Morphine pharmacokinetics in premature and mature newborn infants

Application of intranasal dexmedetomidine in magnetic resonance imaging of preterm infants: The ED50, efficacy and safety analysis

BACKGROUND

Infants undergoing magnetic resonance imaging (MRI) often require pharmacological sedation. Dexmedetomidine serves as a novel sedative agent that induces a unique unconsciousness similar to natural sleep, and therefore has currently been used as the first choice for sedation in infants and young children.

OBJECTIVE

To determine the 50% effective dose (ED50) and 95% confidence interval (95%CI) of intranasal dexmedetomidine for MRI in preterm and term infants, and to observe the incidence of adverse events. To explore whether there were differences in ED50 and 95%CI, heart rate (HR) and blood oxygen saturation (SpO2), the induction time and wake-up time and the incidence of adverse events between the 2 groups, so as to provide guidance for clinical safe medication for the meanwhile.

METHODS

A total of 68 infants were prospectively recruited for MRI examination under drug sedation (1 week ≤ age ≤ 23 weeks or weight ≤ 5kg). The children were divided into 2 groups according to whether they had preterm birth experience (Preterm group, Atterm group). The Dixon up-and-down method was used to explore ED50. The basic vital signs of the 2 groups were recorded, and the heart rate and SpO2 were recorded every 5 minutes until the infants were discharged from the hospital. The induction time, wake-up time and adverse events were recorded.

RESULTS

The ED50 (95%CI) of intranasal dexmedetomidine in the Preterm group and the Atterm group were 2.23 (2.03-2.66) μg/kg and 2.64 (2.49-2.83) μg/kg, respectively (P < .05). the wake-up time was longer in Preterm group (98.00min) than in Atterm group (81.00 min) (P < .05), the incidence of bradycardia in Preterm group was 3/33, which was higher than that in Atterm group (1/35). There was no difference in the induction time between the 2 groups (P > .05), and there was no significant difference in other adverse events.

CONCLUSIONS

Intranasal dexmedetomidine can be safely used for sedation in preterm infants undergoing MRI. Compared with term infants, preterm infants have a lower dose of dexmedetomidine, a higher incidence of bradycardia, and a longer weak-up time.

Different Effects of Two Protocols for Pre-Procedural Analgosedation on Vital Signs in Neonates during and after Endotracheal Intubation

BACKGROUND

Analgosedation is often used for endotracheal intubation in neonates, but no consensus exists on the optimal pre-procedural medication.

AIMS

To compare the time to intubation and vital signs during and after intubation in 2 NICUs using different premedication protocols.

METHODS

Prospective observational study in 2 tertiary NICUs, comparing fentanyl and optional vecuronium for elective neonatal endotracheal intubation (NICU-1) with atropine, morphine, midazolam and optional pancuronium (NICU-2). Primary endpoints were: time to intubate and number of intubation attempts; secondary endpoints were: deviations of heart rate, oxygen saturation and blood pressure from baseline until 20 min post intubation.

RESULTS

45 and 30 intubations were analyzed in NICU-1 and NICU-2. Time to intubation was longer in NICU-1 (7 min) than in NICU-2 (4 min; p=0.029), but the mean number of intubation attempts did not differ significantly. Bradycardias (34 vs. 1, p<0.001) and hypoxemias (136 vs. 48, p<0.001) were more frequent in NICU-1, and tachycardias (59 vs. 72, p<0.001) more frequent in NICU-2. Mean arterial blood pressure (MAP) increased in NICU-1 (+6.18 mmHg) and decreased in NICU-2 (-5.83 mmHg), whereas mean heart rates (HR) decreased in NICU-1 (-19.29 bpm) and increased in NICU-2 (+15.93 bpm). MAP and HR returned to baseline 6-10 min after intubation in NICU-1 and after 11-15 min and 16-20 min in NICU-2, respectively.

CONCLUSIONS

The two protocols yielded significant differences in the time to intubation and in the extent and duration of physiologic changes during and post-intubation. Short acting drugs should be preferred and vital signs should be closely monitored at least 20 min post intubation. More studies are required to identify analgosedation protocols that minimize potentially harmful events during endotracheal intubation.

