Age-Dependent Human Hepatic Carboxylesterase 1 (CES1) and Carboxylesterase 2 (CES2) Postnatal Ontogeny

Physiologically-Based Pharmacokinetic Modeling and Dosing Optimization of Cefotaxime in Preterm and Term Neonates

BACKGROUND

Cefotaxime is commonly used in treating bacterial infections in neonates. To characterize the pharmacokinetic process in neonates and evaluate different recommended dosing schedules of cefotaxime, a physiologically-based pharmacokinetic (PBPK) model of cefotaxime was established in adults and scaled to neonates.

METHODS

A whole-body PBPK model was built in PK-SIM® software. Three elimination pathways are composed of enzymatic metabolism in the liver, passive filtration through glomerulus, and active tubular secretion mediated by renal transporters. The ontogeny information was applied to account for age-related changes in cefotaxime pharmacokinetics. The established models were verified with realistic clinical data in adults and pediatric populations. Simulations in neonates were conducted and 100 % of the dosing interval where the unbound concentration in plasma was above the minimum inhibitory concentration (fT>MIC) was selected as the target index for dosing regimen evaluation.

RESULTS

The developed PBPK models successfully described the pharmacokinetic process of cefotaxime in adults and were scaled to the pediatric population. Good verification results were achieved in both adults' and neonates' PBPK models, indicating a good predictive performance. The optimal dosage regimen of cefotaxime was proposed according to the postnatal age (PNA) and gestational age (GA) of neonates. For preterm neonates (GA < 36 weeks), dosages of 25 mg/kg every 8 h in PNA 0-6 days and 25 mg/kg every 6 h in PNA 7-28 days were suggested. For term neonates (GA ≥ 36 weeks), dosages of 33 mg/kg every 8 h in PNA 0-6 days and 33 mg/kg every 6 h in PNA 7-28 days were recommended.

CONCLUSIONS

Our study may provide useful experience in practicing PBPK model-informed precision dosing in the pediatric population.

Regulation of carboxylesterases and its impact on pharmacokinetics and pharmacodynamics: an up-to-date review

INTRODUCTION

Carboxylesterase 1 (CES1) and carboxylesterase 2 (CES2) are among the most abundant hydrolases in humans, catalyzing the metabolism of numerous clinically important medications, such as methylphenidate and clopidogrel. The large interindividual variability in the expression and activity of CES1 and CES2 affects the pharmacokinetics (PK) and pharmacodynamics (PD) of substrate drugs.

AREAS COVERED

This review provides an up-to-date overview of CES expression and activity regulations and examines their impact on the PK and PD of CES substrate drugs. The literature search was conducted on PubMed from inception to January 2024.

EXPERT OPINION

Current research revealed modest associations of CES genetic polymorphisms with drug exposure and response. Beyond genomic polymorphisms, transcriptional and posttranslational regulations can also significantly affect CES expression and activity and consequently alter PK and PD. Recent advances in plasma biomarkers of drug-metabolizing enzymes encourage the research of plasma protein and metabolite biomarkers for CES1 and CES2, which could lead to the establishment of precision pharmacotherapy regimens for drugs metabolized by CESs. Moreover, our understanding of tissue-specific expression and substrate selectivity of CES1 and CES2 has shed light on improving the design of CES1- and CES2-activated prodrugs.

Inter-individual exposure variability interpretation through reflection of biological age algorithm in physiologically based toxicokinetic model: Application to human risk assessment of di-isobutyl-phthalate

Age-related changes and interindividual variability in the degree of exposure to hazardous substances in the environment are pertinent factors to be considered in human risk assessment. Existing risk assessments remain in a one-size-fits-all approach, often without due consideration of inter-individual toxicokinetic variability factors, such as age. The purpose of this study was to advance from the existing risk assessment of hazardous substances based on toxicokinetics to a precise human risk assessment by additionally considering the effects of physiologic and metabolic fluctuations and interindividual variability in age. Qualitative age-associated physiologic and metabolic changes in humans, obtained through a meta-analysis, were quantitatively modeled to produce the final biological age algorithm (BAA). The developed BAAs (for males) were extended and applied to the reported testicular reproductive toxicity-focused di-isobutyl-phthalate (DiBP)-mono-isobutyl-phthalate (MiBP) physiologically based toxicokinetic (PBTK) model in males. The advanced PBTK model combined with the BAA was applied to the human risk assessment based on MiBP biomonitoring data. As a result, the specialized DiBP external exposure values for each age could be estimated. Additionally, by applying the Monte Carlo simulation, the distribution of internal exposure diversity among individuals according to the same external exposure dose could be estimated. The contributions of physiologic and metabolic factors to the age-dependent toxicokinetic changes were approximately 93.41-99.99 and 0.01-6.59%, respectively. In addition, the relative contribution of metabolic factors was major in infants and continued to decrease as age increased (up to about age 30 years). This study provides a step-by-step platform that can be widely applied to overcome the limitations of existing toxicokinetic models that still require interindividual pharmacokinetic variability explanations. This will be important for the rationalization and explanation of inter-individual variability in the pharmacokinetics of many substances.

