Acyclovir

Acyclovir is a deoxyguanosine analogue which is used to treat severe herpes virus infections.

A viral enzyme performs the first of three phosphorylations, the last two are by cellular enzymes with means that acyclovir is safe in non-infected cells.

73% is excreted in urine, and the CSF concentration reaches 50% plasma levels - thus it can be used in herpes encephalitis.

Routes of administration;

• orally in tablets or suspension
• topically as a cream or an eye ointment
• slow intravenous infusion

Resistance occurs in:

• TK mutations
• TK minus - most common, 1 per 10,000 normally - less neurovirulent
• DNA pol mutation with maintained function

In the immune competent, resistant mutants are no problem. However in immune suppressed patients there is failure to heal, and chronic lesions form, especially in AIDS.