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BJA Advance Access originally published online on October 26, 2007
British Journal of Anaesthesia 2007 99(6):864-870; doi:10.1093/bja/aem294
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© The Board of Management and Trustees of the British Journal of Anaesthesia 2007. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Inter-individual variability in propofol pharmacokinetics in preterm and term neonates

K. Allegaert1,*, M. Y. Peeters3, R. Verbesselt2, D. Tibboel3, G. Naulaers1, J. N. de Hoon2 and C. A. Knibbe4,5

1 Neonatal Intensive Care Unit, Division of Woman and Child
2 Center for Clinical Pharmacology, University Hospital Gasthuisberg, Herestraat 49, 3000 Leuven, Belgium
3 Department of Paediatric Surgery, Erasmus MC—Sophia Children’s Hospital, Rotterdam, The Netherlands
4 Department of Clinical Pharmacy, St Antonius Hospital, PO Box 2500, 3430 EM Nieuwegein, The Netherlands
5 Division of Pharmacology, Leiden/Amsterdam Centre for Drug Research, Leiden University, The Netherlands

* Corresponding author. E-mail: karel.allegaert{at}uz.kuleuven.ac.be

Accepted for publication August 29, 2007.


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 Funding
 References
 
Background: To document covariates which contribute to inter-individual variability in propofol pharmacokinetics in preterm and term neonates.

Methods: Population pharmacokinetics were estimated (non-linear mixed effect modelling) based on the arterial blood samples collected in (pre)term neonates after i.v. bolus administration of propofol (3 mg kg–1, 10 s). Covariate analysis included postmenstrual age (PMA), postnatal age (PNA), gestational age, weight, and serum creatinine.

Results: Two hundred and thirty-five arterial concentration–time points were collected in 25 neonates. Median weight was 2930 (range 680–4030) g, PMA 38 (27–43) weeks, and PNA 8 (1–25) days. In a three-compartment model, PMA was the most predictive covariate for clearance (P<0.001) when parameterized as [CLstd·(PMA/38)11.5]. Standardized propofol clearance (CLstd) at 38 weeks PMA was 0.029 litre min–1. The addition of a fixed value in neonates with a PNA of ≥10 days further improved the model (P<0.001) and resulted in the equation [CLstd·(PMA/38)11.5 +0.03] for neonates ≥10 days. Values for central volume (1.32 litre), peripheral volume 1 (15.4 litre), and peripheral volume 2 (1.29 litre) were not significantly influenced by any of the covariates (P>0.001).

Conclusions: PMA and PNA contribute to the inter-individual variability of propofol clearance with very fast maturation of clearance in neonatal life. This implicates that preterm neonates and neonates in the first week of postnatal life are at an increased risk for accumulation during either intermittent bolus or continuous administration of propofol.

Keywords: anaesthetics i.v., propofol; neonates; pharmacology; sedation


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 Funding
 References
 
Although pharmacokinetics of propofol have been extensively studied in different populations of adult and paediatric age, data in neonates are still very limited. Propofol is a highly lipophilic anaesthetic compound and therefore exhibits rapid distribution from blood into s.c. fat and the central nervous system with subsequent redistribution. Elimination kinetics are triphasic and characterized by fast metabolic clearance. Although the use of propofol became standard of care for i.v. induction of anaesthesia and has been repeatedly reported in the literature, the administration of propofol is still—to the best of our knowledge—off label in neonates.1–10

In adults, urinary excretion of unchanged propofol only marginally (<1%) contributes to overall propofol clearance. Propofol clearance mainly depends on the hepatic blood flow (high extraction drug) with subsequent metabolism. Although multiple hepatic and extrahepatic human cytochrome (CYP) P450 isoforms (hydroxylation) are also involved in propofol metabolism, glucuronidation is the major metabolic pathway of propofol.11 12 During paediatric life, phase I and phase II hepatic metabolism display iso-enzyme-dependent ontogeny, although there are progressive changes in body composition with subsequent effects on the relative distribution volume of lipophilic compounds. These maturational processes are most prominent in early neonatal life.13 14

It is therefore to be anticipated that propofol disposition displays maturation. It was recently documented that propofol clearance (i.v. bolus administration, 3 mg kg–1) in neonates was significantly lower compared with the values in toddlers and children, but we were unable to estimate any maturational trend in neonatal life.5 In the present paper, we report on the covariates which contribute to the inter-individual variability of propofol clearance in a further extended cohort of preterm and term neonates.


