Only one report describing the excretion of AZT in breast milk has been located (85). Six HIV-seropositive women were given a single 200-mg dose of AZT, and serum and breast milk samples were collected 1, 2, 4, and 6 hours later. Peak serum and milk concentrations, ranging between 422.1 and 1019.3 ng/mL and 472.1 and 1043.0 ng/mL, respectively, were measured at approximately 1-2 hours after the dose. The milk:serum ratio (based on AUC) ranged between 1.11 and 1.78. The authors speculated that the milk concentrations were sufficiently high to decrease the viral load in milk, thereby reducing the potential for maternal-infant HIV transmission (85).

Breastfeeding is not recommended in women with HIV infection because HIV-1 is transmitted in milk (80,81,83,86, 87, 88). Until 1999, no studies had been published that examined the effect of any antiretroviral therapy on HIV-1 transmission in milk. In that year, a double-blind placebo-controlled trial investigated the effect of oral AZT in the postpartum period on the transmission of HIV-1 to the breastfeeding infant (56). Infants (at 6 months of age) of women who had received an oral AZT regimen of 300 mg twice daily until labor, 600 mg at beginning of labor, then 300 mg twice daily for 7 days postpartum, had a 38% reduction (relative efficacy 0.38, 95% CI 0.05-0.60; p = 0.027) in vertical transmission of HIV-1 infection despite breastfeeding when compared with controls. Furthermore, no excess of major adverse biological or clinical events was observed in the AZT group compared with controls (56).

The thymidine analog zidovudine is a NRTI that is used for the treatment of HIV infection. Other drugs in this class are abacavir, didanosine, lamivudine, stavudine, and zalcitabine.

Animal Data: Zidovudine has been tested in a number of pregnant animals, including mice, rats, rabbits, and pigtailed macaques (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13). Depending on the dose, maternal and fetal toxicity effects were observed but not structural defects.

Placental Transfer: AZT crosses the placenta to the fetus, both in animals (14, 15, 16) and in humans (1,17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27). Placental transfer of the drug is rapid and is by simple diffusion (17, 18, 19). Seven pregnant HIV-seropositive women with gestational lengths between 14 and 26 weeks were scheduled for therapeutic abortions (20). The women were given AZT, 200 mg orally every 4 hours for five doses, 1-2.75 hours before pregnancy termination. Fetal blood concentrations of the parent compound and its inactive glucuronide metabolite ranged from 100 to 287 ng/mL and from 346 to 963.5 ng/mL, respectively. In six patients (one woman had blood levels below the level of detection), mean AZT concentrations in the maternal blood, amniotic fluid, and fetal blood were 143, 168, and 205 ng/mL, respectively.

In a 1990 study, the pregnancy of an HIV-seropositive woman was terminated at 13 weeks (21). She had been taking AZT, 100 mg 4 times daily, for 6 weeks, with her last dose consumed approximately 4 hours before the abortion. Both AZT and the glucuronide metabolite were found in the amniotic fluid and various fetal tissues. The lower limit of detection for the assay was 0.01 μmol/L (0.01 nmol/g). The concentrations of the parent compound and the metabolite in maternal plasma were 0.35 and 0.90 μmol/L, respectively. In comparison, the fetal concentrations of AZT (corresponding levels of the metabolite are shown in parentheses) were amniotic fluid, 0.31 μmol/L (1.16 μmol/L); liver, 0.14 nmol/g (0.16 nmol/g); muscle, 0.26 nmol/g (0.50 nmol/g); and CNS, 0.01 nmol/g (0.05 nmol/g). The low levels of AZT in the latter system probably indicate that transplacental passage of the drug, at this dose, may be insufficient to treat HIV infection of the fetal CNS (21). The significance of this result is increased by the finding that neurologic and neuropsychological morbidity in infants exposed to HIV in utero is high (21,28).

