In vitro activities of cefiderocol, ceftolozane/tazobactam, ceftazidime/avibactam and other comparative drugs against imipenem-resistant Pseudomonas aeruginosa and Acinetobacter baumannii, and Stenotrophomonas maltophilia, all associated with bloodstream infections in Taiwan

Abstract

Objectives

We investigated the in vitro activities of cefiderocol, ceftazidime/avibactam, ceftolozane/tazobactam and other related drugs against imipenem-resistant Pseudomonas aeruginosa, imipenem-resistant Acinetobacter baumannii and Stenotrophomonas maltophilia isolates.

Methods

Non-duplicated bacteraemia isolates (n = 300) of imipenem-resistant P. aeruginosa (n = 100), imipenem-resistant A. baumannii (n = 100) and S. maltophilia (n = 100) were evaluated. Imipenem-resistant P. aeruginosa and imipenem-resistant A. baumannii isolates were defined as isolates exhibiting imipenem MIC ≥8 mg/L, as determined using the VITEK 2 system. The MICs of 11 other antimicrobial agents for the isolates were determined by the broth microdilution method. Iron-depleted CAMHB was used to determine the MICs of cefiderocol.

Results

The rates of colistin resistance of imipenem-resistant P. aeruginosa and imipenem-resistant A. baumannii were 5% and 10%, respectively. The MIC₉₀ values of cefiderocol, ceftolozane/tazobactam, ceftazidime/avibactam, tigecycline and colistin were as follows: imipenem-resistant P. aeruginosa: 1, 4, 16, >4 and 2 mg/L; imipenem-resistant A. baumannii: 8, >64, >64, 4 and 2 mg/L; and S. maltophilia: 0.25, >64, >64, 2 and >8 mg/L, respectively. For imipenem-resistant A. baumannii isolates, the MICs of cefiderocol, ceftolozane/tazobactam and ceftazidime/avibactam were ≤4 mg/L for 88%, 8% and 1% of the isolates, respectively. Cefiderocol MICs were ≤4 mg/L for the five colistin-resistant imipenem-resistant P. aeruginosa isolates and 70% of the 10 colistin-resistant imipenem-resistant A. baumannii isolates.

Conclusions

Cefiderocol exhibited more potent in vitro activity than ceftolozane/tazobactam and ceftazidime/avibactam against imipenem-resistant P. aeruginosa, imipenem-resistant A. baumannii and S. maltophilia isolates.

Introduction

Healthcare-associated infections caused by MDR non-fermentative Gram-negative bacteria (GNB), including Pseudomonas aeruginosa, Acinetobacter baumannii and Stenotrophomonas maltophilia, have steadily increased and are associated with high morbidity and mortality rates.^1–5 Besides last-resort antibiotics, such as tigecycline and colistin, new agents have recently been considered important therapeutic options for the treatment of MDR-GNB infections.⁶

Both ceftolozane/tazobactam and ceftazidime/avibactam have shown potent in vitro activity against P. aeruginosa, including meropenem-resistant isolates, except for class B carbapenemase producers, but poor activity against A. baumannii and S. maltophilia.^6–8

Recent studies on cefiderocol, a novel siderophore cephalosporin, have shown that this agent exhibits potent antimicrobial activities in vitro against meropenem- and colistin-resistant P. aeruginosa and A. baumannii (including those harbouring VIM, IMP, OXA-23, OXA-40 and OXA-58) and S. maltophilia.^9–12

This study investigated the in vitro activities of cefiderocol, ceftolozane/tazobactam, ceftazidime/avibactam and eight related drugs against imipenem-resistant P. aeruginosa, imipenem-resistant A. baumannii and S. maltophilia isolates.

Methods

Bacterial isolates

We analysed 300 consecutive and non-duplicated isolates of imipenem-resistant P. aeruginosa (n = 100), imipenem-resistant A. baumannii (n = 100) and S. maltophilia (n = 100). The samples were recovered from patients with bacteraemia who were treated at the National Taiwan University Hospital (NTUH), a 2900 bed tertiary care medical centre, from January 2016 to June 2017.

P. aeruginosa, A. baumannii complex and S. maltophilia isolates were identified and their susceptibilities to imipenem were determined using the VITEK 2 system (bioMérieux, Marcy-l’Étoile, France). MALDI-TOF MS (Bruker Biotyper^TM, Bruker Daltonik GmbH, Bremen, Germany) was performed to separate A. baumannii from the A. baumannii complex.¹³ Imipenem-resistant P. aeruginosa and imipenem-resistant A. baumannii isolates were defined as isolates with imipenem MIC ≥8 mg/L.¹⁴ This study was approved by the institutional review board of the hospital (201704021RSB) and the requirement for informed consent from each patient was waived.

