Expression of functional cytochrome P4501A1 in human embryonic hepatic tissues during organogenesis

Physiologically Based Pharmacokinetic Modeling in Neonates: Current Status and Future Perspectives

Rational drug use in special populations is a clinical problem that doctors and pharma-cists must consider seriously. Neonates are the most physiologically immature and vulnerable to drug dosing. There is a pronounced difference in the anatomical and physiological profiles be-tween neonates and older people, affecting the absorption, distribution, metabolism, and excretion of drugs in vivo, ultimately leading to changes in drug concentration. Thus, dose adjustments in neonates are necessary to achieve adequate therapeutic concentrations and avoid drug toxicity. Over the past few decades, modeling and simulation techniques, especially physiologically based pharmacokinetic (PBPK) modeling, have been increasingly used in pediatric drug development and clinical therapy. This rigorously designed and verified model can effectively compensate for the deficiencies of clinical trials in neonates, provide a valuable reference for clinical research design, and even replace some clinical trials to predict drug plasma concentrations in newborns. This review introduces previous findings regarding age-dependent physiological changes and pathological factors affecting neonatal pharmacokinetics, along with their research means. The application of PBPK modeling in neonatal pharmacokinetic studies of various medications is also reviewed. Based on this, we propose future perspectives on neonatal PBPK modeling and hope for its broader application.

Predicting Human Fetal Drug Exposure Through Maternal-Fetal PBPK Modeling and In Vitro or Ex Vivo Studies

Medication (drug) use in human pregnancy is prevalent. Determining fetal safety and efficacy of drugs is logistically challenging. However, predicting (not measuring) fetal drug exposure (systemic and tissue) throughout pregnancy is possible through maternal-fetal physiologically based pharmacokinetic (PBPK) modeling and simulation. Such prediction can inform fetal drug safety and efficacy. Fetal drug exposure can be quantified in 2 complementary ways. First, the ratio of the steady-state unbound plasma concentration in the fetal plasma (or area under the plasma concentration-time curve) to the corresponding maternal plasma concentration (ie, Kp,uu ). Second, the maximum unbound peak (Cu,max,ss,f ) and trough (Cu,min,ss,f ) fetal steady-state plasma concentrations. We (and others) have developed a maternal-fetal PBPK model that can successfully predict maternal drug exposure. To predict fetal drug exposure, the model needs to be populated with drug specific parameters, of which transplacental clearances (active and/or passive) and placental/fetal metabolism of the drug are critical. Herein, we describe in vitro studies in cells/tissue fractions or the perfused human placenta that can be used to determine these drug-specific parameters. In addition, we provide examples whereby this approach has successfully predicted systemic fetal exposure to drugs that passively or actively cross the placenta. Apart from maternal-fetal PBPK models, animal studies also have the potential to estimate fetal drug exposure by allometric scaling. Whether such scaling will be successful is yet to be determined. Here, we review the above approaches to predict fetal drug exposure, outline gaps in our knowledge to make such predictions and map out future research directions that could fill these gaps.

Cytochrome P450 enzymes in the pediatric population: Connecting knowledge on P450 expression with pediatric pharmacokinetics

Cytochrome P450 enzymes play an important role in the pharmacokinetics, efficacy, and toxicity of drugs. Age-dependent changes in P450 enzyme expression have been studied based on several detection systems, as well as by deconvolution of in vivo pharmacokinetic data observed in pediatric populations. The age-dependent changes in P450 enzyme expression can be important determinants of drug disposition in childhood, in addition to the changes in body size and the other physiological parameters, and effects of pharmacogenetics and disease on organ functions. As a tool incorporating drug-specific and body-specific factors, physiologically-based pharmacokinetic (PBPK) models have become increasingly used to characterize and explore mechanistic insights into drug disposition. Thus, PBPK models can be a bridge between findings from basic science and utilization in predictive science. Pediatric PBPK models incorporate additional system specific information on developmental physiology and ontogeny and have been used to predict pharmacokinetic parameters from preterm neonates onwards. These models have been advocated by regulatory authorities in order to support pediatric clinical trials. The purpose of this chapter is to highlight accumulated knowledge and findings from basic research focusing on P450 enzymes, as well as the current status and future challenges of expanding the utilization of pediatric PBPK modeling.

Ontogeny of Drug-Metabolizing Enzymes

Almost 50% of prescription drugs lack age-appropriate dosing guidelines and therefore are used "off-label." Only ~10% drugs prescribed to neonates and infants have been studied for safety or efficacy. Immaturity of drug metabolism in children is often associated with drug toxicity. This chapter summarizes data on the ontogeny of major human metabolizing enzymes involved in oxidation, reduction, hydrolysis, and conjugation of drugs. The ontogeny data of individual drug-metabolizing enzymes are important for accurate prediction of drug pharmacokinetics and toxicity in children. This information is critical for designing clinical studies to appropriately test pharmacological hypotheses and develop safer pediatric drugs, and to replace the long-standing practice of body weight- or surface area-normalized drug dosing. The application of ontogeny data in physiologically based pharmacokinetic model and regulatory submission are discussed.

