Evidence of impaired cisapride metabolism in neonates

Integrated Isogenic Human Induced Pluripotent Stem Cell-Based Liver and Heart Microphysiological Systems Predict Unsafe Drug-Drug Interaction

Three-dimensional (3D) microphysiological systems (MPSs) mimicking human organ function in vitro are an emerging alternative to conventional monolayer cell culture and animal models for drug development. Human induced pluripotent stem cells (hiPSCs) have the potential to capture the diversity of human genetics and provide an unlimited supply of cells. Combining hiPSCs with microfluidics technology in MPSs offers new perspectives for drug development. Here, the integration of a newly developed liver MPS with a cardiac MPS—both created with the same hiPSC line—to study drug–drug interaction (DDI) is reported. As a prominent example of clinically relevant DDI, the interaction of the arrhythmogenic gastroprokinetic cisapride with the fungicide ketoconazole was investigated. As seen in patients, metabolic conversion of cisapride to non-arrhythmogenic norcisapride in the liver MPS by the cytochrome P450 enzyme CYP3A4 was inhibited by ketoconazole, leading to arrhythmia in the cardiac MPS. These results establish integration of hiPSC-based liver and cardiac MPSs to facilitate screening for DDI, and thus drug efficacy and toxicity, isogenic in the same genetic background.

Linking the Epigenome with Exposure Effects and Susceptibility: The Epigenetic Seed and Soil Model

The epigenome is a dynamic mediator of gene expression that shapes the way that cells, tissues, and organisms respond to their environment. Initial studies in the emerging field of "toxicoepigenetics" have described either the impact of an environmental exposure on the epigenome or the association of epigenetic signatures with the onset or progression of disease; however, the majority of these pioneering studies examined the relationship between discrete epigenetic modifications and the effects of a single environmental factor. Although these data provide critical blocks with which we construct our understanding of the role of the epigenome in susceptibility and disease, they are akin to individual letters in a complex alphabet that is used to compose the language of the epigenome. Advancing the use of epigenetic data to gain a more comprehensive understanding of the mechanisms underlying exposure effects, identify susceptible populations, and inform the next generation risk assessment depends on our ability to integrate these data in a way that accounts for their cumulative impact on gene regulation. Here we will review current examples demonstrating associations between the epigenetic impacts of intrinsic factors, such as such as age, genetics, and sex, and environmental exposures shape the epigenome and susceptibility to exposure effects and disease. We will also demonstrate how the "epigenetic seed and soil" model can be used as a conceptual framework to explain how epigenetic states are shaped by the cumulative impacts of intrinsic and extrinsic factors and how these in turn determine how an individual responds to subsequent exposure to environmental stressors.

Drug-induced liver injury in children

PURPOSE OF REVIEW

Drug-induced liver injury (DILI) is an underrecognized cause of pediatric liver disease, accounts for almost 20% of pediatric acute liver failure cases, and is a major reason for liver transplantation in the USA. This article reviews the pathogenesis of DILI, approach to diagnosis and management, and highlights recent pediatric DILI case series.

RECENT FINDINGS

Select individuals have an increased propensity to develop DILI. Known genetic polymorphisms of enzymes and host factors play an important role in medication management and influence the clinical outcome in 20-25% of all drug therapies. Children are more likely to have mitochondrial dysfunction from drugs, increasing their susceptibility to severe liver injury or acute liver failure. Antibiotics and central nervous system agents account for the majority of pediatric DILI in the West, although herbals are becoming more common.

SUMMARY

Clinical features of DILI vary and overlap so exclusion of other conditions, identification of latency period and risk factors, and use of a searchable database can aid evaluation. Treatment consists of cessation of the offending agent and supportive care. Areas needing further research include elucidating mechanisms, identifying at risk individuals, and therapeutic interventions.

Overlapping between CYP3A4 and CYP3A7 expression in the fetal human liver during development

OBJECTIVE

The cytochrome P450 (CYP450) superfamily is implicated in important life processes, including metabolism of many molecules. CYP3A account for the largest portion of CYP450 proteins in human, including CYP3A4, CYP3A5 and CYP3A7. The purpose of this study was to investigate the immunohistochemical expression of CYP3A4 and CYP3A7 in human liver at different post-conceptional (PC) ages.

