Aminoglycoside adaptive resistance: importance for effective dosage regimens

Pseudomonas aeruginosa Phosphate Transporter PitA (PA4292) Controls Susceptibility to Aminoglycoside Antibiotics by Regulating the Proton Motive Force

Pseudomonas aeruginosa is an opportunistic Gram-negative bacterium that causes nosocomial infections in immunocompromised patients. β-lactam and aminoglycoside antibiotics are commonly used in the treatment of P. aeruginosa infections. Previously, we found that mutation in a PA4292 gene increases bacterial resistance to β-lactam antibiotics. In this study, we demonstrated that mutation in PA4292 increases bacterial susceptibility to aminoglycoside antibiotics. We further found enhanced uptake of tobramycin by the ΔPA4292 mutant, which might be due to an increase of proton motive force (PMF). Sequence analysis revealed PA4292 is homologous to the Escherichia coli phosphate transporter PitA. Mutation of PA4292 indeed reduces intracellular phosphate concentration. We thus named PA4292 as pitA. Although the PMF is enhanced in the ΔpitA mutant, the intracellular ATP concentration is lower than that in the isogenic wild-type strain PA14, which might be due to lack of the ATP synthesis substrate phosphate. Overexpression of the phosphate transporter complex genes pstSCAB in the ΔpitA mutant restores the intracellular phosphate concentration, PMF, ATP synthesis, and aminoglycosides resistance. In addition, growth of wild-type PA14 in a low-phosphate medium resulted in higher PMF and aminoglycoside susceptibility compared to cells grown in a high-phosphate medium. Overall, our results demonstrate the roles of PitA in phosphate transportation and reveal the relationship between intracellular phosphate and aminoglycoside susceptibility.

Use of Antibiotics in Preterm Newborns

Due to complex maturational and physiological changes that characterize neonates and affect their response to pharmacological treatments, neonatal pharmacology is different from children and adults and deserves particular attention. Although preterms are usually considered part of the neonatal population, they have physiological and pharmacological hallmarks different from full-terms and, therefore, need specific considerations. Antibiotics are widely used among preterms. In fact, during their stay in neonatal intensive care units (NICUs), invasive procedures, including central catheters for parental nutrition and ventilators for respiratory support, are often sources of microbes and require antimicrobial treatments. Unfortunately, the majority of drugs administered to neonates are off-label due to the lack of clinical studies conducted on this special population. In fact, physiological and ethical concerns represent a huge limit in performing pharmacokinetic (PK) studies on these subjects, since they limit the number and volume of blood sampling. Therapeutic drug monitoring (TDM) is a useful tool that allows dose adjustments aiming to fit plasma concentrations within the therapeutic range and to reach specific drug target attainment. In this review of the last ten years’ literature, we performed Pubmed research aiming to summarize the PK aspects for the most used antibiotics in preterms.

Pharmacokinetic/Pharmacodynamic Modeling and Application in Antibacterial and Antifungal Pharmacotherapy: A Narrative Review

Pharmacokinetics and pharmacodynamics are areas in pharmacology related to different themes in the pharmaceutical sciences, including therapeutic drug monitoring and different stages of drug development. Although the knowledge of these disciplines is essential, they have historically been treated separately. While pharmacokinetics was limited to describing the time course of plasma concentrations after administering a drug-dose, pharmacodynamics describes the intensity of the response to these concentrations. In the last decades, the concept of pharmacokinetic/pharmacodynamic modeling (PK/PD) emerged, which seeks to establish mathematical models to describe the complete time course of the dose-response relationship. The integration of these two fields has had applications in optimizing dose regimens in treating antibacterial and antifungals. The anti-infective PK/PD models predict the relationship between different dosing regimens and their pharmacological activity. The reviewed studies show that PK/PD modeling is an essential and efficient tool for a better understanding of the pharmacological activity of antibacterial and antifungal agents.

