Clinical efficacy and safety of linezolid in intensive care unit patients

Background To characterize the population of critically ill patients and infections treated with linezolid in the intensive care unit (ICU), and to evaluate the clinical efficacy and safety of linezolid therapy. Methods This multi-center, observational, real-world study was conducted across 52 hospitals between June 9, 2018, and December 28, 2019. Patients who met the following inclusion criteria were included: (1) admitted to the ICU, (2) of any age group, and (3) having a clinical or laboratory diagnosis of a Gram-positive bacterial infection. Clinical efficacy was categorized as success (cured or improved), failed, or non-evaluable. Adverse events and serious adverse events were recorded during treatment. Results A total of 366 ICU patients who met the inclusion criteria were evaluated. Linezolid was used as second- and first-line treatment in 232 (63.4%) and 134 (36.6%) patients, respectively. The most common isolated strain was Staphylococcus aureus (methicillin-resistant Staphylococcus aureus: n=37/119, 31.1%; methicillin-susceptible Staphylococcus aureus: n=15/119, 12.6%); this was followed by Enterococci (vancomycin-resistant Enterococci: n=8/119, 6.7%; vancomycin-susceptible Enterococci: n=11/119, 9.2%) and Streptococcus pneumoniae (multidrug-resistant: n=4/119, 3.4%; non-multidrug resistant: n=2/119, 1.7%). The main infection sites where pathogens were detected included the lung (n=216/366, 59.6%), skin and soft tissue (n=104/366, 28.4%), and blood (n=50/366, 13.7%). Clinical success was achieved in 301 (82.2%) patients; 34 (9.3%) were cured and 267 (73.0%) improved; treatment failure and non-evaluable outcomes were observed in 29 (7.9%) in 36 (9.8%) patients, respectively. Linezolid-related adverse events were reported in 8 (2.2%) patients. No treatment-related serious adverse events were reported. Conclusions Based on real-world results, linezolid was found to be effective and safe in the treatment of Gram-positive bacterial infections in critically ill patients.


Introduction
The intensive care unit (ICU) is the "hardest hit " department for hospital-acquired infections, where a range of traumatic procedures can lead to fatal infections. [ 1 , 2 ] Ventilator-related pneumonia, catheter-related bloodstream infections caused by an arterial or venous puncture, and skin and soft tissue infections (SSTIs) secondary to tracheotomy and surgical wounds are the main types of hospital-acquired infections. [3][4][5] Nearly half of these infections are caused by Gram-positive bacteria. [6] The increasing prevalence of multidrug-resistant Grampositive pathogens poses a significant challenge in the ICU. [7] Methicillin-resistant Staphylococcus aureus (MRSA), methicillinsusceptible Staphylococcus aureus (MSSA), and vancomycin-As advanced antibiotics commonly used in the ICU, linezolid and vancomycin are often compared. Although vancomycin is often used as the first choice, side effects pertaining to liver and kidney function, and especially renal injury, lead to certain limitations in its use in ICU patients. [11] Linezolid is more useful than vancomycin in SSTIs and MRSA pneumonia, [12] and is more effective and cost-effective for hospital-acquired MRSA infections. [13] It is also the only antibiotic that is more effective than daptomycin and quinopidine (among others) for vancomycinresistant Enterococcus faecalis infection. [ 14 , 15 ] Considering the role of linezolid in the treatment of Grampositive bacteria, this real-world study was conducted to characterize the population of critically ill patients in the ICU and infections treated with linezolid in the ICU. It also aimed to evaluate the clinical efficacy of linezolid therapy and to assess the safety of Chinese-made linezolid in ICU patients. This study collected data from 52 hospitals and conducted a retrospective analysis to guide better antibiotic prescribing among clinicians.

Patient population
This multi-center, observational, real-world study conducted across 52 hospitals between June 9, 2018 and December 28, 2019 was approved by the ethics committee of the West China Hospital, Sichuan University, as well as the institutional boards of the other 51 centers (No. 30/2017).
Patients consented to participate in the study and met the following inclusion criteria: (1) admitted to the ICU, (2) of any age group, and (3) having a clinical or laboratory diagnosis of a Gram-positive bacterial infection.

Research methods
The research drug was linezolid injection (200 mg/100 mL) under the brand name Hengjie, which was produced by the Jiangsu Hansoh Pharmaceutical Group Co., Ltd. (H20150223). The start of linezolid injection was used as the start time point in the study; patients were followed up once a day until 48 h after the discontinuation of therapy, transfer out of the ICU, or death.

