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Drug resistance rates of difficult to treat Pseudomonas aeruginosa isolates to ceftolozane-tazobactam and ceftazidime-avibactam from a tertiary hospital, Singapore

      To the Editor,
      Drug-resistant Gram-negative organisms including Pseudomonas aeruginosa result in excess mortality, morbidity, and costs to healthcare systems globally. Different terms such as multi-drug-resistant (MDR), extensively drug-resistant (XDR), and pan-drug-resistant (PDR) have been used to describe levels of drug resistance.
      Clinical and Laboratory Standards Institute (CLSI)
      Performance Standards for Antimicrobial Susceptibility Testing.
      Categorical descriptions have been used to refer to acquired resistance to at least one drug in three categories of antimicrobials (MDR), all but two or fewer categories of antimicrobials (XDR), and resistance to all antimicrobials agents (PDR). The term difficult-to-treat (DTR) has also been introduced, referring to resistance to β-lactam antibiotics, and fluoroquinolones.
      The European Committee on Antimicrobial Susceptibility Testing (EUCAST)
      Breakpoint tables for interpretation of MICs and zone diameters Version 11.0.
      Guidelines currently recommend the use of ceftolozane-tazobactam, ceftazidime-avibactam, or imipenem-relebactam for treating DTR P. aeruginosa.
      • Tamma P.D.
      • Aitken S.L.
      • Bonomo R.A.
      • Mathers A.J.
      • van Duin D.
      • Clancy C.J.
      Infectious Diseases Society of America guidance on the treatment of extended-spectrum β-lactamase producing Enterobacterales (ESBL-E), carbapenem-resistant Enterobacterales (CRE), and Pseudomonas aeruginosa with difficult-to-treat resistance (DTR-P. aeruginosa).
      Other treatment options include cefiderocol, aminoglycosides, or colistin.
      Currently, susceptibility testing is typically performed by determining the minimum inhibitory concentration (MIC), or disk diffusion inhibition diameter, and interpreting these based on defined breakpoints. The European Committee on Antimicrobial Susceptibility Testing (EUCAST) and Clinical and Laboratory Standards Institute (CLSI) breakpoints are the two more commonly used guides for interpretation. Interpretations are typically abbreviated to ‘S’, ‘I’ and ‘R’. While ‘S’ and ‘R’ are defined as being susceptible and resistant by both EUCAST and CLSI, the ‘I’ category is defined as being intermediate by CLSI, but as ‘susceptible, increased exposure’ by EUCAST. This is to indicate that therapeutic success is likely when there is increased drug exposure either by way of adjusting drug dosing, or by virtue of high concentration of an antibiotic at the site of infection. For P. aeruginosa, there is no longer an ‘S’ categorisation for multiple antibiotics including ceftazidime, cefepime, piperacillin-tazobactam, imipenem, ciprofloxacin, and aztreonam. The term non-susceptible has often been used to group intermediate and resistant results previously but may no longer be applicable with the change of the ‘I’ definition by EUCAST.
      In this study, we reviewed clinical laboratory data on MDR, XDR, and DTR P. aeruginosa and, in addition, susceptibility data for clinical DTR P. aeruginosa in relation to current first therapy ceftolozane-tazobactam and ceftazidime-avibactam.
      Laboratory drug susceptibility testing data were retrospectively reviewed between 1 January 2019 and 31 May 2021. Pseudomonas aeruginosa for which susceptibility testing was performed were included. During this time period, organism identification was performed via matrix-assisted laser desorption/ionisation-time of flight (MALDI-TOF) (Bruker MALDI Biotyper, Bruker Daltonics, US and VITEK MS, bioMérieux, France). Routine susceptibility testing as performed using Vitek 2 (bioMérieux), including ceftazidime, cefepime, piperacillin-tazobactam, imipenem, meropenem, ciprofloxacin, gentamicin, and amikacin. For drug-resistant isolates in invasive isolates (such as bacteraemia) or on clinicians' request, an extended susceptibility testing panel including aztreonam, ceftazidime-avibactam, and ceftolozane-tazobactam was performed using Sensititre broth microdilution (Thermo Fisher Scientific, USA).
