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.

Nephrotoxicity of concomitant piperacillin/tazobactam and teicoplanin compared with monotherapy

OBJECTIVES

Piperacillin/tazobactam combined with vancomycin has been associated with a decline in renal function when compared with monotherapy. Teicoplanin is a glycopeptide similar to vancomycin. We investigated whether piperacillin/tazobactam combined with teicoplanin is associated with a decline in renal function as well.

METHODS

We conducted a single-centre retrospective cohort study with data from our electronic health records from 9 August 2013 to 15 November 2019, including all adult patients that received either piperacillin/tazobactam, teicoplanin or piperacillin/tazobactam + teicoplanin. The incidence of acute kidney injury (AKI) at 48-72 h served as the primary outcome, whereas change in serum creatinine served as a secondary outcome.

RESULTS

Of the 4202 included patients, 3188 (75.9%) received piperacillin/tazobactam, 791 (18.8%) received teicoplanin and 223 (5.3%) received piperacillin/tazobactam + teicoplanin. The incidence of AKI at 48-72 h after commencement of antibiotic therapy was 5.4% for piperacillin/tazobactam, 3.4% for teicoplanin and 11.7% for piperacillin/tazobactam + teicoplanin (P < 0.001). However, mean serum creatinine at 48-72 h was slightly higher in the piperacillin/tazobactam + teicoplanin group therapy compared with baseline [+1.61% (95% CI -2.25 to 5.70)], indicating a slight decrease in renal function, and decreased for piperacillin/tazobactam [-1.98% (95% CI -2.73 to -1.22)] and teicoplanin [-8.01% (95% CI -9.54 to -6.45)]. After correcting for significant confounders in a multivariate linear regression analysis, these patterns remained.

CONCLUSIONS

Our study suggests that piperacillin/tazobactam + teicoplanin is associated with a higher prevalence of AKI compared with monotherapy. However, as the overall decline in renal function with piperacillin/tazobactam + teicoplanin is very small, its clinical relevance is likely limited. Therefore, piperacillin/tazobactam + teicoplanin can probably be safely combined.

Developmental patterns in human blood-brain barrier and blood-cerebrospinal fluid barrier ABC drug transporter expression

When drugs exert their effects in the brain, linear extrapolation of doses from adults could be harmful for children as the blood-brain barrier (BBB) and blood-CSF barrier (BCSFB) function is still immature. More specifically, age-related variation in membrane transporters may impact brain disposition. As human data on brain transporter expression is scarce, age dependent [gestational age (GA), postnatal age (PNA), and postmenstrual age (PMA)] variation in immunohistochemical localization and staining intensity of the ABC transporters P-glycoprotein (Pgp), breast cancer resistance protein (BCRP), and multidrug resistance-associated proteins 1, 2, 4, and 5 (MRP1/2/4/5) was investigated. Post mortem brain cortical and ventricular tissue was derived from 23 fetuses (GA range 12.9-39 weeks), 17 neonates (GA range 24.6-41.3 weeks, PNA range 0.004-3.5 weeks), 8 children (PNA range 0.1-3 years), and 4 adults who died from a wide variety of underlying conditions. In brain cortical BBB, immunostaining increased with age for Pgp and BCRP, while in contrast, MRP1 and MRP2 staining intensity appeared higher in fetuses, neonates, and children, as compared to adults. BCSFB was positively stained for Pgp, MRP1, and MRP2 and appeared stable across age, while BCRP was not detected. MRP4 and MRP5 were not detected in BBB or BCSFB. In conclusion, human BBB and BCSFB ABC membrane transporters show brain location and transporter-specific maturation.

Human Ontogeny of Drug Transporters: Review and Recommendations of the Pediatric Transporter Working Group

The critical importance of membrane-bound transporters in pharmacotherapy is widely recognized, but little is known about drug transporter activity in children. In this white paper, the Pediatric Transporter Working Group presents a systematic review of the ontogeny of clinically relevant membrane transporters (e.g., SLC, ABC superfamilies) in intestine, liver, and kidney. Different developmental patterns for individual transporters emerge, but much remains unknown. Recommendations to increase our understanding of membrane transporters in pediatric pharmacotherapy are presented.