Clinical efficacy
The primary efficacy indicators were categorized as clinically cured, clinically improved, clinically failed, or non-evaluable. Clinical cure was defined as the absence of signs and symptoms, the absence of subsequent antibiotic medication, or the absence of infection on a culture report. Clinical improvement was defined as the partial disappearance of clinical signs and symptoms and/or the need for more antibiotics. Patients who died or had an unsatisfactory response to linezolid therapy, worsening or recurring signs and symptoms, requirements for a change in antibiotic medication, or a positive culture at the end of therapy were considered to have clinically failed. Insufficient information making it impossible to determine a response led to categorization as non-evaluable. [ 16 , 17 ] Clinical success included cured or improved categories.
Secondary efficacy indicators included organ function status, bacterial clearance rate, negative rate of pathogenic cultures, ICU stay time, duration of hospital stay, and ICU mortality and mortality.

Safety
Adverse events (AEs) referred to adverse medical events that occurred after patients or clinical trial subjects received linezolid, but were not necessarily causally related to treatment. Serious adverse events (SAEs) were defined by meeting one or more of the following criteria: (1) death, (2) life-threatening complications, (3) necessitating hospitalization or prolonged hospitalization, and (4) permanent or severe disability. [ 16 , 17 ] The researchers monitored AEs and SAEs for 30 days after receipt of the medication. The severity of all reported AEs was evaluated by local investigators, regardless of whether they were related to linezolid. The AEs and SAEs were categorized into five levels: definitely related, very likely to be related, possibly related, probably related, and unlikely or not related. The standard protocols for the identification and treatment of AEs were customized by clinicians according to the clinical condition.

Combined therapy
The combined medication included two components, one being the antibiotic itself. As linezolid injection has no activity against Gram-negative bacteria, the investigator could add antibiotics as appropriate in clinically suspected or confirmed cases of Gram-negative bacterial infections. Antifungal drugs were used as appropriate when fungal infections were suspected or diagnosed. The other component was the main drug used for the treatment of the primary disease (if there were other major concomitant diseases). This study mainly evaluated whether linezolid injection demonstrated cross-reactivity with certain drugs.

Data collection
Data were obtained from patients at 52 institutions using a standardized form and process. The data included the following: (1) demographic data; (2) clinical baseline data on the first day of medication including microbiological data and acute physiology and chronic health evaluation (APACHE II), sequential organ failure assessment (SOFA), and Glasgow coma scale (GCS) scores; (3) clinical effectiveness and safety data; and (4) pharmacoeconomic indicators including total costs of linezolid use, antibiotic use, and ICU stay.

Statistical analysis
SPSS version 22.0 was used for statistical analysis. No inferential analyses were performed; only descriptive statistics were employed. All the analyses were explanatory. For continuous variables, numerical data were reported as means ± standard deviation, medians (interquartile range), and minimum and maximum. Absolute and relative frequencies were used to summarize categorical data. Data are presented as n (%) and mean ± standard deviation.

Patient demographics and clinical characteristics
A total of 366 ICU patients who met the inclusion criteria were evaluated. The demographic, clinical, and infection characteristics are shown in Table 1. There were 246 (67.2%) and 120 (32.8%) male and female patients, respectively. The mean age was 57.3 ± 22.9 years, and the majority (66.4%) of patients were over 65 years old. A total of 30 children were included in this study; they had an average age of 6.2 ± 5.9 years. The mean body weight of the cohort was 60.8 ± 17.4 kg, with a mean body mass index of 22.7 ± 3.9 kg/m 2 . Patients from different grade hospitals were well represented; the majority (89.9%) were derived from grade III level A hospitals.
In critically ill patients, the APACHE II score is a comprehensive score that allows assessment of the severity of the disease and predicts the risk of death. In this study, the mean APACHE II score of the entire cohort was 17.9 ± 8.3. The mean SOFA score, which evaluates organ function, was 6.4 ± 4.6. The GCS score is used to describe the extent of impaired consciousness in patients; the higher the score, the better the state of consciousness. The mean GCS score in the study cohort was 11.5 ± 3.5.

Microbiology
Among all included patients, linezolid was used in 119 (32.5%) patients after the isolated bacterial strain was diag-nosed to be Gram-positive; in the remaining 247 (67.5%), antibiotics were prescribed empirically.
The proportions of isolated strains in the three grades of hospitals are shown in Figure 1 . MRSA accounted for the largest proportion in grade III level A and grade II level A hospitals.
Co-administration of two drugs may alter their effectiveness. This interaction may delay, decrease, or increase the absorption of the drug, or cause adverse reactions. In this study, we did not observe any adverse effects when linezolid was co-administered with Gram-negative antibiotics, namely, cephalosporins, ciprofloxacin, meropenem, and gentamicin. In addition, the use of linezolid with antifungal drugs such as aminoglycosides and fluoroquinolones did not appear to affect the effectiveness of the either drug.
Treatment of MSSA infections had the highest clinical success rate ( n = 14/15, 93.3%), while that with MRSA infections came second ( n = 31/37, 83.8%). The number of enterococcal infections was limited, and the clinical success of treatment of VSE infections ( n = 10/11, 90.9%) was higher than that of VRE infec-tions ( n = 6/8, 75%). Among the isolated MRSA-infected patients who were reclassified according to the infection site, the clinical success of treatment of MRSA pulmonary infections with linezolid ( n = 22/27, 81.5%) was higher than that of the treatment of MRSA-related SSTI ( n = 5/7, 71.5%) ( Figure 3 ).

