Comparative pharmacokinetics of antiviral nucleoside analogues

The recent development of nucleoside analogues with antiviral activity has expanded the small but useful armamentarium for the treatment of certain viral diseases such as the human immunodeficiency virus, cytomegalovirus and others. Their intracellular site of action and need for sequential phosphorylation require that traditional pharmacokinetic parameters be used in conjunction with an understanding of intracellular metabolism when designing dosage regimens. This review summarises the available pharmacokinetic literature for zidovudine, didanosine, zalcitabine, aciclovir, ganciclovir, vidarabine and ribavirin. After oral administration, didanosine, aciclovir and ribavirin are < 50% bioavailable and ganciclovir is < 6% absorbed. In contrast, zidovudine and zalcitabine are > 60% bioavailable, although zidovudine undergoes considerable and variable first-pass hepatic glucuronidation while zalcitabine has no first-pass effect. Zidovudine, zalcitabine and didanosine are absorbed rapidly in the fasted state, with peak plasma concentrations exceeding their respective in vitro antiretroviral inhibitory concentrations. All reviewed agents except ribavirin have a relatively short plasma half-life (approximately 0.5 to 4h), with each agent demonstrating a different intracellular enzymatic activation scheme. For example, the rate-limiting step for formation of zidovudine triphosphate is the conversion of the monophosphate to the diphosphate, while didanosine is ultimately converted to dideoxyadenosine triphosphate which has the longest intracellular half-life (approximately 12 to 24h) among these agents. These drugs are not highly protein bound and they distribute into tissues with an apparent volume of distribution at steady-state ranging from 0.3 to 1.2 L/kg. They vary in the extent to which they enter cerebrospinal fluid, ranging from a low of < 25% for didanosine to a high of > 70% of a concurrent plasma concentration for ribavirin and vidarabine. These agents also vary with regard to degree of renal excretion of the parent drug, with the lowest noted for vidarabine (1 to 3%) and the highest for zalcitabine (approximately 75%) and ganciclovir (> 90%). With the increasing number of clinically useful nucleoside analogues, it is essential for the clinician to appreciate the subtle differences among these agents to ensure that optimal therapeutic outcomes may be attained with minimal toxicity.