Pharmacodynamics of Ceftaroline plus Ampicillin against Enterococcus faecalis in an In Vitro Pharmacokinetic/Pharmacodynamic Model of Simulated Endocardial Vegetations

New Antimicrobials and New Therapy Strategies for Endocarditis: Weapons That Should Be Defended

The overall low-quality evidence concerning the clinical benefits of different antibiotic regimens for the treatment of infective endocarditis (IE), which has made it difficult to strongly support or reject any regimen of antibiotic therapy, has led to a discrepancy between the available guidelines and clinical practice. In this complex scenario, very recently published guidelines have attempted to fill this gap. Indeed, in recent years several antimicrobials have entered the market, including ceftobiprole, ceftaroline, and the long-acting lipoglycopeptides dalbavancin and oritavancin. Despite being approved for different indications, real-world data on their use for the treatment of IE, alone or in combination, has accumulated over time. Furthermore, an old antibiotic, fosfomycin, has gained renewed interest for the treatment of complicated infections such as IE. In this narrative review, we focused on new antimicrobials and therapeutic strategies that we believe may provide important contributions to the advancement of Gram-positive IE treatment, providing a summary of the current in vitro, in vivo, and clinical evidence supporting their use in clinical practice.

Treatment of Enterococcus faecalis Infective Endocarditis: A Continuing Challenge

Today, Enterococcus faecalis is one of the main causes of infective endocarditis in the world, generally affecting an elderly and fragile population, with a high mortality rate. Enterococci are partially resistant to many commonly used antimicrobial agents such as penicillin and ampicillin, as well as high-level resistance to most cephalosporins and sometimes carbapenems, because of low-affinity penicillin-binding proteins, that lead to an unacceptable number of therapeutic failures with monotherapy. For many years, the synergistic combination of penicillins and aminoglycosides has been the cornerstone of treatment, but the emergence of strains with high resistance to aminoglycosides led to the search for new alternatives, like dual beta-lactam therapy. The development of multi-drug resistant strains of Enterococcus faecium is a matter of considerable concern due to its probable spread to E. faecalis and have necessitated the search of new guidelines with the combination of daptomycin, fosfomycin or tigecycline. Some of them have scarce clinical experience and others are still under investigation and will be analyzed in this review. In addition, the need for prolonged treatment (6–8 weeks) to avoid relapses has forced to the consideration of other viable options as outpatient parenteral strategies, long-acting administrations with the new lipoglycopeptides (dalbavancin or oritavancin), and sequential oral treatments, which will also be discussed.

Meropenem plus Ceftaroline Is Active against Enterococcus faecalis in an In Vitro Pharmacodynamic Model Using Humanized Dosing Simulations

The standard of care for serious Enterococcus faecalis infections is ampicillin plus ceftriaxone. Ampicillin's inconvenient dosing schedule, drug instability, allergy potential, along with ceftriaxone's high risk for Clostridioides difficile infection and its promotion of vancomycin-resistant enterococci (VRE), led our team to explore alternative options. This work aimed to understand the role of carbapenems in combination with cephalosporins in these infections. We selected two ampicillin and penicillin susceptible E. faecalis strains (AMP-MIC 0.5-2 μg/mL; PCN-MIC 2 μg/mL) and simulated human therapeutic dosing regimens in a 48-h in vitro pharmacodynamic model (IVPD) with ampicillin (2g q4h), ertapenem (1g q24h), meropenem (2g q8h), ceftriaxone (2g q12h), and ceftaroline (600 mg q8h). As expected, ampicillin plus ceftriaxone demonstrated enhanced activity compared with ampicillin monotherapy with no MIC increases in either isolate. Meropenem and ceftaroline demonstrated significant kill against both isolates, with no regrowth or MIC increases occurring. Meropenem plus ceftriaxone also demonstrated significant kill, and while no MIC increases were identified for meropenem, there was minor regrowth and larger standard deviations. Ertapenem combined with either ceftriaxone or ceftaroline enhanced activity at 24 h, but at 48 h, regrowth occurred, and ertapenem MIC increases were noted. Meropenem-based combination therapy against E. faecalis may provide clinicians with another regimen to treat severe E. faecalis infections. Meropenem plus ceftaroline was as active as the standard of care treatment (ampicillin plus ceftriaxone) and may serve as an alternative for serious E. faecalis infections. Further studies are warranted to determine the clinical efficacy.

