FormalPara Key Points

Generally, the antibiotic exposure for suppressing resistance emergence is higher than that associated with clinical efficacy.

The exposure required to suppress resistance emergence varies with bacterial species and specific isolates.

Resistance emergence may be more likely in the presence of a higher bacterial burden.

1 Introduction

The widespread use and misuse of antibiotics have led to the rapid emergence and global dissemination of antibiotic resistance [1, 2]. Currently, up to 70% of Gram-negative bacteria may harbor extended-spectrum β-lactamase (ESBL) enzymes depending on geographical location, conferring resistance to commonly used antibiotics such as piperacillin/tazobactam [3,4,5]. Commonly encountered multidrug-resistant bacteria may include carbapenem-resistant Acinetobacter baumannii, metallo-β-lactamase-producing Pseudomonas aeruginosa, and ESBL-producing Klebsiella pneumoniae, which are associated with increased morbidity and mortality in patients with a bacteraemia (odds ratio [OR] 2.98, 95% confidence interval [CI] 2.36–3.75; p < 0.001) [6].

Few new antibiotics and antibiotic combinations have been developed for the treatment of infections caused by resistant Gram-negative bacteria, such as plazomicin, ceftolozane/tazobactam, ceftazidime/avibactam, and meropenem/vaborbactam, however, resistance to these agents has been previously described [7, 8]. Therefore, given the cost and time required for the development of new antibiotics, methods to minimize resistance emergence to both new and old antibiotics are of paramount importance [9, 10]. One potential method is antibiotic dose optimization.

In vitro studies simulating current antibiotic dosing practices highlight that exposures associated with an increased probability of clinical cure may be insufficient to suppress the emergence of antibiotic-resistant Gram-negative bacteria [11, 12]. Pharmacokinetic/pharmacodynamic (PK/PD) indices relate the antibiotic exposure to the antibiotic susceptibility of an infecting pathogen where susceptibility may be described as the minimum inhibitory concentration (MIC), thereby providing the clinician with a dosing target. In studies investigating the PK/PD targets required for clinical efficacy, common indices include the percentage of the dosing interval that the drug concentration exceeds the pathogen MIC (%T>MIC, e.g. β-lactam antibiotics), the maximum drug concentration to MIC ratio (Cmax/MIC, e.g. aminoglycosides) and the area under the drug concentration versus time curve (reflecting total antibiotic exposure) to MIC ratio (AUC/MIC, e.g. fluoroquinolones). Given that the MIC is a measure of susceptibility for the majority of the bacterial population at a standardized inoculum (5.5 × 105 colony-forming units per millilitre; CFU/mL), some studies have suggested that alternative measures of susceptibility reflecting the potential for resistance to develop may provide an advantage when determining the PK/PD targets required for suppressing resistance emergence. However, the inaccuracies in any MIC obtained by a single MIC test should also be considered due to assay variability [13]. One example is the mutant prevention concentration (MPC), which describes the antibiotic concentration required to suppress the growth of first-generation mutant bacteria that may selectively proliferate at concentrations above the MIC [14, 15]. The antibiotic concentration range between the MIC and the MPC is the mutant selection window (MSW). Antibiotic concentrations within the MSW promote the growth of resistant bacterial pathogens; thus, the antibiotic exposure required to suppress the emergence of resistance should be maintained above the MSW. Moreover, compared with MIC testing, MPC and MSW testing is conducted at higher bacterial burdens > 1×108 CFU/mL, representing a serious bacterial infection that is more likely to facilitate resistance emergence than may be the case in common infection types [13, 16]. Overall, no standardized definitions exist to determine the antibiotic exposures that should be targeted to suppress the emergence of antibiotic resistance against different bacterial burdens. Moreover, there remains a lack of research to define target antibiotic exposures needed to minimize the development of resistance. Thus, this systematic review aims to describe the currently known antibiotic PK/PD indices required to suppress the emergence of Gram-negative bacterial antibiotic resistance.

2 Methods

A systematic review of the literature was performed according to the guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement [17].

2.1 Search Strategy

Four electronic databases (PubMed, EMBASE, SCOPUS, and Web of Science) were searched for studies, published between January 1953 and March 2019, investigating antibiotic exposures required to suppress the emergence of antibiotic resistance in Gram-negative bacteria. We designed the search strategy based on four concept areas as follows:

  1. 1.

