Pharmacokinetics of midazolam in critically ill neonates

Prediction of Pediatric Pharmacokinetics for CYP3A4 Metabolized Drugs: Comparison of the Performance of Two Hepatic Ontogeny Within a Physiologically Based Pharmacokinetic Model

The rapid growth in the use of pediatric physiologically based pharmacokinetic (PBPK) models, particularly for regulatory applications, has focused emphasis on model verification and ensuring system parameters are robust, including how these change with age. Uncertainty remains regarding the ontogeny of some enzymes and transporters, in this study 2 published ontogeny profiles for hepatic CYP3A4 were compared. Clinical pharmacokinetic data on 4 intravenously administered CYP3A4 substrates (alfentanil, fentanyl, midazolam, and sildenafil) used across the pediatric age range was collected from the literature. The PBPK models were verified in the adult population and then used to compare the Salem and a modified Upreti ontogeny profiles for CYP3A4 in terms of parent drug clearance and area under the curve from birth onward. Overall, the modified Upreti ontogeny profile resulted in 15 out of 17 age-related predictions within 2-fold and 12 out of 17 predictions within 1.5-fold ranges of observed values, for the Salem ontogeny these values were 12 out of 17 and 8 out of 17, respectively. The Upreti ontogeny profile performed better than Salem, average fold error and absolute average fold error were 1.14 and 1.35 compared to 1.56 and 1.90, respectively. Identifying the optimal CYP3A4 ontogeny is important for regulatory use of PBPK especially given the number of drugs cleared by this enzyme. This study broadens the evidence from previous studies that Upreti is more favorable than Salem, but further work is needed especially in the neonatal and early infant age range.

Predicting Volume of Distribution in Neonates: Performance of Physiologically Based Pharmacokinetic Modelling

The volume of distribution at steady state (Vss) in neonates is still often estimated through isometric scaling from adult values, disregarding developmental changes beyond body weight. This study aimed to compare the accuracy of two physiologically based pharmacokinetic (PBPK) Vss prediction methods in neonates (Poulin & Theil with Berezhkovskiy correction (P&T+) and Rodgers & Rowland (R&R)) with isometrical scaling. PBPK models were developed for 24 drugs using in-vitro and in-silico data. Simulations were done in Simcyp (V22) using predefined populations. Clinical data from 86 studies in neonates (including preterms) were used for comparison, and accuracy was assessed using (absolute) average fold errors ((A)AFEs). Isometric scaling resulted in underestimated Vss values in neonates (AFE: 0.61), and both PBPK methods reduced the magnitude of underprediction (AFE: 0.82-0.83). The P&T+ method demonstrated superior overall accuracy compared to isometric scaling (AAFE of 1.68 and 1.77, respectively), while the R&R method exhibited lower overall accuracy (AAFE: 2.03). Drug characteristics (LogP and ionization type) and inclusion of preterm neonates did not significantly impact the magnitude of error associated with isometric scaling or PBPK modeling. These results highlight both the limitations and the applicability of PBPK methods for the prediction of Vss in the absence of clinical data.

Prediction of dolutegravir pharmacokinetics and dose optimization in neonates via physiologically based pharmacokinetic (PBPK) modelling

BACKGROUND

Only a few antiretroviral drugs (ARVs) are recommended for use during the neonatal period and there is a need for more to be approved to increase treatment and prophylaxis strategies. Dolutegravir, a selective integrase inhibitor, has potential for treatment of HIV infection and prophylaxis of transmission in neonates.

OBJECTIVES

To model the pharmacokinetics of dolutegravir in neonates and to simulate a theoretical optimal dosing regimen.

METHODS

The physiologically based pharmacokinetic (PBPK) model was built incorporating the age-related changes observed in neonates. Virtual neonates between 0 and 28 days were simulated. The model was validated against observed clinical data for raltegravir and midazolam in neonates, prior to the prediction of dolutegravir pharmacokinetics.

RESULTS

Both raltegravir and midazolam passed the criteria for model qualification, with simulated data within 1.8-fold of clinical data. The qualified model predicted the pharmacokinetics for several multidose regimens of dolutegravir. Regimen 6 involved 5 mg doses with a 48 h interval from Day 1-20, increasing to 5 mg once daily on Week 3, yielding AUC and Ctrough values of 37.2 mg·h/L and 1.3 mg/L, respectively. These exposures are consistent with those observed in paediatric patients receiving dolutegravir.

CONCLUSIONS

Dolutegravir pharmacokinetics were successfully simulated in the neonatal PBPK model. The predictions suggest that during the first 3 weeks of life a 5 mg dose administered every 48 h may achieve plasma exposures needed for therapy and prophylaxis.

Sedative and Analgesic Pharmacokinetics During Pediatric ECMO

Sedatives and analgesics are often administered to critically ill children supported by extracorporeal membrane oxygenation (ECMO) to facilitate comfort and to decrease risks of life-threatening complications. Optimization of sedative and analgesic dosing is necessary to achieve desired therapeutic benefits and must consider interactions between the circuit and patient that may affect drug metabolism, clearance, and impact on target organs. This paper reviews existing in vitro and pediatric in vivo literature concerning the effects of the ECMO circuit on sedative and analgesic disposition and offers dosing guidance for the management of critically ill children receiving these drugs.

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.

Potential of iontophoresis as a drug delivery method for midazolam in pediatrics

Drug delivery to the neonatal and premature pediatric populations is very challenging. This research assessed the potential of delivering midazolam by transdermal iontophoresis as an alternative strategy in pediatric therapy. In vitro experiments used intact and tape-stripped porcine skin as models for the skin barrier function of full-term and premature newborns, respectively. Midazolam transdermal transport was significantly enhanced by applying higher currents, increasing the formulation pH, and optimizing the drug's mole fraction in the vehicle. When the skin barrier was decreased to half of its baseline competence, the passive permeation of midazolam increased by approximately 60-fold; and complete stratum corneum removal led to an additional 20-fold enhancement in permeation. Iontophoresis retained control of the drug transport trough partially compromised skin. However, a very high passive contribution undermined the iontophoretic control when the barrier was fully compromised. Overall, midazolam delivery could be rate-controlled by iontophoresis in most circumstances, and therapeutically useful fluxes could be achieved.