Pharmacotherapy of neonatal opioid withdrawal syndrome: a review of pharmacokinetics and pharmacodynamics

INTRODUCTION

Neonatal opioid withdrawal syndrome (NOWS) often arises in infants born to mothers who used opioids during pregnancy. Morphine, methadone, and buprenorphine are the most common first-line treatments, whereas clonidine and phenobarbital are generally reserved for adjunctive therapy. These drugs exhibit substantial pharmacokinetic (PK) and pharmacodynamic (PD) variability. Current pharmacological treatments for NOWS are based on institutional protocols and largely rely on empirical treatment of patient symptoms.

AREAS COVERED

This article reviews the PK/PD of NOWS pharmacotherapies with a focus on the implication of physiological development and maturation. Body size-standardized clearance is consistently low in neonates, except for methadone. This can be ascribed to underdeveloped metabolic and elimination pathways. The effects of pharmacogenetics have been clarified especially for morphine. The PK/PD relationship of medications used in the treatment of NOWS is generally understudied.

EXPERT OPINION

Providing an appropriate opioid dose in neonates is challenging. Advancements in quantitative pharmacology and PK/PD modeling approaches facilitate identification of key factors driving PK/PD variability and characterization of exposure-response relationships. PK/PD model-informed simulations have been widely employed to define age-appropriate pediatric dosing regimens. The model-informed approach holds promise to aid more rational use of medications in the treatment of NOWS.

Age- and Genotype-Dependent Variability in the Protein Abundance and Activity of Six Major Uridine Diphosphate-Glucuronosyltransferases in Human Liver

The ontogeny of hepatic uridine diphosphate-glucuronosyltransferases (UGTs) was investigated by determining their protein abundance in human liver microsomes isolated from 136 pediatric (0-18 years) and 35 adult (age >18 years) donors using liquid chromatography / tandem mass spectrometry (LC-MS/MS) proteomics. Microsomal protein abundances of UGT1A1, UGT1A4, UGT1A6, UGT1A9, UGT2B7, and UGT2B15 increased by ∼8, 55, 35, 33, 8, and 3-fold from neonates to adults, respectively. The estimated age at which 50% of the adult protein abundance is observed for these UGT isoforms was between 2.6-10.3 years. Measured in vitro activity was generally consistent with the protein data. UGT1A1 protein abundance was associated with multiple single nucleotide polymorphisms exhibiting noticeable ontogeny-genotype interplay. UGT2B15 rs1902023 (*2) was associated with decreased protein activity without any change in protein abundance. Taken together, these data are invaluable to facilitate the prediction of drug disposition in children using physiologically based pharmacokinetic modeling as demonstrated here for zidovudine and morphine.

Predictive Performance of a Recently Developed Population Pharmacokinetic Model for Morphine and its Metabolites in New Datasets of (Preterm) Neonates, Infants and Children

BACKGROUND AND OBJECTIVE

Model validation procedures are crucial when models are to be used to develop new dosing algorithms. In this study, the predictive performance of a previously published paediatric population pharmacokinetic model for morphine and its metabolites in children younger than 3 years (original model) is studied in new datasets that were not used to develop the original model.

METHODS

Six external datasets including neonates and infants up to 1 year were obtained from four different research centres. These datasets contained postoperative patients, ventilated patients and patients on extracorporeal membrane oxygenation (ECMO) treatment. Basic observed versus predicted plots, normalized prediction distribution error analysis, model refitting, bootstrap analysis, subpopulation analysis and a literature comparison of clearance predictions were performed with the new datasets to evaluate the predictive performance of the original morphine pharmacokinetic model.

RESULTS

The original model was found to be stable and the parameter estimates were found to be precise. The concentrations predicted by the original model were in good agreement with the observed concentrations in the four datasets from postoperative and ventilated patients, and the model-predicted clearances in these datasets were in agreement with literature values. In the datasets from patients on ECMO treatment with continuous venovenous haemofiltration (CVVH) the predictive performance of the model was good as well, whereas underprediction occurred, particularly for the metabolites, in patients on ECMO treatment without CVVH.

