Substantial progress has been made in the prevention of mother-to-child transmission (PMTCT) of HIV in resource-advantaged and resource-limited countries. Provision of antiretroviral (ART) drugs to mother and infant has reduced transmission to <2% in resource-advantaged countries.1 Infant PMTCT regimens commonly use zidovudine alone, nevirapine alone, or zidovudine in combination with lamivudine and/or nevirapine. Tenofovir has been proposed as an alternative agent. Tenofovir has been successfully used to prevent HIV transmission in pregnant animal models and has been effective in some studies when given as preexposure prophylaxis to high-risk adults.2–5 Because of its poor bioavailability, tenofovir is administered as the prodrug tenofovir disoproxil fumarate (TDF).6 Studies of the safety and pharmacokinetics of TDF in pregnant women during labor and neonates are limited.7,8 No previous study has looked at repeated infant dosing with TDF during the first week of life. The HIV Prevention Trials Network (HPTN) study 057 evaluated the safety and pharmacokinetics of TDF in HIV-infected pregnant women during labor and their infants in Malawi and Brazil. The primary objectives of the study were to evaluate the safety and pharmacokinetics of intrapartum/neonatal TDF with the goal of establishing an appropriate dosing regimen for HIV-infected women during labor and their infants during the first week of life.
METHODS AND MATERIALS
Study Design and Participants
HPTN 057 was a phase 1, open-label, noncontrolled trial of HIV-infected pregnant women during labor and their infants with 4 cohorts of maternal and infant dosing: cohort 1—maternal 600 mg doses during labor/no infant dosing; cohort 2—no maternal dosing/infant 4 mg/kg doses on days 0, 3, and 5; cohort 3—maternal 900 mg doses during labor/infant 6 mg/kg doses on days 0, 3, and 5; and cohort 4—maternal 600 mg doses during labor/infant 6 mg/kg daily for 7 doses. Subjects first enrolled in cohorts 1 and 2. Based on the results from these cohorts, cohort 3 was enrolled using increased dose sizes, as allowed by the original protocol. After review of the data from cohort 3, the protocol was amended to include a fourth cohort in which the infants received daily dosing. The targeted sample sizes were 30 mother–infant pairs in cohorts 1, 3, and 4 and 20 in cohort 2.
The study was conducted at the Queen Elizabeth Central Hospital in Blantyre, Malawi, and at 4 sites in Brazil: Federal University of Minas Gerais, Belo Horizonte; Irmandade da Santa Casa de Misericórdia, Porto Alegre; Hospital Nossa Senhora da Conceiçao Infectious Diseases Service, Porto Alegre; and Hospital Federal dos Servidores do Estado, Servico de Doenças Infecciosas, Rio de Janeiro. Women were recruited from antenatal clinics where HIV testing, counseling, and local standard of care ART regimens for PMTCT were provided. All women provided written informed consent. Maternal screening laboratory evaluations were performed after 34 weeks of gestation. Eligible mothers were enrolled in the study at presentation for delivery.
Eligibility criteria included age above 18 years and documented HIV infection. We excluded women who received previous treatment with TDF or had an active medical condition that might affect TDF pharmacokinetics or compromise their ability to complete the study. All infants born to study mothers were enrolled in the study. Infants in cohorts 2, 3, and 4 were excluded from dosing if they had birth weight <2000 g, severe congenital malformation or other medical condition incompatible with life or that would interfere with study participation or interpretation as judged by study clinician, grade 2 or higher serum creatinine level, or any other grade 3 or higher toxicity. Mothers were screened for enrollment during pregnancy and enrolled at presentation for delivery. Maternal and infant study visits were undertaken within 24–48 hours and 5–7 days postpartum, at 6 and 12 weeks, and at 6 and 12 months for repeat medical history, physical exams, and laboratory evaluations. If a mother in cohort 1 or 3 had a viral load >400 copies per milliliter at the 6-week study visit or an infant in cohort 1, 2, or 3 was diagnosed as HIV infected by 2 consecutive DNA or RNA polymerase chain reaction tests, HIV genotyping was performed using the ViroSeq HIV Genotyping System (Celera Diagnostics, Alameda, CA).