   Methods
Top
 Abstract
 Introduction
 Methods
Results
 Discussion
 Funding
 References
 
Clinical characteristics, ethics, procedural sedation model, and sample collection
Neonates were included after approval of the study protocol by the local ethical board of the University Hospital Gasthuisberg, Leuven, Belgium, and after informed written consent was obtained from the parents. The decision to prescribe propofol for scheduled short procedural sedation was made by the attending neonatologist, and neonates were only considered for inclusion if an arterial line was available to enable the sequential collection of blood samples. Neonates had to be cardiovascular and respiratory stable, as judged by the attending neonatologist. Clinical characteristics were registered at inclusion [postmenstrual age (PMA, weeks), gestational age (GA, weeks), postnatal age (PNA, days), weight, and serum creatinine].

Besides elective chest tube removal, (semi)elective chest tube placement or endotracheal intubation was the indication for the administration of propofol as a sedative to ameliorate the feasibility of the procedure. Just before the procedure was initiated, propofol (3 mg kg–1 i.v. bolus over 10 s, Diprivan® 1%, Braun, Diegem, Belgium) was administered in addition to the analgesics already administered by either continuous (fentanyl or tramadol) or intermittent (acetaminophen) i.v. infusion.15 This procedural sedation model has already been used to describe the pharmacodynamics of methohexital during chest tube removal in neonates.16 Neither dose nor type of analgesic was standardized since these analgesics were titrated based on systematic evaluation of pain during neonatal stay.15

Blood samples were collected by arterial line up to 24 h after administration of propofol, with a specific emphasis on the very early phase (i.e. 1, 5, 15, 30, 60, 90 min and 2, 4, 8, 12 and 24 h) after initiation of administration. The total volume of blood samples in every individual neonate was limited to 1.8 ml kg–1.

Drug assay
To an equivalent volume of 0.5 ml of whole blood, 50 µl of the internal standard (thymol, 10 µg ml–1) and 1 ml of acetonitrile were added to precipitate the proteins. After vortexing two times for 15 s, the samples were centrifuged for 10 min at 3000 rpm. An aliquot of 200 µl of the supernatant was transferred to a microvial of 300 µl for automatic injection by an autosampler 717plus (Waters). The injection volume was 10 µl. The chromatographic system consisted of a Waters 600E pump, combined with a Waters autosampler 717plus and a fluorescence detector (Hitachi F-1000) with excitation and emission wavelengths set at, respectively, 270 and 310 nm. Chromatographic separation of propofol and the internal standard thymol was performed on a reversed phase high performance liquid chromatography column packed with Lichrosorb RP18 5 µm (125x4.0 mm ID) and the mobile phase was a mixture of acetonitrile and water (+0.1% trifluoroacetic acid) (60/40, v/v) pumped at a flow rate of 1 ml min–1. Linearity was found for standard curves of propofol constructed in whole blood in the range of 0.02–20 µg ml–1. Intra- and interday coefficients of variation were lower than 15%, and the lowest limit of quantitation for propofol, determined as the lowest concentration with coefficient of variation lower than 20%, was 0.02 µg ml–1.