Novel human in vitro vegetation simulation model for infective endocarditis

Infective endocarditis (IE) is a heart valve infection with high mortality rates. IE results from epithelial lesions, inducing sterile healing vegetations consisting of platelets, leucocytes, and fibrin that are susceptible for colonization by temporary bacteremia. Clinical testing of new treatments for IE is difficult and fast models sparse. The present study aimed at establishing an in vitro vegetation simulation IE model for fast screening of novel treatment strategies. A healing promoting platelet and leucocyte-rich fibrin patch was used to establish an IE organoid-like model by colonization with IE-associated bacterial isolates Staphylococcus aureus, Streptococcus spp (S. mitis group), and Enterococcus faecalis. The patch was subsequently exposed to tobramycin, ciprofloxacin, or penicillin. Bacterial colonization was evaluated by microscopy and quantitative bacteriology. We achieved stable bacterial colonization on the patch, comparable to clinical IE vegetations. Microscopy revealed uneven, biofilm-like colonization of the patch. The surface-associated bacteria displayed increased tolerance to antibiotics compared to planktonic bacteria. The present study succeeded in establishing an IE simulation model with the relevant pathogens S. aureus, S. mitis group, and E. faecalis. The findings indicate that the IE model mirrors the natural IE process and has the potential for fast screening of treatment candidates.

Anti-biofilm Approach in Infective Endocarditis Exposes New Treatment Strategies for Improved Outcome

Infective endocarditis (IE) is a life-threatening infective disease with increasing incidence worldwide. From early on, in the antibiotic era, it was recognized that high-dose and long-term antibiotic therapy was correlated to improved outcome. In addition, for several of the common microbial IE etiologies, the use of combination antibiotic therapy further improves outcome. IE vegetations on affected heart valves from patients and experimental animal models resemble biofilm infections. Besides the recalcitrant nature of IE, the microorganisms often present in an aggregated form, and gradients of bacterial activity in the vegetations can be observed. Even after appropriate antibiotic therapy, such microbial formations can often be identified in surgically removed, infected heart valves. Therefore, persistent or recurrent cases of IE, after apparent initial infection control, can be related to biofilm formation in the heart valve vegetations. On this background, the present review will describe potentially novel non-antibiotic, antimicrobial approaches in IE, with special focus on anti-thrombotic strategies and hyperbaric oxygen therapy targeting the biofilm formation of the infected heart valves caused by Staphylococcus aureus. The format is translational from preclinical models to actual clinical treatment strategies.

In vivo synergism of ampicillin, gentamicin, ceftaroline and ceftriaxone against Enterococcus faecalis assessed in the Galleria mellonella infection model

BACKGROUND

The unfavourable safety profile of aminoglycosides and the synergistic effects observed in vitro have prompted the development of novel dual β-lactam therapies, e.g. ampicillin/ceftriaxone or ampicillin/ceftaroline, for the treatment of Enterococcus faecalis endocarditis.

OBJECTIVES

For comparison with in vitro chequerboard assay results, a partial chequerboard setup of ampicillin/gentamicin, ampicillin/ceftriaxone and ampicillin/ceftaroline against E. faecalis was established in the Galleria mellonella larval infection model.

METHODS

Discrimination of synergistic and additive interactions was based on the evaluation of larval survival, bacterial quantity in the haemolymph and a pathology score index (internal to the workgroup). Single and multiple dosing schemes based on the half-life of ampicillin were applied. Pharmacokinetic data of the antibiotics in the larvae were determined via agar plate diffusion assays.

RESULTS

Ampicillin and ceftriaxone exhibited strain-specific synergistic interactions in the larvae under both dosing regimens, while the other two combinations showed additive effects. Ampicillin/ceftaroline was inferior to ampicillin/ ceftriaxone. Not all synergistic effects observed in vitro could be replicated in the larvae.

CONCLUSIONS

Our results suggest superior efficacy of ampicillin/ceftriaxone for the treatment of high-inoculum enterococcal infections, for at least some strains, but question the benefit of the current standard of adding the nephrotoxic gentamicin compared with the safer ceftriaxone. This is the first study to develop a scheme for differentiation between additive and synergistic effects in larvae and apply a multiple-antibiotic dosing scheme based on the pharmacokinetics of ampicillin. The model allows the analysis of synergistic effects of antimicrobials in an in vivo setting, but the clinical correlation warrants further study.

