Tuberculosis (TB) is a common opportunistic infection in children with HIV infection in developing countries. Therefore, coadministration of antiretroviral and antitubercular therapy is frequently indicated. Short-course chemotherapy that includes rifampicin is the standard of care for treating TB. Rifampicin is a strong inducer of cytochrome P450 enzymes, notably CYP3A isoenzymes, and of P-glycoprotein. The protease inhibitor (PI) lopinavir, coformulated with ritonavir in a lopinavir/ritonavir (LPV/r) ratio of 4:1, is primarily metabolized by CYP3A isoenzymes and is also a substrate of P-glycoprotein.1,2 Coadministration with rifampicin resulted in a 90% to 99% reduction in the trough concentration (Cmin) of lopinavir in 2 studies in healthy adult volunteers.3,4 For this reason, coadministration of rifampicin and LPV/r is not recommended.
A study in healthy adult volunteers3 showed that adjusted dose regimens of LPV/r with concomitant rifampicin resulted in acceptable Cmin and peak concentrations of lopinavir. The Cmin was best preserved by adding ritonavir to LPV/r to give a LPV/r ratio of 1:1.
LPV/r-based highly active antiretroviral therapy (HAART) is used in South African children who are aged between 6 months and 3 years and have failed nonnucleoside reverse transcriptase inhibitor (NNRTI)-based HAART or been exposed to NNRTIs for the prevention of mother-to-child transmission. The objective of this study was to evaluate the pharmacokinetic drug-drug interaction between lopinavir and rifampicin by comparing lopinavir concentrations in the plasma of HIV-infected children coadministered rifampicin for TB and the adjusted dose of LPV/r (LPV/r ratio of 1:1) with the concentrations in children treated with standard doses of LPV/r without rifampicin.
A total of 30 children (aged 6 months to 15 years) were enrolled: 15 TB-HIV-coinfected children receiving LPV/r with additional ritonavir (LPV/r ratio of 1:1) as part of HAART and concomitant rifampicin-based antitubercular treatment for at least 4 weeks and 15 HIV-infected children without TB receiving LPV/r-based HAART for at least 4 weeks. Children were recruited at 3 sites: the HIV Clinic at the Red Cross Children's Hospital, Cape Town; the Harriet Shezi Children's Clinic, Chris Hani Baragwanath Hospital, Soweto; and Brooklyn Chest Hospital, Cape Town. Institutional approval of the study was granted by the Research Ethics Committees of the University of Cape Town, Stellenbosch University, and the University of the Witwatersrand.
Exclusion criteria were renal, hepatic, or intestinal disease (including malabsorption or diarrhea); active opportunistic infections; recent exposure to drugs described to have pharmacokinetic interactions with lopinavir; or reported missed doses of LPV/r in the preceding 3 days. The children were accompanied by a parent or legal guardian who provided written informed consent to participate in the study. Rifampicin-based antitubercular treatment regimens were administered in accordance with the National Tuberculosis Treatment Program.
Lopinavir at a dose of 230 mg/m2 plus ritonavir at a dose of 57.5 mg/m2 (LPV/r ratio of 4:1, liquid formulation) were given twice a day in combination with twice-daily dual nucleoside reverse transcriptase inhibitors (NRTIs) to children in the control group; additional ritonavir at a dose of 172.5 mg/m2 (LPV/r ratio of 1:1) was given to children receiving antitubercular treatment. Doses calculated according to body surface area were rounded off to the nearest 0.1 mL. The exact time of the morning dose of LPV/r was recorded, and 8 blood samples were collected at 0 (predose sample), 2, 3, 4, 5, 6, 8, and 12 hours after drug intake. The samples were centrifuged (3000 rpm for 5 minutes) within 30 minutes after sampling, and the plasma was stored immediately at −80°C while awaiting quantification of lopinavir concentrations.
