Administering the appropriate dosage of antiretroviral agents is essential for achieving optimal suppression of viral replication and preventing the emergence of resistance. Extrapolations of adult dosages are commonly utilized when prescribing for adolescents, even though the former are often based on PK data from adults over 30 years of age.12
The current recommended abacavir dose of 300 mg twice daily for adults has been extrapolated to adolescents, resulting in a situation in which those who weigh >37.5 kg receive a smaller mg/kg dose than is recommended in children. The degree of sexual maturity, body mass index, diet, and other physiologic factors impacting drug disposition vary widely among adolescents of similar ages and are likely to influence optimal abacavir dosage requirements in adolescents and young adults. A recent study of abacavir pharmacokinetics in HIV-infected children and adolescents indicates that abacavir clearance does not change significantly during puberty.2
The apparent clearance of abacavir in adolescents was higher than would be expected in adults of similar size, and it did not correlate with weight, gender, or Tanner stage. This finding suggested that abacavir pharmacokinetics should be re-examined in adolescents and young adults.
Overall, this study found a modest, but significant, relationship between apparent abacavir clearance and volume of distribution (weight adjusted) and the age of the subject. This was in spite of the relatively large variability and narrow age range evaluated. However, when examined categorically by age strata, there were no significant differences in PK parameters. There were no age correlations or group differences in abacavir AUC or Cmax. The study was balanced across age strata for gender, ethnicity, associated disease stage, viral load, and CD4 T-cell counts. The two groups were unbalanced with respect to the use of NNRTIs and PIs. A larger proportion of subjects in the younger age group received PI-containing regimens as compared to the older subjects, who were predominantly treated with NRTIs with or without an NNRTI. Given that some PIs have been found to modestly increase abacavir clearance, this may have contributed to this observed association with age. The removal of data relating to these subjects led to loss of power and statistical significance; however, the age trends remained. The sample size for this study was derived to detect an age-related difference in AUCs of at least 25%; the observed difference in the mean values in the two age groups (7.35 μg·h/ml vs. 7.61 μg·h/ml), was much smaller. Given its wide variability, the mean apparent clearance in the younger cohort, of 13.03 ml/min/kg, was not sufficiently different from the older cohort value of 10.43 to reach statistical significance. Given that the AUCs were similar for the two age strata, the indications are that a single standardized dose can be used across both age strata.
Adherence to antiretroviral therapy is always a concern, especially in the adolescent population. Only 50% of the subjects had undetectable viral levels, suggesting that long-term adherence may have been suboptimal. However, a large proportion of the population were receiving antiretroviral therapy consisting solely of abacavir, zidovudine, and lamivudine, a combination that is known to be less effective at controlling viral replication than NNRTI- or PI-containing regimens, especially in the set- ting of high baseline viral loads.13
Potential nonadherence is also unlikely to have an undue influence on the PK results. The PK evaluations were performed around an observed dose, and all three compounds studied have short plasma half-lives, with little accumulation. In addition, all of the C0
values for carbovir-TP and lamivudine-TP were above the limit of detection, indicating no gross nonadherence with the doses just before the PK evaluations.
A comparison of the results of this study with those of previous PK studies of abacavir in children, adolescents, and older adults provides a reference point for determining safe and effective therapy. Older adults receiving abacavir at a dose of 300 mg twice daily (~4 mg/kg) have been shown to have steady-state AUC in the range of 5-6 μg·h/ml.6,12-14
In this study, the mean AUC among all adolescent and young adults receiving the same dose was 7.48 μg·h/ml. Apparent clearance was also in the range previously reported in adults. The apparent clearance of 0.86 l/ min, reported by McDowell et al.
, translates to a weight-adjusted apparent oral clearance of 12.3 ml/min/kg for a 70-kg adult.12
This is very consistent with the overall value from our study (11.7 ml/min/kg).
The AUC in this study was similar to the AUC of 8.4 μg·h/ml previously reported in children and adolescents (PACTG study P1018), but the dose in this study (300 mg twice daily) was lower than in P1018, in which the adolescents received 8 mg/kg (average 460 mg).2
Accordingly, the average apparent clearance adjusted for weight among the younger cohort in this study was lower than for the seven adolescents aged 13-18 years in P1018 (13.0 and 16.7 ml/min/kg, respectively). The lower apparent clearance in this study was not associated with a slower half-life (1.23 h vs. 1.49 h). While the reason for the possible differences between the various study data with respect to clearance is multifactorial, a higher frequency of PI use in P1018 (80%) is likely to have contributed to the differences.