Hepatic Cytochrome P450 Abundance and Activity in the Developing and Adult Göttingen Minipig: Pivotal Data for PBPK Modeling

The Göttingen Minipig is gaining ground as nonrodent species in safety testing of drugs for pediatric indications. Due to developmental changes in pharmacokinetics and pharmacodynamics, physiologically based pharmacokinetic (PBPK) models are built to better predict drug exposure in children and to aid species selection for nonclinical safety studies. These PBPK models require high quality physiological and ADME data such as protein abundance of drug metabolizing enzymes. These data are available for man and rat, but scarce for the Göttingen Minipig. The aim of this study was to assess hepatic cytochrome P450 (CYP) protein abundance in the developing Göttingen Minipig by using mass spectrometry. In addition, sex-related differences in CYP protein abundance and correlation of CYP enzyme activity with CYP protein abundance were assessed. The following age groups were included: gestational day (GD) 84–86 (n = 8), GD 108 (n = 6), postnatal day (PND) 1 (n = 8), PND 3 (n = 8), PND 7 (n = 8), PND 28 (n = 8) and adult (n = 8). Liver microsomes were extracted and protein abundance was compared to that in adult animals. Next, the CYP protein abundance was correlated to CYP enzyme activity in the same biological samples. In general, CYP protein abundance gradually increased during development. However, we observed a stable protein expression over time for CYP4A24 and CYP20A1 and for CYP51A1, a high protein expression during the fetal stages was followed by a decrease during the first month of life and an increase toward adulthood. Sex-related differences were observed for CYP4V2_2a and CYP20A1 at PND 1 with highest expression in females for both isoforms. In the adult samples, sex-related differences were detected for CYP1A1, CYP1A2, CYP2A19, CYP2E1_2, CYP3A22, CYP4V2_2a and CYP4V2_2b with higher values in female compared to male Göttingen Minipigs. The correlation analysis between CYP protein abundance and CYP enzyme activity showed that CYP3A22 protein abundance correlated clearly with the metabolism of midazolam at PND 7. These data are remarkably comparable to human data and provide a valuable step forward in the construction of a neonatal and juvenile Göttingen Minipig PBPK model.

Ontogeny of Hepatic Transporters and Drug-Metabolizing Enzymes in Humans and in Nonclinical Species

The liver represents a major eliminating and detoxifying organ, determining exposure to endogenous compounds, drugs, and other xenobiotics. Drug transporters (DTs) and drug-metabolizing enzymes (DMEs) are key determinants of disposition, efficacy, and toxicity of drugs. Changes in their mRNA and protein expression levels and associated functional activity between the perinatal period until adulthood impact drug disposition. However, high-resolution ontogeny profiles for hepatic DTs and DMEs in nonclinical species and humans are lacking. Meanwhile, increasing use of physiologically based pharmacokinetic (PBPK) models necessitates availability of underlying ontogeny profiles to reliably predict drug exposure in children. In addition, understanding of species similarities and differences in DT/DME ontogeny is crucial for selecting the most appropriate animal species when studying the impact of development on pharmacokinetics. Cross-species ontogeny mapping is also required for adequate translation of drug disposition data in developing nonclinical species to humans. This review presents a quantitative cross-species compilation of the ontogeny of DTs and DMEs relevant to hepatic drug disposition. A comprehensive literature search was conducted on PubMed Central: Tables and graphs (often after digitization) in original manuscripts were used to extract ontogeny data. Data from independent studies were standardized and normalized before being compiled in graphs and tables for further interpretation. New insights gained from these high-resolution ontogeny profiles will be indispensable to understand cross-species differences in maturation of hepatic DTs and DMEs. Integration of these ontogeny data into PBPK models will support improved predictions of pediatric hepatic drug disposition processes. SIGNIFICANCE STATEMENT: Hepatic drug transporters (DTs) and drug-metabolizing enzymes (DMEs) play pivotal roles in hepatic drug disposition. Developmental changes in expression levels and activities of these proteins drive age-dependent pharmacokinetics. This review compiles the currently available ontogeny profiles of DTs and DMEs expressed in livers of humans and nonclinical species, enabling robust interpretation of age-related changes in drug disposition and ultimately optimization of pediatric drug therapy.

