Abstract
The combination of aztreonam plus avibactam is being developed for use in infections caused by metallo-β-lactamase-producing Enterobacteriaceae strains that also produce serine β-lactamases. The in vitro activities of aztreonam-avibactam and comparator antimicrobials were determined against year 2012 and 2013 clinical isolates of Enterobacteriaceae, Pseudomonas aeruginosa, and Acinetobacter baumannii using the broth microdilution methodology recommended by the Clinical and Laboratory Standards Institute (CLSI). A total of 28,501 unique clinical isolates were obtained from patients in 190 medical centers within 39 countries. MIC90 values of aztreonam and aztreonam-avibactam against all collected isolates of Enterobacteriaceae (n = 23,516) were 64 and 0.12 μg/ml, respectively, with 76.2% of the isolates inhibited by ≤4 μg/ml of aztreonam (the CLSI breakpoint) and 99.9% of the isolates inhibited by ≤4 μg/ml of aztreonam-avibactam using a fixed concentration of 4 μg/ml of avibactam. The MIC90 was 32 μg/ml for both aztreonam and aztreonam-avibactam against P. aeruginosa (n = 3,766). Aztreonam alone or in combination with avibactam had no in vitro activity against isolates of A. baumannii. PCR and sequencing were used to characterize 5,076 isolates for β-lactamase genes. Aztreonam was not active against most Enterobacteriaceae isolates producing class A or class C enzymes alone or in combination with class B metallo-β-lactamases. In contrast, >99% of Enterobacteriaceae isolates producing all observed Ambler classes of β-lactamase enzymes were inhibited by ≤4 μg/ml aztreonam in combination with avibactam, including isolates that produced IMP-, VIM-, and NDM-type metallo-β-lactamases in combination with multiple serine β-lactamases.
INTRODUCTION
In recent years, clinical isolates of Enterobacteriaceae, Pseudomonas aeruginosa, and Acinetobacter spp. that are multidrug resistant (MDR) have challenged even the most reliable treatment options commonly used for these serious infections (1–9). The emergence and dissemination of potent β-lactamases such as extended-spectrum β-lactamases (ESBLs) and carbapenemases have become a global problem (10–12). Most worrisome among these isolates are the metallo-β-lactamases (MBLs) (13). Most isolates with these resistance mechanisms will exhibit an MDR or panresistance phenotype that will be difficult to treat empirically with existing therapies.
Aztreonam was approved by the U.S. Food and Drug Administration in 1986 and has been useful for the treatment of infections caused by Enterobacteriaceae and P. aeruginosa (http://packageinserts.bms.com/pi/pi_azactam.pdf). Aztreonam has a unique feature compared to other β-lactams in that it is stable to hydrolysis by Ambler class B metallo-β-lactamases (MBLs) (14, 15). Although aztreonam is active against many Gram-negative bacteria, including some isolates of Enterobacteriaceae that produce MBLs alone, it is inactive against isolates that produce ESBLs, KPC carbapenemases, or plasmid-encoded or stably derepressed chromosomally encoded AmpC β-lactamases (16, 17). Unfortunately, MBL-producing isolates commonly carry additional β-lactamases which may include ESBLs, AmpC enzymes, and serine carbapenemases that can inactivate aztreonam (18, 19).
Avibactam, a non-β-lactam β-lactamase inhibitor, is being developed in combination with aztreonam to restore this drug's activity against isolates expressing MBLs in combination with one or more additional serine β-lactamases. Avibactam does not have any clinically meaningful intrinsic antibacterial activity at the concentration employed for in vitro susceptibility testing but is capable of inhibiting Ambler class A and class C, and some class D, β-lactamases, including ESBLs, and serine carbapenemases (KPC and OXA-48 type) (8, 20–25). Because most Enterobacteriaceae isolates that produce MBLs also coproduce class A or class C β-lactamases, aztreonam-avibactam is a novel combination antimicrobial with potential utility against problematic bacterial pathogens that are increasingly isolated from patient infections.
The in vitro activities of aztreonam-avibactam and comparator agents were assessed against contemporary clinical isolates collected from a global antimicrobial surveillance program during 2012 and 2013. This study investigated the activity of aztreonam-avibactam relative to that of aztreonam alone against a collection of isolates of Enterobacteriaceae, P. aeruginosa, and Acinetobacter baumannii. A specific focus was on species in which an MBL was detected and which also coproduced class A, class C, or class D β-lactamases.
MATERIALS AND METHODS
Organism collection, transport, confirmation of organism identification, susceptibility testing, molecular characterization, quality assurance of data, and development and management of a centralized database were coordinated by International Health Management Associates, Inc. (IHMA) (Schaumburg, IL, USA). Clinical isolates from hospitalized patients were obtained from specimens collected from intra-abdominal infections (IAIs), urinary tract infections (UTIs), skin and skin structure infections (SSSIs), or lower respiratory tract infections (LRTIs), and only one isolate per patient was included. A total of 190 centers across 39 countries participated in this global study covering five major geographical regions, Asia/Pacific, Europe, Latin America, Middle East/Africa, and North America. The central laboratory confirmed the organism identification of all isolates received using matrix-assisted laser desorption ionization–time of flight (MALDI-TOF) mass spectrometry (Bruker Daltonics, Bremen, Germany).
MICs were determined using frozen broth microdilution panels prepared at IHMA. All broth microdilution testing, including panel manufacture, inoculation, incubation, and interpretation, was conducted according to current CLSI guidelines (26). Avibactam was tested at a fixed concentration of 4 μg/ml. Percent susceptibility was calculated according to CLSI interpretive criteria where available, and the FDA interpretive criteria were used for tigecycline in the absence of any CLSI breakpoints (27, 28). Direct comparisons were made between CLSI (includes FDA breakpoints for tigecycline) and EUCAST interpretive criteria using published breakpoints for each drug (27–29). Clinical breakpoints have not been assigned to aztreonam-avibactam. Quality control of broth microdilution panels followed CLSI guidelines using ATCC strains Escherichia coli ATCC 25922 and ATCC 35218, Klebsiella pneumoniae ATCC 700603, and P. aeruginosa ATCC 27853 (26).
E. coli, K. pneumoniae, Klebsiella oxytoca, and Proteus mirabilis were screened and confirmed for extended-spectrum β-lactamase (ESBL) activity according to CLSI guidelines using ceftazidime, cefotaxime, and aztreonam for screening and then confirmation tests using clavulanate in combination with ceftazidime and cefotaxime (26). ESBL phenotypic activity was determined using broth microdilution and disk diffusion if the MICs of the indicator β-lactams were above the highest dilution tested. Quality control of antimicrobial disks and broth microdilution panels followed CLSI guidelines using ATCC strain K. pneumoniae ATCC 700603 as a positive ESBL control (27).