CONCLUSION

The predictive value of the original morphine pharmacokinetic model is demonstrated in new datasets by the use of six different validation and evaluation tools. It is herewith justified to undertake a proof-of-principle approach in the development of rational dosing recommendations - namely, performing a prospective clinical trial in which the model-based dosing algorithm is clinically evaluated.

Metabolism and pharmacokinetics of morphine in neonates: A review

Morphine is an agonist of the µ and k receptors, whose activation results in analgesia. Morphine-like agonists act through the µ opioid receptors to cause pain relief, sedation, euphoria and respiratory depression. Morphine is glucuronidated and sulfated at positions 3 and 6; the plasma concentration ratios correlate positively with birth weight, which probably reflects increased liver weight with increasing birth weight. Moreover, morphine clearance correlates positively with gestational age and birth weight. Steady-state morphine plasma concentrations are achieved after 24-48 hours of infusion, but the glucuronide metabolite plasma concentrations do not reach steady state before 60 hours. The morphine-3-glucuronide metabolite has lower clearance, a shorter half-life and a smaller distribution volume compared with the morphine-6 metabolite, which is the most active morphine-like agonist. Ordinary doses cause constipation, urinary retention and respiratory depression. Neonatal pain relief may require a blood level of approximately 120 ng/ml, whereas lower levels (20-40 ng/ml) seem adequate for children. A bibliographic search was performed using the PubMed database and the keywords "morphine metabolism neonate" and "morphine pharmacokinetics neonate". The initial and final cutoff points were January 1990 and September 2015, respectively. The results indicate that morphine is extensively glucuronidated and sulfated at positions 3 and 6, and that the glucuronidation rate is lower in younger neonates compared with older infants. Although much is known about morphine in neonates, further research will be required to ensure that recommended therapeutic doses for analgesia in neonates are evidence based.

Inter-individual variation in morphine clearance in children

Objectives

The aim of the study was to determine the extent of inter-individual variation in clearance of intravenous morphine in children and to establish which factors are responsible for this variation.

Methods

A systematic literature review was performed to identify papers describing the clearance of morphine in children. The following databases were searched: Medline, Embase, International Pharmaceutical Abstracts, CINAHL, and Cochrane library. From the papers, the range in plasma clearance and the coefficient of variation (CV) in plasma clearance were determined.

Results

Twenty-eight studies were identified. After quality assessment, 20 studies were included. Only 10 studies gave clearance values for individual patients. The majority of the studies were in critically ill patients. Inter-individual variability of morphine clearance was observed in all age groups, but greatest in critically ill neonates (both preterm and term) and infants. In critically ill patients, the CV was 16–9 7 % in preterm neonates, 24–87 % in term neonates, 35 and 134 % in infants, 39 and 55 % in children, and 74 % in adolescents. The CV was 37 and 44 % respectively in non-critically ill neonates and infants. The mean clearance was higher in children (32 and 52 ml min^-1 kg^-1) than in neonates (2 to 16 ml min^-1 kg^-1).

Conclusions

Large inter-individual variation was seen in morphine clearance values in critically ill neonates and infants.

Electronic supplementary material

The online version of this article (doi:10.1007/s00228-015-1843-x) contains supplementary material, which is available to authorized users.

Neonatal liver physiology

In the neonate, the liver is relatively immature and undergoes several changes in its functional capacity during the early postnatal period. The essential liver functions can be classified into three categories: metabolism, detoxification, and bile synthesis. In general, the immature liver function has limited consequences on the healthy term neonate. However, preterm neonates are particularly susceptible to the effects of the immature liver function placing them at risk of hypoglycemia, hyperbilirubinemia, cholestasis, bleeding, and impaired drug metabolism. An appreciation of the dynamic changes in liver function during the neonatal period is essential for successful management of neonates who require medical and surgical interventions. This review will focus on the neonatal liver function as well as the changes that the liver undergoes as it matures.

Pharmacokinetics after a single intravenous dose of the opioid ketobemidone in neonates

BACKGROUND

Ketobemidone is often used as an alternative to morphine in children in the Scandinavian countries. In an earlier study, we have examined the pharmacokinetic properties in children in different age groups but have not focused on neonates. The aim of this clinical trial was to explore the pharmacokinetics of ketobemidone in neonates.