    ‘Anti-bacterial agents’ (MeSH) OR ‘anti-infective agents’ (MeSH)) OR ‘beta-lactams’ (MeSH) OR ‘penicillins’ (MeSH) OR antibacter* OR anti-bacter* OR antimicrobial* OR anti-microbial OR antibiotic* OR anti-biotic* OR beta-lactam* OR penicillin* OR ampicillin OR piperacillin* OR cephalosporin* OR carbapenem* OR imipenem OR meropenem OR doripenem OR ertapenem OR cefepime OR polymyxin* OR colistin OR colisti* OR tobramicin OR tobramycin OR aminoglycoside* OR ceftaroline OR ciprofloxacin OR fosfomycin OR phosphonomycin* OR amikacin OR aztreonam OR gentamicin OR gentamycin OR levofloxacin OR ceftozolane OR moxifloxacin OR tigecycline OR minocycline OR glycylglycine OR tetracycline OR chloramphenicol OR rifampicin OR kanamycin.

  2. 2.

    Resistance OR multi-drug resistance OR drug resistance OR antibiotic resistance OR antimicrobial resistance.

  3. 3.

    Dose OR concentration OR exposure OR pharmacokinetic*.

  4. 4.

    Mutant prevention concentration OR mutant selection window OR suppress OR suppression OR pharmacodynamic*.

Each search was limited to English-language articles only. Finally, the searches (1), (2), (3) and (4) were combined with ‘AND’ as a Boolean Operator. In addition, the reference lists of included studies were searched manually to identify additional records.

2.2 Inclusion and Exclusion Criteria

Exposure for suppression of antibiotic resistance emergence was defined as an exposure that prevents the growth of the test bacteria on antibiotic-containing agar, or an increase in the MIC of the culture. This includes the complete eradication of the bacterial culture within the defined time frame of the experiment.

Inclusion criteria were as follows:

  1. (a)

    Antibiotic: Currently clinically used for the treatment of infections in humans.

  2. (b)

    Microorganism: Gram-negative bacteria.

  3. (c)

    Type of therapy and duration of exposure: Monotherapy over any duration of exposure.

  4. (d)

    Study model: In vitro pharmacodynamic model simulating human pharmacokinetics and/or in vivo animal model, or clinical study.

  5. (e)

    Outcome: Antibiotic exposure and/or the PK/PD ratio required for the emergence of resistance suppression against previously susceptible and/or intermediate (according to the European Committee on Antimicrobial Susceptibility Testing [EUCAST] or Clinical and Laboratory Standards Institute [CLSI] definitions) Gram-negative bacteria.

Exclusion criteria were as follows:

  1. (a)

    Review articles (systematic and narrative) and meta-analyses.

  2. (b)

    Studies describing the pharmacodynamics of combination therapy with two or more antibiotics.

  3. (c)

    Studies describing the emergence of antibiotic resistance in an in vivo microbiota that was distinct from the original experimental infection site.

2.3 Selection of Studies and Data Extraction

The reference management software EndNote X8 (Clarivate Analytics, Philadelphia, PA, USA) was used to manage all data retrieved from the four electronic databases. Two reviewers (CDS and AJH) independently screened all studies by title and abstract for full-text review. Both reviewers resolved any disagreement through consensus, or, if necessary, in consultation with a third reviewer (FBS). Relevant characteristics extracted from the full-text studies were study type (in vitro, in vivo, clinical study), antibiotic tested, bacterial isolate, experimental apparatus, simulated human pharmacokinetic profile (clearance [CL], drug exposures and/or dosing regimens, and elimination half-life [t½]), baseline bacterial burden, method(s) for determination of the resistant subpopulation, and study outcomes in terms of the required drug exposures and PK/PD indices for suppression of emergence of resistance. Details of the protocol for this systematic review were registered on PROSPERO (ID: CRD42018098631).

2.4 Quality Assessment

Both reviewers independently assessed the methodological quality of the included studies and assessed their appropriateness for inclusion in this review. A list of methodological items was developed for quality assessment of the included preclinical (in vitro and in vivo) studies, described in a previous study [18] (Table 1). This list included a total of 11 items to assess a study’s methodological quality, such as the aim of study, microorganism characterization, antibacterial agents, bacterial concentration, pharmacokinetic data, type of pharmacodynamic model used for the study, study observation period, control group, antibiotic concentration determination assay, resistant subpopulation selection, and outcomes. Moreover, there were a total of 17 sub-items and each sub-item was reported as 0 (not described) or 1 (described).

