To obtain information on the serum concentrations of acyclovir and its metabolite in routine health care with respect to the renal function.

This prospective study analyzed data from 27 patients receiving acyclovir intravenously between June 2019 and October 2021. Patients were divided into two subgroups according to the estimated glomerular filtration rate. Serum concentrations of acyclovir and its metabolite 9-(carboxymethoxymethyl) guanine were mainly analyzed on day 5 after the initiation of treatment before the morning dose (trough concentration) and 30 min after the end of the infusion (peak concentration).

Trough acyclovir concentrations ranged from 0.8 to 7.6 mg/L and peak concentrations from 6.3 to 25.7 mg/L, and trough metabolite concentrations ranged from 0.12 to 2.30 mg/L and peak concentrations from 0.47 to 2.70 mg/L, respectively. The ratio of trough metabolite and acyclovir concentrations ranged from 0.07 to 0.63 and the ratio of peak concentrations from 0.03 to 0.24. Patients in the subgroup with reduced renal function were significantly older, smaller, and of lower body weight and received significantly lower doses of acyclovir.

A 10-fold difference in the weight-adjusted apparent clearance of acyclovir was observed between patients. This wide interindividual variability in acyclovir pharmacokinetics can lead not only to toxicity but also to suboptimal acyclovir concentrations in severe infections. Therefore, therapeutic monitoring of serum concentrations of acyclovir and its metabolite may be important for optimizing pharmacotherapy, especially in patients with severe clinical conditions.