What's New In 2024: From the CLSI Subcommittee on Antimicrobial Susceptibility Testing

The Clinical and Laboratory Standards Institute (CLSI) Subcommittee on Antimicrobial Susceptibility Testing held meetings during January and June 2024. Several changes including breakpoint revisions and new breakpoints were introduced by ad hoc working groups (AHWGs) and approved by the full subcommittee.

Phenotypic criteria for recommending carbapenemase testing among Enterobacterales

With multiple β-lactam/β-lactamase inhibitor (BLI) agents targeting specific carbapenemases available clinically, cheaper and easier carbapenemase testing, routine laboratory determination of carbapenemase type could improve clinical care. However, the CLSI’s M100’s Table 2A-1 was previously nonspecific as to which isolates to test for carbapenemase production (eg, “isolates with elevated carbapenem minimal inhibitory concentrations [MIC]”). As such, the phenotypic definition of carbapenem-resistant Enterobacterales (CRE) and laboratory testing recommendations for carbapenemases were discussed over several CLSI meetings, with the goal of creating a CRE definition that was sensitive (ie, would not miss carbapenemase-producing Enterobacterales [CPE]) but was also specific, as to not lead to unnecessary carbapenemase testing in isolates unlikely to have carbapenemases.

To evaluate sensitivity, data from the JMI Laboratories SENTRY Antimicrobial Surveillance Program containing whole genome sequence–confirmed CPE obtained globally were used to determine the percentage of isolates testing resistant to carbapenems. This evaluation determined meropenem MIC was a poor predictor of carbapenemase production, capturing only 64.0% of OXA-48–like carbapenemases and 56.1% of Verona integron–encoded metallo β -lactamase (VIM)–producing isolates (TABLE 1). Meropenem resistance missed 18% of CPE, 10% of Klebsiella pneumoniae carbapenemase (KPC), and 42% of OXA-48. When comparing specific CPE isolates from JMI that underwent testing to ertapenem, imipenem, and meropenem, ertapenem resistance missed only 5.3% of isolates, primarily those with VIM and imipenemase (IMP).¹

To evaluate specificity of the proposed definition, 4 data sets were evaluated: CRACKLE-2 (NCT03646227; unpublished), the Centers for Disease Control and Prevention (CDC) Emerging Infections Program (10.6% of ertapenem-mono-resistant isolates with a carbapenemase), International Health Management Associates² surveillance data comprising of 400 CPE-CRE, and unpublished CRE (n = 21,530) from CDC’s Antimicrobial Resistance Laboratory Network. Conclusions from these various analyses revealed that relying only on meropenem resistance would miss approximately 10% of KPC-producing isolates and more than 40% of OXA-48–like isolates.

Based on these findings, primary language approved for publication in the M100 (35th edition) reads that Enterobacterales “isolates resistant to any carbapenem tested (eg, ertapenem, imipenem, meropenem) except Proteus [species], Providencia [species], or Morganella [species] only resistant to imipenem should undergo carbapenemase testing, using a phenotypic and/ or molecular assay to identify and ideally differentiate the presence of particular carbapenemases (eg, KPC, [New Delhi metallo-β-lactamase 1], OXA-48, VIM, IMP). The decision of testing and reporting is best made by each laboratory in consultation with Antimicrobial Stewardship team and other relevant institutional stakeholders.” Additional details follow in the comment language to be published in the M100 (35th edition).

Cefepime and zidebactam investigational breakpoints
Zidebactam (ZID) is a β-lactam enhancer with a non–β-lactam structure—a bicyclo-acyl-hydrazide chemotype of diazabicyclooctane class.³ Cefepime (FEP)/ZID’s spectrum of activity includes carbapenem-resistant Enterobacterales, carbapenem-resistant Pseudomonas aeruginosa, and carbapenem-resistant Acinetobacter baumannii (CRAB).^4,5 However, FEP (2 g) in combination with ZID (1 g), given as a 60-minute infusion every 8 hours, is currently in phase 3 studies, focused on complicated urinary tract infection and acute pyelonephritis, and therefore does not qualify for formal breakpoints. A phase 2 study in patients with carbapenem-resistant gram-negative infections has begun in India, and a phase 3 study in patients with nosocomial pneumonia is also planned.⁶ Investigational breakpoints were requested by the sponsor (Wockhardt Bio AG) to help facilitate FEP/ZID susceptibility testing for pathogens isolated during compassionate use/expanded access, to aid in analyzing FEP/ZID phase 2/phase 3 results, and to interpret surveillance MIC data.