Table 1 Methodological items for quality assessment of the preclinical (in vitro and in vivo) studies
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2.5 Outcomes and Data Analysis

The included studies were grouped into two categories based on study type: preclinical (in vitro and in vivo) or clinical studies. The outcomes of interest were used to identify the drug exposures and/or the PK/PD indices required to suppress the emergence of resistance. Suppression of emergence of resistance was defined as the presence of a bacterial population without an MIC shift and/or a bacterial burden below the lower limit of quantification (LLQ) on antibiotic-impregnated agar plates at any concentration over the duration of the experiment.

3 Results

Our search strategy identified 10,311 studies, of which 215 were selected for full-text review. Of those 215 studies, 156 were excluded, resulting in 59 studies being included in our systematic review. The complete search strategy and study selection process are presented in Fig. 1.

Fig. 1
figure 1

Systematic literature search and selection process in accordance with the PRISMA guidelines. PRISMA Preferred Reporting Items for Systematic Reviews and Meta-Analyses

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3.1 Study Characteristics

A summary description of the study characteristics and outcome measurements of the included preclinical in vitro 10659 studies is reported in Table 2. The 46 in vitro studies reported on exposures of β-lactams, carbapenems, fluoroquinolones, aminoglycosides, polymyxins, fosfomycin, tetracycline and glycylcyclines for suppression of emergence of resistance. Most in vitro studies (n = 30) used a two-compartment pharmacodynamic model as opposed to a one-compartment pharmacodynamic model (n = 15) to assess the antibiotic exposures against test Gram-negative bacteria. Only Alou et al. [31] used a multiple-compartment bladder infection model for their study. With the exception of that study, all studies had a study duration of at least 24 h. In Table 3, a summary of characteristics of the 11 included preclinical in vivo studies is presented. Most of the in vivo studies (n = 6) reported on β-lactam exposures for suppression of emergence of resistance. All in vivo studies were observed for at least for 24 h. Finally, the characteristics of the included clinical studies are summarized in Table 4. These clinical studies reported PK/PD indices for resistance suppression [76, 77]; however, the bacterial concentration at the site of infection was not determined in those studies.

Table 2 Study characteristics of the preclinical in vitro studies
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Table 3 Study characteristics of the preclinical in vivo studies
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Table 4 Study characteristics of the clinical studies
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3.2 Quality of Studies

As shown in electronic supplementary Table S1, among the preclinical in vitro studies, the quality assessment score range was 14–17, mode 16. Among 46 in vitro studies, 9 reported all the methodological quality assessment items. The studies that scored more than 14 provided information regarding the simulated pharmacokinetic profile. In the case of 11 preclinical in vivo studies, the range of quality assessment scores was 14–16, mode 15 (electronic supplementary Table S2). The total quality score data followed a skewed distribution, and the number of studies was not comparable between the in vitro and in vivo study groups. Quality assessment was not performed for the two clinical studies available for this review.

3.3 Study Outcomes

3.3.1 β-Lactams

As shown in Table 5, total and unbound T>MIC was the relevant PK/PD index describing the suppression of emergence of β-lactam antibiotic resistance in most included studies. Studies that were conducted for up to 72 h using a bacterial burden of 1 × 106 CFU/mL demonstrated that 100% T>MIC was sufficient to suppress the emergence of bacterial resistance for penicillins and cephalosporins. No in vitro study involving carbapenems was conducted for < 120 h. Studies that observed the emergence of resistance for a duration > 120 h suggest that for all β-lactam antibiotics, higher exposure with a minimum concentration (Cmin)/MIC ratio ranging from 1 to ~ 8 may be required to suppress resistance. Importantly, studies with a duration > 120 h also used a higher bacterial burden of > 1×107 CFU/mL compared with studies conducted for < 120 h that used a bacterial burden of ~ 1×106 CFU/mL. Only the study of a dynamic hollow-fibre infection model (HFIM) conducted by Felton et al. [40] directly compared the PK/PD indices required to suppress the emergence of bacterial resistance over the same time period for a high (~ 8×108 CFU/mL) and low (1 × 106 CFU/mL) initial bacterial burden (Pseudomonas aeruginosa). The exposure required to suppress the emergence of resistance for piperacillin/tazobactam administered as a bolus was a Cmin/MIC of 3.4, compared with 4.6 against the high bacterial burden. When a 3 h prolonged infusion of piperacillin/tazobactam was employed, the Cmin/MIC required to suppress the emergence of resistance was 10.4 and 11.9 for the low and high bacterial inoculum, respectively. However, against the low inoculum, achieving a piperacillin/tazobactam exposure of a Cmin/MIC of 3.4 or 10.4, for the bolus and prolonged infusion, respectively, reduced the bacterial burden to below the LLQ over the study duration. In contrast, there was no significant bacterial killing against the high P. aeruginosa inoculum over the study duration [40].