Isolates were selected for molecular characterization of β-lactamase genes based on the following criteria: (i) isolates that resulted in a positive ESBL confirmatory test; (ii) isolates that resulted in a negative confirmatory test but were resistant to ceftazidime (MIC, ≥16 μg/ml); (iii) all Enterobacteriaceae isolates that were nonsusceptible to carbapenems using CLSI criteria (doripenem, meropenem, and imipenem, ≥2 μg/ml; ertapenem, ≥1 μg/ml); (iv) Proteeae isolates with an imipenem MIC value of 2 or 4 μg/ml tested only if also nonsusceptible to another carbapenem; (v) P. aeruginosa isolates that were nonsusceptible to doripenem, imipenem, or meropenem (MIC, ≥4 μg/ml); and (vi) A. baumannii isolates resistant to meropenem or imipenem (MIC, ≥8 μg/ml). Molecular characterization of β-lactamases for genes encoding MBLs (IMP, VIM, NDM, and SPM) and other β-lactamases (SHV, TEM, CTX-M, VEB, PER, GES, CMY, DHA, FOX, ACT-MIR, and OXA-48-like) was done via multiplex PCR, followed by sequencing using methods previously described (30).
Original-spectrum β-lactamase enzymes (OSBLs) that do not hydrolyze expanded-spectrum cephalosporins or carbapenems include TEM-1, SHV-1, and SHV-11. “ESBL positive” was defined as any Enterobacteriaceae, P. aeruginosa, or A. baumannii isolate for which a bla gene encoding an ESBL as defined by the Lahey β-lactamase classification site was detected by PCR (http://www.lahey.org/Studies/). These isolates may also have contained genes encoding other β-lactamases. “ESBL screen-negative” was defined as any E. coli, K. pneumoniae, K. oxytoca, and P. mirabilis isolate with ceftazidime, aztreonam, and cefotaxime MIC values of ≤1 μg/ml and any other Enterobacteriaceae isolate with ceftazidime and aztreonam MIC values of ≤1 μg/ml. “MBL positive” was defined as any Enterobacteriaceae, P. aeruginosa, or A. baumannii isolate for which a bla gene encoding an MBL was detected by PCR. These isolates may also have contained genes encoding other β-lactamases. “MBL negative” was defined as any Enterobacteriaceae, P. aeruginosa, or A. baumannii isolate that did not encode an MBL. These included isolates that were susceptible to all indicated carbapenems, isolates that encoded serine carbapenemases or other serine β-lactamases, and isolates that were resistant to carbapenems by virtue of nonenzymatic mechanisms such as changes in outer membrane permeability.
RESULTS
Of the 28,501 Gram-negative isolates included in this analysis, the percentage of organisms collected by region was as follows: Asia/Pacific, 21.0%; Europe, 46.9%; Latin America, 13.3%; Middle East/Africa, 7.5%; and North America, 11.3%. The source of the organisms by infection type was as follows: intra-abdominal, 20.8%; lower respiratory tract, 25.0%; skin and skin structure, 27.4%; genitourinary, 26.4%; other or unknown sources, 0.4%. MBL-producing isolates were detected among 12 bacterial species, and these were analyzed as a subset in this study.
A total of 23,516 Enterobacteriaceae isolates included all genus and species groups collected in this family during 2012 and 2013, and the β-lactamase subset analysis of MIC frequency distributions for aztreonam and aztreonam-avibactam is shown in Table 1. Among this overall collection of isolates, aztreonam MIC values were distributed across the entire range tested with a MIC50 value of 0.12 μg/ml, which was the MIC90 value observed for aztreonam-avibactam. More than 99.9% of Enterobacteriaceae isolates were inhibited by ≤8 μg/ml of aztreonam-avibactam. Aztreonam-avibactam provided 2-fold-greater in vitro activity than did aztreonam alone against ESBL screen-negative isolates when comparing MIC90 values. The majority of ESBL-positive isolates resulted in an aztreonam MIC value of ≥32 μg/ml with a MIC90 of >128 μg/ml. These MIC values were reduced with the addition of avibactam, for which no MIC values of >32 μg/ml were observed and the MIC90 value of 0.25 μg/ml was 10 doubling dilutions lower than that of aztreonam alone. Higher MIC50 (4-fold) and MIC90 (8-fold) aztreonam-avibactam values were observed among the meropenem-nonsusceptible isolates than among those susceptible to this carbapenem. However, >99% of the meropenem-nonsusceptible isolates had an aztreonam-avibactam MIC of ≤4 μg/ml. The MIC distributions of aztreonam-avibactam MIC values for MBL-negative and MBL-positive isolates were similar to those of the meropenem-susceptible and nonsusceptible populations, respectively.
TABLE 1.