Lopinavir concentrations were quantified by a validated liquid chromatography-tandem mass spectrometry (LC-MS/MS) method using a modification of the method by Chi et al.5 The calibration curve was linear over the range from 0.05 to 20 mg/L. Any sample whose lopinavir results were determined to be >20 mg/L was diluted with drug-free plasma and reanalyzed. The lower limit of quantification (LLOQ) of the lopinavir assay was 0.05 mg/L. Any samples with a lopinavir concentration lower than the LLOQ were reported as <0.05 mg/L and treated as 0 mg/L in the analysis. Accuracy ranged from 94.3% to 103.0%. The intraday and interday precisions ranged from 0.14% to 4.72% and from 1.61% to 4.22%, respectively. The laboratory participates in the International Interlaboratory Control Program of Stichting Kwaliteitsbewaking Klinische Geneesmiddelanalyse en Toxicologie (KKGT; Hague, The Netherlands) on an ongoing basis.
Adults accompanying the participants were asked about treatment adherence to HAART during the 3 days before pharmacokinetic sampling using a questionnaire. Alanine transaminase (ALT) concentrations were measured as a marker of hepatitis within 51 days of pharmacokinetic sampling. Viral load monitoring was performed at 6-month intervals as part of routine management using the NASBA EasyQ (Biomerieux, Boxtel, The Netherlands).
WinNonlin version 4.1 (Pharsight Corporation, Mountain View, CA) was used to characterize the pharmacokinetics of lopinavir from concentration-time curves by noncompartmental analysis. The maximum concentration (Cmax) and the time to Cmax (Tmax) were determined directly from the concentration-time data. The lopinavir Cmin was the concentration measured at the 12-hour time point. The area under the curve from 0 to 12 hours (AUC0-12) was calculated at the steady state with a dosing interval of 12 hours (τ = 12 hours). The Mann-Whitney 2-sample rank sum test for skewed data was used to compare the pharmacokinetic measures between the 2 groups. A probability of ≤0.05 was considered statistically significant for all tests. STATA version 8.2 (Stata Corporation, College Station, TX) was used for the analyses.
The children's general characteristics are summarized in Table 1. The coadministered NRTIs were stavudine plus lamivudine in 22 (73%) children and lamivudine plus zidovudine in 8 (27%) children. The median volume of LPV/r administered to these children was 1.7 mL (range: 1.0 to 2.1 mL). The maximum total volume of LPV/r and additional ritonavir administered was 3.5 mL in children receiving concomitant antitubercular treatment. On the days of pharmacokinetic assessment, drug administration was observed by the investigators and no vomiting was recorded.
Full adherence to their antiretroviral therapy during the 3 days before pharmacokinetic evaluation was reported for all 30 participants. The median duration of antiretroviral treatment was 20 weeks, ranging from 4 to 173 weeks. Twenty-seven children had viral load information up to 6 months after initiation of antiretroviral treatment. Nineteen of 27 children (70%) had viral loads lower than the detectable limit of the assay (<50 copies/mL) after at least 6 months of antiretroviral therapy. The remaining 8 children, 4 children from each group, had viral load log10 values ranging from 2.54 to 4.72. Two of the children receiving antitubercular treatment, and 1 child without TB had slightly higher than normal (5 to 30 U/L) ALT concentrations (35 and 40 U/L and 42 U/L in the 2 groups, respectively). They were all <1.5 times the upper normal limit (UNL) of the ALT normal range, however.
The major lopinavir pharmacokinetic measures are listed in Table 1, and the concentration-time curves are shown in Figure 1. Large interpatient variability was observed. There was a significant reduction in the lopinavir Cmax (P = 0.018) and AUC0-12 (P = 0.036) of children receiving LPV/r at a ratio of 1:1 with concomitant rifampicin-based antitubercular treatment. The difference in lopinavir Cmin between the 2 groups was not significant. Both groups had lopinavir doses slightly higher than the recommended 230 mg/m2. None of the children without concomitant antitubercular treatment had a Cmin lower than 1 mg/L. All but 2 of the children receiving additional ritonavir with rifampicin-based antitubercular treatment had a lopinavir Cmin > 1 mg/L. One of these children had a low dose of additional ritonavir because of a dosing error (49.5 vs. 172.5 mg/m2). The predose sample concentrations for the 2 children were 2.17 and 4.29 mg/L, and the half-life of lopinavir in these children was substantially shorter (0.97 and 1.75 hours) than that observed in any of the other children. One child who was not receiving antitubercular treatment had an undetectable predose lopinavir concentration (<0.05 mg/L) but a normal Cmin. This child was suspected to have missed 1 or more doses before pharmacokinetic sampling. The results of the statistical analysis were unaltered when the 2 children with dosing errors or suspected missed doses were excluded.
