Dobutamine in Paediatric Population: A Systematic Review in Juvenile Animal Models

Objective Although dobutamine is widely used in neonatal clinical practice, the evidence for its use in this specific population is not clear. We conducted a systematic review of the use of dobutamine in juvenile animals to determine whether the evidence from juvenile animal experiments with dobutamine supported the design of clinical trials in neonatal/paediatric population. Methods Studies were identified by searching MEDLINE (1946–2012) and EMBASE (1974–2012). Articles retrieved were independently reviewed by three authors and only those concerning efficacy and safety of the drug in juvenile animals were included. Only original articles published in English and Spanish were included. Results Following our literature search, 265 articles were retrieved and 24 studies were included in the review: 17 focused on neonatal models and 7 on young animal models. Although the aims and design of these studies, as well as the doses and ages analysed, were quite heterogeneous, the majority of authors agree that dobutamine infusion improves cardiac output in a dose dependent manner. Moreover, the cardiovascular effects of dobutamine are influenced by postnatal age, as well as by the dose used and the duration of the therapy. There is inadequate information about the effects of dobutamine on cerebral perfusion to draw conclusions. Conclusion There is enough preclinical evidence to ensure that dobutamine improves cardiac output, however to better understand its effects in peripheral organs, such as the brain, more specific and well designed studies are required to provide additional data to support the design of clinical trials in a paediatric population.


Introduction
The synthetic catecholamine dobutamine is a relatively cardioselective agent with significant cardiac a1and b1/b2adrenoreceptor-mediated direct inotropic effects and limited chronotropic action [1]. Cardiovascular compromise is a frequently condition in critically ill preterm and term infants which contributes to morbidity and mortality in these population. In the diagnosis and treatment of this pathology blood pressure, blood flow and cardiac output are some of the most determinant parameters [2]. The use of inotropes is common in neonates with cardiovascular compromise, being dobutamine introduced for the management of neonatal cardiovascular compromise two decades ago [1].
Dobutamine and dopamine are the inotropes most commonly used in neonatal intensive care; however, as many medicines used in children, especially neonates, have never been tested to the level seen in adult healthcare, the dose is established by simple extrapolation from adults without taking into account age-related differences.
In order to increase availability of medicines authorised for children as well as to increase the information available on the use of medicinal products in the paediatric population, the EU established a regulation on medicinal products for paediatric use (Regulation (EC) No 1901No /2006 [3]. Notably, in the European Medicines Agency priority list (EMA/480197/2010) for studies into off-patent paediatric medicinal products published in 2010, dobutamine appeared as a drug of interest [4].
Nonetheless given the difficulties of conducting pharmacological research in children and more specifically in the neonatal population, the performance of experimental studies in suitable animal species is a common practice. Preclinical findings, and especially juvenile animal data, can be useful to generate information to support the use of a specific drug in paediatric populations. Specifically, it is important that unlike models in adult animals, those based on juvenile and in particular neonatal animals allow for the fact that drug concentration (PK) and drug response (PD) may be different in immature and developing organs. In particular in this case, it is important to focus the review on juvenile and in particular neonatal animals because there are a number of developmental variations of structure and function of the cardiovascular system that suggest a different response to the drug.
The aim of this study was to conduct a systematic review on the use of dobutamine in juvenile animal models and to summarise and assess where there is enough conclusive preclinical evidence to guide a neonatal/paediatric clinical trial to support its use in this population.

Identification of Studies
The literature search was restricted to published results of animal studies and the effects of dobutamine. An electronic search was conducted using MEDLINE (from 1946 to July 2012), and EMBASE (from 1980 to July 2012) with the following combinations of terms: ''dobutamine AND different animal models (using the following strategy: pig* OR swine OR calf OR lamb OR rat OR mouse OR mice OR foal OR horse OR animal*) AND juvenile animals (using the following strategy: juvenile OR infant OR fetal OR foetal OR neonatal OR newborn OR preterm OR pup OR term OR near-term)''.
The dobutamine heading included different related terms such as toxicity, pharmacology, pharmacokinetics, drug administration, adverse reactions, drug concentration, therapy, as well as various different brand and generic drug names.

Inclusion Criteria and Data Extraction
During screening, only studies reported in English or Spanish, and conducted in an ''in vivo'' juvenile/neonatal animal model, were included. Conference papers, book chapters and review papers were excluded, as were studies focused on adult animal models. In addition, studies were included only if the study assessed the effect of dobutamine in juvenile animal models and dobutamine was not tested in combination with other treatments.
After excluding duplicates, titles and abstracts were screened by two authors to assess whether they were relevant for this review and should be subjected to further evaluation; any disagreements were resolved by discussion and if it was not possible to reach a consensus a third author determined eligibility. Finally, the full text of articles considered relevant was evaluated by the three authors.
Relevant information such as animal model and species, age and number of animals, dose and duration of dobutamine therapy, comparison groups and outcomes, was extracted from each study entered into an electronic database and analysed. To assess the quality of the studies the following parameters were taken into account: a) randomisation of the intervention; b) blinding of the intervention; c) comparison to a control group and d) statement of compliance with animal welfare regulations. The heterogeneity of the studies included in the review precluded meta-analysis.