METHODS

Human liver samples from 30 fetuses and newborns were, clustered according with the PC age, routinely processed for immunohistochemical analysis of CYP3A4 and CYP3A7.

RESULTS

CYP3A4 was positive in all but two cases, CYP3A7 was positive in all but one case, which was negative also for CYP3A4.

CONCLUSIONS

Our data on immunohistochemical detection of CYP3A4 and CYP3A7 during development show that CYP3A4 expression is not restricted to the post-natal age, being the immunostaining for both CYP3A4 and CYP3A7 identical after 25 weeks of PC age, thus the relationship between these CYP450 isoforms should be considered much more complex than previous thought. A high interindividual variability was observed among subjects at all gestational age. The variable CYP3A expression suggests the existence of a marked interindividual variability in drug metabolism during the intrauterine life and in perinatal period.

Cytochrome P450 genetic polymorphism in neonatal drug metabolism: role and practical consequences towards a new drug culture in neonatology

The cytochrome P450 superfamily (CYP450) in humans is formed by 57 functional monooxygenases critical for the metabolism of numerous endogenous and exogenous compounds. The superfamily is organized into 18 families and 44 subfamilies. CYP nomenclature is based on the identity of amino acids. The most important functions of the CYP450 are related to metabolism of endogenous compounds, detoxification of exogenous xenobiotics and decomposition of the vast majority of currently used drugs. The expression of CYP450 enzymes in the human body is characterized by a marked substrate and tissue specificity, the most important being localized in the liver, but also present in kidney, lung, brain, breast, prostate and in the small intestine. The human cytochrome P450 3A gene family (CYP3A) accounts for the largest portion of CYP450 proteins in human liver and includes 4 genes: CYP3A4, CYP3A5, CYP3A7, CYP3A43. Multiple and complex genetic variations, marked interindividual, interethnic and gender variability have been reported regarding CYP3A isoform expression and activity. Multiple factors may affect CYP3A expression and activity, such as inducers like rifampicin, phenobarbital, 3-methylcholantrene, beta-naphtoflavone, and dexamethasone. The maturation of organ systems, paralleled by ontogeny of drug-metabolizing enzymes during fetal life and in the first months of postnatal life, surely exerts profound effects on drug disposition, probably being the predominant factor accounting for age-associated changes in drug clearance. In fact, drug dosage in the perinatal period represents a continuous challenge for neonatologists. The purpose of this article is to provide a brief review of the pharmacokinetic differences between neonates and adults, showing the peculiarities of liver CYP450-related drug metabolism in the perinatal period and at birth, and to report the toxic mechanisms of liver injury in neonates, due to the most frequently utilized drugs in NICU centers.

Pharmacogenomics of cardiovascular drugs and adverse effects in pediatrics

Individual response to medication is highly variable. For many drugs, a substantial proportion of patients show suboptimal response at standard doses, whereas others experience adverse drug reactions (ADRs). Pharmacogenomics aims to identify genetic factors underlying this variability in drug response, providing solutions to improve drug efficacy and safety. We review recent advances in pharmacogenomics of cardiovascular drugs and cardiovascular ADRs, including warfarin, clopidogrel, β-blockers, renin-angiotensin-aldosterone system inhibitors, drug-induced long QT syndrome, and anthracycline-induced cardiotoxicity. We particularly focus on the applicability of pharmacogenomic findings to pediatric patients in whom developmental changes in body size and organ function may affect drug pharmacokinetics and pharmacodynamics. Solid evidence supports the importance of gene variants in CYP2C9 and VKORC1 for warfarin dosing and in CYP2C19 for clopidogrel response in adult patients. For the other cardiovascular drugs or cardiovascular ADRs, further studies are needed to replicate or clarify genetic associations before considering uptake of pharmacogenetic testing in clinical practice. With the exception of warfarin and anthracycline-induced cardiotoxicity, there is lack of pharmacogenomic studies on cardiovascular drug response or ADRs aimed specifically at children or adolescents. The first pediatric warfarin pharmacogenomic study indeed indicates differences from adults, pointing out the importance and need for pediatric-focused pharmacogenomic studies.