Investigational susceptibility breakpoints (all ≤ 64/64 µg/mL) were proposed for 3 pathogen groups after review of MIC distributions and MIC₉₀ values: CRE (n = 945; MIC₉₀ = 2-4 µg/mL), carbapenem-resistant P. aeruginosa (n = 1413; MIC₉₀ = 8-16 µg/ mL), and CRAB (n = 1488; MIC₉₀ = 32-64 µg/ mL) using data collected during 4 global SENTRY surveillance studies.^4,5 (TABLE 2). The FEP/ZID MIC method was previously approved by CLSI and the European Committee on Antimicrobial Susceptibility Testing, in which MICs are determined at a 1:1 ratio of FEP and ZID.

Bacterial efficacy of FEP/ZID using a plasma human-simulated regimen in a neutropenic mouse lung⁷ and thigh infections against A baumannii was demonstrated.⁸ Finally, outcomes for 28 patients, many with multidrug-resistant (MDR) P aeruginosa infections, treated with FEP/ZID under compassionate use protocol were evaluated. Almost all patients had evidence of clinical improvement, microbiological cure, or presumed eradication, and survived their hospital stay.⁶

After discussion, the AHWG acknowledged this is an investigational breakpoint, with an area of uncertainty in the range of 32 to 64 µg/mL, especially with P aeruginosa. Additional data from ongoing trials are valuable in determining future breakpoint revisions. Ultimately, proposed susceptibility breakpoints for all 3 pathogens were approved by the full subcommittee during the plenary session.

A baumannii breakpoint updates

Ampicillin/sulbactam
Acinetobacter is a gram-negative, nonmotile bacteria known to cause opportunistic infections.⁹ An AHWG was formed to systematically review all Acinetobacter breakpoints, especially in light of the recent US Food and Drug Administration (FDA) approval of sulbactam-durlobactam (SUL-DUR) in 2023.¹⁰ SUL is a penicillanic acid sulfone BLI with unique intrinsic antibacterial activity against Acinetobacter through inhibition of penicillin-binding protein (PBP)-3 and PBP-1, as ampicillin (AMP) alone does not have in vitro activity against Acinetobacter species.¹¹ As such, dosing recommendations for AMP-SUL against CRAB are based on the SUL component (total daily dose of 6-9 g SUL).¹² AMP-SUL is recommended as first-line treatment of CRAB, as it was the only commercially available formulation in the US until SUL-DUR.^12-15,10

The estimated epidemiological cutoff value (ECOFF/ECV) using data sets (some SUL alone and some as combination (AMP-SUL) with all values converted to AMP-SUL provided an estimated ECOFF/ ECV at 8/4 μg/mL, which supported the current susceptible breakpoint.¹⁶ Humanized SUL dosing in neutropenic mouse infection models (thigh and lung) was used to determine the SUL-specific proportion of time (20%-40%) where the concentration of drug was above MIC (%T > MIC) that resulted in a 1-log₁₀ colony-forming unit reduction in Acinetobacter species.^17-19

Jaruratanasirikul et al used population pharmacokinetic (PK) data and Monte Carlo simulations and found that to achieve both 40% and 60% T > MIC, a SUL dosing regimen of 1 g every 8 hours (given over 4 hours) and 1 g every 6 hours (given over 4 hours) was required for MICs of 4 μg/mL and 8 μg/mL, respectively, for ventilator-associated pneumonia.²⁰ Similar results were described by Setiawan et al, where hospitalized adults required 1 g every 6 to 8 hours (given over 4 hours) to achieve 60% T > MIC to cover MIC of 4 μg/mL.²¹ Multiple PK/pharmacodynamic (PD) studies reported that no dosing regimens were sufficient for SUL MIC greater than 16 μg/mL with low probability of target attainment (PTA).^17,20-23 These data support SUL breakpoints of susceptible at 4 μg/mL and intermediate at 8 μg/mL, allowing for technical variability in MIC.

