L'homme, Rafaëlla F. A PharmD*†; Dijkema, Tim MD‡; van der Ven, Andre J. A. M MD, PhD†§; Burger, David M PharmD, PhD*†
Objective: Single-dose nevirapine (SD-NVP) to prevent mother-to-child transmission (MTCT) of HIV is associated with development of NVP resistance, probably because of its long half-life in combination with a low genetic barrier to resistance. The objective of this study was to find enzyme inducers to reduce the NVP half-life.
Design: The design of this phase 1 pharmacokinetic study was a single-center, open-label, 2-period, 9-group study.
Methods: After administration of a single 200-mg dose of NVP to HIV-seronegative nonpregnant women in periods 1 and 2, blood was sampled twice a week for 21 days. In period 2, additional interventions (single-dose carbamazepine, phenobarbital, or phenytoin; phenytoin for 3 or 7 days; or St. John's wort, vitamin A, or cholecalciferol for 14 days) were administered to all subjects except for the control group.
Results: Thirty-six subjects participated. In 3 intervention groups, the T-half ratio (nevirapine half-life in period 2/half-life in period 1) differed significantly from that in the control group: a single 400-mg dose of carbamazepine (P = 0.021) or 184 mg of phenytoin once daily for 3 (P = 0.021) or 7 days (P = 0.021). The median decreases in the NVP half-life were 18.8, 19.0, and 16.9 hours, respectively.
Conclusions: Interventions with a single dose of 400 mg of carbamazepine or 184 mg of phenytoin for 3 or 7 days effectively reduced the NVP half-life. Appropriately powered safety and feasibility end point studies are warranted before these interventions can be tested in the setting of single-dose NVP for prevention of mother-to-child transmission (PMTCT) of HIV to reduce the development of NVP resistance.
Without treatment, the risk to transmit HIV to a baby during pregnancy or delivery is approximately 25% to 48%. The complexity and costs of several effective strategies to prevent mother-to-child transmission (MTCT) of HIV presently limit their large-scale introduction in low-income countries. The administration of a single dose of the antiretroviral drug nevirapine (NVP) to the mother shortly before delivery and to the newborn within the first 24 to 72 hours after birth1 is simple and affordable for low-income countries, however. This so-called “single-dose nevirapine” (SD-NVP) strategy reduces the risk of MTCT by 50%.
Aside from the suboptimal effectiveness of the SD-NVP strategy, an important disadvantage is that the virus develops resistance against NVP in approximately 20% to 70% of the women.2,3 This can have a great impact. First, the effectiveness of SD-NVP may be diminished in a subsequent pregnancy. Second, efficacy can be diminished when the mother herself has an indication for NVP-based highly active antiretroviral therapy (HAART) in the future.4 Third, there is a possibility of transmitting resistant HIV to others. NVP resistance can develop because of the long elimination half-life of NVP in combination with a low genetic barrier to resistance. In a study, women who developed the K103N mutation after taking SD-NVP had a significantly longer elimination half-life of NVP than those in whom no resistance was detected (74.8 vs. 51.8 hours; P = 0.01).5
A short course of an enzyme inducer may prevent the development of resistance by decreasing the elimination half-life of NVP. The primary elimination pathway for NVP seems to be the oxidative metabolism by cytochrome P450 enzymes CYP3A4 and CYP2B6.6 Several potent CYP3A inducers have been described in literature. Long-term treatment with anticonvulsants, such as carbamazepine, phenobarbital, and phenytoin, increased the clearance of antipyrine, which is a broad marker of enzyme induction, to a similar extent on average.7 St. John's wort increased the clearance of NVP by 35%.8 Pregnane X receptor (PXR)-mediated upregulation of CYP3A4/CYP3A7 and CYP3A5 by retinol and β-carotene points to a potential interference in the metabolism of xenobiotic and endogenous relevant compounds.9 Finally, the fully active dihydroxylated metabolite of cholecalciferol, 1α,25-(OH)2D3, was shown to induce the expression of CYP3A4 genes and, to a lesser extent, CYP2B6 and CYP2C9 genes in normal differentiated primary human hepatocytes.10
The primary objective of this pilot study was to investigate the effect of intervention strategies with a number of the previously described agents on the elimination half-life of NVP. The study was conducted in The Netherlands as a prelude to studies in Tanzania. Appropriately powered safety and feasibility end point studies need to be performed before these interventions can be tested in the setting of SD-NVP for the prevention of MTCT (PMTCT) of HIV to reduce the development of NVP resistance.
The present study was a single-center, open-label, 2-period, 9-group, phase 1 pharmacokinetic study. Nonpregnant healthy women aged 18 to 40 years were eligible for enrollment after pre-entry and laboratory evaluation. Women who tested positive for HIV and/or hepatitis B or C virus were excluded. Subjects were not allowed to take any concomitant drug, including hormonal contraceptives and vitamin supplements (for 2 weeks preceding dosing), except for paracetamol (acetaminophen) and loperamide. The study protocol was reviewed and approved by the Ethics Committee of the Radboud University Medical Center, Nijmegen, The Netherlands. Informed consent was obtained from all women before enrollment.