METHODS

Fifteen full-term neonates (eight females) from 37 gestational weeks at birth and scheduled for elective surgery were included in the trial. Their median age was 3 days (range 1-18 days). Ketobemidone hydrochloride was administered as a single intravenous bolus dose, and ketobemidone concentrations were measured by liquid chromatography-mass spectrometry over 10 h. Pharmacokinetic parameters were calculated with standard compartmental methods.

RESULTS

The median (range) values for ketobemidone clearance, apparent volume of distribution, volume of central compartment, distribution half-life and elimination half-life were 0.46 (0.23-0.84) l/h/kg, 4.64 (3.50-7.31) l/kg, 1.71 (0.16-3.47) l/kg, 2.85 (1.04-10.78) min and 7.26 (3.5-11.3) h.

CONCLUSION

Compared with our previous study in children older than 1 year of age, the elimination of ketobemidone appeared to be slower in full-term neonates. Despite a low pharmacokinetic variability of ketobemidone as observed in the present neonatal patient population, we recommend individualizing the dose of ketobemidone based on observations of analgesic efficacy.

Prediction of morphine dose in humans

BACKGROUND

Morphine is widely used throughout the human life span. Several pharmacokinetic models have been proposed to predict how morphine clearance changes with weight and age. This study uses a large external data set to evaluate the ability of pharmacokinetic models to predict morphine doses.

METHODS

A data set of morphine clearance estimates was created from published reports in premature neonates, full-term neonates, infants, children, and adults. This external data set was used to evaluate published models for morphine clearance as well as other models proposed for use in neonates and infants. Morphine clearance predictions were used to predict morphine dose rates to achieve similar target concentrations in all age groups.

RESULTS

An allometric ¾ power model using weight combined with a sigmoid maturation model using postmenstrual age successfully predicted the morphine dose rate (within 25% of target) in all age groups except infants [predicted dose 30% under target (95% CI, 7-46%)]. Other published models based on empirical allometric scaling all made unacceptable predictions (>100% of target) in at least one age group.

CONCLUSIONS

Clearance based on empirical allometric scaling predicted unacceptable doses. Theory-based allometric scaling combined with a maturation function has been confirmed by external evaluation to provide a sound basis for describing clearance and predicting morphine doses in humans of all ages.

A comparison of simple allometric and maturation models for the prediction of morphine clearance in pediatrics

BACKGROUND

The objective of this study is to predict morphine clearance in children (preterm neonates to 10-month-old infants) by maturation models that include age and weight and to compare the predictive performance of morphine maturation models with simple allometric models.

METHODS

Age, weight, and morphine clearance data were obtained from the literature. A maturation model (n=60) for morphine was developed using data from preterm and term neonates to 5-year-old children. The allometric models were developed using the same data as the maturation model. The predictive performance of the models was tested in 88 children of different age groups.

RESULTS

The maturation and allometric models predicted morphine clearance in children with the same degree of accuracy or error. Out of 88 subjects, the prediction error of 50% or less was observed in slightly >60% of the subjects. Almost 40% of the subjects showed a prediction error >50%, 20% of which showed an error ≥ 100%. Although both the maturation and allometric models provided a good prediction of morphine clearance in many children, they were less accurate for many others.

CONCLUSIONS

High intersubject variability in morphine clearance probably led to less than adequate performance of the models. However, there could be many drugs for which intersubject variability in clearance might not be as high as morphine clearance and in those situations these models could perform reasonably well.

Evaluation of a morphine maturation model for the prediction of morphine clearance in children: how accurate is the predictive performance of the model?

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

The pharmacokinetics of morphine are well known in pre-term and term neonates, infants, younger and older children, as well as in adults. There are circumstances when a pharmacokinetic study may not be possible in children (especially neonates and infants), and as a result one would like to predict drug clearance in children. Several methods, such as allometric scaling and prediction based on incorporation of physiological parameters, have been suggested. Recently a morphine maturation model has been proposed to predict morphine clearance in the paediatric population.

