Pharmacokinetics of midazolam during continuous infusion in critically ill neonates

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.

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.

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.

Use of methohexital for elective intubation in neonates

The effectiveness and safety of a short acting barbiturate, methohexital, was assessed for its use at the time of elective intubation in 18 newborn infants with severe respiratory or cardiac conditions. Evaluation included the speed of action and the degree of relaxation, sedation, and sleep in the first five minutes after administration. All newborn infants were intubated in a fully relaxed and somnolent state. In most infants recovery was completed within five minutes. A slight to moderate oxygen saturation drop was observed during the period of intubation, especially in patients with cyanotic heart disease. The side effects of the drug were twitching and a slight drop in blood pressure. In conclusion, methohexital seems to be a useful drug for short term anaesthesia in neonates, during which, short procedures like elective intubation can be safely performed.

Measurement by HPLC of midazolam and its major metabolite, 1-hydroxymidazolam in plasma of very premature neonates

A simple, selective high-performance liquid chromatographic method with ultraviolet detection at 220 nm is described for quantitation of midazolam and its primary metabolite 1-hydroxymidazolam in 100 microL plasma samples from premature infants. A mobile phase of acetonitrile:tetrahydrofuran:phosphate buffer (0.01 M, pH 6.7) (35:5:60 v/v) was pumped at 1 mL/min through a C8 Symmetry (150 x 3.9 mm) column. Plasma (100 microL) was extracted with 10% v/v isopropyl alcohol in dichloromethane containing 25 ng/mL climazolam (internal standard, IS) followed by back extraction into phosphoric acid (0.02 M). 1-Hydroxymidazolam, midazolam, and climazolam (IS) were eluted at 4.9, 7.4, 8.4 min, respectively. Recoveries were > 70%. Calibration curves in blank plasma were linear (r > 0.999) from 12.5 to 800 ng/mL. Within-day and between-day imprecision (CV%) was 1.8-6.5%, and 4.1-8.8%, respectively. Inaccuracy was 12.3%. Application of the method was demonstrated by a pharmacokinetic analysis of midazolam and 1-hydroxymidazolam in plasma drawn from 15 premature neonates after a single intravenous dose of midazolam (0.1 mg/kg).

Steady-state plasma concentrations of midazolam in critically ill infants and children

OBJECTIVE

To determine the steady-state plasma concentrations of midazolam in critically ill infants and children.

DESIGN

Prospective uncontrolled study conducted over 18 months.

SETTING

Regional pediatric intensive care unit in a children's hospital.

PATIENTS

Thirty-eight infants and children, aged 1 month to 13 years, requiring midazolam as sedation during mechanical ventilation. The patients were divided into three age groups: (1) infants less than 12 months (n = 16); (2) children 1-2 years (n = 12); and (3) children aged 3 years and older (n = 10).

MAIN OUTCOME MEASURES

A single blood sample was collected once steady-state plasma concentrations of midazolam were achieved during a continuous intravenous infusion. Plasma clearance was calculated from the plasma concentrations and infusion rate.

RESULTS

The plasma clearance was higher in children aged 3 years and older (median plasma clearance 13.0 mL/min/kg) than in infants and children 1-2 years old (median plasma clearance 3.1 and 2.3 mL/min/kg, respectively) (Kruskal-Wallis analysis of variance, p < 0.01). The midazolam infusion rates were similar for the three groups studied (Kruskal-Wallis analysis of variance, p > 0.05). The plasma concentrations of midazolam were significantly lower in children 3 years and older (median plasma concentration 128 ng/mL) than in infants and children 1-2 years old (median plasma concentrations 395 and 790 ng/mL, respectively) (Kruskal-Wallis analysis of variance, p < 0.05).

CONCLUSIONS

The plasma clearance in children 3 years and older was higher than in infants and children up to 2 years old. There was considerable interindividual variation in the steady-state plasma concentrations of midazolam in critically ill infants and children.

[Use of intravenous midazolam in status epilepticus in children]

BACKGROUND

Status epilepticus is usually treated by benzodiazepines such as diazepam or clonazepam in association with phenytoin and phenobarbital. Midazolam (MDZ) is a recently developed short-elimination half-life benzodiazepine.

CASE REPORTS

Four children, aged 3 days to 4 years, were hospitalized in the pediatric intensive care unit for life-threatening illness and developed status epilepticus. They were given an i.v. bolus dose of 300 to 500 micrograms/kg of MDZ immediately followed by a continuous i.v. infusion at the dose of 100 to 300 micrograms/kg/h. All seizures stopped within 1 hour after the bolus dose administration. No acute adverse events of MDZ were noted. Withdrawal symptoms in one patient were controlled by progressive reduction of MDZ doses. Neurologic sequelae were noted in three children, secondary to their primitive illness.