The human hepatic cell line HepaRG as a possible cell source for the generation of humanized liver TK-NOG mice

Humanized-liver mice, in which the liver has been repopulated with human hepatocytes, have been used to study aspects of human liver physiology such as drug metabolism, toxicology and hepatitis infection. However, the procurement of human hepatocytes is a major problem in producing humanized-liver mice because of the finite nature of the patient-derived resource.

In order to overcome this limitation, the human hepatic cell line HepaRG® were evaluated as promising donor cells for liver reconstitution in the TK-NOG mouse model.

We demonstrate that, in vivo, transplanted confluent culture or differentiated HepaRG® cells proliferated and differentiated toward both hepatocyte-like and biliary-like cells within the recipient liver. In contrast, proliferative HepaRG® cells could engraft TK-NOG mouse liver but could differentiate only toward biliary-like cells. The differentiation to hepatocyte-like cells was characterized by the detection of human albumin in the recipient mouse serum and was confirmed by immunohistochemical staining for human leukocyte antigen, human albumin, cytochrome P450 3A4, and multidrug resistance-associated protein 2. Biliary-like cells were characterized by positive staining for cytokeratin-19.

These results indicated that the differentiated HepaRG® cells are a possible cell source for generating humanized-liver mice, which are a useful model for in vivo studies of liver physiology.

Ontogeny of mammalian metabolizing enzymes in humans and animals used in toxicological studies

It is well recognized that expression of enzymes varies during development and growth. However, an in-depth review of this acquired knowledge is needed to translate the understanding of enzyme expression and activity into the prediction of change in effects (e.g. kinetics and toxicity) of xenobiotics with age. Age-related changes in metabolic capacity are critical for understanding and predicting the potential differences resulting from exposure. Such information may be especially useful in the evaluation of the risk of exposure to very low (µg/kg/day or ng/kg/day) levels of environmental chemicals. This review is to better understand the ontogeny of metabolizing enzymes in converting chemicals to either less-toxic metabolite(s) or more toxic products (e.g. reactive intermediate[s]) during stages before birth and during early development (neonate/infant/child). In this review, we evaluated the ontogeny of major "phase I" and "phase II" metabolizing enzymes in humans and commonly used experimental animals (e.g. mouse, rat, and others) in order to fill the information gap.

The ontogeny of drug metabolism enzymes and implications for adverse drug events

Profound changes in drug metabolizing enzyme (DME) expression occurs during development that impacts the risk of adverse drug events in the fetus and child. A review of our current knowledge suggests individual hepatic DME ontogeny can be categorized into one of three groups. Some enzymes, e.g., CYP3A7, are expressed at their highest level during the first trimester and either remain at high concentrations or decrease during gestation, but are silenced or expressed at low levels within one to two years after birth. SULT1A1 is an example of the second group of DME. These enzymes are expressed at relatively constant levels throughout gestation and minimal changes are observed postnatally. ADH1C is typical of the third DME group that are not expressed or are expressed at low levels in the fetus, usually during the second or third trimester. Substantial increases in enzyme levels are observed within the first one to two years after birth. Combined with our knowledge of other physiological factors during early life stages, knowledge regarding DME ontogeny has permitted the development of robust physiological based pharmacokinetic models and an improved capability to predict drug disposition in pediatric patients. This review will provide an overview of DME developmental expression patterns and discuss some implications of the data with regards to drug therapy. Common themes emerging from our current knowledge also will be discussed. Finally, the review will highlight gaps in knowledge that will be important to advance this field.

Cytochrome P450 and GlutathioneS-Transferase mRNA Expression in Human Fetal Liver Hematopoietic Stem Cells

During fetal development, the liver serves as the primary hematopoietic organ in which hematopoietic stem cells (HSC) capable of initiating long-term hematopoiesis comprise a large proportion of the hepatic cell population. Although HSC are potential targets for transplacental chemicals, little is known regarding their xenobiotic biotransformation ability. We quantitated the steady-state mRNA expression of six cytochrome P450 (P450) and 11 glutathione S-transferase (GST) isoforms in CD34(+)-selected HSC isolated from second trimester human fetal liver donors, genotyped donors for polymorphic hGSTM1 and hGSTT1 status, and analyzed gene expression in HSC relative to total liver from donors of similar gestational ages. Several P450 isoforms, including CYP1A1, CYP2E1, CYP3A4, and CYP3A5, were expressed at low levels in HSC (relative mRNA expression CYP3A5 > CYP1A1 > CYP2E1 > CYP3A4). CYP1A2 and CYP3A7 were not detected in HSC. The CYP3A4/5 mRNA expression in HSC was accompanied by detectable CYP3A protein and low midazolam oxidation activity. Several GST isoforms, including hGSTM1, hGSTM2, hGSTM4, and hGSTP1, were significantly higher in HSC as compared with total fetal liver. With the exception of hGSTA4, alpha class GST were not detected in HSC. GST expression in HSC was accompanied by substantial GST catalytic activity toward 1-chloro-2,4-dinitrobenzene. In summary, our data indicate that fetal liver CD34(+)-derived HSC constitutively express several P450 isoforms at low levels relative to total hepatic cell populations but have a higher capacity for GST conjugation reactions through mu and pi class isoforms. The functional ramifications of these observations are discussed relative to the sensitivity of human fetal HSC to transplacental chemical injury.