A Review of Combination Antimicrobial Therapy for Enterococcus faecalis Bloodstream Infections and Infective Endocarditis

Enterococci, one of the most common causes of hospital-associated infections, are responsible for substantial morbidity and mortality. Enterococcus faecalis, the more common and virulent species, causes serious high-inoculum infections, namely infective endocarditis, that are associated with cardiac surgery and mortality rates that remained unchanged for the last 30 years. The best cures for these infections are observed with combination antibiotic therapy; however, optimal treatment has not been fully elucidated. It is the purpose of this review to highlight treatment options and their limitations, and provide direction for future investigative efforts to aid in the treatment of these severe infections. While ampicillin plus ceftriaxone has emerged as a preferred treatment option, mortality rates continue to be high, and from a safety standpoint, ceftriaxone, unlike other cephalosporins, promotes colonization with vancomycin resistant-enterococci due to high biliary concentrations. More research is needed to improve patient outcomes from this high-mortality disease.

In vitro bactericidal activity of amoxicillin combined with different cephalosporins against endocarditis-associated Enterococcus faecalis clinical isolates

Background

The combination of amoxicillin with cefazolin could be an interesting regimen for the empirical therapy of severe infective endocarditis, but its activity against enterococci is unknown.

Objectives

To evaluate in vitro the bactericidal activity of the combination of amoxicillin with different cephalosporins including cefazolin.

Methods

Combinations of amoxicillin (at MIC×¼) with cefazolin, cefotaxime, ceftriaxone, cefepime, ceftaroline or ceftobiprole (at the mean free plasma concentration) were studied using time–kill experiments for 10 endocarditis-associated Enterococcus faecalis strains and 2 reference strains.

Results

The combinations amoxicillin/cefazolin, amoxicillin/cefotaxime, amoxicillin/ceftriaxone and amoxicillin/cefepime were synergistic at 12 and 24 h against 12/12 strains and amoxicillin/ceftobiprole and amoxicillin/ceftaroline against 10/12 strains. The combination amoxicillin/cefepime was bactericidal at 24 h against 9/12 strains, the combination amoxicillin/cefazolin against 8/12 strains, the combinations amoxicillin/ceftaroline, amoxicillin/cefotaxime and amoxicillin/ceftobiprole against 7/12 strains and the combination amoxicillin/ceftriaxone against 6/12 strains.

Conclusions

The combination amoxicillin/cefazolin is as synergistic and bactericidal in vitro as amoxicillin/cefotaxime or amoxicillin/ceftriaxone against E. faecalis.

Dual β-Lactam Combinations Highly Active against Mycobacterium abscessus Complex In Vitro

As a consequence of a growing population of immunocompromised individuals, including transplant recipients and cystic fibrosis patients, there has been a dramatic increase in chronic infections caused by Mycobacterium abscessus complex (MABC) strains that are usually recalcitrant to effective antibiotic therapy. The recent rise of macrolide resistance in MABC has further complicated this clinical dilemma, dramatizing the need for novel agents. The repurposing of current antibiotics is one rapid path from discovery to patient care. In this study, we have discovered that dual β-lactams, and specifically the combination of ceftazidime with either ceftaroline or imipenem, are synergistic and have clinically relevant activities, with MIC50s of 0.25 (ceftaroline with 100 µg/ml ceftazidime) and 0.5 µg/ml (imipenem with 100 µg/ml ceftazidime) against clinical MABC isolates. Similar synergy was observed in time-kill studies against the M. abscessus ATCC 19977 strain using clinically achievable concentrations of either imipenem (4 µg/ml) or ceftaroline (2 µg/ml), as the addition of ceftazidime at concentrations of ≥50 µg/ml showed a persistent bactericidal effect over 5 days. Treatment of THP-1 human macrophages infected with three different M. abscessus clinical isolates supported the in vitro findings, as the combination of 100 µg/ml ceftazidime and 0.125 µg/ml ceftaroline or 100 µg/ml ceftazidime and 0.25 µg/ml imipenem dramatically reduced the CFU counts to near baseline levels of infection. This study's finding that there is synergy between certain β-lactam combinations against M. abscessus infection provides optimism toward identifying an optimum dual β-lactam treatment regimen.IMPORTANCE The emergence of chronic MABC infections among immunocompromised populations and their inherent and acquired resistance to effective antibiotic therapy have created clinical challenges in advancing patients for transplant surgery and treating those with disease. There is an urgent need for new treatment regimens, and the repurposing of existing antibiotics provides a rapid strategy to advance a laboratory finding to patient care. Our recent discoveries that dual β-lactams, specifically the combination of ceftazidime with ceftaroline or ceftazidime with imipenem, have significant in vitro MIC values and kill curve activities and are effective against infected THP-1 human macrophages provide optimism for a dual β-lactam treatment strategy against MABC infections. The unexpected synergistic activities reported in this study create a new path of discovery to repurpose the large family of β-lactam drugs.