Quantification of drug metabolising enzymes and transporter proteins in the paediatric duodenum via LC-MS/MS proteomics using a QconCAT technique

Characterising the small intestine absorptive membrane is essential to enable prediction of the systemic exposure of oral formulations. In particular, the ontogeny of key intestinal Drug Metabolising Enzymes and Transporter (DMET) proteins involved in drug disposition needs to be elucidated to allow for accurate prediction of the PK profile of drugs in the paediatric cohort. Using pinch biopsies from the paediatric duodenum (n = 36; aged 11 months to 15 years), the abundance of 21 DMET proteins and two enterocyte markers were quantified via LC-MS/MS. An established LCMS nanoflow method was translated to enable analysis on a microflow LC system, and a new stable-isotope-labelled QconCAT standard developed to enable quantification of these proteins. Villin-1 was used to standardise abundancy values. The observed abundancies and ontogeny profiles, agreed with adult LC-MS/MS-based data, and historic paediatric data obtained via western blotting. A linear trend with age was observed for duodenal CYP3A4 and CES2 only. As this work quantified peptides on a pinch biopsy coupled with a microflow method, future studies using a wider population range are very feasible. Furthermore, this DMET ontogeny data can be used to inform paediatric PBPK modelling and to enhance the understanding of oral drug absorption and gut bioavailability in paediatric populations.

Physiologically-based pharmacokinetic modeling of nafamostat to support dose selection for treatment of pediatric patients with COVID-19

Pediatric patients with coronavirus disease 2019 (COVID-19) are increasing, and severe cases such as multisystem inflammatory syndrome are being reported. Nafamostat, a repurposing drug, is currently being explored for the treatment of COVID-19 in adults. However, the data supporting its exposure in pediatrics remains scarce. Physiologically-based pharmacokinetic (PBPK) modeling enables the prediction of drug exposure in pediatrics based on ontogeny of metabolic enzymes and age dependent anatomical and physiological changes. The study aimed to establish a PBPK model of nafamostat in adults, then scale the adult PBPK model to children for predicting pediatric exposures of nafamostat and an optimal weight-based nafamostat dose in pediatric population. The developed model adequately described adult exposure data in healthy volunteers following i.v. administration with three doses (10, 20, and 40 mg). Scaling adult PBPK models to five pediatric groups predicted that as age advances from neonate to adult, the exposure of nafamostat slightly increased from neonate to infant, steadily decreased from infant to child, and then increased from child to adult after the administration of 0.2 mg/kg/h for 14 days, a dosing regimen being conducted in a clinical trial for COVID-19. Based on the fold change of predicted area under the curve for the respective pediatric group over those of adults, weight-based dosages for each pediatric group may be suggested. The novel PBPK model described in this study may be useful to investigate nafamostat pharmacokinetics in a pediatric subgroup further.

Construction and Characterization of CRISPR/Cas9 Knockout Rat Model of Carboxylesterase 2a Gene

Carboxylesterase (CES) 2, an important metabolic enzyme, plays a critical role in drug biotransformation and lipid metabolism. Although CES2 is very important, few animal models have been generated to study its properties and functions. Rat Ces2 is similar to human CES2A-CES3A-CES4A gene cluster, with highly similar gene structure, function, and substrate. In this report, CRISPR-associated protein-9 (CRISPR/Cas9) technology was first used to knock out rat Ces2a, which is a main subtype of Ces2 mostly distributed in the liver and intestine. This model showed the absence of CES2A protein expression in the liver. Further pharmacokinetic studies of diltiazem, a typical substrate of CES2A, confirmed the loss of function of CES2A both in vivo and in vitro. At the same time, the expression of CES2C and CES2J protein in the liver decreased significantly. The body and liver weight of Ces2a knockout rats also increased, but the food intake did not change. Moreover, the deficiency of Ces2a led to obesity, insulin resistance, and liver fat accumulation, which are consistent with the symptoms of nonalcoholic fatty liver disease (NAFLD). Therefore, this rat model is not only a powerful tool to study drug metabolism mediated by CES2 but also a good disease model to study NAFLD. SIGNIFICANCE STATEMENT: Human carboxylesterase (CES) 2 plays a key role in the first-pass hydrolysis metabolism of most oral prodrugs as well as lipid metabolism. In this study, CRISPR/Cas9 technology was used to knock out Ces2a gene in rats for the first time. This model can be used not only in the study of drug metabolism and pharmacokinetics but also as a disease model of nonalcoholic fatty liver disease (NAFLD) and other metabolic disorders.

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.

Human carboxylesterases and fluorescent probes to image their activity in live cells

Human carboxylesterases (CESs) are serine hydrolases that are responsible for the phase I metabolism of an assortment of ester, amide, thioester, carbonate, and carbamate containing drugs. CES activity is known to be influenced by a variety of factors including single nucleotide polymorphisms, alternative splicing, and drug-drug interactions. These different factors contribute to interindividual variability of CES activity which has been demonstrated to influence clinical outcomes among people treated with CES-substrate therapeutics. Detailed exploration of the factors that influence CES activity is emerging as an important area of research. The use of fluorescent probes with live cell imaging techniques can selectively visualize the real-time activity of CESs and have the potential to be useful tools to help reveal the impacts of CES activity variations on human health. This review summarizes the properties of the five known human CESs including factors reported to or that could potentially influence their activity before discussing the design aspects and use considerations of CES fluorescent probes in general in addition to highlighting several well-characterized probes.