Xenobiotic-Metabolizing Genes and Small-for-Gestational-Age Births

BACKGROUND

Little is known about the role of xenobiotic-metabolizing gene variants as risk factors for small-for-gestational-age (SGA) births or as modifiers for the effects of exposures such as maternal smoking.

METHODS

We conducted 2 joint studies: a case-control design including 493 cases (birth weight below the 10th percentile according to gestational age and sex) and 472 controls (at or above the 10th percentile) and a family-based study (mother, father, and newborn) with approximately 250 case trios and a similar number of control trios. Logistic regression and a log-linear model were used to analyze the association between genetic variants such as CYP1A1*2A, CYP1A1*2B, CYP1A1*4, GSTT1, GSTM1, and XRCC3 and SGA. The interaction between genetic variants and maternal smoking was also studied.

RESULTS

The odds ratio (OR) for the association of complete maternal GSTT1 deletion with SGA was 0.63 (95% confidence interval = 0.41-0.97), and that for the complete newborn GSTM1 deletion was 0.74 (0.55-0.98). Newborns with the partial GSTT1 deletion had an OR of 1.40 (1.01-1.95), and newborns homozygous for CYP1A1*2A had an OR of 4.28 (1.02-18.0). These results were coherent with the trio-based results. Significant interactions were observed between maternal smoking in the third trimester and CYP1A1*2A (P = 0.03), XRCC3 (P = 0.03), and newborn GSTT1 (P = 0.01).

CONCLUSIONS

Certain genetic variants involved in the metabolism of xenobiotics increase the risk of SGA, as well as modify the effects of maternal smoking by increasing or decreasing its risk.

Androgen metabolism in thymus of fetal and adult rats

Cytochrome P450 (P450) monooxygenases play a role in target tissue metabolic activation of xenobiotics and/or endogenous compounds, such as vasoactive molecules or hormones. Indeed, tissue-specific metabolism of steroids is important in a variety of organs, including thymus, and may alter tissue-specific functions. Steroids have been shown to regulate thymus growth and function, but surprisingly little is known about expression of the responsible enzyme systems in thymus tissue, nor is the thymus-specific biotransformation of testosterone known. We therefore investigated gene and protein expression, total protein content, and enzyme activity of major P450 isoforms and other key steroid-metabolizing enzymes in thymus tissue of adult and fetal rats. We detected 6 beta-hydroxytestosterone (HT), 7 alpha-HT, 16 alpha-HT, 2 alpha-HT, and androstenedione to be major testosterone metabolites in the adult thymus. The high production of 7 alpha-HT and 16 alpha-HT correlated well with the gene and protein expression of CYP2A1/2 and CYP2B1/2 in thymus of adult animals. When compared with fetal thymic tissue, CYP2A1/2, 17beta-hydroxysteroid dehydrogenase isoform 1 (17 beta-HSDH1) and the androgen receptor were 8-, 3-, and 3-fold more highly expressed in adult rats, whereas 17 beta-HSDH2, 17 beta-HSDH3, and 5 alpha-reductase were reduced to 12%, 0%, and 32% of those in fetal thymus. In conclusion, we demonstrated that rat thymus expresses a variety of cytochrome P450 monooxygenases and other steroid-metabolizing enzymes, and it successfully metabolizes testosterone. Changes of the underlying steroid-metabolizing enzyme systems may aid in understanding the role of androgens in altering biological functions of the thymus.

Ontogeny of drug metabolizing enzymes in the neonate

Fetal exposure to xenobiotics is modulated to a considerable degree by the metabolic capabilities of the mother and the placenta. However, once liberated from the uterine environment the neonate is instantly exposed to a wide array of new macromolecules in the form of byproducts of cellular metabolism, dietary constituents, environmental toxins and pharmacologic agents. The rapid and efficient biotransformation of these compounds by Phase I and Phase II drug-metabolizing enzymes is an essential process if the infant is to avoid the accumulation of reactive compounds that could produce cellular injury or tissue dysfunction. Genetic polymorphisms and environmental factors are known to contribute dramatically to individual variation in the activity of drug-metabolizing enzymes. More recently, it has become apparent that programmed, developmental, regulatory events occur - independent of genotype - which further add to individual variation in drug metabolism. An appreciation of the impact of ontogeny on the expression and functional activity of the major drug-metabolizing enzymes enables the practicing clinician to predict the ultimate consequence of drug administration in the neonate to help guide optimal drug therapy.

