Nonlinear dobutamine pharmacokinetics in a pediatric population

Pharmacokinetic study (phase I-II) of a new dobutamine formulation in preterm infants immediately after birth

BACKGROUND

Dobutamine is particularly suited to treatment of haemodynamic insufficiency caused by increased peripheral vascular resistance and myocardial dysfunction in the preterm infant. Knowledge of the elimination half-life is essential to estimate the steady state when its efficacy/safety can be evaluated.

METHODS

Analysis of pharmacokinetic data in ten preterm newborns treated with a new neonatal formulation of dobutamine (IMP) after screening for haemodynamic insufficiency within the first 72 h from birth. Blood samples were withdrawn at the end of IMP infusion and at a random time after the end of infusion (5 min, 15 min, 45 min, 2 h and 6 h). IMP concentration in each sample was measured by ultra-high performance liquid chromatography with electrochemical detection.

RESULTS

Median duration of IMP infusion was 37.7 h (IQR 21.2). Calculated IMP half-life ranged between 3.06 and 36.1 min (median 10.6 min), leading to a time to reach the steady-state concentration between 15 min and >2 h. Adverse events were not related to IMP.

CONCLUSIONS

The wide variability in dobutamine metabolism in preterm infants requires awareness about the risk of under- or overtreatment. A delay of up to 3 h might be required before drawing blood samples to evaluate the effective dose.

IMPACT

Small trials suggest dobutamine as the optimal drug in the preterm infant with haemodynamic insufficiency after birth. Age-related differences in drug pharmacokinetics may result in suboptimal treatments. The lack of formal studies in preterms results in inadequate data on efficacy and safety. This study provides data on the variability of the elimination half-life of dobutamine in the very preterm infant during transitional circulation. There is a wide variation in the time to reach the plasma concentration corresponding to steady state, the moment when its efficacy/safety can be reliably evaluated. This information is crucial for planning future trials on cardiovascular support.

Population pharmacokinetics and pharmacodynamics of dobutamine in neonates on the first days of life

AIMS

To describe the pharmacokinetics (PK) and concentration-related effects of dobutamine in critically ill neonates in the first days of life, using nonlinear mixed effects modelling.

METHODS

Dosing, plasma concentration and haemodynamic monitoring data from a dose-escalation study were analysed with a simultaneous population PK and pharmacodynamic model. Neonates receiving continuous infusion of dobutamine 5-20 μg kg-1 min-1 were included. Left ventricular ejection fraction (LVEF) and cardiac output of right and left ventricle (RVO, LVO) were measured on echocardiography; heart rate (HR), mean arterial pressure (MAP), peripheral arterial oxygen saturation and cerebral regional oxygen saturation were recorded from patient monitors.

RESULTS

Twenty-eight neonates with median (range) gestational age of 30.4 (22.7-41.0) weeks and birth weight (BW) of 1618 (465-4380) g were included. PK data were adequately described by 1-compartmental linear structural model. Dobutamine clearance (CL) was described by allometric scaling on BW with sigmoidal maturation function of postmenstrual age (PMA). The final population PK model parameter mean typical value (standard error) estimates, standardised to median BW of 1618 g, were 41.2 (44.5) L h-1 for CL and 5.29 (0.821) L for volume of distribution, which shared a common between subject variability of 29% (17.2%). The relationship between dobutamine concentration and RVO/LVEF was described by linear model, between concentration and LVO/HR/MAP/cerebral fractional tissue oxygen extraction by sigmoidal Emax model.

CONCLUSION

In the postnatal transitional period, PK of dobutamine was described by a 1-compartmental linear model, CL related to BW and PMA. A concentration-response relationship with haemodynamic variables has been established.

A sensitive method for the simultaneous UHPLC-MS/MS analysis of milrinone and dobutamine in blood plasma using NH4F as the eluent additive and ascorbic acid as a stabilizer

The purpose of this work was to develop and validate an HPLC-MS/MS method suitable for quantifying two important cardiovascular drugs, milrinone and dobutamine, in neonatal and paediatric patients' blood plasma samples. Sufficiently low LLOQ levels were required to obtain adequate pharmacokinetic data for the evaluation of optimal dosing. Since the specifics of the patient group set some restrictions on the available sample volume, the method was designed to use only 20 µL of plasma for the analysis. Analytes were separated chromatographically in a biphenyl column using a conventional water-methanol-formic acid eluent with the addition of ammonium fluoride. The latter provided a significant signal enhancement in positive ion mode detection for both analytes allowing the LLOQ to reach below 1 ng/mL. Matrix matched calibration was linear in the range of 1-300 ng/mL, between-run accuracy remained within 107-115%, and precision within 4.8-7.4% for both analytes over the calibration range (including LLOQ level). Dobutamine degradation in plasma samples was prevented by the usage of ascorbic acid. The method was applied to plasma samples of neonates from two pharmacokinetic/pharmacodynamics studies (n = 38).

