Transplacental transfer of zidovudine in the near-term pregnant baboon

Predicting drug-metagenome interactions: Variation in the microbial β-glucuronidase level in the human gut metagenomes

Characterizing the gut microbiota in terms of their capacity to interfere with drug metabolism is necessary to achieve drug efficacy and safety. Although examples of drug-microbiome interactions are well-documented, little has been reported about a computational pipeline for systematically identifying and characterizing bacterial enzymes that process particular classes of drugs. The goal of our study is to develop a computational approach that compiles drugs whose metabolism may be influenced by a particular class of microbial enzymes and that quantifies the variability in the collective level of those enzymes among individuals. The present paper describes this approach, with microbial β-glucuronidases as an example, which break down drug-glucuronide conjugates and reactivate the drugs or their metabolites. We identified 100 medications that may be metabolized by β-glucuronidases from the gut microbiome. These medications included morphine, estrogen, ibuprofen, midazolam, and their structural analogues. The analysis of metagenomic data available through the Sequence Read Archive (SRA) showed that the level of β-glucuronidase in the gut metagenomes was higher in males than in females, which provides a potential explanation for the sex-based differences in efficacy and toxicity for several drugs, reported in previous studies. Our analysis also showed that infant gut metagenomes at birth and 12 months of age have higher levels of β-glucuronidase than the metagenomes of their mothers and the implication of this observed variability was discussed in the context of breastfeeding as well as infant hyperbilirubinemia. Overall, despite important limitations discussed in this paper, our analysis provided useful insights on the role of the human gut metagenome in the variability in drug response among individuals. Importantly, this approach exploits drug and metagenome data available in public databases as well as open-source cheminformatics and bioinformatics tools to predict drug-metagenome interactions.

The role of xenobiotic-metabolizing enzymes in the placenta: a growing research field

Introduction: The placenta is a temporary and unique organ that allows for the physical connection between a mother and fetus; this organ regulates the transport of gases and nutrients mediating the elimination of waste products contained in the fetal circulation. The placenta performs metabolic and excretion functions, on the basis of multiple enzymatic systems responsible for the oxidation, reduction, hydrolysis, and conjugation of xenobiotics. These mechanisms give the placenta a protective role that limits the fetal exposure to harmful compounds. During pregnancy, some diseases require uninterrupted treatment even if it is detrimental to the fetus. Drugs and other xenobiotics alter gene expression in the placenta with repercussions for the fetus and mother's well-being.Areas covered: This review provides a brief description of the human placental structure and function, the main drug and xenobiotic transporters and metabolizing enzymes, placenta-metabolized substrates, and alterations in gene expression that the exposure to xenobiotics may cause.Expert opinion: Research should be focused on the identification and validation of biological markers for the assessment of the harmful effects of some drugs in pregnancy, including the evaluation of polymorphisms and methylation patterns in chorionic villous samples and/or amniotic fluid.

Species Differences in the Metabolism and Disposition of Antiviral Nucleoside Analogues: 2. Zidovudine

Preclinical and clinical studies on the disposition of zidovudine, a thymidine analogue with potent activity against human immunodeficiency virus, identified significant species differences in the metabolism and elimination of the drug. Zidovudine was extensively metabolized to the 5′-O-glucuronide in man and other primates. Rabbits and dogs were intermediate in their extent of biotransformation to the glucuronide conjugate, whereas rats and mice excreted the drug largely unchanged. Decreased metabolism was compensated by increased renal elimination, such that plasma elimination phase half-lives for zidovudine were similar (0.6–1.1 h) in all species. Rapid and extensive absorption and considerable penetration into tissues were also observed for all species studied. Only in the brain and testes were drug levels less than in plasma, although effective antiviral concentrations of zidovudine were achieved in brain and CSF. This review summarizes the variety of studies of the absorption, distribution, metabolism, and elimination of zidovudine in several species, including humans.

