Alternative empiric therapy to carbapenems in management of drug resistant gram negative pathogens: a new way to spare carbapenems

Background: Increasing prevalence of carbapenem resistance in Gram negative bacteria due to excessive and indiscriminate use of carbapenems has forced the medical fraternity to find out ways to spare carbapenems. This retrospective study was aimed to explore a new fixed dose combination (FDC) of ceftriaxone+sulbactam with adjuvant disodium edetate as a carbapenem sparing drug in the management of moderate to severe bacterial infections of lower respiratory tract infections (LRTIs), urinary tract infections (UTIs) and intra-abdominal infections (IAIs). Methods: A retrospective analysis involves those patients in whom FDC or meropenem was used empirically for the management of these infections caused by multidrug resistant pathogens. Results: The average age of evaluated patients was 58.17±13.98 years. Out of 107 patients, 95 patients selected for the evaluations in which LRTIs, UTIs and IAIs were diagnosed in 43 (45.26%), 32 (33.68%) and 20 (21.05%) patients, respectively. The most common pathogen was Escherichia coli (38.94%), followed by Klebsiella species (26.31%), Pseudomonas species (18.94%) and Acinetobacter species (15.78%). According to the susceptibility results, FDC appeared as the most active antibacterial agent against E. coli (94.54%) followed by Acinetobacter species (93.33%), Pseudomonas species (88.88%) and Klebsiella species (84%). On the other hand, meropenem susceptibility to E. coli was 86.47% followed by Acinetobacter species (78.57%), Pseudomonas species (66.66%) and Klebsiella species (64%). Further our results revealed that FDC has >75% clinical success compared to meropenem (~61% clinical success). Conclusion: These results depict non-inferiority of new FDC in the treatment of moderate to severe Gram negative bacterial infections caused by carbapenem resistant organisms and therefore, it should be considered as an alternative to carbapenem for treating LRTIs, UTIs and IAIs.


Introduction
Hospital acquired or nosocomial infections (HAIs) are infections occurring during a stay in hospital that are not present at the time of hospital admission. Lower respiratory tract infections (LRTIs), urinary tract infections (UTIs) and intra-abdominal infections (IAIs) are amongst the most prevalent HAIs [26,36]. LTRIs are thought to be leading cause of death all over the world (WHO, 2008). UTIs are the most common infections among the women, particularly under 50 years of age [27]. IAIs, especially complicated IAIs, represent an important cause of morbidity and are frequently associated with poor prognosis [35]. A wide variety of bacterial pathogens are accounting for such infections including Escherichia coli, Pseudomonas aeruginosa, Proteus mirabilis, Klebsiella pneumoniae, Enterobacter spp., and coagulase-negative Staphylococci [21,27]. Empiric antibiotic therapy is commonly used for the treatment of these infections and is usually presumptive and instituted before knowledge of the etiology of specific disease. Among various classes of drugs, β-lactams are one of the most frequently prescribed empirical antimicrobial drugs for the treatment of such infections [24]. However, in recent years, rise in resistance to β-lactam drugs has been noticed because of the extended spectrum doi: 10.7243/2052-5958-3-2 β-lactamases (ESBLs) enzymes which hydrolyse most of the β-lactam antibiotics [10,22,23].
To cater the antibiotic resistance due to extended spectrum beta-lactamases (ESBLs), carbapenem drugs have been introduced in clinical settings. Although, carbapenem drugs play a vital role in the management of the infections caused by ESBLs producing organisms due to their broad spectrum activity and stability to hydrolysis against ESBLs [31], carbapenem resistance among the members of the Enterobacteriaceae, Pseudomonas and Acinetobacter has been reported globally [11,15,22,30]. The common mechanisms of carbapenem resistance are carbapenem hydrolyzing enzymes, changes in outer membrane proteins, over expression of efflux pumps [32]. Carbapenem resistant organisms are associated with high mortality and morbidity rates and have the potential to spread widely [37]. Increasing use of carbapenem drugs has also a direct alliance with carbapenem resistant Gram negative bacteria [18]. Until recently, most of the older antibiotics have become less useful due to the spread of such carbapenem resistant bacteria. As a result of the increasing resistance towards antibiotics over the past few years, it is no wonder that we are now facing the prospect of losing the battle against many bacterial diseases. Although the new antibiotics have to come along to take their place, the drug development pipeline for new antibiotics has been drying out. The drying antibiotic pipeline particularly against Gram negative bacteria has forced us to look into opportunities for improving usage of the existing antimicrobial agents.
One recently introduced approach to improve the existing antimicrobial agents is the use of antibiotic adjuvant therapy, which potentiates the activity of antibiotics. Adjuvant therapies include antibiotic combinations, synergy between antibiotics and non-antibiotics, inhibition of resistance by altering the physiology of antibiotic-insensitive cells, such as those in biofilms.
Introduction of a novel AAE (antibiotic adjuvant entity) with use of EDTA as adjuvant for chelation and catalytic action to existing antibiotics has been seen as a ray of hope. A new FDC (ceftriaxone, sulbactam with adjuvant EDTA) a novel antibiotic adjuvant entity (AAE) has been reported to have proven efficacy in a wide range of infections [7,8].
This retrospective observational study was aimed to explore a new Fixed Dose Combination (FDC) of Ceftriaxone+Sulbactam with adjuvant Disodium edetate as a carbapenem sparing drug in the management of moderate to severe bacterial infections of LRTIs, UTIs and IAIs.