Scaling clearance in paediatric pharmacokinetics: All models are wrong, which are useful?

Linked Articles

This article is commented on in the editorial by Holford NHG and Anderson BJ. Why standards are useful for predicting doses. Br J Clin Pharmacol 2017; 83: 685–7. doi: 10.1111/bcp.13230

Aim

When different models for weight and age are used in paediatric pharmacokinetic studies it is difficult to compare parameters between studies or perform model‐based meta‐analyses. This study aimed to compare published models with the proposed standard model (allometric weight^0.75 and sigmoidal maturation function).

Methods

A systematic literature search was undertaken to identify published clearance (CL) reports for gentamicin and midazolam and all published models for scaling clearance in children. Each model was fitted to the CL values for gentamicin and midazolam, and the results compared with the standard model (allometric weight exponent of 0.75, along with a sigmoidal maturation function estimating the time in weeks of postmenstrual age to reach half the mature value and a shape parameter). For comparison, we also looked at allometric size models with no age effect, the influence of estimating the allometric exponent in the standard model and, for gentamicin, using a fixed allometric exponent of 0.632 as per a study on glomerular filtration rate maturation. Akaike information criteria (AIC) and visual predictive checks were used for evaluation.

Results

No model gave an improved AIC in all age groups, but one model for gentamicin and three models for midazolam gave slightly improved global AIC fits albeit using more parameters: AIC drop (number of parameters), –4.1 (5), –9.2 (4), –10.8 (5) and –10.1 (5), respectively. The 95% confidence interval of estimated CL for all top performing models overlapped.

Conclusion

No evidence to reject the standard model was found; given the benefits of standardised parameterisation, its use should therefore be recommended.

Anticonvulsant effectiveness and hemodynamic safety of midazolam in full-term infants treated with hypothermia

BACKGROUND

Midazolam is used as an anticonvulsant in neonatology, including newborns with perinatal asphyxia treated with hypothermia. Hypothermia may affect the safety and effectiveness of midazolam in these patients.

OBJECTIVES

The objective was to evaluate the anticonvulsant effectiveness and hemodynamic safety of midazolam in hypothermic newborns and to provide dosing guidance.

METHODS

Hypothermic newborns with perinatal asphyxia and treated with midazolam were included. Effectiveness was studied using continuous amplitude-integrated electroencephalography. Hemodynamic safety was assessed using pharmacokinetic-pharmacodynamic modeling with plasma samples and blood pressure recordings (mean arterial blood pressure) under hypothermia.

RESULTS

No effect of therapeutic hypothermia on pharmacokinetics could be identified. Add-on seizure control with midazolam was limited (23% seizure control). An inverse relationship between the midazolam plasma concentration and mean arterial blood pressure could be identified. At least one hypotensive episode was experienced in 64%. The concomitant use of inotropes decreased midazolam clearance by 33%.

CONCLUSIONS

Under therapeutic hypothermia, midazolam has limited add-on clinical anticonvulsant effectiveness after phenobarbital administration. Due to occurrence of hypotension requiring inotropic support, midazolam is less suitable as a second-line anticonvulsant drug under hypothermia.

Inter-individual variation in midazolam clearance in children

OBJECTIVES

To determine the extent of inter-individual variation in clearance of midazolam in children and establish which factors are responsible for this variation.

METHODS

A systematic literature review was performed to identify papers describing the clearance of midazolam 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

25 articles were identified. Only 13 studies gave the full range of clearance values for individual patients. The CV was greater in critically ill patients (18%-170%) than non-critically ill patients (13%-54%). Inter-individual variation was a major problem in all age groups of critically ill patients. The CV was 72%-106% in preterm neonates, 18%-73% in term neonates, 31%-130% in infants, 21%-170% in children and 47%-150% in adolescents. The mean clearance was higher in children (1.1-16.7 mL/min/kg) than in neonates (0.78-2.5 mL/min/kg).

CONCLUSIONS

Large inter-individual variation was seen in midazolam clearance values in critically ill neonates, infants, children and adolescents.

Clinical pharmacology of midazolam in neonates and children: effect of disease-a review

Midazolam is a benzodiazepine with rapid onset of action and short duration of effect. In healthy neonates the half-life (t_1/2) and the clearance (Cl) are 3.3-fold longer and 3.7-fold smaller, respectively, than in adults. The volume of distribution (Vd) is 1.1 L/kg both in neonates and adults. Midazolam is hydroxylated by CYP3A4 and CYP3A5; the activities of these enzymes surge in the liver in the first weeks of life and thus the metabolic rate of midazolam is lower in neonates than in adults. Midazolam acts as a sedative, as an antiepileptic, for those infants who are refractory to standard antiepileptic therapy, and as an anaesthetic. Information of midazolam as an anaesthetic in infants are very little. Midazolam is usually administered intravenously; when minimal sedation is required, intranasal administration of midazolam is employed. Disease affects the pharmacokinetics of midazolam in neonates; multiple organ failure reduces the Cl of midazolam and mechanical ventilation prolongs the t_1/2 of this drug. ECMO therapy increases t_1/2, Cl, and Vd of midazolam several times. The adverse effects of midazolam in neonates are scarce: pain, tenderness, and thrombophlebitis may occur. Respiratory depression and hypotension appear in a limited percentage of infants following intravenous infusion of midazolam. In conclusion, midazolam is a safe and effective drug which is employed as a sedative, as antiepileptic agent, for infants who are refractory to standard antiepileptic therapy, and as an anaesthetic.