Table 5 Study outcomes of the preclinical in vitro studies
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Only one study has combined data for different β-lactam antibiotics (ceftazidime, cefepime and meropenem) to determine the exposure required for the suppression of emergence resistance for two K. pneumoniae and two P. aeruginosa isolates [58]. Classification and regression-tree analysis showed a β-lactam antibiotic Cmin/MIC ≥ 3.8 is required to suppress the emergence of resistance against most isolates tested. Further evidence of a difference in the exposure required to suppress the emergence of resistance between bacterial species is demonstrated in experiments with ceftolozane/tazobactam. The approximate human dosing regimen of ceftolozane/tazobactam required to suppress the emergence of resistance for three Escherichia coli isolates (CTX-M-15, MIC 0.25 mg/L; CMY-10, MIC 1 mg/L; and a wild-type isolate, MIC 0.25 mg/L) and two P. aeruginosa isolates (a wild-type, MIC 0.5 mg/L; and a MexA/MexB efflux pump overexpressing isolate; MIC 4 mg/L) was ≥ 1/0.5 g and ≥ 2/1 g administered every 8 h as a 1 h infusion, respectively [42, 45]. In addition to the bacterial species, inoculum and MIC, specific bacterial phenotypes may influence the threshold required to suppress the emergence of resistance. An in vitro study in a two-compartment pharmacodynamic model by Cappelletty [24] compared mucoid and non-mucoid P. aeruginosa (both MIC 8 mg/L). Against the non-mucoid isolate, a cefepime exposure of 85% T>MIC suppressed the emergence of resistance. In contrast, the same exposure could not suppress the emergence of resistance against the mucoidal isolate.

The results of the in vivo studies are similar to the results of the in vitro studies conducted over 24 h. Meropenem and imipenem exposures of 40% T>MIC were reported to suppress the emergence of antibiotic resistance against three susceptible P. aeruginosa isolates (meropenem MICs 0.125, 0.25 and 1 mg/L; imipenem MICs 1, 1 and 1 mg/L) in a neutropenic murine model over 24 h (Table 6) [68]. In contrast, the exposures required to suppress the emergence of resistance in vitro for meropenem in studies conducted over > 120 h were a Cmin/MIC of between 2 and 6 [54, 58].

Table 6 Study outcomes of preclinical in vivo studies
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3.3.2 Aminoglycosides

The relevant PK/PD ratio describing the suppression of emergence of aminoglycoside resistance is the Cmax/MIC ratio (Table 5). Achieving a netilmicin Cmax/MIC ratio of ≥ 8 mg/L suppressed the emergence of resistance in vitro, in the dynamic HFIM against E. coli (MIC 1 mg/L), K. pneumoniae (MIC 0.125 mg/L) and P. aeruginosa (MIC ≤ 8 mg/L).

Using the dynamic in vitro HFIM model, Tam et al. [34] demonstrated that an amikacin Cmax/MIC of 13 or a Cmax/MIC of 20 was required to suppress the emergence of resistance when administered twice or once daily, respectively, against A. baumannii (MIC 2 mg/L) over 72 h. Similarly, Ghazi et al. [55] reported that a simulated exposure following 400 mg of nebulized amikacin administered twice daily in an in vitro dynamic one-compartment model prevented the regrowth of resistant subpopulation of a clinical isolate of A. baumannii (MIC 2 mg/L).

