Species differences in the disposition of acyclovir

Area of Science:

• Pharmacology
• Drug Metabolism
• Toxicology

Background:

• Acyclovir is a widely used antiviral medication.
• Understanding its pharmacokinetic profile across different species is crucial for drug development and efficacy.
• Species-specific disposition can influence therapeutic outcomes and potential toxicity.

Purpose of the Study:

• To investigate the disposition of acyclovir in various animal species.
• To characterize the absorption, distribution, metabolism, and excretion (ADME) of acyclovir.
• To identify potential species-specific differences in acyclovir pharmacokinetics.

Main Methods:

• Acyclovir disposition was studied in mice, rats, dogs, guinea pigs, rabbits, and rhesus monkeys.
• Intravenous and oral dosing routes were employed.
• Plasma concentration-time profiles were analyzed using radioimmunoassay.
• Tissue distribution, protein binding, and urinary excretion were quantified.
• Metabolites were identified using high-performance liquid chromatography.

Main Results:

• Acyclovir demonstrated good distribution into tissues, including the brain, with plasma protein binding ≤36%.
• Dogs exhibited a biphasic plasma decline with a half-life of 2.3 ± 0.1 hours and a volume of distribution (Vd β) of 1.2 ± 0.2 L/kg.
• Significant species variations in gastrointestinal absorption were noted; dogs and mice showed better oral absorption than rats and rhesus monkeys.
• Over 95% of parenteral 14C-acyclovir was recovered as unchanged drug in the urine of mice, rats, and dogs.
• Minor metabolites, 9-carboxymethoxymethylguanine (CMMG) and 8-hydroxy-9-(2-hydroxyethoxymethyl)guanine, were identified, comprising up to 40% of urinary radioactivity in guinea pigs, rabbits, and rhesus monkeys.

Conclusions:

• Acyclovir is well-distributed and primarily excreted unchanged in several species, but significant interspecies differences exist in its absorption and metabolism.
• These pharmacokinetic variations may impact dosing strategies and therapeutic efficacy across different species.
• Further research into species-specific metabolic pathways could inform the development of related antiviral agents.