Prenatal and neonatal drug metabolism in man

Hepatobiliary disposition of 17-OHPC and taurocholate in fetal human hepatocytes: a comparison with adult human hepatocytes

Little information is available in the literature regarding the expression and activity of transporters in fetal human liver or cultured cells. A synthetic progesterone structural analog, 17α-hydroxyprogesterone caproate (17-OHPC), is used in the prevention of spontaneous abortion in women with a history of recurrent miscarriage (habitual abortion). 17-OHPC has been reported to traverse the placental barrier and gain access to fetal circulation. In this study, the role of transporters in the disposition of 17-OHPC in fetal and adult human hepatocytes was examined. Progesterone metabolites have been reported to induce trans-inhibition of bile acid transporter, ABCB11. Thus, we investigated the effect of 17-OHPC or its metabolites on [(3)H]taurocholic acid transport in sandwich-cultured human fetal and adult hepatocytes. 17-OHPC was taken up rapidly into the cells and transported out partially by an active efflux process that was significantly inhibited by cold temperature, cyclosporine, verapamil, and rifampin. The active efflux mechanism was observed in both adult and fetal hepatocyte cultures. 17-OHPC produced a concentration-dependent inhibition of taurocholate efflux into canaliculi in sandwich-cultured adult and fetal human hepatocytes. However, given the high concentrations required to cause inhibition of these transport processes, no adverse effects would be anticipated from therapeutic levels of 17-OHPC. We also evaluated the expression of various hepatic transporters (ABCB1, ABCB4, SLCO1B1, SLCO1B3, SLCO2B1, ABCB11, SLC10A1, ABCC2, ABCC3, ABCC4, and ABCG2) in fetal and adult hepatocytes. With the exception of ABCB4, all transporters examined were expressed, albeit at lower mRNA levels in fetal hepatocytes compared with adults.

Metabolism of 17alpha-hydroxyprogesterone caproate, an agent for preventing preterm birth, by fetal hepatocytes

Preterm delivery (i.e., delivery before 37 completed weeks of gestation) is a major determinant of neonatal morbidity and mortality. Until recently, no effective therapies for prevention of preterm birth existed. In a recent multicentered trial, 17alpha-hydroxyprogesterone caproate (17-OHPC) was shown to reduce the rate of preterm birth by 33% in a group of high-risk women. Limited pharmacologic data exist for this drug. Previous studies have shown that CYP3A is involved in the metabolism of 17-OHPC. In this study, we evaluated the metabolism of 17-OHPC in adult and fetal human hepatocytes and in expressed cytochrome P450 enzymes. 17-OHPC was metabolized by expressed CYP3A7 and by fetal hepatocytes. The metabolite profile was qualitatively different between expressed CYP3A4 and CYP3A7. Expressed CYP3A4 demonstrated a significantly higher (>10 times) capacity to metabolize 17-OHPC than CYP3A7. Based on retention times, two unique metabolites were observed in the fetal and adult hepatocyte systems along with one common metabolite. The intrinsic clearance of 17-OHPC by fetal hepatocytes was observed to be one-half of that in adults. In summary, this study demonstrates that fetal hepatocytes and, in particular, the fetal form of CYP3A (i.e., CYP3A7) can metabolize 17-OHPC.

Population approaches in paediatrics

Population pharmacokinetic (PK) approach is now often used to evaluate PK characteristics of a new compound during its clinical development. Recently, new legislation governing the development and authorization of medicines for use in children aged 0-17 years was introduced in the European Union. Among the strategies proposed in relation to clinical aspects, use of population PKs is stated. In this manuscript, comparison between standard PK and population PK methods will be briefly addressed to understand why the second is particularly adapted to perform PK studies in paediatrics. Then, specific patients' characteristics (covariates) in paediatrics will be presented. Examples of PK and PK-pharmacodynamic (PK-PD) studies will be finally given. The number of population PK studies published still exceeds largely those of PK-PD.