Antimicrobial susceptibility testing

Frozen broth microdilution plates, prepared by International Health Management Associates (IHMA; Schaumburg, IL, USA), were used to determine the MICs of the 11 agents, including cefiderocol (0.03–64 mg/L), ceftolozane/tazobactam (0.03–64 and 4 mg/L, respectively) and ceftazidime/avibactam (0.03–64 and 4 mg/L, respectively) (Table 1).^10,11 To determine the MICs of cefiderocol, iron-depleted CAMHB (ID-CAMHB) was used,¹¹ whereas other drugs were tested in CAMHB according to the current CLSI guidelines.¹⁴ The MIC of cefiderocol was determined as the first panel well in which the isolate growth was significantly reduced (i.e. a button of <1 mm in diameter or light/faint turbidity) relative to the strong growth observed in the ID-CAMHB control growth well (i.e. a colony of ≥2 mm in diameter).¹¹ The MICs of other antimicrobial agents were defined as the lowest concentration that inhibited the visible growth of the microorganism.¹⁴Escherichia coli ATCC 25922 and P. aeruginosa ATCC 27853 were the quality control strains used on each testing day.

Table 1.

In vitro activities of cefiderocol (S-649266), ceftazidime/avibactam, ceftolozane/tazobactam and other related drugs against imipenem-resistant Pseudomonas aeruginosa, imipenem-resistant Acinetobacter baumannii and Stenotrophomonas maltophilia associated with bloodstream infections

Species (no. of isolates)	MIC (mg/L)			No. (%) of isolates with indicated susceptibility categorya
	range	MIC50	MIC90	S	I	R
P. aeruginosa (n = 100)
cefiderocol	≤0.03–8	0.12	1	NA	NA	NA
ceftazidime	0.5 to >64	8	64	54 (54)	12 (12)	34 (34)
cefepime	1 to >16	8	>16	67 (67)	18 (18)	15 (15)
ceftolozane/tazobactam	0.25–32	0.5	4	95 (95)	4 (4)	1 (1)
ceftazidime/avibactam	1 to >64	4	16	83 (83)	0 (0)	17 (17)
meropenem	0.25 to >64	8	32	25 (25)	19 (19)	56 (56)
aztreonam	1 to >32	16	>32	44 (44)	19 (19)	37 (37)
ciprofloxacin	≤0.25 to >4	0.5	>4	57 (57)	11 (11)	32 (32)
amikacin	≤4–32	4	8	97 (97)	3 (3)	0 (0)
tigecycline	≤0.25 to >4	>4	>4	NA	NA	NA
colistin	≤0.5 to >8	1	2	95 (95)	0 (0)	5 (5)
Meropenem-susceptible P. aeruginosa (MICs ≤2 mg/L, n = 25)
cefiderocol	≤0.03–2	0.06	0.5	NA	NA	NA
ceftazidime	0.5–32	2	32	21 (84)	1 (4)	3 (12)
cefepime	1–16	4	8	24 (96)	1 (4)	0 (0)
ceftolozane/tazobactam	0.25–32	0.5	1	24 (96)	0 (0)	1 (4)
ceftazidime/avibactam	1–8	2	8	25 (100)	0 (0)	0 (0)
meropenem	0.25–2	2	2	25 (100)	0 (0)	0 (0)
aztreonam	1–16	8	16	19 (76)	6 (24)	0 (0)
ciprofloxacin	≤0.25 to >4	0.25	>4	14 (56)	4 (16)	7 (28)
amikacin	≤4–32	4	8	24 (96)	1 (4)	0 (0)
tigecycline	≤0.25 to >4	4	>4	NA	NA	NA
colistin	≤0.5–4	1	2	23 (92)	0 (0)	2 (8)
Meropenem-resistant P. aeruginosa (MICs ≥4 mg/L, n = 75)
cefiderocol	≤0.03–8	0.12	2	NA	NA	NA
ceftazidime	1 to >64	16	>64	33 (44)	11 (14.7)	31 (41.3)
cefepime	1 to >16	8	>16	43 (57.3)	17 (22.7)	15 (20.0)
ceftolozane/tazobactam	0.25–8	1	4	71 (94.7)	4 (5.3)	0 (0)
ceftazidime/avibactam	1 to >64	4	32	58 (77.3)	0 (0)	17 (22.7)
meropenem	4 to >64	8	32	0 (0)	19 (25.3)	56 (74.7)
aztreonam	1 to >32	16	>32	25 (33.3)	13 (17.3)	37 (49.3)
ciprofloxacin	≤0.25 to >4	0.5	>4	43 (57.3)	7 (9.3)	25 (33.3)
amikacin	≤4–32	4	8	73 (97.3)	2 (2.7)	0 (0)
tigecycline	0.5 to >4	>4	>4	NA	NA	NA
colistin	≤0.5 to >8	1	2	72 (96)	0 (0)	3 (4)
A. baumannii (n = 100)
cefiderocol	0.06 to >64	0.5	8	NA	NA	NA
ceftazidime	8 to >64	>64	>64	14 (14)	2 (2)	84 (84)
cefepime	4 to >16	>16	>16	3 (3)	6 (6)	91 (91)
ceftolozane/tazobactam	2 to >64	32	>64	NA	NA	NA
ceftazidime/avibactam	2 to >64	64	>64	NA	NA	NA
meropenem	8 to >64	>64	>64	0 (0)	0 (0)	100 (100)
aztreonam	4 to >32	>32	>32	NA	NA	NA
ciprofloxacin	≤0.25 to >4	>4	>4	6 (6)	1 (1)	93 (93)
amikacin	≤4 to >64	>64	>64	19 (19)	10 (10)	71 (71)
tigecycline	≤0.25 to >4	1	4	NA	NA	NA
colistin	≤0.5 to >8	1	2	90 (90)	0 (0)	10 (10)
S. maltophilia (n = 100)
cefiderocol	≤0.03–1	0.06	0.25	NA	NA	NA
ceftazidime	1 to >64	32	>64	15 (15)	17 (17)	68 (68)
cefepime	4 to >16	>16	>16	NA	NA	NA
ceftolozane/tazobactam	0.5 to >64	32	>64	NA	NA	NA
ceftazidime/avibactam	1 to >64	16	>64	NA	NA	NA
meropenem	8 to >64	>64	>64	NA	NA	NA
aztreonam	8 to >32	32	>32	NA	NA	NA
ciprofloxacin	1 to >4	4	>4	NA	NA	NA
amikacin	≤4 to >64	>64	>64	NA	NA	NA
tigecycline	≤0.25–4	≤0.25	2	NA	NA	NA
colistin	≤0.5 to >8	>8	>8	NA	NA	NA

