Our data confirm that the antiretroviral drugs zidovudine, lamivudine, and nevirapine administered to pregnant and nursing women can be measured in the infants’ plasma shortly after birth and in breast milk expressed while nursing. Once the immediate newborn period has passed, lamivudine and nevirapine remain present in biologically significant concentrations in the infants’ plasma, while zidovudine is not detected above the lower limit of quantitation of the assay used. Three previous studies have described concentrations of zidovudine, lamivudine, and nevirapine associated with the administration of HAART to breast-feeding mothers (5
). Only one of these studies measured infant antiretroviral concentrations, and in that study, infant treatment with prophylactic daily zidovudine dosing prevented an evaluation of zidovudine exposure due to breast milk alone (27
). The data reported here are the first from a cohort of women receiving nevirapine-based HAART and their infants studied on the day of delivery, as well as at several subsequent times over the first 6 months of nursing.
Our data demonstrate that the magnitude of infant drug concentrations from exposure to maternally administered drug differs for each individual antiretroviral agent studied, as well as with the time postpartum. For all antiretrovirals, infant concentrations were highest on the day of delivery compared to the other postnatal sampling times, consistent with transplacental passage of the antiretrovirals from mother to fetus prior to delivery. The infant concentrations of zidovudine, lamivudine, and nevirapine at delivery observed in this study are consistent in magnitude with those seen in other studies reporting infant antiretroviral concentrations at delivery following chronic maternal dosing during pregnancy and labor (18
). In our current study, infant lamivudine and nevirapine concentrations declined over the study period despite constant breast milk concentrations, consistent with previously observed developmental increases in nevirapine and lamivudine clearances over the first 6 months of life (17
). The decreased drug concentrations at the 24-week visit may have been affected by the initiation of weaning prior to the collection date, though we do not have precise information on when weaning began for each woman.
The infant drug concentrations at the week 2 through week 14 postnatal visits reflect drug exposure from breast feeding during adherent maternal HAART administration and exclusive breast feeding. At these times, maternal plasma zidovudine concentrations were low, reflecting the very short (~1 h) half-life of zidovudine in adults and the resulting rapid clearance of an administered dose from plasma during a dosing interval. The median breast milk-to-plasma ratio for zidovudine was around 50%. While the standard daily zidovudine infant dose used for prevention of mother-to-child transmission in the first 4 to 6 weeks of life is 8 mg/kg (or 8,000 μg/kg), we estimated a median daily infant zidovudine dose from breast milk of 1.35 μg/kg, more than 1,000 times lower than the standard prophylactic dose (3
). Given this extremely small breast milk dose, it is not surprising that zidovudine was not detectable in any of the infant samples obtained after the day of delivery. A limitation of our study is that very few of our samples were obtained within the first 2 h after maternal dosing, when maternal plasma and breast milk zidovudine concentrations are highest. We also did not assay for intracellular concentrations of phosphorylated zidovudine in the infants due to the large amount of blood required for this assay. The active form of zidovudine is its intracellular triphosphorylated metabolite, which has a longer half-life than unmetabolized plasma zidovudine. It is possible that some zidovudine may reach the infant from breast milk, entering and persisting inside the cells while plasma concentrations quickly fall below the limit of quantitation of our assay, especially if nursing takes place soon after maternal dosing.
The lamivudine plasma concentrations in the mothers were consistently higher than those of zidovudine, which is consistent with the longer plasma half-life of 5 to 7 h for lamivudine. The lamivudine concentrations in breast milk were generally greater than those in plasma, which is consistent with previous findings (8
). Lamivudine concentrations in breast milk declined more slowly than in plasma, leading to a gradual increase in the lamivudine milk-to-plasma ratio as the dosing interval progressed. The change in the lamivudine milk-to-plasma ratio over the course of the dosing interval demonstrates why breast milk and maternal plasma drug concentration data must be analyzed taking into account the time since maternal dosing (34
). The estimated infant median daily dose of lamivudine from breast milk was 182 μg/kg, which is approximately 2% of the recommended daily treatment dose of lamivudine of 8 mg/kg divided into two doses in children over 3 months of age. Exposure to this amount of lamivudine via breast milk, as measured from weeks 2 to 24, resulted in a median infant lamivudine concentration of 23 ng/ml, which is just above the upper limit of the range of the lamivudine 50% inhibitory concentration (IC50
) for wild-type HIV (0.6 to 21 ng/ml) (4
Nevirapine has a long half-life (20 to 30 h) with chronic dosing in adults. Maternal nevirapine concentrations were in the range typically seen with chronic nevirapine therapy in adults (10
). The median nevirapine breast milk-to-plasma ratio was just over 70% and remained constant over the maternal dosing interval. The median estimated daily dose of nevirapine administered to the infants from breast milk was just over 600 μg/kg/day. Studies investigating the efficacy of direct administration of nevirapine to breast-feeding infants to prevent breast milk HIV transmission are ongoing (13
). The dose used in these studies is 4 mg/kg (or 4,000 μg/kg) once a day, and our estimated breast milk dose is about 15% of this dose (29
). Using a previously developed model of nevirapine pharmacokinetics in infants, the predicted nevirapine concentrations were somewhat lower than those observed in the current study. This analysis suggests that our estimated dose of presumed daily breast milk intake derived from U.S. data may be conservative, underestimating the actual daily nevirapine dose from breast milk in these Kenyan infants. During weeks 2, 6, and 14, the median infant nevirapine concentration was 896.9 ng/ml, well above the median HIV IC50
of 17 ng/ml for nevirapine but below the suggested target trough nevirapine concentration of 3,000 ng/ml (1
; Nevirapine package insert, revised April 2007 [Boehringer Ingelheim]). The maternal plasma, breast milk, and infant concentrations and the breast milk/plasma ratio observed for nevirapine in this study are very similar to those recently reported in a study of efavirenz transfer from breast milk. In that study, the median maternal plasma efavirenz concentration was 6,030 ng/ml, the median breast milk efavirenz concentration was 3,450 ng/ml, the mean efavirenz breast milk/plasma ratio was 0.54, and the median infant efavirenz concentration was 870 ng/ml, just below the suggested target trough efavirenz concentration of 1,000 ng/ml (1
). These data suggest that the transfer of either nonnucleoside reverse transcriptase inhibitor from maternal breast milk to infants would have similar potential for beneficial or adverse effects.