Human Variability in Carboxylesterases and carboxylesterase-related Uncertainty Factors for Chemical Risk Assessment

Carboxylesterases (CES) are an important class of enzymes involved in the hydrolysis of a range of chemicals and show large inter-individual variability in vitro. An extensive literature search was performed to identify in vivo probe substrates for CES1 and CES2 together with their protein content and enzymatic activity. Human pharmacokinetic (PK) data on Cmax, clearance, and AUC were extracted from 89 publications and Bayesian meta-analysis was performed using a hierarchical model to derive CES-related variability distributions and related uncertainty factors (UF). The CES-related variability indicated that 97.5% of healthy adults are covered by the kinetic default UF (3.16), except for clopidogrel and dabigatran etexilate. Clopidogrel is metabolised for a small amount by the polymorphic CYP2C19, which can have an impact on the overall pharmacokinetics, while the variability seen for dabigatran etexilate might be due to differences in the absorption, since this can be influenced by food intake. The overall CES-related variability was moderate to high in vivo (<CV 50%), which might be due to possible polymorphism in the enzyme but also to the small sample size available per chemical. The presented CES-related variability can be used in combination with in vitro data to derive pathway-specific distributions.

Development and Application of a Life-Stage Physiologically Based Pharmacokinetic (PBPK) Model to the Assessment of Internal Dose of Pyrethroids in Humans

To address concerns around age-related sensitivity to pyrethroids, a life-stage physiologically based pharmacokinetic (PBPK) model, supported by in vitro to in vivo extrapolation (IVIVE) was developed. The model was used to predict age-dependent changes in target tissue exposure of 8 pyrethroids; deltamethrin (DLM), cis-permethrin (CPM), trans-permethrin, esfenvalerate, cyphenothrin, cyhalothrin, cyfluthrin, and bifenthrin. A single model structure was used based on previous work in the rat. Intrinsic clearance (CLint) of each individual cytochrome P450 or carboxylesterase (CES) enzyme that are active for a given pyrethroid were measured in vitro, then biologically scaled to obtain in vivo age-specific total hepatic CLint. These IVIVE results indicate that, except for bifenthrin, CES enzymes are largely responsible for human hepatic metabolism (>50% contribution). Given the high efficiency and rapid maturation of CESs, clearance of the pyrethroids is very efficient across ages, leading to a blood flow-limited metabolism. Together with age-specific physiological parameters, in particular liver blood flow, the efficient metabolic clearance of pyrethroids across ages results in comparable to or even lower internal exposure in the target tissue (brain) in children than that in adults in response to the same level of exposure to a given pyrethroid (Cmax ratio in brain between 1- and 25-year old = 0.69, 0.93, and 0.94 for DLM, bifenthrin, and CPM, respectively). Our study demonstrated that a life-stage PBPK modeling approach, coupled with IVIVE, provides a robust framework for evaluating age-related differences in pharmacokinetics and internal target tissue exposure in humans for the pyrethroid class of chemicals.

In vitro age-related differences in rats to organophosphates

The mechanism of toxic action for organophosphates (OPs) is the persistent inhibition of acetylcholinesterase (AChE) resulting in accumulation of acetylcholine and subsequent hyperstimulation of the nervous system. Organophosphates display a wide range of acute toxicities. Differences in the OP's chemistries results in differences in the compound's metabolism and toxicity. Acute toxicities of OPs appear to be principally dependent on compound specific efficiencies of detoxication, and less dependent upon efficiencies of bioactivation and sensitivity of AChE. Serine esterases, such as carboxylesterase (CaE) and butyrylcholinesterase (BChE), play a prominent role in OP detoxication. Organophosphates can stoichiometrically inhibit these enzymes, removing OPs from circulation thus providing protection for the target enzyme, AChE. This in vitro study investigated age-related sensitivity of AChE, BChE and CaE to twelve structurally different OPs in rat tissues. Sensitivity of esterases to these OPs was assessed by inhibitory concentration 50s (IC50s). The OPs displayed a wide range of inhibitory potency toward AChE with IC50s in the low nM-μM range with no differences among ages; however, the CaE IC50s generally increased with age reflecting greater protection in adults. These results suggest age-related differences in acute toxicities of OPs in mammals are primarily a result of their detoxication capacities.

Recent advances in the ontogeny of drug disposition

Developmental changes that occur throughout childhood have long been known to impact drug disposition. However, pharmacokinetic studies in the paediatric population have historically been limited due to ethical concerns arising from incorporating children into clinical trials. As such, much of the early work in the field of developmental pharmacology was reliant on difficult-to-interpret in vitro and in vivo animal studies. Over the last 2 decades, our understanding of the mechanistic processes underlying age-related changes in drug disposition has advanced considerably. Progress has largely been driven by technological advances in mass spectrometry-based methods for quantifying proteins implicated in drug disposition, and in silico tools that leverage these data to predict age-related changes in pharmacokinetics. This review summarizes our current understanding of the impact of childhood development on drug disposition, particularly focusing on research of the past 20 years, but also highlighting select examples of earlier foundational research. Equally important to the studies reviewed herein are the areas that we cannot currently describe due to the lack of research evidence; these gaps provide a map of drug disposition pathways for which developmental trends still need to be characterized.