Study design
This retrospective study was designed to evaluate the efficacy of new FDC in comparison to meropenem in the treatment of the patients who are diagnosed with moderate to severe Gram negative hospital acquired bacterial infections. It was conducted at Asian institute of medical sciences, Faridabad for a period of 18 months between June 2013 to December 2014. All patients admitted to the hospital for more than 48 hours with single pathogen infection and showing sensitivity towards new FDC or Meropenem were considered eligible for the study.
Hospital case sheet files of all the patients were reviewed to collect the necessary data like clinical signs and symptoms at the time of admission of patients and during the course of treatment and finally at the end of therapy. All the necessary lab investigations like sputum, bronchoalveolar lavage (BAL), endotracheal (ET) secretions, urine and blood culture and sensitivity reports, hematology, biochemistry and other relevant investigations carried out at baseline and end of treatment were evaluated from all the enrolled patients.

Bacterial identification
Identification of bacteria was carried out according to the methods described by Cheesbrough (2000).

Demographic analysis and antibiotic therapy
The detailed demographic and baseline characteristics of all patients including number of evaluable patients, age, types of infections who were analyzed in this study are given in Table 1. The patients were assigned to receive either meropenem (1.0 g, every 8 h) or new Elores, (ceftriaxone+sulbactam with adjuvant EDTA, 1.5 g or 3.0 g, every 12 h) through intravenous administration. For those patients who were more severe or failed to respond to FDC, colistin with a loading dose of 9 MIU followed by BD doses of 4.5 MIU were used along with previous antibiotic.
The antibiotic therapy of both of the drugs (FDC or meropenem) was initially started empirically based on the clinical symptoms and treating physicians decision and was continued or shifted to other therapy based on the in vitro microbiological susceptibility tests and clinical outcomes. Based on the in vitro antibiotic susceptibility, all the patients were divided into 3 groups for ease of evaluations:

In-vitro antibiotic susceptibility testing
All isolates, recovered from all cultures were subjected to susceptibility testing for FDC, meropenem and colistin using the disk diffusion method according to the recommendations of CLSI (2013). The discs of meropenem (10 μg), FDC (45 μg) and colistin (10 μg) were obtained from Himedia (Mumbai, India). The zones of inhibition surrounding the various antibiotic discs were measured and compared with CLSI. Sensitivity of isolated organisms against antibiotics were reported as sensitive (S) and intermediate (I) based on the breakpoints.

Clinical evaluation of patients
The clinical efficacy of the therapy was evaluated and classified as cured (resolution of clinical signs and symptoms or improvement not requiring further antibacterial therapy), or failure (persistence of clinical signs and symptoms or worsening in signs and symptoms that required alternative antimicrobial therapy after 72 h of treatment). The overall efficacy rate was defined as the proportion of the patients cured. Bacterial efficacy was evaluated based on the following four categories: complete eradication if elimination of the original causative pathogens, persistence if the original causative pathogens were repeatedly isolated, substitution if new organisms were isolated on repeated culture and re-infection if re-appearance of the original causative pathogens after eradication and with clinical symptoms of infection.

Study design and demographic analysis
A total of 107 patients data were evaluated retrospectively in this study. Variables of each patient such as age, causative agents, dosage and regime of antibiotic therapy were recorded. Out of these 107 patients, 95 patients with single bacterial infection were treated either FDC or meropenem. Twelve patients who were either culture negative or treatment failure or expired during course of study were excluded from the study. The average age of patients was 58.17±13.98 years.

In vitro antibiotic susceptibility
On evaluation of culture and sensitivity data, it was observed that clinical samples of the patients were cultured on an appropriate medium, and significant growth was found in 95 samples (88.78%) which yielded clinically significant pathogen that could be implied as a causative agent. Among bacterial infections, the most common pathogen that was isolated was E. coli which was isolated from 37 patients (38.94%), followed

Antibiotics treatment and their efficacy evaluation
On day 3 of treatment, progress of the therapy was measured in terms of improvement in the clinical signs and symptoms and microbiological results. Patient with susceptible pathogens with clinical improvement were continued on respective empirical therapies. Patients who were not showing response towards FDC, colistin was given as an add on therapy.
In G1, out of 20 patients which received FDC, 15 patients (75%) were clinically cured with complete bacteriological eradication within 7-8 days whereas 5 patients who failed to respond to FDC on day 3 were shifted to FDC+colistin combination therapy and all these patients got cured in additional 8 days therapy making a total of 11 days. The duration of antibiotic treatment in these 15 patients treated with FDC was 7.2±1.01 days ( Table 3). In G2, among 65 patients whose pathogens were susceptible to both FDC and meropenem, 31 patients received meropenem (G2A) and 34 received FDC (G2B   Among those patients who did not respond to either meropenem or FDC, colistin was added to ongoing therapy and complete clinical success was observed treatment duration 8.67±0.71 days (Table 6 and Figure 1).