Pediatric microdose and microtracer studies using 14C in Europe

Important information gaps remain on the efficacy and safety of drugs in children. Pediatric drug development encounters several ethical, practical, and scientific challenges. One barrier to the evaluation of medicines for children is a lack of innovative methodologies that have been adapted to the needs of children. This article presents our successful experience of pediatric microdose and microtracer studies using (14) C-labeled probes in Europe to illustrate the strengths and limitations of these approaches.

Pharmacotherapy in neonatal and pediatric extracorporeal membrane oxygenation (ECMO)

ECMO support is an established life saving therapy for potentially reversible respiratory and/or cardiac failure. Improvement of outcome depends on effective treatment of the primary diagnosis and complications. Adequate drug therapy is important in reaching these goals. Pharmacokinetic and pharmacodynamic data in neonates and older children on ECMO are sparse. Most studies show altered volume of distribution and clearance for the drugs studied. This article gives an overview of the available PK and PD studies in neonates and children on ECMO, suggests possible mechanisms of altered PK and PD and identifies areas of interest for further research.

Feasibility of sedation and analgesia interruption following cannulation in neonates on extracorporeal membrane oxygenation

Purpose

In most extracorporeal membrane oxygenation (ECMO) centers patients are heavily sedated to prevent accidental decannulation and bleeding complications. In ventilated adults not on ECMO, daily sedation interruption protocols improve short- and long-term outcome. This study aims to evaluate safety and feasibility of sedation interruption following cannulation in neonates on ECMO.

Methods

Prospective observational study in 20 neonates (0.17–5.8 days of age) admitted for ECMO treatment. Midazolam (n = 20) and morphine (n = 18) infusions were discontinued within 30 min after cannulation. Pain and sedation were regularly assessed using COMFORT-B and visual analog scale (VAS) scores. Midazolam and/or morphine were restarted and titrated according to protocolized treatment algorithms.

Results

Median (interquartile range, IQR) time without any sedatives was 10.3 h (5.0–24.1 h). Median interruption duration for midazolam was 16.5 h (6.6–29.6 h), and for morphine was 11.2 h (6.7–39.4 h). During this period no accidental extubations, decannulations or bleeding complications occurred.

Conclusions

This is the first study to show that interruption of sedatives and analgesics following cannulation in neonates on ECMO is safe and feasible. Interruption times are 2–3 times longer than reported for adult ICU patients not on ECMO. Further trials are needed to substantiate these findings and evaluate short- and long-term outcomes.

Population pharmacokinetics and metabolism of midazolam in pediatric intensive care patients

OBJECTIVE

To determine the pharmacokinetics and metabolism of midazolam in pediatric intensive care patients.

DESIGN

Prospective population pharmacokinetic study.

SETTING

Pediatric intensive care unit.

PATIENTS

Twenty-one pediatric intensive care patients aged between 2 days and 17 yrs.

INTERVENTIONS

The pharmacokinetics of midazolam and metabolites were determined during and after a continuous infusion of midazolam (0.05-0.4 mg/kg/hr) for 3.8 hrs to 25 days administered for conscious sedation.

MEASUREMENTS AND MAIN RESULTS

Blood samples were taken at different times during and after midazolam infusion for determination of midazolam, 1-OH-midazolam, and 1-OH-midazolam-glucuronide concentrations via high-performance liquid chromatography-ultraviolet detection. A population analysis was conducted via a two-compartment pharmacokinetic model by the NPEM program. The final population model was used to generate individual Bayesian posterior pharmacokinetic parameter estimates. Total body clearance, apparent volume distribution in terminal phase, and plasma elimination half-life were (mean +/- sd, n = 18): 5.0 +/- 3.9 mL/kg/min, 1.7 +/- 1.1 L/kg, and 5.5 +/- 3.5 hrs, respectively. The mean 1-OH-midazolam/midazolam ratio and (1-OH-midazolam + 1-OH-midazolam-glucuronide)/midazolam ratio were 0.14 +/- 0.21 and 1.4 +/- 1.1, respectively. Data from three patients with renal failure, hepatic failure, and concomitant erythromycin-fentanyl therapy were excluded from the final pharmacokinetic analysis.

