Clinical and Laboratory Standards Institute and European Committee on Antimicrobial Susceptibility Testing Position Statements on Polymyxin B and Colistin Clinical Breakpoints

Recent data on polymyxin pharmacokinetics, pharmacodynamics, toxicity, and clinical outcomes suggest these agents have limited clinical utility. Pharmacokinetics-pharmacodynamics data show a steady-state concentration of 2 μg/mL is required for killing bacteria with colistin minimum inhibitory concentrations of 2 μg/mL. Less than 50% of patients with normal renal function achieve this exposure, and it is associated with high risk of nephrotoxicity. This exposure does not achieve bacterial stasis in pneumonia models. Randomized and observational studies consistently demonstrate increased mortality for polymyxins compared with alternative agents. The Clinical and Laboratory Standards Institute (CLSI) and European Committee on Antimicrobial Susceptibility Testing (EUCAST) are 2 global organizations that establish interpretive criteria for in vitro susceptibility data. CLSI has recently taken the step to eliminate the "susceptible" interpretive category for the polymyxins, whereas EUCAST maintains this interpretive category. This viewpoint describes the opinions of these organizations and the data that were used to inform their perspectives.

Utilising sigmoid models to predict the spread of antimicrobial resistance at the country level

BackgroundThe spread of antimicrobial resistance (AMR) is of worldwide concern. Public health policymakers and pharmaceutical companies pursuing antibiotic development require accurate predictions about the future spread of AMR.AimWe aimed to identify and model temporal and geographical patterns of AMR spread and to predict future trends based on a slow, intermediate or rapid rise in resistance.MethodsWe obtained data from five antibiotic resistance surveillance projects spanning the years 1997 to 2015. We aggregated the isolate-level or country-level data by country and year to produce country-bacterium-antibiotic class triads. We fitted both linear and sigmoid models to these triads and chose the one with the better fit. For triads that conformed to a sigmoid model, we classified AMR progression into one of three characterising paces: slow, intermediate or fast, based on the sigmoid slope. Within each pace category, average sigmoid models were calculated and validated.ResultsWe constructed a database with 51,670 country-year-bacterium-antibiotic observations, grouped into 7,440 country-bacterium-antibiotic triads. A total of 1,037 triads (14%) met the inclusion criteria. Of these, 326 (31.4%) followed a sigmoid (logistic) pattern over time. Among 107 triads for which both sigmoid and linear models could be fit, the sigmoid model was a better fit in 84%. The sigmoid model deviated from observed data by a median of 6.5%; the degree of deviation was related to the pace of spread.ConclusionWe present a novel method of describing and predicting the spread of antibiotic-resistant organisms.

Feasibility of de-linking reimbursement of antimicrobials from sales: the Australian perspective as a qualitative case study

Background

There is a disparity in the economic return achievable for antimicrobials compared with other drugs because of the need for stewardship. This has led to a decline in pharmaceutical companies’ willingness to invest in the development of these drugs and a consequent global interest in funding models where reimbursement is de-linked from sales.

Objectives

To explore the perspective of stakeholders regarding the feasibility of de-linked reimbursement of antimicrobials in Australia.

Methods

Semi-structured interviews were conducted with 18 participants sourced from the pharmaceutical industry and individuals representing public-sector payers or regulators. Interviews were transcribed verbatim, coded and thematically analysed using the framework method.

Results

Five key themes were identified in the interviews: funding silos are a barrier to de-linking reimbursement; varying levels of supporting evidence are (currently) required for funding depending upon setting; funding status or cost is used as a stewardship tool; a de-linked model may cost more; and concerns regarding governance and access to antimicrobials exist in the private sector.

Conclusions

Australia’s current multi-tiered funding of medicines across different levels of government was perceived as a barrier to de-linked reimbursement. Participants felt that the responsibility for antimicrobial funding and stewardship should be integrated and centralized. Implementing a nationally funded de-linked reimbursement model for new antimicrobials would require a review of funding decision-making criteria, given that most MDR infections are off-label indications and could not then be funded through the Australian Pharmaceutical Benefits Scheme. Findings from this study could be applicable to other countries with reimbursement frameworks similar to Australia.

Daptomycin in the treatment of enterococcal bloodstream infections and endocarditis: a EUCAST position paper

SCOPE

This position paper describes the view adopted by EUCAST on the role of daptomycin in the treatment of serious infections caused by Enterococcus species.

