerular filtration.9-

Carboxymethoxymethylguanine is the

only significant metabolite of acyclovir re-

covered from the urine.

Antiviral Agents