Systematic review: cardiovascular safety profile of 5-HT(4) agonists developed for gastrointestinal disorders

BACKGROUND

The nonselective 5-HT(4) receptor agonists, cisapride and tegaserod have been associated with cardiovascular adverse events (AEs).

AIM

To perform a systematic review of the safety profile, particularly cardiovascular, of 5-HT(4) agonists developed for gastrointestinal disorders, and a nonsystematic summary of their pharmacology and clinical efficacy.

METHODS

Articles reporting data on cisapride, clebopride, prucalopride, mosapride, renzapride, tegaserod, TD-5108 (velusetrag) and ATI-7505 (naronapride) were identified through a systematic search of the Cochrane Library, Medline, Embase and Toxfile. Abstracts from UEGW 2006-2008 and DDW 2008-2010 were searched for these drug names, and pharmaceutical companies approached to provide unpublished data.

RESULTS

Retrieved articles on pharmacokinetics, human pharmacodynamics and clinical data with these 5-HT(4) agonists, are reviewed and summarised nonsystematically. Articles relating to cardiac safety and tolerability of these agents, including any relevant case reports, are reported systematically. Two nonselective 5-HT(4) agonists had reports of cardiovascular AEs: cisapride (QT prolongation) and tegaserod (ischaemia). Interactions with, respectively, the hERG cardiac potassium channel and 5-HT(1) receptor subtypes have been suggested to account for these effects. No cardiovascular safety concerns were reported for the newer, selective 5-HT(4) agonists prucalopride, velusetrag, naronapride, or for nonselective 5-HT(4) agonists with no hERG or 5-HT(1) affinity (renzapride, clebopride, mosapride).

CONCLUSIONS

5-HT(4) agonists for GI disorders differ in chemical structure and selectivity for 5-HT(4) receptors. Selectivity for 5-HT(4) over non-5-HT(4) receptors may influence the agent's safety and overall risk-benefit profile. Based on available evidence, highly selective 5-HT(4) agonists may offer improved safety to treat patients with impaired GI motility.

RNA sequencing reveals dynamic changes of mRNA abundance of cytochromes P450 and their alternative transcripts during mouse liver development

Cytochromes P450 (P450s) are a superfamily of enzymes that have critical functions in liver to catalyze the biotransformation of numerous drugs. However, the functions of most P450s are not mature at birth, which can markedly affect the metabolism of drugs in newborns. Therefore, characterization of the developmental profiles and regulatory mechanisms of P450 expression is needed for more rational drug therapy of pediatric patients. An animal model is indispensable for studying the mechanisms of postnatal development of the P450s. Hence we used RNA sequencing (RNA-Seq) to provide a "true quantification" of mRNA expression of all P450s in mouse liver during development. Liver samples of male C57BL/6 mice at 12 different ages from prenatal to adulthood were used. Total mRNAs of the 103 mouse P450s displayed two rapid increasing stages after birth, reflecting critical functional transition of liver during development. Four ontogenic expression patterns were identified among the 71 significantly expressed P450s, which categorized genes into neonatal-, adolescent-, adolescent/adult-, and adult-enriched groups. The 10 most highly expressed subfamilies of mouse P450s in livers of adult mice were CYP2E, -2C, -2D, -3A, -4A, -2F, -2A, -1A, -4F, and -2B, which showed diverse expression profiles during development. The expression patterns of multiple members within a P450 subfamily were often classified to different groups. RNA-Seq also enabled the quantification of known transcript variants of CYP2C44, CYP2C50, CYP2D22, CYP3A25, and CYP26B1 and identification of novel transcripts for CYP2B10, CYP2D26, and CYP3A13. In conclusion, this study reveals the mRNA abundance of all the P450s in mouse liver during development and provides a foundation for mechanistic studies in the future.