A 1989 report described the treatment of a 30-year-old HIV-seropositive woman who was treated before and throughout gestation with AZT, 1200 mg/day (22). IV AZT, 0.12 mg/kg/h, was infused 24 hours before labor induction at 39 weeks. An uncomplicated vaginal delivery occurred resulting in the birth of a normal male infant. The newborn's renal and hepatic functions were normal, and no other toxicity, such as anemia or macrocytosis, was noted. At birth, concentrations of AZT in the maternal blood, amniotic fluid, and cord blood were 0.28, 3.82, and 0.47 mcg/mL, respectively. AZT concentrations in the infant's blood at 6, 24, 36, and 48 hours were 0.46, 0.51, 0.44, and 0.27 mcg/mL, respectively, indicating that in the first 24 hours, elimination of the drug from the newborn was negligible (22). Levels of the inactive metabolite were also determined concurrently; in each sample, the metabolite concentration was higher than that of the parent compound. The infant was doing well and growing normally at 6 months of age.

Two HIV-positive women at 18 and 21 weeks were treated with AZT (1000 mg/day) for 3 days before elective abortion (23). The final 200-mg dose was consumed 2-3 hours before abortion. Both AZT and its metabolite were found in the two women and in amniotic fluid and fetal blood, with fetal:maternal ratios for AZT of 1.10 and 6.00, respectively, and for the metabolite of 0.84 and 3.75, respectively.

Three experimental in vitro models using perfused human placentas to predict the placental transfer of NRTIs (didanosine, stavudine, zalcitabine, and AZT) were described in a 1999 publication (29). For each drug, the predicted fetal:maternal plasma drug concentration ratios at steady state with each of the three models were close to those actually observed in pregnant macaques. Based on these results, the authors concluded that their models would accurately predict the mechanism, relative rate, and extent of in vivo human placental transfer of NRTIs (29).

Human Data: The pharmacokinetics of AZT during human and nonhuman primate pregnancies have been determined in a number of studies (24,26,27,30, 31, 32, 33). AZT was administered to seven HIV-positive women beginning at 28-35 weeks with a 200-mg IV dose on day 1, followed by 200 mg orally 5 times a day from day 2 until labor (24). The mean maternal plasma concentrations of AZT and its inactive glucuronide metabolite at delivery were 0.29 and 0.87 mcg/mL, respectively. Similar amounts were measured in umbilical cord venous blood, with mean concentrations of 0.28 and 1.01 mcg/mL, respectively, suggesting that the fetuses and newborns were unable to metabolize AZT (24). No significant drug-induced adverse effects were observed in the women or their newborns, and no congenital malformations were noted. Fetal growth was normal in six and accelerated in one, and the slightly lower than normal hemoglobin values were not considered clinically significant (24).

In pregnant macaques or humans, the pharmacokinetics of AZT were not affected by pregnancy (macaques) (30), nor did AZT affect the transplacental pharmacokinetics of didanosine (macaques) (31), lamivudine (humans) (32), or stavudine (macaques) (33). Two other studies involving four women infected with HIV measured peak AZT maternal serum levels and elimination half-lives that were statistically similar to those of nonpregnant adults (26,27). However, in three women, the AUC during pregnancy was significantly less than after pregnancy (4.5 μmol/L vs. 6.8 μmol/L), and the apparent total body clearance was significantly greater (2.5 L/h/kg vs. 1.7 L/h/kg) (26). Moreover, the difference in the apparent volume of distribution during and after pregnancy reached near significance (3.9 L/kg vs. 2.6 L/kg) (26).

The effect of AZT on the transplacental passage of HIV is unknown. A 1990 review of AZT in pregnancy focused on the issue of whether the drug prevented HIV passage to the fetus and concluded that available data were insufficient to provide an answer to this question (22). However, a 1993 study observed that, although AZT is transferred to the fetus relatively intact, the approximately 50% retained by the placenta is extensively metabolized, with one of the metabolites being zidovudine triphosphate, the product responsible for the antiviral activity of the parent drug (34). The effect of this may be a reduction in the risk of HIV transmission to the fetus (34).

Brief descriptions of the treatment and pregnancy outcome of 12 HIV-seropositive women were provided in a 1991 abstract (35). AZT therapy was started before conception in 4 women and between 21 and 34 weeks (mean 25 weeks) gestation in 8. The mean duration of therapy was 8 weeks (range 1-24 weeks). Three of the women who had conceived while taking AZT underwent therapeutic abortions of grossly normal fetuses between 10 and 12 weeks. Five women had delivered grossly normal infants with a mean birth weight of 2900 g, and the infants of the remaining four women were undelivered between 24 and 36 weeks. Three other abstracts that appeared in 1992 and 1993 described 46 pregnant women treated with AZT without producing toxic effects or anomalies in their fetuses (36, 37, 38).