A novel maturation function for clearance of the cytochrome P450 3A substrate midazolam from preterm neonates to adults

BACKGROUND AND OBJECTIVE

Major changes in cytochrome P450 (CYP) 3A activity may be expected in the first few months of life with, later, relatively limited changes. In this analysis we studied the maturation of in vivo CYP3A-mediated clearance of midazolam, as model drug, from preterm neonates of 26 weeks gestational age (GA) to adults.

METHODS

Pharmacokinetic data after intravenous administration of midazolam were obtained from six previously reported studies. Subjects were premature neonates (n = 24; GA 26-33.5 weeks, postnatal age (PNA) 3-11 days, and n = 24; GA 26-37 weeks, PNA 0-1 days), 23 children after elective major craniofacial surgery (age 3-23 months), 18 pediatric intensive-care patients (age 2 days-17 years), 18 pediatric oncology patients (age 3-16 years), and 20 healthy male adults (age 20-31 years). Population pharmacokinetic modeling with systematic covariate analysis was performed by use of NONMEM v6.2.

RESULTS

Across the entire lifespan from premature neonates to adults, bodyweight was a significant covariate for midazolam clearance. The effect of bodyweight was best described by use of an allometric equation with an exponent changing with bodyweight in an exponential manner from 0.84 for preterm neonates (0.77 kg) to 0.44 for adults (89 kg), showing that the most rapid maturation occurs during the youngest age range.

CONCLUSIONS

An in-vivo maturation function for midazolam clearance from premature neonates to adults has been developed. This function can be used to derive evidence-based doses for children, and to simulate exposure to midazolam and possibly other CYP3A substrates across the pediatric age range in population pharmacokinetic models or physiologically based pharmacokinetic models.

Sedation and analgesia in mechanically ventilated preterm neonates: continue standard of care or experiment?

Attention to comfort and pain control are essential components of neonatal intensive care. Preterm neonates are uniquely susceptible to pain and agitation, and these exposures have a negative impact on brain development. In preterm neonates, chronic pain and agitation are common adverse effects of mechanical ventilation, and opiates or benzodiazepines are the pharmacologic agents most often used for treatment. Questions remain regarding the efficacy, safety, and neurodevelopmental impact of these therapies. Both preclinical and clinical data suggest troubling adverse drug reactions and the potential for adverse longterm neurodevelopmental impact. The negative impacts of standard pharmacologic agents suggest that alternative agents should be investigated. Dexmedetomidine is a promising alternative therapy that requires further interprofessional and multidisciplinary research in this population.

Abnormal Movements of Japanese Infants following Treatment with Midazolam in a Neonatal Intensive Care Unit: Incidence and Risk Factors

Background. This study was conducted to investigate the incidence of, and factors associated with, myoclonus-like abnormal movements of Japanese infants following treatment with midazolam in a neonatal intensive care unit (NICU). Methods. We retrospectively investigated abnormal movements and associated risk factors in Japanese infants (less than 1 year old) who received continuous intravenous midazolam treatment in the NICU of the Neonatal Medical Center, Kumamoto City Hospital, Japan, between April 2007 and March 2009. Results. The study included 94 infants who received 119 sessions of midazolam treatment in total. Nine infants (9.6%) developed abnormal movements attributable to midazolam. These nine patients had a significantly lower gestational age at birth, a significantly lower number of weeks after conception at the start of midazolam treatment, and significantly lower body weight compared with patients free of abnormal movements. Logistic regression analysis revealed neonatal asphyxia as a factor associated with an elevated risk of abnormal movements (P = 0.03). Conclusion. The incidence of abnormal movements after midazolam treatment was about 9.6% among the Japanese NICU infants. This result suggests that neonatal asphyxia may be involved in the onset of abnormal movements in infants treated with midazolam.

Prediction of drug clearance in children 3 months and younger: an allometric approach

BACKGROUND

Sometimes it might not be possible to conduct a pharmacokinetic (PK) study in neonates and infants. Under these circumstances, one would like to predict PK parameters in this age group. Because drug clearance is the most important PK parameter, the objective of this study was to describe an allometric method to predict drug clearance in children ≤3 months.

METHODS

In total, 43 drugs (107 observations) were randomly selected for this study. The age of the children ranged from 0 to 1 year. Children were divided into two groups: ≤3 months and ≥3 months to 1 year. Drug clearance (CL) in children was predicted using the following equation: CL in the child=adult CL×(weight of the child/70)(0.75 or 1.0 or 1.2).

RESULTS

The results of the study indicated that the exponent 1.2 performs better in the prediction of drug clearance than exponent 1.0 or 0.75 for children ≤3 months. By contrast, exponent 1.0 provided better prediction for children ≥3 months to 1 year than exponent 1.2. Exponent 0.75 provided the worst results leading to substantial prediction error in children 0-1 year (in many instances more than 1000% prediction error).

CONCLUSIONS

Overall, it appears that exponent 1.2 is the best method out of three methods for reasonably accurate prediction of drug clearance in children ≤3 months old. However, exponent 1.2 will underpredict drug clearance in children older than 3 months. The suggested approach could be used to support the choice of the initial dose in clinical trials for children ≤3 months old.

A maturation model for midazolam clearance

BACKGROUND

Physiological-based pharmacokinetic models have been used to describe midazolam clearance (CL) maturation. There are no maturation descriptors of CL from neonate to adulthood based on reported estimates at different ages.

METHODS

Published CL estimates after intravenous administration from time-concentration profiles were used to construct a maturation model based on size and age. Curve fitting was performed using nonlinear mixed effects models.