Clinical data were largely uninformative for breakpoint reevaluation, with no clear correlation between SUL MICs and outcomes. In the end, the susceptible breakpoint (≤ 8/4 μg/mL) for AMP-SUL was not revised but the dose (1 g every 6 hours as an extended infusion given over ≥ 3 hours) on which the breakpoint is based will be added to the 35th edition of the M100 (TABLE 2).

Minocycline is a second-generation tetracycline that binds the 30S ribosomal subunit, inhibiting protein synthesis.²⁴ It is FDA approved for infections caused by Acinetobacter, including CRAB and extensively drug-resistant strains. Approved adult dosing for the intravenous formulation is one 200-mg dose followed by 100 mg every 12 hours, with a maximum dose of 400 mg in 24 hours (eg, 200 mg every 12 hours).²⁵ An estimated ECOFF of 0.5 μg/mL was established using 2013-2022 SENTRY data.^16,26 Available PK/PD data demonstrated adequate antibacterial activity, in vivo efficacy, and PTA values against isolates with MICs less than or equal to 2 μg/mL.^27,28 In addition, the higher-labeled dose of 200 mg every 12 hours is necessary to achieve at least 90% PTA at MICs at least 1 μg/mL. Bacterial growth and poor efficacy (20%-40% survival) were observed at MICs 4 to 32 mg/L. Multiple PK/PD studies reported the ratio of the area under the concentration–time curve to the MIC ( fAUC/ MIC) variation among different models.

Clinical outcomes data demonstrated insufficient evidence to support the current breakpoints. In one study of patients (n = 14) with pneumonia or bacteremia caused by MDR Acinetobacter, clinical failure resulted when isolates had minocycline MICs of 3 to 4 μg/mL (via ETEST, bioMérieux, Marcyl’Etoile, France), even in combination with other therapy.²⁹ Limited data characterizing the tolerability of higher doses are available; however, safety data for nonantimicrobial use have described doses up to 10 mg/kg/ day for 72 hours as safe and well tolerated with a single patient who experienced a dose-limiting hepatic enzyme elevation.^24,30

MIC breakpoints were voted to be reduced to less than or equal to 1 μg/mL (susceptible); 2 μg/mL (intermediate), and greater than or equal to 4 μg/mL (resistant), based on dosing of 200 mg every 12 hours (TABLE 2). Doxycycline and tetracycline breakpoints will be archived on the CLSI website with a note that they are under review.

References

1. JMI Laboratories. Accessed August 3, 2023. https://www.jmilabs.com/

2. IHMA International Health Management Associates. Accessed August 3, 2023. https://www.ihma.com/

3. Papp-Wallace KM, Nguyen NQ, Jacobs MR, et al. Strategic approaches to overcome resistance against gram-negative pathogens using β-lactamase inhibitors and β-lactam enhancers: activity of three novel diazabicyclooctanes WCK 5153, zidebactam (WCK 5107), and WCK 4234. J Med Chem. 2018;61(9):4067-4086. doi:10.1021/acs.jmedchem.8b00091

4. Sader HS, Duncan LR, Arends SJR, Carvalhaes CG, Castanheira M. Antimicrobial activity of aztreonam-avibactam and comparator agents when tested against a large collection of contemporary Stenotrophomonas maltophilia isolates from medical centers worldwide. Antimicrob Agents Chemother. 2020;64(11):e01433-20. doi:10.1128/AAC.01433-20

5. Sader HS, Castanheira M, Huband M, Jones RN, Flamm RK. WCK 5222 (cefepime-zidebactam) antimicrobial activity against clinical isolates of gram-negative bacteria collected worldwide in 2015. Antimicrob Agents Chemother. 2017;61(5):e00072-17. doi:10.1128/AAC.00072-17

6. Meeting minutes. Presented at: Clinical and Laboratory Standards Institute Breakpoint Working Group Meeting; June 23, 2024; Chicago, IL.