MIC cumulative frequency distribution of aztreonam and aztreonam-avibactam tested against Gram-negative pathogens collected worldwide (2012 and 2013)
| Organism (n) and drug | % frequency distribution by MIC (μg/ml)a: |
||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ≤0.015 | 0.03 | 0.06 | 0.12 | 0.25 | 0.5 | 1 | 2 | 4 | 8 | 16 | 32 | 64 | 128 | >128 | |
| Enterobacteriaceae | |||||||||||||||
| All (23,516) | |||||||||||||||
| Aztreonam | 7.7 | 13.4 | 38.7 | 60.9 | 69.2 | 71.8 | 73.3 | 74.4 | 76.2 | 78.0 | 81.1 | 86.2 | 92.4 | 96.3 | 100 |
| Aztreonam-avibactam | 18.0 | 47.7 | 77.9 | 90.5 | 95.7 | 98.1 | 99.3 | 99.7 | 99.9 | 99.9 | >99.9 | >99.9 | >99.9 | >99.9 | 100 |
| ESBL screen negative (16,830) | |||||||||||||||
| Aztreonam | 10.7 | 18.7 | 53.9 | 84.6 | 95.6 | 98.7 | >99.9 | >99.9 | >99.9 | >99.9 | >99.9 | >99.9 | 100 | ||
| Aztreonam-avibactam | 22.5 | 57.8 | 88.7 | 97.8 | 99.7 | >99.9 | 100 | ||||||||
| ESBL positive (4,087) | |||||||||||||||
| Aztreonam | <0.1 | <0.1 | 0.1 | 0.2 | 0.3 | 0.6 | 1.1 | 3.5 | 8.3 | 12.6 | 22.2 | 40.6 | 66.6 | 84.6 | 100 |
| Aztreonam-avibactam | 6.9 | 26.3 | 59.8 | 82.1 | 92.4 | 96.9 | 98.5 | 99.2 | 99.7 | 99.9 | >99.9 | 100 | |||
| Meropenem susceptible (22,939) | |||||||||||||||
| Aztreonam | 7.8 | 13.7 | 39.7 | 62.4 | 70.8 | 73.4 | 74.9 | 76.1 | 77.8 | 79.7 | 82.9 | 88.0 | 94.1 | 97.7 | 100 |
| Aztreonam-avibactam | 18.3 | 48.7 | 79.4 | 91.8 | 96.4 | 98.4 | 99.4 | 99.7 | 99.9 | >99.9 | >99.9 | >99.9 | >99.9 | >99.9 | 100 |
| Meropenem nonsusceptible (577) | |||||||||||||||
| Aztreonam | 0.7 | 0.9 | 1.6 | 2.4 | 4.9 | 6.8 | 8.1 | 8.7 | 9.7 | 10.2 | 12.8 | 15.9 | 24.8 | 38.3 | 100 |
| Aztreonam-avibactam | 2.9 | 6.4 | 17 | 35.9 | 66.7 | 86.0 | 94.3 | 97.2 | 99.3 | 99.7 | 99.8 | 100 | |||
| MBL negative (23,425) | |||||||||||||||
| Aztreonam | 7.7 | 13.5 | 38.9 | 61.1 | 69.3 | 71.9 | 73.4 | 74.5 | 76.3 | 78.1 | 81.3 | 86.4 | 92.5 | 96.3 | 100 |
| Aztreonam-avibactam | 18 | 47.8 | 78 | 90.6 | 95.7 | 98.2 | 99.3 | 99.7 | 99.9 | >99.9 | >99.9 | >99.9 | >99.9 | >99.9 | 100 |
| MBL positive (91) | |||||||||||||||
| Aztreonam | 4.4 | 5.5 | 9.9 | 13.2 | 22.0 | 29.7 | 35.2 | 37.4 | 38.5 | 39.6 | 45.1 | 53.8 | 68.1 | 89.0 | 100 |
| Aztreonam-avibactam | 11.0 | 20.9 | 37.4 | 56.0 | 80.2 | 86.8 | 93.4 | 96.7 | 100 | ||||||
| P. aeruginosa | |||||||||||||||
| All (3,766) | |||||||||||||||
| Aztreonam | 0.1 | 0.1 | 0.1 | 0.2 | 1.0 | 2.3 | 3.1 | 5.8 | 29.4 | 58.5 | 78.0 | 91.0 | 95.9 | 97.9 | 100 |
| Aztreonam-avibactam | 0.1 | 0.1 | 0.2 | 0.5 | 1.6 | 3.3 | 4.0 | 7.4 | 35.4 | 73.6 | 86.2 | 96.0 | 99.2 | 99.6 | 100 |
| MBL positive (118) | |||||||||||||||
| Aztreonam | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.9 | 0.9 | 0.9 | 12.7 | 22.0 | 60.2 | 86.4 | 91.5 | 94.9 | 100 |
| Aztreonam-avibactam | 0.0 | 0.0 | 0.0 | 0.0 | 0.9 | 0.9 | 0.9 | 0.9 | 13.6 | 44.1 | 76.3 | 91.5 | 96.6 | 99.2 | 100 |
| A. baumannii | |||||||||||||||
| All (1,219) | |||||||||||||||
| Aztreonam | 0.2 | 0.2 | 0.3 | 0.4 | 0.5 | 0.6 | 0.7 | 0.8 | 1.4 | 3.7 | 12.8 | 32.5 | 56.0 | 77.2 | 100 |
| Aztreonam-avibactam | 0.2 | 0.4 | 0.4 | 0.5 | 0.5 | 0.7 | 0.7 | 0.9 | 2.3 | 6.2 | 19.2 | 38.1 | 65.5 | 88.8 | 100 |
| MBL positive (3) | |||||||||||||||
| Aztreonam | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 33.3 | 100 | |
| Aztreonam-avibactam | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 33.3 | 33.3 | 100 | |
MIC90 values are in bold.
The activities of aztreonam and aztreonam-avibactam tested against a subset of 10 species of Enterobacteriaceae in which MBLs were found are shown in Table 2. MIC90 values for aztreonam-avibactam ranged from 0.03 to 4 μg/ml, with the lowest MIC values observed among Proteeae and the highest value observed for MBL-positive Enterobacter cloacae. Eleven of 21,244 isolates from the 10 different species of Enterobacteriaceae that were tested (considering only species where an MBL was found) had reduced susceptibility to aztreonam-avibactam (MIC values ranging from 16 to >128 μg/ml). These isolates consisted of six different species (Enterobacter aerogenes, E. cloacae, E. coli, K. pneumoniae, P. mirabilis, and Serratia marcescens) and came from seven different countries, including four from Russia. None of these isolates produced an MBL, but they expressed a variety of β-lactamases, including CTX-M-15, -22, and -28; SHV-110; VEB-2; and OXA-48, all of which are inhibited by avibactam. Other isolates in the collection also produced these β-lactamases and tested with low MICs of aztreonam-avibactam. Therefore, the mechanism for the increased MICs of aztreonam-avibactam in these 10 isolates remains undefined and is likely due to a combination of factors.
TABLE 2.