Expression of cytochromes p450 in fetal, infant, and juvenile liver of cynomolgus macaques

Preclinical data of fetal, infant, and juvenile animals are important for the prediction of drug toxicity in fetuses and children. However, expression of drug-metabolizing enzymes, including cytochromes P450 (CYPs), have not been fully investigated in fetal, infant, or juvenile liver of the cynomolgus macaque, an animal species important for preclinical studies. In this study, hepatic expression of 20 cynomolgus macaque CYPs (mfCYPs) in the CYP1-4 subfamilies that are relevant to drug metabolism was measured in fetuses, infants, and juveniles using DNA microarrays. Expression of most mfCYPs, including those moderately or abundantly expressed in postnatal livers such as mfCYP2A23, mfCYP2A24, mfCYP2B6, mfCYP2C9, mfCYP2C19, mfCYP2C76, mfCYP2D17, mfCYP2E1 mfCYP3A4, and mfCYP3A5, was much less abundant in fetal livers, but increased substantially after birth. In contrast, expression of mfCYP2C8 in fetal livers was not substantially different from postnatal livers. Since human CYP3A7 is expressed more abundantly in fetal livers than in adult livers, mfCYP3A7, an ortholog of human CYP3A7, was analyzed by quantitative polymerase chain reaction. Expression of mfCYP3A7 in fetal livers was much lower than that in postnatal livers, and greatly increased after birth, unlike the expression of human CYP3A7. These results indicate that expression of most mfCYPs examined was low in fetal livers, but increased greatly in postnatal livers, with a few exceptions such as mfCYP2C8.

Cisapride: what can we learn from the rise and fall of a prokinetic?

Cisapride, the prototype serotonergic agent, evolved from a body of research that defined the key roles of serotonergic receptors in gastrointestinal motor and sensory function. Impressed by its in vitro properties and encouraged by clinical trial data, cisapride became the drug of choice for the treatment of a wide range of motility disorders and clinicians appeared impressed by its efficacy and comfortable with its side-effect profile. Once serious cardiac events began to be reported in association with cisapride therapy, dark clouds rapidly gathered and soon enveloped the drug, leading to its widespread withdrawal from markets. What lessons can we learn from the story of cisapride? How can its brief but spectacular rise and equally sensational demise inform the development of new drugs which are so sorely needed in the management of motility and functional gastrointestinal disorders? This review explores the background to the development of cisapride, its history in clinical trials and the experience with adverse events and, in so doing, attempts to identify lessons for the future in the therapeutics of enteric neuromodulatory drugs.

Advances of molecular clinical pharmacology in gastroenterology and hepatology

During developmental age, differences in pharmacodynamic reactions to several drugs may reflect polymorphisms of genes encoding drug-transporting proteins, receptors, drug targets, and gene products, whose disturbed activity sometimes plays an important role in certain diseases. Administration of drugs with a narrow therapeutic index may quite easily be associated with changes in pharmacokinetics and development of adverse drug reactions, which occasionally may cause fatalities. In such cases, polypragmasy and resulting drug interactions may enhance effects of changes in drug-metabolizing enzymes' activities. Phenotyping and genotyping of patients slowly are finding their place in some therapeutic regimens used in clinical gastroenterology and hepatology. At present, some assays to measure, for example, thiopurine S-methyltransferase activity are already commercially available. Polymorphisms of CYP450 enzymes, interleukins, and altered gene expression play an important role in some patients' various gastrointestinal tract and liver diseases. Herbal drugs also affect proinflammatory and antiinflammatory cytokine and nitric oxide balance in the body. Therapeutic use of recombined proteins, such as infliximab, natalizumab, onercept, humanized antibody to integrin α-4 β-7, or IFN-β in some large-bowel diseases increased therapeutic efficacy. IFN-α used in the patients with chronic hepatitis C improved cellular immunity in these subjects and exerted antiviral activity. Practical application of progress in pharmacogenetics, pharmacokinetics, pharmacodynamics, and use of bioproducts in novel therapeutic regimens has opened therapeutic frontiers and increased clinical safety.