      Susceptibility testing results were compiled and re-interpreted based on the latest EUCAST and CLSI breakpoints as shown in Table 1.
      Clinical and Laboratory Standards Institute (CLSI)
      Performance Standards for Antimicrobial Susceptibility Testing.
      ,
      The European Committee on Antimicrobial Susceptibility Testing (EUCAST)
      Breakpoint tables for interpretation of MICs and zone diameters Version 11.0.
      The isolates were further defined as MDR, XDR, or DTR isolates. MDR was defined as being non-susceptible to three or more antibiotic categories. XDR was defined as being non-susceptible to four or more antibiotic categories. DTR was defined as being non-susceptible to ceftazidime, cefepime, piperacillin-tazobactam, imipenem, meropenem, and ciprofloxacin. Aztreonam was not included in our definition as it is not part of first line testing in our laboratory. As ceftazidime, cefepime, piperacillin-tazobactam, imipenem and ciprofloxacin no longer have an ‘S’ categorisation based on EUCAST breakpoints, isolates categorised as ‘I’ with EUCAST breakpoints for these antibiotics are considered to be susceptible. However, the EUCAST ‘I’ categorisation for meropenem was considered to be non-susceptible. This is because the ‘I’ cut-off for ceftazidime, cefepime, piperacillin-tazobactam, imipenem, and ciprofloxacin are usually within the epidemiological cut-off (ECOFF) and may be considered as wild-type.
      For meropenem, the ‘I’ categorisation falls outside of the ECOFF.
      Table 1Pseudomonas aeruginosa breakpoints by EUCAST and CLSI
      AntibioticEUCAST
      For ease of comparison, breakpoints are presented in the format used by CLSI.
      CLSI
      S≤IR≥S≤IR≥
      Ceftazidime0.0010.002–81681632
      Cefepime0.0010.002–81681632
      Piperacillin-tazobactam0.0010.002–16321632–64128
      Imipenem0.0010.002–48248
      Meropenem24–816248
      Ciprofloxacin0.0010.002–0.510.512
      Aztreonam0.0010.002–163281632
      Ceftazidime-avibactam8N/A168N/A16
      Ceftolozane-tazobactam4N/A84816
      Amikacin16N/A32163264
      CLSI, Clinical and Laboratory Standards Institute; EUCAST, European Committee on Antimicrobial Susceptibility Testing; I, susceptible increased exposure (EUCAST) or intermediate (CLSI); R, resistant; S, susceptible.
      a For ease of comparison, breakpoints are presented in the format used by CLSI.
      A total of 4715 P. aeruginosa were included. Only isolates for which susceptibility data was available for ceftazidime, cefepime, piperacillin-tazobactam, imipenem, meropenem, ciprofloxacin, gentamicin, and amikacin were included. Of these, 159 non-duplicate isolates would be classified as MDR, an additional 234 would be classified as XDR. The cumulative antibiogram is presented in Table 2 stratified by these drug-resistance classifications.
      Table 2Overall antimicrobial susceptibility of Pseudomonas aeruginosa isolates
      AntimicrobialEUCASTCLSI
      SIRSIR
      Pseudomonas aeruginosa (4715 isolates)
       CeftazidimeN/A82.5%17.5%82.5%5.0%12.6%
       CefepimeN/A86.6%13.4%86.6%4.4%9.0%
       Piperacillin-tazobactamN/A76.2%23.8%76.2%9.1%14.7%
       ImipenemN/A83.8%16.2%83.8%0.0%16.2%
       Meropenem82.4%8.0%9.7%82.4%5.3%12.3%
       CiprofloxacinN/A81.1%18.9%81.1%6.0%13.0%
       Amikacin95.5%N/A4.5%95.5%1.0%3.5%
       Gentamicin
      Gentamicin breakpoints were removed from EUCAST in 2021. For the purposes of this study, previous breakpoints in EUCAST v9.0 breakpoints were used for gentamicin.
