Clinical relevance of in vitro synergistic activity of antibiotics for multidrug-resistant Gram-negative infections: A systematic review

Current treatment options for pneumonia caused by carbapenem-resistant Acinetobacter baumannii

PURPOSE OF REVIEW

The purpose of this review is to briefly summarize the challenges associated with the treatment of pneumonia caused by carbapenem-resistant Acinetobacter baumannii (CRAB), discuss its carbapenem-resistance, and review the literature supporting the current treatment paradigm and therapeutic options.

RECENT FINDINGS

In a multicenter, randomized, and controlled trial the novel β-lactam-β-lactamase inhibitor sulbactam-durlobactam was compared to colistin, both in addition to imipenem-cilastatin. The drug met the prespecified criteria for noninferiority for 28-day all-cause mortality while demonstrating higher clinical cure rates in the treatment of CRAB pneumonia. In an international, randomized, double-blind, placebo controlled trial colistin monotherapy was compared to colistin combined with meropenem. In this trial, combination therapy was not superior to monotherapy in the treatment of drug-resistant gram-negative organisms including CRAB pneumonia.

SUMMARY

CRAB pneumonia is a preeminent public health threat without an agreed upon first line treatment strategy. Historically, there have been drawbacks to available treatment modalities without a clear consensus on the first-line treatment regimen. CRAB pneumonia is a top priority for the continued development of antimicrobials, adjuvant therapies and refinement of current treatment strategies.

Comparative in vitro activities of eravacycline in combination with colistin, meropenem, or ceftazidime against various Achromobacter spp. strains isolated from patients with cystic fibrosis

The Achromobacter species is an emerging pathogen causing chronic bacterial infections in patients with certain conditions, such as cystic fibrosis (CF), hematologic and solid organ malignancies, renal failure, and certain immune deficiencies. In the present study, we assessed the in vitro bactericidal activities of eravacycline, either alone or in combination with colistin, meropenem, or ceftazidime, using 50 Achromobacter spp. strains isolated from CF patients. We also investigated the synergistic interactions of these combinations using microbroth dilutions against 50 strains of Achromobacter spp. Bactericidal, and we assessed the synergistic effects of the tested antibiotic combinations using the time-kill curve (TKC) technique. Our studies show that meropenem alone is the most effective antibiotic of those tested. Based on the TKCs, we found that eravacycline-colistin combinations display both bactericidal and synergistic activities for 24 h against 5 of the 6 Achromobacter spp. strains, including colistin-resistant ones, at 4xMIC of colistin. Although we did not observe synergistic interactions with eravacycline-meropenem or eravacycline-ceftazidime combinations, we did not observe antagonism with any combination tested.This study's findings could have important implications for antimicrobial therapy with tested antibiotics.

Mortality due to Multidrug-Resistant Gram-Negative Bacteremia in an Endemic Region: No Better than a Toss of a Coin

The incidence of multidrug-resistant (MDR) bloodstream infections (BSIs) is associated with high morbidity and mortality. Little evidence exists regarding the epidemiology of BSIs and the use of appropriate empirical antimicrobial therapy in endemic regions. Novel diagnostic tests (RDTs) may facilitate and improve patient management. Data were assessed from patients with MDR Gram-negative bacteremia at a university tertiary hospital over a 12-month period. In total, 157 episodes of MDR Gram-negative BSI were included in the study. The overall mortality rate was 50.3%. Rapid molecular diagnostic tests were used in 94% of BSI episodes. In univariate analysis, age (OR 1.05 (95% CI 1.03, 1.08) p < 0.001), Charlson Comorbidity Index (OR 1.51 (95% CI 1.25, 1.83) p < 0.001), procalcitonin ≥ 1(OR 3.67 (CI 95% 1.73, 7.79) p < 0.001), and monotherapy with tigecycline (OR 3.64 (95% CI 1.13, 11.73) p = 0.030) were the only factors associated with increased overall mortality. Surprisingly, time to appropriate antimicrobial treatment had no impact on mortality. MDR pathogen isolation, other than Klebsiella pneumoniae and Acinetobacter baumanii, was associated with decreased mortality (OR 0.35 (95% CI 0.16, 0.79) p = 0.011). In multivariate analysis, the only significant factor for mortality was procalcitonin ≥ 1 (OR 2.84 (95% CI 1.13, 7.11) p = 0.025). In conclusion, in an endemic area, mortality rates in MDR BSI remain notable. High procalcitonin was the only variable that predicted death. The use of rapid diagnostics did not improve mortality rate.