Effect of domperidone on the QTc interval in premature infants

OBJECTIVE

To evaluate the effect of domperidone use on corrected QT interval in premature infants.

STUDY DESIGN

A prospective study of premature infants receiving domperidone was included in this study. A baseline electrocardiogram was obtained just before and 3, 7 and 14 days after initiation of domperidone. Corrected QT was considered prolonged if it exceeded the upper limit for age.

RESULT

A total of 40 premature infants were enrolled in this study. The mean birth weight of 1109+/-332 g, mean gestational age of 28.8+/-2.4 years and mean age at the onset of domperidone were 32.8+/-2 days. No difference in corrected QT interval was observed between just before and 3, 7 and 14 days after the start of the treatment. Two infants had corrected QT interval prolongation without any clinical side effect that resolved spontaneously.

CONCLUSION

Our experience suggests that domperidone administered cautiously in modest doses does not result in arrhythmias or conduction defects in premature infants statistically. Additional data are needed to give optimal advice regarding the safety of domperidone treatment in premature infants.

Approaches for assessing risks to sensitive populations: lessons learned from evaluating risks in the pediatric population

Assessing the risk profiles of potentially sensitive populations requires a "tool chest" of methodological approaches to adequately characterize and evaluate these populations. At present, there is an extensive body of literature on methodologies that apply to the evaluation of the pediatric population. The Health and Environmental Sciences Institute Subcommittee on Risk Assessment of Sensitive Populations evaluated key references in the area of pediatric risk to identify a spectrum of methodological approaches. These approaches are considered in this article for their potential to be extrapolated for the identification and assessment of other sensitive populations. Recommendations as to future research needs and/or alternate methodological considerations are also made.

Therapeutic product disposition in at-risk populations

In an emergency situation, such as a chemical, biological, radionuclide, nuclear or explosion (CBRNE) event, all patient populations are at increased risk of serious adverse events. Therapeutic product (TP) safety and efficacy depend on the disposition of the product through absorption, distribution, metabolism and excretion. The ability of a patient to benefit from or merely tolerate a TP can be modified by many factors, including but not limited to culture, diet, disease, environmental contaminants, genetic predisposition, stress and socioeconomic status and recent life experiences. Metabolism is considered to have the greatest effect on safety and efficacy, as chemicals not metabolised can accumulate to toxic levels. Inter-individual variances in most drug metabolism enzymes may range up to greater than 1000-fold. The fetus, neonates, infants, individuals with hormonal change, infection or prior exposure to licit or illicit products and the elderly are more susceptible to increased risk of serious adverse health effects. The critically ill are the most at risk. The at-risk populations for a serious adverse event are dependent then on the CBRNE event, their physical and cognitive states and the inter-individual intrinsic and extrinsic factors that affect TP disposition.

Population pharmacokinetics of domperidone in preterm neonates

PURPOSE

A population pharmacokinetic analysis was performed to define domperidone pharmacokinetic parameters in preterm neonates, as no pharmacokinetic data are available in this population.

METHODS

An oral domperidone solution was administered (0.75 mg/kg per day) in 32 preterm neonates (64 samples). Domperidone plasma concentration was measured by high-performance liquid chromatography (HPLC) assay, and a one-compartment model with first-order absorption was fitted to the data using NONMEM version V level 1.1.

RESULTS

The mean peak and trough plasma concentration values of domperidone were, respectively, 25.3 +/- 20.5 ng/ml and 15.4 +/- 11.4 ng/ml (mean +/- standard deviation). The pharmacokinetic parameters (interindividual variability%) were clearance (Cl/F) = 0.92 L/h (51.6%), volume of distribution (Vd/F) = 0.405 L (68%), and absorption constant rate (Ka) = 0.0843 h(-1) (55.8%). The clearance is not lower than values reported in adults. No influence of covariates (postnatal age, prematurity, weight, gender) on domperidone pharmacokinetic parameters was found.