Physiologically-Based Pharmacokinetic Modeling of Remdesivir and Its Metabolites to Support Dose Selection for the Treatment of Pediatric Patients With COVID-19

Severe coronavirus disease 2019 (COVID‐19) disease, including multisystem inflammatory syndrome, has been reported in children. This report summarizes development of a remdesivir physiologically‐based pharmacokinetic (PBPK) model that accurately describes observed adult remdesivir and metabolites exposure and predicts pediatric remdesivir and metabolites exposure. The adult PBPK model was applied to predict pediatric remdesivir and metabolites steady‐state exposures using the Pediatric Population Model in SimCYP and incorporated the relevant physiologic and mechanistic information. Model development was based on adult phase I exposure data in healthy volunteers who were administered a 200‐mg loading dose of remdesivir intravenous (IV) over 0.5 hours on Day 1, then 100‐mg daily maintenance doses of IV over 0.5 hours starting on Day 2 and continuing through Days 5 or 10. Simulations indicated that use of the adult therapeutic remdesivir dosage regimen (200‐mg loading dose on Day 1 then 100‐mg daily maintenance dose starting on Day 2) in pediatric patients ≥ 40 kg and a weight‐based remdesivir dosage regimen (5‐mg/kg loading dose on Day 1 then 2.5‐mg/kg daily maintenance dose starting on Day 2) in pediatric patients weighing 2.5 to < 40 kg is predicted to maintain therapeutic exposures of remdesivir and its metabolites. The comprehensive PBPK model described in this report supported remdesivir dosing in planned pediatric clinical studies and dosing in the emergency use authorization and pediatric compassionate use programs that were initiated to support remdesivir as a treatment option during the pandemic.

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.

Physiologically-Based Pharmacokinetic/Pharmacodynamic Model of MBQ-167 to Predict Tumor Growth Inhibition in Mice

MBQ-167 is a dual inhibitor of the Rho GTPases Rac and Cdc42 that has shown promising results as an anti-cancer therapeutic at the preclinical stage. This drug has been tested in vitro and in vivo in metastatic breast cancer mouse models. The aim of this study is to develop a physiologically based pharmacokinetic/pharmacodynamic (PBPK-PD) model of MBQ-167 to predict tumor growth inhibition following intraperitoneal (IP) administration in mice bearing Triple Negative and HER2+ mammary tumors. PBPK and Simeoni tumor growth inhibition (TGI) models were developed using the Simcyp V19 Animal Simulator. Our developed PBPK framework adequately describes the time course of MBQ-167 in each of the mouse tissues (e.g., lungs, heart, liver, kidneys, spleen, plasma) and tumor, since the predicted results were consistent with the experimental data. The developed PBPK-PD model successfully predicts tumor shrinkage in HER2+ and triple-negative breast tumors after the intraperitoneal administration of 1 and 10 mg/kg body weight (BW) dose level of MBQ-167 three times a week. The findings from this study suggest that MBQ-167 has a higher net effect and potency inhibiting Triple Negative mammary tumor growth compared to HER2+ and that liver metabolism is the major route of elimination of this drug.

Ontogeny of Phase I Metabolism of Drugs

Capturing ontogeny of enzymes involved in phase I metabolism is crucial to improve prediction of dose-concentration and concentration-effect relationships throughout infancy and childhood. Once captured, these patterns can be integrated in semiphysiologically or physiology-based pharmacokinetic models to support predictions in specific pediatric settings or to support pediatric drug development. Although these translational efforts are crucial, isoenzyme-specific ontogeny-based models should also incorporate data on variability of maturational and nonmaturational covariates (eg, disease, treatment modalities, pharmacogenetics). Therefore, this review provides a summary of the ontogeny of phase I drug-metabolizing enzymes, indicating current knowledge gaps and recent progresses. Furthermore, we tried to illustrate that straightforward translation of isoenzyme-specific ontogeny to predictions does not allow full exploration of scenarios of potential variability related to maturational (non-age-related variability, other isoenzymes or transporters) or nonmaturational (disease, pharmacogenetics) covariates, and necessitates integration in a "systems" concept.

LncRNA KCNQ1OT1 ameliorates the liver injury induced by acetaminophen through the regulation of miR-122-5p/CES2 axis

Long noncoding RNAs (lncRNAs) have been shown to be implicated in acetaminophen (APAP)-induced liver injury (AILI). We applied this study to investigate the role and functional mechanism of KCNQ1 overlapping transcript 1 (KCNQ1OT1) in AILI. The AILI model was established by APAP treatment in mice. The liver injury was preliminarily evaluated by ALT and AST activities via the detection kits. The quantitative real-time polymerase chain reaction (qRT-PCR) was exploited for detecting the expression of KCNQ1OT1, microRNA-122-5p (miR-122-5p), and carboxylesterase 2 (CES2). Protein levels were analyzed via Western blot. 3-(4, 5-dimethylthiazol-2-y1)-2, 5-diphenyl tetrazolium bromide (MTT) assay, and flow cytometry were separately applied to determine cell proliferation and apoptosis rate. Inflammation was assessed by enzyme-linked immunosorbent assay (ELISA). Dual-luciferase reporter assay was implemented to testify the intergenic combination. The function of KCNQ1OT1 in vivo was explored through KCNQ1OT1 knockdown in mice. APAP triggered the downregulation of KCNQ1OT1 and CES2 in mice serums. KCNQ1OT1 upregulation could relieve the AILI in HepaRG cells, which were abrogated by CES2 downregulation. KCNQ1OT1 served as a sponge of miR-122-5p and miR-122-5p directly targeted CES2. KCNQ1OT1 overexpression abated the AILI through the miR-122-5p/CES2 axis in HepaRG cells in vitro and mice in vivo. The collective results clarified that KCNQ1OT1 weakened the AILI in vitro and in vivo by the miR-122-5p/CES2 axis, providing an explicit molecular mechanism and selectable therapeutic strategy of AILI.