Expression of cytochrome P450 1A1 and its contribution to oxidation of a potential human carcinogen 1-phenylazo-2-naphthol (Sudan I) in human livers

Cytochrome P450 1A1 (CYP1A1) is one of the most important enzymes implicated in the metabolic activation of carcinogens. To date, there is still conflicting evidence for the expression of enzymatically functional CYP1A1 in human liver. In the present work, we clearly demonstrate that CYP1A1 capable of metabolizing a carcinogen 1-phenylazo-2-naphthol (Sudan I) is expressed in livers of eight American Caucasian donors. Using two independent methods (immunoblotting and N-terminal sequencing), CYP1A1 protein was detected and quantified in all human hepatic microsomes tested in the study. Its levels, ranging from 0.97 to 3.0 pmol/mg protein, correlated with activities catalyzed by this enzyme [7-ethoxyresorufin O-deethylation (EROD) and oxidation of Sudan I], indicating the presence of enzymatically active CYP1A1. Even though levels of CYP1A1 expression are low, <0.7% of total hepatic CYP, the CYP1A1 contribution to oxidation of carcinogenic Sudan I in the test set of human liver microsomes ranges from 12 to 30%.

The murine Cyp1a1 gene is expressed in a restricted spatial and temporal pattern during embryonic development

In adult mice the cytochrome P450 Cyp1a1 gene is not constitutively expressed but is highly inducible by foreign compounds acting through the aryl hydrocarbon (Ah) receptor. However, the expression profile of the Cyp1a1 gene in the developing embryo is not well under-stood. Using established transgenic mouse lines where 8.5 kb of the rat CYP1A1 promoter is cloned upstream of the lacZ reporter gene (1), we describe the expression of the CYP1A1-driven reporter gene in all tissues through-out stages E7-E14 of embryonic development. In contrast to the absence of constitutive Cyp1a1 and lacZ transgene expression in tissues of the adult mouse, a constitutive cell-specific and time-dependent pattern of CYP1A1 promoter activity was observed in the embryo. This expression pattern was confirmed as reflecting the endogenous gene by measuring Cyp1a1 mRNA levels and protein expression by immunohistochemistry. The number of cells displaying endogenous CYP1A1 activity could be increased in the embryo upon xenobiotic challenge, but only within areas where the CYP1A1 promotor was already active. When reporter mice were bred onto a genetic background expressing a lower affinity form of the Ah receptor (DBA allele), transgene and murine Cyp1a1 protein expression were both attenuated in the adult mouse liver upon xenobiotic challenge. By comparison, constitutive CYP1A1 promoter activity in the embryo was identical in the presence of either the high or low affinity Ah receptor. These novel data suggest that the Cyp1a1 protein may play a role in murine development and that regulation of the Cyp1a1 gene during this period is either through the action of a high affinity Ah receptor ligand or by an alternative regulatory pathway.

Embryonic turkey liver: activities of biotransformation enzymes and activation of DNA-reactive carcinogens

Avian embryos are a potential alternative model for chemical toxicity and carcinogenicity research. Because the toxic and carcinogenic effects of some chemicals depend on bioactivation, activities of biotransformation enzymes and formation of DNA adducts in embryonic turkey liver were examined. Biochemical analyses of 22-day in ovo turkey liver post-mitochondrial fractions revealed activities of the biotransformation enzymes 7-ethoxycoumarin de-ethylase (ECOD), 7-ethoxyresorufin de-ethylase (EROD), aldrin epoxidase (ALD), epoxide hydrolase (EH), glutathione S-transferase (GST), and UDP-glucuronyltransferase (GLUT). Following the administration of phenobarbital (24 mg/egg) on day 21, enzyme activities of ECOD, EROD, ALD, EH and GLUT, but not of GST, were increased by two-fold or higher levels by day 22. In contrast, acute administration of 3-methylcholanthrene (5 mg/egg) induced only ECOD and EROD activities. Bioactivation of structurally diverse pro-carcinogens was also examined using (32)P-postlabeling for DNA adducts. In ovo exposure of turkey embryos on day 20 of gestation to 2-acetylaminofluorene (AAF), 4,4'-methylenebis(2-chloroaniline) (MOCA), benzo[a]pyrene (BaP), and 2-amino-3,8-dimethylimidazo[4,5- f]quinoxaline (MeIQx) resulted in the formation of DNA adducts in livers collected by day 21. Some of the DNA adducts had (32)P-postlabeling chromatographic migration patterns similar to DNA adducts found in livers from Fischer F344 rats exposed to the same pro-carcinogens. We conclude that 21-day embryonic turkey liver is capable of chemical biotransformation and activation of genotoxic carcinogens to form DNA adducts. Thus, turkey embryos could be utilized to investigate potential chemical toxicity and carcinogenicity.