CONCLUSION

This pilot study designed with a limited sampling strategy showed that domperidone plasma concentrations were consistent with those reported in adults, suggesting that domperidone dosage regimen currently used in preterm neonates is suitable.

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.

Pharmacogenomics and the future of drug therapy

With the completion of the human genome project, many investigators are striving to translate the resulting wealth of new information into new and improved clinical practices. Pharmacogenomics represents one of the most promising of these applications for adult- and pediatric-based therapies. This article provides a historical perspective, but most importantly, uses this background to illustrate important principles of the field. The application of pharmacogenomics to asthma therapy is presented as an example of the current status of pharmacogenomics as it is being applied to an important pediatric health problem. Finally, a discussion of future promises and challenges to the application of pharmacogenomics is presented, including economic and ethical issues.

Age-specific pulmonary cytochrome P-450 3A1 expression in postnatal and adult rats

A major cause of death and illness in children under the age of five, most living in polluted cities, is respiratory disease. Previous studies have shown that neonatal animals are more susceptible to bioactivated pulmonary cytotoxicants than adults, despite lower expression of the pulmonary cytochrome P-450s (CYP450s) thought to be involved in bioactivation. One CYP450 that is well documented in the bioactivation of many drugs and environmental toxicants in adult lung, but whose expression has not been evaluated during postnatal pulmonary development, is CYP450 3A (CYP3A). We compared age-specific expression of CYP3A1 in 7-day-old and adult male Sprague-Dawley rats. Unlike those shown for previously studied pulmonary CYP450s, expression levels for CYP3A1 mRNA in differentiating airway cells of postnatal rats are the same as in fully differentiated airway cells of adults. CYP3A1 protein expression (28%) and enzymatic activity (23%) were lower in postnatal airways compared with adults. Although other CYP450 immunoreactive proteins are primarily expressed in nonciliated cells, immunoreactive CYP3A1 protein was expressed in both ciliated and nonciliated cells in postnatal and adult rat proximal airways. CYP3A1 protein is detected diffusely throughout ciliated and nonciliated cells in 7-day-old rats, whereas it is only detected in the apex of these cells in adult rats. This study demonstrates that the lungs of postnatal rats have detectable levels of CYP3A1 and that CYP3A1 mRNA expression appears not to be age dependent, whereas steady-state CYP3A1 protein levels and enzyme activity show an age-dependent pattern.

Developmental and tissue-specific expression of human flavin-containing monooxygenases 1 and 3

Substantial changes occur in drug and toxicant disposition during early life stages that can impact therapeutic efficacy and adverse reactions to drugs and toxicants. Of the many parameters involved, alterations in drug metabolism are of major importance. Although the cytochrome P450-dependent monooxygenases are accepted as playing a substantial role in drug and toxicant metabolism, the flavin-containing monooxygenases (FMOs) also have an important role. Apparently unique to the human, FMO3 is the most abundant FMO family member in the adult human liver, whereas FMO1 dominates in most animal models. However, early studies documented that FMO1 is the most abundant FMO enzyme in the human fetal liver, whereas FMO3 is essentially absent. This review focuses on recent studies characterising human FMO ontogeny and, in particular, the 'switch' in hepatic FMO enzyme expression. Because it is so closely related, tissue-specific expression patterns also are examined. Finally, a summary of what is known in animal models is presented as a point of comparison.