The study protocol was approved by at least one local ethics review committee affiliated with every study site, by committees affiliated with US collaborating institutions, and by other local and/or national regulatory bodies where applicable and was in accordance with the Helsinki Declaration of 1975, as revised in 2000. The study was registered with ClinicalTrials.gov (NCT00120471).
Study Dosing and Pharmacokinetic Sample Collection
Maternal TDF was administered as 300-mg tablets (Gilead Sciences, Foster City, CA). Infant TDF was administered as an oral suspension reconstituted from powder (Gilead Sciences) to a concentration of 20 mg/mL. Mothers in cohorts 1 and 3 had plasma samples collected before TDF was administered and 1, 2, 4, 8, 12, 18–24, and 36–48 hours after the dose. Plasma samples were also collected at the time of delivery from mothers who received TDF. Amniotic fluid was collected from mothers who received TDF and delivered by cesarean section. Mothers who were breast-feeding had a breast milk sample collected.
A cord blood plasma sample was collected at each study delivery. Cohort 1 infants had plasma samples collected at 4, 12, 18–24, and 36–48 hours after delivery. Cohort 2 and 3 infants had plasma samples collected before administration of the initial study dose and 2, 10, and 18–24 hours after the dose, before the day 3 dose and 2 and 10 hours after the dose, and before the day 5 dose and 2, 10, 18–24, and 36–48 hours after the dose. Cohort 4 infants had plasma samples collected before administration of the initial study dose, 2 and 10 hours after the dose, and just before the next dose; before the day 3 dose, 2 and 10 hours after the dose, and before the next dose; and before the day 6 dose and 2, 10, and 24 hours after the dose.
Tenofovir concentrations were measured by liquid chromatographic–tandem mass spectrometric assay. Specimens (50 μL) with added isotopic internal standards were protein precipitated, filtered, evaporated to dryness, and reconstituted in 0.5% acetic acid in water; 10 µL of reconstituted material was injected into the mass spectrometer and analyzed in positive electrospray multiple reaction monitoring mode. The multiple reaction monitoring transitions employed were as follows: TDF, m/z 288 > 176 and 13C5-TDF, m/z 293 > 181. Chromatographic separation was achieved on a Zorbax Eclipse XDB C18 Column (Agilent, Santa Clara, CA) using a gradient of 0.5% acetic acid in water to 0.5% acetic acid in methanol. The solvent flow rate was 0.5 mL/min. The assay was linear over a range of 5–1000 ng/mL with average r2 value of 0.9984. The precision was ≤6.9%, and the accuracy was less than or equal to ±9.4%.
Clinical and Laboratory Monitoring
Clinical and laboratory events were classified using the Division of AIDS Table for Grading the Severity of Adult and Pediatric Adverse Events, Version 1.0, dated December 2004 and Clarification dated August 2009.9 A Protocol Safety Review Team was established to review clinical and laboratory data reports through regularly scheduled conference calls and as needed. The Protocol Safety Review Team could pause protocol enrollment and dosing if 2 or more mothers or infants experienced the same grade 3 or higher adverse event assessed to be related to study drug dosing. An HPTN Study Monitoring Committee monitored the study regularly, focusing on quality of trial conduct and study safety data.
The predose concentration (Cpredose), maximum plasma concentration (Cmax), corresponding time (Tmax), minimum plasma concentration (Cmin), and final postdose sample concentration (Ctrough) were determined by direct inspection. For concentrations below the assay limit of detection, a value of one-half of the detection limit (2.5 ng/mL) was used in summary calculations. Tenofovir area under the concentration–time curve (AUC) during the dose interval (from time 0 to the final sample) was estimated using the trapezoidal rule. Apparent clearance from plasma was calculated as dose divided by AUC. The terminal slope of the curve (λz) was estimated from the terminal portion of the concentration–time curves. Half-life was calculated as 0.693 divided by λz, and apparent volume of distribution was determined by apparent clearance divided by λz. Tenofovir concentrations were analyzed by noncompartmental pharmacokinetic analysis using WinNonlin and Excel. The pharmacokinetic target was to maintain infant tenofovir concentration throughout the first week of life above 50 ng/mL, the mean trough tenofovir concentration in adults receiving chronic dosing with TDF.