Fetal blood sampling in baboons (Papio spp.): important procedural aspects and literature review

BACKGROUND

The baboons (Papio cynocephalus) have similarities with human placentation and fetal development. Fetal blood sampling allows investigators to assess fetal condition at a specific point in gestation as well as transplacental transfer of medications. Unfortunately, assessing fetal status during gestation has been difficult and fetal instrumentation associated with high rate of pregnancy loss. Our objectives are to describe the technique of ultrasound guided cordocentesis (UGC) in baboons, report post-procedural outcomes, and review existing publications.

METHODS

This is a procedural paper describing the technique of UGC in baboons. After confirming pregnancy and gestational age via ultrasound, animals participating in approved research protocols that required fetal assessment underwent UGC.

RESULTS

We successfully performed UGC in four animals (five samples) using this technique. Animals were sampled in the second and third trimesters with fetal blood sampling achieved by sampling a free cord loop, placental cord insertion site or the intrahepatic umbilical vein. All procedures were without complication and these animals delivered at term.

CONCLUSIONS

Ultrasound guided fetal umbilical cord venipuncture is a useful and safe technique to sample the fetal circulation with minimal risk to the fetus or mother. We believe this technique could be used for repeated fetal venous blood sampling in the baboons.

Metabolism and pharmacokinetics of the combination Zidovudine plus Lamivudine in the adult Erythrocebus patas monkey determined by liquid chromatography-tandem mass spectrometric analysis

Because of their similarity to humans, non-human primates constitute useful preclinical models in which to examine potential human drug toxicities. Antiretroviral nucleoside reverse transcriptase inhibitor (NRTI) toxicity is currently under investigation in Erythrocebus patas monkeys, and whereas NRTI pharmacokinetics have been studied in other monkey species, pharmacokinetics for Zidovudine plus Lamivudine (AZT/3TC) dosing have not been reported in the patas. Here we present 24 h serum pharmacokinetic parameters after a single oral exposure to the combination of AZT (40 mg) and 3TC (24 mg), doses equivalent to a human daily dose of Combivir. The patas (n=3) AZT/3TC pharmacokinetic profiles were similar to those seen in other primate species. Average maximum serum concentrations (Cmax) for AZT and 3TC were 2.35 and 2.65 microg/ml, respectively, and were observed at 0.83 h (Tmax). Cmax was 13.34 microg/ml for the AZT-glucuronide (AZT-G) and was 0.023 microg/ml for the potentially toxic minor metabolite 3'-amino-3'-deoxythymidine (AMT), both occurring at about 1 h after dosing. Similar elimination half-times, 0.70 and 0.68 h(-1), were found for AZT and AZT-G, respectively, while 3TC was eliminated about half as fast (0.33 h(-1)) resulting in AUC(0-infinity) values of 6.97 microg/ml h for 3TC, 2.99 microg/ml h for AZT, 20.5 microg/ml h for AZT-G and 0.002 for AMT 6.97 microg/ml h. This study shows similar metabolism and pharmacokinetics for oral administration of AZT/3TC in the adult patas monkey, other primate species and humans. The data validate the use of the patas monkey for studies of NRTI toxicity.

Perinatal genotoxicity and carcinogenicity of anti-retroviral nucleoside analog drugs