CONCLUSIONS

We describe population and individual midazolam pharmacokinetic parameter estimates in pediatric intensive care patients by using a population modeling approach. Lower midazolam elimination was observed in comparison to other studies in pediatric intensive care patients, probably as a result of differences in study design and patient differences such as age and disease state. Covariates such as renal failure, hepatic failure, and concomitant administration of CYP3A inhibitors are important predictors of altered midazolam and metabolite pharmacokinetics in pediatric intensive care patients. The derived population model can be useful for future dose optimization and Bayesian individualization.

Pharmacokinetics and metabolism of oral midazolam in preterm infants

AIMS

To characterize the pharmacokinetics and metabolism of oral midazolam in 15 preterm infants.

METHODS

After an oral dose (0.1 mg kg(-1)), blood was drawn up to 24 h after administration. Midazolam and 1-OH-midazolam concentrations were determined with GC-MS. In 8 out of these 15 patients the pharmacokinetics of intravenous midazolam was also studied.

RESULTS

Apparent oral clearance, apparent volume of distribution, plasma half-life and 1-OH-Midazolam/Midazolam AUC ratio were [median (range)]: 2.7 [0.67-15.5] ml kg(-1) min(-1), 1.4 [0.3-12.1] l kg(-1), 7.6 [1.2-15.1], h and 0.03 [0.01-0.96], respectively. Absolute bioavailability was 0.49 [0.12-1.0].

CONCLUSIONS

Midazolam oral clearance is markedly decreased in preterm infants as compared with older children, probably because of immature CYP3A4 activity.

Pharmacokinetics and metabolism of intravenous midazolam in preterm infants

BACKGROUND

Midazolam, a benzodiazepine, is finding expanded use in neonatal intensive care units. We studied the pharmacokinetics and metabolism of midazolam after a single intravenous dose in preterm infants.

METHODS

The pharmacokinetics of midazolam and its hydroxylated metabolite (1-OH-midazolam) after a single 0.1 mg/kg intravenous dose of midazolam were determined in 24 preterm infants (gestational age, 26 to 34 weeks; postnatal age, 3 to 11 days). Blood samples were obtained before drug administration and at 0.5, 1, 2, 4, 6, 12, and 24 hours after the start of the infusion. Midazolam and 1-OH-midazolam concentrations were determined by use of gas chromatography-mass spectrometry.

RESULTS

Total body clearance, apparent volume of distribution, and plasma half-life of midazolam were (median [range]): 1.8 (0.7-6.7) ml/kg per min, 1.1 (0.4-4.2) L/kg, and 6.3 (2.6-17.7) h, respectively. In 19 of 24 preterm infants, 1-OH-midazolam concentrations could be detected: 1-OH-midazolam (1-OH-M) maximal concentration of drug in plasma (C(max)), time to reach C(max) (T(max)), and 1-OH-M/M area under the concentration-time curve from time zero to the last sampling time point (AUC(0-t)) ratio were [median (range)]: 8.2 (<0.5-68.2) ng/ml, 6 (1-12) h, and 0.09 (<0.001-1), respectively. Midazolam plasma clearance was increased in those infants who had indomethacin (INN, indometacin) exposure.

DISCUSSION

Consequent to immature hepatic cytochrome P450 3A4 (CYP3A4) activity, midazolam clearance and 1-OH-midazolam concentrations are reduced markedly in preterm infants as compared to concentrations in previous reports from studies in older children and adults. Indomethacin exposure and its apparent impact on midazolam clearance support alteration of drug disposition produced by a patent ductus arteriosus or by the direct effects of indomethacin on hemodynamic or renal function.