One hundred twenty-two mother–infant pairs were enrolled in the study, 73 in Malawi and 49 in Brazil. The clinical characteristics of the subjects and their pregnancy outcomes are presented in Table 1.
Maternal pharmacokinetic parameters and median concentration–time plots with 600 mg (cohort 1) and 900 mg (cohort 3) TDF doses are presented in the Supplemental Digital Content,https://links.lww.com/QAI/A456. Median AUC to the last time point was 25% greater with the larger dose, but AUC with both doses exceeded the AUC0–24 in nonpregnant HIV-infected adults receiving 300 mg daily dosing (2550–3100 ng·h/mL).10 Amniotic fluid samples were collected at delivery from 24 mothers delivering by cesarean section. Median (range) time between maternal dosing and collection of amniotic fluid was 4.4 (1.2–11.4) hours. Median (range) tenofovir concentration in amniotic fluid was 248 (20–725) ng/mL compared with 147 (39–617) ng/mL in maternal plasma from these mothers at the time of delivery. Figure 1 presents tenofovir concentrations in amniotic fluid and maternal plasma at delivery and their ratio plotted against the time between maternal dosing and delivery. Breast milk samples were obtained from 25 mothers in cohorts 1 and 2. Tenofovir was detectable in 3 of 4 samples collected within 2 days of delivery, with concentrations ranging from 6.3 to 17.8 ng/mL, and in 1 of 21 samples collected 4–6 days after delivery, with a concentration of 15.7 ng/mL.
Cord Blood Concentrations
In cohorts 1 and 4 (maternal 600 mg doses), median (range) cord blood tenofovir concentration was 82 (below lower limit of quantitation–249) ng/mL and the median (range) ratio of the cord blood to maternal delivery tenofovir concentration was 0.60 (0–1.97). In cohort 3 (maternal 900 mg doses), median (range) cord blood tenofovir concentration was 122 (below lower limit of quantitation–538) ng/mL and the median (range) ratio of the cord blood to maternal delivery tenofovir concentration was 0.59 (0–3.06). Cord blood concentrations exceeded the 50 ng/mL target in 19 infants (63%) in cohort 1, 31 (86%) in cohort 3, and 24 (73%) in cohort 4. Figure 2 presents tenofovir concentrations in cord blood and maternal plasma at delivery and their ratio plotted against the time between maternal dosing and delivery.
Median infant concentration–time plots are presented in Figure 3, and infant pharmacokinetic parameters are presented in Table 2. In cohort 1 (maternal 600 mg doses, no infant doses), 1 infant exceeded 50 ng/mL at 4 hours after birth and all subsequent infant samples were below. In cohort 2 (no maternal doses, 4 mg/kg infant doses on days 0, 3, and 5), trough tenofovir concentrations after each dose exceeded 50 ng/mL in less than 10% of infants. In cohort 3 (maternal 900 mg doses, 6 mg/kg infant doses on days 0, 3, and 5), trough tenofovir concentrations exceeded 50 ng/mL in 6%–13% of infants. In contrast, in cohort 4 (maternal 600 mg doses, 6 mg/kg infant daily doses), trough tenofovir concentrations exceeded 50 ng/mL in 74%–97% of infants and 21 infants exceeded the 50 ng/mL target in all trough samples. The lowest tenofovir concentration observed in a cohort 4 infant was 31 ng/mL.