Ontogeny of hepatic microsomal 3-hydroxylation of quinine in Adélie Penguins

Ontogeny of hepatic cytochrome P450 (CYP) content and of quinine 3-hydroxylation, a biomarker of human CYP3A activity, was investigated in Adélie Penguins (Pygoscelis adeliae) by comparing liver microsomes from chicks aged 13-28 days (n=10) with those from adults. The total CYP content in chick microsomes was significantly lower than in adults and correlated with the age of the chicks (r=0.894, P<0.001). Kinetic parameters (mean+/-S.D.) for quinine 3-hydroxylation in chick microsomes were lower than the corresponding values in adult penguins (Km, 122+/-27 vs. 160+/-73 microM; Vmax, 119+/-35 vs. 160+/-72 pmol/min/mg protein) but the differences were not significant, and neither Km nor Vmax correlated with age of the chicks. The pattern of inhibition of quinine 3-hydroxylation by specific human CYP inhibitors suggests that the CYP enzyme mediating quinine 3-hydroxylation in penguin chicks is similar to human CYP3A, but is a different isoform from that in adult penguins. The results imply that Adélie Penguin chicks may have a different susceptibility to environmental pollutants from adult penguins.

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.

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.

The ontogeny of human drug-metabolizing enzymes: phase I oxidative enzymes

Although some patterns are beginning to emerge, our knowledge of human phase I drug-metabolizing enzyme developmental expression remains far from complete. Expression has been observed as early as organogenesis, but this appears restricted to a few enzymes. At least two of the enzyme families that are expressed in the fetal liver exhibit a temporal switch in the immediate perinatal period (e.g., CYP3A7 to CYP3A4/3A5 and FMO1 to FMO3), whereas others show a progressive change in isoform expression through gestation (e.g., the class I alcohol dehydrogenases). Many of the phase I drug-metabolizing enzyme exhibit dynamic perinatal expression changes that are regulated primarily by mechanisms linked to birth and secondarily to maturity. A few of these enzymes are not detectable until well after birth, suggesting that birth is necessary but not sufficient for the onset of expression (e.g., CYP1A2). Tissue-specific expression adds to the complexity during ontogeny. For example, CYP3A7 expression is restricted to the fetal liver. However, with few exceptions, complete temporal relationship information during development is not known. Furthermore, most studies have concentrated on hepatic expression and much less is known about extrahepatic developmental events.

Pharmacogenetics and pharmacogenomics

This article introduces pharmacogenetics and pharmacogenomics in the context of pharmacotherapy in the pediatric ICU setting. As an independent discipline (if it can be considered as such), pediatric or developmental pharmacogenetics is essentially at a neonatal stage. Available pharmacokinetic data derived from studies of drugs that are largely dependent on a single CYP pathway for their elimination provide initial assessments of the developmental profile of that particular CYP isoform. Essentially then, pharmacogenetics in a pediatric context refers to the changes in phenotype that occur as a child grows and develops. Furthermore, the apparent drug biotransformation "phenotype" may be influenced by disease (infection), environmental factors (diet and environmental contaminants) and concurrent medications; however, drug response is a function of the complex interplay among genes involved in drug transport, drug biotransformation, receptors, and signal transduction processes, among others. Therefore, optimization of pediatric pharmacotherapy necessarily requires that developmental changes in each of these areas and not just drug biotransformation be investigated thoroughly before the promise of pharmacogenetics and pharmacogenomics for rational therapeutics can be realized in children.

Cytochrome-P450-dependent biotransformation of xenobiotics in human and rodent embryonic tissues

Profound species differences and developmental stage differences as well as a lack of solid data prevent broad, sweeping generalizations in terms of statements that can be made concerning the prenatal expression of individual P450 isoforms. It is clear, however, that several of such isoforms are expressed at levels that can be toxicologically significant. At present, the greatest interest appears to be in P450s 1A1, 1B1, 2E1, and 3A7, each of which has been reported to be expressed at toxicologically significant levels or at least at potentially toxicologically significant levels during organogenesis. Reports of the expression of other P450 isoforms at later stages of gestation also have appeared in the recent literature.