Glucuronidation in therapeutic drug monitoring

BACKGROUND

Glucuronidation is a major drug-metabolizing reaction in humans. A pharmacological effect of glucuronide metabolites is frequently neglected and the value of therapeutic drug monitoring has been questioned. However, this may not always be true.

METHODS

In this review the impact of glucuronidation on therapeutic drug monitoring has been evaluated on the basis of a literature search and experience from the own laboratory.

RESULTS

The potential role of monitoring glucuronide metabolite concentrations to optimize therapeutic outcome is addressed on the basis of selected examples of drugs which are metabolized to biologically active/reactive glucuronides. Furthermore indirect effects of glucuronide metabolites on parent drug pharmacokinetics are presented. In addition, factors that may modulate the disposition of these metabolites (e.g. genetic polymorphisms, disease processes, age, and drug-drug interactions) are briefly mentioned and their relevance for the clinical situation is critically discussed.

CONCLUSION

Glucuronide metabolites can have indirect as well as direct pharmacological or toxicological effects. Although convincing evidence to support the introduction of glucuronide monitoring into clinical practice is currently missing, measurement of glucuronide concentrations may be advantageous in specific situations. If the glucuronide metabolite has an indirect effect on the pharmacokinetics of the parent compound, monitoring of the parent drug may be considered. Furthermore pharmacogenetic approaches considering uridine diphosphate (UDP) glucuronosyltransferases polymorphisms may become useful in the future to optimize therapy with drugs subject to glucuronidation.

Incorporating children's toxicokinetics into a risk framework

Children's responses to environmental toxicants will be affected by the way in which their systems absorb, distribute, metabolize, and excrete chemicals. These toxicokinetic factors vary during development, from in utero where maternal and placental processes play a large role, to the neonate in which emerging metabolism and clearance pathways are key determinants. Toxicokinetic differences between neonates and adults lead to the potential for internal dosimetry differences and increased or decreased risk, depending on the mechanisms for toxicity and clearance of a given chemical. This article raises a number of questions that need to be addressed when conducting a toxicokinetic analysis of in utero or childhood exposures. These questions are organized into a proposed framework for conducting the assessment that involves problem formulation (identification of early life stage toxicokinetic factors and chemical-specific factors that may raise questions/concerns for children); data analysis (development of analytic approach, construction of child/adult or child/animal dosimetry comparisons); and risk characterization (evaluation of how children's toxicokinetic analysis can be used to decrease uncertainties in the risk assessment). The proposed approach provides a range of analytical options, from qualitative to quantitative, for assessing children's dosimetry. Further, it provides background information on a variety of toxicokinetic factors that can vary as a function of developmental stage. For example, the ontology of metabolizing systems is described via reference to pediatric studies involving therapeutic drugs and evidence from in vitro enzyme studies. This type of resource information is intended to help the assessor begin to address the issues raised in this paper.

Pharmacokinetics in the newborn

In addition to differences in the pharmacodynamic response in the infant, the dose and the pharmacokinetic processes acting upon that dose principally determine the efficacy and/or safety of a therapeutic or inadvertent exposure. At a given dose, significant differences in therapeutic efficacy and toxicant susceptibility exist between the newborn and adult. Immature pharmacokinetic processes in the newborn predominantly explain such differences. With infant development, the physiological and biochemical processes that govern absorption, distribution, metabolism, and excretion undergo significant growth and maturational changes. Therefore, any assessment of the safety associated with an exposure must consider the impact of these maturational changes on drug pharmacokinetics and response in the developing infant. This paper reviews the current data concerning the growth and maturation of the physiological and biochemical factors governing absorption, distribution, metabolism, and excretion. The review also provides some insight into how these developmental changes alter the efficiency of pharmacokinetics in the infant. Such information may help clarify why dynamic changes in therapeutic efficacy and toxicant susceptibility occur through infancy.