Abacavir metabolism takes place primarily through the hepatic pathways of carboxylation and glucuronidation, with minimal renal clearance. Evaluation of metabolite ratios can provide mechanistic insights regarding PK differences, helping to distinguish between absorption and metabolism processes. The abacavir-carboxylate-to-abacavir ratios as well as the abacavir-glucuronide-to-abacavir ratios were not different between the age cohorts. Overall, there was no significant impact of age or gender on metabolite levels. However, the study population contained a large proportion of minority subjects and showed higher abacavir-glucuronide AUC among Hispanics than among non-Hispanic African Americans. There are well-documented genetic polymorphisms of UDP-glucuronosyltransferase enzymes, including UGT2B7, that are known to play a role in abacavir metabolism. These polymorphisms impact the metabolite profile of abacavir15,16
and are likely to contribute to the interindividual variability among the subjects. They are possibly involved in the ethnicity effect observed in this study.17
Zidovudine is also predominantly metabolized by UGT2B7. It is therefore not surprising that there was a rather strong association between abacavir and zidovudine AUC. However, what was unexpected was the high correlation between abacavir and lamivudine AUC, given that lamivudine predominantly undergoes renal elimination. These linked exposures suggest that absorption may be an important determinant for intersubject variability. Also, it is potentially a matter for concern that subjects with low abacavir exposure are more likely to experience low zidovudine and lamivudine exposure and effects. This may contribute to the high rate of viral breakthrough in patients receiving abacavir, zidovudine, and lamivudine as initial antiretroviral therapy or when de-intensifying to triple nucleoside regimens following viral suppression.18,19
Using the combined-cartridge liquid chromatography-tandem mass spectrometry assay, we were able to measure all of the TPs of the NRTIs in Trizivir in one injection with liquid chromatography-tandem mass spectrometry. While the procedure is thus somewhat simplified, it still remains complex and one not to be undertaken lightly. Still, this study has shown that the measurement of all three TPs is feasible. The average intracellular lamivudine-TP concentrations in this study of 3.73 pmol/106
cells predose and 5.38 pmol/106
cells at 4 h after the dose were also within the range of average concentrations reported in previous studies (1.7-8.6 pmol/106
Similarly, the zidovudine-TP concentrations in this study, of 32.7 fmol/106
cells predose and 74.0 fmol/106
cells at 4 h after the dose, were within the range of 6.3-58 fmol/106
cells seen in previous studies.20-23
We did find a correlation between the concentrations of the three TP anabolites, possibly because of shared steps of metabolism, including nucleoside diphosphate kinase.21
However, we did not note any gender-related effects on zidovudine-TP or on lamivudine-TP as previously reported.20
Despite the wide interindividual variability in carbovir-TP concentrations achieved, the ranges have been determined in patients responding to treatment (see above). Theoretically, assessment of steady-state carbovir-TP concentrations to determine whether they are within the reported ranges could allow assessment of a patient’s adherence to the abacavir regimen. However, the carbovir-TP ranges can differ between laboratories due to differences in cell processing and assay methodologies used, preventing routine clinical use of these assays.
This study represents the first evaluation of intracellular carbovir-TP concentrations in an HIV-infected adolescent population. The adolescent group exhibited carbovir-TP concentrations and carbovir-TP AUCs—predose and 4 h after the dose—that were similar to those in the young adult group. The overall median carbovir-TP values predose and 4 h after the dose (28.0 and 38.0 fmol/106
cells, respectively) were at the lower end of the range reported from previous studies (4-290 fmol/106
There was tremendous interpatient variability in carbovir-TP concentrations, the range of variability being as much as 60-fold in samples both predose and 4 h after the dose. The majority of the subjects had higher carbovir-TP concentrations in their samples taken 4 h after the dose (≥20% higher than predose concentrations in 83% (25 of the 30 subjects)). The median predose intracellular carbovir-TP concentration was more than double the ex vitro
carbovir-TP reverse transcriptase Ki
of 21 nmol/l (~10 fmol/106
Moreover, the median carbovir-TP Cave
exceeded the Ki
by approximately fourfold. This suggests that carbovir-TP exercises a substantial inhibitory effect on the incorporation of deoxyguanosine triphosphate into DNA by HIV-1 reverse transcriptase due to abacavir in these patients. In contrast to some reports, we found no apparent gender-related differences in carbovir-TP concentrations or plasma abacavir concentrations. There were also no significant correlations between plasma abacavir PK parameters and carbovir-TP concentrations. This is in contrast to the findings of Piliero et al
who observed a significant linear relationship between plasma abacavir AUC and carbovir-TP AUC.
Prior studies have indicated an average intracellular carbovir-TP half-life of between 12 and 20 h, allowing once-daily dosing.27,28
With only two samples available per patient, and the short (12-h) dose interval, this study was unable to estimate the half-life of carbovir-TP in individual subjects. However, the magnitude of carbovir-TP fluctuation during the dose interval, characterized by the C4
ratio (median value 1.9), was consistent with a relatively short half-life. If the half-life were 20 h, one would expect a ratio of 1.3, or possibly lower if the 4-h concentrations were to miss the peak. Given that the “predose” sample for study purposes was collected after an unsupervised dose, nonadherence would have preferentially impacted the predose concentrations and inflated this ratio. Significant nonadherence in this population, although possible, is unlikely to have occurred, given that the lamivudine-TP ratio of 1.3 is close to the value expected as indicated by prior studies and lamivudine-TPs intracellular half-life. Whether the larger fluctuation in carbovir-TP concentrations is a characteristic of this study population or of adolescents in general requires further inquiry. This may have clinical relevance when using extended-dose intervals. Further study would also be needed to rule out the possibility of adolescents having a shorter carbovir-TP half-life than adults do.
In conclusion, this study revealed modest associations of age with abacavir clearance and with volume of distribution, but these did not reach statistical significance in age group comparisons. This study is the first comprehensive evaluation of these antiretroviral agents and their metabolites in adolescents and young adults. The abacavir pharmacokinetics parameters were highly variable among the subjects, and it is likely that genetics and concomitant medications contributed to this variability.29
A novel aspect of this study was the assessment of intracellular levels of carbovir, zidovudine, and lamivudine-TP. Importantly, NRTI-TP concentrations were similar in the two age groups, while carbovir-TP concentration showed greater fluctuation than has been reported in other studies. Overall, plasma and intracellular carbovir-TP exposure were similar in this study population of adolescents and young adults and in accordance with previous reports in older individuals, thereby indicating that the current recommended dosage of 8 mg/kg up to a maximum of 300 mg twice daily is both safe and efficacious in adolescents.