Translational in vitro and in vivo PKPD modelling for apramycin against Gram-negative lung pathogens to facilitate prediction of human efficacious dose in pneumonia

OBJECTIVES

New drugs and methods to efficiently fight carbapenem-resistant Gram-negative pathogens are sorely needed. In this study we characterized the preclinical pharmacokinetics and pharmacodynamics of the clinical-stage drug candidate apramycin in time kill and mouse lung infection models. Based on in vitro and in vivo data, we developed a mathematical model to predict human efficacy.

METHODS

Three pneumonia-inducing Gram-negative species Acinetobacter baumannii, Pseudomonas aeruginosa, and Klebsiella pneumoniae were studied. Bactericidal kinetics were evaluated with time-kill curves; in vivo pharmacokinetics were studied in healthy and infected mice, with sampling in plasma and epithelial lining fluid after subcutaneous administration; in vivo efficacy was measured in a neutropenic mouse pneumonia model. A pharmacokinetic-pharmacodynamic model, integrating all the data, was developed and simulations were performed.

RESULTS

Good lung penetration of apramycin in epithelial lining fluid (ELF) was shown (AUC_ELF/AUC_plasma = 88%). Plasma clearance was 48% lower in lung infected mice compared to healthy mice. For two out of five strains studied, a delay in growth (∼5h) was observed in vivo but not in vitro. The mathematical model enabled integration of lung pharmacokinetics to drive mouse PKPD. Simulations predicted that 30 mg/kg of apramycin once daily would result in bacteriostasis in patients.

CONCLUSION

Apramycin is a candidate for treatment of carbapenem-resistant Gram-negative pneumonia as demonstrated in an integrated modeling framework for three bacterial species. We show that mathematical modelling is a useful tool for simultaneous inclusion of multiple data sources, notably plasma and lung in vivo PK and simulation of expected scenarios in a clinical setting, notably lung infections.

Dosing of Aminoglycosides in Chronic Kidney Disease and End-Stage Renal Disease Patients Treated with Intermittent Hemodialysis

<b><i>Background:</i></b> The dosing of aminoglycosides (AGs) in patients with kidney disease is challenging due to their markedly prolonged half-life, which renders pulse dosing schedules unsuitable. We performed a review of the literature that describes the pharmacokinetics of, and dosing recommendations for, AG for patients with abnormal renal functions and various renal replacement therapy modalities, focusing on patients treated with intermittent hemodialysis (iHD). <b><i>Summary:</i></b> During one iHD session, dialysis removes a remarkable amount of the drug regardless of the dialyzer type. In patients with severely reduced kidney functions, the distribution phase is prolonged, which needs to be taken into account when drawing samples shortly after drug administration or following an iHD session. <b><i>Key Messages:</i></b> The doses recommended for the pulse dosing of patients without kidney disease leads to unacceptably high overall systemic exposure for patients with severely reduced kidney functions even with dosing intervals extended up to 48 h. Therefore, lower doses accompanied by extended dosing intervals must be applied for this patient group. The clinical evidence and current recommendations support the dosing of AG following, rather than before, HD sessions. In patients with end-stage kidney disease, the samples for TDM of AGs should not be drawn earlier than 2 h after end of the infusion and 4 h after the end of iHD session to allow full (re)distribution of the drug.

Gentamicin Dosing in Neonates with Normal Renal Function: Trough and Peak Levels

BACKGROUND AND OBJECTIVE: Gentamicin is commonly used in neonates, and it requires drug concentration monitoring. The objective of this study was to determine the extent of high trough (≥ 2 mg/l) and therapeutic peak serum gentamicin concentrations (5-12 mg/l) using our current gentamicin regimen and to adjust the dosing regimen accordingly and reassess.

METHODS

This was a prospective cohort study of neonates, with normal renal function, who were prescribed gentamicin. Group 1: March 2014-July 2017-gentamicin intravenous (IV) 2.5 mg/kg given every 36 h if < 30 weeks gestational age (GA) and every 24 h if ≥ 30 weeks GA; Group 2: August 2019-February 2020-gentamicin IV 3.5 mg/kg given every 36 h if < 30 weeks GA and every 24 h if ≥ 30 weeks GA. We assessed the number of neonates with aberrant trough and peak serum gentamicin concentrations.