Population pharmacokinetic analysis
Data analysis
The analysis was performed using non-linear mixed effect modelling (GloboMax LLC, Hanover, MD, USA, version V release 1.1)9 by use of the first-order conditional estimation (Method 1) with {eta}–{epsilon} interaction. S-plus (Insightful software, Seattle, WA, USA, version 6.2) was used to visualize the data. Discrimination between different models was made by comparison of the objective function. A value of P<0.005, representing a decrease of 7.8 points in the objective function ({chi}2 distribution), was considered statistically significant. In addition, goodness of fit plots, including observations vs individual predictions, observations vs population predictions and weighted residuals vs time and population predictions vs weighted residuals were used for diagnostic purposes. Furthermore, the confidence interval of the parameter estimates, the correlation matrix, and visual improvement of the individual plots were used to evaluate the model.

Covariate analysis
The covariates PMA, PNA, GA, body weight, gender, and renal function (serum creatinine) were plotted subsequently against the individual post hoc parameter estimates and the weighted residuals to visualize potential relationships.

On the basis of these plots, covariates were tested for their influence and the nature of their influence which consisted of an exploration of a linear (centralized around the median value of the covariate), power, and subpopulation incorporation of each of the covariates for the involved pharmacokinetic parameter. For the subpopulation implementation, variation in the value separating the subpopulations was also explored. The most optimal parameterization for each covariate on a specific pharmacokinetic parameter was chosen based on the objective function, although a value of P<0.05 representing a decrease of 3.8 points in the objective function was considered statistically significant.

Starting from the basic model without covariates, the covariate model was first built up using forward inclusion. The contribution of each covariate was confirmed by stepwise backward deletion. In the final model, all covariates associated with a significant increase in objective function ({chi}2 distribution) after elimination were maintained. The choice of the model was further evaluated as described in Data analysis.

Validation
The internal validity of the population pharmacokinetic model was assessed by the bootstrap re-sampling method (repeated random sampling to produce another data set of the same size but with a different combination of individuals). Parameters obtained with the bootstrap replicates (250 times) were compared with the estimates obtained from the original data set.

Pharmacokinetic model
The parameters of a two- and three-compartment model were fitted to the log-transformed data using ADVAN3 TRANS4 and ADVAN11 TRANS4. Propofol pharmacokinetics were adequately described by a three-compartment model, parameterized in terms of volume of the central compartment (V1), elimination clearance (CL), inter-compartmental clearance between central and peripheral 1 (Q2), peripheral volume 1 (V2), inter-compartmental clearance between central and peripheral 2 (Q3), peripheral volume 2 (V3). The individual value (post hoc value) of the parameters of the ith subject was modelled by


Formula 294M1

(1)
where {theta}mean is the population mean and {eta}i is assumed to be a random variable with zero mean and variance {omega}.2 The residual error was described with a proportional error model. This means for the jth observed log-transformed concentration of the ith individual, the relation (Yij):


Formula 294M2

(2)
where cpred is the predicted transformed propofol concentration and {varepsilon}ij the random variable with zero mean and variance {sigma}.2


   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 Funding
 References
 
Two hundred and thirty-five arterial concentration–time points were collected in 25 neonates. The clinical characteristics of the neonates are reported in Table 1. The sequential model building process is summarized in Table 2. On the basis of the criteria described in Methods, a three-compartment model was preferred over a two-compartment model. Scatter plots showing the relationship between propofol clearance and PMA (weeks), GA (weeks), PNA (days), and body weight (kg) of the basis model are provided in Figure 1A–D.


Figure 1
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  		Fig 1 Scatter plot showing relationship between propofol clearance and (A) PMA (weeks), (B) GA (weeks), (C) PNA (days), and (D) body weight (kg) of the basic model.