Physiologically Based Pharmacokinetic Modeling in Risk Assessment: Case Study With Pyrethroids

The assessment of potentially sensitive populations is an important application of risk assessment. To address the concern for age-related sensitivity to pyrethroid insecticides, life-stage physiologically based pharmacokinetic (PBPK) modeling supported by in vitro to in vivo extrapolation was conducted to predict age-dependent changes in target tissue exposure to 8 pyrethroids. The purpose of this age-dependent dosimetry was to calculate a Data-derived Extrapolation Factor (DDEF) to address age-related pharmacokinetic differences for pyrethroids in humans. We developed a generic human PBPK model for pyrethroids based on our previously published rat model that was developed with in vivo rat data. The results demonstrated that the age-related differences in internal exposure to pyrethroids in the brain are largely determined by the differences in metabolic capacity and in physiology for pyrethroids between children and adults. The most important conclusion from our research is that, given an identical external exposure, the internal (target tissue) concentration is equal or lower in children than in adults in response to the same level of exposure to a pyrethroid. Our results show that, based on the use of the life-stage PBPK models with 8 pyrethroids, DDEF values are essentially close to 1, resulting in a DDEF for age-related pharmacokinetic differences of 1. For risk assessment purposes, this indicates that no additional adjustment factor is necessary to account for age-related pharmacokinetic differences for these pyrethroids.

Carboxylesterase 1 and Precision Pharmacotherapy: Pharmacogenetics and Nongenetic Regulators

Carboxylesterase (CES) 1 is the most abundant drug-metabolizing enzyme in human livers, comprising approximately 1% of the entire liver proteome. CES1 is responsible for 80%-95% of total hydrolytic activity in the liver and plays a crucial role in the metabolism of a wide range of drugs (especially ester-prodrugs), pesticides, environmental pollutants, and endogenous compounds. Expression and activity of CES1 vary markedly among individuals, which is a major contributing factor to interindividual variability in the pharmacokinetics (PK) and pharmacodynamics (PD) of drugs metabolized by CES1. Both genetic and nongenetic factors contribute to CES1 variability. Here, we discuss genetic polymorphisms, including single-nucleotide polymorphisms (SNPs), and copy number variants and nongenetic contributors, such as developmental status, genders, and drug-drug interactions, that could influence CES1 functionality and the PK and PD of CES1 substrates. Currently, the loss-of-function SNP G143E (rs71647871) is the only clinically significant CES1 variant identified to date, and alcohol is the only potent CES1 inhibitor that could alter the therapeutic outcomes of CES1 substrate medications. However, G143E and alcohol can only explain a small portion of the interindividual variability in the CES1 function. A better understanding of the regulation of CES1 expression and activity and identification of biomarkers for CES1 function in vivo could lead to the development of a precision pharmacotherapy strategy to improve the efficacy and safety of many CES1 substrate drugs. SIGNIFICANCE STATEMENT: The clinical relevance of CES1 has been well demonstrated in various clinical trials. Genetic and nongenetic regulators can affect CES1 expression and activity, resulting in the alteration of the metabolism and clinical outcome of CES1 substrate drugs, such as methylphenidate and clopidogrel. Predicting the hepatic CES1 function can provide clinical guidance to optimize pharmacotherapy of numerous medications metabolized by CES1.

Toxicokinetics of cis- and trans-Permethrin: Influence of Isomer, Maturation, and Sex

Permethrin exposure of children and adults is widespread in many populations, but knowledge of its relative toxicokinetics (TK) and health risks in immature age groups is lacking. Studies were conducted in rats to determine the influence of immaturity and sex (on plasma and target organ dosimetry of each of the insecticide’s 2 isomers, cis- and trans-permethrin [CIS and TRANS]). Postnatal day 15, 21, and 90 (adult), Sprague Dawley rats were orally administered a graduated series of doses of CIS and TRANS in corn oil. Serial sacrifices were conducted over 24 h to obtain plasma, brain, liver, skeletal muscle, and fat profiles of CIS and TRANS. Levels of TRANS decreased relatively rapidly, despite administration of relatively high doses. Concentrations of each isomer in plasma, brain, and other tissues monitored were inversely proportional to the animals’ age. The youngest pups exhibited 4-fold higher plasma and brain area under the curves than did adults. Little difference was observed in the TK of CIS or TRANS between adult male and female rats, other than higher initial plasma and liver CIS levels in females. Elevated exposure of the immature brain appears to be instrumental in increased susceptibility to the acute neurotoxicity of high-dose permethrin (Cantalamessa [1993]), but it remains to be established whether age-dependent TK is relevant to long-term, low-level risks.