In Vitro Synergistic Activity of Antimicrobial Combinations against Carbapenem- and Colistin-Resistant Acinetobacter baumannii and Klebsiella pneumoniae

Polymyxins are commonly used as the last resort for the treatment of MDR Acinetobacter baumannii and Klebsiella pneumoniae nosocomial infections; however, apart from the already known toxicity issues, resistance to these agents is emerging. In the present study, we assessed the in vitro synergistic activity of antimicrobial combinations against carbapenem-resistant and colistin-resistant A. baumannii and K. pneumoniae in an effort to provide more options for their treatment. Two hundred A. baumannii and one hundred and six K. pneumoniae single clinical isolates with resistance to carbapenems and colistin, recovered between 1 January 2021 and 31 July 2022,were included. A. baumannii were tested by the MIC test strip fixed-ratio method for combinations of colistin with either meropenem or rifampicin or daptomycin. K. pneumoniae were tested for the combinations of colistin with meropenem and ceftazidime/avibactam with aztreonam. Synergy was observed at: 98.99% for colistin and meropenem against A. baumannii; 91.52% for colistin and rifampicin; and 100% for colistin and daptomycin. Synergy was also observed at: 73.56% for colistin and meropenem against K. pneumoniae and; and 93% for ceftazidime/avibactam with aztreonam. The tested antimicrobial combinations presented high synergy rates, rendering them valuable options against A. baumannii and K. pneumoniae infections.

Evaluation of in vitro activity of fosfomycin, and synergy in combination, in Gram-negative bloodstream infection isolates in a UK teaching hospital

Introduction. Fosfomycin has retained activity against many multi-drug resistant (MDR) Gram-negatives, and may be useful against extended spectrum beta-lactamase (ESBL) producing and carbapenem-resistant Enterobacterales to improve clinical outcomes.Hypothesis/Gap Statement. There are few data from the UK on the susceptibility of invasive Gram-negative isolates to fosfomycin, especially in the era of increasing use of oral fosfomycin for urinary tract infections (UTIs).Aim. We evaluated fosfomycin susceptibility against 100 consecutive Gram-negative bloodstream isolates, both individually, and in combination with other mechanistically similar and differing antibiotics. The aim was to investigate the synergy between antibiotic combinations against several E. coli, K. pneumoniae and P. aeruginosa isolates with variable levels of resistance.Methodology. Disc diffusion and MIC test strip methods applying revised EUCAST guidelines for Fosfomycin were used, followed by the MTS™ 'cross synergy' method for 'resistant' isolates as defined below: (a) Fosfomycin resistant by MIC test strip; (b) MDR isolates defined as being resistant to ≥3 classes of antibiotics (based on routine sensitivity testing; beta lactams were considered as a single class), and/or (c) AMP C or ESBL or carbapenemase producers (or carbapenem resistant). FIC Index (Fractional Inhibitory Concentration Index) calculations were used to interpret findings, whereby: FIC = (MICA combination A+B/ MIC agent A) + (MICB combination A+B/ MIC agent B). A result of ≤0.5 was taken to indicate 'synergy', >0.5 and ≤1.0 to indicate 'additive' effect, >1.0 and ≤4.0 to indicate 'indifference', and >4.0 to indicate 'antagonism'.Results. We found that 95/100 isolates were susceptible to fosfomycin by MIC test strip, with 88/100 isolates susceptible to fosfomycin by disc, based on EUCAST guideline breakpoints. A total of 30/100 isolates (the more 'resistant' of the 100) were eligible for synergy testing according to our definitions (see Methodology), with the remaining 70 isolates not tested further. Seventeen out of 30 were MDR, 2/30 were AMP C producers and 9/30 were ESBL producers. Overall, 34/300 (11 %) of all combination tests showed synergy and 161/300 (54 %) were additive. Synergy was most commonly detected between fosfomycin and beta-lactam antibiotics, including piperacillin/tazobactam (10/30; 33 %), ceftazidime/avibactam (10/30; 30 %), and temocillin (8/30; 27 %). An additive effect was most commonly detected with aztreonam (25/30; 83 %) and meropenem (25/30; 83 %), but 100 % indifference was found with tigecycline (30/30). No antagonism was identified with any antibiotic combination.Conclusion. Fosfomycin non-susceptibility by MIC test strip was unusual. Synergy was variable when combining fosfomycin with other antibiotics against the more 'resistant' isolates. Synergistic/additive effects were detected for beta-lactam/fosfomycin combinations in >80 % of all such combinations, suggesting beta-lactams may be the preferred partner for fosfomycin. Agents with a discordant site of action were more likely to result in indifference. Antagonism was not detected.