The current worldwide spread of the human immunodeficiency virus-1 (HIV-1) to the heterosexual population has resulted in approximately 800,000 children born yearly to HIV-1-infected mothers. In the absence of anti-retroviral intervention, about 25% of the approximately 7,000 children born yearly to HIV-1-infected women in the United States are HIV-1 infected. Administration of zidovudine (AZT) prophylaxis during pregnancy reduces the rate of infant HIV-1 infection to approximately 7%, and further reductions are achieved with the addition of lamivudine (3TC) in the clinical formulation Combivir. Whereas clinically this is a remarkable achievement, AZT and 3TC are DNA replication chain terminators known to induce various types of genotoxicity. Studies in rodents have demonstrated AZT-DNA incorporation, HPRT mutagenesis, telomere shortening, and tumorigenicity in organs of fetal mice exposed transplacentally to AZT. In monkeys, both AZT and 3TC become incorporated into the DNA from multiple fetal organs taken at birth after administration of human-equivalent protocols to pregnant dams during gestation, and telomere shortening has been found in monkey fetuses exposed to both drugs. In human infants, AZT-DNA and 3TC-DNA incorporation as well as HPRT and GPA mutagenesis have been documented in cord blood from infants exposed in utero to Combivir. In infants of mice, monkeys, and humans, levels of AZT-DNA incorporation were remarkably similar, and in newborn mice and humans, mutation frequencies were also very similar. Given the risk-benefit ratio, these highly successful drugs will continue to be used for prevention of vertical viral transmission, however evidence of genotoxicity in mouse and monkey models and in the infants themselves would suggest that exposed children should be followed well past adolescence for early detection of potential cancer hazard.

Pharmacokinetics of intravenous acyclovir, zidovudine, and acyclovir-zidovudine in pregnant rats

The pharmacokinetics and placental transfer of acyclovir and zidovudine monotherapies and acyclovir-zidovudine combination therapy were compared in the pregnant rat. Timed-pregnancy Sprague-Dawley rats were used for the study. Doses of 60 mg of each drug/kg of body weight in monotherapy and in combination therapy were given by intravenous bolus, and samples of maternal plasma, amniotic fluid, fetal tissue, and placental tissue were collected over a period of 8 h postdose. Concentrations of each drug in the various matrices were measured by high-performance liquid chromatography. All data were analyzed by using WinNonlin. A one-compartment model with first-order elimination was used to fit the AZT plasma data from the combination therapy rats, but the plasma data from the other groups were fit to a two-compartment model. Tissue data were analyzed by noncompartmental analysis to generate area-under-the-concentration-time-curve values. Implementation of the combination therapy altered the pharmacokinetics of each drug compared to its monotherapy pharmacokinetics. The combination of these two drugs may potentiate fetal and amniotic fluid exposures to each drug.

Incorporation of 3'-azido-3'-deoxythymidine (AZT) into fetal DNA and fetal tissue distribution of drug after infusion of pregnant late-term rhesus macaques with a human-equivalent AZT dose

In the United States, the nucleoside analogue drug 3'-azido-3'deoxythymidine (AZT; also called zidovudine or ZDV) is given to most pregnant women who produce a positive test result for HIV-1. To investigate transplacental distribution and genotoxicity of AZT, near-term pregnant rhesus (Macaca mulatta) monkeys and their fetuses were studied. Four pregnant monkeys were continuously infused with 8 mg AZT/kg body weight for the 4 hours just prior to hysterotomy at term. This short-term AZT exposure resulted in AZT incorporation into DNA of fetal liver, lung, heart, skeletal muscle, brain, testis, and placenta, which varied between 29 and 1944 molecules of AZT/10(6) nucleotides. In contrast, values for AZT and combined metabolites, determined by radioactivity, varied between 0.94 and 5.20 microg AZT equivalents/g tissue. A fifth animal, (H076), was infused with 17.3 mg AZT/kg body weight for approximately 3 hours, followed by 1 hour without drug before hysterotomy. Similar to the 4 other monkeys, variable levels of AZT (16-147 molecules of AZT/10(6) nucleotides) were incorporated into organ DNA of H076, whereas organ tissues contained less-variable levels of AZT and metabolites (0.86-2.05 microg AZT equivalents/g tissue). For H076, at hysterotomy 1 hour after discontinuation of drug, values for AZT and the 3'-azido-3'-deoxythymidine-beta-D-glucuronide (AZTG) in fetal blood and amniotic fluid were twofold and threefold higher than those in maternal blood. Most AZT pharmacokinetic parameters in the fifth monkey were similar to those previously reported for the first 4 monkeys and those observed in a similar study of pregnant women. These data show that a short-term AZT infusion in pregnant rhesus monkeys, which have similar AZT pharmacokinetics to those present in a pregnant human, results in incorporation of drug into the DNA of placenta and most fetal organs. Data imply that the human fetus may also be subject to incorporation of AZT into DNA even after short-term AZT infusion to the mother just before delivery.