RESULTS

There were 16 publications reporting an estimate of CL after intravenous administration in children, although few estimates were available from 44-80 weeks postmenstrual age (PMA). CL maturation, standardized to a 70 -kg person was described using the Hill equation. Mature CL was 523 (CV 32%, 95%CI 469, 597) ml·min(-1) ·70 kg(-1) . The maturation half-time was 73.6 (95%CI 59.4, 80.0) weeks PMA and the Hill coefficient 3 (95%CI 2.2, 4.1). Predicted CL changes with age based on this model were in close agreement with physiologically based pharmacokinetic (PBPK) models. A comparison with a published PBPK model predictions revealed a root mean squared prediction error (precision) of 4.0% (95%CI 1.1, 5.8) and bias was -0.9% (95%CI -4.3, 2.6).

CONCLUSIONS

Previously published pharmacokinetic parameters can be used to develop maturation models that address gaps in current knowledge regarding the influence of age on a drug's disposition. If a midazolam sedation target concentration of 0.1 mg·l(-1) , similar to that given to adults, is assumed, then we might anticipate steady-state infusion rates of 0.014 mg·kg(-1) ·h(-1) in neonates, 0.05 mg·kg(-1) ·h(-1) in a 1-year-old, 0.06 mg·kg(-1) ·h(-1) in a 5-year-old and 0.05 mg·kg(-1) ·h(-1) in a 12-year-old child. Age-related pharmacodynamic differences that will affect dose and the impact of active metabolites on response are not yet quantified.

Influence of antiepileptic drugs on amplitude-integrated electroencephalography

Amplitude-integrated electroencephalography monitors different aspects of cerebral function in neonatal intensive care units. To examine the influence of various antiepileptic drugs on the background patterns and voltage of amplitude-integrated electroencephalography recordings, we screened 191 tracing segments originating from 77 newborns treated with antiepileptic drugs. The influences of lorazepam, diazepam, and phenobarbital given as bolus doses, and midazolam and lidocaine given in continuous infusion, were examined. Voltages and patterns before and after drug administration were assessed. Time taken to return to previous voltage was assessed in clinically significant cases. Chi-square and Wilcoxon tests were used for statistical analyses. Significant changes were evident after lorazepam, diazepam, phenobarbital, and midazolam administration. Depending on the voltage-assessment method, a clinically significant depression of the lower voltage border occurred in 25-35% of tracings, and of the upper border in 16-32%. In 12% of tracings, change to a worse pattern was noted. The average time for recovery to predrug administration voltage was 2.5 hours (range, 15 minutes to 15 hours). Changes in amplitude-integrated electroencephalography tracings occur after antiepileptic drugs are infused. These changes include deterioration of pattern and depression of voltage that may persist for a considerable period. The potential depressing effects of these drugs should be taken into consideration when assessing amplitude-integrated electroencephalogram tracings.

Do ventilated neonates require pain management?

Mechanical ventilation is a stressful experience in neonates resulting in changes in neuroendocrine parameters, pain scores, and physiologic responses. Assisted ventilation in neonates is presumed to be associated with chronic repetitive pain, which in turn is associated with adverse long-term sequelae. Reasons to routinely sedate ventilated neonates include improved ventilator synchrony, improved pulmonary function, and decreased neuroendocrine responses, including cortisol, beta-endorphine, and catecholamines. Reasons not to treat include the well-known adverse side effects of pain medication, especially the opiates, including hypotension from morphine, chest wall rigidity from fentanyl, and tolerance, dependence, and withdrawal from both opiates and benzodiazepines. Additionally, adverse effects such as death and IVH are not improved with preemptive treatment. Chronic pain assessment is poorly validated and difficult to assess in this population, and most studies have evaluated only acute pain scores. If patients are treated, opiates are the most common class of drugs, with morphine the most well studied. Fentanyl may be advantageous in hypotensive, younger neonates because it has fewer cardiovascular effects. The benzodiazepines, midazolam and lorazepam, have been used in ventilated neonates, but midazolam has been associated with adverse effects in one small study so concern remains regarding its use. Significant gaps in our knowledge exist, especially in regard to long-term effects of treatment, or lack thereof, and in the assessment of the chronic pain associated with assisted ventilation.

Prediction of Drug Clearance in Children: Impact of Allometric Exponents, Body Weight, and Age

In recent years, with the advent of pediatric exclusivity and requirements for conducting clinical studies involving children, emphasis has been placed on finding safe and efficacious doses of drugs for children. It has been suggested that one can predict the clearance (CL) of a drug in children according to this equation: CL in the child = Adult CL * (Weight of the child/70)0.75. In light of the controversy surrounding the exponent of 0.75 for the prediction of clearance, the objectives of the study were as follows: (1) to develop allometric equations based on body weight or age to predict clearance of a drug in children; (2) to determine if the fixed exponent of 0.75 is a suitable exponent for the prediction of clearance in children from adult data, as compared with the allometric exponent generated for individual drugs; (3) to determine if the allometric equation generated on the basis of age predicts clearance in children better or worse than the allometric equation generated on the basis of body weight; and (4) to propose a new approach based on the findings of the current evaluation. Five methods were used to predict drug clearance in children. Six drugs were used in the evaluation, and drug clearance in each child was predicted for a given drug. Besides evaluating the exponent of 0.75, allometric equations were developed using double log plots of clearance versus body weight or age. The exponents of the allometric equations were then used to predict drug clearance by replacement of 0.75 in the aforementioned equation. The results of the study indicate that 0.75 is not the best exponent for prediction of drug clearance in children, and a more suitable approach is to develop an allometric relationship for a given drug in children. For all 6 drugs, there were 77 children in whom the clearance was predicted. There were 48 observations for which error in the predicted clearance was 50% or more with use of the exponent 0.75, whereas there were only 13 observations with prediction error > or = 50% when 0.75 was replaced by an allometric exponent developed for a given drug. In order to predict drug clearance in children with reasonable accuracy, an allometric equation should be developed for every drug and the exponent 0.75 should be replaced by the exponent of the allometric equation developed for that drug.

