| Summary: | Background: Administration of (val)acyclovir in combination with mycophenolate mofetil (MMF) can result in increased acyclovir plasma levels due to competition with acyclovir for renal tubular secretion. Several studies of pharmacokinetic interactions between acyclovir and MMF have been previously described, however, they present conflicting results. Case report: We present two cases of male patients to whom a pharmacokinetic interaction occurred between (val)acyclovir and mycophenolate mofetil. Results: Case A developed an acyclovir through level of 3.5 mg/L on day 3 of his acyclovir treatment, which resulted in neurotoxicity attributed to acyclovir overdosing because of the pharmacokinetic interaction with MMF. Case B developed an acyclovir through level of 3.2 mg/L on day 4 of his acyclovir treatment, which was attributed to the pharmacokinetic interaction with MMF in combination with his poor renal function. No adverse side-effects occurred. Conclusion: Clinical decision support systems should be used to generate alerts when (val)acyclovir and MMF are co-administered in patients with renal impairment (eGFR < 60 ml/min/1,73 m2). By using early-stage therapeutic drug monitoring, dosages can be adjusted if necessary.
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