BACKGROUND

High-dose daptomycin is considered effective in the treatment of enterococcal bloodstream infection (BSI) and endocarditis, although published clinical experience with the latter condition is limited.

METHODS

EUCAST reviewed the available published data on pharmacokinetics-pharmacodynamics (PK-PD), resistance selection, clinical efficacy and safety for the use of 10-12 mg/kg/day of daptomycin for these conditions, noting that the doses licensed by the European Medicines Agency are only 4-6 mg/kg/day, and only for infections caused by Staphylococcus aureus.

FINDINGS AND RECOMMENDATIONS

The PK-PD evidence shows that, even with doses of 10-12 mg/kg/day, it is not possible to treat infections caused by isolates at the upper end of the wild-type distributions of Enterococcus faecalis (with MICs of 4 mg/L) and E. faecium (with MICs of 4 or 8 mg/L). For this reason, and because there are ongoing issues with the reliability of laboratory testing, EUCAST lists daptomycin breakpoints for Enterococcus species as "IE"-insufficient evidence. EUCAST advises increased vigilance in the use of high-dose of daptomycin to treat enterococcal BSI and endocarditis. Additional PK-PD studies and prospective efficacy and safety studies of serious Enterococcal infections treated with high-dose daptomycin may permit the setting of breakpoints in the future.

Estimating the utilisation of unregistered antimicrobials in Australia

OBJECTIVE

To identify and estimate the usage of unregistered antimicrobial drugs in Australian clinical practice.

METHODS

A descriptive pharmaco-epidemiological study, utilising three data sources: analysis of Special Access Scheme (SAS) applications for unregistered antimicrobials included in clinical guidelines over a five year period, analysis of antimicrobials dispensed from South Australian public hospital pharmacy departments over a two year period and analysis of National Antimicrobial Utilisation Surveillance Program (NAUSP) data for reported inpatient usage of unregistered antimicrobials in Australian hospitals over the last 5 years.

RESULTS

59 unregistered antimicrobials were identified using the mixed methods. 18,362 Special Access Scheme applications were submitted between May 2012 and April 2017 to access the 20 unregistered antimicrobials identified in the Therapeutic Guidelines® (eTG complete); 51.4% were determined by the prescriber to be for life-threatening indications. Annual applications more than doubled over the five years. 34 unregistered antimicrobials were dispensed from South Australian public hospitals between July 2015 and June 2017. On average, 1.1% of total antimicrobial usage (Defined Daily Doses) per month was accessed via the SAS, of which 87.7% were for outpatients or discharged patients. 34 unregistered antimicrobials for systemic use identified in the NAUSP database were used in Australian hospitals between 2013 and 2018.

CONCLUSION

The use of unregistered antimicrobials in Australian clinical practice is not uncommon. With increasing antimicrobial resistance, there will be a continued reliance on older less-used antimicrobial agents and an increasing need for novel drugs, therefore regulatory pathways need to facilitate security of supply and assurance of medicine quality and safety.

Clinical indications treated with unregistered antimicrobials: regulatory challenges of antimicrobial resistance and access to effective treatment for patients

Objective Increasing antimicrobial resistance and a concurrent paucity of new antimicrobials marketed increases the risk that patients will develop infections resistant to currently available drugs. This study aimed to determine the range of clinical indications for which unregistered antimicrobials are prescribed at two tertiary hospitals in South Australia to identify any trends over a 2-year period. The effects of recent regulatory changes to the Special Access Scheme (SAS) were assessed. Methods Data were extracted from application forms submitted to the Therapeutic Goods Administration to access unregistered antimicrobials via the SAS pathway at two Australian tertiary hospitals for the period July 2015–June 2017. Average weighted antimicrobial prices were retrieved from the hospital iPharmacy (DXC Technology, Macquarie Park, NSW, Australia) dispensing system. To estimate the effect of a new access pathway (Category C), the SAS classification for each application was retrospectively assessed over time with each regulatory change. Results Between July 2015 and June 2017, 477 SAS applications for 29 different antimicrobials were submitted for 353 patients at the two hospitals. The most common indications were tuberculosis (43.6%) and refractory Helicobacter pylori (10%). Regulatory changes reduced the proportion of applications requiring preapproval for access. Conclusions Although the introduction of a new pathway has decreased the administrative burden when accessing unregistered antimicrobials, this study highlights the range of clinical conditions for which there are no registered drugs available in Australia. What is known about the topic? With increasing antimicrobial resistance and a paucity of novel antimicrobials entering the market, access to older, previously less-used antimicrobials is increasingly important in clinical practice. Accessing unregistered antimicrobials is common practice in Australian hospitals, but the range of clinical indications for which they are used is unclear. What does this paper add? Increasing antimicrobial resistance and a concurrent paucity of new antimicrobials being marketed globally is increasing the risk that patients may develop infections that cannot be treated with registered products. This study describes the range of clinical conditions for which registered antimicrobials are not available or appropriate, illustrating the challenges associated with sustainable access to effective treatments. What are the implications for practitioners? Access to effective antimicrobials in a timely manner is essential for optimal patient outcomes. Reliance on unregistered products is associated with increased risks regarding timely access to safe, quality-assured, effective medicines.