      90.7%N/A9.3%90.7%2.3%7.0%
      MDR Pseudomonas aeruginosa (159 isolates)
       CeftazidimeN/A25.2%74.8%25.2%35.2%39.6%
       CefepimeN/A57.2%42.8%57.2%22.0%20.8%
       Piperacillin-tazobactamN/A8.2%91.8%8.2%33.3%58.5%
       ImipenemN/A52.8%47.2%52.8%0.0%47.2%
       Meropenem35.8%38.4%25.8%35.8%23.3%40.9%
       CiprofloxacinN/A50.3%49.7%50.3%28.3%21.4%
       Amikacin98.1%N/A1.9%98.1%0.6%1.3%
       Gentamicin
      Gentamicin breakpoints were removed from EUCAST in 2021. For the purposes of this study, previous breakpoints in EUCAST v9.0 breakpoints were used for gentamicin.
      85.%N/A14.5%85.%6.9%7.5%
      XDR Pseudomonas aeruginosa (234 isolates)
       CeftazidimeN/A5.6%94.4%5.6%22.6%71.8%
       CefepimeN/A19.7%80.3%19.7%18.4%62.0%
       Piperacillin-tazobactamN/A3.4%96.6%3.4%28.2%68.4%
       ImipenemN/A25.2%74.8%24.8%0.4%74.8%
       Meropenem16.2%19.7%64.1%16.2%13.2%70.5%
       CiprofloxacinN/A2.6%97.4%2.6%20.5%76.9%
       Amikacin61.1%N/A38.9%61.1%6.8%32.1%
       Gentamicin
      Gentamicin breakpoints were removed from EUCAST in 2021. For the purposes of this study, previous breakpoints in EUCAST v9.0 breakpoints were used for gentamicin.
      31.2%N/A68.8%31.2%8.1%60.7%
      Non-duplicate DTR Pseudomonas aeruginosa (43 isolates)
      Only 43 DTR isolates were included as they were either bacteraemia isolates (4 isolates), or extended susceptibility testing for ceftolozane-tazobactam and ceftazidime-avibactam was performed on clinicians' request.
       AztreonamN/A55.8%44.2%N/A55.8%44.2%
       Amikacin41.9%N/A58.1%41.9%7.0%51.2%
       Gentamicin
      Gentamicin breakpoints were removed from EUCAST in 2021. For the purposes of this study, previous breakpoints in EUCAST v9.0 breakpoints were used for gentamicin.
      23.3%N/A76.7%23.3%7.0%69.8%
       Ceftazidime-avibactam11.6%N/A88.4%11.6%N/A88.4%
       Ceftolozane-tazobactam16.3%N/A83.7%16.3%14.0%69.8%
       Susceptible to either ceftazidime-avibactam or ceftolozane-tazobactam20.9%N/AN/A20.9%N/AN/A
      CLSI, Clinical and Laboratory Standards Institute; DTR, difficult-to-treat; EUCAST, European Committee on Antimicrobial Susceptibility Testing; I, susceptible increased exposure (EUCAST) or intermediate (CLSI); MDR, multi-drug-resistant; R, resistant; S, susceptible; XDR, extensively-drug-resistant.
      a Gentamicin breakpoints were removed from EUCAST in 2021. For the purposes of this study, previous breakpoints in EUCAST v9.0 breakpoints were used for gentamicin.
      b Only 43 DTR isolates were included as they were either bacteraemia isolates (4 isolates), or extended susceptibility testing for ceftolozane-tazobactam and ceftazidime-avibactam was performed on clinicians' request.
      In total, 145 non-duplicate isolates were classified as DTR. Susceptibility testing results for 43 isolates to ceftazidime-avibactam (11.6% susceptible) and ceftolozane-tazobactam (16.3% susceptible) were available (Table 2). Of note, the categorisation of aztreonam was similar for both EUCAST and CLSI breakpoints. Overall susceptibility to ceftolozane-tazobactam and ceftazidime-avibactam was low. The minimum inhibitory concentration (MIC)50 and MIC90 are as follows: aztreonam 16 mg/L and >32 mg/L; ceftazidime-avibactam >32 mg/L and >32 mg/L; ceftolozane tazobactam: >16 mg/L and >16 mg/L.