A Literature Review of the Pharmacokinetics and Pharmacodynamics of Dobutamine in Neonates

Since its discovery in 1975 dobutamine has been used off-label for treating hemodynamic insufficiency in newborns and children. We present a structured literature review of pharmacokinetic and pharmacodynamic data for dobutamine in the pediatric population. Structured searches were conducted to identify relevant articles according to pre-defined inclusion criteria. Where possible, results for the pharmacodynamic and pharmacokinetic effect of dobutamine were reported as pooled data. Forty-six papers met the inclusion criteria. With regard to pharmacodynamic data a number of studies reported significant increases in a number of clinical parameters such as heart rate, blood pressure, cardiac output across a wide range of pediatric populations. With regard to pharmacokinetic data studies reported that the infusion rate was positively correlated to plasma dobutamine concentration. There was great variability with regard to dobutamine clearance between individuals and as to whether it followed first- or zero-order elimination kinetics. While the pharmacodynamic effects of dobutamine appear to reflect the pharmacological profile of the drug, the pharmacokinetic data are difficult to interpret due to inhomogeneity between study populations ages, comorbidities, dobutamine dosages and methodologies. High-quality prospective pharmacokinetic and pharmacodynamic data especially in newborns are urgently required prior to a large randomized study.

Kinetic interactions of dopamine and dobutamine with human catechol-O-methyltransferase and monoamine oxidase in vitro

Dopamine and dobutamine are often infused together into acutely ill patients requiring temporary support of cardiac and renal function, but whether these catecholamines affect the metabolic clearance of each other is not established. We determined the kinetics of dopamine and dobutamine as substrates and inhibitors of each other, i.e., apparent V(max), K(m), and K(i), with crude preparations of human blood mononuclear cell catechol-O-methyltransferase (COMT) and platelet monoamine oxidase (MAO) at pH 7.4 and 37 degrees C. Values of V(max) for dopamine and dobutamine as substrates for COMT were 0.45 and 0.59 nmol of 3-O-methyl product formed per milligram of protein per minute, whereas those for K(m) were 0.44 and 0.05 mM, respectively. Dopamine and dobutamine were competitive inhibitors of each other in this reaction. The K(i) for dopamine as an inhibitor of dobutamine methylation was 1.5 mM, whereas that for dobutamine as an inhibitor of dopamine methylation was 0.015 mM. Dopamine but not dobutamine was a substrate for MAO. The V(max) for dihydroxyphenylacetaldehyde formation from dopamine was 0.29 nmol/mg protein/min and the K(m) for dopamine was 0.38 mM. Dobutamine was a noncompetitive inhibitor of dopamine oxidation in this reaction (K(i) congruent with 1.19 mM). The high apparent K(m) and K(i) values derived for dopamine and dobutamine when tested with these two human enzymes in vitro suggest that these catecholamines do not interfere with the metabolism of each other when both are infused together at therapeutic concentrations.

Sulfoconjugation and renal excretion contribute to the interpatient variation of exogenous catecholamine clearance in critically ill children

OBJECTIVE

To delineate the contributions of sulfoconjugation, renal excretion, and patient age to the wide interpatient variability in exogenous dobutamine and dopamine plasma clearance.

DESIGN

Simultaneous plasma free and sulfoconjugated dobutamine and/or dopamine, respective urine free catecholamine, and serum creatinine were determined on stable critically ill children receiving unchanged continuous infusions of dobutamine and/or dopamine for at least 1 hr. Free dobutamine and dopamine clearance rates were calculated.

SETTING

Pediatric and neonatal intensive care units in university settings.

PATIENTS

Forty-seven stable critically ill neonates and children.

INTERVENTIONS

Continuous infusions of dobutamine and/or dopamine: nine patients received dopamine only, 27 patients received dobutamine only, and 11 patients received both simultaneously.

MEASUREMENTS AND MAIN RESULTS

Fractions of plasma dobutamine and dopamine sulfoconjugated were 0.73 +/- 0.05 and 0.76 +/- 0.05, respectively. Free plasma dobutamine and dopamine clearances were 102 +/- 15 mL/kg/min and 250 +/- 38 mL/kg/min, respectively. Linear regression analyses demonstrated relationships of the fraction of plasma dobutamine and dopamine sulfoconjugated to the respective free plasma clearances (r2 = .30, p < .01, and r2 = 0.29, p < .01, respectively), and, more impressively, to the natural logarithm of the respective free plasma clearances (r2 = 0.58, p < .001, and r2 = 0.39, p < .01). Patients with serum creatinine concentrations >2 mg/dL had lower free plasma dobutamine and dopamine clearance rates than those patients with serum creatinine of <2 mg/dL (6 +/- 1 vs. 107 +/- 15 mL/kg/min for dobutamine and 40 +/- 38 vs. 270 +/- 39 mL/kg/min for dopamine, respectively, p < .05 for both by Mann-Whitney U test). No relationship was noted between free catecholamine clearance and age.