MIC distribution for aztreonam and aztreonam-avibactam tested against 10 species (21,244 isolates) of Enterobacteriaceae in which MBLs were identifieda
| Organism and phenotype (n) | Drug | MIC (μg/ml): |
||
|---|---|---|---|---|
| MIC50 | MIC90 | MIC range | ||
| Citrobacter freundii (769) | Aztreonam | 0.25 | 64 | ≤0.015 to >128 |
| Aztreonam-avibactam | 0.06 | 0.25 | ≤0.015 to 4 | |
| MBL negative (763) | Aztreonam | 0.25 | 64 | ≤0.015 to >128 |
| Aztreonam-avibactam | 0.06 | 0.25 | ≤0.015 to 4 | |
| MBL positive (6) | Aztreonam | 0.12 to >128 | ||
| Aztreonam-avibactam | 0.06 to 0.25 | |||
| E. aerogenes (989) | Aztreonam | 0.12 | 32 | ≤0.015 to >128 |
| Aztreonam-avibactam | 0.06 | 0.5 | ≤0.015 to >128 | |
| MBL negative (988) | Aztreonam | 0.12 | 32 | ≤0.015 to >128 |
| Aztreonam-avibactam | 0.06 | 0.5 | ≤0.015 to >128 | |
| MBL positive (1) | Aztreonam | 0.03 | ||
| Aztreonam-avibactam | ≤0.015 | |||
| Enterobacter asburiae (215) | Aztreonam | 0.12 | 32 | ≤0.015 to >128 |
| Aztreonam-avibactam | 0.06 | 0.25 | ≤0.015 to 8 | |
| MBL negative (212) | Aztreonam | 0.12 | 32 | ≤0.015 to >128 |
| Aztreonam-avibactam | 0.06 | 0.25 | ≤0.015 to 8 | |
| MBL positive (3) | Aztreonam | 0.5 to >128 | ||
| Aztreonam-avibactam | 0.12 to 2 | |||
| E. cloacae (1,543) | Aztreonam | 0.25 | 64 | ≤0.015 to >128 |
| Aztreonam-avibactam | 0.06 | 1 | ≤0.015 to 32 | |
| MBL negative (1,522) | Aztreonam | 0.25 | 64 | ≤0.015 to >128 |
| Aztreonam-avibactam | 0.06 | 1 | ≤0.015 to 32 | |
| MBL positive (21) | Aztreonam | 32 | 128 | 0.5 to 128 |
| Aztreonam-avibactam | 0.25 | 4 | 0.03 to 4 | |
| E. coli (8,452) | Aztreonam | 0.12 | 64 | ≤0.015 to >128 |
| Aztreonam-avibactam | 0.03 | 0.12 | ≤0.015 to 16 | |
| MBL negative (8,447) | Aztreonam | 0.12 | 64 | ≤0.015 to >128 |
| Aztreonam-avibactam | 0.03 | 0.12 | ≤0.015 to 16 | |
| MBL positive (5) | Aztreonam | 0.12 to 64 | ||
| Aztreonam-avibactam | 0.06 to 2 | |||
| K. oxytoca (1,377) | Aztreonam | 0.25 | 32 | ≤0.015 to >128 |
| Aztreonam-avibactam | 0.03 | 0.12 | ≤0.015 to 4 | |
| MBL negative (1,372) | Aztreonam | 0.25 | 32 | ≤0.015 to >128 |
| Aztreonam-avibactam | 0.03 | 0.12 | ≤0.015 to 4 | |
| MBL positive (5) | Aztreonam | 0.06 to 2 | ||
| Aztreonam-avibactam | 0.03 to 0.25 | |||
| K. pneumoniae (5,613) | Aztreonam | 0.12 | >128 | ≤0.015 to >128 |
| Aztreonam-avibactam | 0.06 | 0.25 | ≤0.015 to 32 | |
| MBL negative (5,577) | Aztreonam | 0.12 | >128 | ≤0.015 to >128 |
| Aztreonam-avibactam | 0.06 | 0.25 | ≤0.015 to 32 | |
| MBL positive (36) | Aztreonam | 128 | >128 | 0.06 to >128 |
| Aztreonam-avibactam | 0.12 | 0.5 | 0.03 to 0.5 | |
| P. mirabilis (1,630) | Aztreonam | ≤0.015 | 0.25 | ≤0.015 to >128 |
| Aztreonam-avibactam | ≤0.015 | 0.03 | ≤0.015 to >128 | |
| MBL negative (1,622) | Aztreonam | ≤0.015 | 0.25 | ≤0.015 to >128 |
| Aztreonam-avibactam | ≤0.015 | 0.03 | ≤0.015 to >128 | |
| MBL positive (8) | Aztreonam | ≤0.015 to 8 | ||
| Aztreonam-avibactam | ≤0.015 to 0.03 | |||
| Providencia stuartii (106) | Aztreonam | 0.03 | 4 | ≤0.015 to 64 |
| Aztreonam-avibactam | ≤0.015 | 0.03 | ≤0.015 to 0.5 | |
| MBL negative (102) | Aztreonam | 0.03 | 4 | ≤0.015 to 64 |
| Aztreonam-avibactam | ≤0.015 | 0.03 | ≤0.015 to 0.5 | |
| MBL positive (4) | Aztreonam | 0.5 to 32 | ||
| Aztreonam-avibactam | ≤0.015 to 0.12 | |||
| S. marcescens (550) | Aztreonam | 0.12 | 4 | ≤0.015 to >128 |
| Aztreonam-avibactam | 0.12 | 0.25 | ≤0.015 to 16 | |
| MBL negative (548) | Aztreonam | 0.12 | 4 | ≤0.015 to >128 |
| Aztreonam-avibactam | 0.12 | 0.25 | ≤0.015 to 16 | |
| MBL positive (2) | Aztreonam | 0.25 to 0.5 | ||
| Aztreonam-avibactam | 0.06 to 0.12 | |||
The 10 species included in this data set were those collected during 2012 and 2013 from which at least one isolate carried a metallo-β-lactamase.
The in vitro activities of aztreonam and aztreonam-avibactam were determined against 4,985 isolates of the Gram-negative nonfermenters P. aeruginosa (n = 3,766) and A. baumannii (n = 1,219) and are shown in Table 1. Against isolates of P. aeruginosa, the MIC90 values for both aztreonam and aztreonam-avibactam were 32 μg/ml. Similarly, for isolates of A. baumannii, the MIC90 values for both agents were >128 μg/ml.
Variation in the activities of aztreonam against all tested species of Enterobacteriaceae was observed in the five regions, with MIC90 values ranging from 16 μg/ml in North America to 128 μg/ml in Latin America (Table 3). Regional variation in the number of MBL-producing isolates was observed among this collection, which ranged from one isolate in Latin America to 38 isolates in Europe. The MBL enzymes observed in each region varied, with 61% of isolates in the Asia/Pacific region having IMP-type enzymes and 76% in Europe having VIM-type enzymes. Only NDM- and VIM-type enzymes were found in Africa, the Middle East, Latin America, and the United States. The variability in MIC values was much less for aztreonam-avibactam, with isolates from all regions showing MIC50 values of 0.03 to 0.06 μg/ml and MIC90 values of 0.12 to 0.25 μg/ml among the overall collection of isolates in each region.
TABLE 3.