Comparison and characterization of immunoglobulin G subclasses among primate species

Little information is available on the immunoglobulin G (IgG) subclasses expressed in the sera of nonhuman primate species. To address this issue, we compared the IgG subclasses found in humans (IgG1, IgG2, IgG3, and IgG4) to those of nonhuman primates, such as baboons and macaques. Cross-reactive antihuman IgG subtype-specific reagents were identified and used to analyze purified IgG from sera by solid-phase enzyme-linked immunosorbent assay. Protein A-purified human IgG obtained from sera was composed of IgG1, IgG2, IgG3, and IgG4, whereas baboon and macaque IgG was composed of IgG1, IgG2, and IgG4. Protein G-purified human IgG was composed of IgG1, IgG2, IgG3, and IgG4, whereas baboon and macaque IgG was composed of IgG1, IgG2, and IgG4. To test the possibility that baboon and macaque IgG3 is actually present, but is outcompeted for binding to proteins A and G by the other more abundant IgG subclasses, we repurified the IgG from sera that did not bind either protein A or protein G. We found a baboon IgG3 population in the sera that did not bind protein A, but bound protein G. No IgG3 subtype was detectable in macaque sera. These data suggest that baboon sera, like human sera, contain four IgG subtypes, whereas macaque sera exhibit only three of the human subclass analogs. In addition, the IgG subtype-specific reagents were shown to be useful in determining the IgG subclass distribution following vaccination of baboons with hepatitis B surface antigen.

Implications of the kinetics of zidovudine in the pregnant baboon following oral administration

Zidovudine (ZDV) therapy in pregnancy reduces mother-to-child transmission of HIV. The action of ZDV in the fetus is thought to be an important contributor to efficacy. Previous research in primates has demonstrated that continuous infusion of ZDV to the mother leads to sustained plasma concentrations in the fetus; however, it has not been determined what concentrations of ZDV are achieved in the fetus following oral administration. The pharmacokinetics of drug distribution to the fetus following oral administration of a 100-mg dose of ZDV to the mother are reported from 6 chronically catheterized baboons. The first order elimination half-life of ZDV from both the mother and fetus was approximately 1.2 hours. The area under the concentration-time curve for the fetus was 77% (r2 = 0.98; p < .001) that of the mother and the estimated peak drug levels in the fetus were 52% (r2 = 0.83; p < .01) those in the mother. The rapid transfer and short half-life of ZDV leads to a drug concentration-time profile that would not sustain levels in the fetus with dosing every 4 hours. After comparing these findings with existing data from pregnant and nonpregnant humans, it seems likely that current dose recommendations for ZDV in pregnancy would not maintain levels of the active intracellular metabolite of ZDV in all fetuses. This may explain in part the 8% failure rate of ZDV prophylaxis. The correlation between fetal and maternal plasma concentrations of ZDV would allow titration of dose based on maternal drug levels to achieve fetal levels within the therapeutic range.

The baboon as a nonhuman primate model for assessing the effects of maternal immunization with Haemophilus influenzae type b polysaccharide vaccines

These studies were performed to assess the utility of the baboon as a nonhuman primate model to evaluate vaccines for use in humans. Specifically, we examined the antibody response of baboons immunized during the third trimester of pregnancy with Haemophilus influenzae type b (Hib) polyribosylribitol phosphate (PRP) conjugate and unconjugated polysaccharide vaccines. Some of the vaccinated mothers failed to respond to a single immunization with unconjugated Hib PRP. Specific Hib PRP immunoglobulin G (IgG) but not IgM antibodies cross the baboon placenta and are detected in the offspring. Higher-titer baboon anti-Hib PRP did not express two previously defined cross-reactive human anti-Hib PRP idiotypes and was biased towards lambda light-chain expression. Spectrotype analysis indicated that baboon anti-Hib PRP was restricted in heterogeneity and oligoclonal.