European Society of clinical microbiology and infectious diseases (ESCMID) guidelines for the treatment of infections caused by Multidrug-resistant Gram-negative bacilli (endorsed by ESICM -European Society of intensive care Medicine)

SCOPE

These ESCMID guidelines address the targeted antibiotic treatment of 3^rd generation cephalosporin-resistant Enterobacterales (3GCephRE) and carbapenem-resistant Gram-negative bacteria, focusing on the effectiveness of individual antibiotics and on combination vs. monotherapy.

METHODS

An expert panel was convened by ESCMID. A systematic review was performed including randomized controlled trials and observational studies, examining different antibiotic treatment regimens for the targeted treatment of infections caused by the 3GCephRE, carbapenem-resistant Enterobacterales (CRE), carbapenem-resistant Pseudomonas aeruginosa (CRPA) and carbapenem-resistant Acinetobacter baumanni (CRAB). Treatments were classified as head-to-head comparisons between individual antibiotics and monotherapy vs. combination therapy regimens, including defined monotherapy and combination regimens only. The primary outcome was all-cause mortality, preferably at 30 days and secondary outcomes included clinical failure, microbiological failure, development of resistance, relapse/recurrence, adverse events and length of hospital stay. The last search of all databases was conducted in December 2019, followed by a focused search for relevant studies up until ECCMID 2021. Data were summarized narratively. The certainty of the evidence for each comparison between antibiotics and between monotherapy vs. combination therapy regimens was classified by the GRADE recommendations. The strength of the recommendations for or against treatments was classified as strong or conditional (weak).

RECOMMENDATIONS

The guideline panel reviewed the evidence per pathogen, preferably per site of infection, critically appraising the existing studies. Many of the comparisons were addressed in small observational studies at high risk of bias only. Notably, there was very little evidence on the effects of the new, recently approved, beta-lactam beta-lactamase inhibitors on infections caused by carbapenem-resistant Gram-negative bacteria. Most recommendations are based on very-low and low certainty evidence. A high value was placed on antibiotic stewardship considerations in all recommendations, searching for carbapenem-sparing options for 3GCephRE and limiting the recommendations of the new antibiotics for severe infections, as defined by the sepsis-3 criteria. Research needs are addressed.

Can Drug Repurposing be Effective Against Carbapenem-Resistant Acinetobacter baumannii?

Carbapenem-resistant Acinetobacter baumannii has been classified as a top priority for the development of new therapies due to its resistance to most antibiotics. Drug repurposing may be a fast and inexpensive strategy for treating this pathogen. This review aims to critically evaluate repurposed drugs for the treatment of infections caused by carbapenem-resistant A. baumannii, correlating their antimicrobial activity with data available for toxicity and side effects. Some drugs have been suggested as promising candidates for repurposing; however, in some cases, high toxicity and low plasma concentrations reduce applicability in clinical practice. The most favorable applicability is offered by fusidic acid and colistin, possibly combined with a third agent, promising to be well tolerated and achieving satisfactory plasma concentrations.

Comparison of Bleeding Risk Between Colistin-Tigecycline and Colistin-Carbapenem Treatment Regimens: A Retrospective Cohort Study

Background

Antibiotic combination is commonly used to treat multidrug-resistant pathogens. Reports have indicated that tigecycline use is associated with hypofibrinogenemia. However, whether the bleeding risk of tigecycline is higher than that of other antibiotics remains unknown. The aim of this study was to compare the bleeding risk between colistin–tigecycline and colistin–carbapenem treatment.

Methods

This retrospective cohort study enrolled adult patients treated with colistin along with tigecycline or carbapenems (doripenem, imipenem–cilastatin, or meropenem) for ˃72 hours during hospitalization. The primary outcome was major bleeding events, which were determined by a hemoglobin drop of ≥2 g/d and receipt of blood transfusions with whole blood or packed red blood cells. Multivariate logistic regression was applied to determine risk factors for bleeding events.

Results

In total, 106 and 268 patients in the colistin–tigecycline and colistin–carbapenem groups met the criteria for analysis, respectively. The two groups did not differ significantly in demographic data, except for alanine aminotransferase (ALT), serum creatinine (S_Cr) and ulcer disease. The colistin–tigecycline group had a higher ALT, S_Cr and a lower proportion of ulcer disease. Major bleeding events did not differ significantly between the colistin–tigecycline and colistin–carbapenem groups (12.26% vs 9.33%, P = 0.40). Antibiotic duration [OR = 1.06 (1.02–1.11), P=0.007)] and anticoagulant use [OR = 2.16 (1.05–4.42), P=0.04] were associated with major bleeding events.