Regional activity of aztreonam-avibactam and comparator antibacterial agents tested against 23,516 isolates of Enterobacteriaceaea
| Region and category (n) | Drug | MIC (μg/ml) |
% susceptible by criteria: |
|||
|---|---|---|---|---|---|---|
| MIC50 | MIC90 | MIC range | CLSI | EUCAST | ||
| Asia/Pacific (5,009) | Aztreonam | 0.12 | 64 | ≤0.015 to >128 | 73.5 | 69.3 |
| Aztreonam-avibactamb | 0.06 | 0.25 | ≤0.015 to 32 | NA | NA | |
| Meropenem | 0.03 | 0.12 | ≤0.004 to >8 | 98.4 | 98.6 | |
| Cefepime | ≤0.12 | >16 | ≤0.12 to >16 | 82.7 | 75.1 | |
| Ceftazidime | 0.25 | 64 | ≤0.015 to >128 | 75.2 | 71.0 | |
| Piperacillin-tazobactam | 2 | 64 | ≤0.25 to >128 | 85.5 | 80.9 | |
| Amikacin | 2 | 8 | ≤0.25 to >32 | 97.1 | 95.1 | |
| Tigecycline | 0.5 | 2 | ≤0.015 to >8 | 93.3 | 82.7 | |
| Levofloxacin | 0.12 | >4 | ≤0.03 to >4 | 73.5 | 70.6 | |
| Colistin | ≤0.12 | >4 | ≤0.12 to >4 | NA | 83.9 | |
| MBL negative (4,976) | Aztreonam | 0.12 | 64 | ≤0.015 to >128 | 73.8 | 69.5 |
| Aztreonam-avibactamb | 0.06 | 0.12 | ≤0.015 to 32 | NA | NA | |
| Meropenem | 0.03 | 0.06 | ≤0.004 to >8 | 98.8 | 99.1 | |
| Cefepime | ≤0.12 | >16 | ≤0.12 to >16 | 83.1 | 75.6 | |
| Ceftazidime | 0.25 | 64 | ≤0.015 to >128 | 75.7 | 71.5 | |
| Piperacillin-tazobactam | 2 | 64 | ≤0.25 to >128 | 85.8 | 81.3 | |
| Amikacin | 2 | 8 | ≤0.25 to >32 | 97.3 | 95.4 | |
| Tigecycline | 0.5 | 2 | ≤0.015 to >8 | 93.3 | 82.8 | |
| Levofloxacin | 0.12 | >4 | ≤0.03 to >4 | 73.7 | 70.9 | |
| Colistin | ≤0.12 | >4 | ≤0.12 to >4 | NA | 83.9 | |
| MBL positive (33) | Aztreonam | 32 | 128 | ≤0.015 to >128 | 33.3 | 30.3 |
| Aztreonam-avibactamb | 0.12 | 1 | ≤0.015 to 4 | NA | NA | |
| Meropenem | 8 | >8 | 0.5 to >8 | 18.2 | 33.3 | |
| Cefepime | >16 | >16 | 8 to >16 | 15.2 | 0.0 | |
| Ceftazidime | >128 | >128 | 64 to >128 | 0.0 | 0.0 | |
| Piperacillin-tazobactam | >128 | >128 | 0.5 to >128 | 30.3 | 18.2 | |
| Amikacin | 4 | >32 | 1 to >32 | 66.7 | 57.6 | |
| Tigecycline | 1 | 2 | 0.06 to 4 | 93.9 | 66.7 | |
| Levofloxacin | 4 | >4 | 0.06 to >4 | 45.5 | 27.3 | |
| Colistin | ≤0.12 | >4 | ≤0.12 to >4 | NA | 84.9 | |
| Europe (10,924) | Aztreonam | 0.12 | 64 | ≤0.015 to >128 | 77.9 | 75.3 |
| Aztreonam-avibactamb | 0.06 | 0.12 | ≤0.015 to >128 | NA | NA | |
| Meropenem | 0.03 | 0.12 | ≤0.004 to >8 | 97.4 | 97.8 | |
| Cefepime | ≤0.12 | >16 | ≤0.12 to >16 | 85.1 | 79.2 | |
| Ceftazidime | 0.25 | 64 | ≤0.015 to >128 | 78.6 | 75.0 | |
| Piperacillin-tazobactam | 2 | 128 | ≤0.25 to >128 | 83.3 | 77.6 | |
| Amikacin | 2 | 8 | ≤0.25 to >32 | 96.4 | 93.5 | |
| Tigecycline | 0.5 | 2 | ≤0.015 to >8 | 90.5 | 79.6 | |
| Levofloxacin | 0.06 | >4 | ≤0.03 to >4 | 73.5 | 76.4 | |
| Colistin | ≤0.12 | >4 | ≤0.12 to >4 | NA | 81.8 | |
| MBL negative (10,886) | Aztreonam | 0.12 | 64 | ≤0.015 to >128 | 78.0 | 75.4 |
| Aztreonam-avibactamb | 0.06 | 0.12 | ≤0.015 to >128 | NA | NA | |
| Meropenem | 0.03 | 0.12 | ≤0.004 to >8 | 92.7 | 98.0 | |
| Cefepime | ≤0.12 | >16 | ≤0.12 to >16 | 85.4 | 79.5 | |
| Ceftazidime | 0.25 | 64 | ≤0.015 to >128 | 78.6 | 75.3 | |
| Piperacillin-tazobactam | 2 | 64 | ≤0.25 to >128 | 83.6 | 77.9 | |
| Amikacin | 2 | 8 | ≤0.25 to >32 | 96.6 | 93.7 | |
| Tigecycline | 0.5 | 2 | ≤0.015 to >8 | 90.6 | 79.7 | |
| Levofloxacin | 0.06 | >4 | ≤0.03 to >4 | 78.8 | 76.6 | |
| Colistin | ≤0.12 | >4 | ≤0.12 to >4 | NA | 81.9 | |
| MBL positive (38) | Aztreonam | 32 | >128 | 0.06 to >128 | 42.1 | 36.8 |
| Aztreonam-avibactamb | 0.25 | 1 | ≤0.015 to 2 | NA | NA | |
| Meropenem | >8 | >8 | 0.25 to >8 | 7.9 | 15.8 | |
| Cefepime | >16 | >16 | ≤0.12 to >16 | 10.5 | 2.6 | |
| Ceftazidime | >128 | >128 | 16 to >128 | 0.0 | 0.0 | |
| Piperacillin-tazobactam | >128 | >128 | 16 to >128 | 2.6 | 0.0 | |
| Amikacin | 16 | >32 | 1 to >32 | 52.6 | 34.2 | |
| Tigecycline | 1 | 8 | 0.25 to >8 | 79.0 | 52.6 | |
| Levofloxacin | >4 | >4 | 0.5 to >4 | 18.4 | 13.2 | |
| Colistin | ≤0.12 | >4 | ≤0.12 to >4 | NA | 73.7 | |
| Latin America (3,149) | Aztreonam | 0.12 | 128 | ≤0.015 to >128 | 67.6 | 64.4 |
| Aztreonam-avibactamb | 0.06 | 0.25 | ≤0.015 to 16 | NA | NA | |
| Meropenem | 0.03 | 0.12 | ≤0.004 to >8 | 95.8 | 96.9 | |
| Cefepime | ≤0.12 | >16 | ≤0.12 to >16 | 76.5 | 68.3 | |
| Ceftazidime | 0.25 | 64 | ≤0.015 to >128 | 70.0 | 65.0 | |
| Piperacillin-tazobactam | 4 | 128 | ≤0.25 to >128 | 81.0 | 74.3 | |
| Amikacin | 2 | 8 | ≤0.25 to >32 | 94.4 | 90.0 | |