Ontogeny of hepatic and renal systemic clearance pathways in infants: part I

Dramatic developmental changes in the physiological and biochemical processes that govern drug pharmacokinetics and pharmacodynamics occur during the first year of life. These changes may have significant consequences for the way infants respond to and deal with drugs. The ontogenesis of systemic clearance mechanisms is probably the most critical determinant of a pharmacological response in the developing infant. In recent years, advances in molecular techniques and an increased availability of fetal and infant tissues have afforded enhanced insight into the ontogeny of clearance mechanisms. Information from these studies is reviewed to highlight the dynamic and complex nature of developmental changes in clearance mechanisms in infants during the first year of life. Hepatic and renal elimination mechanisms constitute the two principal clearance pathways of the developing infant. Drug metabolising enzyme activity is primarily responsible for the hepatic clearance of many drugs. In general, when compared with adult activity levels normalised to amount of hepatic microsomal protein, hepatic cytochrome P450-mediated metabolism and the phase II reactions of glucuronidation, glutathione conjugation and acetylation are deficient in the neonate, but sulfate conjugation is an efficient pathway at birth. Parturition triggers the dramatic development of drug metabolising enzymes, and each enzyme demonstrates an independent rate and pattern of maturation. Marked interindividual variability is associated with their developmental expression, making the ontogenesis of hepatic metabolism a highly variable process. By the first year of life, most enzymes have matured to adult activity levels. When compared with adult values, renal clearance mechanisms are compromised at birth. Dramatic increases in renal function occur in the ensuing postpartum period, and by 6 months of age glomerular filtration rate normalised to bodyweight has approached adult values. Maturation of renal tubular functions exhibits a more protracted time course of development, resulting in a glomerulotubular imbalance. This imbalance exists until adult renal tubule function values are approached by 1 year of age. The ontogeny of hepatic biliary and renal tubular transport processes and their impact on the elimination of drugs remain largely unknown. The summary of the current understanding of the ontogeny of individual pathways of hepatic and renal elimination presented in this review should serve as a basis for the continued accruement of age-specific information concerning the ontogeny of clearance mechanisms in infants. Such information can only help to improve the pharmacotherapeutic management of paediatric patients.

Disposition of morphine in tissues of the pregnant rat and foetus following single and continuous intraperitoneal administration to the mother

Foetal exposure to maternally administered opiates such as morphine represent a serious human health problem but disposition studies in man are difficult to perform. Morphine disposition was therefore investigated in pregnant rats and their foetuses near term as a model. Disposition was examined either following intraperitoneal dosing as a single dose or continuous infusion. A high-pressure liquid chromatography assay for morphine in plasma and tissue was developed and validated. Following the single morphine dose, foetal distribution was rapid and concentrations in foetal and placental tissue were from 2.6 (whole foetus) to 27.6 (placenta) times higher compared with maternal plasma. The rank order of the area under the concentration vs time curve (AUC) of morphine in tissues was: placenta > or = foetal liver > foetal brain > whole foetus > maternal brain. The foetal brain to maternal brain AUC ratio for morphine was 9.5, suggesting large differences in their blood-brain barrier permeability. Following continuous administration of morphine there were significant linear relationships between maternal plasma and tissue concentrations with the same rank order as the single dose study. However, following continuous administration the relative amount of morphine in placenta and foetal liver was reduced by half and one-third, respectively, compared with the single dose study. These results document why the rat foetus is particularly susceptible to the pharmacodynamic effects of morphine following maternal administration.