Developmental expression of cytochrome P450 enzymes in human liver

Drug-metabolizing cytochrome P450 enzymes, the major phase I enzymes, are active in human liver already at very early stages of intrauterine development, although presumably at fairly low concentrations and in low numbers. During maturation, these enzymes go through various developmental programmes towards adulthood. The major increase both in abundance as well as in number of different enzymes takes place after birth, probably during the first year of life. Detailed information concerning these developmental changes is still limited. The major drug-metabolizing P450 enzymes appear to be primarily members of the CYP3A subfamily in all stages of development. The balance between different members of this subfamily, however, undergoes significant switches from the foetal predominant CYP3A7 to the major adult form CYP3A4. The ontogeny of the other cytochrome P450 enzymes is less well characterized, but the major switch-on appears to occur mainly after birth. Developmental expression of P450 enzymes is one of the key factors determining the pharmacokinetic status of developing individuals both pre- and postnatally.

Xenobiotic-metabolizing cytochrome P450 enzymes in the human feto-placental unit: role in intrauterine toxicity

Practically all lipid-soluble xenobiotics enter the conceptus through placental transfer. Many xenobiotics, including a number of clinically used drugs, are known to cause unwanted effects in the embryo or fetus, including in utero death, initiation of birth defects, and production of functional abnormalities. It is well established that numerous xenobiotics are not necessarily toxic as such, but are enzymatically transformed in the body to reactive and toxic intermediates. The cytochrome P450 (CYP) enzymes are known to catalyze oxidative metabolism of a vast number of compounds, including many proteratogens, procarcinogens, and promutagens. About 20 xenobiotic-metabolizing CYP forms are known to exist in humans. Most of these forms are most abundant in the liver, but examples of exclusively extrahepatic CYP forms also exist. Unlike rodents, the liver of the human fetus and even embryo possesses relatively well-developed metabolism of xenobiotics. There is experimental evidence for the presence of CYP1A1, CYP1B1, CYP2C8, CYP2D6, CYP2E1, CYP3A4, CYP3A5, and CYP3A7 in the fetal liver after the embryonic phase (after 8 to 9 weeks of gestation). Significant xenobiotic metabolism occurs also during organogenesis (before 8 weeks of gestation). Also, some fetal extrahepatic tissues, most notably the adrenal, contain substantial levels of CYP enzymes. The full-term human placenta is devoid of many CYP activities present in liver. Placental CYP1A1 is highly inducible by maternal cigarette smoking. Other forms present in full-term placenta include CYP4B1 and CYP19 (steroid aromatase), which also contribute to the oxidation of some xenobiotics. At earlier stages of pregnancy, the placenta may express a wider array of CYP genes, including CYP2C, CYP2D6, and CYP3A7. Due to the small size of the fetus and low abundance of CYPs in placenta, the contribution of feto-placental metabolism to overall gestational pharmacokinetics of drugs is probably minor. In contrast, several toxic outcomes have been ascribed to altered metabolic patterns in the feto-placental unit, including a putative association between reduced placental oxidative capacity and birth defects. Examples of human teratogens that are substrates for CYP enzymes include thalidomide, phenytoin, ethanol, and several hormonal agents. Recent studies have improved our understanding of the expression and regulation of individual CYP genes in the fetus and placenta, and the stage is set for applying this knowledge with more precision to the role of xenobiotic metabolism in abnormal intrauterine development in humans.

Pulmonary cytochrome P450 monooxygenase and Clara cell differentiation in mice

Various studies indicate that cytodifferentiation of Clara cells and development of pulmonary cytochrome P450 (CYP) monooxygenases occur postnatally. The timing of these events is species-specific. Neonatal mice are more susceptible than adult mice are to Clara cell injury by naphthalene, but little is known about the postnatal development of Clara cells and CYP in mice. This study was designed to determine the developmental pattern of Clara cell differentiation and CYP expression in mice. Lungs from mice aged 16 days gestation to 63 days postnatal (DPN) were studied. Clara cell secretory protein (CC10) expression in nonciliated cells was detected earlier in proximal airways than in distal airways, but reached adult levels at 14 DPN in all airway levels. Cilia-associated tubulin expression closely followed the onset of CC10 expression, as did expression of CYP reductase. CYP2B protein expression appeared and differentiated earlier in bronchi than in bronchioles and reached adult levels at 14 and 28 DPN, respectively. CYP2F2 expression appeared earlier in proximal airways, but did not reach adult levels of expression until after 28 DPN. CYP activity, measured by naphthalene metabolism, increased with age and corresponded to CYP2F2 protein expression. We conclude that in the mouse, (1) Clara cell maturation is a postnatal event, (2) Clara cell differentiation is complete at the same age in proximal and distal airways, (3) CYP reductase protein expression occurs at the same time as CC10 expression, but CYP2B and CYP2F2 lag behind, and (4) stereoselective naphthalene monooxygenase activity corresponds with CYP2F2 protein expression.