Conclusion

Colistin–tigecycline treatment was not associated with a higher bleeding risk. Antibiotic duration and concurrent use of anticoagulant were the risk factors of bleeding events.

Identifying global research gaps to mitigate antimicrobial resistance: A scoping review

OBJECTIVE

Identify research gaps relevant to the global effort to combat antimicrobial resistance.

METHODS

Web of Science, PubMed, Scopus, and Ovid MEDLINE were searched for reviews on antimicrobial resistance published between January 1, 2015 and December 31, 2019. Recommendations for future research were identified.

FINDINGS

Seventy-four reviews met inclusion criteria; 300 research gaps and recommendations were identified. The largest number were from the human health sector (105; 35%) followed by environmental health (72; 23%), animal health (66; 22%), food and feed (14; 5%), and plants and crops (8; 3%); 35 (12%) involved more than one sector. The largest number of gaps concerned surveillance of resistance (68; 23%), followed by study design or methodology (52; 17%), interventions (41; 14%), risk assessment and modeling (35; 12%), ecological (26; 9%) and biochemical (28; 9%) aspects of resistance, interface between reservoirs of resistant pathogens (24; 8%), and economic (15; 5%) and awareness- and behavior-related (11; 4%) aspects of antimicrobial resistance.

CONCLUSIONS

Important research gaps remain in our complete understanding of antimicrobial resistance, and more research is needed about its development, transmission, and impact across the interface of human, animal, and environmental reservoirs.

Systematic review and meta-analysis of in vitro efficacy of antibiotic combination therapy against carbapenem-resistant Gram-negative bacilli

The superiority of combination therapy for carbapenem-resistant Gram-negative bacilli (CR-GNB) infections remains controversial. In vitro models may predict the efficacy of antibiotic regimens against CR-GNB. A systematic review and meta-analysis was performed including pharmacokinetic/pharmacodynamic (PK/PD) and time-kill (TK) studies examining the in vitro efficacy of antibiotic combinations against CR-GNB [PROSPERO registration no. CRD42019128104]. The primary outcome was in vitro synergy based on the effect size (ES): high, ES ≥ 0.75, moderate, 0.35 < ES < 0.75; low, ES ≤ 0.35; and absent, ES = 0). A network meta-analysis assessed the bactericidal effect and re-growth rate (secondary outcomes). An adapted version of the ToxRTool was used for risk-of-bias assessment. Over 180 combination regimens from 136 studies were included. The most frequently analysed classes were polymyxins and carbapenems. Limited data were available for ceftazidime/avibactam, ceftolozane/tazobactam and imipenem/relebactam. High or moderate synergism was shown for polymyxin/rifampicin against Acinetobacter baumannii [ES = 0.91, 95% confidence interval (CI) 0.44-1.00], polymyxin/fosfomycin against Klebsiella pneumoniae (ES = 1.00, 95% CI 0.66-1.00) and imipenem/amikacin against Pseudomonas aeruginosa (ES = 1.00, 95% CI 0.21-1.00). Compared with monotherapy, increased bactericidal activity and lower re-growth rates were reported for colistin/fosfomycin and polymyxin/rifampicin in K. pneumoniae and for imipenem/amikacin or imipenem/tobramycin against P. aeruginosa. High quality was documented for 65% and 53% of PK/PD and TK studies, respectively. Well-designed in vitro studies should be encouraged to guide the selection of combination therapies in clinical trials and to improve the armamentarium against carbapenem-resistant bacteria.

Oral Fixed-Dose Combination Pharmaceutical Products: Industrial Manufacturing Versus Personalized 3D Printing

Fixed-dose combination (FDC) products containing at least two different active pharmaceutical ingredients are designed to treat more effectively different pathologies as they have demonstrated to enhance patient compliance. However, the combination of multiple drugs within the same dosage form can bring many physicochemical and pharmacodynamic interactions. The manufacturing process of FDC products can be challenging, especially when it is required to achieve different drug release profiles within the same dosage form to overcome physicochemical drug interactions. Monolithic, multiple-layer, and multiparticulate systems are the most common type of FDCs. Currently, the main manufacturing techniques utilized in industrial pharmaceutical companies rely on the use of combined wet and dry granulation, hot-melt extrusion coupled with spray coating, and compression of bilayered tablets. Nowadays, personalized medicines are gaining importance in clinical settings and 3D printing is taking a highlighted role in the manufacturing of complex and personalized 3D solid dosage forms that could not be manufactured using conventional techniques. In this review, it will be discussed in detail current marketed FDC products and their application in several diseases with an especial focus on antimicrobial drugs. Current industrial conventional techniques will be compared with 3D printing manufacturing of FDCs. Graphical Abstract.

