Without the use of preventive measures, the risk of mother-to-child transmission (MTCT) of HIV-1 is estimated to vary between 25% and 48%. Several preventive strategies have been evaluated, but most of them are too expensive to implement in resource-limited countries. The regimen of a single dose of nevirapine to the mother just before delivery and a single dose of nevirapine to the newborn between 24 and 72 hours after birth reduces the risk of MTCT by 50% and is affordable in many situations.1
Recent studies, however, have shown that this single dose to the mother can induce nevirapine resistance in 20% to 30% of the mothers.2 The development of this resistance may have major implications. First, it is uncertain whether a subsequent course of nevirapine is still effective for the prevention of MTCT when these women become pregnant again. Second, the efficacy of subsequent treatment with a nevirapine-based highly active antiretroviral therapy (HAART) regimen is diminished when the patient is harboring a resistant virus.3 Finally, nevirapine-resistant strains may be transmitted to other people.
The mechanism of the occurrence of nevirapine resistance after a single dose is most likely related to the long elimination half-life of nevirapine and the low genetic barrier to resistance. Small pharmacokinetic studies have demonstrated that the elimination half-life after a single dose of nevirapine is approximately 60 hours.4 This implies that plasma concentrations of nevirapine are detectable in mothers for several days after delivery. The subtherapeutic but detectable plasma levels present the perfect environment for the occurrence of resistance, because the concentrations may be subinhibitory for several days.
The primary objective of this study was to investigate the intersubject variability and potential influencing factors in the decay of plasma nevirapine concentrations after a single 200-mg dose. A secondary objective was the evaluation of the use of saliva as an alternative to blood sampling for measurement of nevirapine concentrations. The study was conducted in The Netherlands as a prelude to a similar study in Tanzania.
The present study was a single-center, open-label, single-dose, single-period 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. The study protocol was reviewed and approved by Ethics Committee of the Radboud University Medical Centre, Nijmegen, The Netherlands. Informed consent was obtained from all women before enrollment.
All study subjects received a single oral dose of 200 mg of nevirapine on day 0, and the Principal Investigator directly observed medication ingestion. Sampling of blood and saliva was done just before and 3, 7, 10, 14, 17, and 21 days after sampling. Stimulated saliva was obtained by a salivette (Sarstedt, Etten-Leur, The Netherlands) using a dental cotton roll impregnated with citric acid (20 mg), which stimulates the salivary flow. Study subjects were asked to chew on the cotton roll for approximately 1 minute. Saliva was obtained by centrifugation of the cotton roll. The plasma and saliva samples were stored at −40°C until analysis. Plasma and saliva concentrations of nevirapine were determined by a validated high-performance liquid chromatography (HPLC) assay with ultraviolet (UV) detection.5 The lower and upper limits of quantification were 0.15 and 15 mg/L, respectively. The intra- and interday precision ranged from 1.3% to 3.9% and from 1.9% to 3.0%, respectively. The accuracy of the assay ranged from 91.5% to 102.6%.
The typical median inhibitory concentration (IC50) value of nevirapine is 0.1 mg/L; corrected for 60% protein binding, this corresponds to a plasma level of approximately 0.2 mg/L. It is currently unknown, however, at what plasma level nevirapine selects for resistance. Clinical studies have determined that effective concentrations of nevirapine are greater than 3 to 4 mg/L, whereas levels less than 0.1 to 0.2 mg/L do not have selective pressure. Therefore, any plasma level between 0.2 and 3.0 mg/L has been defined by us as subtherapeutic. The following patient factors were tested for an association with the time to undetectable nevirapine plasma concentration: age, height, weight, body surface area, alcohol use, smoking habits, and oral contraceptive use.
Forty-four nonpregnant healthy women were enrolled in the protocol. The median age, height, and body weight (interquartile range) were 26 (21-33) years, 1.72 (1.68-1.75) m, and 64 (59-75) kg, respectively. Other than 1 Asian woman and 1 woman of mixed background, the remaining 42 women were white.
The pharmacokinetic parameters of the study subjects are presented in Table 1. The median elimination half-life (t½) for nevirapine in plasma was 56.7 hours, with a range of 25.6 to 164 hours. Maximum nevirapine plasma levels on day 3 (first postdose measurement) ranged from 0.36 to 1.59 mg/L, with a median value of 0.71 mg/L. The median time to the first undetectable nevirapine plasma concentration was 17 days. There were 7 subjects in whom nevirapine was still detectable on day 21, however, the last day of sampling (Fig. 1).