For gentamicin, a Cmax/MIC ratio of 30 when administered twice daily was reported to suppress the resistance emergence of P. aeruginosa (MIC 2 mg/L); however, even a Cmax/MIC ratio > 36 when administered as a single-daily dose was unable to suppress the emergence of resistance [34]. The PK/PD indices identified in vitro were similar to those found in a mouse lung infection model study by Maciá et al. [67], who demonstrated that a plasma tobramycin Cmax/MIC ratio of 19 when administered four times daily prevented the emergence of resistance against a wild-type P. aeruginosa (MIC 1 mg/L) in a murine pneumonia infection model (Table 6). In a separate murine pneumonia model, Louie et al. [71] reported that an AUC24/MIC ratio ≥ 110.6 in the epithelial lining fluid of the lungs was required to suppress the resistance emergence of wild-type P. aeruginosa (MIC 1 mg/L).

3.3.3 Fluoroquinolones

AUC/MIC and AUC/MPC were commonly described PK/PD ratios associated with the suppression of emergence of resistance for fluoroquinolones (Table 5). In vitro studies for ciprofloxacin conducted for 24–72 h have reported that an AUC24/MPC of between 11 and ~ 58 is required to suppress the emergence of resistance against E. coli, K. pneumoniae and P. aeruginosa isolates. The AUC24/MIC ratio associated with suppression of resistance varies from 300 to 1400, with no apparent relationship with the AUC/MPC ratio or the bacterial species. On the other hand, a mouse lung infection model study by Maciá et al. [67] reported that a ciprofloxacin AUC24/MIC ratio of 385 prevented the emergence of resistance against a wild-type P. aeruginosa isolate (MIC 0.125 mg/L) (Table 6). However, that study also demonstrated that the same ciprofloxacin exposure failed to suppress the emergence of resistance of a hypermutable P. aeruginosa strain (MIC 0.125 mg/L). Additionally, a clinical study reported that a higher AUC24/MIC ratio of 582 suppressed the emergence of resistance of susceptible E. coli, Enterobacter cloacae, Haemophilus influenzae and Serratia marcescens in patients (Table 7) [76].

Table 7 Clinical studies of antibiotic exposures and PK/PD indices for suppression of resistance emergence against Gram-negative bacteria
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3.3.4 Tetracyclines

As shown in Table 5, only one preclinical dynamic two-compartment in vitro study, conducted by Alfouzan et al. [53], has determined the relationship between minocycline exposure and suppression of emergence resistance of A. baumannii. This study suggested that an unbound AUC24/MIC ratio of 20–25 was required to suppress the emergence of resistance against three A. baumannii isolates (MICs 0.5, 3 and 4 mg/L).

3.3.5 Polymyxins

A literature search conducted for this review identified only two preclinical in vitro studies investigating polymyxin B (Table 5) [30, 43]. An in vitro dynamic HFIM study by Tam et al. [30] suggested that a polymyxin B dose of 20 mg/kg body weight (AUC24/MIC ~ 808) administered twice daily, which is approximately eightfold the current recommended doses, suppressed the emergence of polymyxin B resistance in a wild-type P. aeruginosa (MIC 1 mg/L), but failed to suppress the emergence of resistance in three other carbapenem-resistant P. aeruginosa clinical isolates (MIC ranged from 0.5 to 1 mg/L). On the other hand, a dynamic one-compartment in vitro study by Hagihara et al. [43] reported that an unbound AUC12/MIC ratio of ~ 8 at a simulated twice-daily polymyxin B dose of 1 mg/kg was required to suppress the emergence of polymyxin resistance in three clinical isolates of carbapenem-resistant A. baumannii (MICs 1 mg/L).