Use of Saliva in Therapeutic Drug Monitoring of Caffeine in Preterm Infants

Caffeine is frequently used to treat apnea of prematurity in preterm infants. Because caffeine has a narrow therapeutic window, plasma concentrations are generally monitored weekly. It would be advantageous to monitor this therapy without blood sampling; saliva might offer this possibility. Paired plasma-saliva and saliva-saliva observations were made in preterm infants (n = 140, gestational ages between 24 and 34 weeks) who received caffeine for the treatment of apnea of prematurity. Three methods were used to collect saliva: no stimulation, dilute citric acid on collection gauze, and dilute citric acid in the cheek pouch before collection. Plasma and saliva caffeine concentrations were determined using high-performance liquid chromatography (HPLC). For all collection methods, the plots of the plasma/saliva outcomes showed linear relationships. The correlation between caffeine concentration in plasma and saliva and the reproducibility of saliva sampling was better with stimulation of saliva production using citric acid in the cheek pouch (r = 0.89) than with no stimulation (r = 0.68) or with stimulation using citric acid on the collection swab (r = 0.79). Monitoring of caffeine therapy in saliva can be applied reliably for routine use in clinical practice, but its reliability and reproducibility depend on the saliva sampling method used; saliva stimulation with citric acid in the cheek pouch is the best method studied.

Cytochrome P450 3A: ontogeny and drug disposition

The maturation of organ systems during fetal life and childhood exerts a profound effect on drug disposition. The maturation of drug-metabolising enzymes is probably the predominant factor accounting for age-associated changes in non-renal drug clearance. The group of drug-metabolising enzymes most studied are the cytochrome P450 (CYP) superfamily. The CYP3A subfamily is the most abundant group of CYP enzymes in the liver and consists of at least 3 isoforms: CYP3A4, 3A5 and 3A7. Many drugs are mainly metabolised by the CYP3A subfamily. Therefore, maturational changes in CYP3A ontogeny may impact on the clinical pharmacokinetics of these drugs. CYP3A4 is the most abundantly expressed CYP and accounts for approximately 30 to 40% of the total CYPcontent in human adult liver and small intestine. CYP3A5 is 83% homologous to CYP3A4, is expressed at a much lower level than CYP3A4 in the liver, but is the main CYP3A isoform in the kidney. CYP3A7 is the major CYP isoform detected in human embryonic, fetal and newborn liver, but is also detected in adult liver, although at a much lower level than CYP3A4. Substrate specificity for the individual isoforms has not been fully elucidated. Because of large interindividual differences in CYP3A4 and 3A5 expression and activity, genetic polymorphisms have been suggested. However, although some gene mutations have been identified, the impact of these mutations on the pharmacokinetics of CYP3A substrates has to be established. Ontogeny of CYP3A activity has been studied in vitro and in vivo. CYP3A7 activity is high during embryonic and fetal life and decreases rapidly during the first week of life. Conversely, CYP3A4 is very low before birth but increases rapidly thereafter, reaching 50% of adult levels between 6 and 12 months of age. During infancy, CYP3A4 activity appears to be slightly higher than that of adults. Large interindividual variations in CYP3A5 expression and activity were observed during all stages of development, but no apparent developmental pattern of CYP3A5 activity has been identified to date. Profound changes occur in the activity of CYP3A isoforms during all stages of development. These changes have, in many instances, proven to be of clinical significance when treatment involves drugs that are substrates, inhibitors or inducers of CYP3A. Investigators and clinicians should consider the impact of ontogeny on CYP3A in both pharmacokinetic study design and data interpretation, as well as when prescribing drugs to children.