S, susceptible; I, intermediate; R, resistant; NA, not applicable.

a

Susceptibilities of the isolates to the antimicrobial agents tested were determined according to the MIC interpretive breakpoints recommended by the CLSI.¹⁴

The susceptibility of imipenem-resistant P. aeruginosa, imipenem-resistant A. baumannii and S. maltophilia to the antimicrobial agents tested was determined according to the MIC interpretive breakpoints recommended by the CLSI.¹⁴ For cefiderocol and tigecycline, there were no MIC interpretive breakpoints recommended by the CLSI, EUCAST or US FDA for the three bacterial species tested.^14,15 For ceftazidime/avibactam, MIC breakpoints ≤8/4 mg/L were available for P. aeruginosa (but not for A. baumannii or S. maltophilia) as recommended by the CLSI, EUCAST and US FDA.^14,15 For A. baumannii, there were no CLSI MIC breakpoints for ceftolozane/tazobactam and aztreonam.¹⁴ Among the 11 agents tested against S. maltophilia isolates, CLSI MIC breakpoints were available only for ceftazidime (susceptible, MIC ≤8 mg/L; intermediate, 16 mg/L; and resistant, ≥32 mg/L).¹⁴

Results

The MIC ranges of cefiderocol, ceftolozane/tazobactam and ceftazidime/avibactam for E. coli ATCC 25922 were 0.25–0.5, 0.12–0.25 and 0.12–0.25 mg/L, respectively, whereas those for P. aeruginosa ATCC 27853 were 0.12–0.25, 0.25–0.5 and 1–4 mg/L, respectively. The MICs of the 11 agents tested against the two control strains (E. coli ATCC 25922 and P. aeruginosa ATCC 27853) were all within the MIC ranges recommended by the CLSI.^14,16