| Tigecycline | 0.5 | 2 | ≤0.015 to >8 | 91.9 | 79.6 | |
| Levofloxacin | 0.25 | >4 | ≤0.03 to >4 | 66.5 | 64.4 | |
| Colistin | ≤0.12 | >4 | ≤0.12 to >4 | NA | 83.2 | |
| MBL negative (3,148) | Aztreonam | 0.12 | 128 | ≤0.015 to >128 | 67.6 | 64.4 |
| Aztreonam-avibactamb | 0.06 | 0.25 | ≤0.015 to 16 | NA | NA | |
| Meropenem | 0.03 | 0.12 | ≤0.004 to >8 | 95.5 | 96.9 | |
| Cefepime | ≤0.12 | >16 | ≤0.12 to >16 | 76.5 | 68.3 | |
| Ceftazidime | 0.25 | 64 | ≤0.015 to >128 | 70.0 | 65.0 | |
| Piperacillin-tazobactam | 4 | 128 | ≤0.25 to >128 | 81.0 | 74.3 | |
| Amikacin | 2 | 8 | ≤0.25 to >32 | 94.4 | 90.0 | |
| Tigecycline | 0.5 | 2 | ≤0.015 to >8 | 91.9 | 79.5 | |
| Levofloxacin | 0.25 | >4 | ≤0.03 to >4 | 66.5 | 64.4 | |
| Colistin | ≤0.12 | >4 | ≤0.12 to >4 | NA | 83.2 | |
| MBL positive (1) | Aztreonam | 0.03 | 100 | 100 | ||
| Aztreonam-avibactamb | ≤0.015 | NA | NA | |||
| Meropenem | 1 | 100 | 100 | |||
| Cefepime | 4 | 100 | 0.0 | |||
| Ceftazidime | 0.25 | 100 | 100 | |||
| Piperacillin-tazobactam | 1 | 100 | 100 | |||
| Amikacin | 8 | 100 | 100 | |||
| Tigecycline | 0.5 | 100 | 100 | |||
| Levofloxacin | 0.5 | 100 | 100 | |||
| Colistin | ≤0.12 | NA | 100 | |||
| Middle East/Africa (1,803) | Aztreonam | 0.12 | 64 | ≤0.015 to >128 | 73.3 | 71.3 |
| Aztreonam-avibactamb | 0.03 | 0.12 | ≤0.015 to >128 | NA | NA | |
| Meropenem | 0.03 | 0.12 | ≤0.004 to >8 | 98.1 | 98.2 | |
| Cefepime | ≤0.12 | >16 | ≤0.12 to >16 | 78.2 | 72.3 | |
| Ceftazidime | 0.25 | 64 | ≤0.015 to >128 | 74.4 | 71.4 | |
| Piperacillin-tazobactam | 2 | 64 | ≤0.25 to >128 | 84.1 | 78.2 | |
| Amikacin | 2 | 8 | ≤0.25 to >32 | 96.8 | 93.4 | |
| Tigecycline | 0.5 | 2 | 0.06 to 8 | 93.6 | 82.3 | |
| Levofloxacin | 0.12 | >4 | ≤0.03 to >4 | 74.4 | 72.7 | |
| Colistin | ≤0.12 | >4 | ≤0.12 to >4 | NA | 84.4 | |
| MBL negative (1,787) | Aztreonam | 0.12 | 64 | ≤0.015 to >128 | 73.6 | 71.6 |
| Aztreonam-avibactamb | 0.03 | 0.12 | ≤0.015 to 16 | NA | NA | |
| Meropenem | 0.03 | 0.12 | ≤0.004 to >8 | 99.0 | 99.1 | |
| Cefepime | ≤0.12 | >16 | ≤0.12 to >16 | 78.6 | 72.8 | |
| Ceftazidime | 0.25 | 64 | ≤0.015 to >128 | 74.9 | 71.9 | |
| Piperacillin-tazobactam | 2 | 64 | ≤0.25 to >128 | 84.8 | 78.9 | |
| Amikacin | 2 | 8 | ≤0.25 to >32 | 97.0 | 93.6 | |
| Tigecycline | 0.5 | 2 | 0.06 to 8 | 93.6 | 82.7 | |
| Levofloxacin | 0.06 | >4 | ≤0.03 to >4 | 74.5 | 72.9 | |
| Colistin | ≤0.12 | >4 | ≤0.12 to >4 | 84.6 | 84.6 | |
| MBL positive (16) | Aztreonam | 16 | 128 | ≤0.015 to 128 | 37.5 | 37.5 |
| Aztreonam-avibactamb | 0.12 | 2 | ≤0.015 to 2 | NA | NA | |
| Meropenem | >8 | >8 | 4 to >8 | 0.0 | 0.0 | |
| Cefepime | >16 | >16 | ≤0.12 to >16 | 31.3 | 18.8 | |
| Ceftazidime | >128 | >128 | 0.5 to >128 | 18.8 | 18.8 | |
| Piperacillin-tazobactam | >128 | >128 | >128 | 0.0 | 0.0 | |
| Amikacin | 4 | >32 | 1 to >32 | 81.3 | 68.8 | |
| Tigecycline | 2 | 2 | 0.25 to 4 | 93.8 | 37.5 | |
| Levofloxacin | 2 | >4 | 0.06 to >4 | 62.5 | 43.8 | |
| Colistin | ≤0.12 | >4 | ≤0.12 to >4 | NA | 68.8 | |
| North America (2,631) | Aztreonam | 0.12 | 16 | ≤0.015 to >128 | 86.3 | 84.6 |
| Aztreonam-avibactamb | 0.03 | 0.12 | ≤0.015 to 4 | NA | NA | |
| Meropenem | 0.03 | 0.12 | ≤0.004 to >8 | 98.4 | 98.6 | |
| Cefepime | ≤0.12 | 2 | ≤0.12 to >16 | 94.0 | 89.8 | |
| Ceftazidime | 0.25 | 32 | ≤0.015 to >128 | 86.7 | 84.5 | |
| Piperacillin-tazobactam | 2 | 32 | ≤0.25 to >128 | 89.2 | 86.1 | |
| Amikacin | 2 | 4 | ≤0.25 to >32 | 98.4 | 96.8 | |
| Tigecycline | 0.5 | 2 | ≤0.015 to >8 | 92.4 | 82.7 | |
| Levofloxacin | 0.06 | >4 | ≤0.03 to >4 | 80.6 | 79.3 | |
| Colistin | ≤0.12 | >4 | ≤0.12 to >4 | NA | 84.0 | |
| MBL negative (2,628) | Aztreonam | 0.12 | 16 | ≤0.015 to >128 | 86.4 | 84.6 |
| Aztreonam-avibactamb | 0.03 | 0.12 | ≤0.015 to 4 | NA | NA | |
| Meropenem | 0.03 | 0.12 | ≤0.004 to >8 | 98.5 | 98.7 | |
| Cefepime | ≤0.12 | 2 | ≤0.12 to >16 | 94.1 | 89.9 | |
| Ceftazidime | 0.25 | 32 | ≤0.015 to >128 | 86.8 | 84.6 | |
| Piperacillin-tazobactam | 2 | 32 | ≤0.25 to >128 | 89.3 | 86.2 | |
| Amikacin | 2 | 4 | ≤0.25 to >32 | 98.5 | 96.8 | |
| Tigecycline | 0.5 | 2 | ≤0.015 to >8 | 92.4 | 82.8 | |
| Levofloxacin | 0.06 | >4 | ≤0.03 to >4 | 80.6 | 79.3 | |
| Colistin | ≤0.12 | >4 | ≤0.12 to >4 | NA | 84.0 | |
| MBL positive (3) | Aztreonam | 0.12 to >128 | 33.3 | 33.3 | ||
| Aztreonam-avibactamb | 0.06 to 0.5 | NA | NA | |||