WHAT THIS STUDY ADDS

The current study evaluates the predictive performance of the morphine maturation model for the prediction of morphine clearance in pre-term, term, infants and children up to 5 years of age. The results of the study highlight the shortcomings of the model to predict morphine clearance in children in aforementioned age groups.

AIMS

Recently, a maturation model that incorporates a sigmoidal E(max) type model has been proposed for the estimation of morphine clearance in paediatric patients. The primary objective of this report is to evaluate the predictive performance of the morphine maturation model for the prediction of morphine clearance in children of different ages. The secondary objective of this report is to evaluate the predictive performance of exponent 0.75 on bodyweight in the absence of the sigmoidal part of the morphine maturation model.

METHODS

In order to evaluate the predictive performance of the morphine maturation model, the clearance values of morphine for individual children (preterm neonates to 5-year-old children; n= 147) were obtained from the literature. The predicted clearance of morphine in an individual child, obtained from the maturation model as well as from the fixed exponent 0.75 was compared with the observed clearance in that individual child.

RESULTS

The morphine maturation model's predictive power in neonates, infants and younger children is poor and the inclusion of the sigmoidal part in the model only helps in reducing the substantial error introduced in the prediction due to the application of exponent 0.75 on bodyweight. Furthermore, the real benefit of the sigmoidal E(max) part of the model disappears by 1 year of age.

CONCLUSIONS

The morphine maturation model has a poor predictive power of morphine clearance in preterm and term neonates, infants and very young children and may not be of any practical value for the prediction of morphine clearance in this age group.

Morphine glucuronidation in preterm neonates, infants and children younger than 3 years

BACKGROUND AND OBJECTIVE

A considerable amount of drug use in children is still unlicensed or off-label. In order to derive rational dosing schemes, the influence of aging on glucuronidation capacity in newborns, including preterms, infants and children under the age of 3 years was studied using morphine and its major metabolites as a model drug.

METHODS

A population pharmacokinetic model was developed with the nonlinear mixed-effects modelling software NONMEM V, on the basis of 2159 concentrations of morphine and its glucuronides from 248 infants receiving intravenous morphine ranging in bodyweight from 500 g to 18 kg (median 2.8 kg). The model was internally validated using normalized prediction distribution errors.

RESULTS

Formation clearances of morphine to its glucuronides and elimination clearances of the glucuronides were found to be primarily influenced by bodyweight, which was parameterized using an allometric equation with an estimated exponential scaling factor of 1.44. Additionally, a postnatal age of less than 10 days was identified as a covariate for formation clearance to the glucuronides, independent of birthweight or postmenstrual age. Distribution volumes scaled linearly with bodyweight.

CONCLUSIONS

Model-based simulations show that in newborns, including preterms, infants and children under the age of 3 years, a loading dose in microg/kg and a maintenance dose expressed in microg/kg1.5/h, with a 50% reduction of the maintenance dose in newborns younger than 10 days, results in a narrow range of morphine and metabolite serum concentrations throughout the studied age range. Future pharmacodynamic investigations are needed to reveal target concentrations in this population, after which final dosing recommendations can be made.

Pharmacokinetics after an intravenous single dose of the opioid ketobemidone in children

BACKGROUND

Ketobemidone is often used as an alternative to morphine in children in the Scandinavian countries. The aim of this clinical trial was to explore the pharmacokinetics of ketobemidone in children because these properties have not been reported previously.

METHODS

Thirty children, newborn to 10 years, scheduled for elective surgery were included in the trial. Ketobemidone hydrochloride was administered as a single intravenous bolus dose and ketobemidone and norketobemidone concentrations were measured by LC-MS over 8 h. Pharmacokinetic parameters were determined using compartmental methods.