Safety and Tolerance
Tenofovir was well tolerated by study mothers and infants. Clinical and laboratory adverse events were common but were thought to be consistent with the background rate of such events in a population of HIV-infected pregnant women and their newborns from Malawi and Brazil followed for 1 year. Of the 99 mothers in cohorts 1, 3, and 4 who were exposed to tenofovir, 8 had single serious adverse events, none of which was considered to be related to tenofovir exposure. One mother died 11 weeks after tenofovir dosing from respiratory failure because of bronchopneumonia and AIDS. Thirty-three infants had 50 serious adverse events. Low serum albumin on day 2 of life in a cohort 1 infant was considered possibly related to tenofovir exposure; all other serious adverse events of infants were considered not related to tenofovir exposure. Eleven infant deaths occurred between 29 and 38 weeks after delivery, with 3 attributed to pneumonia, 3 to gastroenteritis, 2 to marasmus, 2 to sepsis, and 1 to meningitis. Five (4.1%) of the 122 infants were infected with HIV, of whom 4 were positive at birth. The other infected infant first tested positive at the 12-week visit and was breast-feeding. ARTs for PMTCT were limited to intrapartum single-dose nevirapine at delivery for 4 of the mothers of infected infants, whereas the mother of the fifth infected infant received 1 week of zidovudine before delivery and single-dose nevirapine at delivery.
HIV resistance genotyping was performed for mothers who received TDF in cohorts 1 and 3 and HIV-infected infants in cohorts 1–3 using plasma samples collected 6 weeks after delivery. Samples were available from 62 (93.9%) of the 66 mothers in cohorts 1 and 3; 16 samples were not analyzed because of low viral load (<400 copies/mL HIV RNA). Genotyping results were obtained for 35 (76.1%) of the remaining 46 samples (19 from cohort 1 and 16 from cohort 3). Genotyping results were also obtained from 3 HIV-infected infants. The K65R tenofovir resistance mutation was not detected in any of the maternal or infant samples.
Tenofovir, administered as the oral prodrug TDF, is a potent nucleotide analogue that has been approved for treatment of HIV infection in adults and children and has been successfully used for prevention of HIV transmission in high-risk adults.6,11,12 Adult studies of oral TDF for both treatment and prophylaxis have used 300 mg daily doses, which result in mean peak concentrations of approximately 300 ng/mL, mean trough concentrations of 50–60 ng/mL, and mean elimination half-life of 14–16 hours.13,14 Use of TDF by women in labor and their neonates has been proposed for PMTCT of HIV and early treatment of HIV-infected neonates.15
Two previous studies, Pediatric AIDS Clinical Trials Group (PACTG) 394 and Agence Nationale de Recherche sur le Sida (ANRS) 12109, have assessed tenofovir exposure with single TDF doses administered to mothers during labor and their infants after birth.7,8,16 Both found that 600 mg doses administered during labor resulted in maternal tenofovir concentrations equivalent to those seen in nonpregnant adults receiving standard 300 mg doses. Median cord blood concentrations were 76 and 100 ng/mL in these studies, and most infants had cord blood tenofovir concentrations exceeding 50 ng/mL. The most important determinant of cord blood tenofovir concentration was the time interval between maternal tenofovir dosing and delivery. Our study confirms that tenofovir doses of 600 mg administered during labor result in maternal exposure similar to that with standard 300-mg doses in nonpregnant adults and cord blood tenofovir concentrations above 50 ng/mL in most newborns. Consistent with the ANRS study, we found that the cord blood to maternal tenofovir concentration ratio increases during the first 4 hours after intrapartum administration and is stable at approximately 60%–70% thereafter.7
Our study is the first to look at amniotic fluid concentrations of tenofovir. Although we were limited by being able to collect samples only from mothers undergoing cesarean section, our data clearly show that tenofovir accumulates in amniotic fluid. The amniotic fluid to maternal tenofovir concentration ratio continues to increase beyond 4 hours after maternal dosing with amniotic fluid concentrations exceeding maternal plasma concentrations by several folds. Tenofovir is excreted predominantly via the kidney as unmetabolized drug.10 Once tenofovir crosses the placenta and enters the fetal circulation, it may be excreted by the fetal kidneys into the amniotic fluid or transported back across the placenta to the maternal circulation. Tenofovir may be reabsorbed by the fetus from swallowed amniotic fluid, although the extent of gastrointestinal absorption of tenofovir by the fetus, whose gastrointestinal track is characterized by neutral gastric pH and a slow transit time, is not known. In adults, unconjugated tenofovir exhibits very limited oral bioavailability, requiring administration conjugated to disoproxil fumarate as a prodrug to achieve adequate oral bioavailability.10
