Study design and patients
Magic Bullet is an investigator-driven, open-label, randomized controlled noninferiority trial conducted from May 2012 to November 2015. Thirty-two hospitals participated in the study—17 in Spain, 10 in Greece, and 5 in Italy. The study protocol has been published previously .
Patients were eligible if they were 18 years of age or older, developed VAP after at least 96 h on mechanical ventilation or less than 96 h on mechanical ventilation if they had previously received antibiotic treatment for at least 5 days, and were hospitalized for more than 7 days. VAP was defined as new or progressive pulmonary infiltrates on a chest X-ray suggestive of pneumonia with no other probable cause in combination with at least one of the following characteristics: fever, an elevated (> 10,000/mm) total peripheral white blood cell (WBC) count, immature neutrophils (bands) higher than 15% regardless of the total peripheral WBC count, leukopenia with a total WBC count less than 4500/mm, new onset of expectorated or suctioned respiratory secretions characterized by a purulent appearance indicative of bacterial pneumonia, or a Modified Clinical Pulmonary Infection Score (CPIS) > 4.
Specific exclusion criteria included the isolation of colistin- or meropenem-resistant GNB cultures from respiratory samples from surveillance cultures in the 7 days before inclusion, as well as the use of meropenem. The complete inclusion and exclusion criteria are detailed in Additional file 1.
The study was conducted in accordance with the Declaration of Helsinki and with the legal norm directive 2001/20/EC of the European Parliament relating to the implementation of Good Clinical Practice. The institutional ethics committee at each study site approved the protocol, and the regulatory authorities of the three countries provided authorization. All patients or their legal surrogates provided written informed consent prior to study enrollment.
The study was monitored for safety by the Data Safety Monitoring Board (DSMB) and assessed when 50% of the sample size was recruited to detect and report early evidence of a prespecified or unanticipated benefit or detriment to trial participants that may be attributable to one of the treatments under evaluation to determine recommendations concerning continuation or conclusion of the trial . The trial was registered with ClinicalTrials.gov, number NCT01292031, and with EudraCT, number 2010-023310-31.
Randomization and masking
After confirmation of the inclusion/exclusion criteria by the local principal investigator in an electronic case report form, patients were randomized to receive colistin or meropenem in a 1:1 allocation using an automatic system integrated into the electronic case report form. The overall study population included all patients who were enrolled, randomly assigned, and received at least one dose of the study medication constituted the modified intention-to-treat (MITT) population.
Randomization was stratified by patient severity at the time of VAP diagnosis measured with the Acute Physiology and Chronic Health Evaluation (APACHE) score (≤ 15 or > 15) and by clinical site. Given the open-label trial design, masking was not applicable.
Patients received either colistin methanesulfonate through a 1-h intravenous infusion at a loading dose of 4.5 million units (MIU), followed by 3 MIU maintenance doses every 8 h in a 30-min infusion, or meropenem at a dose of 2 g administered intravenously (i.v.) every 8 h in a 30-min infusion. In addition, all patients received levofloxacin (500 mg i.v. every 12 h in a 30-min infusion). For patients with moderate renal dysfunction at baseline, the colistin dosage was adjusted as follows: 2 MIU every 12 h with a creatinine clearance (CrCl) of 50–90 ml/min, and 2 MIU every 24 h with a CrCl of 10–50 ml/min following the formula of Gounden et al. . For patients with a CrCl of 10–50 ml/min, the dose of meropenem was reduced to 1 g every 12 h, and levofloxacin was reduced to 250 mg every 12 h.
The combination with levofloxacin in both groups was based on the results of the clinical trial of Heyland et al. , which suggested that for patients at a high risk of MDR-GNB, meropenem and ciprofloxacin combination therapy is safe and may be associated with better microbiological and clinical outcomes.