Except for oral contraceptive use, none of the other patient characteristics seemed to be related to the time to an undetectable nevirapine concentration in plasma. There were 17 women who reported taking oral contraceptives, and they had a median time to the first undetectable nevirapine plasma level of 21 days. This was significantly longer than for the remaining 27 women who reported not taking oral contraceptives (14 days; P < 0.001). The difference in the median plasma half-life of nevirapine in oral contraceptive users versus nonusers was not significant (69.7 vs. 52.8 hours, respectively; P = 0.053).
Saliva nevirapine concentrations were approximately half of the values observed in plasma. Nevirapine levels in saliva were significantly correlated with nevirapine levels in plasma at the first day of sampling: [NVP]saliva = −0.002 + 0.495 × [NVP]plasma (R2 = 0.531, F = 47.496, P < 0.001). Time to an undetectable nevirapine concentration was shorter in saliva than in plasma: median values were 14 and 17 days, respectively (see Fig. 1).
In this study of 44 healthy, non-pregnant, HIV-1-uninfected Dutch women, a single dose of 200 mg of nevirapine had an average half-life of 56.7 hours (or 52.8 hours in women who did not use contraceptives). Nevirapine levels remained detectable in plasma for a median of 17 days (range: 10 to >21 days). In 16% of women, nevirapine was detectable at the last measured time point, 21 days after the single dose. Thus, a single dose of nevirapine was associated with persistent measurable drug levels beyond 3 weeks after administration.
It is clear that our study population of healthy nonpregnant Dutch women of childbearing age is not similar to the setting in Tanzania (or other sub-Saharan African countries), where HIV-infected pregnant women are black and have different dietary habits, body weights, and comedications, for example. Nevertheless, the median nevirapine half-life that we observed in our group of 44 subjects (56.7 hours) is not too different from the average value as reported by Musoke et al4 in a smaller group of pregnant HIV-infected Ugandan women receiving single-dose nevirapine (61.3 hours). We were not able to identify any significant patient factor (other than oral contraceptive use) that was associated with an influence on nevirapine half-life. Oral contraceptives may be able to inhibit hepatic metabolism of nevirapine, although this effect was previously not observed (and thus not expected by us) in a formal drug-drug interaction study.6 It must be noted, however, that our study was not designed to address causality between oral contraceptive use and nevirapine half-life.
Most importantly, our data describe the window of opportunity for the virus to select for nevirapine-resistant mutations. The longer the time to an undetectable nevirapine level in plasma, the longer the virus has time to replicate. In 2000, Jackson et al7 reported that among the 15 women in the HIVNET 006 trial in whom virus was tested for the K103N mutation, the 3 women who developed the mutation had a significantly longer elimination half-life of nevirapine than the 12 women in whom no resistance was detected (74.8 vs. 51.8 hours; P = 0.01). Thus, one of the most rational interventions is the addition of other antiretroviral agents after delivery to cover this window of opportunity for the virus to select for nevirapine resistance. Recently, preliminary data were presented that short courses (4-7 days) of zidovudine plus lamivudine (Combivir) added to single-dose nevirapine in the prevention of MTCT significantly reduced the development of nevirapine resistance when compared with no intervention.8 One could speculate that this may not be sufficient to prevent the development of all nevirapine mutations. Indeed, nevirapine resistance was not fully absent in the intervention arms. Extending the duration of administering additional antiretroviral agents after delivery may also increase the development of resistance to these drugs, however.
It may be attractive to use methods other than collecting blood samples for measurement of exposure to nevirapine. As reported earlier, nevirapine can be detected in saliva.9 In our study, we observed a strong correlation between nevirapine levels in saliva versus plasma, and in almost all subjects, an undetectable nevirapine level was detected 3 days earlier in saliva than in plasma (see Fig. 1). Collecting saliva samples for measurement of nevirapine levels has the advantage of taking samples at home (by the patient herself, with no skilled personnel needed), less discomfort for the patient, and less infection risk for health care workers who draw the sample.
In conclusion, most women who received a single nevirapine dose of 200 mg still had detectable plasma concentrations of nevirapine after more than 2 weeks. This information is valuable for designing intervention studies to prevent the development of nevirapine resistance.
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Keywords:© 2005 Lippincott Williams & Wilkins, Inc.
nevirapine; pharmacokinetics; resistance; developing countries