Herpes simplex virus develops resistance to aciclovir by selection of mutants deficient in thymidine kinase which are usually of diminished virulence with reduced infectivity and latency. Resistance is rare in immunocompetent patients on short courses of oral therapy but it is more preva lent in immunocompromised patients who have often received prolonged courses of treatment. Herpes zoster resistance develops by a similar mechanism and has been reported in immunocompromised patients undergoing prolonged therapy with aciclovir.

5.2 Pharmacokinetic properties
Absorption

Aciclovir is slowly and incompletely absorbed from the gastrointestinal tract. The peak plasma concentration occurs about 2 hours following ingestion.

Distribution

There is a wide distribution to various tissues, including the CSF where concentrations achieved are about 50% of those achieved in plasma. Protein binding is reported to range from 9-33%. Aciclovir crosses the placenta and is excreted in breast milk in concentrations approximately 3 times higher than those in maternal serum.

Metabolism and Elimination

Renal excretion is the major route of elimination by both glomerular filtration and tubular secretion. The terminal or beta-phase half-life is reported to be about 2-3 hours for adults without renal impairment. As aciclovir persists in the plasma of patients with renal insufficiency, in chronic renal failure this value is increased and may be up to 19.5 hours in anuric patients. As renal function decreases, a greater percentage of the drug is eliminated by metabolic conversion to carboxymethoxymethyl guanine. During haemodialysis the half-life is reduced to 5.7 hours, with 60% of a dose of aciclovir being removed in 6 hours. Faecal excretion may account for about 2% of a dose.

In neonates and young infants (0 to 3 months of age) treated with doses of 10 mg/kg administered by infusion over a one-hour period every 8 hours the Cssmax was found to be 61.2 microMol (13.8 micrograms/ml) and Cssmin to be 10.1 microMol (2.3 micrograms/ml). A separate group of neonates treated with 15 mg/kg every 8 hours showed approximate dose proportional increases, with a Cmax of 83.5 micromolar (18.8 microgram/ml) and Cmin of 14.1 micromolar (3.2 microgram/ml).

5.3 Preclinical safety data
Systemic administration of aciclovir in internationally accepted standard tests did not produce embryotoxic or teratogenic effects in rats, rabbits or mice. In a non-standard test in rats, foetal abnormalities were observed, but only following such high subcutaneous doses that maternal toxicity was produced. The clinical relevance of these findings is uncertain.

Two generation studies in mice do not reveal any effect of aciclovir on fertility.

The results of a wide range of mutagenicity tests in vitro and in vivo indicate that aciclovir does not pose a genetic risk to man. Aciclovir was not found to be carcinogenic in long term studies in the rat and the mouse. Largely reversible adverse effects on spermatogenesis in association with overall toxicity in rats and dogs have been reported only at doses of aciclovir greatly in excess of those employed therapeutically. Aciclovir has been shown to have no definite effect upon sperm count, morphology or motility in man.

6. Pharmaceutical particulars
6.1 List of excipients
Also contains: colloidal anhydrous silica, magnesium stearate, polyvidone, sodium starch glyc