Updates include novel and revised breakpoints for azithromycin, polymyxins, daptomycin, and cefiderocol.
The Clinical and Laboratory Standards Institute (CLSI) Subcommittee on Antimicrobial Susceptibility Testing (AST) held its 2019 meetings in January and June, introducing several new and revised breakpoints. Although the subcommittee approved certain breakpoints in June 2019, they are subject to change prior to official approval of the meeting minutes at the January 2020 meeting.
Azithromycin for Neisseria gonorrhoeae
Current recommendations from the US Centers for Disease Control and Prevention and the World Health Organization support combination therapy with ceftriaxone (250 mg, intramuscular) plus azithromycin (1 g, oral) for the treatment of uncomplicated gonorrhea.1 The hope for dual therapy of agents with different mechanisms of action is to slow the emergence and spread of resistance to ceftriaxone, as resistance of N gonorrhoeae to other antimicrobial classes (eg, fluoroquinolones) has been documented in recent years.1,2 Azithromycin minimal inhibitory concentration (MIC) distribution data and clinical efficacy data were reviewed at the June 2018 meeting, and a “susceptible-only” breakpoint of ≤1 mg/L was justified by the wild-type susceptibility distribution and lack of sufficient clinical efficacy data for isolates with MICs between 2 and 8 mg/L. This new breakpoint was subsequently published in the 2019 (29th edition) of the M100 document (Table 1).3 A comment that azithromycin use is required as part of a combination regimen was also included.
Polymyxins (polymyxin B and colistin)
New antibiotics are not available in many parts of the world, so polymyxins are often last-resort agents used to treat multidrug-resistant pathogens despite poor clinical outcomes after their use in many studies.4,5 Even though newer, more effective antibiotics are available in the United States, recent estimated sales data reveal that colistin use is still significantly higher than that of the newer (ie, ceftazidime/avibactam, meropenem/vaborbactam, plazomicin) and more expensive branded agents.6
Previously, the CLSI had polymyxin B breakpoints for Pseudomonas aeruginosa but no interpretive criteria for Enterobacteriaceae. The CLSI’s “silence” on Enterobacteriaceae—no current breakpoint and only an epidemiological cutoff value (ECV)—was thought not to be in patients’ best interests because ECVs cannot be used by hospital laboratories nor published as clinical breakpoints. After a review of pharmacokinetic/pharmacodynamic (PK/ PD), outcomes and MIC distribution data, the CLSI determined that a breakpoint could not be <2 mg/L, as this would cut into the wild-type distribution of Enterobacteriaceae. Polymyxin B and colistin breakpoints were ultimately set for Enterobacteriaceae, P aeruginosa, and Acinetobacter spp at ≤2 mg/L (intermediate) and ≥4 mg/L (resistant), with no susceptible category. Rationale for an intermediate-only breakpoint suggests a black box warning that these agents are of limited clinical efficacy even for isolates with MIC values <2 mg/L. The subcommittee also suggested that several warnings/ comments be added to polymyxin B/colistin breakpoints: (1) “Clinical and PK/PD data demonstrate this agent is of limited clinical efficacy”; (2) “If available, alternative [nonpolymyxin] agents are strongly preferred. If these agents are not available, this breakpoint presumes use of colistin in combination with  or more additional, active antimicrobials”; (3) “Colistin (methanesulfonate) should be given with a loading dose and maximum renally adjusted doses”; (4) “When given systemically, this drug is unlikely to be effective for pneumonia.” The committee approved the breakpoints at the June 2019 meeting, with official publication set for the next M100 document.
Daptomycin for Enterococcus spp
Daptomycin is approved by the US Food and Drug Administration (FDA) for vancomycin-susceptible Enterococcus faecalis. However, clinicians frequently use it in the treatment of vancomycin-resistant Enterococcus faecium, for which treatment options are lacking. In June 2018, the AST Subcommittee revised the daptomycin breakpoints for Enterococcus spp to include a range of MICs (2-4 mg/L) that would be considered “susceptible-dose dependent” (SDD). The AST Subcommittee made the approval after reviewing several studies that found worse clinical outcomes with daptomycin MICs of 3 to 4 mg/L compared with MICs ≤2 mg/L when treating patients with vancomycin-resistant Enterococcus bacteremia.7 More recently, at the January 2019 meeting, the AST Subcommittee revisited this issue and reviewed unpublished microbiological data from a 3-center study showing a difference in susceptibility among Enterococcus spp. Revised daptomycin breakpoints were approved to separate E faecium and other Enterococcus spp in an effort to more accurately reflect the susceptibility differences between E faecium and other Enterococcus spp. A comment to be included with the breakpoints for E faecium states, “The SDD category is based on a dosage regimen of 8-12 mg/kg/day in adults and is intended for serious infections due to Enterococcus faecium. Consultation with an infectious diseases specialist is recommended.” Similarly, the AST Subcommittee approved the new daptomycin breakpoints for other Enterococcus spp (Table 13), with a comment that these breakpoints are based on a regimen of 6 mg/kg/mL.