Renal function is low immediately after birth, and both glomerular filtration and tubular secretion increase dramatically over the first weeks and months of life, affecting excretion of renally excreted drugs.17 When designing this study, our expectation was that tenofovir elimination would be prolonged immediately after birth and increase over the subsequent weeks and months of life, as has been demonstrated in the rhesus macaque.18 However, both previous studies and our study demonstrate that the half-life of washout elimination of transplacentally acquired tenofovir is not prolonged compared with that in adults. Renal elimination of tenofovir occurs through a combination of glomerular filtration and tubular secretion. The tubular concentration of tenofovir will be determined by the balance between tenofovir secretion into the renal tubule by organic acid transporters (1 and 3) and efflux out of the tubule by the transporter multidrug resistance protein 4.19 The unexpectedly rapid elimination of tenofovir by the neonate suggests that immediately after birth, human newborns may demonstrate a balance between renal tubular organic acid transporter secretory influx activity and multidrug resistance protein 4 efflux activity similar to that in adults.
The previous studies of tenofovir pharmacokinetics in neonates include cohorts that received single doses of tenofovir shortly after birth. In the PACTG 394 study, a 4 mg/kg dose, half of the normal infant and child 8 mg/kg dose, resulted in a median peak concentration of 101 ng/mL and by 24 hours after dosing most subjects had tenofovir concentrations below 50 ng/mL.8 The authors suggest that multiple or higher doses of tenofovir would be needed to maintain concentrations effective for viral suppression.8 The ANRS 12109 study used a 13 mg/kg dose derived from simulations using a population model from an initial maternal dosing–only cohort.7 In this cohort, tenofovir was administered only to mothers in labor, followed by collection of sparse blood samples from mothers and infants. The data were analyzed using a population pharmacokinetic approach, and the resulting model was used in simulations to determine optimal size and timing of a neonatal dose. No tenofovir doses were administered directly to the infant in this cohort; so, neonatal bioavailability, absorption rate, and volume of distribution could not be estimated. The model incorporated the assumptions that neonates have the same bioavailability and absorption rate as their mothers and that neonatal volume of distribution is proportional to that of their mothers, scaled by the ratio of neonatal to maternal weight. The simulations resulted in a suggested infant dose of 13 mg/kg, which was then investigated in a second cohort of mothers and infants. In these infants, the median peak tenofovir concentration was 290 ng/mL and the median tenofovir concentration 24–36 hours after the dose was 76 ng/mL.16
Our study is the first to investigate administration of multiple TDF doses during the first week after birth. Our initial cohorts enrolled concurrently with the PACTG 394 study, and our first infant dosing group (cohort 2) used the same 4 mg/kg dose. We initially hoped that 3 doses administered on days 0, 3, and 5 after birth would be sufficient to maintain plasma tenofovir concentrations above 50 mg/mL, and the initial protocol included a dose escalation to 6 mg/kg. However, trough tenofovir concentrations fell below the 50 ng/mL target in approximately 90% of infants receiving tenofovir doses of either 4 or 6 mg/kg on days 0, 3, and 5 after birth. We then added a fourth cohort with daily 6 mg/kg dosing for 1 week, which achieved median tenofovir Cmax from 206 to 350 ng/mL and trough plasma tenofovir concentrations above 50 ng/mL in nearly all infants. Based on these data, we recommend daily dosing of 6 mg/kg TDF oral suspension for HIV treatment or prophylaxis of a neonate during the first week of life.