The drugs used in this study were commercial preparations of colistin and meropenem. Colistin is manufactured by G.E.S., Genéricos Españoles, Madrid (Spain), and meropenem is manufactured by Fresenius Kabi España, Barcelona (Spain). Both drugs were purchased and labeled for the study. Antimicrobials with activity against methicillin-resistant Staphylococcus aureus (vancomycin or linezolid) were administered at the investigator’s discretion .
A respiratory sample for microbiological diagnosis was obtained from all patients before randomization. The method used (bronchoalveolar lavage (BAL) or tracheal aspirate) was freely selected by the investigator. All samples were processed quantitatively. The cutoff values used to diagnose pneumonia were > 106 colony-forming units per ml (CFU/ml) for conventional tracheal aspirates and > 104 CFU/ml for BAL fluid. A set of two blood cultures was also obtained to determine the presence of bacteremia. Once the microbiological results from baseline samples were available, the therapy was adapted to the culture results. As a general rule, physicians were advised to prescribe a single antibiotic with the narrowest spectrum that had activity against the infecting organism as soon as possible.
To evaluate a microbiological cure, a tracheal aspirate or a sputum culture was collected if the patient remained on mechanical ventilation or if the patient breathed spontaneously, at 72 h, at 8 days, at the end of treatment, and at 28 days after randomization. Pathogen identification and susceptibility testing for all isolates were performed by the local laboratory. To test the susceptibility to colistin in most hospitals participating in the study, commercial microdilution was used (83%), and in the remaining hospitals, gradient strips were used (17%). Colistin monitoring was not performed for dosing adjustments.
Assessments of clinical symptoms and physical findings, sample collection, and evaluations of efficacy and safety variables, including renal function monitoring (CrCl and renal replacement therapy) and adverse events, were conducted at the baseline evaluation, at 72 h, at 8 days, at the end of treatment, and at 28 days after randomization.
The primary outcome of efficacy was determined according to the proportion of patients in the MITT population with microbiologically confirmed (microbiologically modified intention-to-treat, mMITT) VAP who died from any cause 28 days after randomization.
The secondary outcomes included 28-day all-cause mortality; clinical cure; renal function at the end of treatment, including post hoc renal failure based on the RIFLE score ; adverse events in the MITT population; and microbiological cure in the mMITT population. A post hoc analysis of mortality in the group with CR-GNB VAP was added. Clinical cure was defined as complete resolution of all signs and symptoms of VAP at 28 days after randomization. Microbiological cure was defined as eradication of the pathogen causing VAP at 28 days after randomization.
Adverse events included any serious treatment-emergent adverse events (SAEs), suspected SAEs related to a study medication (according to the investigator’s opinion), and serious unexpected adverse events up to 28 days after the last dose of the study medication.
The noninferiority of colistin was confirmed if the lower limit of the two-sided 95% confidence interval (CI) for the difference in mortality rates (colistin minus meropenem) in the mMITT population was less than or equal to 10% . On the assumption of a success rate of 20% mortality , a sample of 198 patients who could be evaluated in each treatment group (a total of 396) was required to demonstrate noninferiority at the two-sided significance level of 5% with a power of 80%. Considering that the microbiological diagnosis of VAP is obtained in approximately 80% of episodes , a 25% increase in the sample size is required. Thus, a total of 496 individuals were needed. The primary analysis was performed by intention-to-treat in the mMITT and MITT populations.
Continuous variables were reported as the means (standard deviations (SD)) or the medians (interquartile ranges), and categorical variables were reported as numbers (%). For dichotomous efficacy outcomes, risk ratios (RRs) and risk differences for the primary and secondary endpoints with 95% CIs were calculated with Cochran’s Mantel-Haenszel method for estimation of the common treatment effect. A Kaplan–Meier curve was plotted for time to death until day 28 for the mMITT and MITT populations. A p value < 0.05 was considered significant.
Statistical analysis was performed using SPSS version 19.0 for Windows (SPSS Inc., Chicago, IL, USA).