The infant nevirapine concentrations we observed in the day-of-delivery samples reflect transplacental passage of nevirapine and were roughly equivalent to those in maternal plasma at the time of delivery. Based on the range of breast milk nevirapine concentrations observed in this study, we estimate that by day 3 of life, 50% of infants ingested from breast milk a total nevirapine dose exceeding the 2-mg/kg infant postnatal prophylactic dose and by day 4 of life, the breast milk nevirapine dose exceeded 2 mg/kg in over 70% of infants. As a result of this combined transplacental and breast milk nevirapine exposure over the course of the first week of life, nursing infants whose mothers received a chronic prenatal and postnatal nevirapine HAART regimen will have nevirapine concentrations that exceed those seen in infants exposed to the mother-infant single-dose prophylactic perinatal regimen. Administration of the single-dose postnatal infant prophylactic nevirapine may be unnecessary in these infants if the mother is adherent to combination therapy before and after delivery. However, there was no evidence that the nevirapine dose given to the infants in this study was harmful, and programmatically it may be difficult to determine which mothers are adherent to ART.
In countries with high per capita incomes where triple-combination antiretroviral regimens, elective cesarean section, and safe formula feeding are readily available and acceptable to pregnant women, rates of mother-to-child transmission as low as 0.6% have been reported (22
). In resource-limited settings where some or all of these interventions are not available, the use of shorter, less-intensive antiretroviral regimens has been shown to result in smaller but still very significant reductions in antenatal and intrapartum mother-to-child HIV transmission (7
). In resource-limited settings where formula feeding is not safe or practical and breast feeding substantially improves infant survival, prevention of postnatal HIV transmission via breast milk remains a major challenge (33
). Although exclusive breast feeding has been shown to be associated with lower rates of HIV transmission than mixed feeding, the risk of transmission is not eliminated. Recent studies of the strategy of exclusive breast feeding for 4 to 6 months followed by early cessation of breast feeding and the use of replacement feeding and complementary foods have been associated with increased risks after weaning of morbidity and mortality from infectious diseases and malnutrition (30
). The development of practical and effective strategies to allow continued breast feeding while minimizing the risk of mother-to-child HIV transmission is urgently needed.
One proposed strategy for the prevention of breast milk HIV transmission is the administration of HAART to nursing mothers. Treatment of nursing mothers with zidovudine, lamivudine, and nevirapine from 28 weeks of gestation through 1 month postpartum has been shown to reduce breast milk HIV viral RNA loads at delivery and at the end of the first postpartum week compared to the levels in untreated women, although breast milk HIV DNA may be not be suppressed (8
). Several studies of the efficacy of maternal HAART in preventing breast milk HIV transmission are under way. However, animal and human studies have demonstrated that antiretroviral agents are transferred into breast milk (5
). Because of the frequency of feeding and the reduced clearance of antiretrovirals in infants, even low concentrations of antiretrovirals in breast milk may result in biologically significant antiretroviral concentrations in the nursing infant (14
). An understanding of infant drug exposure resulting from antiretrovirals received via breast milk is necessary before the use of antiretrovirals in nursing women becomes widespread.
Our data clearly show that lamivudine and nevirapine, but not zidovudine, are transmitted in biologically significant concentrations via breast milk to nursing infants when their mothers receive these drugs. The resulting infant antiretroviral drug exposure may have benefits, such as prevention of HIV infection or partial suppression of HIV replication in infants who become HIV infected. Alternatively, this drug exposure could result in potential drug side effects or, in infants who become HIV infected, the emergence of HIV drug resistance. Recent data from the parent Kisumu Breastfeeding Study demonstrate the emergence of HIV type 1 genotypic resistance mutations to nucleoside reverse transcriptase inhibitors (primarily to lamivudine but, to a lesser degree, to zidovudine as well) and nonnucleoside reverse transcriptase inhibitors among children who are HIV infected at birth or during the first 6 months while breast feeding and whose mothers received treatment with zidovudine, lamivudine, and nevirapine during pregnancy and while nursing (35
). While enhanced prophylactic strategies such as this are likely to reduce perinatal and postpartum HIV infections among infants, the extent of infant drug exposure via breast milk and the effects on infants of this exposure, especially the emergence of resistance mutations in those infants who become HIV infected despite maternal treatment, are important considerations for HIV treatment programs providing maternal HAART during breast feeding for prevention of mother-to-child transmission and for pediatric treatment programs that treat HIV-infected infants who have been exposed to these drugs via breast feeding.