 


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  		Table 1 Individual clinical characteristics of the 25 neonates included in the current study

 


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  		Table 2 Sequential model building process

 
The introduction of PMA as a covariate for clearance using a power equation [CLstd·(PMA/38)b] further improved the model (objective function 251.28, P<0.001) and resulted in a more significant reduction of the objective function compared with PNA (linear, –245.42, P<0.05) or body weight (power model, 245.94, P<0.05) (Table 2). Standardized propofol clearance (CLstd) at 38 weeks PMA was 0.029 litre min–1 with a power scaling parameter b of 11.5 (Table 3). The addition of a fixed value in neonates with a PNA of ≥10 days further improved the model (objective function –277.5, P<0.001, Table 2) and resulted in the equation [CLstd·(PMA/38)b+a] for neonates ≥10 days. Variation of day 10 in days 8, 11, 12, or 13 resulted in a less significant reduction in objective function (P<0.001, P<0.05, P<0.05, and P<0.05, respectively). Upon introduction of both PMA and PNA as covariates in the final PMA+PNA model, the inter-individual variability for clearance was reduced from 322% to 84% (Table 3).


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  		Table 3 Parameter estimates of the basic pharmacokinetic model, the postmenstrual+postnatal age model, and the stability of the parameters using the bootstrap validation (BS). Values in parentheses are coefficient of variation of the parameter (CV); CL, elimination clearance; CLstd, elimination clearance in a standardized individual of 38 weeks postmenstrual age; b, power scaling parameter; a, plus clearance constant if postnatal age ≥10 days; V1, central volume; Q2, inter-compartmental clearance between central and peripheral 1; V2, peripheral volume 1; Q3, inter-compartmental clearance between central and peripheral 2; V3, peripheral volume 2; the inter-individual variability is calculated as the square root of the exponential variance of {eta} minus 1; {varepsilon}, residual error proportional calculated as square root of the variance; –2LL, objective function

 
All parameter estimates including the values on inter-individual and residual variability of the basic pharmacokinetic model, the final PMA+PNA model, and the stability of these parameters using bootstrap validation are presented in Table 3. Diagnostic plots (Fig. 2A–D) are provided to allow the evaluation of the optimal, that is, the final PMA+PNA model to the data.


Figure 2
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  		Fig 2 Diagnostic plots, allowing the evaluation of the optimal model to the data. (A) Observed vs individually predicted propofol concentrations; (B) observed vs population predicted concentrations; (C) weighted residuals vs time; (D) weighted residuals vs population predicted concentrations. Solid lines represent the line of unity.

 

   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
Funding
 References
 
This study is the first population analysis in preterm and term neonates exploring the pharmacokinetics and its covariates following i.v. single bolus administration of propofol based on 253 observations collected in 25 neonates. It was recently documented that propofol clearance in neonates is significantly lower compared with similar observations in toddlers and children but additional maturational trends could not be unveiled.5 On the basis of the current observations in a further extended cohort and using a population pharmacokinetic approach, both PMA and PNA contribute as covariates of the inter-individual variability in propofol clearance in neonates.

Since propofol is a highly lipophilic drug with a high hepatic extraction ratio, clearance mainly depends on hepatic blood flow with subsequent metabolic clearance. This results in more aqua-soluble compounds subsequently eliminated by renal route. Glucuronidation is the major metabolic pathway of propofol, although multiple human cytochrome P450 (CYP) isoforms also contribute to propofol metabolism.11 12 We hypothesize that the differences in propofol clearance between neonates and infants from 4 months onwards (Table 4) at least in part reflect the still incomplete maturation of hepatic and extrahepatic phase I and phase II enzyme activities in the first month of life.5–8 The additional impact of PNA (≥10 days) on propofol clearance hereby likely reflects ontogeny of glucuronidation activity since there are in vivo observations on the maturation of acetaminophen and morphine glucuronidation in early neonatal life, both illustrating the relevance of early neonatal life, that is, first week of postnatal life, on glucuronidation activity in neonates.13 17–19


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  		Table 4 The pharmacokinetic estimates and clinical characteristics observed in neonates after i.v. bolus administration of propofol were compared with the estimates described by Raoof and colleagues and Murat and colleagues [standard deviation (SD)] in infants and toddlers. The estimates in neonates were based on the final PMA+PNA model in a 10-day-old neonate with PMA of 38 weeks and a weight of 3 kg (Vss, distribution volume at steady state)