Induction of the rat hepatic cytosolic arylamine oxidase by a series of polychlorinated biphenyls: association with the Ah locus

The objective of the present study was to identify the polychlorinated biphenyl congeners of Aroclor 1254 that are responsible for the induction of the cytosolic bioactivation of aromatic amines. Various chlorobiphenyls, ranging from di- to hexa-substituted, were administered to rats and the ability of the hepatic cytosol to bioactivate 2-aminoanthracene and 2-aminofluorene was investigated. These studies revealed that the induction of the cytosolic activation of aromatic amines increased with the increasing extent of chlorination; moreover, planar congeners were more effective inducers of this activity compared to their non-planar isomers. This observation prompted us to investigate whether the cytosolic activation of aromatic amines is associated with the Ah receptor. Treatment of mice with Aroclor 1254 stimulated the cytosolic activation of aromatic amines in C57BL6 mice, an Ah-responsive strain, whereas it had no effect in DBA2 mice, a non-responsive strain. These findings indicate that the bioactivation of aromatic amines by the liver cytosol is linked to the Ah receptor.

Pharmacogenetics in pediatrics. Implications for practice

Cumulative experience with pharmacotherapy in children indicates that it is difficult to prescribe medications rationally solely on the basis of patient age. Furthermore, the apparent drug biotransformation phenotype may be influenced by disease (e.g., infection), environmental factors (e.g., diet and environmental contaminants), and concurrent medications. Therefore, characterization of drug biotransformation pathways during development and, at a given developmental stage, the effects of known modulators of drug biotransformation are essential for optimum treatment. This is particularly true when one considers that altered drug biotransformation may contribute significantly to therapeutic failure (e.g., graft rejection with inadequate serum and tissue concentrations of cyclosporin and myelotoxicity consequent to a relative inability to metabolize normal doses of certain antineoplastic agents). Accordingly, the goals of coordinated clinical and basic investigations should be to characterize important drug biotransformation pathways for compounds under development and intended for use in pediatrics and to identify the population extremes or "outliers" to aid in selection of an appropriate dosage range for efficacy studies. Acquired knowledge should then be incorporated into the drug-design process to further maximize the efficacy-toxicity ratio. The development of acceptable, preferably noninvasive, phenotyping procedures for all age ranges including neonates, infants, and older children is a major challenge for investigators but, if met, will be rewarded with improved pediatric pharmacotherapy.

Biotransformation of all-trans-retinol and all-trans-retinal to all-trans-retinoic acid in rat conceptal homogenates

Catalysis of the oxidation of all-trans-retinol (vitamin A1) or of all-trans-retinal to all-trans-retinoic acid (all-trans-RA) by rat conceptal enzymes was investigated during organogenesis. Products of the reaction were identified and quantified with HPLC by comparing their elution times with those of authentic standard retinoids. Under the incubation and assay conditions utilized, all-trans-retinol and all-trans-retinal were converted to readily detectable quantities of all-trans-RA. Rat conceptal homogenates from gestational days 10.5, 11.5 and 12.5 each exhibited enzymatic activity for oxidation of all-trans-retinol and all-trans-retinal to all-trans-RA. Enzymatic catalysis was verified by showing that: (1) both reactions were coenzyme dependent; (2) the rates of reactions increased as concentrations of conceptal protein increased; (3) both reactions were abolished by heating the tissue homogenates (100 degrees, 5 min); and (4) both reactions exhibited substrate saturation. Under the same experimental conditions, formation of all-trans-RA from all-trans-retinol was much slower than from all-trans-retinal, suggesting that oxidation of all-trans-retinol to all-trans-retinal was the rate-limiting step for biotransformation of all-trans-retinol to all-trans-RA in embryonic tissues. When NAD or NADP were replaced by NADH or NADPH, the rate of oxidation of all-trans-retinol was reduced markedly, indicating that the reaction was catalyzed primarily by an NAD/NADP-dependent dehydrogenase(s). Carbon monoxide (CO:O2 = 90:10) did not inhibit the reaction. NAD appeared to be a more effective cofactor than NADP in catalyzing oxidation of all-trans-retinal to all-trans-RA. When NAD was omitted, formation of all-trans-RA from all-trans-retinal was reduced by approximately 55%. Replacing NAD by NADH or NADPH also reduced the conversion of all-trans-retinal to all-trans-RA by about 60%. These observations suggested at least two pathways for the generation of all-trans-RA from all-trans-retinal in embryos: oxidation catalyzed by an NAD/NADP-dependent dehydrogenase(s) and oxidation catalyzed by an oxidase(s) that did not require NAD, NADH, NADP or NADPH. Conversion of all-trans-retinol to all-trans-RA was inhibited strongly by low concentrations of citral, but not by high concentrations of sodium azide, 4-methylpyrazole, or metyrapone. Similarly, oxidation of all-trans-retinal was inhibited strongly by citral but not by metyrapone.(ABSTRACT TRUNCATED AT 400 WORDS)