Colistin plus meropenem for carbapenem-resistant Gram-negative infections: in vitro synergism is not associated with better clinical outcomes

OBJECTIVES

In vitro models showing synergism between polymyxins and carbapenems support combination treatment for carbapenem-resistant Gram-negative (CRGN) infections. We tested the association between the presence of in vitro synergism and clinical outcomes in patients treated with colistin plus meropenem.

METHODS

This was a secondary analysis of AIDA, a randomized controlled trial comparing colistin with colistin-meropenem for severe CRGN infections. We tested in vitro synergism using a checkerboard assay. Based on the fractional inhibitory concentration (ΣFIC) index for each colistin-meropenem combination, we categorized results as synergistic, antagonistic or additive/indifferent. The primary outcome was clinical failure at 14 days. Secondary outcomes were 14- and 28-day mortality and microbiological failure.

RESULTS

The sample included 171 patients with infections caused by carbapenem-resistant Acinetobacter baumannii (n = 131), Enterobacteriaceae (n = 37) and Pseudomonas aeuruginosa (n = 3). In vitro testing showed synergism for 73 isolates, antagonism for 20 and additivism/indifference for 78. In patients who received any colistin plus meropenem, clinical failure at 14 days was 59/78 (75.6%) in the additivism/indifference group (reference category), 54/73 (74.0%) in the synergism group (adjusted odds ratio (aOR) 0.76, 95% CI 0.31-1.83), and 11/20 (55%) in the antagonism group (aOR 0.77, 95% CI 0.22-2.73). There was no significant difference between groups for any secondary outcome. Comparing the synergism group to patients treated with colistin monotherapy, synergism was not protective against 14-day clinical failure (aOR 0.52, 95% CI 0.26-1.04) or 14-day mortality (aOR1.09, 95% CI 0.60-1.96).

DISCUSSION

In vitro synergism between colistin and meropenem via checkerboard method did not translate into clinical benefit.

Comparative Evaluation of the In Vitro Activities of WCK 5222 (Cefepime-Zidebactam) and Combination Antibiotic Therapies against Carbapenem-Resistant Pseudomonas aeruginosa

The in vitro activity of WCK 5222 (cefepime-zidebactam) was compared to that of several available combination therapies among 30 clinical carbapenem-resistant Pseudomonas aeruginosa (CRP) strains using gradient diffusion strips. The combinations included nonsusceptible β-lactams (cefepime, ceftolozane-tazobactam, and meropenem) with amikacin and fosfomycin. WCK 5222 MICs ranged from 2 to 32 mg/liter, and 97% were ≤16 mg/liter, while 105/146 (72%) combinations demonstrated inhibition below established susceptibility breakpoints. WCK 5222 monotherapy may be preferred over the combinations assessed for CRP infections.

Epidemiology, Treatment, and Prevention of Nosocomial Bacterial Pneumonia

Septicaemia likely results in high case-fatality rates in the present multidrug-resistant (MDR) era. Amongst them are hospital-acquired pneumonia (HAP) and ventilator-associated pneumonia (VAP), two frequent fatal septicaemic entities amongst hospitalised patients. We reviewed the PubMed database to identify the common organisms implicated in HAP/VAP, to explore the respective risk factors, and to find the appropriate antibiotic choice. Apart from methicillin-resistant Staphylococcus aureus and Pseudomonas aeruginosa, extended-spectrum β-lactamase-producing Enterobacteriaceae spp., MDR or extensively drug-resistant (XDR)-Acinetobacter baumannii complex spp., followed by Stenotrophomonas maltophilia, Chryseobacterium indologenes, and Elizabethkingia meningoseptica are ranked as the top Gram-negative bacteria (GNB) implicated in HAP/VAP. Carbapenem-resistant Enterobacteriaceae notably emerged as an important concern in HAP/VAP. The above-mentioned pathogens have respective risk factors involved in their acquisition. In the present XDR era, tigecycline, colistin, and ceftazidime-avibactam are antibiotics effective against the Klebsiella pneumoniae carbapenemase and oxacillinase producers amongst the Enterobacteriaceae isolates implicated in HAP/VAP. Antibiotic combination regimens are recommended in the treatment of MDR/XDR-P. aeruginosa or A. baumannii complex isolates. Some special patient populations need prolonged courses (>7-day) and/or a combination regimen of antibiotic therapy. Implementation of an antibiotic stewardship policy and the measures recommended by the United States (US) Institute for Healthcare were shown to decrease the incidence rates of HAP/VAP substantially.