RESULTS

Six children were excluded from pharmacokinetic analysis because of incomplete blood sampling. The values of ketobemidone clearance (l/h/kg) given as median (range) were 0.84 (0.29-3.0) in Group A (0-90 days), 0.89 (0.55-1.35) in Group B (1-2.5 years) and 0.74 (0.50-0.99) in Group C (7-10 years). The corresponding values for apparent volume of distribution (l/kg) were 4.4 (3.7-6.9) (Group A), 2.6 (2.0-5.6) (Group B) and 3.9 (2.7-5.0 (Group C), and for elimination half-life (h) 3.0 (1.4-8.9) (Group A), 2.0 (1.2-4.7) (Group B) and 3.7 (2.4-6.9) (Group C), respectively. In the two neonates the elimination half-life was almost 9 h. The metabolite norketobemidone did not reach levels above the limit of quantification (0.07 ng/ml) in any of the patients.

CONCLUSION

The pharmacokinetic parameters of ketobemidone in children older than 1 month appear to be similar to those in adults. Because of the large interindividual variability of the pharmacokinetics in neonates, further studies especially in this age group are warranted.

Disposition of morphine in plasma and cerebrospinal fluid varies during neonatal development in pigs

The pharmacological effects of morphine are mediated via the central nervous system (CNS) but its clearance from the CNS in neonates has not been investigated. We have proposed that neonatal development of the blood-brain barrier affected CNS clearance mechanisms and CNS exposure to morphine. Male piglets (n=5) aged one, three and six weeks were given morphine sulfate (0.5 mg kg−1, i.v.). Serial blood and cerebrospinal fluid (CSF) samples were withdrawn over 360 min after morphine administration. Morphine concentration was measured by radioimmunoassay. A three-compartment model was fitted to individual data. Estimated parameters were reported as median and range. The peak morphine concentrations in plasma were not significantly different in the one-, three- or six-week-old piglets. Plasma clearance at one week (4.5, 3.8-8.6 mL min−1 kg−1) was significantly lower than at three weeks (30.0, 19.1- 39.0 mL min−1 kg−1) and six weeks (37.0, 29.7–82.8 mL min−1 kg−1). The peak morphine concentration in CSF at one week (59.84, 31–67 ng mL−1) was higher than at three weeks (18.8, 17.7–25 ng mL−1) and six weeks (24.51, 16.5–84 ng mL−1), while CSF clearance was lower at one week (1.0, 0.18-9 mL min−1 kg−1) compared with three weeks (6.2, 2.3–9.3 mL min−1 kg−1) and six weeks (3.95, 1.3–85.7 mL min−1 kg−1). Apparent plasma: CSF transfer ratio at one week was greater than at three and six weeks. The reduced plasma and CSF morphine clearance in early infancy resulted in elevated systemic and central morphine exposure in neonatal pigs.

A Mechanistic Approach for the Scaling of Clearance in Children

BACKGROUND AND OBJECTIVE

Clearance is an important pharmacokinetic concept for scaling dosage, understanding the risks of drug-drug interactions and environmental risk assessment in children. Accurate clearance scaling to children requires prior knowledge of adult clearance mechanisms and the age-dependence of physiological and enzymatic development. The objective of this research was to develop and evaluate ontogeny models that would provide an assessment of the age-dependence of clearance.

METHODS

Using in vitro data and/or in vivo clearance values for children for eight compounds that are eliminated primarily by one process, models for the ontogeny of renal clearance, cytochrome P450 (CYP) 3A4, CYP2E1, CYP1A2, uridine diphosphate glucuronosyltransferase (UGT) 2B7, UGT1A6, sulfonation and biliary clearance were developed. Resulting ontogeny models were evaluated using six compounds that demonstrated elimination via multiple pathways. The proportion of total clearance attributed to each clearance pathway in adults was delineated. Each pathway was individually scaled to the desired age, inclusive of protein-binding prediction, and summed to generate a total plasma clearance for the child under investigation. The paediatric age range included in the study was premature neonates to sub-adults.

RESULTS

There was excellent correlation between observed and predicted clearances for the model development (R2 = 0.979) and test sets (Q2 = 0.927). Clearance in premature neonates could also be well predicted (development R2 = 0.951; test Q2 = 0.899).

CONCLUSION

Paediatric clinical trial development could greatly benefit from clearance scaling, particularly in guiding dosing regimens. Furthermore, since the proportion of clearance via different elimination pathways is age-dependent, information could be gained on the developmental extent of drug-drug interactions.