Analgesia and local anesthesia during invasive procedures in the neonate

BACKGROUND

Preterm and full-term neonates admitted to the neonatal intensive care unit or elsewhere in the hospital are routinely subjected to invasive procedures that can cause acute pain. Despite published data on the complex behavioral, physiologic, and biochemical responses of these neonates and the detrimental short- and long-term clinical outcomes of exposure to repetitive pain, clinical use of pain-control measures in neonates undergoing invasive procedures remains sporadic and suboptimal. As part of the Newborn Drug Development Initiative, the US Food and Drug Administration and the National Institute of Child Health and Human Development invited a group of international experts to form the Neonatal Pain Control Group to review the therapeutic options for pain management associated with the most commonly performed invasive procedures in neonates and to identify research priorities in this area.

OBJECTIVE

The goal of this article was to review and synthesize the published clinical evidence for the management of pain caused by invasive procedures in preterm and full-term neonates.

METHODS

Clinical studies examining various therapies for procedural pain in neonates were identified by searches of MEDLINE (1980-2004), the Cochrane Controlled Trials Register (The Cochrane Library, Issue 1, 2004), the reference lists of review articles, and personal files. The search terms included specific drug names, infant-newborn, infant-preterm, and pain, using the explode function for each key word. The English-language literature was reviewed, and case reports and small case series were discarded.

RESULTS

The most commonly performed invasive procedures in neonates included heel lancing, venipuncture, IV or arterial cannulation, chest tube placement, tracheal intubation or suctioning, lumbar puncture, circumcision, and SC or IM injection. Various drug classes were examined critically, including opioid analgesics, sedative/hypnotic drugs, nonsteroidal anti-inflammatory drugs and acetaminophen, injectable and topical local anesthetics, and sucrose. Research considerations related to each drug category were identified, potential obstacles to the systematic study of these drugs were discussed, and current gaps in knowledge were enumerated to define future research needs. Discussions relating to the optimal design for and ethical constraints on the study of neonatal pain will be published separately. Well-designed clinical trials investigating currently available and new therapies for acute pain in neonates will provide the scientific framework for effective pain management in neonates undergoing invasive procedures.

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.

Prediction of Cytochrome P450-Mediated Hepatic Drug Clearance in Neonates, Infants and Children

Correct dosing of drugs in neonates, infants and children is hampered by a general lack of knowledge about drug disposition in this population. Suggested methods to improve our knowledge without performing conventional full-scale investigations include population pharmacokinetic studies, allometric scaling of drug disposition according to bodyweight and in silico prediction of pharmacokinetics. The last method entails scaling of pharmacokinetic parameters according to age-dependent changes in drug absorption and elimination capacity, plasma protein binding and physiological characteristics of the subjects. Maturation (or ontogeny) of the drug-metabolising part of the cytochrome P450 (CYP) enzyme system is thus an important factor in the calculations for most drugs. The aim of this commentary is to test and critically examine the proposed methods to estimate hepatic clearance (CL) as a function of age (0-20 years), with CYP3A-mediated metabolism as the case in point. Midazolam and alfentanil were used as model drugs. Allometric scaling failed to predict the CL of midazolam and alfentanil in neonates. Calculations using in vitro findings on CYP maturation gave better estimates for neonates but very divergent ones for older infants and children. This was chiefly due to very different data on CYP3A4/5 ontogeny in three published studies. In the age range where full adult CYP activity per gram of liver could be assumed, allometric scaling and in silico predictions gave similar results. These predictions were also in approximate agreement with clinical data.The findings with the two model drugs can very probably be generalised to most drugs cleared by CYP-dependent hepatic metabolism. Allometric scaling accounts for development of body size and function but not for the fact that the drug-metabolising capacity of the liver is generally low at birth. The crucial question in the prediction of CL is thus when the activity of the applicable CYP isoform(s) attains adult levels. There are still not enough data on this, particularly when different studies even on the same CYP isoform have given very divergent results. It may also be pointed out that CYP ontogeny is an area where we have at least some information. There are several other important developmental changes about which we know practically nothing. Thus, while allometric scaling is generally unreliable for prediction in neonates and infants, the alternative method of in silico prediction can at present be used only to obtain tentative initial estimates of drug CL. Neither of the methods can be used as a substitute for actual clinical studies.

Minimum effective dose of midazolam for sedation of mechanically ventilated neonates

OBJECTIVE

To determine the minimal effective dose (MED) of intravenous midazolam, required for appropriate sedation in 95% of patients, 1 h after drug administration.

METHODS

A double-blind dose-finding study using the continual reassessment method, a Bayesian sequential design. Twenty-three newborn infants hospitalized in intensive care unit participated. Inclusion criteria were: (i) post-natal age <28 days, (ii) gestational age >33 weeks, (iii) intubation and ventilatory support required for respiratory distress syndrome, (iv) need for sedation (i.e. one of the six following criteria: agitation or grimacing or crying facial expression before tracheal suctioning, agitation or grimacing or crying facial expression during tracheal suctioning). Each neonate was allocated to a loading dose, ranging from 75 to 200 microg/kg, and a maintenance dose ranging from 37.5 to 100 microg/kg/h.

RESULTS

The primary endpoint was the level of sedation 1 h after the onset of infusion. The sedation procedure was classified as a success if all the following clinical criteria were met: no agitation, no grimacing and no crying facial expression before as well as during tracheal suctioning. Based on the 23 patients, the final estimated probability of success was 76.9% (95% credibility interval: 56.6-91.4%) for the 200 microg/kg loading dose. no significant adverse effect was observed.

CONCLUSIONS

Continual reassessment is a new approach, suitable for dose-finding study in neonates. this method overcomes some of the ethical, statistical and practical problems associated with this population. Using this method, the MED was estimated to be the 200 mug/kg loading dose of midazolam.

Prediction of drug disposition in infants and children by means of physiologically based pharmacokinetic (PBPK) modelling: theophylline and midazolam as model drugs

AIMS

To create a general physiologically based pharmacokinetic (PBPK) model for drug disposition in infants and children, covering the age range from birth to adulthood, and to evaluate it with theophylline and midazolam as model drugs.