Variation of MIC measurements: the contribution of strain and laboratory variability to measurement precision

Objectives

Although testing of antimicrobial agents for susceptibility has inherent variability like any assay, it is generally held that there are also real differences in susceptibility between strains. In the routine laboratory, variability of the MIC measurement may be sufficient to mask real strain differences. We determined which factors contributed to the variability, using linezolid against Staphylococcus aureus as one example.

Methods

Twenty-five S. aureus strains were sent to five different laboratories in quadruplicate in a blinded fashion. Laboratories determined MICs of linezolid using Etest. Results of 22 strains corresponding to 440 observations were available for analysis. Sources of variability were explored and quantified using an ANOVA approach.

Results

The overall geometric mean MIC was 1.8 mg/L, comparable to that of the published WT distribution of 1.7 mg/L (www.eucast.org). The total variation amounted to ∼1.3 2-fold dilutions for a one-sided CI of 95% and two 2-fold dilutions for a CI of 99%. Variation between laboratories and variation between strains contributed 10% and 48%, and in a subset analysis averaging 17% and 26%, respectively. Strain-to-strain variation (biological variation) could not be reliably determined, even with four replicates.

Conclusions

This analysis serves as an example of an approach to discerning various sources of MIC variation. Here, at best, a single measurement of an MIC may provide an indication of whether it likely belongs to the WT distribution. Only repeated measurements of MICs for individual strains within one laboratory may provide an indication of differences in susceptibility between strains.

Polymyxin Susceptibility Testing and Breakpoint Setting

Susceptibility testing of polymyxins has been subject to intensive review and revision in recent years. A joint working group was established by the Clinical and Laboratory Standards Institute and the European Committee on Antimicrobial Susceptibility Testing to establish a reference method. Issues examined included the effects of divalent cations, binding to laboratory materials, and addition of polysorbate 80. The working group recommended the use of broth microdilution without the addition of polysorbate 80 as the reference method. Published studies have shown that other testing methods, including agar dilution, disk diffusion and gradient diffusion, have unacceptably high levels of very major errors compared to the reference method, and are not recommended for routine laboratory use. Most data were for the testing of colistin; less information was available for polymyxin B. The joint working group was also asked to consider the setting of clinical breakpoints for relevant pathogens. This task involved examination of the available pharmacokinetic-pharmacodynamic, pharmacokinetic-toxicodynamic and population clinical pharmacokinetic data. All current pharmacokinetic-pharmacodynamic targets are based on MICs generated using the reference broth dilution procedure.

Probenecid and food effects on flucloxacillin pharmacokinetics and pharmacodynamics in healthy volunteers

OBJECTIVES

To measure the effect of probenecid, fasting and fed, on flucloxacillin pharmacokinetic and pharmacodynamic endpoints.

METHODS

Flucloxacillin 1000 mg orally was given to 11 volunteers alone while fasting ('flucloxacillin alone'), and with probenecid 500 mg orally while fasting ('probenecid fasting') and with food ('probenecid fed'). Flucloxacillin pharmacokinetic and pharmacodynamic endpoints were compared.