Common errors of drug administration in infants: causes and avoidance

Drug administration errors are common in infants. Although the infant population has a high exposure to drugs, there are few data concerning pharmacokinetics or pharmacodynamics, or the influence of paediatric diseases on these processes. Children remain therapeutic orphans. Formulations are often suitable only for adults; in addition, the lack of maturation of drug elimination processes, alteration of body composition and influence of size render the calculation of drug doses complex in infants. The commonest drug administration error in infants is one of dose, and the commonest hospital site for this error is the intensive care unit. Drug errors are a consequence of system error, and preventive strategies are possible through system analysis. The goal of a zero drug error rate should be aggressively sought, with systems in place that aim to eliminate the effects of inevitable human error. This involves review of the entire system from drug manufacture to drug administration. The nuclear industry, telecommunications and air traffic control services all practise error reduction policies with zero error as a clear goal, not by finding fault in the individual, but by identifying faults in the system and building into that system mechanisms for picking up faults before they occur. Such policies could be adapted to medicine using interventions both specific (the production of formulations which are for children only and clearly labelled, regular audit by pharmacists, legible prescriptions, standardised dose tables) and general (paediatric drug trials, education programmes, nonpunitive error reporting) to reduce the number of errors made in giving medication to infants.

Pharmacokinetics and metabolism of rectally administered paracetamol in preterm neonates

AIM

To investigate the pharmacokinetics, metabolism, and dose-response relation of a single rectal dose of paracetamol in preterm infants in two different age groups.

METHODS

Preterm infants stratified by gestational age groups 28-32 weeks (group 1) and 32-36 weeks (group 2) undergoing painful procedures were included in this study. Pain was assessed using a modified facies pain score.

RESULTS

Twenty one infants in group 1 and seven in group 2 were given a single rectal dose of 20 mg/kg body weight. Therapeutic concentrations were reached in 16/21 and 1/7 infants in groups 1 and 2, respectively. Peak serum concentrations were significantly higher in group 1. Median time to reach peak concentrations was similar in the two groups. As serum concentration was still in the therapeutic range for some infants in group 1, elimination half life (T1/2) could not be determined in all infants: T1/2 was 11.0 +/- 5.7 in 11 infants in group 1 and 4.8 +/- 1.2 hours in group 2. Urinary excretion was mainly as paracetamol sulphate. The glucuronide:sulphate ratio was 0.12 +/- 0.09 (group 1) and 0.28 +/- 0.35 (group 2). The pain score did not correlate with therapeutic concentrations.

CONCLUSIONS

A 20 mg/kg single dose of paracetamol can be safely given to preterm infants in whom sulphation is the major pathway of excretion. Multiple doses in 28-32 week old neonates would require an interval of more than 8 hours to prevent progressively increasing serum concentrations.

Pregnancy and epilepsy

Pregnant women with epilepsy are at risk for a variety of complications. This article reviews the extensive literature on pregnancy and epilepsy with special emphasis on the management of pregnancy. Information is presented concerning seizure frequency in pregnancy, effects of epileptic seizures on the fetus, occurrence of complications during pregnancy and delivery, the incidence of fetal congenital malformations, and infant development. Recommendations are given concerning prenatal counseling, antiepileptic drug management, breast feeding, vitamin K supplementation and folic acid supplementation.

Teratology: principles and practice

Gross anomalies of structure and/or function affect 2 to 4 per cent of all human newborns and are the leading cause of perinatal mortality in this country. Despite their significance, the etiology of most such defects remains unclear. A majority are unassociated with any identifiable cause; a small but significant percentage are attributed to heritable disorders of either a Mendelian (single gene) or chromosomal nature. Drugs and environmental exposures are currently implicated in only a small percentage of affected pregnancies. Nevertheless, an awareness of the principles of teratogenesis, an appreciation for the role--though imprecise--of placental transfer and fetal drug disposition, and avoidance of known teratogens currently offers our best hope for the study and prevention of birth defects.

Uptake of gentamicin in foetal and newborn rats after a single intraperitoneal injection

The aim of the present investigation has been to examine 1) the placental passage and uptake of gentamicin in foetal rat tissues and 2) uptake and organ distribution of this drug in neonatal rats with special reference to the kidney. After a single intraperitoneal injection of tritium-labelled gentamicin to the mother, we found a rapid passage of radioactivity across the rat placenta, but no pronounced tendency to accumulation in the foetus. After administration of 10 micrograms or 100 mg gentamicin per kg body weight to pregnant rats at day 11 or 13, respectively, of gestation, the maximal concentrations of the drug recovered in individual foetuses were 0.05 and 300 ng per mg wet weight. Ten and 90 min. after administration the uptake of gentamicin in the kidney cortex of the mother was 25 and 118 fold higher, respectively, than the corresponding uptake in the foetal kidney after 19 days of gestation. After a single intraperitoneal injection, gentamicin uptake in the kidney cortex of neonatal rats resembled that seen in adult animals, with a rapid initial phase followed by a relatively stable plateau, although the maximal uptake was only about 10% as large in the neonates.