METHODS

Physiological data for neonates, 0.5-, 1-, 2-, 5-, 10- and 15-year-old children, and adults, of both sexes were compiled from the literature. The data comprised body weight and surface area, organ weights, vascular and interstitial spaces, extracellular body water, organ blood flows, cardiac output and glomerular filtration rate. Tissue: plasma partition coefficients were calculated from rat data and unbound fraction (f u) of the drug in human plasma, and age-related changes in unbound intrinsic hepatic clearance were estimated from CYP1A2 and CYP2E1 (theophylline) and CYP3A4 (midazolam) activities in vitro. Volume of distribution (V dss), total and renal clearance (CL and CL R) and elimination half-life (t(1/2)) were estimated by PBPK modelling, as functions of age, and compared with literature data.

RESULTS

The predicted V dss of theophylline was 0.4-0.6 l kg(-1) and showed only a modest change with age. The median prediction error (MPE) compared with literature data was 3.4%. Predicted total CL demonstrated the time-course generally reported in the literature. It was 20 ml h(-1) kg(-1) in the neonate, rising to 73 ml h(-1) kg(-1) at 5 years and then decreasing to 48 ml h(-1) kg(-1) in the adult. Overall, the MPE was - 4.0%. Predicted t(1/2) was 18 h in the neonate, dropping rapidly to 4.6-7.2 h from 6 months onwards, and the MPE was 24%. The predictions for midazolam were also in good agreement with literature data. V dss ranged between 1.0 and 1.7 l kg(-1) and showed only modest change with age. CL was 124 ml h(-1) kg(-1) in the neonate and peaked at 664 ml h(-1) kg(-1) at 5 years before decreasing to 425 ml h(-1) kg(-1) in the adult. Predicted t(1/2) was 6.9 h in the neonate and attained 'adult' values of 2.5-3.5 h from 1 year onwards.

CONCLUSIONS

A general PBPK model for the prediction of drug disposition over the age range neonate to young adult is presented. A reference source of physiological data was compiled and validated as far as possible. Since studies of pharmacokinetics in children present obvious practical and ethical difficulties, one aim of the work was to utilize maximally already available data. Prediction of the disposition of theophylline and midazolam, two model drugs with dissimilar physicochemical and pharmacokinetic characteristics, yielded results that generally tallied with literature data. Future use of the model may demonstrate further its strengths and weaknesses.

Analgesia and anesthesia for neonates: Study design and ethical issues

OBJECTIVE

The purpose of this article is to summarize the clinical, methodologic, and ethical considerations for researchers interested in designing future trials in neonatal analgesia and anesthesia, hopefully stimulating additional research in this field.

METHODS

The MEDLINE, PubMed, EMBASE, and Cochrane register databases were searched using subject headings related to infant, newborn, neonate, analgesia, anesthesia, ethics, and study design. Cross-references and personal files were searched manually. Studies reporting original data or review articles related to these topics were assessed and critically evaluated by experts for each topical area. Data on population demographics, study characteristics, and cognitive and behavioral outcomes were abstracted and synthesized in a systematic manner and refined by group members. Data synthesis and results were reviewed by a panel of independent experts and presented to a wider audience including clinicians, scientists, regulatory personnel, and industry representatives at the Newborn Drug Development Initiative workshop. Recommendations were revised after extensive discussions at the workshop and between committee members.

RESULTS

Designing clinical trials to investigate novel or currently available approaches for analgesia and anesthesia in neonates requires consideration of salient study designs and ethical issues. Conditions requiring treatment include pain/stress resulting from invasive procedures, surgical operations, inflammatory conditions, and routine neonatal intensive care. Study design considerations must define the inclusion and exclusion criteria, a rationale for stratification, the confounding effects of comorbid conditions, and other clinical factors. Significant ethical issues include the constraints of studying neonates, obtaining informed consent, making risk-benefit assessments, defining compensation or rewards for participation, safety considerations, the use of placebo controls, and the variability among institutional review boards in interpreting federal guidelines on human research. For optimal study design, investigators must formulate well-defined study questions, choose appropriate trial designs, estimate drug efficacy, calculate sample size, determine the duration of the studies, identify pharmacokinetic and pharmacodynamic parameters, and avoid drug-drug interactions. Specific outcome measures may include scoring on pain assessment scales, various biomarkers and their patterns of response, process outcomes (eg, length of stay, time to extubation), intermediate or long-term outcomes, and safety parameters.

CONCLUSIONS

Much more research is needed in this field to formulate a scientifically sound, evidence-based, and clinically useful framework for management of anesthesia and analgesia in neonates. Newer study designs and additional ethical dilemmas may be defined with accumulating data in this field.

Burst suppression on amplitude-integrated electroencephalogram may be induced by midazolam: a report on three cases

Continuous amplitude-integrated electroencephalogram (aEEG) recording with a cerebral function monitor is a useful tool to evaluate prognoses following perinatal asphyxia in term infants. Drugs may change the pattern of the conventional EEG. This report presents three infants treated with midazolam for status epilepticus and repetitive seizures who proved resistant to other anticonvulsants (phenobarbitone, lidocaine). The infants developed burst suppression patterns on aEEG concurrent with high serum levels of midazolam (900-7093 microg l(-1)). Following discontinuation of midazolam treatment, serum levels normalized and background patterns returned to normal voltage traces.

CONCLUSION

These findings indicated that midazolam can cause burst suppression on aEEG. Therefore, the prognostic value of aEEG is limited in case of high serum levels of midazolam. Serum levels of midazolam should be measured in infants who have burst suppression patterns on aEEG during midazolam treatment.

