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 t1/2-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 t1/2-values approach more closely to the true t1/2-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.
Footnotes
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Send reprint requests to: Dr. David G. Johns, Building 37, Room 5B22, Laboratory of Medicinal Chemistry, Division of Basic Sciences, National Cancer Institute/National Institutes of Health, Bethesda, MD 20892-4255.
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Received September 11, 1996; accepted March 25, 1997.
- Abbreviations used are::
- ddN
- 2′,3′-dideoxynucleoside
- HIV
- human immunodeficiency virus
- ddNTP
- ddN-5′-triphosphate
- ddATP
- 2′,3′-dideoxyadenosine-5′-triphosphate
- F-ddATP
- 2′-β-fluoro-ddATP
- ddI
- 2′,3′-dideoxyinosine
- F-ddA
- 2′-β-fluoro-2′,3′-dideoxyadenosine
- ddGTP
- 2′,3′-dideoxyguanosine-5′-triphosphate
- PBM cells
- peripheral blood mononuclear cells
- D4T
- 2′,3′-didehydro-3′-deoxythymidine
- PHA
- phytohemagglutinin
- tm
- measured (uncorrected)t1/2
- tD
- cell doubling time
- 3TC-TP
- 2′-deoxy-3′-thiacytidine-5′-triphosphate
- AZT
- 3′-azido-3′-deoxythymidine
- ddC
- 2′,3′-dideoxycytidine
- The American Society for Pharmacology and Experimental Therapeutics
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