3.3.6 Fosfomycin

The likely PK/PD ratio to best describe fosfomycin exposures required to suppress emergence of resistance remains unknown. As shown in Table 6, a preclinical rabbit tissue cage infection model study by Pan et al. [74] suggested that an AUC24/MPC ratio of > 10 prevented the emergence of resistant bacteria against laboratory reference strains of P. aeruginosa American Type Culture Collection (ATCC) 27853 (MIC 4 mg/L) and E. coli ATCC 25922 (MIC 2 mg/L). Conversely, resistance was identified in all 15 rabbits for both P. aeruginosa and E. coli when the T>MPC was < 70%. One in vitro dynamic HFIM study by Docobo-Pérez et al. [47] identified a fosfomycin AUC24/MIC ratio of ≥ 3136 suppressed the resistant subpopulation of a susceptible CTX-M-15-producing E. coli (MIC 1 mg/L) clinical isolate. In contrast, this study also reported that another CTX-M-15-producing E. coli isolate with the same MIC (MIC 1 mg/L) was not suppressed by a similar fosfomycin exposure. On the other hand, a dynamic one-compartment model study by VanScoy et al. [48] suggested that a new PK/PD index, the percentage of the dosing interval that the fosfomycin concentrations remained above the resistance inhibitory concentration (RIC; f %T>RIC), was a better predictor to describe the exposure required for the suppression of emergence of resistance (Table 5). According to their study, a dose ≥ 2 g every 6 h suppressed the emergence of one susceptible E. coli ATCC strain (MIC 1 mg/L) and two E. coli clinical isolates (MIC for both isolates 1 mg/L) when the % fT>RIC was more than 32.8.

4 Discussion

Our systematic review, based on 56 preclinical (in vitro and in vivo) and 2 clinical studies, details the antibiotic exposures reported to suppress the emergence of antibiotic resistant Gram-negative bacteria. The results highlight the potential for intraspecies variability of antibiotic exposures required for suppression of resistance that may be independent of the MIC.

The included preclinical studies in this systematic review have reported that the β-lactam exposures of fCmin/MIC ≥ 6 achieved by intermittent infusion suppressed the emergence of resistance of highly susceptible isolates [29, 40, 49, 54, 58] and with lower initial inoculum (~ 104 CFU/mL) [40]. However, the preclinical study has also shown that resistance emergence to β-lactam antibiotics may occur with an exposure less than a Cmin/MIC of 4 against the clinically relevant bacterial densities as high as ~ 108 CFU/ml [58] frequently encountered in patients with ventilator-associated pneumonia (VAP) [138]. In contrast, the exposure needed for optimal clinical cure varies between > 45% fT>MIC [78] and a Cmin/MIC ≥ 12 [79,80,81,82]; however, this is likely dependent on the infectious type and patient illness severity [83]. Thus, the antibiotic dose to suppress the emergence of β-lactam antibiotic resistance for most patients is higher than that required for clinical effect [84,85,86,87,88,89,90,91]. The multicentre Defining Antibiotic Levels in Intensive Care Patients (DALI) study highlights the significant interpatient variability of β-lactam antibiotic pharmacokinetics in the critically ill patient population, with only 60.4% of patients with currently used dosing regimens achieving 100% fT>MIC [92]. Current pharmacokinetic models based on a limited number of patients would suggest that empiric dosing regimens for many β-lactam antibiotics are inadequate to suppress the emergence of resistance. The dose of meropenem, piperacillin/tazobactam and cefepime, for a patient with a creatinine CL of ~ 90 mL/min/1.73 m2, would need to be increased to 1 g administered 6-hourly, 4/0.5 g administered 4-hourly, and 2 g administered 6-hourly, respectively, to meet the minimum exposures required to suppress the emergence of resistance [84,85,86,87,88,89,90,91]. However, even these doses may not achieve the exposures required to suppress the emergence of resistance in patients infected with higher MIC pathogens. The COMParative Activity of Carbapenem Testing (COMPACT) study highlights that the carbapenem MIC of infectious pathogens for patients in intensive care is higher than for other patients. In addition, due to altered pharmacokinetics present in critically ill patients, the antibiotic exposure required for both clinical effect and suppression of emergence of resistance is higher as a result of reduced susceptibility of bacteria [93]. Although the higher doses required to suppress the emergence of resistance raises the concern for potential toxicity, the antibiotic exposure required for toxicity is generally high (piperacillin Cmin ~ 361 mg/L [94,95,96], cefepime Cmin ≥ 22 mg/L [97] and meropenem Cmin ≥ 64 mg/L).