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

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

Plasma concentrations of midazolam in neonates receiving extracorporeal membrane oxygenation

Drug disposition is affected during extracorporeal membrane oxygenation (ECMO). This study investigates the dose-concentration relationship of midazolam in neonates requiring ECMO during continuous infusion into the circuit (extracorporeally; n = 10) and intravenously (n = 10). Data on hourly doses and sedation scores were collected for 120 hours. Plasma concentrations were analyzed at times 0, 2, 4, 6, 12, 18, and 24, and every 12 hours thereafter. Both groups were clinically similar. Mean (standard deviation) dose for all patients was 250 (185) microg/kg/h, four times greater than previously reported. Doses administered in the first 24 hours were significantly greater extracorporeally [361 (300)] compared with intravenous [258 (190) microg/kg/h, p < 0.001]. Mean (standard deviation) plasma concentrations in all patients at 24, 48, and 72 hours were 1.4 (0.9), 1.8 (1.2), and 2.6 (1.8) microg/ml, respectively. Satisfactory sedation levels were achieved in all patients. Comparison of the actual observed with predicted (simulated) midazolam concentrations suggested significant attenuation of plasma levels during the first 24 hours of ECMO. However, at 48 hours, observed concentrations exceeded those predicted, suggesting accumulation. We conclude that in the first 24 hours of ECMO, because of an expanded circulating volume and sequestration by the circuit, significantly more midazolam is required to achieve adequate sedation. Subsequently, and because of circuit saturation, maintenance doses should be reduced.

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.

Epileptic manifestations induced by midazolam in the neonatal period

Antiepileptic drugs may cause worsening of epilepsy by aggravating pre-existing seizures or by triggering new seizure types. There are several reports of adverse effects related to midazolam, but only a few authors reported epileptic manifestations. We report four newborns seen at the Neonatal Intensive Care Unit of our University Hospital, who developed seizures a few seconds after the administration of midazolam. It is difficult to identify the patients at risk, but it is important to be aware and recognize this situation.

Flumazenil's Reversal of Myoclonic-like Movements Associated with Midazolam in Term Newborns

Sedation is an important aspect of care for critically ill newborns. Proper sedation reduces stress during procedures such as mechanical ventilation. Midazolam, a short-acting benzodiazepine, is widely administered as a sedative in newborn intensive care units but is not without side effects. Three term newborns developed myoclonic-like abnormal movements after receiving midazolam. In one, flumazenil controlled the abnormal movements. Flumazenil is a potent benzodiazepine antagonist that competitively blocks the central effects of benzodiazepines. It can reverse the sedative effects of benzodiazepines occurring after diagnostic or therapeutic procedures or after benzodiazepine overdose. Flumazenil may be considered in cases of abnormal movements associated with midazolam. However, further studies are needed to provide guidelines for the administration of this drug in newborns.

Pharmacokinetics of midazolam in critically ill pediatric patients

Midazolam is frequently used to produce sedation in critically ill pediatric patients. We studied the pharmacokinetics of midazolam in 22 patients (age 8 days to 16 years). The intravenous infusion rate to produce sedation ranged from 49-385 mcg/kg/hr. The blood samples were obtained at steady-state and midazolam was measured by gas chromatography with electron capture. The steady-state plasma concentrations of midazolam ranged from 49-385 ng/mL. The total clearance, apparent volume of distribution, and elimination half-life ranged from 0.1-3.1 L/kg/hr, 0.2-3.5 L/kg, and 0.3-10.9 hours, respectively. The marked interpatient variability in pharmacokinetics explains in part, the substantial variation in dosage requirements of midazolam to produce sedation in critically ill pediatric patients.

Midazolam nasal spray reduces procedural anxiety in children

OBJECTIVE

Anxiety and pain even in minor procedures are still great problems in pediatrics, not least in pediatric oncology. Conscious sedation is indicated when other means to overcome a child's fear fail. The aim of this study was to investigate whether intranasal administration of midazolam given before insertion of a needle in a subcutaneously implanted central venous port could reduce anxiety, discomfort, pain, and procedure problems.

METHOD

Forty-three children with cancer participated in this randomized, double-blind, placebo-controlled, crossover study in which nasal administration of midazolam spray,.2 mg/kg body weight, was compared with placebo. Children, parents, and nurses completed a visual analog scale questionnaire to evaluate efficacy.

RESULTS

Parents and nurses reported reduced anxiety, discomfort, and procedure problems for children in the midazolam group and would prefer the same medication at next procedure. They also reported pain reduction. Children reported reduced anxiety and procedure problems but reduction of pain and discomfort was not significant. No serious or unexpected side effects occurred. Nasal discomfort was the most common side effect (17/38 approximately 45%) and the primary reason for dropouts (8/43 approximately 19%). Anxiety varied with age but not with gender. When anxiety increased, the differences between midazolam and placebo increased.

CONCLUSION

Nasal midazolam spray offers relief to children anxious about procedures, such as insertion of a needle in a subcutaneously implanted intravenous port, venous blood sampling, venous cannulation, etc. Its use, however, may be limited by nasal discomfort in some patients for whom rectal and oral routes might be alternatives.

Prevention and management of pain and stress in the neonate

This statement is intended for health care professionals caring for neonates (preterm to one month of age). The objectives of this statement are to: increase awareness that neonates experience pain; provide a physiological basis for neonatal pain and stress assessment and management by health care professionals; make recommendations for reduced exposure of the neonate to noxious stimuli and to minimize associated adverse outcomes; and recommend effective and safe interventions that relieve pain and stress.

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.

Use of midazolam for refractory status epilepticus in pediatric patients

Status epilepticus is more common among children than young adults. Children might be less likely to die and might be resistant to permanent neurologic damage due to status epilepticus, but significant sequelae also have been demonstrated. Aggressive intervention and rapid termination of seizures contribute significantly to better prognosis and reduced mortality from status epilepticus. Initial treatment of status epilepticus typically consists of either diazepam or lorazepam, immediately followed by phenytoin or phenobarbital. However, approximately 100% to 15% of status epilepticus episodes are refractory to these conventional therapies. Traditionally, refractory status epilepticus is treated with barbiturate coma or general anesthetics, both of which require invasive cardiorespiratory and hemodynamic monitoring and are associated with significant complications. Midazolam is a water-soluble benzodiazepine with a fast onset of action, a short half-life, and inactive metabolites that has been very effective in terminating seizures refractory to diazepam, lorazepam, phenytoin, and phenobarbital in pediatric patients. Midazolam is a valuable treatment option for refractory status epilepticus, especially in pediatric patients.