 
Guitton and colleagues demonstrated that multiple human cytochrome P450 isoforms are involved in liver metabolism of propofol based on in vitro microsomal adult hepatic samples. It is therefore to be anticipated that, besides maturation of glucuronidation, ontogeny of phase I process (cytochrome 2B6) also contributes to the differences in propofol clearance.12

The study of maturational aspects of propofol metabolism in neonates and children can therefore be of relevance beyond the drug-specific results, since these observations might provide us with additional insights into the ontogeny of various phase I and phase II processes in neonatal and paediatric life.13 Instead of weight, PMA is the most relevant covariate for clearance of propofol with a power scaling parameter of 11.5 (CV 15%) and an additional fixed effect in neonates ≥10 days of PNA. The power scaling parameter of 11.5 hereby reflects that fast maturational changes in propofol clearance during perinatal life in this cohort related more to age than weight of which the power scaling parameter was estimated to be 0.75 by Knibbe and colleagues,20 considering children older than 1 yr, that is, once metabolic activity is at an adult level of activity.

The current observations are of clinical relevance. The overall reduced clearance and the important inter-individual variability (322%) clearance of propofol observed in neonates make accumulation more likely during either continuous or repeated bolus administration.21 22 Preterm neonates (PMA) and neonates in the first week of postnatal life (PNA) are even more prone to display reduced clearance. This fast maturational increase in propofol clearance is also reflected in Figure 1A–D.

Simulated population propofol concentrations (line) in neonates aged 27 weeks (black), 38 weeks (light grey), and 43 weeks (dark grey) PMA and either <10 days (dashed line) or ≥10 days (solid line) PNA after a bolus dose of 3 mg kg–1 in 10 s re-illustrate the impact of maturation on propofol clearance in neonates (Fig. 3). The similar concentration–time profile at the PMA of 27 weeks but ≥10 days and at the PMA of 38 weeks but <10 days hereby illustrates the additional independent impact of PNA on propofol clearance.


Figure 3
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  		Fig 3 Simulated population propofol concentrations (line) in neonates aged 27, 38, and 43 weeks PMA and <10 days and ≥10 days PNA after a bolus dose of 9 mg in 10 s.

 
In conclusion, this population pharmacokinetic study on propofol disposition in preterm and term neonates confirmed the overall lower propofol clearance in neonates compared with older paediatric populations. Significant inter-individual variability (322%) in propofol clearance has been documented. PMA and PNA both contribute to the inter-individual variability of propofol clearance observed with very fast maturation of clearance in neonatal life. This implicates that PMA and neonates in the first week of postnatal life are at an increased risk for accumulation during either intermittent bolus or continuous administration of propofol.


   Funding
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Funding
References
 
The University Hospitals Leuven, Belgium (KA, Clinical Research Fund).


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Funding
 References
 
1 Rigby-Jones AE, Nolan JA, Priston MJ, et al. Pharmacokinetics of propofol infusions in critically ill neonates, infants and children in an intensive care unit. Anesthesiology (2002) 97:1393–400.[CrossRef][Web of Science][Medline]

2 Shangguan WN, Lian Q, Aarons L, et al. Pharmacokinetics of single bolus of propofol in Chinese children of different ages. Anesthesiology (2006) 104:27–32.[CrossRef][Web of Science][Medline]

3 Jones RD, Chan K, Andrew LJ. Pharmacokinetics of propofol in children. Br J Anaesth (1990) 65:661–7.[Abstract/Free Full Text]

4 Prins S, Peeters MY, Houmes RJ, et al. Propofol 6 % as sedative in children under 2 years of age following major craniofacial surgery. Br J Anaesth (2005) 94:630–5.[Abstract/Free Full Text]

5 Allegaert K, Debeer A, Rayyan M, Devlieger H, Naulaers G. Propofol pharmacokinetics in neonates. Crit Care (2007) 11:S170.[CrossRef]