Description of the Studies
Although the research was focused on articles in Spanish and English, none of the article retrieved was written in Spanish, so all the articles included in the review were in English.
A total of 265 articles were retrieved from the electronic search. After application of the inclusion criteria, only 24 studies were included in the systematic review; 17 focused on neonatal models and 7 on young animal models. The corresponding flow diagram is shown in Figure 1.
The characteristics of the studies focused on neonatal animal models are described in Table 1. Only one study focused on preterm animals. In the 17 articles in neonatal models included in the review, nine were performed in piglets, four in lambs, three in puppies and one in foals. A variety of animal models were used including: four studies in hypoxic animals, seven in healthy animals, two studies comparing hypoxic and normoxic conditions and the other four considering different specific conditions (pulmonary hypertension; endotoxic shock; hypotension and hypoplastic left heart syndrome). Moreover, 5 studies focused only on the effects of dobutamine whereas the other 12 studied the effects of different drugs and compared them with those of dobutamine: as a comparison drug, dopamine was used in ten studies; epinephrine in four; isoproterenol in four; norepinephrine in two; milrinone in two; nitroprusside in one and vasopressin in one. The dose range used in the studies varied from 0.5 to 80 mg/ kg/min. The duration of the treatment and the infusion pattern also varied between the studies. Table 2 summarises the characteristics of the seven studies focused on juvenile animals (infants or adolescents): six of them were in piglets and one in healthy foals. The animal models differ, using healthy animals in four studies and disease models in the other three. In five of them the effect of dobutamine was compared with that of other drugs (namely, dopamine in four studies; epinephrine in two; isoproterenol in one; norepinephrine in two; milrinone in two and phenylephrine in one), while just two articles focused on the effects of dobutamine in a model of postresuscitation left ventricular dysfunction. The dose ranged from 2 to 32 mg/kg/min.
In an effort to detect any patterns in the results we looked at the studies performed in pig, since it is the specie more frequently used. In this regard, most authors agree that dobutamine infusion improves cardiac output mainly at doses higher than 10 mg/kg/ min [6,[8][9][10][11][12][13][14][16][17][18][19]. It seems that at short term infusion this improvement was mainly due to an increase in heart rate [9,11,12,16,18], whereas at long term the increment in stroke volume is more prevalent [6,8,9,17].
Further, it seems that the inotropic effect of dobutamine is present both during normoxia and hypoxia, but sometimes in a more attenuated way in the latter condition [12]. Only one study focused on preterm animals and in this case an increase in heart rate was observed without changes in mean arterial blood pressure or cardiac output at any of the doses used [11].

Other Organ Effects
Some studies evaluated the effects of dobutamine on other organs, mainly at renal, mesenteric and pulmonary levels, with results varying as a function of dose and duration of treatment.
There are 5 studies, all in neonatal animal models, on the effects of dobutamine on cerebral perfusion, which is mainly assessed by measurement of carotid blood flow. In this regard, two studies showed an increase in carotid blood flow at dose higher than 10 mg/kg/min, independent of the duration of the treatment [5,6] and in one study that measured cerebral blood flow through radiolabeled microspheres, a dose-dependent increase (5-15 mg/ kg/min) was observed [11]. However, in another study there were no changes in carotid blood flow regardless of the dose used (5-20 mg/kg/min) [8], and other authors noted an increase in carotid vascular resistance at lowest doses (2 and 5 mg/kg/min) [12].
Some authors studied carotid oxygenation by various methods: one study found no changes at any given doses (5-20 mg/kg/min) [8], while another showed an increase at 5 and 10 mg/kg/min [12]. No follow-up studies were identified on neurodevelopment in animals treated with dobutamine.
At the renal level, two studies concluded that the renal response to dobutamine depends on maturity [12,28]. With respect to treatment duration, two studies in newborn piglets observed an increase in renal artery blood flow after 60 min of dobutamine infusion with no changes at shorter infusion times [9,19]. In most   studies there were no changes in renal blood flow after dobutamine administration [6,8,11,25], but some authors studying juvenile and adult animals observed increases in renal blood flow and decreases in renal vascular resistance, especially when the drug was infused continuously [12,18,28]. At the pulmonary level, some studies observed a decrease in pulmonary vascular resistance [6,8,9,24,26]. In contrast, however, one study in juvenile animals reported no changes in either pulmonary vascular resistance or pulmonary artery pressure [18].
Considering models of specific diseases, the effects of dobutamine are mixed, depending on the nature of the disease. For example, in a pulmonary hypertension model, dobutamine decreased the pulmonary artery pressure and pulmonary vascular resistance at 5 and 10 mg/kg/min [20]. In a neonatal piglet model of hypoplastic left heart syndrome an increase in the Qp/Qs ratio was observed, possibly due to differential effects on systemic and pulmonary vascular resistance [10], and in a model of right ventricular injury in young swine indices of pulmonary vascular impedance were found to decrease [14].
In regard to perfusion to other organs, it should be noted that an increase in mesenteric blood flow was observed with continuous dobutamine infusion over 2 hours at doses higher than 10 mg/kg/ min [6,8,9,19], though there were no changes at short infusion times (10-15 min.) [9,12,19]. Moreover, in a study in newborn piglets an increase in intestinal and renal oxygen saturation was observed at 5 and 10 mg/kg/min [5], whereas in a study in adolescent pigs with sepsis a deterioration in liver function was observed at 10 mg/kg/min [17]. More specifically, one study described platelet aggregation dysfunction in hypoxic-reoxygenated newborn piglets treated with 10 and 20 mg/kg/min of dobutamine [7].
Only one study evaluated the use of dobutamine in premature animals (piglets born at 90% gestation) and no changes in specific organ perfusion were detected [11].
Considering the articles included in the review, none of them reported pharmacokinetics and pharmacodynamics (PK/PD) responses of dobutamine in neonatal and juvenile animals or focused on toxicity in immature systems.