Pharmacology of Common Analgesic and Sedative Drugs Used in the Neonatal Intensive Care Unit

In this review of analgesic and sedation medication in neonates, important classes of old and newer medications commonly used in the neonatal intensive care unit setting are discussed. In addition to drug metabolism, efficacy, and safety for individual drugs, new insights into multimodal analgesic approaches suggest ways in which multiple analgesic drug classes can be combined to maximize efficacy and minimize toxicity. Opiate pharmacogenetics and the potential for a precision therapeutics approach is explored, with a final description of gaps in knowledge and a call for future research of pain and sedation control in the neonatal population.

The Impact of Carboxylesterases in Drug Metabolism and Pharmacokinetics

BACKGROUND

Carboxylesterases (CES) play a critical role in catalyzing hydrolysis of esters, amides, carbamates and thioesters, as well as bioconverting prodrugs and soft drugs. The unique tissue distribution of CES enzymes provides great opportunities to design prodrugs or soft drugs for tissue targeting. Marked species differences in CES tissue distribution and catalytic activity are particularly challenging in human translation.

METHODS

Review and summarization of CES fundamentals and applications in drug discovery and development.

RESULTS

Human CES1 is one of the most highly expressed drug metabolizing enzymes in the liver, while human intestine only expresses CES2. CES enzymes have moderate to high inter-individual variability and exhibit low to no expression in the fetus, but increase substantially during the first few months of life. The CES genes are highly polymorphic and some CES genetic variants show significant influence on metabolism and clinical outcome of certain drugs. Monkeys appear to be more predictive of human pharmacokinetics for CES substrates than other species. Low risk of clinical drug-drug interaction is anticipated for CES, although they should not be overlooked, particularly interaction with alcohols. CES enzymes are moderately inducible through a number of transcription factors and can be repressed by inflammatory cytokines.

CONCLUSION

Although significant advances have been made in our understanding of CESs, in vitro - in vivo extrapolation of clearance is still in its infancy and further exploration is needed. In vitro and in vivo tools are continuously being developed to characterize CES substrates and inhibitors.

A Time-Embedding Network Models the Ontogeny of 23 Hepatic Drug Metabolizing Enzymes

Pediatric patients are at elevated risk of adverse drug reactions, and there is insufficient information on drug safety in children. Complicating risk assessment in children, there are numerous age-dependent changes in the absorption, distribution, metabolism, and elimination of drugs. A key contributor to age-dependent drug toxicity risk is the ontogeny of drug metabolism enzymes, the changes in both abundance and type throughout development from the fetal period through adulthood. Critically, these changes affect not only the overall clearance of drugs but also exposure to individual metabolites. In this study, we introduce time-embedding neural networks in order to model population-level variation in metabolism enzyme expression as a function of age. We use a time-embedding network to model the ontogeny of 23 drug metabolism enzymes. The time-embedding network recapitulates known demographic factors impacting 3A5 expression. The time-embedding network also effectively models the nonlinear dynamics of 2D6 expression, enabling a better fit to clinical data than prior work. In contrast, a standard neural network fails to model these features of 3A5 and 2D6 expression. Finally, we combine the time-embedding model of ontogeny with additional information to estimate age-dependent changes in reactive metabolite exposure. This simple approach identifies age-dependent changes in exposure to valproic acid and dextromethorphan metabolites and suggests potential mechanisms of valproic acid toxicity. This approach may help researchers evaluate the risk of drug toxicity in pediatric populations.

Toxicokinetics of Deltamethrin: Dosage Dependency, Vehicle Effects, and Low-Dose Age-Equivalent Dosimetry in Rats

There is increasing concern that infants and children may be at increased risk of neurological effects of pyrethroids, the most widely used class of insecticide. The objectives of this investigation were to (1) characterize the dose-dependent toxicokinetics (TK) of deltamethrin (DLM) for exposures ranging from environmentally relevant to acutely toxic; (2) determine the influence of an aqueous versus oil vehicle on oral absorption and bioavailability; and (3) determine whether DLM exhibits low-dose, age-equivalent internal dosimetry. Serial arterial plasma samples were obtained for 72 h from adult, male Sprague Dawley rats given 0.05-5.0 mg DLM/kg as an oral bolus in corn oil (CO). DLM exhibited linear, absorption rate-limited TK. Increases in maximum plasma concentration (Cmax) and AUC∘∞ were directly proportional to the dose. Oral bioavailability was quite limited. The vehicle and its volume had modest effect on the rate and extent of systemic absorption in adult rats. Postnatal day (PND) 15, 21, and 90 (adult) rats received 0.10, 0.25, or 0.50 mg DLM/kg orally in CO and were sacrificed periodically for plasma, brain, and liver collection. Age-dependent differences between PND 15 and 90 plasma Cmax and AUC∘24 values progressively diminished as the dose decreased, but there was a lack of low dose age equivalence in these brain and liver dosimeters. Other maturational factors may account for the lack of the low-dose age equivalence in brain and liver. This investigation provides support for the premise that the relatively low metabolic capacity of immature subjects may be adequate to effectively eliminate trace amounts of DLM and other pyrethroids from the plasma.