In a surveillance study of Michigan Medicaid recipients involving 229,101 completed pregnancies conducted between 1985 and 1992, 2 newborns had been exposed to AZT during the 1st trimester (F. Rosa, personal communication, FDA, 1993). No major birth defects were observed.

Data on 45 newborns (2 sets of twins) of 43 pregnant women, enrolled in studies conducted by 17 institutions participating in AIDS Clinical Trials Units and who had been treated with AZT, were described in 1992 (39). AZT dosage ranged from 300 to 1200 mg/day, with 24 of the women taking the drug during at least two trimesters. All the infants were born alive. No congenital abnormalities were observed in 12 infants who had been exposed in utero to AZT during the 1st trimester, although 1 newborn with elevated 17α-hydroxyprogesterone levels had clitoral enlargement. Normal levels of the hormone were measured in this infant at 4 months of age. Two term infants were growth restricted, but 38 other singleton term infants had a mean birth weight of 3287 g. Seven infants had hemoglobin values <13.5 g/dL; three of these were delivered prematurely. The authors concluded that the few cases of anemia and growth restriction might have been, at least partially, caused by maternal AZT therapy (39). Another report involving 29 pregnant patients treated with AZT at government-sponsored AIDS clinical trial centers appeared in 1992, but no data on the outcome of these pregnancies were given (40).

The outcomes of 104 pregnancies in which AZT was used were described in a 1994 report (41). Sixteen of the pregnancies are terminated during the 1st trimester—8 spontaneous and 8 elective abortions. Among the remaining 88 cases, sss8 infants had birth defects, 4 after 1st trimester exposure and 4 after exposure during the 2nd or 3rd trimesters. None of the defects could be attributed to AZT exposure (41): multiple minor anomalies (low-set ears, retrognathia, hirsutism, triangular face, blue sclera, prominent sacral dimple); multiple minor anomalies (type not specified); extra digits on both hands, hare lip (central), and cleft palate; fetal alcohol syndrome; atrial septal defect (asymptomatic) with pectus excavatum; microcephaly, chorioretinitis (Toxoplasma gondii infection); pectus excavatum; and albinism with congenital ptosis, growth restriction, and oligohydramnios (indicates chromosomal analysis normal) (41).

The Antiretroviral Pregnancy Registry reported, for the period January 1989 through July 2009, prospective data (reported before the outcomes were known) involving 4702 live births that had been exposed during the 1st trimester to one or more antiretroviral agents (42). Congenital defects were noted in 134, a prevalence of 2.8% (95% confidence interval [CI] 2.4-3.4). In the 6100 live births with earliest exposure in the 2nd/3rd trimesters, there were 153 infants with defects (2.5%, 95% CI 2.1-2.9). The prevalence rates for the two periods did not differ significantly. There were 288 infants with birth defects among 10,803 live births with exposure anytime during pregnancy (2.7%, 95% CI 2.4-3.0). The prevalence rate did not differ significantly from the rate expected in a nonexposed population. There were 9735 outcomes exposed to AZT (3167 in the 1st trimester and 6568 in the 2nd/3rd trimesters) in combination with other antiretroviral agents. There were 264 birth defects (97 in the 1st trimester and 167 in the 2nd/3rd trimesters). In reviewing the birth defects of prospective and retrospective (pregnancies reported after the outcomes were known) registered cases, the Registry concluded that, except for isolated cases of NTDs with efavirenz exposure in retrospective reports, there was no other pattern of anomalies (isolated or syndromic) (42) (see Lamivudine for required statement).

