Introduction

Infections with resistant pathogens are associated with increased mortality, morbidity, and length and cost of hospital stay [2]. Therefore, development of new antibiotics with high potency, stability against the mechanisms of resistance, and favorable pharmacokinetic and pharmacodynamic characteristics has become an urgent priority.

Linezolid is a synthetic antibacterial agent of oxazolidinone class with in vitro activity against aerobic Gram-positive bacteria, including methicillin-resistant Staphylococcus aureus (MRSA), methicillin-resistant coagulase-negative staphylococci (MRCoNS), vancomycin-resistant enterococci (VRE), penicillin-resistant Streptococcus pneumoniae (PRSP), and other drug-resistant Gram-positive bacteria [4, 16]. It is chemically unrelated to other antibacterial agents and selectively inhibits bacterial protein synthesis through binding to domain V of the 23S rRNA on the bacterial 50S ribosomal subunit and prevents the formation of a functional 70S initiation complex, an essential component of the translation process. Linezolid has demonstrated excellent tissue penetration, equivalent bioavailability between the oral and intravenous formulations, and it lacks cross-resistance with current antibiotic therapies due to its unique mechanism of action.

An increasing incidence of serious infections due to resistant Gram-positive bacteria in children has become particularly evident in recent years in neonatal and pediatric intensive care and hematology/oncology units [22]. Resistant phenotype spread has become a difficult therapeutic problem, and linezolid appears to be a very promising alternative for the treatment of resistant pathogens. The indications for administration of linezolid to children so far include the treatment of VRE, nosocomial and community-acquired pneumonia due to MRSA or PRSP, and complicated skin/soft tissue infections due to MRSA and MRCoNS [18, 27].

Several randomized controlled trials (RCTs) have been conducted to compare the effectiveness of linezolid with that of antibiotics usually given for the treatment of Gram-positive infections in adults. As a result, many meta-analyses were conducted for the efficacy and safety of linezolid in adults [1, 5, 7, 15, 28]. However, there was no meta-analysis done for the efficacy and safety of linezolid in children. The aim of the study reported here was to compare the efficacy and safety of linezolid versus antibiotics usually given for the treatment of Gram-positive infections in children by performing a meta-analysis of relevant RCTs. This meta-analysis follows the PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analyses) statement [19], and a protocol has been registered at the PROSPERO (International Prospective Register of Systematic Reviews) with protocol number CRD42013004732 (http://www.crd.york.ac.uk/PROSPERO).

Methods

Search strategy and selection criteria

An extensive search of PubMed (up to September 2013), Cochrane Central Register, and Scopus of RCTs was performed to identify relevant RCTs for our meta-analysis. Search terms included “linezolid,” “child,” “randomized clinical trials,” “efficacy,” and “safety.” No language restrictions were used. Specific search terms for each database were the following:

  • PubMed search terms (linezolid [substance] OR “zyvox” [all fields]) AND (child [all fields] OR neonate [all fields] OR infant [all fields]) AND (randomized controlled trial [pt] OR controlled clinical trial [pt] OR randomized controlled trials [mh] OR random allocation [mh] OR double-blind method [mh] OR single-blind method [mh] OR clinical trial [pt] OR clinical trials [mh] OR (“clinical trial” [tw]) OR ((singl* [tw] OR doubl* [tw] OR trebl* [tw] OR tripl* [tw]) AND (mask* [tw] OR blind* [tw])) OR (“Latin square” [tw]) OR placebos [mh] OR placebos [mh] OR placebo* [tw] OR random* [tw] OR research design [mh:noexp] OR follow-up studies [mh] OR prospective studies [mh] OR cross-over studies [mh] OR prospectiv* [tw] OR volunteer* [tw]) NOT (animal [mh] NOT human [mh])

  • CENTRAL (Cochrane) search terms (linezolid AND child)

  • SCOPUS search terms (linezolid AND (child OR neonate OR infant) AND clinical trial).

Randomized controlled trials were selected using a methodological filter [23]. To identify relevant completed studies that were unpublished, we searched clinical trial registries (ISRCTN Register, Netherland’s Trial Register, UMIN Clinical Trials Registry, Australian New Zealand Clinical Trials Registry, and ClinicalTrial.gov) up to September 2013. We obtained the results of these studies by contacting the manufacturer through one of their expert opinion leaders.