Development and Evaluation of a Generic Physiologically Based Pharmacokinetic Model for Children

BACKGROUND

Clinical trials in children are being encouraged by regulatory authorities in light of the immense off-label and unlicensed use of drugs in the paediatric population. The use of in silico techniques for pharmacokinetic prediction will aid in the development of paediatric clinical trials by guiding dosing regimens, ensuring efficient blood sampling times, maximising therapeutic effect and potentially reducing the number of children required for the study. The goal of this study was to extend an existing physiologically based pharmacokinetic (PBPK) model for adults to reflect the age-related physiological changes in children from birth to 18 years of age and, in conjunction with a previously developed age-specific clearance model, to evaluate the accuracy of the paediatric PBPK model to predict paediatric plasma profiles.

METHODS

The age-dependence of bodyweight, height, organ weights, blood flows, interstitial space and vascular space were taken from the literature. Physiological parameters that were used in the PBPK model were checked against literature values to ensure consistency. These included cardiac output, portal vein flow, extracellular water, total body water, lipid and protein. Five model compounds (paracetamol [acetaminophen], alfentanil, morphine, theophylline and levofloxacin) were then examined by gathering the plasma concentration-time profiles, volumes of distribution and elimination half-lives from different ages of children and adults. First, the adult data were used to ensure accurate prediction of pharmacokinetic profiles. The model was then scaled to the specific age of children in the study, including the scaling of clearance, and the generated plasma concentration profiles, volumes of distribution and elimination half-lives were compared with literature values.

RESULTS

Physiological scaling produced highly age-dependent cardiac output, portal vein flow, extracellular water, total body water, lipid and protein values that well represented literature data. The pharmacokinetic profiles in children for the five compounds were well predicted and the trends associated with age were evident. Thus, young neonates had plasma concentrations greater than the adults and older children had concentrations less than the adults. Eighty-three percent, 97% and 87% of the predicted plasma concentrations, volumes of distribution and elimination half-lives, respectively, were within 50% of the study reported values. There was no age-dependent bias for term neonates to 18 years of age when examining volumes of distribution and elimination half-lives.

CONCLUSION

This study suggests that the developed paediatric PBPK model can be used to scale pharmacokinetics from adults. The accurate prediction of pharmacokinetic parameters in children will aid in the development of dosing regimens and sampling times, thus increasing the efficiency of paediatric clinical trials.

Efficacy and safety of continuous intravenous infusion of remifentanil in preterm infants undergoing laser therapy in retinopathy of prematurity: clinical experience

BACKGROUND

Preterm infants often require surgery. As experimental evidence suggests that premature infants may experience pain and this could even result in fatal complications, the anaesthesiologist must face problems related to lowbirth weight, high risk of hypothermia, concomitant pulmonary disease and metabolic and receptor immaturity. Recently remifentanil has been considered an optimal analgesic drug in a preterm infant undergoing mechanical ventilation and frequent surgical manoeuvres, but no clinical studies have been reported in the literature. The aim of our study was to evaluate the efficacy of a continuous intravenous infusion of remifentanil in premature infants undergoing laser therapy for retinopathy of prematurity (ROP).

METHODS

Six premature infants with ROP were scheduled for laser therapy. The procedure was performed in the neonatal intensive care unit. Transcutaneous carbon dioxide, pulse oximetry, respiratory rate, ECG and noninvasive blood pressure were continuously monitored. Infusion of remifentanil started with a dose of 0.75-1 microg x kg-1x min-1, 1 h before surgery. A midazolam bolus dose (0.20 mg x kg-1) was administered and the remifentanil infusion was increased to 3-5 microg x kg-1x min-1 taking into account haemodynamic and respiratory changes or spontaneous movements.

RESULTS

Increased dosage was necessary only for 10 min during the procedure. No changes in temperature and ventilatory settings were observed and after 2 h from the surgical procedure the preterm infants were back to their preoperative status.

CONCLUSIONS

A continuous infusion of remifentanil allowed optimal control of surgical stress and a return to preoperative status and ventilatory settings without side-effects.