6 Raoof AA, van Obbergh LJ, Verbeeck RK. Propofol pharmacokinetics in children with biliary atresia. Br J Anaesth (1995) 74:46–9.[Abstract/Free Full Text]

7 Murat I, Billard V, Vernois J, et al. Pharmacokinetics of propofol after a single dose in children aged 1–3 years with minor burns. Comparison of three data analysis approaches. Anesthesiology (1996) 84:526–32.[CrossRef][Web of Science][Medline]

8 Saint-Maurice C, Cockshott ID, Douglas EJ, et al. Pharmacokinetics of propofol in young children after a single dose. Br J Anaesth (1989) 63:667–70.[Abstract/Free Full Text]

9 Ghanta S, Abdel-Latif ME, Lui K, Ravindranathan H, Awad J, Oei J. Propofol compared with the morphine, atropine, and suxamethonium regimen as induction agents for neonatal endotracheal intubation: a randomized, controlled trial. Pediatrics (2007) 119:e1248–55.[Abstract/Free Full Text]

10 Rigby-Jones A, Priston M, Wolf A, Sneyd J. Paediatric propopol pharmacokinetics: a multicentre study. Paediatr Anaesth (2007) 17:610.

11 Favetta P, Degoute CS, Perdrix JP, et al. Propofol metabolites in man following propofol induction and maintenance. Br J Anaesth (2002) 88:653–8.[Abstract/Free Full Text]

12 Guitton J, Buronfosse T, Desage M, et al. Possible involvement of multiple human cytochrome P450 isoforms in the liver metabolism of propofol. Br J Anaesth (1998) 80:788–95.[Abstract/Free Full Text]

13 Kearns GL, Abdel-Rahman SM, Alander SW, et al. Developmental pharmacology—drug disposition, action, and therapy in infants and children. N Engl J Med (2003) 349:1157–67.[Free Full Text]

14 Rakhmanina NY, van den Anker JN. Pharmacological research in pediatrics: from neonates to adolescents. Adv Drug Deliv Rev (2006) 58:4–14.[CrossRef][Web of Science][Medline]

15 Allegaert K, Tibboel D, Naulaers G, et al. Systematic evaluation of pain in neonates: effect on the number of intravenous analgesics prescribed. Eur J Clin Pharmacol (2003) 59:87–90.[Web of Science][Medline]

16 Allegaert K, Naulaers G, Debeer A, et al. The use of methohexital during chest tube removal in neonates. Paediatr Anaesth (2004) 14:308–12.[CrossRef][Medline]

17 Bouwmeester NJ, Anderson BJ, Tibboel D, et al. Developmental pharmacokinetics of morphine and its metabolites in neonates, infants and young children. Br J Anaesth (2004) 92:208–17.[Abstract/Free Full Text]

18 Choonara IA, McKay P, Hain R, Rane A. Morphine metabolism in children. Br J Clin Pharmacol (1989) 28:599–604.[Web of Science][Medline]

19 Allegaert K, de Hoon J, Verbesselt R, Vanhole C, Devlieger H, Tibboel D. Intra- and inter-individual variability of glucuronidation of paracetamol during repeated administration in neonates. Acta Paediatr (2005) 94:1273–9.[Web of Science][Medline]

20 Knibbe CA, Zuideveld KP, Aarts LP, et al. Allometric relationships between the pharmacokinetics of propofol in rats, children and adults. Br J Clin Pharmacol (2005) 59:705–11.[CrossRef][Web of Science][Medline]

21 Parke TJ, Stevens JE, Rice AS, et al. Metabolic acidosis and fatal myocardial failure after propofol infusion in children: five case reports. Br Med J (1992) 305:613–6.[Abstract/Free Full Text]

22 Wolf AR, Potter F. Propofol infusion in children: when does an anesthetic tool become an intensive care liability? Paediatr Anaesth (2004) 14:435–8.[CrossRef][Medline]


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