| Meropenem | >8 | 0.0 | 0.0 | |||
| Cefepime | >16 | 0.0 | 0.0 | |||
| Ceftazidime | >128 | 0.0 | 0.0 | |||
| Piperacillin-tazobactam | >128 | 0.0 | 0.0 | |||
| Amikacin | >32 | 0.0 | 0.0 | |||
| Tigecycline | 0.5 to 2 | 100 | 33.3 | |||
| Levofloxacin | 0.5 to >4 | 33.3 | 33.3 | |||
| Colistin | ≤0.12 to 1 | NA | 100 | |||
CLSI susceptibilities were defined by CLSI document M100-S24 (27), where applicable. Tigecycline susceptibilities in the CLSI category were defined by the FDA (28). NA, no breakpoint defined. EUCAST susceptibilities were defined by breakpoint tables for interpretation of MICs (29).
No breakpoint criteria have been defined for aztreonam-avibactam.
Globally, a total of 5,076 clinical isolates were molecularly characterized for β-lactamase genes. Two hundred twelve of these isolates carried genes that encoded MBLs from three families and were detected in 12 species (Table 4). The Enterobacteriaceae isolates carried a wide variety of MBL gene types whereas P. aeruginosa isolates carried genes for only IMP and VIM and A. baumannii carried only NDM genes. It was interesting that the most common Enterobacteriaceae isolates that carried an MBL gene were Klebsiella spp. (41 isolates, 45.1%), as expected, and Enterobacter spp. (25 isolates, 27.5%), which are less commonly associated with this resistance mechanism.
TABLE 4.
Species and enzyme family distribution of metallo-β-lactamases for Gram-negative pathogens
| Organism (n) | No. of isolates with MBL: |
||
|---|---|---|---|
| VIM (141a) | IMP (37b) | NDM (34c) | |
| C. freundii (6) | 2 | 3 | 1 |
| E. aerogenes (1) | 1 | ||
| Enterobacter asburiae (3) | 2 | 1 | |
| E. cloacae (21) | 11 | 4 | 6 |
| E. coli (5) | 2 | 3 | |
| K. oxytoca (5) | 3 | 1 | 1 |
| K. pneumoniae (36) | 10 | 8 | 18 |
| P. mirabilis (8) | 6 | 1 | 1 |
| Providencia stuartii (4) | 4 | ||
| S. marcescens (2) | 1 | 1 | |
| P. aeruginosa (118) | 101 | 17 | |
| A. baumannii (3) | 3 | ||
Includes 29 isolates with VIM-1; 95 isolates with VIM-2; 7 isolates with VIM-4; 4 isolates with VIM-5; one isolate each with VIM-23, -26, -31, and -32; and two isolates with VIM-type enzymes.
Includes 8 isolates with IMP-1; 3 isolates with IMP-4; 3 isolates with IMP-7; 2 isolates with IMP-14; 8 isolates with IMP-26; 7 isolates with IMP-8; one isolate each with IMP-16, -18, -19, -47, and -48; and one isolate with an IMP-type enzyme.
Includes 32 isolates with NDM-1, 1 isolate with NDM-5, and 1 isolate with NDM-6.
Ninety molecularly characterized Enterobacteriaceae isolates observed to produce β-lactamases of different Ambler classes had different spectra of activity for aztreonam and, to a lesser extent, aztreonam-avibactam (Table 5). All combinations of enzymes from Ambler classes A, B, C, and D were observed. The majority of these characterized isolates produced ESBLs and/or class C enzymes and/or serine carbapenemases. Excluding Enterobacter spp. (six isolates) and S. marcescens (one isolate), which possess chromosomal AmpC β-lactamases, only nine isolates of Enterobacteriaceae expressed a class B enzyme (MBL) alone, highlighting the necessity of utilizing avibactam in combination with aztreonam based upon the MIC values observed in isolates expressing multiple β-lactamases. Similar MIC values for the two drugs were observed among isolates carrying only an MBL and those with additional OSBLs. The 71 isolates that coproduced an MBL and a class A, class C, or class D β-lactamase produced aztreonam and aztreonam-avibactam MIC90 values of >128 μg/ml (MIC range, 0.5 to >128 μg/ml) and 1 μg/ml (MIC range, ≤0.015 to 4 μg/ml), respectively. Of particular note were isolates that expressed a serine carbapenemase in combination with an MBL. There were four isolates that produced both an MBL and a KPC carbapenemase. All four of these isolates were K. pneumoniae isolates that came from Greece and coproduced KPC-2 and VIM-2, among a variety of other β-lactamases, including ESBLs and plasmid-encoded AmpC-type enzymes, and all were inhibited by aztreonam-avibactam with a MIC of 0.5 μg/ml. In addition, three isolates of E. cloacae produced OXA-48 in combination with an MBL. Two isolates had VIM-4 (aztreonam-avibactam MIC, 0.25 μg/ml), and one had VIM-31 (MIC, 1 μg/ml). The two VIM-4 producers also expressed a plasmid-mediated AmpC (CMY-4), and one of them had an additional ESBL (SHV-12).
