Abstract

Measurement of intracellular drug levels in cell culture systems can be of predictive value in establishing rational clinical dosage schedules. Such in vitro measurements carried out with anti-HIV agents of the 2′,3′-dideoxynucleoside (ddN) class have shown that many of the pharmacologically active ddNTP metabolites of these agents have relatively long intracellular half-lives and little or no host-cell cytotoxicity. As a consequence, replication of drug-exposed cells continues at an unperturbed rate so that a systematic dilution error occurs in the measurement of ddNTP decay half-times. The aim of this study is to present a simple general formulation for the correction of measured t_1/2-values for ddNTPs and for other agents with similar intracellular pharmacokinetic properties. Two factors of practical interest emerge: first, the error is greater for agents with slow intracellular clearance rates than for agents with rapid rates; and second, for cell lines with long doubling times, the measured t_1/2-values approach more closely to the true t_1/2-values, until with the extreme case (quiescent or "G₀" cells), the observed and true decay times are identical. The greatest dilution errors are seen with adenodine-based agents such as ddATP and 2′-F-ddATP, while the smallest errors are seen with rapidly cleared agents of the dideoxythymidine class.