IN VITRO METABOLISM STUDY OF BUPRENORPHINE: EVIDENCE FOR NEW METABOLIC PATHWAYS

Buprenorphine (BUP) is a synthetic derivative of the morphine alkaloid thebaine. BUP is metabolized by N-dealkylation to form the active metabolite nor-buprenorphine (Nor-BUP), and both undergo subsequent glucuronidation. Although BUP has been used clinically for years, its metabolism has still not been fully elucidated. The aim of this study was to clarify the identity of the human hepatic cytochromes P450 (P450s) involved in BUP metabolism and to investigate other potential metabolites. The metabolism of BUP was examined using human liver microsomes (HLM) and Ad293 P450-transfected cell lines, as well as CYP 3A4 and 2C8 recombinant isoforms. The kinetic parameters of metabolite formation were calculated for HLM and competent isoforms. Individual contribution of P450 isoforms in BUP metabolism as well as Nor-BUP production was evaluated using chemical inhibition experiments, as well as the relative activity factor approach. The analytical method used was based on liquid chromatography-mass spectrometry. Among the 13 P450 isoforms tested, CYP 3A4, 2C8, 3A5, and 3A7 produced Nor-BUP. Based on the results of chemical inhibition, CYP 3A4 accounts for about 65% of Nor-BUP production and CYP 2C8 for about 30%. BUP utilization by either HLM or P450-transfected cells revealed that another oxidative metabolic pathway exists, which was found to involve CYP 2C9, 2C18, 2C19, and mainly CYP 3A. Incubation of BUP or Nor-BUP with HLM led to the formation of new metabolites, identified by tandem mass spectrometry as being hydroxy-BUP and hydroxy-Nor-BUP. Hydroxy-BUP was produced by the CYP 3A, but not the 2C isoforms.

Diets containing soy protein isolate increase hepatic CYP3A expression and inducibility in weanling male rats exposed during early development

Hepatic CYP3A enzymes were studied in weanling male Sprague-Dawley rats exposed to diets from gestational d 4 in which the sole protein source was either casein (CAS) or soy protein isolate (SPI). At age 25 d, rats were gavaged with corn oil or one of the CYP3A inducers, dexamethasone (DEX) and clotrimazole (CLT), at a dose of 50 mg/kg. Little CYP3A1 (CYP3A23), CYP3A2, or CYP3A9 mRNA was observed in CAS-fed weanling rats but CYP3A18 mRNA was readily detectable in Northern blots. In contrast, consumption of SPI without inducer treatment resulted in the expression of CYP3A1 (CYP3A23), and CYP3A2 mRNAs, expression of CYP3A apoprotein in hepatic microsomes, and a 2-fold greater turnover of the CYP3A substrate midazolam (P < 0.05). DEX induced CYP3A1, CYP3A2, and CYP3A9 (P < 0.05), but not CYP3A18 mRNA expression in rats fed both diets. Hepatic CYP3A apoprotein expression and midazolam 4-hydroxylation in SPI-fed rats was greater than that of CAS-fed rats after DEX treatment (P < 0.05). CLT also induced CYP3A2 mRNA 2-fold in rats fed both diets but CYP3A apoprotein expression in microsomes from SPI-fed CLT rats was double that of CLT-treated rats fed CAS (P < 0.05). The elevation of CYP3A apoprotein due to SPI and the CYP3A apoprotein induction by DEX and CLT treatment yielded no significant diet x inducer interaction. Analysis of heterologous nuclear RNA expression by RT-PCR using intron-specific primers for CYP3A1 revealed a 14-fold increase in RNA transcription in CAS-fed rats after treatment with DEX (P < 0.05) but no increase in rats fed SPI compared with rats fed CAS even though CYP3A1 mRNA and CYP3A apoprotein were significantly elevated. These data demonstrate that exposure to SPI during early development can increase CYP3A expression via post-transcriptional mechanisms and suggest that early soy consumption has potential effects on the metabolism of a wide variety of CYP3A substrates.

Developing drugs for pediatric use: a role for juvenile animal studies?

The increased interest and/or need to perform pediatric clinical trials to allow the marketing and safe use of a wider range of medicines in children has raised the profile of the need to conduct juvenile animal studies. It is argued that such studies may identify "unique" toxicities not seen from available adult animal and clinical data. This paper will review the current situation from an industrial, regulatory, and scientific perspective. Areas of important consideration include functional (dynamic) and kinetic differences between children and adults and specific considerations associated with testing in young animals. This paper will also review what are we currently doing? Whether we really need these studies? and What challenges lie in the future?