The failure of maternal AZT therapy to prevent the transmission of HIV-1 infection to one of the mother's female twins was described in a 1990 report (43). The woman was treated with AZT, 400 mg/day, from 18 weeks until delivery. A cesarean section was performed at 27 weeks because of premature labor that was not responsive to tocolytic therapy. Twin A was diagnosed with culture-proven HIV-1 and cytomegalovirus infection. At the time of the report, the child was 9 months old with limited sight, severe failure to thrive, and encephalopathy. An HIV-1 culture in twin B was negative, but analysis for HIV-1 antigens continued to be positive through 20 weeks of age. A number of possible explanations were proposed by the authors concerning the failure of AZT to protect twin A from infection. Included among these were passage of the virus before the onset of treatment at 18 weeks, intrauterine transfer of the virus via an amniocentesis performed at 14 weeks, viral resistance to the drug, low fetal tissue drug levels, maternal noncompliance (doubtful), and acquisition of the virus during birth (43).

A 1992 review on the treatment of HIV-infected pregnant women stated that most obstetric experts offered AZT therapy in cases of AIDS, AIDS-related complex, or when the CD4+ cell counts were below 200 cells/μL (44). Although no fetal toxicity secondary to AZT had been reported, the author recommended caution with 1st trimester use of the agent and noted the potential for fetal bone marrow depression and resulting anemia.

A clinical trial, the subject of several reviews and editorials (45, 46, 47, 48, 49, 50, 51), conducted from 1991 to 1993 and published in 1994, found that the risk of maternal-infant transmission of HIV disease could be decreased by 67.5% by treatment of pregnant women (who had mildly symptomatic HIV disease) with AZT (52). The randomized, double-blind, placebo-controlled trial enrolled untreated HIV-infected pregnant women, at 14-34 weeks, which had CD4+ T-lymphocyte counts above 200 cells/mm³ and no clinical indications for antenatal antiretroviral therapy. The maternal AZT treatment regimen consisted of antepartum oral therapy (100 mg orally 5 times daily) and intrapartum IV dosing (2 mg/kg for 1 hour, then 1 mg/kg/h until delivery). The newborns were treated with oral AZT (2 mg/kg every 6 hours) for 6 weeks. A total of 477 women were enrolled, 409 of whom delivered 415 live-born infants during the study period. Among those with known HIV-infection status were 180 infants from the AZT-treated group and 183 placebo-treated controls. The authors of this study used statistical methods to predict the number of infants who would be HIV-infected at 18 months of age, thus allowing a faster analysis of their data. They estimated that the number of HIV-infected children would be 8.3% (95% CI 3.9%-12.8%) in the AZT group and 25.5% (95% CI 18.4%-32.5%) in the placebo group (52). This was a 67.5% (95% CI 40.7%-82.1%) reduction in the risk of HIV transmission (p = 0.00006). No differences in growth, prematurity, or the number and patterns of major or minor congenital abnormalities were observed between the two groups. Thirty-three live-born infants had congenital defects, 17 of 206 (8.3%) in the treatment group and 16 of 209 (7.7%) in the nontreated controls. Cardiac malformations were observed in 10 infants (5 in each group), CNS defects in 5 (3 in the AZT group, 2 in controls), and 9 unspecified defects in each group. The total incidence of congenital malformations is higher than expected in the general population, but this probably reflects the population studied. The only drug-related adverse effect observed in the newborns was a decrease in hemoglobin concentration in those exposed to AZT in utero. The maximum difference in hemoglobin concentration between the groups, 1 g/dL, occurred at 3 weeks of age, but by 12 weeks of age, the hemoglobin values were similar (52).

Although AZT appeared to be effective in reducing transmission of HIV-1 to the fetus, some infants became infected despite treatment. Possible reasons proposed for these failures included (a) virus transmission before treatment began, (b) ineffective suppression of maternal viral replication, (c) poor maternal compliance with the drug regimen, and (d) virus resistance to AZT (52).

A series of studies and editorials appeared in 1998-2000 that evaluated or discussed the effect of short courses of AZT on the perinatal transmission of HIV in various populations (53, 54, 55, 56, 57, 58, 59). The most effective therapy, however, involved starting treatment of the mother at 28 weeks, with 6 weeks of treatment in the infant (58,59).

A 2000 review described seven clinical trials that have been effective in reducing perinatal transmission, five with AZT alone, one with AZT plus lamivudine, and one with nevirapine (60). Six of the trials were in less-developed countries. Prolonged use of AZT in the mother and infant was the most effective for preventing vertical transmission of HIV, but also was the most expensive. The combination of AZT and lamivudine, consisting of antepartum, intrapartum, and postpartum maternal therapy with continued therapy in the infant for 1 week, may have been as effective as prolonged AZT (60).