Safety and tolerability
Linezolid-related AEs were reported in 8 (2.2%) patients. No treatment-related SAEs were reported. A total of 3 patients experienced rash (chest rashes: n = 2; neck rash: n = 1), all three events were thought to be possibly related to linezolid. Overall, 2 patients were found to have lactic acidosis; one case was considered very likely to be related and the other was possibly related. A total of two patients were found to have thrombocytopenia, which was very likely to be related. Only one patient experienced possibly related acute kidney injury. Although the adverse reactions were attenuated after stopping linezolid, they persisted.   Linezolid was discontinued in 4 patients due to AEs, and there were 17 fatalities (unrelated to the study drug) throughout the trial period. There were no adverse effects recorded in children receiving linezolid therapy. Table 4 summarizes the occurrence of AEs during linezolid therapy.

Discussion
Our results provide insight into a real-world experience of linezolid use against various Gram-positive infections, including MSSA, MRSA, and VRE, in ICU patients who were severely ill and were therefore exposed to the risk of multiple nosocomial infections. Most patients included in this study were older than 65 years; patients younger than 18 years were also included. Patients were recruited from 52 hospitals in central and southwest China that had representative ICUs, enabling the analysis of a wide range of illnesses and microbiologic data. In this trial, linezolid use resulted in excellent clinical success rates in patients with Gram-positive bacterial pneumonia and SSTI. It also showed good safety and tolerability in ICU patients, including adults and children. Linezolid has shown similar high clinical success rates as both, first-line and second-line treatment; this provides more choices for clinical decision-making.
In China, despite a downward trend in recent years, multidrug-resistant Gram-positive bacteria remain one of the most important threats to human health. [18] According to the results of the CHINET study, the prevalence of nosocomial drugresistant Gram-positive infections in China is 35.3%, while that of acquired infections in the ICU is even higher. [18] The low immune status of ICU patients and the use of more invasive devices such as endotracheal tubes, central venous catheters, arterial cannulas, and urinary catheters are major risk factors for Gram-positive infections. Given the correspondingly high infection burden, the precise selection of antimicrobial drugs in the ICU is crucial. [19][20][21] Linezolid demonstrates significant microbial clearance for pulmonary Gram-positive bacteria, and especially MRSA, due to its characteristic high lung tissue and lung epithelial surface fluid permeability. [22] In our study, linezolid showed high clinical success in the treatment of MRSA pneumonia; the clinical success rate in patients with MRSA strains isolated from the lung reached 81.5%. Vancomycin is one of the first-line treatments for MRSA pneumonia. Previous systematic reviews have shown that linezolid and vancomycin have similar clinical efficacy in MRSA pneumonia. [ 11 , 23-25 ] Based on the above evidence, the American Thoracic Association/Infectious Diseases Association guidelines recommend vancomycin or linezolid in the treatment of MRSA pneumonia, and prefer vancomycin as the first choice. [26] However, a recent meta-analysis of seven randomized controlled trials showed that linezolid-treated patients had higher clinical cure rates and microbial clearance than those treated with vancomycin. [27] The findings also indicated that the treatment outcome was poor when linezolid was used after vancomycin treatment failure. In critically ill patients, prolonged endotracheal intubation is a major cause of MRSA infections. In a recent prospective observational clinical study conducted across four European ICUs, [28] intravenous linezolid had better efficacy than vancomycin in reducing the burden of endotracheal MRSA in patients undergoing prolonged mechanical ventilation.
Although the number of patients with MRSA strains isolated from skin and soft tissue was limited, the results showed a high clinical success rate, particularly in terms of clinical cure. SSTIs such as impetigo, abscesses, and surgical site infections are common. Mortality and treatment costs are high for severe SSTIs, especially for those involving deep tissue. [29] Line- zolid and vancomycin are effective antibiotics for their treatment. A meta-analysis of nine randomized controlled trials compared linezolid with vancomycin for the treatment of SSTIs. [30] Linezolid was found to be more clinically effective than vancomycin, with fewer skin complications and shorter hospital stays. More importantly, although linezolid is more expensive than vancomycin, the cost of treatment was lower than that of the latter. [31] VRE is also a serious threat and has been shown to be an independent risk factor for death in patients with enterococcal bloodstream infections. [ 32 , 33 ] Treatment options for enterococcal infections remain limited as they are highly resistant to a wide range of antibiotics. The main drugs currently available for treatment are linezolid and daptomycin. Nine previous clinical studies have shown no significant difference between daptomycin and linezolid in terms of clinical effectiveness or microbial clearance. [34] Notably, linezolid is associated with a higher risk of thrombocytopenia than daptomycin. Although linezolid was less commonly selected for patients in whom VRE strains were isolated, our results showed that it offered a good clinical success rate in VRE.
In our study, lower clinical success was observed in pneumonia or SSTIs complicated with bloodstream infections; this may be attributed to the high SOFA and APACHE II scores, which indicate a higher baseline rate of treatment failure. Findings from previous studies are consistent with our results. A 5-year retrospective study on Gram-positive bacteria isolated from bloodstream infections in critically ill patients found the SOFA score to be an independent risk factor for death in these infections. [35] There was no significant difference between the three treatment regimens involving glycopeptides, linezolid, and daptomycin.
The known primary AEs associated with vancomycin use include nephrotoxicity and ototoxicity, [ 36 , 37 ] which are especially noted in ICU patients with unstable renal function. Conversely, linezolid treatment has been linked to myelosuppression, peripheral and optic neuropathy, and lactic acidosis, especially in cases of long-term use. [38] Linezolid-related myelosuppression often results in severe thrombocytopenia, which can lead to platelet transfusions, bleeding, and even an increased risk of death. The incidence of thrombocytopenia in previous studies was generally low, at approximately 2.4%; the incidence in critically ill patients and in those with low-body weight was 13.9-60.5%. [39] The incidence of thrombocytopenia was low in our study and inconsistent with data from previous studies; it was observed in only two patients and the symptoms resolved after drug discontinuation. The lower incidence may have been related to the following factors: (1) linezolid use was avoided in at-risk patients (as assessed by clinicians), and (2) linezolid was only used for a short time in this study; previous studies have shown that the development of thrombocytopenia is associated with long-term use.
Linezolid is eliminated by renal (30%) and non-renal (65%) mechanisms; it is therefore less likely to cause renal injury. [40] Only one patient in this cohort developed acute kidney injury; however, the renal function did not recover after discontinuation of linezolid. However, a recent meta-analysis found that linezolid use in patients with renal impairment was associated with a higher incidence of thrombocytopenia; this indicated the need for clinicians to use linezolid more cautiously in these patients. [40] Despite the increasing clinical use of linezolid in adults, realworld data on linezolid use in pediatric patients is considerably limited. A total of 30 patients in this study were aged younger than 18 years. Linezolid demonstrated high efficacy and safety in the treatment of pediatric patients, with 17 children (56.7%) of clinical improved, 12 children of clinical cured (40.0%), and only 1 child of clinical failure. There were no AEs in any of the 30 patients; this suggested that linezolid can be used safely and effectively in pediatric critically ill patients. However, linezolid did not offer a significant advantage for the treatment of serious Gram-positive bacterial infections in children in previous studies; it showed neither superior nor inferior efficacy to standard antimicrobial drug therapy. [41] Further qualitative studies on the use of linezolid in children are therefore needed.