Glucuronidation in humans. Pharmacogenetic and developmental aspects

During human development impressive changes in drug disposition occur. An important determinant of drug clearance is metabolism, something that is not only determined by ontogenic regulation but also by genetic processes which add to the variability of drug metabolism during different stages of childhood. Therefore, an understanding of the developmental regulation of different metabolic pathways, together with information on the genetic determinants of drug metabolism, will increase the knowledge of inter- and intraindividual variability in drug disposition during childhood. Conjugation has historically received less attention than cytochrome P450 metabolism. An important group of conjugation reactions are catalysed by the uridine 5'-diphosphate (UDP)-glucuronosyltransferases (UGTs); to date at least 10 different UGT isoforms have been identified. The UGTs are not only involved in the metabolism of many drugs [e.g. morphine, paracetamol (acetaminophen)] but also capable of the biotransformation of important endogenous substrates (e.g. bilirubin, ethinylestradiol) and several xenobiotics. Isoform specificity for these substrates has, however, not been fully characterised. Serious adverse events associated with chloramphenicol toxicity in the neonate have highlighted the importance of developmental changes in UGT activity. However, isoform-specific differences preclude the generalisation of a simple developmental pattern for UGT activity. UGT2B7 is the only UGT isoform for which ontogeny has been characterised both in vitro and in vivo, using morphine as the probe drug. However, no general developmental pattern for the individual UGT isoforms which might be of value for the clinician is currently available. Genetic polymorphisms have been identified for the UGT family. Not only for the UGT1A gene, which reduces bilirubin glucuronidation, leading to genetic hyperbilirubinaemia (the Crigler-Najjar and Gilbert's syndromes), but also for 3 other UGT isoforms. However, the impact of these genetic differences on drug metabolism remains to be established because of overlapping isoform specificity of the drugs studied, as well as a lack of specific probe substrates to test the activity of individual UGT isoforms in relation to these gene mutations. Clearly, an information gap exists regarding the developmental and genetic aspects of UGT regulation and its potential impact on therapy. More research is needed on the pharmacogenetics and ontogeny of the UGTs for effective translation of scientific information into clinically applicable knowledge.

[Excessive terfenadine level due to drinking grapefruit juice]

Doctors who prescribe drugs should be aware not only of potential drug-drug interactions, but also of potential food-drug interactions. In particular, the ingestion of grapefruit juice leads to enhanced systemic bioavailability of terfenadine and of several important and frequently prescribed other drugs. Components of grapefruit juice interfere with the function of intestinal cytochrome P450 3A4, which degrades terfenadine. The resulting enhanced blood levels of terfenadine may lead to heart rate disturbances. Selling the drugs involved over the counter is consequently risky.

Effects of the angiotensin converting enzyme (ACE) inhibitor, captopril, on the cardiovascular, endocrine, and renal responses to furosemide in conscious lambs

To test the hypothesis that angiotensin II modulates the physiological responses to furosemide in the newborn, various parameters of cardiovascular, renal, and endocrine function were measured before and after iv injection of furosemide to eight conscious, chronically instrumented lambs in the presence and absence of the angiotensin converting enzyme (ACE) inhibitor, captopril. During ACE inhibition, the rise in heart rate and decrease in renal blood flow in response to furosemide did not occur, and the natriuretic and diuretic responses to furosemide were attenuated by approximately two-thirds. There was also an increase in the urinary excretion of prostaglandin F2 and prostaglandin F1 alpha as well as an increase in the excretion of prostaglandin E2 after furosemide, in the presence of ACE inhibition. Therefore, the cardiovascular, renal, and endocrine responses to furosemide in conscious lambs were significantly altered by ACE inhibition.

Renal nerves do not modulate the renal and endocrine responses to furosemide in conscious lambs

To test the hypothesis that renal sympathetic nerves influence the renal and renin responses to furosemide in conscious newborn animals, experiments were carried out in chronically instrumented lambs with either bilateral renal denervation (denervated, n = 7, age = 13 +/- 3 (SD) days) or intact renal sympathetic nerves (intact, n = 6, age = 13 +/- 4 (SD) days), at least 4 days after surgery. Basal glomerular filtration rate (GFR), urinary flow, and sodium excretion rates were similar in intact and denervated lambs (p > 0.5). A prompt diuretic and natriuretic response and a decrease in GFR occurred after i.v. furosemide (2 mg/kg); these responses were similar in intact and denervated lambs. Basal plasma renin activity (PRA) was not altered by renal denervation. PRA increased 10 min after furosemide (p < 0.001) and remained elevated at 90 min; the response was similar in both groups of lambs. Basal plasma aldosterone levels were elevated in denervated (191 +/- 232 (SD) pg/mL) compared with intact lambs (62 +/- 24 (SD) pg/mL). Plasma aldosterone levels increased after furosemide administration in both groups of animals. These data provide evidence to suggest that renal sympathetic nerves do not appear to modulate the renal and endocrine responses to furosemide, at least at a dose of 2 mg/kg. Our observations also support the premise that early in life, aldosterone counteracts the effects of renal denervation on fluid and electrolyte homeostasis.