A study published in 1999 evaluated the safety, efficacy, and perinatal transmission rates of HIV in 30 pregnant women receiving various combinations of antiretroviral agents (61). Many of the women were substance abusers. AZT was used by 26 women in various combinations that included didanosine, lamivudine, indinavir, nelfinavir, nevirapine, saquinavir, and delavirdine. Antiretroviral therapy was initiated at a median of 14 weeks (range preconception to 32 weeks). Despite previous histories of extensive antiretroviral experience and of vertical transmission of HIV, combination therapy was effective in treating maternal disease and in preventing transmission to the current newborns. The outcomes of the pregnancies included one stillbirth, one case of microcephaly, and five infants with birth weights <2500 g, two of which were premature (61).

In another 1999 study, the safety and efficacy of a short course of nevirapine was compared with AZT for the prevention of perinatal transmission of HIV-1 (62). At the onset of labor, women were randomly assigned to receive either a single dose of nevirapine (200 mg) plus a single dose (2 mg/kg) to their infants within 72 hours of birth (N = 310) or AZT (600 mg then 300 mg every 3 hours until delivery) plus 4 mg/kg twice daily for 7 days to their infants (N = 308). Nearly all (98.8%) of the women breastfed their infants immediately after birth. Up to age 14-16 weeks, significantly fewer infants in the nevirapine group were HIV-1-infected, lowering the risk of infection or death, compared with AZT, by 48% (95% CI 24%-65%) (62). The prevalence of maternal and infant adverse effects was similar in the two groups. In an accompanying study, the nevirapine regimen was shown to be cost-effective in various seroprevalence settings (63).

In an unusual case, a woman was exposed to HIV through self-insemination with fresh semen obtained from a man with a high HIV ribonucleic acid viral load (>750,000 copies/mL plasma) (64). Ten days later, she was started on a prophylactic regimen of AZT (600 mg/day), lamivudine (300 mg/day), and indinavir (2400 mg/day). Pregnancy was confirmed 14 days after insemination. The indinavir dose was reduced to 1800 mg/day 4 weeks after the start of therapy because of the development of renal calculi. All antiretroviral therapy was stopped after 9 weeks because of negative tests for HIV. She gave birth at 40 weeks to a healthy 3490-g male infant, without evidence of HIV disease, who was developing normally at 2 years of age (64).

A case of a life-threatening anemia following in utero exposure to antiretroviral agents was described in 1998 (65). A 30-year-old woman with HIV infection was treated with AZT, didanosine, and trimethoprim-sulfamethoxazole (3 times weekly) during the 1st trimester. Vitamin supplementation was also given. Because of an inadequate response, didanosine was discontinued and lamivudine and zalcitabine were started in the 3rd trimester. Two weeks before delivery, the HIV viral load was undetectable. At term, a pale, male infant was delivered who developed respiratory distress shortly after birth. Examination revealed a hyperactive precordium and hepatomegaly without evidence of hydrops. The hematocrit was 11% with a reticulocyte count of zero. An extensive workup of the mother and infant failed to determine the cause of the anemia. Bacterial and viral infections, including HIV, parvovirus B19, cytomegalovirus, and others, were excluded. The infant received a transfusion and was apparently doing well at 10 weeks of age. Because no other cause of the anemia could be found, the authors attributed the condition to bone marrow suppression, most likely to AZT. A contribution of the other agents to the condition, however, could not be excluded (65).

A study of the inhibitory effects of AZT on hematopoiesis was published in 1996 (66). The researchers compared the effect of increasing concentrations of AZT on hematopoietic progenitors from women of childbearing age (from bone marrow aspirates), mid-trimester-aborted fetuses (from bone marrow and liver), and term newborns (from cord blood). The inhibitory effect of AZT was more pronounced on fetal and neonatal erythroid progenitors than those from the bone marrow of the women. AZT had no effect on granulocyte colony-stimulating factor or erythropoietin. The authors concluded that neonatal anemia after in utero exposure to AZT was due to reduced clonal maturation of erythroid progenitors (66).