Two reviewers (MI and FA) independently searched the literature and examined relevant RCTs for further assessment of data on effectiveness and safety. Eligible studies were randomized controlled trials that assessed the clinical efficacy and safety of linezolid in children in comparison with other antimicrobial agents for treatment of infections caused by Gram-positive bacteria. However, no eligible studies evaluating only efficacy or safety were retrieved from the databases. Hospital admission of patients was not required for eligibility. We did not place any restrictions on participants, length of follow-up, or the class of antimicrobial agents in the control group. Experimental trials and trials focusing on pharmacokinetic or pharmacodynamic variables, trials referring only to the in vitro activity of linezolid, and unpublished studies that were incomplete were excluded.

Data extraction

The following data were extracted from every study: year of publication, clinical setting, patient population, number of patients [by intention to treat (ITT), and those assessed clinically and microbiologically], characteristics of patients (age, sex, ethnic origin, and disease), antimicrobial agents and doses used, clinical and microbiological outcomes, adverse effects, treatment duration, time from treatment to test of cure, and funding. The ITT population comprised patients who received at least one dose of the medications studied in the individual RCTs. The clinically assessed population comprised patients that fulfilled all inclusion and exclusion criteria in the individual RCTs, had complete follow-up with a final assessment at the preset test of cure visit, and received one course of study drug for the minimum duration, with no more than one dose of potentially effective concomitant therapy after the first dose of linezolid. The microbiologically assessed population was a subset of the clinically assessable population, and had also microbiologically documented infections.

Two reviewers (MI and FA) independently extracted the relevant data. Any disagreement was resolved by consensus in meetings with all investigators.

Definitions of infections

Infections were defined according to the definitions used in the individual randomized controlled trials. Definition of infection did not differ substantially between the studies included in the meta-analysis.

For skin and soft tissue infections (SSTIs), both complicated and uncomplicated SSTIs could be included in the meta-analysis. At least one of the following symptoms of infection needed to have been present: drainage or discharge, erythema, fluctuance, heat or localized warmth, pain or tenderness to palpation, swelling, or induration.

For catheter-related bacteremia, presence of indwelling venous or arterial catheter, at least one positive blood culture obtained from the catheter growing a Gram-positive organism and clinical profile was to be compatible with a diagnosis of catheter-related bacteremia with at least two of the following findings: fever, hypothermia, leukocytosis, leukopenia or left shift of band neutrophils, increased pulse (>98th percentile for age), and increased respiratory rate (>2SD of normal for age). In addition to the inclusion signs and symptoms, other signs of septic shock (decreased peripheral perfusion or hypotension) and petechiae or purpura could also be used to meet inclusion for patients with catheter-related bacteremia.

With regard to the definition of bacteremia of unknown origin, a clinical profile compatible with a diagnosis of bacteremia without an identified source was considered as equivalent to a diagnosis of bacteremia of unknown origin. In addition to the inclusion signs and symptoms, other signs of septic shock (decreased peripheral perfusion or hypotension) and petechiae or purpura could also be used to meet inclusion for patients with bacteremia of unknown origin.

For a diagnosis of pneumonia, baseline chest radiograph needed to show new or progressive infiltrates, consolidation with or without effusion, and two of the following signs and symptoms: cough, new/worsened purulent sputum production, rales, pulmonary consolidation, or signs of respiratory distress (e.g., dyspnea, tachypnea, cyanosis, intercostal retractions, labored breathing, grunting, or nasal flaring). Patients with pneumonia could also have, as part of these two inclusion criteria, fever, hypothermia, leukocytosis, leukopenia or left shift of band neutrophils, increased pulse (>98th percentile for age), increased respiratory rate (>2SD of normal for age), requirement for mechanical ventilation, an increase in ventilator settings, altered mental status, lethargy, or irritability in infants <1 year of age.

Quality assessment

We assessed the quality of every published randomized controlled trial on the basis of five elements of study design and reporting the following: generation of random numbers, details of double-blinding procedure, information on withdrawals, allocation concealment, and discussion of other potential sources of bias [11]. One point was awarded for the specification of each criterion, with a maximum score of 5. High-quality RCTs scored 3 or more points, whereas low-quality RCTs scored 2 or fewer points, according to a modified Jadad score [21].

Analyzed outcomes

The primary outcome was (1) treatment success in the clinically assessable population, which was defined as cure (resolution of symptoms and signs of infection) or clinically significant improvement of patients and (2) treatment success in the microbiologically assessable population. Secondary outcomes included adverse events or discontinuation of treatment that was probably related to effects of the study drug and eradication of pathogens present at baseline. Treatment success and adverse events were assessed in all patients.