7. Avery LM, Abdelraouf K, Nicolau DP. Assessment of the in vivo efficacy of WCK 5222 (cefepime-zidebactam) against carbapenem-resistant Acinetobacter baumannii in the neutropenic murine lung infection model. Antimicrob Agents Chemother. 2018;62(11):e00948-18. doi:10.1128/AAC.00948-18

8. Almarzoky Abuhussain SS, Avery LM, Abdelraouf K, Nicolau DP. In vivo efficacy of humanized WCK 5222 (cefepime-zidebactam) exposures against carbapenem-resistant Acinetobacter baumannii in the neutropenic thigh model. Antimicrob Agents Chemother. 2019;63(1):e01931-18. doi:10.1128/AAC.01931-18

9. Brady MF, Jamal Z, Pervin N. Acinetobacter. In: StatPearls. StatPearls Publishing; 2024. Accessed August 12, 2024. http://www.ncbi.nlm.nih.gov/books/NBK430784/

10. Parkinson J. FDA approves sulbactam-durlobactam for bacterial pneumonia. Contagion Live. May 23, 2023. Accessed August 13, 2024. https://www.contagionlive.com/view/fda-approves-sulbactam-durlobactam

11. Papp-Wallace KM, Senkfor B, Gatta J, et al. Early insights into the interactions of different β-lactam antibiotics and β-lactamase inhibitors against soluble forms of Acinetobacter baumannii PBP1a and Acinetobacter sp. PBP3. Antimicrob Agents Chemother. 2012;56(11):5687-5692. doi:10.1128/AAC.01027-12

12. Tamma PD, Aitken SL, Bonomo RA, Mathers AJ, van Duin D, Clancy CJ. Infectious Diseases Society of America guidance on the treatment of AmpC β-lactamase–producing Enterobacterales, carbapenem-resistant Acinetobacter baumannii, and Stenotrophomonas maltophilia infections. Clin Infect Dis. 2022;74(12):2089-2114. doi:10.1093/cid/ciab1013

13. Levin AS. Multiresistant Acinetobacter infections: a role for sulbactam combinations in overcoming an emerging worldwide problem. Clin Microbiol Infect. 2002;8(3):144-153. doi:10.1046/j.1469-0691.2002.00415.x

14. Durand-Réville TF, Guler S, Comita-Prevoir J, et al. ETX2514 is a broad-spectrum β-lactamase inhibitor for the treatment of drug-resistant gram-negative bacteria including Acinetobacter baumannii. Nat Microbiol. 2017;2:17104. doi:10.1038/nmicrobiol.2017.104

15. Paul M, Carrara E, Retamar P, et al. European Society of Clinical Microbiology and Infectious Diseases (ESCMID) guidelines for the treatment of infections caused by multidrug-resistant Gram-negative bacilli (endorsed by European society of intensive care medicine). Clin Microbiol Infect. 2022;28(4):521-547. doi:10.1016/j.cmi.2021.11.025

16. ECOFFinder. Clinical and Laboratory Standards Institute. Accessed August 12, 2024. https://clsi.org/meetings/susceptibility-testing-subcommittees/ecoffinder/

17. Yokoyama Y, Matsumoto K, Ikawa K, Watanabe E, Morikawa N, Takeda Y. Population pharmacokinetic-pharmacodynamic target attainment analysis of sulbactam in patients with impaired renal function: dosing considerations for Acinetobacter baumannii infections. J Infect Chemother. 2015;21(4):284-289. doi:10.1016/j.jiac.2014.12.005

