ABSTRACT
Eravacycline is a novel, fully synthetic fluorocycline antibiotic being developed for the treatment of serious infections, including those caused by resistant Gram-positive pathogens. Here, we evaluated the in vitro activities of eravacycline and comparator antimicrobial agents against a recent global collection of frequently encountered clinical isolates of Gram-positive bacteria. The CLSI broth microdilution method was used to determine in vitro MIC data for isolates of Enterococcus spp. (n = 2,807), Staphylococcus spp. (n = 4,331), and Streptococcus spp. (n = 3,373) isolated primarily from respiratory, intra-abdominal, urinary, and skin specimens by clinical laboratories in 37 countries on three continents from 2013 to 2017. Susceptibilities were interpreted using both CLSI and EUCAST breakpoints. There were no substantive differences (a >1-doubling-dilution increase or decrease) in eravacycline MIC90 values for different species/organism groups over time or by region. Eravacycline showed MIC50 and MIC90 results of 0.06 and 0.12 μg/ml, respectively, when tested against Staphylococcus aureus, regardless of methicillin susceptibility. The MIC90 values of eravacycline for Staphylococcus epidermidis and Staphylococcus haemolyticus were equal (0.5 μg/ml). The eravacycline MIC90s for Enterococcus faecalis and Enterococcus faecium were 0.06 μg/ml and were within 1 doubling dilution regardless of the vancomycin susceptibility profile. Eravacycline exhibited MIC90 results of ≤0.06 μg/ml when tested against Streptococcus pneumoniae and beta-hemolytic and viridans group streptococcal isolates. In this surveillance study, eravacycline demonstrated potent in vitro activity against frequently isolated clinical isolates of Gram-positive bacteria (Enterococcus, Staphylococcus, and Streptococcus spp.), including isolates collected over a 5-year period (2013 to 2017), underscoring its potential benefit in the treatment of infections caused by common Gram-positive pathogens.
INTRODUCTION
Multidrug-resistant (MDR) Gram-positive organisms are major human pathogens, causing both health care-associated and community-acquired infections. Clinically important antimicrobial-resistant Gram-positive pathogens include methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant enterococci (VRE), and Streptococcus pneumoniae. In fact, all three have recently been highlighted among the Gram-positive pathogens classified as serious or high public health threats by the Centers for Disease Control and Prevention (CDC) (1) and the World Health Organization (WHO) (2).
Eravacycline is a novel, fully synthetic fluorocycline antibiotic recently approved by U.S. Food and Drug Administration (FDA) and the European Medicines Agency (EMA) for the treatment of complicated intra-abdominal infections (cIAI), including those caused by MDR pathogens (3, 4; https://clinicaltrials.gov/ct2/show/NCT01844856). Additionally, eravacycline has been demonstrated to have in vivo efficacy as a treatment in murine models of systemic, thigh, and lung infection and pyelonephritis (4, 6, 7).
Eravacycline is comprised of a tetracycline core with two novel modifications: a fluorine atom at the C-7 position and a pyrrolidinoacetamido group at the C-9 position, both of which are on the D ring (4, 8). These novel modifications confer enhanced in vitro activity compared to that of other tetracyclines against resistant Gram-negative and Gram-positive bacteria, and the pyrrolidinoacetamido group allows for increased ribosomal binding and steric hindrance to avoid ribosome protection-based tetracycline resistance.
Eravacycline inhibits bacterial protein synthesis (i.e., acyl-tRNA transfer) by binding to the 30S ribosomal subunit (9). Eravacycline demonstrates potent broad-spectrum activity against Gram-positive cocci and Gram-negative bacilli (except Pseudomonas aeruginosa and Burkholderia spp.), including anaerobes, as well as atypical bacterial pathogens and Neisseria gonorrhoeae (3, 10–15), and does not exhibit a loss of antibacterial activity against isolates expressing tetracycline ribosomal protection genes or most tetracycline efflux resistance genes (9, 10, 13).
The objective of the current study was to determine the in vitro activity of eravacycline relative to that of other antimicrobial agents using a representative global collection of clinical isolates of Gram-positive bacteria.
RESULTS AND DISCUSSION
A total of 10,511 Gram-positive aerobic isolates collected between 2013 and 2017 were included in this study. The MIC distributions and the cumulative percentage of selected isolates of Gram-positive bacteria tested inhibited by eravacycline are shown in Table 1. The MIC90 of eravacycline for isolates of S. aureus was 0.12 μg/ml irrespective of whether the isolates were MRSA or methicillin-susceptible S. aureus (MSSA). The eravacycline MIC90 values for the coagulase-negative staphylococci Staphylococcus epidermidis and Staphylococcus haemolyticus, including the methicillin-resistant subsets, were ≤0.5 μg/ml. The eravacycline MIC90 for Enterococcus faecalis was 0.06 μg/ml, with a 1-doubling-dilution shift being seen for vancomycin-resistant E. faecalis. The eravacycline MIC90 for Enterococcus faecium was 0.06 μg/ml, regardless of its vancomycin susceptibility. Eravacycline exhibited MIC90 results of ≤0.06 μg/ml when tested against beta-hemolytic and viridans group streptococci as well as an MIC90 of 0.015 μg/ml for Streptococcus pneumoniae.