CONCLUSION

Sulfoconjugation and renal excretion are important determinants of the wide interpatient variability in plasma free dobutamine and dopamine clearance rates.

Dobutamine pharmacokinetics during dobutamine stress echocardiography

Many patients fail to achieve target heart rate during dobutamine stress echocardiography (DSE). We evaluated the pharmacokinetics of dobutamine during DSE to determine whether patients with an impaired chronotropic response have higher rates of dobutamine clearance and consequently relatively lower plasma dobutamine levels. Plasma dobutamine levels, heart rate, and left ventricular (LV) ejection fraction (EF) were measured in 13 male patients referred for DSE at baseline and at the end of stepped 3-minute dobutamine infusions of 5, 10, 20, and 30 microg/kg/min. Dobutamine levels increased with doses: 27 +/- 10, 111 +/- 17, 275 +/- 17, and 403 +/- 28 ng/ml (mean +/- SEM). There was no relation observed between the plasma dobutamine level achieved at the 30-microg infusion dose and the increase in heart rate from baseline (r = 0.066; p = 0.83). Baseline LVEF and a measure of chronotropic beta responsivity were identified as independent predictors of dobutamine clearance, together accounting for 73% of the variance in dobutamine clearance. In conclusion, (1) there is a dose-dependent increase in plasma dobutamine levels during DSE, (2) dobutamine clearance is positively related to baseline LVEF and is partially mediated by a beta-receptor mechanism, and (3) an impaired chronotropic response during DSE is not due to failure to achieve a sufficiently high dobutamine level. We conclude that in patients who lack an adequate heart rate response during the early stages of DSE (e.g., up to 20 microg/kg/min infusion), administration of atropine rather than progressively higher amounts of dobutamine may provide a more effective strategy to achieve target heart rate.

Pharmacokinetics of cardiovascular drugs in children. Inotropes and vasopressors

Infants and children with congenital or acquired heart disease and children with systemic disease often require pharmacological support of their failing circulation. Catecholamines may serve as inotropic (enhance myocardial contractility) or vasopressor (elevate systemic vascular resistance) agents. Noncatecholamine inotropic agents, such as the cardiac glycosides or the bipyridines, may be used in place of, or in addition to, catecholamines. Developmental changes in neonates, infants and children will affect the response to inotropic or pressor therapy. Maturation of the gastrointestinal tract, liver and kidneys alters absorption, metabolism and elimination of drugs, although there are few clear examples of this among the vasoactive drugs considered in this review. Changes in body composition affect the volume of distribution (Vd) and clearance (CL) of drugs. Developmentally based pharmacodynamic differences also affect the responses to both therapeutic and toxic effects of inotropes. These pharmacodynamic differences are based in part upon developmental changes in myocardial structure, cardiac innervation and adrenergic receptor function. For example, the immature myocardium has fewer contractile elements and therefore a decreased ability to increase contractility; it also responds poorly to standard techniques of manipulating preload. Available data suggest that dopamine and dobutamine pharmacokinetics are similar to those in adults. Wide interindividual variability has been noted. A consistent relationship between CL and age has not been demonstrated, although one investigator demonstrated an almost 2-fold increase in the CL of dopamine in children under the age of 2 years. The CL of dopamine appears to be reduced in children with renal and hepatic failure. Fewer data are available regarding the pharmacokinetics of epinephrine (adrenaline), norepinephrine (noradrenaline) and isoprenaline (isoproterenol). Digoxin pharmacokinetics have been extensively evaluated in infants and children. The Vd for digoxin is increased in infants and children. Children beyond the neonatal period display increased CL of digoxin, approaching adult values during puberty. Although it was previously thought that children both needed and tolerated higher serum concentrations of digoxin than adults, more recent studies indicate that adequate clinical response can be achieved with serum concentrations similar to those aimed for in adults, with decreased toxicity. Evaluation of studies of digoxin pharmacokinetics is complicated by the presence of an endogenous substance with digoxin-like activity on radioimmunoassay. Limited studies of amrinone pharmacokinetics in infants and children indicate a dramatically larger Vd, and a decreased elimination half-life in older infants and children, compared with values observed in adults.

Evaluation, treatment, and transport of pediatric patients with shock

Early recognition and correct treatment of shock remain the most important keys to preventing the death and disability frequently caused by this condition in children. The pediatrician plays a vital role in this process and in referral of the patient for transport to tertiary care centers, where shock is best managed. The transport environment creates special challenges in initial stabilization and ongoing treatment of shock. Discussion centers on clinical clues to recognition, on simple measures available to increase tissue oxygenation, and on the issues of pretransport and transport treatment. Support of airway and breathing, vascular access, and correct fluid therapy remain the cornerstones of successful treatment.