      In our population of DTR P. aeruginosa, the susceptibility rates to the new β-lactam-β-lactamase inhibitors, ceftazidime-avibactam and ceftolozane-tazobactam, was low with less than 20% of tested isolates. Although ceftazidime-avibactam and ceftolozane-tazobactam are recommended as first line treatment for DTR P. aeruginosa, their utility appears to be limited in our setting.
      • Tamma P.D.
      • Aitken S.L.
      • Bonomo R.A.
      • Mathers A.J.
      • van Duin D.
      • Clancy C.J.
      Infectious Diseases Society of America guidance on the treatment of extended-spectrum β-lactamase producing Enterobacterales (ESBL-E), carbapenem-resistant Enterobacterales (CRE), and Pseudomonas aeruginosa with difficult-to-treat resistance (DTR-P. aeruginosa).
      Given the limited susceptibility, there is ongoing reliance on using polymyxins for treating DTR P. aeruginosa isolates. Other series have reported higher rates of susceptibility to these antibiotics.
      • Karlowsky J.A.
      • Lob S.H.
      • Raddatz J.
      • et al.
      In vitro activity of imipenem/relebactam and ceftolozane/tazobactam against clinical isolates of Gram-negative bacilli with difficult-to-treat resistance and multidrug-resistant phenotypes—study for monitoring antimicrobial resistance trends, United States 2015-2017.
      This may be due to differences in predominant resistance mechanisms leading to the DTR phenotype. High rates of carbapenemase production has been reported in another local hospital with a preponderance of metallo-β-lactamases.
      • Teo J.Q.-M.
      • Lim J.C.
      • Tang C.Y.
      • et al.
      Ceftolozane/tazobactam resistance and mechanisms in carbapenem-nonsusceptible Pseudomonas aeruginosa.
      Ceftolozane-tazobactam may be a potential marker for carbapenemase production but routine testing for carbapenemase production is not usually performed.
      In our setting, we have previously identified clonal transmission of NDM-producing P. aeruginosa.
      • Chew K.L.
      • Octavia S.
      • Ng O.T.
      • et al.
      Challenge of drug resistance in Pseudomonas aeruginosa: clonal spread of NDM-1-positive ST-308 within a tertiary hospital.
      NDM as well as other metallo-β-lactamases described in P. aeruginosa may retain activity against aztreonam in light of limited monobactam hydrolysis. Nonetheless, the MICs for aztreonam in our DTR isolates were elevated compared to expected wild-type populations. Of the 43 DTR isolates with aztreonam susceptibility testing performed, 55.8% were categorised as ‘I’ based on both EUCAST and CLSI breakpoints. All had MIC of 16 mg/L. As there is no ‘S’ categorisation for aztreonam, if we take these isolates as wild-type/susceptible, the number of isolates classified as DTR would be significantly reduced. Aztreonam-avibactam has been suggested as a combination treatment for carbapenemase-producing Enterobacterales but do not have similar efficacy in P. aeruginosa.
      • Biedenbach D.J.
      • Kazmierczak K.
      • Bouchillon S.K.
      • Sahm D.F.
      • Bradford P.A.
      In vitro activity of aztreonam-avibactam against a global collection of Gram-negative pathogens from 2012 and 2013.
      ,
      • Chew K.L.
      • Tay M.K.L.
      • Cheng B.
      • Lin R.T.P.
      • Octavia S.
      • Teo J.W.P.
      Aztreonam-avibactam combination restores susceptibility of aztreonam in dual-carbapenemase-producing-Enterobacteriaceae.
      Our data highlight that DTR P. aeruginosa remains problematic even with the availability of new treatment options. Despite changes in international guidelines in the management of DTR P. aeruginosa, the treatment options remain limited in light of high levels of resistance to ceftolozane-tazobactam and ceftazidime-avibactam. Specific testing of resistance mechanisms was not performed in our collection of isolates, but high prevalence of metallo-β-lactamases in carbapenem non-susceptible P. aeruginosa has been demonstrated in another hospital in Singapore.
      • Teo J.Q.-M.
      • Lim J.C.
      • Tang C.Y.