The study design is shown in Table 1. In periods 1 and 2, sampling of blood was done just before; 8 hours after; and 3, 7, 10, 14, 17, and 21 days after intake of NVP. Plasma samples were stored at −40°C until analysis. Plasma NVP levels were determined in all samples by validated high-performance liquid chromatography (HPLC) assay with ultraviolet (UV) detection (Thermo, Breda, The Netherlands). The lower and upper limits of quantification were 0.03 and 15 mg/L, respectively. The intra- and interday precision ranged from 0.4% to 11.4% and from 0.0% to 2.1%, respectively. The accuracy of the assay ranged from 100.1% to 104.8%. NVP half-life was calculated per group in periods 1 and 2 using all quantifiable NVP levels. This exploratory pilot study with 4 participants per group was not powered to perform statistical tests.
In period 2, several medication levels were determined for safety reasons. Plasma concentrations of carbamazepine (group 2), phenobarbital (group 3), and phenytoin (groups 4-6) were determined in the blood sampled 8 hours after intake on day 0 by the validated immunoassay TDxFLx (Abbott Diagnostics, Amsterdam, The Netherlands). In addition, trough levels of phenytoin were determined in the blood samples of day 3 (groups 5 and 6) and day 7 (group 6). Concentrations of retinol (group 8) and 25(OH)D (group 9) were determined in the blood sampled 8 hours after intake on day 0 by validated HPLC assay. Furthermore, trough levels of retinol (group 8) and 25(OH)D (group 9) were determined in the blood samples of day 7 and 14.
Thirty-six nonpregnant healthy women were enrolled in the protocol. The median age, height, and body weight (interquartile range) were 22 (20-24) years, 1.71 (1.67-1.75) m, and 64.5 (60.0-73.0) kg, respectively. All women were white. One woman randomized to group 7 dropped out of the study during period 1 for personal reasons.
The median elimination half-life of NVP in period 1 was 53.9 (range: 34.2-104.2) hours. Pharmacokinetic parameters of NVP in periods 1 and 2 are illustrated per medication group in Table 2. Nonparametric statistical tests (Mann-Whitney U test) were performed on the T-half ratio (NVP half-life in period 2/NVP half-life in period 1). T-half ratios of 3 intervention groups differed significantly from that of group 1 (control): a single 400-mg dose of carbamazepine (group 2; P = 0.021), 184 mg of phenytoin once daily for 3 days (group 5; P = 0.021), and 184 mg of phenytoin once daily for 7 days (group 6; P = 0.021). In these 3 intervention groups, the median (range) decreases in the NVP half-life in period 2 were 18.8 [15.6-38.0] hours, 19.0 [11.4-25.4] hours, and 16.9 [10.9-37.4] hours, respectively. The decrease in the NVP half-life has led to faster undetectable NVP levels in groups 2, 5, and 6 with a single dose of carbamazepine or with phenytoin for 3 and 7 days, respectively. In these 3 intervention groups, the median (range) decreases in time to undetectable NVP levels in period 2 were 4.0 [3.0-7.0] days, 7.0 [7.0-7.0] days, and 8.5 [7.0-11.0] days, respectively. There were no large differences between maximum NVP plasma levels 8 hours after intake in the 2 periods.
Eight hours after intake of a single dose, carbamazepine levels in group 2, ranging from 3.78 to 6.03 mg/L, were all close to the lowest level of the therapeutic range of 4 to 10 mg/L. Phenobarbital levels in group 3 eight hours after intake of a single dose ranged from 5.10 to 5.83 mg/L, and were thus less than the therapeutic range of 10 to 40 mg/L. Phenytoin levels in groups 4 through 6, ranging from 2.36 to 6.27 mg/L, were all less than the therapeutic range of 8 to 20 mg/L. Retinol and 25(OH)D levels were all within the normal range of 0.70 to 3.00 μmol/L and 25 to 100 nmol/L, respectively.
One of 36 women in period 1 and 2 of 36 women in period 2 showed mild elevation of the liver enzyme alanine aminotransferase (ALT) (84, 95, and 114 U/L, respectively). Two of 4 women taking carbamazepine and 5 of 12 women taking phenytoin experienced mild vertigo. All 4 women in the phenobarbital group experienced mild somnolence. Adverse events were transient and did not influence the activities of daily living.
The present pilot study shows that the elimination half-life of NVP can be effectively decreased by interventions with a single 400-mg dose of carbamazepine or 184 mg of phenytoin for 3 or 7 days, most likely because of enzyme induction.