We suspect that the explanation for the discrepancy in our recommended dose size of 6 mg/kg and the ANRS 12109 recommended dose of 13 mg/kg lies with limitations of their initial models, which used values for neonatal bioavailability, absorption, and volume of distribution extrapolated from maternal parameters rather than estimated from infant dosing data. The similarity in infant tenofovir exposure after administration of the 13 mg/kg single dose in the ANRS 12109 study and the first 6 mg/kg dose administered in cohorts 3 and 4 of our study is most likely explained by the limitations in tenofovir bioavailability in neonates. The bioavailability of tenofovir in TDF is only 39% when given with food and 25% when administered in the fasted state.10 In the absence of an intravenous formulation to compare with the oral formulation, we cannot directly measure tenofovir bioavailability in neonates, but the lack of an increase in tenofovir exposure with a TDF dose over twice as large suggests saturable gut absorption kinetics. Both studies administered TDF as an oral solution; so, formulation differences do not seem to have contributed to the disparate results.
Our study has several limitations. Our subjects came from Brazil and Malawi, and it is possible that tenofovir pharmacokinetics may be different in neonates from other populations. Although we sampled more intensively than in previous studies, our infant sampling schedule was limited compared with those used in adult studies and focused on tenofovir elimination. Tenofovir is metabolized intracellularly to its active form, tenofovir diphosphate, and we did not measure intracellular diphosphate concentrations. The ANRS 12109 study measured intracellular tenofovir diphosphate concentrations once per subject between 10 and 45 hours after dosing and found that most infants had intracellular diphosphate concentrations equivalent to those seen in adults receiving chronic tenofovir dosing.16 Their data were insufficient to characterize the time course of neonatal tenofovir diphosphate elimination. More research is needed to describe neonatal accumulation and elimination of the intracellular phosphate moieties of tenofovir and also nucleoside reverse transcriptase inhibitors, such as zidovudine and lamivudine, although such studies are made difficult by the large sample volumes needed to determine intracellular phosphate concentrations. Our data are also limited by the narrow age range of our subjects. Tenofovir is an approved treatment of HIV-infected children over 2 years of age at a dose of 8 mg/kg once daily. Although our study strongly supports the use of a 6 mg/kg daily dose during the first week of life, it provides no information on when that dose can be increased to the usual 8 mg/kg pediatric dose. In addition, the infant TDF formulation used in this study is no longer commercially available, having been replaced by a TDF powder formulation intended for mixing with soft foods. The bioavailability of this formulation when used in newborns must be investigated before it can be used in research studies or clinical care.
In conclusion, based on these data, we recommend use of TDF doses of 600 mg in pregnant women during labor and daily dosing of 6 mg/kg in neonates during the first week of life. Larger studies incorporating these dosing guidelines should be conducted to delineate the safety and efficacy of tenofovir for PMTCT and for early treatment of neonatal HIV infection.
The authors thank the mothers and their infants who participated in the study; the HPTN 057 study coordinators, counselors, clinicians, pharmacists, and data quality and laboratory staff; and Jim Rooney, Yvonne Bryson, Edmund Capparelli, George Siberry, Elizabeth Brown, Scharla Estep, and Melissa Allen for their help with this study. The authors also thank Gilead Sciences, Inc, for providing the TDF used in this study.
1. Townsend CL, Cortina-Borja M, Peckham CS, et al.. Low rates of mother-to-child transmission of HIV following effective pregnancy interventions in the United Kingdom and Ireland, 2000-2006. AIDS. 2008;22:973–981.
2. Van Damme L, Corneli A, Ahmed K, et al.. Preexposure prophylaxis for HIV infection among African women. N Engl J Med. 2012;367:411–422.
3. Baeten JM, Donnell D, Ndase P, et al.. Antiretroviral prophylaxis for HIV prevention in heterosexual men and women. N Engl J Med. 2012;367:399–410.