RESULTS

Probenecid, fasting and fed, increased free plasma flucloxacillin area under the concentration-time curve (zero to infinity) ∼1.65-fold (p < 0.01) versus flucloxacillin alone. Probenecid fed prolonged time to peak flucloxacillin concentrations ∼2-fold versus the other two regimens (p < 0.01). Probenecid fasting or fed increased free flucloxacillin concentrations exceeding 30%, 50% and 70% of the first 6, 8 and 12 h post-dose by 1.58- to 5.48-fold compared with flucloxacillin alone. As an example of this pharmacodynamic improvement, the probability of target attainment of free concentrations above the minimum inhibitory concentration for Staphylococcus aureus (0.5 mg/L) for 50% of a 6-hour dose interval was > 80% for flucloxacillin plus probenecid (fasting or fed) and < 20% for flucloxacillin alone.

CONCLUSIONS

Probenecid increased flucloxacillin exposure, with predicted pharmacodynamic effects greater than pharmacokinetic effects because of the altered shape of the concentration-time curve. Probenecid may improve the applicability of oral flucloxacillin regimens.

Benzylpenicillin versus flucloxacillin for penicillin-susceptible Staphylococcus aureus bloodstream infections from a large retrospective cohort study

In clinical practice, differing opinions exists regarding the optimal management of patients with penicillin-susceptible Staphylococcus aureus (PSSA) bloodstream infection (BSI). The aim of this study was to compare the 30-day mortality of patients treated with benzylpenicillin or flucloxacillin for PSSA BSI from a large prospectively collected data set from Australia and New Zealand. A logistic regression model and propensity score treatment analysis using inverse probability of treatment weighting were used. A total of 915 patients were included in the study, with an overall mortality rate of 12.9% (118/915) [benzylpenicillin 10.5% (33/315) and flucloxacillin 14.2% (85/600)]. Endocarditis was associated with benzylpenicillin treatment choice, whereas skin and soft-tissue infection was associated with flucloxacillin treatment choice. In the multivariate analysis, increased 30-day mortality was associated with flucloxacillin compared with benzylpenicillin [odds ratio (OR) = 1.6, 95% confidence interval (CI) 1.0-2.5; P = 0.05). When adjusted for treatment choice in the propensity score analysis, flucloxacillin was again associated with increased 30-day mortality (OR = 1.06, 95% CI 1.01-1.1; P = 0.03). An increase in 30-day mortality associated with flucloxacillin use suggests a potential benefit for benzylpenicillin therapy in patients with PSSA BSI.

MIC-based dose adjustment: facts and fables

Over recent decades, several publications have described optimization procedures for antibiotic therapy in the individual patient based on antimicrobial MIC values. Most methods include therapeutic drug monitoring and use a single MIC determination plus the relevant pharmacokinetics/pharmacodynamics to adjust the dose to optimize antimicrobial drug exposure and antibacterial effects. However, the use of an MIC obtained by a single MIC determination is inappropriate. First, routine clinical laboratories cannot determine MICs with sufficient accuracy to guide dosage owing to the inherent assay variation in the MIC test. Second, the variation in any MIC determination, whatever method is used, must be accounted for. If dose adjustments are made based on therapeutic drug monitoring and include MIC determinations, MIC variation must be considered to prevent potential underdosing of patients. We present the problems and some approaches that could be used in clinical practice.

Antimicrobial resistance in clinical Escherichia coli isolated from companion animals in Australia

Multidrug-resistant (MDR) Escherichia coli have become a major public health concern to both humans and animal health. While the frequency of antimicrobial resistance (AMR) in clinical E. coli is monitored regularly in human medicine, current frequency of AMR in companion animals remains unknown in Australia. In this study we conducted antimicrobial susceptibility testing (AST) and where possible, determined potential risk factors for MDR infection among 883 clinical Escherichia coli isolated from dogs (n=514), cats (n=341) and horses (n=28). AST was undertaken for 15 antimicrobial agents according to the Clinical Laboratory Standards Institute (CLSI) guidelines and interpreted using epidemiological cut-off values (ECOFFs) as well as CLSI veterinary and human clinical breakpoints. The AST revealed complete absence of resistance to carbapenems while resistance to amikacin was observed at a low level in isolates from dogs (1.6%) and cats (1.5%) compared to horses (10.7%). Among dog isolates, resistance to fluoroquinolones ranged from 9.1%-9.3% whereas among cat isolates, it ranged from 3.2%-5%. Among dog isolates, the proportion showing a 3rd generation cephalosporin (3GC) non-wild type phenotype was significantly higher (P<0.05) in skin and soft tissue infection (SSTI, n=122) isolates (17.2%-20.5%) compared to urinary tract infection (UTI, n=392) isolates (9.9%-10.2%). The frequency of multidrug resistance was 18.1%, 11.7% and 42.9% in dog, cat and horse isolates, respectively. Risk factor analysis revealed that MDR E. coli isolated from UTI were positively associated with chronicity of infection and previous antimicrobial treatment. Dogs and cats with chronic UTI that had been previously treated with antimicrobials were eight times and six times more likely to be infected with MDR E. coli compared to dogs and cats with non-chronic UTI, and no history of antimicrobial treatment, respectively. This study revealed that pre-existing disease condition and prior antimicrobial use were the major risks associated with UTI with MDR E. coli in companion animals.