Clinical Pharmacology

The dose of a drug is a major determinant of its safety, and establishing a safe dose of a novel drug is a prime objective during clinical development. The design of pre-marketing clinical trials precludes the representation of important subpopulations such as children, the elderly and people with co-morbidities. Therefore, postmarketing surveillance (PMS) activities are required to monitor the safety profile of drugs in real clinical practice. Furthermore, individual variations in pharmacogenetic profiles, the immune system, drug metabolic pathways and drug-drug interactions are also important factors in the occurrence of adverse drug reactions. Thus, the safety of a drug is a major clinical consideration before and after it is marketed. A multidisciplinary approach is required to enhance the safety profile of drugs at all stages of development, including PMS activities. Clinical pharmacology encompasses a range of disciplines and forms the backbone of drug safety consideration during clinical drug development. In this review we give an overview of the clinical drug development process and consider its limitations. We present a discussion of several aspects of clinical pharmacology and their application to enhancing drug safety. Pharmacokinetic-pharmacodynamic modelling provides a method of predicting a clinically safe dose; consideration of drug pharmacokinetics in special populations may enhance safe therapeutics in a wider spectrum of patients, while pharmacogenetics provides the possibility of genotype-specific therapeutics. Pharmacovigilance activities are also discussed. Given the complex nature and unpredictability of type B reactions, PMS activities are crucial in managing the risks drugs pose to the general population. The various aspects of clinical pharmacology discussed make a strong case for this field as the backbone of optimising and promoting safe development and use of drugs.

In vitro and pharmacophore insights into CYP3A enzymes

The cytochrome P450 3A (CYP3A) enzymes have a major role in the metabolism of drugs in humans. Their wide substrate specificity and induction by a vast array of structurally diverse compounds presents the possibility of metabolic drug-drug interactions. Understanding the enzymes themselves is crucial. Over the past decade, this has occurred mostly with in vitro studies, although more recent approaches incorporate computational models to predict CYP inhibition and substrate potential. The three-dimensional displacement, or pharmacophore, of chemical features in space that are derived from inhibition data have produced pharmacophores for CYP3A4, CYP3A5 and CYP3A7, and provide new insights into ligand binding for each enzyme.

Human variability in CYP3A4 metabolism and CYP3A4-related uncertainty factors for risk assessment

CYP3A4 constitutes the major liver cytochrome P450 isoenzyme and is responsible for the oxidation of more than 50% of all known drugs. Human variability in kinetics for this pathway has been quantified using a database of 15 compounds metabolised extensively (>60%) by this CYP isoform in order to develop CYP3A4-related uncertainty factors for the risk assessment of environmental contaminants handled via this route. Data were analysed from published pharmacokinetic studies (after oral and intravenous dosing) in healthy adults and other subgroups using parameters relating primarily to chronic exposure [metabolic and total clearances, area under the plasma concentration-time curve (AUC)] and acute exposure (Cmax). Interindividual variability in kinetics was greater for the oral route (46%, 12 compounds) than for the intravenous route (32%, 14 compounds). The physiological and molecular basis for the difference between these two routes of exposure is discussed. In relation to the uncertainty factors used for risk assessment, the default kinetic factor of 3.16 would be adequate for adults, whereas a CYP3A4-related factor of 12 would be required to cover up to 99% of neonates, which have lower CYP3A4 activity.

Drugs and the liver: advances in metabolism, toxicity, and therapeutics

Biotransformation of drugs is one of the major functions of liver. Hepatic drug metabolism develops early in organogenesis and continues in postnatal life through puberty. Genetic and developmental studies on hepatic drug metabolism show that immaturity, polymorphisms, and altered balance of different hepatic enzymatic activities affect pharmacologic inactivation and alter the risk of toxic effects of drugs on the hepatic parenchyma. Although drug-induced liver disease is less common in children, several reports of hepatotoxicity are published every year. Furthermore, the increasing use of nonregulated remedies (eg, herbal preparations or recreational drugs) increases the risk of unpredictable and potentially severe reactions. Many significant advances in the treatment of hepatic diseases have been achieved recently. However, differences in clinical features, natural history, and response to treatment between children and adults require evaluation of new therapeutic options in focused pediatric clinical trials.