Data analysis and statistical methods

The meta-analysis was performed with random effects models in Review Manager (version 5.1). We chose to use random effect models because of the obvious heterogeneity across the trials included in the meta-analysis (different infections, control drugs). Pooled odds ratios (ORs) and 95 % confidence intervals (CIs) for all primary and secondary outcomes (including ITT, clinically assessed and microbiologically assessed populations) were calculated using the Mantel-Haenszel method.

The statistical heterogeneity between RCTs was assessed by Q statistic generated from the x 2 test (p < 0.10 was defined to indicate significant heterogeneity). The extent of heterogeneity was estimated with the I 2 measure, and published guidelines were used to define low (I 2 = 25–49 %), moderate (I 2 = 50–74 %), and high (I 2 > 75 %) heterogeneity [12]. For each analysis, the I 2 and p values of statistical heterogeneity are presented.

Assessment of publication bias was not undertaken because of the small number of included studies.

Results

Main characteristics of the pooled trials

Search results are depicted as a flow diagram in Fig. 1. One hundred fifty-nine potential articles were identified; 16 studies met the inclusion criteria according to information in the title and abstract and were assessed for eligibility, of which 14 were excluded. Six were excluded because they were part of RCTs already included in the analysis [3, 14, 16, 20, 24, 31]. Five studies had no control regimen and were thus not included in the analysis. Three of these were published [8, 18, 26], while two were unpublished at the time of our analysis (NCT 00035854 and NCT 00035269). Finally, three reports had data not only for children ([30] and two unpublished trials NCT 00037050 and NCT 00035425). We obtained the results of these studies by contacting the manufacturer through one of their opinion leaders. Data from the first study were excluded because we could not obtain the relevant information from the researchers, whereas data from the two unpublished trials were excluded due to the small sample size for ages 0–18 years old (one study included only eight subjects and the other nine subjects in this age category). Finally, two randomized trials met the inclusion criteria of our study, yielding a total of 815 patients [17, 29].

Fig. 1
figure 1

Study selection

Selected randomized controlled trials

The main characteristics of the analyzed RCTs are shown in Table 1. The range for the quality score of the included RCTs was 2–3. According to the information provided for each study, the effectiveness of linezolid was examined in the clinically and microbiologically assessable population.

Table 1 Main characteristics of the trials included in the meta-analysis

Treatment success in clinically evaluable (CE) populations

The primary clinical outcomes that were included in the meta-analysis are shown in Fig. 2(a). Success of empirical treatment in clinically assessed patients was achieved at 85 % for both linezolid-treated patients and comparator-treated patients. Linezolid was slightly more effective than comparator regimens, with no statistical heterogeneity between studies, but the difference was not significant (OR = 1.39, 95 % CI 0.98–1.98, I 2 = 0 %, p = 0.71).

Fig. 2
figure 2

a, b Meta-analysis of treatment success for clinically (a) and microbiologically (b) assessed patients. The vertical line indicates no difference between the two treatment groups. The size of each square denotes the proportion of information provided by each trial. Pooled odds ratios were calculated from random effects models with the Mantel-Haenszel method

Treatment success in microbiologically evaluable (ME) populations

Data of the two RCTs included in the meta-analysis on microbiologically assessed patients are shown in Fig. 2(b). For microbiologically assessable populations, the difference in treatment efficacy between the study and control groups was not significant (OR = 1.12, 95 % CI 0.61–2.03, I 2 = 0 %, p = 0.67).

In addition, data for eradication of the pathogen isolated at baseline were available only for S. aureus (Fig. 3(a)) and MRSA (Fig. 3(b)). In vitro linezolid had a similar effect on these bacteria studied, with increased eradication of them, but no significant differences were recorded [S. aureus, (OR = 1.07 95 % CI 0.52–2.20, I 2 = 0, p = 0.94) and MRSA (OR = 1.21, 95 % CI 0.18–8.27, I 2 = 0 %, p = 0.66)].

Fig. 3
figure 3

a, b Meta-analysis of eradication of Staphylococcus aureus (a) and MRSA (b) for microbiologically assessed patients. The vertical line indicates no difference between the two treatment groups. Pooled odds ratios were calculated from random effects models with the Mantel-Haenszel method

Adverse effects

Data on adverse effects possibly related to the study regimens were reported in these two trials. Overall, linezolid seems to be as safe as comparators (OR = 0.61, 95 % CI 0.25–1.48, I 2 = 87 %, p = 0.006), but the disparity between studies was high (Fig. 4). Our meta-analysis did not show any differences between the studied medications for any adverse effects possibly or probably related to the study drug.