An approach for dose finding of drugs in infants: sedation by midazolam studied using the continual reassessment method

AIMS

No drug has been demonstrated to provide simultaneously appropriate sedation, safety and lack of disturbance of the measured parameters during cardiac catheterization in infants. The objective of this study was to estimate the dose of midazolam, administered rectally, that would provide a 90% probability of adequate sedation in infants during cardiac catheterization. A sedation score > or =4 (six-point scale) 30 to 60 min after dosing was rated as a success.

METHODS

A double-blind, continual reassessment method using a Bayesian approach has been used. Sixteen infants were administered a single midazolam dose, within a 0.1 to 0.6 mg kg(-1) dose range.

RESULTS

Consecutive failures led to allocation of the highest dose to 15 out of 16 patients. The final estimated probability of failure of the 0.6 mg kg(-1) dose was 81% (95% CI: 78.5 to 84%). The time to reach a score > or =4 was longer than expected and the median duration-time at score > or =4 was shorter (15 min) than expected.

CONCLUSIONS

Delayed absorption and low rectal bioavailability may explain these data. Higher doses or different routes of administration may lead to the expected sedation, but the safety of doses higher than 0.6 mg kg(-1) administered rectally has not been evaluated. The therapeutic strategy for sedation of this category of infants in the hospital has now been changed based on the present results in that rectal midazolam has been abandoned in this indication.

Clinical pharmacology of midazolam in infants and children

Midazolam is a parenteral benzodiazepine with sedative, amnesic, anxiolytic, muscle relaxant and anticonvulsant properties. The drug exerts its clinical effect by binding to a receptor complex which facilitates the action of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). Midazolam has a faster onset and shorter duration of action than other benzodiazepines such as diazepam and lorazepam. The most serious adverse events associated with midazolam in children include hypoventilation, decreased oxygen saturation, apnoea and hypotension. It is water soluble in the commercially prepared formulation but becomes lipid soluble at physiological pH and can then cross the blood brain barrier. It is metabolised in the liver by the cytochrome P450 system, and its chief metabolite is 1-hydroxymethyl midazolam. The latter is conjugated to the glucuronide form, and it has only minimal biological activity. Midazolam is excreted primarily by the kidney. Its half-life in children over 12 months is reported to be 0.8 to 1.8 hours, with a clearance of 4.7 to 19.7 ml/min/kg. Doses given to children must be calculated on a mg/kg basis. For children 6 months to 5 years of age the initial dose is 0.05 to 0.1 mg/kg. A total dose up to 0.6 mg/kg titrated slowly may be necessary to achieve the desired endpoint. For children 6 to 12 years of age the initial dose is 0.025 to 0.05 mg/kg with a total dose up to 0.4 mg/kg to achieve the desired end-point.

Toxicokinetics in infants and children in relation to the ADI and TDI

Age-dependent developmental changes in toxicokinetics occur in both rats and humans, particularly in relation to renal function and hepatic xenobiotic metabolism. These processes are immature in humans at birth, especially in the pre-term neonate, but mature rapidly over the first months of life. In consequence the duration of immaturity primarily corresponds to the period of suckling. Similar developmental changes occur in the neonatal rat over the first weeks of life. Rat pups start to consume some of the adult diet in the third week of life, prior to weaning, so that there is a potential for consumption of the adult diet during the period of immaturity. There is an extensive database on the pharmacokinetics of therapeutic drugs in infants and children. The elimination/clearance of many drugs is higher in children than in adults and this difference would apply to other xenobiotics. In consequence, children frequently will have lower body burdens than adults for the same daily intake of a chemical when this is expressed on a body weight basis, as used to describe the ADI (Acceptable Daily Intake) or TDI (Tolerable Daily Intake) (e.g. mg/kg body weight/day). Therefore, an increased safety or uncertainty factor for post-suckling infants and children is not required in relation to age-related differences in toxicokinetics. Indeed, the higher clearance of many xenobiotics (toxicokinetics) by children compared with adults may compensate, at least in part, for increased organ sensitivity (toxicodynamics) during development.

Haemodynamic responses and population pharmacokinetics of midazolam following administration to ventilated, preterm neonates

OBJECTIVE

To evaluate the effects of intravenous midazolam on haemodynamic variables and cerebral blood flow velocity (CBFV) and to determine the pharmacokinetics using a population approach in very low birthweight (VLBW) ventilated infants.

METHODOLOGY

Physiological variables were measured at predetermined times in 10 infants with birthweight < or = 1500 g following a bolus dose of intravenous midazolam (0.1 mg/kg). Heart rate, mean arterial blood pressure (MAP) and transcutaneous CO2 (TcPCO2) were recorded and CBFV was assessed by Doppler ultrasound. Midazolam concentrations were also measured and pharmacokinetic parameters determined using a population modelling package.

RESULTS

No change in heart rate occurred during the study period, while the MAP decreased by 3 mmHg 5 min after midazolam administration compared to baseline values. A non-significant fall in TcPCO2 was seen at 20 min. Mean CBFV decreased from the baseline by 12% at 5 min, then returning to predose values. Midazolam concentrations were in the range shown to be effective in sedation of paediatric intensive care infants with the elimination being delayed in comparison to older children.

CONCLUSIONS

As only minor cerebral and haemodynamic effects were found with the use of midazolam in stable ventilated preterm infants, it appears to be a safe, short-term sedative agent.