RESULTS

Forty-eight neonates < 30 weeks GA and 34 ≥ 30 weeks GA were given 2.5 mg/kg gentamicin. Eleven (23%) neonates < 30 weeks GA and four (13%) ≥ 30 weeks GA had subtherapeutic peak concentrations (< 5 mg/l); none had supratherapeutic (> 12 mg/l) or toxic trough concentrations (≥ 2 mg/l). Forty-four neonates < 30 weeks GA and 54 ≥ 30 weeks GA were given 3.5 mg/kg gentamicin. Eighty-four (86%) had non-toxic trough concentrations (< 2 mg/l). One (1%) < 30 weeks GA neonate had subtherapeutic (< 5 mg/l) and one (1%) neonate ≥ 30 weeks GA had supratherapeutic (> 12 mg/l) peak concentrations.

CONCLUSIONS

Gentamicin regimen of 2.5 mg/kg given every 36 h for neonates < 30 weeks GA and every 24 h for neonates ≥ 30 weeks GA was suboptimal at achieving therapeutic gentamicin peak. Increasing the dosage to 3.5 mg/kg achieved therapeutic peak concentrations in 98% and non-toxic trough concentrations in 86% of all neonates (prior to dose interval adjustment).

Pharmacokinetics of culture-directed antibiotics for the treatment of peritonitis in automated peritoneal dialysis: A systematic narrative review

The objectives of this study were to provide a summary of the pharmacokinetic data of some intraperitoneal (IP) antibiotics that could be used for both empirical and culture-directed therapy, as per the ISPD recommendations, and examine factors to consider when using IP antibiotics for the management of automated peritoneal dialysis (APD)-associated peritonitis. A literature search of PubMed, EMBASE, Scopus, MEDLINE and Google Scholar for articles published between 1998 and 2020 was conducted. To be eligible, articles had to describe the use of antibiotics via the IP route in adult patients ≥18 years old on APD in the context of pharmacokinetic studies or case reports/series. Articles describing the use of IP antibiotics that had been recently reviewed (cefazolin, vancomycin, gentamicin and ceftazidime) or administered for non-APD-associated peritonitis were excluded. A total of 1119 articles were identified, of which 983 abstracts were screened. Seventy-three full-text articles were assessed for eligibility. Eight records were included in the final study. Three reports had pharmacokinetic data in patients on APD without peritonitis. Each of cefepime 15 mg/kg IP, meropenem 0.5 g IP and fosfomycin 4 g IP given in single doses achieved drug plasma concentrations above the minimum inhibitory concentration for treating the susceptible organisms. The remaining five records were case series or reports in patients on APD with peritonitis. While pharmacokinetic data support intermittent cefepime 15 mg/kg IP daily, only meropenem 0.5 g IP and fosfomycin 4 g IP are likely to be effective if given in APD exchanges with dwell times of 15 h. Higher doses may be required in APD with shorter dwell times. Information on therapeutic efficacy was derived from case reports/series in individual patients and without therapeutic drug monitoring. Until more pharmacokinetic data are available on these antibiotics, it would be prudent to shift patients who develop peritonitis on APD to continuous ambulatory peritoneal dialysis, where pharmacokinetic information is more readily available.

Triclosan depletes the membrane potential in Pseudomonas aeruginosa biofilms inhibiting aminoglycoside induced adaptive resistance

Biofilm-based infections are difficult to treat due to their inherent resistance to antibiotic treatment. Discovering new approaches to enhance antibiotic efficacy in biofilms would be highly significant in treating many chronic infections. Exposure to aminoglycosides induces adaptive resistance in Pseudomonas aeruginosa biofilms. Adaptive resistance is primarily the result of active antibiotic export by RND-type efflux pumps, which use the proton motive force as an energy source. We show that the protonophore uncoupler triclosan depletes the membrane potential of biofilm growing P. aeruginosa, leading to decreased activity of RND-type efflux pumps. This disruption results in increased intracellular accumulation of tobramycin and enhanced antimicrobial activity in vitro. In addition, we show that triclosan enhances tobramycin effectiveness in vivo using a mouse wound model. Combining triclosan with tobramycin is a new anti-biofilm strategy that targets bacterial energetics, increasing the susceptibility of P. aeruginosa biofilms to aminoglycosides.