This study with small groups was not designed to perform statistical tests. Because of large and consistent decreases in the NVP half-life, however, we decided to perform a nonparametric statistical test on the primary parameter, T-half ratio, to determine whether the effect was actually significant.
The interventions with St. John's wort tea and vitamin A and D for 14 days did not show a decrease in the NVP half-life. For St. John's wort, this may be explained by the fact that the product used in this study (tea) contains a relatively low amount of hyperforine,11 which is the constituent likely responsible for enzyme induction.12 For vitamin D, one explanation might be that after intake of cholecalciferol by healthy women without a vitamin D deficiency, in vivo conversion to the fully active dihydroxylated metabolite 1α,25-(OH)2D3 is not optimal. A single 200-mg dose of phenobarbital was not enough to reduce the NVP half-life substantially. Lengthening treatment would not be wise because of slow elimination by the nursing infant. It seemed to be necessary to take 184 mg of phenytoin once daily for at least 3 days to reduce the NVP half-life significantly. In the African setting of PMTCT, it should be safe to use phenytoin during breast-feeding13 because of its poor passage into breast milk. Surprisingly, the simple intervention with a single 400-mg dose of carbamazepine seemed to reduce the NVP half-life effectively. It is unknown whether this effect is attributable to enzyme activation, enzyme induction, or upregulation of the efflux transporter P-glycoprotein (P-gp),14 although data are conflicting with regard to NVP being a substrate for P-gp.15 Carbamazepine passes into breast milk but is generally considered safe for use during breast-feeding.13 Rifampicin is a potent inducer of CYP3A, decreasing the NVP area under the curve (AUC) by 37% to 58%6, and was also considered for use in this pilot study. Rifampicin was not part of the interventions, however, because widespread use together with SD-NVP may promote the development of resistance against rifampicin in patients (co)infected with tuberculosis.
NVP plasma levels 8 hours after intake were not influenced by interventions with enzyme inducers. This is consistent with a delay of the effect on enzyme induction, because increased protein synthesis is required, and this takes a few days for maximum results. These interventions are unlikely to influence the protective effect of NVP on MTCT of HIV during labor, because there was no influence on maximum NVP levels. Because this finding is based on a study with small groups of healthy women, it needs to be confirmed in clinical practice.
It is clear that our study population of healthy nonpregnant Dutch women of child-bearing age is not similar to the setting in sub-Saharan African countries, where HIV-infected pregnant women are black and have different dietary habits, body weights, and concomitant medications. The CYP2B6 T/T genotype at position 516 is more common in African Americans than in European Americans and is associated with greater efavirenz exposure16 and, to a lesser extent, greater NVP exposure.17 The median NVP half-life in our group of 36 white women (53.9 hours) does not differ much from the mean NVP half-life in a smaller group of pregnant HIV-infected Ugandan women receiving SD-NVP (61.3 hours).18 The results from this pilot study are valuable, because we were able to identify 3 potential interventions (single 400-mg dose of carbamazepine or 184 mg of phenytoin for 3 or 7 days) to be studied in the setting of PMTCT to confirm our finding that the NVP half-life is diminished and to test the hypothesis that development of NVP resistance should decrease. The most rational intervention to start with is a single dose of carbamazepine because of its simplicity.
The addition of other antiretroviral agents after delivery to cover the window of opportunity for the virus to select for NVP resistance is a different approach. Recently, preliminary data were presented that short courses (4-7 days) of zidovudine + lamivudine (ZDV + 3TC [Combivir, GlaxoSmithKline, Brentford, UK]) added to SD-NVP in the PMTCT significantly reduced the development of NVP resistance when compared with no intervention.19 The substantial increase in costs and complexity is not desirable, however. Recent data from a study in Zambia showed that it is already difficult to carry out the simple SD-NVP intervention accurately.20
It is currently unknown at what plasma level NVP selects for resistance. NVP levels that are undetectable or, in the context of HAART, greater than 3.0 to 3.4 mg/L21,22 do not have selective pressure. The intervention with 4 to 7 days of Combivir covers the potential zone of selective pressure less than 3.0 to 3.4 mg/L only partially, which explains why NVP resistance was not fully absent in the intervention arms.19 A significant decrease in the NVP half-life by the addition of an enzyme inducer should reduce the duration of the potential zone of resistance but may also not be sufficient to prevent the development of all NVP mutations. It might be interesting to study whether combining these 2 approaches of decreasing the NVP half-life and coadministering other antiretroviral agents would be of additional value.
In conclusion, the interventions with a single 400-mg dose of carbamazepine or 184 mg of phenytoin once daily for 3 or 7 days effectively reduced the NVP half-life after intake of a single 200-mg dose of NVP, leading to faster undetectable NVP levels. Appropriately powered safety and feasibility end point studies are warranted before these interventions can be tested in the setting of SD-NVP for PMTCT in Africa to confirm our finding of a decreased NVP half-life and to test the hypothesis that the development of NVP resistance would be reduced.
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