Urinary effects of morphine in preterm infants

AIM

To describe the association between morphine administration in preterm infants, hydronephrosis, and renal dysfunction.

METHODS

The findings were based on serial ultrasound examinations and blood studies.

RESULTS

Two preterm infants had bladder distension and hydronephrosis after they received intravenous morphine for analgesia. Morphine was used at a low dose. Each patient had a normal urine output and normal serum creatinine before the signs and symptoms of urinary retention were observed. Within 24 h of morphine administration, each infant concurrently developed oliguria and elevation of the serum creatinine. Cessation of morphine and urinary drainage resulted in rapid and complete resolution of the hydronephrosis and the elevated creatinine.

CONCLUSION

Morphine, even at low dosages, can be associated with hydronephrosis in hospitalized preterm infants.

Pharmacokinetics in the newborn

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

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

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

Human variability in glucuronidation in relation to uncertainty factors for risk assessment

The appropriateness of the default uncertainty factor for human variability in kinetics has been investigated for glucuronidation using an extensive database of substrates metabolised primarily by this pathway. Inter-individual variability was quantified for 15 compounds from published pharmacokinetic studies (after oral and intravenous dosing) in healthy adults and other subgroups using parameters relating to chronic exposure (metabolic and total clearances, area under the plasma concentration time-curve (AUC)) and acute exposure (C(max)). Low inter-individual variability (about 30-35%) was found for all parameters (clearance corrected or not corrected for body weight, metabolic clearance, oral AUC and C(max)) after either iv or oral administration to healthy adults. The overall variability of 31% for glucuronidation in healthy adults supported the validity of the default kinetic uncertainty factor of 3.16 for this group, because it would cover more than 99% of individuals. Comparisons between potentially sensitive subgroups and healthy adults using differences in means and variability indicated that neonates showed the greatest impairment of glucuronidation, and that the 3.16 kinetic default factor applied to the mean data for adults would be inadequate for this subpopulation. The in vivo data have been used to derive pathway-related default factors for compounds eliminated largely via glucuronidation.

Morphine pharmacokinetics and pain assessment in premature newborns

OBJECTIVES

To determine morphine pharmacokinetics in premature neonates varying in postconceptional age (PCA) and evaluate behavioral pain response in relationship to serum morphine concentrations.

METHODS

Premature neonates (n = 48), stratified by weeks of PCA (group 1 = 24-27 weeks, group 2 = 28-31 weeks, group 3 = 32-35 weeks, and group 4 = 36-39 weeks) received morphine infusions. Blood samples were drawn at 48, 60, and 72 hours and at discontinuation of morphine, followed by 3 samples obtained during the next 24 hours. Newborns were videotaped during heel lances and restful states, with morphine at steady-state concentrations and without morphine. Pain was assessed by using the Neonatal Facial Coding System (NFCS). Statistical analysis included regression between NFCS score changes from baseline to painful procedure with and without morphine.

RESULTS

Morphine clearance for groups 1, 2, 3, and 4 was calculated as 2.27 +/- 1.07, 3.21 +/- 1.57, 4.51 +/- 1.97, and 7.80 +/- 2.67 mL/kg/min, respectively, and correlated with PCA (r = 0.63, P <.001). Pain measured by facial expression was diminished; however, it did not correlate with morphine concentrations.

CONCLUSION

Morphine clearance in premature neonates is less than reported, increasing with PCA. Facial activity discloses morphine analgesia; however, it is unrelated to morphine concentrations.

The use of intravenous opioids to provide analgesia in critically ill, premature neonates: a research critique

The intravenous (i.v.) administration of opioids to neonates is a common method of providing analgesia in the intensive care setting. This paper reviews and critiques the studies that have investigated this intervention. The studies are grouped into four major areas: pharmacokinetic studies; i.v. opioids for anesthesia; i.v. opioids for sedation or analgesia in mechanically ventilated neonates; and cardiorespiratory effects of i.v. opioids. An attempt is made to synthesize current knowledge about the analgesic effects and side effects associated with the use of i.v. opioids in neonates. The intent is to stimulate additional research on the management of neonatal pain using opioid analgesics.