For the 100 imipenem-resistant P. aeruginosa isolates, 75% were not susceptible to meropenem (MICs ≥4 mg/L) and ≥95% of the isolates were susceptible to ceftolozane/tazobactam, amikacin and colistin. Cefiderocol and ceftazidime/avibactam MICs were ≤4 mg/L for 99% and 62% of these imipenem-resistant P. aeruginosa isolates, respectively (Figure 1a). Cefiderocol MICs were ≤1 mg/L (0.03, 0.06, 0.12 and 1 mg/L, respectively) for four and MIC = 4 mg/L for one of the five colistin-resistant imipenem-resistant P. aeruginosa isolates. The MIC of cefiderocol was 8 mg/L for only one imipenem-resistant P. aeruginosa isolate, for which the MICs of ceftolozane/tazobactam and ceftazidime/avibactam were 4 and 32 mg/L, respectively. For imipenem-resistant P. aeruginosa isolates, 83% were susceptible to ceftazidime/avibactam (Figure 1a). Ceftazidime/avibactam-resistant P. aeruginosa isolates were only found among meropenem-resistant and imipenem-resistant P. aeruginosa isolates (22.7%) (Table 1). The MIC₉₀ of ceftazidime/avibactam (16 mg/L) for imipenem-resistant P. aeruginosa isolates was 4-fold lower than that of ceftazidime alone (64 mg/L).

Figure 1.

Distribution and cumulative percentages of isolates susceptible to cefiderocol, ceftolozane/tazobactam and ceftazidime/avibactam for (a) imipenem-resistant Pseudomonas aeruginosa (n = 100), (b) imipenem-resistant Acinetobacter baumannii (all were meropenem-resistant) (n = 100) and (c) Stenotrophomonas maltophilia (n = 100) associated with bloodstream infections.

All imipenem-resistant A. baumannii isolates were resistant to meropenem (MIC ≥8 mg/L). Cefiderocol, ceftolozane/tazobactam and ceftazidime/avibactam MICs were ≤4 mg/L for 88%, 8% and 1% of these isolates, respectively (Figure 1b). Cefiderocol MICs were ≤4 mg/L for 62 (83.8%) of the 74 imipenem-resistant A. baumannii isolates that were not susceptible to ceftazidime, cefepime, meropenem, ciprofloxacin and amikacin. Cefiderocol MICs were ≤4 mg/L for 70% of the 10 colistin-resistant imipenem-resistant A. baumannii isolates (Figure 2). Additionally, the cefiderocol MIC was ≤4 mg/L in four (57.1%) of the seven imipenem-resistant A. baumannii isolates, which were resistant to ceftazidime, cefepime, ciprofloxacin, amikacin and colistin. The MICs of ceftolozane/tazobactam and ceftazidime/avibactam were both ≥64 mg/L for the 12 imipenem-resistant A. baumannii isolates, with cefiderocol MIC ≥8 mg/L. The MIC₉₀ of ceftazidime/avibactam for imipenem-resistant A. baumannii isolates (>64 mg/L) was the same as that of ceftazidime alone (>64 mg/L).

Figure 2.

Proportions of carbapenem (imipenem and meropenem)-resistant A. baumannii isolates with cefiderocol MIC ≤4 mg/L among the isolates with different resistance (including intermediate and resistant isolates) patterns to ceftazidime (CAZ), cefepime (FEP), ciprofloxacin (CIP), amikacin (AMK) and colistin (CST).

Among the 100 S. maltophilia isolates, 15% were susceptible to ceftazidime. In addition, the MIC₉₀ values of tigecycline and colistin were 2 and >8 mg/L, respectively. Cefiderocol MICs were ≤1 mg/L for all isolates. Ceftolozane/tazobactam MICs were ≤4 mg/L (MIC₅₀ = 32 mg/L and MIC₉₀ >64 mg/L) for only 21% of the isolates, whereas ceftazidime/avibactam MICs were ≤4 mg/L (MIC₅₀ = 16 mg/L and MIC₉₀ >64 mg/L) for only 14% of the isolates (Figure 1c). The MIC₉₀ of ceftazidime/avibactam for S. maltophilia isolates (>64 mg/L) was similar to that of ceftazidime alone (>64 mg/L).

Discussion

Cefiderocol is currently in late-stage clinical development for the treatment of complicated urinary tract infections, nosocomial pneumonia and carbapenem-resistant GNB infections.^6,9–11 In this study, susceptibility to cefiderocol was determined in terms of the percentages of isolates with MICs ≤4 mg/L based on the pharmacokinetic and Monte Carlo simulation studies, showing that cefiderocol has a 90% probability of achieving 75% fT_>MIC for an organism with an MIC of 4 mg/L.¹⁷ In addition, cefiderocol possesses bactericidal properties and efficacy against isolates of Enterobacteriaceae, P. aeruginosa, A. baumannii and S. maltophilia in animals (MICs ≤4 mg/L).^11,17,18

