Objectives: Zidovudine (ZDV) requires intracellular phosphorylation to ZDV triphosphate (ZDV-TP) prior to the inhibition of HIV replication. The effect of ZDV dose on the formation of intracellular phosphorylated metabolites may help define the optimum daily dose of ZDV, which is still unknown.
Design and methods: The plasma and intracellular phosphorylated metabolite concentrations of ZDV were determined over a 12 h period following oral administration of 100 and 300 mg ZDV to 10 HIV-seropositive patients at steady state during two dosing regimens (i.e., 100 mg three times daily and 300 mg twice daily). The intracellular ZDV phosphates, ZDV monophosphate (ZDV-MP), ZDV diphosphate (ZDV-DP) and ZDV-TP were measured in peripheral blood mononuclear cells using a combination of high-performance liquid chromatography and radioimmunoassay.
Results: There was a greater than threefold increase in maximum plasma concentration (Cmax) following 300 mg ZDV when compared with 100 mg ZDV (mean +/- SD, 2.59 +/- 0.52 versus 0.70 +/- 0.14 mumol/l). The area under the concentration time curve (AUC0-12 h) was also significantly increased (4.59 +/- 0.79 versus 1.42 +/- 0.51 mumol/l x h) following 300 mg ZDV dose. For total intracellular ZDV phosphate metabolites the AUC0-12 h was doubled (7.64 +/- 3.67 versus 3.71 +/- 1.83 pmol/10(6) cells x h) in patients taking 300 mg ZDV compared with 100 mg. The AUC0-12 h for ZDV-MP was significantly increased at the higher dose (6.47 +/- 3.14 versus 2.77 +/- 1.70 pmol/10(6) cells x h), whereas the active moiety ZDV-TP was variable and not significantly different (0.42 +/- 0.42 versus 0.61 +/- 0.81 pmol/10(6) cells x h) following 100 and 300 mg ZDV.
Conclusions: Administration of 100 mg ZDV orally produces significantly less of the potentially toxic metabolite, ZDV-MP, and comparative, although variable, concentrations of the active metabolite ZDV-TP when compared with 300 mg ZDV orally. This finding supports clinical data indicating the efficacy of low-dose (300 mg daily) ZDV. The measurement of intracellular phosphorylated metabolites advances our understanding of the clinical pharmacology of ZDV.