Discussion
Carbapenems are β-lactam antimicrobial agents that are relatively resistant to hydrolysis by most β-lactamases including Amp-C and have been considered as the last resort drugs all over the world for the management of serious infections [1,39]. However, increasing carbapenem resistance among Gram negative bacteria has been documented greatly in recent years [16,17,22]. Amongst various mechanism of carbapenem resistance, acquired metallo-β-lactamases (MBL) have recently emerged as one of the most worrisome resistance mechanisms owing to their capacity to hydrolyze all β-lactams, including carbapenems [13].
To combat such increasing carbapenem resistance, novel beta-lactam and beta lactamase inhibitor combination (BL+BLI) has received much attention as a carbapenem alternative drug in recent past [9,12,15,20,33]. However, only BL+BLI combinations did not exhibit significant activity against some ESBLs and majority of MBL producing Gram negative organisms. Hence a novel antibiotic combination of ceftriaxone+sulbactam with adjuvant disodium edetate (proven for efficacy and safety to treat the patients with infections caused by such organisms has been used in current investigations. In the present study, 95 culture positive patients in ICU with  moderate to severe infections treated with this new FDC or meropenem were retrospectively analyzed. The general distribution pattern of nosocomial infections that emerged in our study showed LRTIs (45.26%) to be the most common, followed by UTIs (33.68%) and IAIs (21.05%). This is in agreement with the study performed by earlier researcher [14]. The most frequently isolated pathogen was E. coli (38.94%), followed by Klebsiella spp. (26.31%), Pseudomonas spp. (18.94%) and Acinetobacter spp. (15.78%). This data is in line with many previous studies where E. coli, Klebsiella spp., Pseudomonas spp., and Acinetobacter spp. were reported to be causative agents of LRTIs. UTIs and IAIs [19,29].
In the present study, in vitro antimicrobial susceptibility results revealed that 84% to 91% of members of Enterobacteriaceae family and 86 to 94% of non-Enterobacteriaceae family members were sensitive to new FDC. Consistent with these results, earlier reports also demonstrated enhanced activity of this new FDC against members of Enterobacteriaceae and non-Enterobacteriaceae [4,34]. On other hands, 64% to 86% isolates of Enterobacteriaceae family whereas and 66% to 78% members of non-Enterobacteriaceae were susceptible to Meropenem, which is in accordance with previous observations [4,34].
Further, in FDC treated patients an overall success rate was >75% against ~61% in meropenem treated. In G1 15/20 patients and in G2B 26/34 (75.9%) who were susceptible to FDC and were administered with FDC achieved clinical and microbiological success suggesting the consistency in in vitro and in vivo results. On the other hand, 5/20 in G1 and 8/34 in G2B (24%) pathogens who did not respond to FDC in first 3 days were successfully treated when colistin was given with FDC, exploring the new therapeutic options in these failure cases. In G2A to whom meropenem was given empirically, only 19/31 patients (61.29%) achieved clinical cure whereas 12 patients failed to respond to meropenem indicating false susceptibility of carbapenem in vitro which is in agreement with previous reports where false susceptibility of carbapenems has been observed in MBL producing strains [5,25]. Another possibility of the decreased susceptibility of meropenem to members to these isolates implied the presence of outer membrane protein mutation [2,28]. Further when these patients were shifted to Meropenem+Colistin, 5/12 patients (41.67%) showed no improvement and were cured when shifted to FDC+Colistin thus indicating presence of MBL strains to which FDC is reported to be active [4,34]. This data advocates use of this new FDC as a better alternative to meropenem. Overall, our results indicate the approximately 17% superiority of new FDC alone over meropenem therapy. It     has been noticed that 15 to 20% mortality rates in patients of nosocomial infections has been associated [14]. However, this percentage was very low in our case. There was no significant difference in treatment regimen of two therapies however, dosing frequency and cost of drug was lower in FDC treated group as compared to meropenem treated.

Conclusion
Our data showed that FDC can be a good option as carbapenem sparing drug and combination of FDC and colistin can successfully treat complicated multi drug resistant cases of LRTIs, UTIs, and IAIs without mortality within 8-11 days.