A 1999 report from France described the possible association of AZT and lamivudine (NRTIs) use in pregnancy with mitochondrial dysfunction in the offspring (67). Mitochondrial disease is relatively rare in France (estimated prevalence is 1 in 5000-20,000 children). From an ongoing epidemiological survey of 1754 mother-child pairs exposed to AZT and other agents during pregnancy, however, eight children with possible mitochondrial dysfunction were identified. None of the eight infants were infected with HIV, but all received prophylaxis for up to 6 weeks after birth with the same antiretroviral regimen as given during pregnancy. Four of the cases were exposed to AZT alone and four to a combination of AZT and lamivudine. Two from the combination group died at about 1 year of age. All eight cases had abnormally low respiratory-chain enzyme activities. The authors concluded that their results supported the hypothesis of a causative association between mitochondrial respiratory-chain dysfunction and NRTIs. Moreover, the toxicity may have been potentiated by combination of these agents (67).

In a paper following the above study, investigators noted that NRTIs inhibit DNA polymerase-γ, the enzyme responsible for mitochondrial DNA replication (68). They then hypothesized that this inhibition would induce depletion of mitochondrial DNA and mitochondrial DNA-encoded mitochondrial enzymes, thus resulting in mitochondrial dysfunction (69). Moreover, they stated that support for their hypothesis was suggested by the closeness of the clinical manifestations of inherited mitochondrial diseases with the adverse effects attributed to NRTIs. These adverse effects included polyneuropathy, myopathy, cardiomyopathy, pancreatitis, bone marrow suppression, and lactic acidosis. They also postulated that this mechanism was involved in the development of a lipodystrophy syndrome of peripheral fat wasting and central adiposity, a condition that has been thought to be related to protease inhibitors (68).

A commentary on the above two studies concluded that the evidence for NRTI-induced mitochondrial dysfunction was equivocal (69). First, the clinical presentations in the infants were varied and not suggestive of a single cause; indeed, three of the infants were symptom-free and one had Leigh's syndrome, a classic mitochondrial disease (69). Second, the clinical features, in some cases, were not suggestive of mitochondrial dysfunction. Although three had neurologic symptoms, none had raised levels of lactate in the cerebrospinal fluid. Moreover, histologic or histochemical features of mitochondrial disease were only found in two cases. Finally, low mitochondrial DNA, which would have been direct evidence of NRTI toxicity, was not found in the three cases in which it was measured (69).

A case of combined transient mitochondrial and peroxisomal β-oxidation dysfunction after exposure to NRTIs (AZT and lamivudine) combined with protease inhibitors (ritonavir and saquinavir) throughout gestation was reported in 2000 (70). A male infant was delivered at 38 weeks. He received postnatal prophylaxis with AZT and lamivudine for 4 weeks until the drugs were discontinued because of anemia. Other adverse effects that were observed in the infant (age at onset) were hypocalcemia (shortly after birth); group B streptococcal sepsis, ventricular extrasystoles, prolonged metabolic acidosis, and lactic acidemia (8 weeks); a mild elevation of long-chain fatty acids (9 weeks); and neutropenia (3 months). The metabolic acidosis required treatment until 7 months of age, whereas the elevated plasma lactate resolved over 4 weeks. Cerebrospinal fluid lactate was not determined nor was a muscle biopsy conducted. Both the neutropenia and the cardiac dysfunction had resolved by 1 year of age. The elevated plasma fatty acid level was confirmed in cultured fibroblasts, but other peroxisomal functions (plasmalogen biosynthesis and catalase staining) were normal. Although mitochondrial dysfunction has been linked to NRTI agents, the authors were unable to identify the cause of the combined abnormalities in this case. The child was reported to be healthy and developing normally at 26 months of age (70).