      • et al.
      Ceftolozane/tazobactam resistance and mechanisms in carbapenem-nonsusceptible Pseudomonas aeruginosa.
      Newer β-lactamase inhibitors such as relebactam and vaborbactam have limited inhibitory activity against metallo-β-lactamases. Other options such as cefiderocol may offer hope but additional data are required.
      • Bassetti M.
      • Echols R.
      • Matsunaga Y.
      • et al.
      Efficacy and safety of cefiderocol or best available therapy for the treatment of serious infections caused by carbapenem-resistant Gram-negative bacteria (CREDIBLE-CR): a randomised, open-label, multicentre, pathogen-focused, descriptive, phase 3 trial.
      Colistin and polymyxin will likely remain the mainstays of treatment for such infections, despite issues with clinical efficacy and drug toxicity.

      Conflicts of interest and sources of funding

      The authors state that there are no conflicts of interest to disclose.

      References

        • Clinical and Laboratory Standards Institute (CLSI)
        Performance Standards for Antimicrobial Susceptibility Testing.
        30th ed. CLSI Supplement M100, Wayne, PA: CLSI2020
        • The European Committee on Antimicrobial Susceptibility Testing (EUCAST)
        Breakpoint tables for interpretation of MICs and zone diameters Version 11.0.
        2021
        • Tamma P.D.
        • Aitken S.L.
        • Bonomo R.A.
        • Mathers A.J.
        • van Duin D.
        • Clancy C.J.
        Infectious Diseases Society of America guidance on the treatment of extended-spectrum β-lactamase producing Enterobacterales (ESBL-E), carbapenem-resistant Enterobacterales (CRE), and Pseudomonas aeruginosa with difficult-to-treat resistance (DTR-P. aeruginosa).
        Clin Infect Dis. 2021; 72: 1109-1116
      1. The European Committee on Antimicrobial Susceptibility Testing (EUCAST). MIC and zone distributions and ECOFFs. Cited Aug. 2021
        • Karlowsky J.A.
        • Lob S.H.
        • Raddatz J.
        • et al.
        In vitro activity of imipenem/relebactam and ceftolozane/tazobactam against clinical isolates of Gram-negative bacilli with difficult-to-treat resistance and multidrug-resistant phenotypes—study for monitoring antimicrobial resistance trends, United States 2015-2017.
        Clin Infect Dis. 2021; 72: 2112-2120
        • Teo J.Q.-M.
        • Lim J.C.
        • Tang C.Y.
        • et al.
        Ceftolozane/tazobactam resistance and mechanisms in carbapenem-nonsusceptible Pseudomonas aeruginosa.
        mSphere. 2021; 6 (e01026-20)
        • Chew K.L.
        • Octavia S.
        • Ng O.T.
        • et al.
        Challenge of drug resistance in Pseudomonas aeruginosa: clonal spread of NDM-1-positive ST-308 within a tertiary hospital.
        J Antimicrob Chemother. 2019; 74: 2220-2224
        • Biedenbach D.J.
        • Kazmierczak K.
        • Bouchillon S.K.
        • Sahm D.F.
        • Bradford P.A.
        In vitro activity of aztreonam-avibactam against a global collection of Gram-negative pathogens from 2012 and 2013.
        Antimicrob Agents Chemother. 2015; 59: 4239-4248
        • Chew K.L.
        • Tay M.K.L.
        • Cheng B.
        • Lin R.T.P.
        • Octavia S.
        • Teo J.W.P.
        Aztreonam-avibactam combination restores susceptibility of aztreonam in dual-carbapenemase-producing-Enterobacteriaceae.
        Antimicrob Agents Chemother. 2018; 62: e00414-e00418
        • Bassetti M.
        • Echols R.
        • Matsunaga Y.
        • et al.
        Efficacy and safety of cefiderocol or best available therapy for the treatment of serious infections caused by carbapenem-resistant Gram-negative bacteria (CREDIBLE-CR): a randomised, open-label, multicentre, pathogen-focused, descriptive, phase 3 trial.
        Lancet Infect Dis. 2021; 21: 226-240