This study demonstrates that the intracellular active metabolites, ZDV-TP and 3TC-TP, persist within the cell for long periods of time at detectable levels that are likely to inhibit viral replication. Although the ZDV-MP concentration fluctuated greatly over the 24-h dose interval, the other metabolites, including ZDV-TP and 3TC-TP, only showed modest fluctuation over this interval. Pharmacokinetic modeling yielded a half-life for ZDV-TP of 9.1 h, while that for 3TC-TP was 17.7 h. These half-lives are similar to previously reported values from adult populations: 7 to 11 h for ZDV-TP (1
) and 16 to 32 h for 3TC-TP (1
We witnessed considerable intersubject variability in the rates of phosphorylation, with the ZDV-TP and 3TC-TP predose concentrations spanning an ~40-fold range (coefficients of variation of 74% for ZDV-TP QD, 97% for ZDV-TP BID, 90% for 3TC-TP QD, and 97% for 3TC-TP BID). This broad variability is a concern when trying to extend the dose interval since a substantial number of subjects would have very low concentrations with the QD dosing.
A priori, we had chosen a ratio of BID/QD predose concentrations of ZDV-TP and 3TC-TP of <2 to support additional studies of QD ZDV and 3TC. When the same total daily doses were administered either QD or BID, the ratios of predose levels between all of the 3TC metabolites all differed by less than a factor of 2. The same was true for ZDV, ZDV-MP, and ZDV-DP. The ZDV-TP ratio was significantly different from 1 but not different from 2. Despite the ratios of predose concentrations falling below our threshold value of 2 for all of the metabolites, the lower levels of the active metabolite ZDV-TP, the substantial intersubject variability, the relatively short ZDV-TP half-life, and findings from preclinical and relatively recent adult clinical studies require reassessment of these criteria. Preclinical studies using ZDV as a continuous infusion or as a QD dose in a murine model of AIDS encephalopathy demonstrated improved antiviral effect of continuous infusion ZDV compared to QD dosing (3
). Similarly, Drusano et al. reported findings that did not support a QD dosing of ZDV in an in vitro hollow-fiber model system that has been predictive for other nucleosides (6
In addition, clinical studies of pharmacokinetics and pharmacodynamics comparing ZDV at 300 mg BID versus 600 mg QD have been performed in adults. The COD10001 study demonstrated that mean half-lives were similar in the two dosing regimens (6.3 h for the QD regimen versus 5.48 h for the BID regimen), and both groups had similar declines in viral loads over 14 days of monotherapy (22
). In COD20002, 32 antiretroviral naive subjects were randomized, in a 1:1 ratio, to receive either ZDV 600 mg QD or 300 mg BID as monotherapy. Viral loads over the first 14 days demonstrated a trend suggesting that the subjects on the BID dosing regimen achieved lower viral loads (P
= 0.056) and more rapid declines in viral load (P
= 0.065) compared to the QD group (22
). These studies differ from the present study in several significant ways. In the present study, the QD and BID regimen were given in addition to either a protease inhibitor or a non-NRTI or both as part of a HAART regimen. In addition, we opted to study subjects who had been receiving their current HAART regimen for at least 4 weeks and were at steady state. Although our ZDV-TP half-life was somewhat longer at 9.1 h in adolescents, it was still short enough to result in lower predose ZDV-TP concentrations with QD dosing. In addition, we did not investigate viral dynamics since many of our subjects had undetectable viral loads at the start of the study. Thus, despite a potential gain in adherence, the lower levels of ZDV-TP taken in consideration with findings from other studies have dissuaded us from pursuing further study of a QD ZDV regimen.
In our study, we were able to demonstrate that ZDV-TP measurements on the standard BID dosing regimen were correlated with ZDV-TP concentrations on a QD dose. This finding may be important if future technologies allow easy measurement of ZDV-TP. If monitoring of ZDV-TP becomes practical, one may be able to individualize patient dosing regimens in patients with high ZDV-TP levels on a BID regimen and to assess a QD regimen. In adolescent patients, removing barriers to adherence by simplifying dosing regimens is critically important and requires additional research efforts. However, because the measurement of ZDV-TP is a complex research laboratory procedure, extending the ZDV dose interval in patients with high predose ZDV-TP is not practical or readily available.
Another possible strategy to make daily ZDV more effective would be to use increased doses of ZDV to achieve higher ZDV-TP levels. Although we did not study doses other than 600 mg QD and 300 mg BID in our study, Fletcher and coworkers demonstrated systemic exposure to ZDV-TP increased using a 700-mg daily dose compared to a 500-mg daily dose (1
). In addition, Anderson et al. noted a 125% increase in ZDV-TP concentrations when the ZDV dose was increased by 130% (1
). These data indicate that, despite substantial subject variability, measurements of ZDV-TP do change as a function of dose. However, higher doses may be associated with greater toxicity and cost and would need to be studied.
This is the first evaluation of intracellular NRTI metabolites in adolescent HIV-infected subjects. Our findings are in accord with previous studies in older populations. The estimates of the ZDV and 3TC metabolites were in the range of those previously reported, and peak ZDV-TP concentrations were in the range previously reported (1
; Rogers, unpublished). Likewise, 3TC-TP concentrations were similar to those reported (1
; Rogers, unpublished). The proportion of 3TC metabolites as the 3TC-TP moiety was higher than seen by Moore et al. (14
). In that study 3TC accounted for only 15 to 20% of the intracellular phosphates compared to the ca. 45% observed in the present study. Unlike Anderson et al. (1
), we were unable to find a correlation between concentrations of ZDV-TP and 3TC-TP. Given variability in measurements and different methodologies among studies, these findings suggest similar metabolic handling of ZDV and 3TC in this young population.
Our study had several limitations. Because of imbalance in the evaluability of pharmacokinetic data during the early stages of accrual, randomization to the two groups was stopped, compromising the crossover design and limiting the interpretability of the tests for interaction and order effects. Given the high variability in metabolite concentrations, our study was too small to assess for potential age or gender effects as reported by Anderson et al., and no analyses by gender were performed (1
). In addition, the BID predose sample was obtained prior to the scheduled morning dose and did not follow an observed dose.
In conclusion, we have demonstrated that most patients have detectable levels of the TP of ZDV and 3TC 24 h after dosing and that half-lives on a QD regimen were similar to previously reported values when the drugs were given BID. There was considerable variability, and some patients had nonmeasurable concentrations of ZDV-TP after 24 h. The predose intracellular MP and DP metabolite concentrations of ZDV and 3TC on QD were all within 70% of those seen with BID therapy. However, ZDV-TP and 3TC-TP concentrations with QD therapy were lower as a percentage of the BID values, with the ZDV-TP changes reaching marginal significance due to its shorter half-life. Despite these similarities between the QD and BID ZDV dosing regimens, the lower concentrations of ZDV-TP and the lack of a targeted nadir linked with virologic efficacy, previously published literature, and the increasing number of QD antiviral agents do not support further study of a QD dose of 600 mg of ZDV.