There are conflicting results with the aminoglycoside exposure required to suppress the emergence of resistance. A Cmax/MIC ratio of between 20 (amikacin administered once daily against A. baumannii) and 32 (gentamicin administered twice daily against P. aeruginosa) have been reported to suppress the emergence of resistance [34, 55]. Moreover, a Cmax/MIC ratio of 15 (amikacin administered once daily) was associated with microbiological success in the group of patients with VAP [98]. In contrast, a lower Cmax/MIC of 8–10 has been shown to improve clinical cure rates in patients with nosocomial pneumonia [99], urinary tract infection, lower respiratory tract infection, cutaneous infection, and intra-abdominal infection [100]. On the other hand, an AUC24/MIC ratio of 110 in a murine pneumonia model has also been associated with the suppression of emergence of resistance [71]. This is similar to the AUC24/MIC of 120 associated with improved clinical cure in patients with P. aeruginosa bacteraemia [101]. Thus, it would appear that the exposure required to suppress the emergence of antibiotic resistance in vitro is higher than that required for clinical efficacy. Moreover, to adequately achieve target peak drug concentrations, the recommended weight-based 7 mg/kg of tobramycin and gentamicin once-daily regimen was based on PK data derived from a general patient population for the treatment of Gram-negative infections [102, 103]. As a result, current aminoglycoside once-daily dosing regimens may not achieve the exposure required for clinical cure in critically ill patients due to differences in the PK, as shown by Rea et al. [104]. A study by Roger et al. [105] in 63 ICU patients with severe sepsis reported that increasing the dose to 8 mg/kg did not have an appreciable impact to increase the rate of PK/PD target attainment (the rate of target attainment was 100%). However, this study did not determine the infecting pathogen MIC in most cases, and used the EUCAST breakpoint of P. aeruginosa for assessing the Cmax/MIC ratio [105].

Fluoroquinolone resistance emergence is thought to be primarily related to de novo mutations. Since concentrations above the MPC suppress selective proliferation of first-step mutants, the MPC may appropriately describe the concentration of a fluoroquinolone that will suppress the emergence of resistance [106, 107]. Fluoroquinolone exposures within the MSW, the concentration range between the MIC and MPC, have been shown to promote resistance emergence [107]. Thus, fluoroquinolone dosing should aim to minimize the time the concentration remains within the MSW to minimize the risk of amplifying resistant bacterial populations. A pharmacodynamic target fTMSW of < 20% is associated with suppression of emergence of resistance [106] that is clinically achievable; an intravenous ciprofloxacin dose of 400 mg administered three times daily would likely be sufficient to suppress the emergence of resistance against a bacterial isolate with an MIC of ≤ 0.125 mg/L for most patients [108,109,110].

The suppression of polymyxin B resistance emergence against carbapenem-resistant isolates has been described for A. baumannii (AUC24/MIC ~ 80) [43], but not for P. aeruginosa with an AUC24/MIC exposure > 800 [30]. Thus, targeting the exposure required for optimal bactericidal activity (AUC12/MIC > 50) may also suppress the emergence of resistance against A. baumannii [111]. This is clinically achievable with polymyxin B doses of 1.5 mg/kg administered twice daily [112]; however, this may be limited by nephrotoxicity that may occur with daily doses > 250 mg [113]. Polymyxin B monotherapy may be insufficient to suppress the emergence of resistance when used to treat P. aeruginosa infections. Importantly, no dosing regimen of colistin has been shown to suppress the emergence of resistance, which may be related to the slow conversion of the prodrug to the active compound [114, 115].

A fosfomycin exposure of an AUC24/MIC ≥ 3136 has been shown to suppress the emergence of resistance for E. coli and E. cloacae isolates with an MIC ≤ 8 mg/L. With a breakpoint of 8 mg/L, it may be possible to treat and prevent the emergence of resistance for systemic infections in most patients who receive a dose of 8 g administered three times daily [116], or to treat a urinary tract infection with a single 3 g oral dose [60]. However, fosfomycin resistance may emerge during treatment for K. pneumoniae or non-fermenting Gram-negative bacterial infections.