Cefiderocol disk diffusion breakpoints
Cefiderocol is a novel cephalosporin not yet approved by the FDA. This agent has demonstrated in vitro activity against gram-negative bacteria, including carbapenem-resistant Enterobacteriaceae, P aeruginosa, Acinetobacter spp, and Stenotrophomonas maltophilia.8 At the June 2018 meeting, the AST Subcommittee approved cefiderocol MIC breakpoints based on microbiological, PK/PD, and clinical data from the sponsor.9 At the January 2019 meeting, the sponsor presented data for MIC/disk correlation studies and proposed disk diffusion breakpoints. These breakpoints for cefiderocol and Enterobacteriaceae, P aeruginosa, Acinetobacter spp, and S maltophilia (Table 2) were approved at the January 2019 meeting and confirmed at the June 2019 meeting following review of reproducibility data. It was also proposed that the subcommittee continue to monitor these interpretive criteria as laboratories begin to test the drug.
Fosfomycin susceptibility testing
Fosfomycin susceptibility testing issues have been a topic of discussion among a fosfomycin ad hoc working group (AHWG). Prompted by a recent publication, the AHWG was originally tasked with making a recommendation on colonies within the zone of inhibition for interpreting disk diffusion testing against Escherichia coli.10 Ultimately, the AHWG recommended leaving the current CLSI recommendation as is, without additional comments about inner colonies, until further data are available.
In 2019, the AHWG evaluated unpublished data presented by Eric Wenzler, PharmD, BCPS, BCIDP, AAHIVP, and colleagues that questioned whether the current recommendation of addition of glucose 6-phosphate (G6P) for in vitro susceptibility testing is physiologically accurate, as urine appears to be devoid of G6P.11 The Enterobacteriaceae organisms, E coli and Klebsiella pneumoniae, both use 2 nutrient transport uptake systems, the glycerol 3-phosphate transporter and a hexose phosphate transporter (UhpT), to bring fosfomycin into the bacterial cell.12 Because the UhpT system is induced by its substrate, G6P, in vitro testing with G6P supplementation may not accurately reflect in vivo conditions if the urine is devoid of G6P where the site of action is expected for cystitis. Other recent PK/PD and clinical outcomes data also highlight variability of oral fosfomycin concentrations in the urine and unexpectedly high failure rates in randomized clinical trial settings.13-15 The data presented by Wenzler evaluated agar dilution MIC testing and urinary bactericidal titers (UBTs) with and without G6P. MICs without G6P were universally higher compared with those supplemented with G6P, whereas UBTs without added G6P were much lower compared with UBT with G6P. After discussion, the subcommittee advised retaining G6P until further data are available and also recommended revisiting broth microdilution testing for fosfomycin.
Colistin testing methods
Because of difficulty in polymyxin susceptibility testing, novel methods for testing have recently been explored.16,17 Data comparing 2 new methods of colistin susceptibility testing, colistin disk broth elution (CBDE) and colistin agar test (CAT), were presented and subsequently approved as provisional methods for both Enterobacteriaceae and P aeruginosa until further data from additional manufacturers can be evaluated. Using the CBDE method, 10-μg colistin disks are incubated at room temperature for 20 minutes in tubes containing prespecified volumes of cation-adjusted Mueller-Hinton broth to achieve varying concentrations of colistin.17 The bacterial inoculum is then added and vortexed. The broth tubes are incubated for 18 to 20 hours, and the MIC is read based on presence/absence of growth in each tube. In the CAT method, Mueller-Hinton agar plates are prepared with increasing concentrations of colistin (eg, 0, 0.5, 1.0, 2.0, 4.0 mg/L). These colistin agar plates are then inoculated with either a 1-μL or 10-μL loopful of a 1:10 dilution of inoculum and incubated for 16 to 20 hours. Plates are then read where any visual growth of organism is read as positive.
Mahoney is a fourth-year pharmacy student at the University of Minnesota in Minneapolis.
Hirsch is an assistant professor at the University of Minnesota College of Pharmacy. Her translational research program is focused on the detection and treatment of resistant bacterial infections. She is an appointed adviser to the CLSI Subcommittee on AST. *She is a member of the Society of Infectious Diseases Pharmacists.