Cumulative percentage of clinical isolates of staphylococci, enterococci, and streptococci tested from 2013 to 2017 inhibited by eravacycline, by MIC
Tables 2, 3, and 4 provide details on the in vitro activities of eravacycline and the comparator agents against staphylococci, enterococci, and streptococci, respectively, including percent susceptibility according to the CLSI and EUCAST breakpoints. The highest rates of nonsusceptibility in MRSA were reported for azithromycin, clindamycin, and levofloxacin (75.9%, 38.3%, and 65.9%, respectively, by CLSI criteria), while resistance rates were <1% for linezolid, daptomycin, and vancomycin (Table 2). For compounds of the tetracycline class, tigecycline and minocycline, resistance rates were approximately 2 to 12% across FDA/CLSI and EUCAST breakpoints. Comparatively, due to overall lower breakpoints for eravacycline, the nonsusceptible rate was nearly 20% by the FDA criteria and 4.5% by the EUCAST criteria, but the MIC90 value of eravacycline was 2-fold lower than that of tigecycline. Similarly, for E. faecalis the nonsusceptibility rates to linezolid and daptomycin were <1% and 5.6%, respectively, while the rates were 2% and 53%, respectively, for E. faecium (Table 3). Vancomycin retained activity against E. faecalis, with a resistance rate of 4.9%, but it was generally ineffective against E. faecium, in which the rate of resistance exceeded 40%. Both species of enterococci were resistant to minocycline, with nonsusceptibility rates ranging from 49 to 72%. While eravacycline and tigecycline nonsusceptibility rates were about 1 to 5%, the MIC90 of tigecycline was 2 doubling dilutions higher than that of eravacycline. Notably, the rates of resistance for the comparators in this study were similar to those seen in other global surveillance studies (16, 17).
In vitro activity of eravacycline and comparator agents against staphylococci, cumulative 2013 to 2017 datad
In vitro activity of eravacycline and comparator agents against enterococci, cumulative 2013 to 2017 datac
In vitro activity of eravacycline and comparator agents against streptococci, cumulative 2013 to 2017 datag
When isolates were allocated to their respective geographic regions, eravacycline MIC90s were within 1 doubling dilution for all Gram-positive genera/species (see Table S3 in the supplemental material). Similarly, there were no significant differences (a >1-doubling-dilution increase or decrease in MIC90s) observed in the in vitro activity of eravacycline for any genera/species of Gram-positive bacteria stratified by study period (2013 to 2014, 2015, 2016, 2017) (Table S4) or stratified by specimen source (Table S5). A detailed trend analysis could not be conducted, given that there were changes in participating laboratories and the panel of antimicrobial agents tested over the time period studied (2013 to 2017). Overall, eravacycline activity was similar over time and across geographic regions and specimen sources.
Eravacycline consistently demonstrated 2- to 4-fold lower MIC90 values than tigecycline for populations of Gram-positive pathogens. Previous in vitro studies comparing eravacycline and tigecycline have reported similar 2- to 4-fold improvements in the MIC90 (4, 6, 7, 15). Susceptibility rates, due to a difference in breakpoints, were similar between these two antibiotics. As tigecycline EUCAST breakpoints have recently been lowered for Gram-negative organisms, perhaps a review of the breakpoints for Gram-positive organisms is also warranted for this agent.
This global surveillance investigation highlights the broad-spectrum potency of eravacycline against Gram-positive bacteria, including resistant isolates. As cIAIs are well-known to be polymicrobial, involving synergistic Gram-positive, Gram-negative, and anaerobic organism interactions, this study underscores the potential benefit of eravacycline for the empirical treatment of cIAIs. Furthermore, eravacycline may have a role in the treatment of other infections caused predominantly by Gram-positive pathogens, but the clinical utility in such disease states should be investigated.
MATERIALS AND METHODS
Bacterial isolates.From 2013 to 2017, 10,511 clinical isolates of Enterococcus spp. (n = 2,807), Staphylococcus spp. (n = 4,331), and Streptococcus spp. (n = 3,373) were collected by laboratories in 37 countries on three continents (Asia/Pacific, Europe, North America). The identity of each isolate was confirmed using matrix-assisted laser desorption ionization–time of flight (MALDI-TOF) mass spectrometry (Bruker Biotyper; Bruker Daltonics, Bremen, Germany).