Discussion
In the present study we focused on whether the evidence available regarding the use of dobutamine in juvenile animal was sufficient and adequate to support a clinical trial and the use of this drug in paediatric/neonatal populations.
The use of well-designed neonatal animal models, to address and predict effects of dobutamine on developing organs and therefore to obtain information on the potential different safety profile from those seen in adults, are important to support a clinical trial, due to developmental variations of structure and function of the cardiovascular system. In the context of this systematic review, although there are quite a number of studies in this field in different animal species, data from term and preterm pigs are particularly relevance due to pig is a representative model for the developmental cardiovascular physiology of humans [29][30][31]. However, the marked variability in the dose used, timing and duration of treatment and study design produces discrepancies between the results of studies.
Despite the heterogeneity of the studies, most of them show that dobutamine improves cardiac output in a dose-dependent manner, both in neonatal and juvenile models. As cardiac output is a crucial parameter in neonatal cardiovascular function, its improvement may be of importance in decreasing morbidity and mortality associated with cardiovascular compromise in this population.
However, precisely the variability in these studies means that the mechanism by which this improvement is achieved is not clear. Some of the studies analysed report an increase in heart rate [5,9,[11][12][13]16,18,23,26,28], whereas others show an improvement in stroke volume [6,8,9,17] or even an increase in mean arterial pressure [6,11,19,26,27]. Focusing on studies in pigs, as an animal model, this point seems clearer, being at short term infusion tachycardia more marked [9,11,16,18,23] and at long term infusion mainly due to an increase in stroke volume than tachycardia [6,8,9,17].
The existing information about the effects on cerebral perfusion and neurodevelopmental outcomes is clearly inadequate. Although there has been some research in this area, in neonatal animal models [5,6,8,11,12], which use mainly carotid blood flow measurements to study cerebral perfusion, the results are not consistent. This is probably due to the duration of treatment, the dose regimen and the existence of differences in measurement methods (NIRS, radiolabeled microspheres, electromagnetic flow probes, ultrasonic flow probes). The variability in measurement methods used, with advantages and disadvantages between them and in many cases practical and technological limitations, make difficult to compare them and to obtain clear conclusions. Although more studies are needed to clarify the effect of dobutamine at a cerebral level to translate clear conclusions to neonatal care, some of the studies included in the review suggest a trend towards an increase in the blood flow at doses higher than 10 mg/kg/min. So in the design of future studies this point should be considered.
In addition, only one study investigated the effect of dobutamine in preterm animals [11], with insufficient power to extract reliable conclusions to predict human clinical outcomes in this particular population.
Finally, the data on adverse effects reported in some studies (such as liver damage or platelet aggregation dysfunction) related to the use of dobutamine in specific diseases [7,17] are not conclusive. These studies should be analysed carefully, suggesting that more research is needed in this area.

Limitations
Systematic reviews are vulnerable to various different forms of bias. Our search was limited to studies published in English referenced in electronic databases, while there may also be relevant studies that are unpublished or written in other languages. Moreover, the quality of any review is determined by that of the studies included, so any bias in studies included associated with a lack of randomization or blinded assessment could have an impact on the conclusions. In this review, the quality of included studies was overall poor with less than half of them randomly allocating animals to groups, and only one being a blinded study [6].

Conclusions
On the one hand there is enough preclinical evidence to conclude that dobutamine improves cardiac output. On the other hand, although there has been some research on the effects of dobutamine in juvenile animal models, the heterogeneity across studies makes it difficult to obtain clear evidence to better understand its effects in peripheral organs, above all in the brain. Hence, it is necessary to perform more standardised and higher quality studies to support a clinical trial and to allow clear conclusions to be drawn concerning the rational use of dobutamine in paediatric/neonatal population.
Moreover, it is important to conduct studies in juvenile and neonatal animals designed to address specific questions of potential toxicity in developing systems, to perform PK/PD studies, at different postnatal ages, to develop evidence for individualized dosing schemes for children and also to collect data on the longterm safety of dobutamine that cannot be obtained from clinical trials.