The results of a SIDERO-WT-2014 study revealed that the MIC₉₀ values of cefiderocol for meropenem-resistant P. aeruginosa (meropenem MIC ≥4 mg/L) were 0.5 and 1 mg/L for the isolates obtained from North America (n = 151) and Europe (n = 202), respectively.¹⁰ The MIC₉₀ of cefiderocol for meropenem-resistant A. baumannii was 1 mg/L for the isolates obtained from both North America (n = 136) and Europe (n = 244).¹⁰ The MICs of cefiderocol for all meropenem-resistant P. aeruginosa isolates and 99% (376/380) of meropenem-resistant A. baumannii isolates obtained from both regions were ≤4 mg/L.¹⁰ The MIC₉₀ values of cefiderocol for S. maltophilia were 0.5 and 0.25 mg/L for the isolates obtained from North America (n = 152) and Europe (n = 276), respectively.¹⁰

In a recent study conducted by Hackel et al.,¹¹ cefiderocol at a concentration of ≤4 mg/L inhibited 99.2% (260/262) of MDR P. aeruginosa isolates, 89.7% (330/368) of MDR A. baumannii isolates and all (217/217) of the S. maltophilia isolates. Importantly, several isolates with cefiderocol MICs of 32 and >256 mg/L for MDR A. baumannii and MDR P. aeruginosa, respectively, were found.¹¹ Our findings on the susceptibility to cefiderocol of meropenem-resistant P. aeruginosa, imipenem-resistant A. baumannii and S. maltophilia to cefiderocol were partly in agreement with those of recent large-scale studies.^10–12 In this study, cefiderocol MICs >4 mg/L were found in 12 imipenem-resistant A. baumannii isolates.

The overall susceptibility of P. aeruginosa to ceftazidime/avibactam was also improved relative to ceftazidime alone.^6,9 However, P. aeruginosa isolates that were not susceptible to ceftazidime and carbapenems showed varying susceptibility to ceftazidime/avibactam, with resistance approaching 50% in some studies.^6,9 Ceftolozane/tazobactam showed potent in vitro activity against P. aeruginosa isolates with MIC₉₀ values ≤4 mg/L.⁹ In a large European collection, 86.3% of the isolates were inhibited at ≤8 mg/L, although subsequent studies have shown greater variation in the susceptibility of MDR-GNB pathogens.^6,9 The susceptibility of meropenem-resistant or MDR A. baumannii and S. maltophilia isolates to ceftolozane/tazobactam and ceftazidime/avibactam was generally poor at an MIC₉₀ ≥64 mg/L, which was in accordance with our results.^6,9

The mechanism(s) responsible for the high MICs of cefiderocol (MIC >4 mg/L) for imipenem-resistant P. aeruginosa and imipenem-resistant A. baumannii isolates have been investigated but have not been clarified.^11,19 The mechanism of resistance to cefiderocol was hypothesized to involve either a decrease in the production of iron transport system components or mutations in the binding site for the iron transport system on the outer membrane of GNB.^11,19 A deficiency of the iron transporter PiuA in P. aeruginosa resulted in a 16-fold increase in cefiderocol MIC.²⁰ The overproduction of the efflux pump MexA-MexB-OprM in P. aeruginosa has been demonstrated to have no significant impact on the activity of cefiderocol.²⁰ The adaptation-based resistance to other siderophore-conjugated antibacterial agents, such as MB-1, has been observed for some P. aeruginosa isolates in murine thigh infection models, but such adaptation-based resistance to cefiderocol has not been observed.^11,17,19

The current study has several limitations. First, the susceptibility of these isolates to imipenem was not determined using the same susceptibility testing plates. Incorporation of imipenem in the same broth microdilution susceptibility plates would ensure no occurrence of isolate mix-up and confirm the accuracy of VITEK 2 MICs. Second, molecular biomarkers in the 12 imipenem-resistant A. baumannii isolates with high cefiderocol MICs (>4 mg/L) were not explored. Finally, this was a single-centre study conducted in Taiwan. The generalizability of the study findings to other centres and regions where the circulating MDR-GNB, particularly A. baumannii strains, might differ requires further confirmation.

In conclusion, besides colistin, cefiderocol showed the most potent in vitro activity among the other related agents, including ceftolozane/tazobactam and ceftazidime/avibactam, against a recent collection of bacteraemia-associated isolates of imipenem-resistant P. aeruginosa, imipenem-resistant A. baumannii and S. maltophilia.

Funding

This work was supported by Shionogi & Co., Ltd.

Transparency declarations

None to declare.

Author notes

Chun-Hsing Liao and Po-Ren Hsueh authors contributed equally.