Limitations
This study has several limitations. First, this is an observational real-world study designed to realistically describe the use of linezolid in critically ill patients. It adopted broad inclusion and exclusion criteria and laid more emphasis on the actual effect in clinical patients compared with traditional randomized controlled studies. However, it could not compare the efficacy of different therapeutic drugs. Second, the sample size included in this study was insufficient; it will continue to expand while compared with other drugs in the future. Third, the first-and second-line treatments in this study referred to the first-and second-line drugs used during the period of ICU stay; the drugs used before ICU admission were not known. Fourth, the etiological evidence for Gram-positive bacteria was lower; this was mainly due to the low detection rate. Fifth, although the incidence of AEs in this study was generally consistent with those of prior phase III clinical trials, linezolid-related AEs may be subjectively underestimated by clinicians due to the complex multiple index abnormalities in ICU patients. Therefore, although linezolid showed good safety in this retrospective study, clinicians need to be cautious in evaluating possible future AEs.

Conclusions
The treatment of Gram-positive bacterial infections in the ICU is extremely important, and the selection of appropriate antibiotics based on patients' clinical characteristics is the key to prognosis. Based on real-world results, linezolid has been found to be effective and safe in the treatment of Gram-positive bacterial infections in critically ill patients. Linezolid showed better clinical success in pulmonary infections or SSTIs caused by Staphylococcus aureus . Our results will provide intensivists with a reference for the selection of medication. However, due to the limitations of this study including those pertaining to sample size, clinicians will need to individually evaluate patient conditions before using linezolid in the clinic. In addition, they will need to be vigilant regarding possible side effects during administration.