Taken together, the PK/PD targets required to suppress the emergence of resistance are higher than that required for clinical efficacy; however, existing data on exposures required to suppress resistance are confounded by various factors. First, many studies determining the exposure required to suppress the emergence of resistance have been conducted in vitro against high bacterial burdens of ~ 1×108 CFU/mL. This may reflect certain infectious syndromes such as VAP [16], but not necessarily a primary bacteraemia that may have a bacterial burden of up to 1 × 104 CFU/mL [117]. The bacterial burden is a key consideration given that the probability of a pre-existing resistant subpopulation increases with a larger bacterial burden. Second, the lack of an immune response in in vitro studies limits the potential application to clinical practice. Reducing the bacterial burden to below 1 × 105 CFU/mL, as demonstrated in a murine thigh infection [118] and a pneumonia [119] model, may result in bacterial CL over 24–48 h. Thus, antibiotic exposures required to suppress the emergence of resistance in vivo may only require the bacterial burden to be reduced to below the threshold for immune CL. However, bacteria also develop mechanisms to evade the host immune response by modulating immune signalling [120] and forming biofilms [121]. Lastly, the antibiotic exposure required for the perceived suppression of emergence of resistance was less for experiments with a duration < 72 h compared with longer durations. This observation may be in keeping with a study of patients with VAP where a treatment duration of 15 days was associated with an increased risk of resistance emergence compared with 8 days of therapy (42.1% vs. 62.3%; p = 0.04) [122]. Moreover, critically ill patients with P. aeruginosa infections receiving > 15 days of meropenem (OR 10, 95% CI 1.98–551), piperacillin/tazobactam (OR 4.7, 95% CI 1.8–12.4), ciprofloxacin (OR 14.5, 95% CI 2.8–75) or ceftazidime (OR 2.6, 95% CI 1.1–6) were at an increased risk of emergence of resistance [123].

Nevertheless, there are some limitations in our included studies. First, there was a high degree of experimental design heterogeneity among the included studies, such as different initial bacterial inoculums between studies (e.g. 105–108 CFU/mL), differing experimental durations (e.g. < 24–240 h), and different pharmacokinetic simulations performed. Thus, it was difficult to draw a definite conclusion on the PK/PD indices or required antibiotic exposures for suppression of the emergence of resistance. Second, preclinical studies have mostly used susceptible bacterial strains with a low MIC that may not be representative of that bacterial species. However, a maximum effect (Emax) model revealed that the less susceptible strains displayed lower Emax and higher half maximal effective concentration (EC50) for tobramycin effect against P. aeruginosa [124]. Third, bacterial growth conditions with the idealized laboratory conditions are extraordinarily different from growth within patients. Thus, it is not unexpected that there are evolved genomic differences between laboratory reference strains and corresponding clinical isolates [125, 126]. Fourth, few genomic data were available for bacterial strains that have been used for preclinical studies. Nonetheless, this represents current clinical practice where bacterial genomic data are not available for routine patient care. Last, we did not include in vivo studies that described the emergence of antibiotic resistance in anatomical sites distinct from the infecting site (e.g. the impact of antibiotic administration on the gastric microbiota). This may be an important consideration for future infection with a resistant organism; however, it is unclear how improved dosing regimens of systemically administered antibiotics may reduce the risk of resistance emergence in different anatomical sites where commensal bacteria may colonize [127, 128].

Due to heterogeneity of the included studies in this systematic review, no clear guidelines for clinical targets that should be used to suppress emergence of resistance can be drawn from the current data. As such, we believe that the following investigations should be prioritized for future research: (1) preclinical model testing of whether PK/PD targets associated with suppression of emergence of resistance remain accurate in bacterial strains with higher MICs; (2) preclinical studies using suboptimal PK/PD exposures and the correlation of phenotypic emergence of resistance with genomic data; and (3) clinical and bacteriological outcome data associated with achieving the aforementioned PK/PD targets associated with suppression of emergence of resistance.

5 Conclusions

This systematic review found that the antibiotic exposures for various classes of antibiotics reported to suppress the emergence of Gram-negative bacteria resistance were generally higher than exposures achievable by recommended antibiotic dosing regimens for clinical cure: β-lactam, Cmin/MIC ≥ 4; aminoglycosides, Cmax/MIC ratio ≥ 20; fluoroquinolones, AUC24/MPC ≥ 35; tetracyclines, AUC24/MIC ratio ≥ 50; polymyxin B, AUC24/MIC ≥ 808; and fosfomycin, AUC24/MIC ≥ 3136. In addition, the use of high antibiotic dosing that targets the thresholds required to suppress the emergence of resistance should be balanced with the potential risk for concentration-dependent adverse events. Optimization of alternative dosing regimens, such as the use of prolonged or continuous infusions of β-lactam antibiotics, should be considered to improve the probability of achieving the required antibiotic exposure to attain PK/PD indices for suppression of emergence of resistance; however, factors such as bacterial burden, MIC, and altered pharmacokinetics should be considered for optimization purposes.