A study involving Erythrocebus patas monkeys exposed in utero to AZT was thought to be relevant to the reports of mitochondrial dysfunction in humans (71). Ten pregnant monkeys in the last half of gestation were given daily oral doses of AZT (1.5 mg/kg/day [N = 3] or 6 mg/kg/day [N = 3]) or no AZT (controls [N = 4]). The doses of AZT were 21% and 86% of the human dose, respectively, based on body weight for a 70-kg pregnant woman. All fetuses were delivered by cesarean section 24 hours after the last AZT dose and 3-5 days before term. No gross defects were evident in the heart left ventricle tissue or skeletal muscle tissue from AZT-exposed or control fetuses. Mitochondria observed by electron microscopy in the two tissue sites were similar in the AZT low-dose and control groups. In contrast, numerous abnormalities were observed in both heart and muscle tissue mitochondria from fetuses exposed to the AZT high-dose group. Moreover, there were dose-dependent alterations in oxidative phosphorylation enzyme assays and in the specific activities of NADH dehydrogenase (complex I), succinate dehydrogenase (complex II), and cytochrome-c oxidase (complex IV) and a dose-dependent depletion of mitochondrial DNA levels. The data were consistent with AZT-induced cardiac and skeletal muscle mitochondrial myopathy (71).

Based on the findings of the above research, a prospective study of the left ventricular structure and function of 382 noninfected (36 exposed to AZT) and 58 HIV-infected (12 exposed to AZT) infants born to HIV-infected women was published in 2000 (72). The median length of in utero exposure to AZT was 103 days (105 days for those not infected and 68 days for those infected at birth). Echocardiographic studies (mean left ventricular fractional shortening, contractility, end-diastolic dimension, and left ventricular mass) were conducted every 4-6 months during the first 14 months of life. All echocardiograms were examined without knowledge of the child's clinical status or medications. No statistical differences were found among the four echocardiographic measures in the four groups of children. The investigators identified at least four limitations of their study, including a small sample size that was unable to estimate the frequency of an uncommon toxic effect, the lack of an assessment of a possible dose effect, the possibility that the sickest children were missed because they were unable to attend follow-up visits, and the effects of other drug therapy in the HIV-infected subgroup that could obscure the effects of AZT. Nonetheless, they concluded that perinatal exposure to AZT was not associated with acute or chronic abnormalities in left ventricular structure or function (72).

A 2000 case report described the adverse pregnancy outcomes, including NTDs, of two pregnant women with HIV infection who were treated with the anti-infective combination trimethoprim/sulfamethoxazole for prophylaxis against Pneumocystis carinii, concurrently with antiretroviral agents (73). Exposure to AZT occurred in one of these cases. A 32-year-old woman with a 3-year history of HIV and recent diagnosis of AIDS was treated before and throughout gestation with the anti-infective combination plus AZT and zalcitabine. Folic acid 10 mg/day was added after the diagnosis of pregnancy (gestational age not specified). At term, a female infant was delivered by cesarean section without HIV infection, but with a bony mass in the lumbar spine (identified by ultrasound at 32 weeks). A diagnostic evaluation revealed that the second lumbar vertebra consisted of hemivertebrae and projected posteriorly into the spinal canal. A malformed and displaced first lumbar vertebra was also noted. Surgery was planned to correct the defect. The authors attributed the NTDs in both cases to the antifolate activity of trimethoprim (73).

The case of a 6-month-old male infant who was diagnosed with acute lymphoblastic leukemia (ALL) was described in a 2000 case report (74). His mother had been diagnosed with HIV infection during the 5th month of gestation. She had been treated with oral AZT for the last 3 months of pregnancy. The 3.5-kg infant had been delivered by cesarean section and treated with AZT prophylaxis (2 mg/kg 4 times a day) for 6 weeks. All tests for HIV in the infant were negative. He achieved complete remission after chemotherapy for the ALL and was currently receiving the maintenance phase of chemotherapy at age 16 months. The relationship between AZT exposure and the ALL was unknown (74).

Because of the AZT-induced carcinogenicity observed in animal studies, a 1999 study evaluated the short-term risk for tumors in a total of 727 children who had been exposed in utero (antepartum) and/or during the neonatal period to HIV and AZT (75). The children were participants in one of two multicenter clinical studies: Pediatric AIDS Clinical Trials Group (PACTG) 076/219 (N = 115) or the Women and Infants Transmission Study (WITS) (N = 612). The mean infant follow-up in the PACTG 076/219 group was 38.3 months (366.9 person-years follow-up), whereas it was 14.5 months (743.7 person-years follow-up) for WITS participants. The range for all children was 1 month to 6 years. No tumors of any nature were reported in the children (relative risk 0, 95% CI 0-17.6) (75).