TABLE 5.
Comparative MICs for 90 MBL-producing Enterobacteriaceae isolates with or without additional β-lactamase enzymes
| Group (n)a | MIC (μg/ml) for drug: |
|||
|---|---|---|---|---|
| Aztreonamb |
Aztreonam-avibactamb |
|||
| MIC90 | MIC range | MIC90 | MIC range | |
| All MBL producers (90) | >128 | ≤0.015 to >128 | 1 | ≤0.015 to 4 |
| MBL only (9)c | ≤0.015 to 1 | ≤0.015 to 0.25 | ||
| MBL + OSBL (10)c | 0.25 | 0.06 to 0.5 | 0.25 | 0.03 to 0.25 |
| MBL + ESBL (8) | 64 to >128 | 0.06 to 0.25 | ||
| MBL + ESBL + OSBL (23) | 128 | 0.5 to >128 | 0.5 | ≤0.015 to 2 |
| MBL + AmpC (14) | 64 | 0.03 to 128 | 2 | ≤0.015 to 4 |
| MBL + AmpC + OSBL (10) | 64 | 0.12 to 64 | 2 | ≤0.015 to 4 |
| MBL + ESBL + AmpC (1) | 32 | 0.03 | ||
| MBL + ESBL + AmpC + OSBL (8) | 16 to >128 | 0.03 to 0.25 | ||
| MBL + KPC (1) | >128 | 0.5 | ||
| MBL + KPC + AmpC + OSBL (1) | >128 | 0.5 | ||
| MBL + KPC + ESBL + AmpC + OSBL (2) | >128 | 0.5 | ||
| MBL + OXA-48 (1) | 2 | 1 | ||
| MBL + OXA-48 + AmpC (1) | 16 | 0.25 | ||
| MBL + OXA-48 + ESBL + AmpC + OSBL (1) | 128 | 0.25 | ||
MBLs include IMP (20 isolates), VIM (40 isolates), and NDM (30 isolates); ESBLs include SHV, TEM, CTX-M, and VEB.
Ranges shown for n < 10 (MIC90, not calculated); individual MICs shown for n = 1.
Does not include species with endogenous AmpC enzymes.
DISCUSSION
MBL-producing pathogens are becoming more widely distributed worldwide and in numerous species of Enterobacteriaceae, including those producing multiple β-lactamase enzymes (10–19). Overall, aztreonam-avibactam had very potent in vitro activity against clinical isolates of Enterobacteriaceae, including those that expressed MBLs. Broad-spectrum activity of aztreonam-avibactam was observed among all species of Enterobacteriaceae tested, several of which encoded multiple β-lactamases. In this study, the activity of aztreonam-avibactam relative to aztreonam alone was particularly increased against Enterobacteriaceae isolates producing Ambler class A, class C, and class D β-lactamases (ESBL, KPC, AmpC, and OXA-48). The addition of avibactam to aztreonam was not able to restore susceptibility in isolates of P. aeruginosa and A. baumannii, suggesting that resistance to aztreonam in these species is primarily driven by other mechanisms.
Investigators have observed findings similar to our results using various β-lactam–avibactam combinations and enzyme-specific profiling analysis (31–36). In these studies, the combination of ceftazidime with avibactam has been shown to provide improved in vitro activity against Enterobacteriaceae isolates which produce ESBL, AmpC, KPC, and OXA-48 enzymes but lacks activity against MBL-producing isolates. Avibactam, in combination with third- and fourth-generation cephalosporins and meropenem against E. coli and K. pneumoniae isolates producing class A and C β-lactamases, provided 100% restoration of activity for the cephalosporins and only modestly improved activity relative to that of meropenem alone. K. pneumoniae isolates producing OXA-48 β-lactamase have had a significant reduction in MIC values with the combination of avibactam with imipenem, cefepime, and ceftazidime. Aztreonam-avibactam demonstrated similar results for OXA-48-producing Enterobacteriaceae isolates tested in our collection which had additional β-lactamases, including ESBL and AmpC enzymes (37).
Country- or species-specific studies have also provided useful data for analyzing the potential role of avibactam. A study from China observed potentiated activity with the addition of avibactam to aztreonam for all Enterobacteriaceae species tested, including those with ESBL phenotypes, stably derepressed AmpC β-lactamases, and IMP and NDM enzymes (36). Limited activity against P. aeruginosa and blaOXA-containing A. baumannii was observed in that study, which was similar to our findings. Avibactam has been shown to be a potent inhibitor of KPC enzymes using enzymatic activity methods in vitro (25). The activity of avibactam in combination with several β-lactams, including aztreonam, has previously been evaluated against a collection of KPC-producing K. pneumoniae isolates with a reduction of resistant MIC values into the susceptible range for all of the tested agents (33). The isolates in our study which had an MBL and a KPC were all inhibited at a concentration of 0.5 μg/ml of aztreonam-avibactam, including isolates with additional AmpC and/or ESBL enzymes. This was a ≥1,028-fold reduction in comparison to the aztreonam MIC tested alone. Murine models of septicemia and thigh infection provided additional in vivo data supporting the activity of avibactam against these highly resistant strains of KPC-producing K. pneumoniae (32).
The continued development of safe and effective β-lactamase inhibitors allows for the continued utility of β-lactams against MDR pathogens that are developing, transferring, and disseminating resistance mechanisms at an alarming rate. Older-generation β-lactam β-lactamase inhibitors are becoming less reliable due to the more aggressive enzymes that are now being produced by contemporary Gram-negative pathogens. Assuredly, resistance mechanisms will continue to develop and resistance rates will increase against the currently defined “drugs of last resort.” The combination of aztreonam plus avibactam should be further explored for its potential role against MDR Gram-negative pathogens, including MBL-expressing Enterobacteriaceae strains, which are becoming problematic on a global scale.
ACKNOWLEDGMENTS
We gratefully acknowledge the contributions of the clinical trial investigators, laboratory personnel, and all members of the AstraZeneca Surveillance program. We also thank the molecular personnel at IHMA for their significant contributions required for the manuscript.
D.J.B., S.K.B., and D.F.S. are employees of International Health Management Associates, Inc. None of the IHMA authors have personal financial interests in the sponsor of this paper (AstraZeneca Pharmaceuticals). P.A.B. is an employee of AstraZeneca.
All authors provided analysis input and have read and approved the final manuscript.
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