Antimicrobial use Guidelines for Treatment of Respiratory Tract Disease in Dogs and Cats: Antimicrobial Guidelines Working Group of the International Society for Companion Animal Infectious Diseases

Respiratory tract disease can be associated with primary or secondary bacterial infections in dogs and cats and is a common reason for use and potential misuse, improper use, and overuse of antimicrobials. There is a lack of comprehensive treatment guidelines such as those that are available for human medicine. Accordingly, the International Society for Companion Animal Infectious Diseases convened a Working Group of clinical microbiologists, pharmacologists, and internists to share experiences, examine scientific data, review clinical trials, and develop these guidelines to assist veterinarians in making antimicrobial treatment choices for use in the management of bacterial respiratory diseases in dogs and cats.

Epidemiology and Mortality of Staphylococcus aureus Bacteremia in Australian and New Zealand Children

IMPORTANCE

Staphylococcus aureus bacteremia (SAB) in children causes significant morbidity and mortality, but the epidemiology in children is not well characterized.

OBJECTIVE

To describe the epidemiology of SAB in children and adolescents younger than 18 years from Australia and New Zealand.

DESIGN, SETTING, AND PARTICIPANTS

A prospective cohort study, using data from the Australian New Zealand Cooperative on Outcomes in Staphylococcal Sepsis cohort for 1153 children with SAB from birth to less than 18 years in pediatric and general hospitals across Australia and New Zealand, collected between January 1, 2007, and December 31, 2012. Multivariate analysis was performed to identify risk factors for mortality. Incidence calculations were calculated separately for Australasian children younger than 15 years using postcode population denominator data from Australian and New Zealand census data.

MAIN OUTCOMES AND MEASURES

Demographic data, hospital length of stay, principal diagnosis, place of SAB onset (community or hospital), antibiotic susceptibility and principal antibiotic treatment, and 7- and 30-day mortality.

RESULTS

Of the 1153 children with SAB, complete outcome data were available for 1073 children (93.1%); of these, males accounted for 684 episodes (63.7%) of SAB. The median age was 57 months (interquartile range, 2 months to 12 years). The annual incidence of SAB for Australian children was 8.3 per 100 000 population and was higher in indigenous children (incident rate ratio, 3.0 [95% CI, 2.4-3.7]), and the incidence for New Zealand children was 14.4 per 100 000 population and was higher in Māori children (incident rate ratio, 5.4 [95% CI, 4.1-7.0]). Community-onset SAB occurred in 761 cases (70.9%), and 142 cases (13.2%) of the infections were methicillin-resistant S aureus (MRSA). Bone or joint infection was most common with 348 cases (32.4%), and endocarditis was uncommon with 30 cases (2.8%). Seven- and 30-day mortality rates were 2.6% (n = 28) and 4.7% (n = 50), respectively. Risk factors for mortality were age younger than 1 year; Māori or Pacific ethnicity; endocarditis, pneumonia, or sepsis; and receiving no treatment or treatment with vancomycin. Mortality was 14.0% (6 of 43) in children with methicillin-susceptible S aureus (MSSA) treated with vancomycin compared with 2.6% (22 of 851) in children treated with alternative agents (OR, 6.1 [95% CI, 1.9-16.7]). MRSA infection was associated with increased length of stay but not mortality.

CONCLUSIONS AND RELEVANCE

In this large cohort study of the epidemiology of SAB in children, death was uncommon, but the incidence was higher for infants and varied by treatment, ethnicity, and clinical presentation. This study provides important information on the epidemiology of SAB in children and risk factors for mortality.