4. Van Rompay KK, Berardi CJ, Aguirre NL, et al.. Two doses of PMPA protect newborn macaques against oral simian immunodeficiency virus infection. AIDS. 1998;12:F79–F83.
5. Van Rompay KK, Marthas ML, Lifson JD, et al.. Administration of 9-[2-(phosphonomethoxy)propyl]adenine (PMPA) for prevention of perinatal simian immunodeficiency virus infection in rhesus macaques. AIDS Res Hum Retroviruses. 1998;14:761–773.
6. Tenofovir Prescribing Information. Foster City, CA: Gilead Sciences, Inc; 2012.
7. Hirt D, Urien S, Ekouevi DK, et al.. Population pharmacokinetics
of tenofovir in HIV-1-infected pregnant women and their neonates (ANRS 12109). Clin Pharmacol Ther. 2009;85:182–189.
8. Flynn PM, Mirochnick M, Shapiro DE, et al.. Pharmacokinetics
and safety of single-dose tenofovir disoproxil fumarate and emtricitabine in HIV-1-infected pregnant women and their infants. Antimicrob Agents Chemother. 2011;55:5914–5922.
9. National Institutes of Health. Division of AIDS (DAIDS) revised toxicity tables for grading severity of pediatric adverse experiences. US National Institutes of Health DAIDS HIV Vaccine and Research Program, version 1.0. Washington, DC. 2004. Available at: http://rcc.tech-res-intl.com
. Accessed April 02, 2012.
10. Kearney BP, Flaherty JF, Shah J. Tenofovir disoproxil fumarate: clinical pharmacology and pharmacokinetics
. Clin Pharmacokinet. 2004;43:595–612.
11. Grant RM, Lama JR, Anderson PL, et al.. Preexposure chemoprophylaxis for HIV prevention in men who have sex with men. N Engl J Med. 2010;363:2587–2599.
12. Abdool Karim Q, Abdool Karim SS, Frohlich JA, et al.. Effectiveness and safety of tenofovir gel, an antiretroviral microbicide, for the prevention of HIV infection in women. Science. 2010;329:1168–1174.
13. Blum MR, Chittick GE, Begley JA, et al.. Steady-state pharmacokinetics
of emtricitabine and tenofovir disoproxil fumarate administered alone and in combination in healthy volunteers. J Clin Pharmacol. 2007;47:751–759.
14. Ramanathan S, Shen G, Cheng A, et al.. Pharmacokinetics
of emtricitabine, tenofovir, and GS-9137 following coadministration of emtricitabine/tenofovir disoproxil fumarate and ritonavir-boosted GS-9137. J Acquir Immune Defic Syndr. 2007;45:274–279.
15. Foster C, Lyall H, Olmscheid B, et al.. Tenofovir disoproxil fumarate in pregnancy and prevention of mother-to-child transmission of HIV-1: is it time to move on from zidovudine? HIV Med. 2009;10:397–406.
16. Hirt D, Ekouevi DK, Pruvost A, et al.. Plasma and intracellular tenofovir pharmacokinetics
in the neonate (ANRS 12109 trial, step 2). Antimicrob Agents Chemother. 2011;55:2961–2967.
17. Kearns GL, Abdel-Rahman SM, Alander SW, et al.. Developmental pharmacology—drug disposition, action, and therapy in infants and children. N Engl J Med. 2003;349:1157–1167.
18. Van Rompay KK, Durand-Gasselin L, Brignolo LL, et al.. Chronic administration of tenofovir to rhesus macaques from infancy through adulthood and pregnancy: summary of pharmacokinetics
and biological and virological effects. Antimicrob Agents Chemother. 2008;52:3144–3160.
19. Ray AS, Cihlar T, Robinson KL, et al.. Mechanism of active renal tubular efflux of tenofovir. Antimicrob Agents Chemother. 2006;50:3297–3304.