Non-polymyxin-based combinations as potential alternatives in treatment against carbapenem-resistant Acinetobacter baumannii infections

Proguanil and chlorhexidine augment the antibacterial activities of clarithromycin and rifampicin against Acinetobacter baumannii

The emergence of Acinetobacter baumannii infections as a significant healthcare concern in hospital settings, coupled with their association with poorer clinical outcomes, has prompted extensive investigation into novel therapeutic agents and innovative treatment strategies. Proguanil and chlorhexidine, both categorized as biguanide compounds, have displayed clinical efficacy as antimalarial and topical antibacterial agents, respectively. In this study, we conducted an investigation to assess the effectiveness of combining proguanil and chlorhexidine with clarithromycin or rifampicin against both laboratory strains and clinical isolates of A. baumannii. The combination therapy demonstrated rapid bactericidal activity against planktonic multidrug-resistant A. baumannii, exhibiting efficacy in eradicating mature biofilms and impeding the development of antibiotic resistance in vitro. Additionally, when administered in conjunction with clarithromycin or rifampicin, proguanil enhanced the survival rate of mice afflicted with intraperitoneal A. baumannii infections, and chlorhexidine expedited wound healing in mice with skin infections. These findings are likely attributable to the disruption of A. baumannii cell membrane integrity by proguanil and chlorhexidine, resulting in heightened membrane permeability and enhanced intracellular accumulation of clarithromycin and rifampicin. Overall, this study underscores the potential of employing proguanil and chlorhexidine in combination with specific antibiotics to effectively combat A. baumannii infections and improve treatment outcomes in clinically challenging scenarios.

Evaluation of Fosfomycin-Sulbactam Combination Therapy against Carbapenem-Resistant Acinetobacter baumannii Isolates in a Hollow-Fibre Infection Model

Static concentration in vitro studies have demonstrated that fosfomycin- or sulbactam-based combinations may be efficacious against carbapenem-resistant Acinetobacter baumannii (CRAB). In the present study, we aimed to evaluate the bacterial killing and resistance suppression potential of fosfomycin-sulbactam combination therapies against CRAB isolates in a dynamic infection model. We simulated clinically relevant dosing regimens of fosfomycin (8 g every 8 h, 1 h infusion) and sulbactam (12 g continuous infusion or 4 g every 8 h, 4 h infusion) alone and in combination for 7 days in a hollow-fibre infection model (HFIM) against three clinical isolates of CRAB. The simulated pharmacokinetic profiles in the HFIM were based on fosfomycin and sulbactam data from critically ill patients. Fosfomycin monotherapy resulted in limited bacterial killing. Sulbactam monotherapies resulted in ~ 3 to 4 log10 kill within the first 8 to 32 h followed by regrowth of up to 8 to 10 log10 CFU/mL. A combination of fosfomycin and continuous infusion of sulbactam led to a ~2 to 4 log10 reduction in bacterial burden within the first 24 h, which was sustained throughout the duration of the experiments. A combination of fosfomycin and extended infusion of sulbactam produced a ~4 log10 reduction in colony count within 24 h. This study demonstrated that fosfomycin in combination with sulbactam is a promising option for the treatment of MDR A. baumannii. Further studies are needed to further assess the potential clinical utility of this combination.

In search for a synergistic combination against pandrug-resistant A. baumannii; methodological considerations

PURPOSE

Pending approval of new antimicrobials, synergistic combinations are the only treatment option against pandrug-resistant A. baumannii (PDRAB). Considering the lack of a standardized methodology, the aim of this manuscript is to systematically review the methodology and discuss unique considerations for assessing antimicrobial combinations against PDRAB.

METHODS

Post-hoc analysis of a systematic review (conducted in PubMed and Scopus from inception to April 2021) of studies evaluating antimicrobial combination against A. baumannii, based on antimicrobials that are inactive in vitro alone.

RESULTS

Eighty-four publications were reviewed, using a variety of synergy testing methods, including; gradient-based methods (n = 11), disk-based methods (n = 6), agar dilution (n = 2), checkerboard assay (n = 44), time-kill assay (n = 50), dynamic in vitro PK/PD models (n = 6), semi-mechanistic PK/PD models (n = 5), and in vivo animal models (n = 11). Several variations in definitions of synergy and interpretation of each method were observed and are discussed. Challenges related to testing combinations of antimicrobials that are inactive alone (with regards to concentrations at which the combinations are assessed), as well as other considerations (assessment of stasis vs killing, clinical relevance of re-growth in vitro after initial killing, role of in vitro vs in vivo conditions, challenges of clinical testing of antimicrobial combinations against PDRAB infections) are discussed.

CONCLUSION

This review demonstrates the need for consensus on a standardized methodology and clinically relevant definitions for synergy. Modifications in the methodology and definitions of synergy as well as a roadmap for further development of antimicrobial combinations against PDRAB are proposed.