Fig. 4
figure 4

Meta-analysis of total adverse effects probably or possibly related to study medications. The vertical line indicates no difference between the two treatment groups. Pooled odds ratios were calculated from random effects models with the Mantel-Haenszel method

Assessment of specific adverse events [diarrhea (Fig. 5(a)), nausea (Fig. 5(b)), vomiting (Fig. 5(c))] showed that the occurrences of neither diarrhea (OR = 0.85, 95 % CI 0.49–1.49, I 2 = 0 %, p = 0.47) nor nausea (OR = 1.27, 95 % CI 0.51–3.20, I 2 = 0 %, p = 0.50) nor vomiting (OR = 0.63, 95 % CI 0.18–2.24, I 2 = 32 %, p = 0.22) were significantly different among treatment groups.

Fig. 5
figure 5

ac Meta-analysis with respect to adverse effects of diarrhea (a), nausea (b), and vomiting (c). The vertical line indicates no difference between the two treatment groups. Pooled odds ratios were calculated from random effects models with the Mantel-Haenszel method

Discussion

Linezolid does not appear to be more or less effective than standard antimicrobial regimens for the treatment of serious infections caused by Gram-positive bacteria in children. Linezolid seemed to be better than comparators with increased eradication rates for S. aureus and MRSA, but these differences were not statistically significant. As a result, it is not only the absence of statistical significance but also the low quality of the available studies that leads to an inconclusive result and makes prominent the need for new and well-designed studies.

Systematic reviews previously conducted for the use of linezolid in children have shown that linezolid appears to be effective and safe in the treatment of pediatric patients with serious infections for approved and off-label indications, including pneumonia and endocarditis, as well as skin/soft tissue, central nervous system, and osteoarticular infections caused by antibiotic-resistant Gram-positive organisms.

However, novel indications for linezolid use need to be established in newer studies with an emphasis on adverse effects and the development of resistance [6, 9]. In vitro, linezolid demonstrates modest efficacy against some Gram-negative bacteria (such as Legionella, Bordetella, Moraxella catarrhalis, and Haemophilus influenzae). Based on this observation, there could be discussion and research for a broader antibacterial spectrum [10, 13, 25].

There were no differences in the total adverse effects, diarrhea, nausea, and vomiting, between the two groups in our study. We recorded a substantial heterogeneity in adverse events at total; this finding was probably because of the various infections, doses, comparator drugs, and population included. Previous data reported that the most common adverse effects of linezolid are diarrhea, loose stools, nausea, vomiting, headache, rash, itching, and fever [24]. However, post-marketing surveillance has noted some rare but serious adverse events. Given these rare potentially serious adverse events, the safety profile of linezolid must be well monitored during treatment [24]. Physicians must be aware of the symptoms and signs of toxicity so that linezolid can be immediately discontinued if these occur. Although there are no official guidelines for monitoring, regular monitoring of linezolid should include (1) symptoms and signs of lactic acidosis, which are nonspecific, but include nausea, vomiting, mental status changes, tachycardia, and hypotension, and, possibly, regular monitoring of the serum bicarbonate levels; (2) development of cytopenia through blood count monitoring; and (3) development of peripheral and optic neuropathy symptoms. Adverse events of linezolid may be more common when the drug is used for longer than 28 days, which is the treatment length currently approved by the US Food and Drug Administration [20, 24, 27].

There are limitations that should be considered in our meta-analysis or the studies that were analyzed. First, there are some missing data from original reports, which the authors could not receive from the investigators performing the trials, and thus may have introduced bias to the reported outcomes of effectiveness. Apart from the missing data, the populations of the two studies differed significantly on their diagnosis. Τhe first study included children that, during their hospitalization, presented with complicated infections, while in the second, the population consisted of children with uncomplicated skin and soft tissue infections. Furthermore, vancomycin serum concentrations not routinely monitored in these two trials might have contributed to lower treatment success of the regimen, thus influencing the outcomes in favor of linezolid. Finally, randomized controlled trials set selective inclusion criteria that can limit their generalizability to unselected populations.

Randomized controlled trials assessing the efficacy of linezolid in children for treatment of infections caused by Gram-positive bacteria will help to establish the position of linezolid in the range of antimicrobial treatment available. This research is needed to clarify whether linezolid should be used for such infections when other antibiotic regimens fail.