The long-term effects of in utero exposure to AZT were the subject of a study published in 1999 (76). HIV-uninfected children (N = 234) born to 231 HIV-infected women enrolled in the PACTG 076 were evaluated under the PACTG 219 protocol (122 exposed to AZT, 112 in the placebo group). The main outcome measures included physical growth, immunologic parameters, cognitive/developmental function, tumors, and mortality data. Children were evaluated every 6 months up to 24 months and then yearly thereafter or as clinically indicated. The median age at the time of last follow-up was 4.2 years (range 3.2-5.6 years). There were no significant differences between those exposed to AZT and those not exposed in terms of the sequential outcome measurements. In addition, there were no deaths or malignancies. In the 137 (59%) children who had at least one ophthalmologic examination, 72 were in the AZT group and 65 were in the placebo group. Although there was no significant difference (p = 0.99) between the groups in abnormal ophthalmic findings, astigmatism was noted in two (AZT group), ptosis in one (AZT group), and epicanthal folds in one (placebo group). Two other AZT-exposed children had ophthalmic abnormalities: (a) bilateral “thinned vessels, discs look slightly pale” in one with normal vision, and (b) one with a fundus reported as “copper-beaten look” that was not thought to be related to metabolic disease. One other asymptomatic, healthy 4-year-old child in the AZT group had a mild cardiomyopathy on echocardiogram (76).

New York Medicaid data were used in a study published in 2000 to determine if there was an association between prenatal AZT use and congenital anomalies (77). The study cohort included 1932 live-born infants delivered from 1993 to 1996 to HIV-infected women in the state of New York, 29.5% of whom were exposed in utero to AZT. The prevalence of any anomaly in the study cohort was 2.76 (95% CI 2.36-3.17) compared with the general New York State population. When AZT-exposed outcomes were compared with those not exposed, the adjusted odds ratios (aORs) for major congenital malformations by trimester of first prescription were 1.20 (95% CI 0.58-2.51) (1st trimester), 1.47 (95% CI 0.85-2.55) (2nd trimester), and 1.84 (95% CI 1.04-3.25) (3rd trimester). There was an increased unadjusted OR for CNS defects when compared with those not exposed to AZT (7.98 [95% CI 1.56-37.46]). However, this finding was based on only four such defects and must be interpreted cautiously (77).

High semen levels of AZT have been reported (78). Six males with HIV disease were treated with 200 mg of the antiviral agent orally every 4-6 hours. AZT concentrations in semen 3.0-4.5 hours after a dose ranged from 1.68 to 6.43 μmol/L, representing semen:serum ratios of 1.3-20.4. The semen levels were above the in vitro minimum inhibitory concentration for HIV-1. A 1994 study reported that AZT reversed the effects of HIV-1 disease progression on semen quality, including ejaculate volume, sperm concentration and total count, and the number of abnormal sperm forms (79). Moreover, AZT therapy significantly reduced the semen white blood cell count, the principal HIV-1 host cells in ejaculates of HIV-1-infected males. The researchers concluded that this might explain why infected males treated with AZT have a reduced viral load in their semen and a lower rate of sexual transmission (79).

Two reviews, one in 1996 and the other in 1997, concluded that all women currently receiving antiretroviral therapy should continue to receive therapy during pregnancy and that treatment of the mother with monotherapy should be considered inadequate therapy (80,81). The same conclusion was reached in a 2003 review with the added admonishment that therapy must be continuous to prevent emergence of resistant viral strains (82). In 2009, the updated U.S. Department of Health and Human Services guidelines for the use of antiretroviral agents in HIV-1-infected patients continued the recommendation that therapy, with the exception of efavirenz, should be continued during pregnancy (83). If indicated, AZT should not be withheld in pregnancy because the expected benefit to the HIV-positive mother outweighs the unknown risk to the fetus. Updated guidelines for the use of antiretroviral drugs to reduce perinatal HIV-1 transmission also were released in 2010 (84). Women receiving antiretroviral therapy during pregnancy should continue the therapy, but, regardless of the regimen, AZT administration is recommended during the intrapartum period to prevent vertical transmission of HIV to the newborn (84).