Genetics of the patent ductus arteriosus (PDA) and pharmacogenetics of PDA treatment

Patent ductus arteriosus (PDA) is a frequent, complex, and difficult to treat clinical syndrome among preterm infants in the neonatal intensive care unit. In addition to known clinical risk factors, there are emerging data about genetic predisposition to PDA in both animal and human models. Clinical response and toxicity from drugs used to treat PDA are highly variable. Developmental and genetic aspects of pharmacokinetics and pharmacodynamics influence exposure and response to pharmacologic therapies. Given the variable efficacy and toxicity of known drug therapies, novel therapeutic targets for PDA treatment offer the promise of precision medicine. This review addresses the known genetic contributions to prolonged ductal patency, variability in response to drug therapy for PDA, and potential novel drug targets for future PDA treatment discovery.

Age-Related Changes in Expression and Activity of Human Hepatic Mitochondrial Glutathione Transferase Zeta1

Glutathione transferase zeta1 (GSTZ1) catalyzes glutathione (GSH)-dependent dechlorination of dichloroacetate (DCA), an investigational drug with therapeutic potential in metabolic disorders and cancer. GSTZ1 is expressed in both hepatic cytosol and mitochondria. Here, we examined the ontogeny and characterized the properties of human mitochondrial GSTZ1. GSTZ1 expression and activity with DCA were determined in 103 human hepatic mitochondrial samples prepared from livers of donors aged 1 day to 84 years. DNA from each sample was genotyped for three common GSTZ1 functional single nucleotide polymorphisms. Expression of mitochondrial GSTZ1 protein increased in an age-dependent manner to a plateau after age 21 years. Activity with DCA correlated with expression, after taking into account the somewhat higher activity of samples that were homo- or heterozygous for GSTZ1A. In samples from livers with the GSTZ1C variant, apparent enzyme kinetic constants for DCA and GSH were similar for mitochondria and cytosol after correcting for the loss of GSH observed in mitochondrial incubations. In the presence of 38 mM chloride, mitochondrial GSTZ1 exhibited shorter half-lives of inactivation compared with the cytosolic enzyme (P = 0.017). GSTZ1 protein isolated from mitochondria was shown by mass spectrometry to be identical to cytosolic GSTZ1 protein in the covered primary protein sequence. In summary, we report age-related development in the expression and activity of human hepatic mitochondrial GSTZ1 does not have the same pattern as that reported for cytosolic GSTZ1. Some properties of cytosolic and mitochondrial GSTZ1 differed, but these were not related to differences in amino acid sequence or post-translationally modified residues.

Pharmacogenetics: Applications to Pediatric Patients

Individual genomic differences may affect drug disposition and effects of many drugs, and identification of biomarkers are crucial to personalize dosage and optimize response. In children, developmental changes associated with growth and maturation translate into different relationships between genotype and phenotype and different responses to treatment compared to adults. This review aims to summarize some developmental aspects of pharmacogenetics, based on practical examples.

Challenges and Opportunities for Increasing the Knowledge Base Related to Drug Biotransformation and Pharmacokinetics during Growth and Development

It is generally acknowledged that there is a need and role for informative pharmacokinetic models to improve predictions and simulation as well as individualization of drug therapy in pediatric populations of different ages and developmental stages. This special issue contains more than 20 papers responding to the challenge of providing new information on scaling factors, ontogeny functions for drug metabolizing enzymes and transporters, the mechanisms underlying the observed developmental trajectories for these gene products, age-dependent changes in physiologic processes affecting drug disposition in children, as well as in vitro and in vivo studies describing the relative contribution of ontogeny and genetic factors as sources of variability in drug disposition in children. Considered together, these contributions serve to illustrate some of the current limitations regarding sample availability, number, and quality, but also provide a framework that allows for the potential value of the results of a given study to be interpreted within the context of these limitations. Among the challenges for the future are improving our understanding of the mechanisms regulating age-dependent changes in factors influencing drug disposition and response, thereby facilitating generalization to systems lacking detailed data, better integrating age-dependent changes in pharmacokinetics with age-dependent changes in pharmacodynamics, and allowing better predictability and individualization of drug disposition and response across the pediatric age spectrum.

A framework and case studies for evaluation of enzyme ontogeny in children's health risk evaluation

Knowledge of the ontogeny of Phase I and Phase II metabolizing enzymes may be used to inform children's vulnerability based upon likely differences in internal dose from xenobiotic exposure. This might provide a qualitative assessment of toxicokinetic (TK) variability and uncertainty pertinent to early lifestages and help scope a more quantitative physiologically based toxicokinetic (PBTK) assessment. Although much is known regarding the ontogeny of metabolizing systems, this is not commonly utilized in scoping and problem formulation stage of human health risk evaluation. A framework is proposed for introducing this information into problem formulation which combines data on enzyme ontogeny and chemical-specific TK to explore potential child/adult differences in internal dose and whether such metabolic differences may be important factors in risk evaluation. The framework is illustrated with five case study chemicals, including some which are data rich and provide proof of concept, while others are data poor. Case studies for toluene and chlorpyrifos indicate potentially important child/adult TK differences while scoping for acetaminophen suggests enzyme ontogeny is unlikely to increase early-life risks. Scoping for trichloroethylene and aromatic amines indicates numerous ways that enzyme ontogeny may affect internal dose which necessitates further evaluation. PBTK modeling is a critical and feasible next step to further evaluate child-adult differences in internal dose for a number of these chemicals.