Monoclonal antibody directed detection of cytochrome P-450 (PCN) in human fetal liver

Monoclonal antibodies (MAbs) raised to rat liver cytochrome P-450s induced by phenobarbital, 3-methylcholanthrene, and pregnenolone-16 alpha-carbonitrile were used to detect these epitope specific P-450s in human abortion fetuses 14-24 weeks of age. This was performed using a Western blot technique. In parallel, ECOD was determined in the same tissue specimens. Of seven different MAbs used MAb PCN 2-13-1/C2 was the only one that immunodetected a cytochrome P-450 band with Western blot analyses of human fetal liver microsomes. This band was consistently detected in all fetal liver specimens studied although the intensity varied among samples. No bands were detected in microsomal preparations from adrenal and renal tissues obtained from the same fetuses. The human adult liver microsomal specimens also contained a MAb PCN 2-13-1/C2 identified cytochrome P-450 band. ECOD activity was detected in all but one of the human fetal livers and varied between 0.22 and 47.5 pmol min-1 mg protein-1, as compared to 113 to 489 pmol min-1 mg protein-1 in human adult livers. In all of the fetuses except one the adrenal ECOD activity (0.63-37.0 pmol min-1 mg protein-1) exceeded that in the liver. The renal ECOD activities were, however, low. The hepatic and adrenal ECOD activities correlated with each other (r = 0.95). Although the ECOD activity is a function of several different P-450s there was also a correlation (r = 0.78) between the ECOD activity and the MAb immunodetected protein band intensity in Western blots of human fetal liver microsomes. The presence of a MAb PCN 2-13-1/C2 identified band in fetal liver microsomes may be indicative of a steroid-dependent effect in fetal life.

The role of metabolism in the testing of developmental toxicants

The delivery of potential developmental toxicants to the conceptus is dependent on several metabolic and pharmacokinetic factors. Within the maternal-embryo/fetal unit, maternal, placental, and embryo/fetal factors must be considered. These factors include blood flow, permeability, biotransformation, and elimination. Pharmacokinetic models based on data gathered from appropriate in vivo and in vitro studies may be used to describe the effect of these factors on toxicant delivery to the conceptus. Several known human developmental toxicants are discussed in terms of the metabolic and pharmacokinetic factors controlling their delivery to the conceptus. Metabolic events, including activation and/or detoxification, have been reported for the majority of the toxicants examined. Thus it would appear that the role of metabolism should be considered during the testing of potential developmental toxicants.

Comparison of the placental transfer of thiopental and diazepam in caesarean section

Drug concentrations were measured in whole blood obtained from mother and child after induction of general anaesthesia with thiopental or diazepam and delivery by Caesarean section. In 27 cases given thiopental 3 mg/kg intravenously the 5-min child/mother concentration ratio rose with increasing injection-delivery (I-D) interval up to 8-10 min. The concentration in the newborn at 2 h showed a similar trend. In 30 cases given diazepam 0.3 mg/kg for sleep induction, there were some low values in cases delivered within 4 min after the injection. However, higher neonatal concentrations and child/mother ratios were observed when the operation lasted 4-5 min, and there was no further increase in cases with longer I-D intervals. There is evidence to suggest that net transfer to the fetus proceeds at a slower rate with thiopental than with diazepam. However, in the present series of low risk elective Caesarean sections, there was no appreciable difference between the induction agents with regard to their effect on the newborn infant.