CONCLUSIONS

High doses of MDZ are effective for treating refractory status epilepticus; optimal dosage and duration of treatment remains to be determined.

The effect of propofol sedation in pregnancy on neonatal condition

A woman who suffered an intracerebral haemorrhage secondary to Moyamoya disease in her 33rd week of pregnancy was sedated for ventilation with propofol for 48 h until she underwent emergency Caesarean section. There appeared to be no adverse metabolic effects on the neonate from the use of propofol in the mother although sedation was prolonged.

Placebo-controlled trial of midazolam sedation in mechanically ventilated newborn babies

Although midazolam is used for sedation of mechanically ventilated newborn babies, this treatment has not been evaluated in a randomised trial. We have done a prospective placebo-controlled study of the effects of midazolam on haemodynamic variables and sedation as judged by a five-item behaviour score. 46 newborn babies on mechanical ventilation for respiratory distress syndrome were randomly assigned to receive midazolam (n = 24) or placebo (n = 22) as a continuous infusion. Doses of midazolam were calculated to obtain plasma concentrations between 200 and 1000 ng/mL within 24 h of starting treatment and to maintain these values throughout the study. Haemodynamic and ventilatory variables were noted every hour, as were complications and possible side-effects of treatment. Mean (SD) duration of inclusion was 78.7 (30.9) h. 1 patient in the treatment group and 7 in the placebo group were withdrawn because of inadequate sedation (p < 0.05). Midazolam gave a significantly better sedative effect than placebo, as estimated by the behaviour score (p < 0.05). Heart rate and blood pressure were reduced by treatment but remained within the normal range for gestational age and there was no effect on ventilatory indices. The incidence of complications was similar in the two groups. No midazolam-related side-effects were noted. Continuous infusion of midazolam at doses adapted to gestational age induces effective sedation in newborn babies on mechanical ventilation, with positive effects on haemodynamic variables. The course of the respiratory distress syndrome was not influenced by this treatment. Midazolam was given over only a few days and the limited effects on heart rate and blood pressure that we report should not encourage long-term administration.

New anticonvulsant drugs. Focus on flunarizine, fosphenytoin, midazolam and stiripentol

In the past decade, several new antiepileptic drugs have been tested. Most recently, 5 new antiepileptic drugs have been launched onto European and US markets. These include vigabatrin, oxcarbazepine and lamotrigine in Europe, and felbamate and gabapentin in the US. In addition to these, 3 additional drugs are in the clinical investigational stage: flunarizine, fosphenytoin and stiripentol. A fourth agent is midazolam, which was originally introduced in 1986, but recently has shown effectiveness in the treatment of status epilepticus. Flunarizine is a selective calcium channel blocker that has shown anticonvulsant properties in both animal and human studies. It is a long-acting anticonvulsant that clinical studies have shown to have effects similar to those of phenytoin and carbamazepine in the treatment of partial, complex partial and generalised seizures. Fosphenytoin was developed to eliminate the poor aqueous solubility and irritant properties of intravenous phenytoin. It is rapidly converted to phenytoin after intravenous or intramuscular administration. In clinical studies, this prodrug showed minimal evidence of adverse events and no serious cardiovascular or respiratory adverse reactions. It may have a clear advantage over the present parenteral formulation of phenytoin. Midazolam is a benzodiazepine that is more potent than diazepam as a sedative, muscle relaxant and in its influence on electroencephalographic measures. It has been shown to be an effective treatment for refractory seizures in status epilepticus. Stiripentol has anticonvulsant properties as well as the ability to inhibit the cytochrome P450 system. There are significant metabolic drug interactions between stiripentol and phenytoin, carbamazepine and phenobarbital (phenobarbitone). Stiripentol has been studied in patients with partial seizures, refractory epilepsy and refractory absence seizures with some efficacious results.

Anaesthetic agents and excretion in breast milk

This review is an update on anesthetic agents and their excretion into breast milk; it presents the reported effects on suckling infants, and discusses the precautions which should be considered. For most anaesthetic agents, there is very sparse information about breast milk excretion and even less published knowledge about the possible effects on the suckling infant. Generally, when an anaesthetic agent is given on a single-dose basis, there is no evidence that it is excreted in breast milk in clinically significant amounts, even if there are detectable concentrations of the drug in the milk. Most anaesthetics are rapidly cleared from the mother, and, consequently, it should be possible to allow suckling as soon as practically feasible after surgery. However, repeated administration of certain opiates and benzodiazepines has been reported to cause adverse effects in neonates, with premature neonates apparently being more susceptible. Thus, in long-term treatment with these drugs, the importance of uninterrupted breast feeding should be assessed against possible adverse drug effects in the neonate.