To our knowledge, this is the first pediatric study to evaluate lopinavir pharmacokinetics and the drug-drug interaction between an adjusted dose regimen of LPV/r and rifampicin in a TB-HIV-coinfected population. We found a reduction in lopinavir Cmax and AUC0-12 in the children receiving additional ritonavir with rifampicin. The key pharmacokinetic measure, Cmin, was similar between the 2 groups, however. Twenty-eight (93%) of the 30 children had a lopinavir Cmin greater than the recommended minimum therapeutic concentration (1 mg/L).6,7
It is important to conduct pharmacokinetic studies in children rather than to extrapolate from adults, particularly from healthy volunteers. There are important age-related factors affecting pharmacokinetics; of particular relevance to lopinavir is the activity of the cytochrome P450 enzyme system.8 Disease status could alter the pharmacokinetics of antiretroviral drugs in patients with TB-HIV coinfection, for example, by altering the concentrations of the drug-transporting proteins.
Our study in children confirmed the finding of the previous study in adult healthy volunteers3 that the reduction of lopinavir Cmin caused by rifampicin can be attenuated by adding additional ritonavir to LPV/r (LPV/r ratio of 1:1). In the adult study,3 there were no significant differences in AUC0-12 or Cmax in the same group of participants when receiving a standard dose of LPV/r or when receiving additional ritonavir (LPV/r ratio of 1:1) with concomitant rifampicin, although we found a significant reduction of Cmax and AUC0-12 between the 2 groups. Although the median Cmin was 15% lower in children receiving additional ritonavir during antituberculosis treatment, the difference between the 2 groups was not significant.
None of the children had their treatment interrupted because of elevations of ALT. In contrast, 38% of adult healthy normal volunteers were withdrawn as a result of adverse events, and 28% had raised liver enzyme concentrations during treatment with an adjusted dose of LPV/r together with rifampicin.3 We found that the combination of additional ritonavir (LPV/r ratio of 1:1) with rifampicin was generally well tolerated in children. Further investigation regarding the toxicity and efficacy of LPV/r with extra ritonavir- and rifampicin-based antitubercular treatment needs to be carried out, however.
Rifabutin can be used in antitubercular treatment instead of rifampicin in patients on LPV/r-based HAART, because rifabutin does not affect lopinavir concentrations.9 This approach is not a feasible option for most TB control programs in developing countries, because rifabutin is currently prohibitively expensive. There is no suitable rifabutin formulation for pediatric practice, which precludes the use of rifabutin in extremely young children. Furthermore, in high-burden countries, TB control programs rely on standard treatment regimens in fixed-dose combinations administered by nurses.
The main limitations of our study are the small sample size and the unpaired study design. Lopinavir concentrations display large interpatient variability, as illustrated by the coefficient of variation in the lopinavir Cmin of 86.1% for the rifampicin group and 111.4% for the control group. This limits the power of our study. It is difficult to recruit large numbers of pediatric patients for pharmacokinetic studies, however.
In conclusion, an adjusted dose regimen of LPV/r (LPV/r ratio of 1:1) achieved adequate lopinavir Cmin in most HIV-infected children when coadministered with rifampicin-based antitubercular treatment. Because of the large interpatient variability observed, however, therapeutic drug monitoring should be considered for children receiving concomitant rifampicin. Unfortunately, this is seldom available in developing countries, where the need is greatest.
The authors gratefully acknowledge the contributions of the staff at the Red Cross Children's Hospital, Harriet Shezi Children's Clinic, Brooklyn Chest Hospital, and Clinical Pharmacology laboratory of the University of Cape Town, South Africa as well as the contribution from Prof. Gregory Hussey.
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