Maternal-fetal pharmacokinetics of zidovudine in rats

The disposition of zidovudine (AZT) was investigated in near-term (day 20) pregnant rats after intravenous bolus administration of AZT at 50 mg/kg. A compartmental pharmacokinetic model was developed to describe AZT concentrations in maternal plasma (1), placenta (2), fetus (3), amniotic fluid (4), and the maternal tissue compartment (5). Model equations were fitted simultaneously to all concentration data by NONLIN least-squares regression. The model that best described the AZT concentration data (F test, AIC, sum of weighted squared residuals) incorporated bidirectional transfer between maternal plasma reversible placenta, placenta reversible fetus, placenta reversible amniotic fluid, and maternal plasma reversible tissue compartment. Transfer rate constants (1/h) were as follows: k12, 0.58 +/- 0.41; k21, 47.64 +/- 46.61; k23, 67.50 +/- 42.03; k32, 13.09 +/- 8.80; k24, 0.62 +/- 0.03; k42, 0.32 +/- 0.06; k15, 5.75 +/- 7.00; k51, 4.12 +/- 1.01; and k10, 1.51 +/- 0.80. AZT rapidly distributed into tissue and placenta compartments. However, AZT accumulated more slowly into amniotic fluid. Intercompartmental distributional clearances suggest that the mechanism of maternal-placental, placental-fetal, and fetal-amniotic fluid transfer of AZT was by passive diffusion. This maternal-fetal model for AZT may offer a useful approach for describing the placental transfer kinetics of other antiviral nucleosides as well.

Serologic crossreactions among primate immunoglobulins

We have generated and characterized 50 murine monoclonal antibodies (mAb) specific for baboon IgG. We examined crossreactivity of these mAb to baboon IgM and immunoglobulin (Ig) of various other primates including human, chimpanzee, rhesus monkey, cynomolgus monkey, and African green monkey. Those mAB that crossreacted with human IgG were further examined using myeloma proteins for specificity to human Ig subclasses. One mAB crossreacted with all four human IgG subclasses and with human IgM. We further analyzed this reactivity utilizing Bence Jones proteins representative of various light (L) chain germline gene family products. This mAB reacted with Bence Jones proteins indicating the recognition of a kappa (k) L chain specificity associated with the kappa I, kappa III, and kappa IV subgroups, but not with kappa II. Based on the differences between kappa II germ line gene encoded L chains and the other kappa L chain subgroups, we ascribe this reactivity to six amino acids that define a discontinuous epitope.

Studies on the placental transfer of cell-free human immunodeficiency virus and p24 antigen in an ex vivo human placental model

OBJECTIVE

We studied whether the human placenta has the structural integrity to impede transplacental passage of cell-free human immunodeficiency virus (HIV)-1 or p24 antigen from the maternal to the fetal circulation.

METHODS

Nine term human placentas from uncomplicated vaginal or cesarean section deliveries were studied ex vivo with a placental perfusion apparatus to determine whether cell-free HIV-1 at 200-2000 tissue culture infectious dose (TCID50/mL) would pass to the fetal circulation. Passage of virus or p24 was assessed by infectivity titration and/or p24 antigen capture enzyme immunoassay.

RESULTS

Infectious HIV-1 was not detected in any of the fetal perfusate samples taken periodically during experiments. Low concentrations of HIV-1 p24 antigen, however, were detected in fetal perfusate samples from three placentas.

CONCLUSIONS

The term human placenta effectively impedes passage of cell-free HIV-1 from the maternal to the fetal circulation. However, it may be permeable to passage of p24 antigen.