18. Housman ST, Hagihara M, Nicolau DP, Kuti JL. In vitro pharmacodynamics of human-simulated exposures of ampicillin/sulbactam, doripenem and tigecycline alone and in combination against multidrug-resistant Acinetobacter baumannii. J Antimicrob Chemother. 2013;68(10):2296-2304. doi:10.1093/jac/dkt197

19. O’Donnell JP, Bhavnani SM. The pharmacokinetics/pharmacodynamic relationship of durlobactam in combination with sulbactam in in vitro and in vivo infection model systems versus Acinetobacter baumannii-calcoaceticus complex. Clin Infect Dis. 2023;76(suppl 2):S202-S209. doi:10.1093/cid/ciad096

20. Jaruratanasirikul S, Wongpoowarak W, Wattanavijitkul T, et al. Population pharmacokinetics and pharmacodynamics modeling to optimize dosage regimens of sulbactam in critically ill patients with severe sepsis caused by Acinetobacter baumannii. Antimicrob Agents Chemother. 2016;60(12):7236-7244. doi:10.1128/AAC.01669-16

21. Setiawan E, Cotta MO, Abdul-Aziz MH, et al. Population pharmacokinetics and dosing simulations of ampicillin and sulbactam in hospitalised adult patients. Clin Pharmacokinet. 2023;62(4):573-586. doi:10.1007/s40262-023-01219-5

22. Onita T, Ikawa K, Ishihara N, et al. Pulmonary pharmacokinetic and pharmacodynamic evaluation of ampicillin/sulbactam regimens for pneumonia caused by various bacteria, including Acinetobacter baumannii. Antibiotics. 2023;12(2):303. doi:10.3390/antibiotics12020303

23. Onita T, Ikawa K, Nakamura K, et al. Prostatic pharmacokinetic/pharmacodynamic evaluation of ampicillin-sulbactam for bacterial prostatitis and preoperative prophylaxis. J Clin Pharmacol. 2021;61(6):820-831. doi:10.1002/jcph.1800

24. Ritchie DJ, Garavaglia-Wilson A. A review of intravenous minocycline for treatment of multidrug-resistant Acinetobacter infections. Clin Infect Dis. 2014;59(suppl 6):S374-380. doi:10.1093/cid/ciu613

25. Minocin. Prescribing information. Rempex Pharmaceuticals; 2023. Accessed August 12, 2024. https://www.accessdata.fda.gov/drugsatfda_docs/label/2015/050444s049lbl.pdf

26. Flamm RK, Shortridge D, Castanheira M, Sader HS, Pfaller MA. In vitro activity of minocycline against U.S. isolates of Acinetobacter baumannii-Acinetobacter calcoaceticus species complex, Stenotrophomonas maltophilia, and Burkholderia cepacia complex: results from the SENTRY Antimicrobial Surveillance Program, 2014 to 2018. Antimicrob Agents Chemother. 2019;63(11):e01154-19. doi:10.1128/AAC.01154-19

27. Lepak AJ, Trang M, Hammel JP, et al. Development of modernized Acinetobacter baumannii susceptibility test interpretive criteria for recommended antimicrobial agents using pharmacometric approaches. Antimicrob Agents Chemother. 2023;67(4):e0145222. doi:10.1128/aac.01452-22

28. Lodise TP, Van Wart S, Sund ZM, et al. Pharmacokinetic and pharmacodynamic profiling of minocycline for injection following a single infusion in critically ill adults in a phase IV open-label multicenter study (ACUMIN). Antimicrob Agents Chemother. 2021;65(3):e01809-20. doi:10.1128/AAC.01809-20

29. Goff DA, Bauer KA, Mangino JE. Bad bugs need old drugs: a stewardship program’s evaluation of minocycline for multidrug-resistant Acinetobacter baumannii infections. Clin Infect Dis. 2014;59(suppl 6):S381-S387. doi:10.1093/cid/ciu593

30. Fagan SC, Waller JL, Nichols FT, et al. Minocycline to improve neurologic outcome in stroke (MINOS): a dose-finding study. Stroke. 2010;41(10):2283-2287. doi:10.1161/STROKEAHA.110.582601