Table S1 in the supplemental material summarizes the numbers of isolates collected in each of the four study periods (2013 to 2014, 2015, 2016, and 2017) by geographic region. Overall, approximately 54% of the isolates came from Europe, 35% of the isolates came from North America, and 10% came from the Asia-Pacific region. In total, there were 3,180, 2,082, 3,176, 956, and 1,117 isolates, respectively, from respiratory, intra-abdominal, urinary, skin, and other specimen sources (Table S2).
Isolates were limited to one per patient, determined by the participating laboratory algorithms to be clinically significant, and collected irrespective of their antimicrobial susceptibility profile and independent of patient gender or age. The study was not designed to directly compare the prevalence of antimicrobial-resistant pathogens across specific geographic locations but, rather, was designed to evaluate the in vitro activities of eravacycline and the comparator antimicrobial agents against a global collection of frequently encountered clinical isolates of Gram-positive bacteria collected from 2013 to 2017.
Antimicrobial susceptibility testing.The in vitro susceptibilities of the isolates were determined using the CLSI-defined broth microdilution method in 96-well broth microdilution panels (18, 19). The antimicrobial agents used in panel production were acquired as laboratory-grade powders from their respective manufacturers or from a commercial source. The list of antimicrobial agents tested in each of the four study periods varied slightly, in that some agents, in addition to those tested in the 2013 to 2014 period, were included in the 2015, 2016, and 2017 testing periods. Of note, ampicillin, clindamycin, meropenem, and oxacillin were tested only in 2015, 2016, and 2017. The eravacycline MICs for Gram-positive bacteria were read following the current CLSI standard for dilution method testing; MIC endpoints were read following panel incubation at 35°C in ambient air for 16 to 20 h (Enterococcus and Staphylococcus spp.) or 35°C in ambient air for 20 to 24 h (Streptococcus spp.) (19). Quality control testing for eravacycline and the other antimicrobial agents was performed on each day of testing, as specified by the CLSI, using the CLSI-defined control strains E. faecalis ATCC 29212, S. aureus ATCC 29213, and S. pneumoniae ATCC 49619 (19).
MICs were interpreted using 2019 CLSI MIC breakpoints (19) and 2019 EUCAST MIC breakpoints (20), with the following exceptions. FDA MIC interpretative breakpoints were used for tigecycline (21) and eravacycline in place of CLSI MIC breakpoints, which are not currently published for these agents. Additionally, tigecycline breakpoints for vancomycin-susceptible Enterococcus faecalis were applied to vancomycin-resistant isolates and to Enterococcus faecium; EUCAST eravacycline breakpoints for the Streptococcus anginosus group were applied to beta-hemolytic streptococci; EUCAST tigecycline breakpoints for beta-hemolytic streptococci were applied to the S. anginosus group; and EUCAST eravacycline breakpoints for S. aureus were applied to coagulase-negative Staphylococcus species.
ACKNOWLEDGMENTS
We thank all laboratories participating in this eravacycline global surveillance study for their contributions, as well as Sophie Magnet for her coordination of the laboratory work.
Funding for this research was provided by Tetraphase Pharmaceuticals, Inc., Watertown, MA, USA, which also included compensation fees for services in relation to preparing the manuscript.
C.F. and M.O. are employees of Tetraphase Pharmaceuticals. J.N. is a former employee of Tetraphase Pharmaceuticals. I.M. and S.H. are employees of IHMA Europe Sàrl. S.H.L. works for IHMA, Inc. Both IHMA laboratories have received research funding from Tetraphase Pharmaceuticals, Inc. J.A.K. is a consultant to IHMA, Inc. The authors employed by IHMA and J.A.K. do not have personal financial interests in the sponsor of this paper (Tetraphase Pharmaceuticals, Inc.). M.B. has participated in advisory boards and/or received speaker honoraria from Achaogen, Angelini, Astellas, AstraZeneca, Bayer, Basilea, Cidara, Gilead, Menarini, MSD, Nabriva, Paratek, Pfizer, The Medicines Company, Tetraphase, and Vifor. G.R.C. has received consulting fees from Cempra Pharmaceuticals, PRA International, Furiex Pharmaceuticals, Inimex Pharmaceuticals, Dr. Reddy’s Laboratories, Cerexa/Forest Laboratories, AstraZeneca, GlaxoSmithKline, Merck, ContraFect, Theravance, and Astellas; has received research grants from Theravance, Innocoll, and The Medicines Company; and has served on the advisory boards of Pfizer, Polymedix, Tetraphase Pharmaceuticals, Seachaid Pharmaceuticals, BioCryst Pharmaceuticals, Durata, Achaogen, ContraFect, and Nabriva.
FOOTNOTES
- Received 22 August 2019.
- Returned for modification 20 October 2019.
- Accepted 9 December 2019.
- Accepted manuscript posted online 16 December 2019.
Supplemental material is available online only.
- Copyright © 2020 Morrissey et al.
This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 International license.