Systematic Review of Antimicrobial Combination Options for Pandrug-Resistant Acinetobacter baumannii

Antimicrobial combinations are at the moment the only potential treatment option for pandrug-resistant A. baumannii. A systematic review was conducted in PubMed and Scopus for studies reporting the activity of antimicrobial combinations against A. baumannii resistant to all components of the combination. The clinical relevance of synergistic combinations was assessed based on concentrations achieving synergy and PK/PD models. Eighty-four studies were retrieved including 818 eligible isolates. A variety of combinations (n = 141 double, n = 9 triple) were tested, with a variety of methods. Polymyxin-based combinations were the most studied, either as double or triple combinations with cell-wall acting agents (including sulbactam, carbapenems, glycopeptides), rifamycins and fosfomycin. Non-polymyxin combinations were predominantly based on rifampicin, fosfomycin, sulbactam and avibactam. Several combinations were synergistic at clinically relevant concentrations, while triple combinations appeared more active than the double ones. However, no combination was consistently synergistic against all strains tested. Notably, several studies reported synergy but at concentrations unlikely to be clinically relevant, or the concentration that synergy was observed was unclear. Selecting the most appropriate combinations is likely strain-specific and should be guided by in vitro synergy evaluation. Furthermore, there is an urgent need for clinical studies on the efficacy and safety of such combinations.

Carbapenem resistance in Acinetobacter baumannii, and their importance in hospital-acquired infections: a scientific review

Carbapenem is an important therapy for serious hospital-acquired infections and for the care of patients affected by multidrug-resistant organisms, specifically Acinetobacter baumannii; however, with the global increase of carbapenem-resistant A. baumannii, this pathogen has significantly threatened public health. Thus, there is a pressing need to better understand this pathogen in order to develop novel treatments and control strategies for dealing with A. baumannii. In this review, we discuss an overview of carbapenem, including its discovery, development, classification and biological characteristics, and its importance in hospital medicine especially in critical care units. We also describe the peculiarity of bacterial pathogen, A. baumannii, including its commonly reported virulence factors, environmental persistence and carbapenem resistance mechanisms. In closing, we discuss various control strategies for overcoming carbapenem resistance in hospitals and for limiting outbreaks. With the appearance of strains that resist carbapenem, the aim of this review is to highlight the importance of understanding this increasingly problematic healthcare-associated pathogen that creates significant concern in the field of nosocomial infections and overall public health.

Semi-mechanistic PK/PD modelling of meropenem and sulbactam combination against carbapenem-resistant strains of Acinetobacter baumannii

Due to limited treatment options for carbapenem-resistant Acinetobacter baumannii (CR-AB) infections, antibiotic combinations are commonly used. In this study, we explored the potential efficacy of meropenem-sulbactam combination (MEM/SUL) against CR-AB. The checkerboard method was used to screen for synergistic activity of MEM/SUL against 50 clinical CR-AB isolates. Subsequently, time-kill studies against two CR-AB isolates were performed. Time-kill data were described using a semi-mechanistic pharmacokinetic/pharmacodynamic (PK/PD) model. Subsequently, Monte Carlo simulations were performed to estimate the probability of 2-log kill, 1-log kill or stasis at 24-h following combination therapy. The MEM/SUL demonstrated synergy against 28/50 isolates. No antagonism was observed. The MIC50 and MIC₉₀ of MEM/SUL were decreased fourfold, compared to the monotherapy MIC. In the time-kill studies, the combination displayed synergistic killing against both isolates at the highest clinically achievable concentrations. At concentrations equal to the fractional inhibitory concentration, synergism was observed against one isolate. The PK/PD model adequately delineated the data and the interaction between meropenem and sulbactam. The effect of the combination was driven by sulbactam, with meropenem acting as a potentiator. The simulations of various dosing regimens revealed no activity for the monotherapies. At best, the MEM/SUL regimen of 2 g/4 g every 8 h demonstrated a probability of target attainment of 2-log₁₀ kill at 24 h of 34%. The reduction in the MIC values and the achievement of a moderate PTA of a 2-log₁₀ reduction in bacterial burden demonstrated that MEM/SUL may potentially be effective against some CR-AB infections.

Semi-mechanistic PK/PD modelling of fosfomycin and sulbactam combination against carbapenem-resistant Acinetobacter baumannii

Due to limited treatment options for carbapenem-resistant Acinetobacter baumannii (CR-AB) infections, antibiotic combinations are now considered potential treatments for CR-AB. This study aimed to explore the utility of fosfomycin-sulbactam combination (FOS/SUL) therapy against CR-AB isolates.Synergism of FOS/SUL against 50 clinical CR-AB isolates were screened using the checkerboard method. Thereafter, time-kill studies against two CR-AB isolates were performed. The time-kill data were described using a semi-mechanistic pharmacokinetic/pharmacodynamic (PK/PD) model. Monte Carlo simulations were then performed to estimate the probability of stasis, 1-log kill and 2-log kill after 24-hours with combination therapy.The FOS/SUL combination demonstrated a synergistic effect against 74% of isolates. No antagonism was observed. The MIC₅₀ and MIC₉₀ of FOS/SUL were decreased four- to eight-fold, compared to the monotherapy MIC₅₀ and MIC₉₀ In the time-kill studies, the combination displayed bactericidal activity against both isolates and synergistic activity against one isolate, at the highest clinically achievable concentrations. Our PK/PD model was able to describe the interaction between fosfomycin and sulbactam in vitro Bacterial kill was mainly driven by sulbactam, with fosfomycin augmentation. FOS/SUL regimens that included sulbactam 4 g every 8 hours, demonstrated a probability of target attainment of 1-log₁₀ kill at 24 h of ∼69-76%, as compared to ∼15-30% with monotherapy regimens at the highest doses.The reduction in the MIC values and the achievement of a moderate PTA of a 2-log₁₀ reduction in bacterial burden demonstrated that FOS/SUL may potentially be effective against some CR-AB infections.