Determination of Human Hepatic CYP2C8 and CYP1A2 Age-Dependent Expression to Support Human Health Risk Assessment for Early Ages

Predicting age-specific metabolism is important for evaluating age-related drug and chemical sensitivity. Multiple cytochrome P450s and carboxylesterase enzymes are responsible for human pyrethroid metabolism. Complete ontogeny data for each enzyme are needed to support in vitro to in vivo extrapolation (IVIVE). This study was designed to determine age-dependent human hepatic CYP2C8 expression, for which only limited ontogeny data are available, and to further define CYP1A2 ontogeny. CYP2C8 and 1A2 protein levels were measured by quantitative Western blotting using liver microsomal samples prepared from 222 subjects with ages ranging from 8 weeks gestation to 18 years after birth. The median CYP2C8 expression was significantly greater among samples from subjects older than 35 postnatal days (n = 122) compared with fetal samples and those from very young infants (fetal to 35 days postnatal, n = 100) (0.00 vs. 13.38 pmol/mg microsomal protein; p < 0.0001). In contrast, the median CYP1A2 expression was significantly greater after 15 months postnatal age (n = 55) than in fetal and younger postnatal samples (fetal to 15 months postnatal, n = 167) (0.0167 vs. 2.354 pmol/mg microsomal protein; p < 0.0001). CYP2C8, but not CYP1A2, protein levels significantly correlated with those of CYP2C9, CYP2C19, and CYP3A4 (p < 0.001), consistent with CYP2C8 and CYP1A2 ontogeny probably being controlled by different mechanisms. This study provides key data for the physiologically based pharmacokinetic model-based prediction of age-dependent pyrethroid metabolism, which will be used for IVIVE to support pyrethroid risk assessment for early life stages.

Age-Dependent Absolute Abundance of Hepatic Carboxylesterases (CES1 and CES2) by LC-MS/MS Proteomics: Application to PBPK Modeling of Oseltamivir In Vivo Pharmacokinetics in Infants

The age-dependent absolute protein abundance of carboxylesterase (CES) 1 and CES2 in human liver was investigated and applied to predict infant pharmacokinetics (PK) of oseltamivir. The CES absolute protein abundance was determined by liquid chromatography-tandem mass spectrometry proteomics in human liver microsomal and cytosolic fractions prepared from tissue samples obtained from 136 pediatric donors and 35 adult donors. Two surrogate peptides per protein were selected for the quantification of CES1 and CES2 protein abundance. Purified CES1 and CES2 protein standards were used as calibrators, and the heavy labeled peptides were used as the internal standards. In hepatic microsomes, CES1 and CES2 abundance (in picomoles per milligram total protein) increased approximately 5-fold (315.2 vs. 1664.4) and approximately 3-fold (59.8 vs. 174.1) from neonates to adults, respectively. CES1 protein abundance in liver cytosol also showed age-dependent maturation. Oseltamivir carboxylase activity was correlated with protein abundance in pediatric and adult liver microsomes. The protein abundance data were then used to model in vivo PK of oseltamivir in infants using pediatric physiologically based PK modeling and incorporating the protein abundance-based ontogeny function into the existing pediatric Simcyp model. The predicted pediatric area under the curve, maximal plasma concentration, and time for maximal plasma concentration values were below 2.1-fold of the clinically observed values, respectively.

Association of Oseltamivir Activation with Gender and Carboxylesterase 1 Genetic Polymorphisms

Oseltamivir, an inactive anti-influenza virus prodrug, is activated (hydrolysed) in vivo by carboxylesterase 1 (CES1) to its active metabolite oseltamivir carboxylate. CES1 functions are significantly associated with certain CES1 genetic variants and some non-genetic factors. The purpose of this study was to investigate the effect of gender and several CES1 genetic polymorphisms on oseltamivir activation using a large set of individual human liver samples. CES1-mediated oseltamivir hydrolysis and CES1 genotypes, including the G143E (rs71647871), rs2244613, rs8192935, the -816A>C (rs3785161) and the CES1P1/CES1P1VAR, were determined in 104 individual human livers. The results showed that hepatic CES1 protein expression in females was 17.3% higher than that in males (p = 0.039), while oseltamivir activation rate in the livers from female donors was 27.8% higher than that from males (p = 0.076). As for CES1 genetic polymorphisms, neither CES1 protein expression nor CES1 activity on oseltamivir activation was significantly associated with the rs2244613, rs8192935, -816A>C or CES1P1/CES1P1VAR genotypes. However, oseltamivir hydrolysis in the livers with the genotype 143G/E was approximately 40% of that with the 143G/G genotype (0.7 ± 0.2 versus 1.8 ± 1.1 nmole/mg protein/min, p = 0.005). In summary, the results suggest that hepatic oseltamivir activation appears to be more efficient in females than that in males, and the activation can be impaired by functional CES1 variants, such as the G143E. However, clinical implication of CES1 gender differences and pharmacogenetics in oseltamivir pharmacotherapy warrants further investigations.