Monitoring of Tobramycin Exposure: What is the Best Estimation Method and Sampling Time for Clinical Practice?

OBJECTIVES

The objective of this article is to investigate the influence of blood sampling times on tobramycin exposure estimation and clinical decisions and to determine the best sampling times for two estimation methods used for therapeutic drug monitoring.

METHODS

Adult patients with cystic fibrosis, treated with once-daily intravenous tobramycin, were intensively sampled over one 24-h dosing interval to determine true exposure (AUC_0-24). The AUC_0-24s were then estimated using both log-linear regression and Bayesian forecasting methods for 21 different sampling time combinations. These were compared to true exposure using relative prediction errors. The differences in subsequent dose recommendations were calculated.

RESULTS

Twelve patients, with a median (range) age of 25 years (18-36) and weight of 66.5 kg (50.6-76.4) contributed 96 tobramycin concentrations. Five hundred and eighty-eight estimated AUC_0-24s were compared to 12 measured true AUC_0-24 values. Median relative prediction errors ranged from - 34.7 to 45.5% for the log-linear regression method and from - 14.46 to 11.23% for the Bayesian forecasting method across the 21 sampling combinations. The most unbiased exposure estimation was provided from concentrations sampled at 100/640 min after the start of the infusion using log-linear regression and at 70/160 min using Bayesian forecasting. Subsequent dosing recommendations varied greatly depending on the estimation method and the sampling times used.

CONCLUSION

Sampling times markedly influence bias in AUC_0-24 estimation, leading to greatly varied dose adjustments. The impact of blood sampling times on dosing decisions is reduced when using Bayesian forecasting.

Hydrogels Embedded With Melittin and Tobramycin Are Effective Against Pseudomonas aeruginosa Biofilms in an Animal Wound Model

We demonstrate that the antimicrobial peptide, melittin, is effective alone and in combination with the aminoglycosides tobramycin to kill Pseudomonas aeruginosa growing as biofilms both in vitro and in vivo. Melittin and tobramycin show enhanced in vitro activity in combination at micromolar concentrations, resulting in a 2-log10 reduction in the number of cells within mature PAO1 P. aeruginosa biofilms after 6-h of treatment. Alternatively, either agent alone resulted in half-a-log10 reduction. Time-killing assays demonstrated that the combination of melittin and tobramycin was effective at 2-h whereas tobramycin was not effective until after 6-h of treatment. We also found the combination was more effective than tobramycin alone against biofilms of 7 P. aeruginosa cystic fibrosis clinical isolates, resulting in a maximum 1.5-log10 cellular reduction. Additionally, melittin alone was effective at killing biofilms of 4 Staphylococcus aureus isolates, resulting in a maximum 2-log10 cellular reduction. Finally, melittin in combination with tobramycin embedded in an agarose-based hydrogel resulted in a 4-fold reduction in bioluminescent P. aeruginosa colonizing mouse wounds by 4-h. In contrast, tobramycin or melittin treatment alone did not cause a statistically significant reduction in bioluminescence. These data demonstrate that melittin in combination with tobramycin embedded in a hydrogel is a potential treatment for biofilm-associated wound infections.

Optimisation of antimicrobial dosing based on pharmacokinetic and pharmacodynamic principles

While suboptimal dosing of antimicrobials has been attributed to poorer clinical outcomes, clinical cure and mortality advantages have been demonstrated when target pharmacokinetic (PK) and pharmacodynamic (PD) indices for various classes of antimicrobials were achieved to maximise antibiotic activity. Dosing optimisation requires a good knowledge of PK/PD principles. This review serves to provide a foundation in PK/PD principles for the commonly prescribed antibiotics (β-lactams, vancomycin, fluoroquinolones and aminoglycosides), as well as dosing considerations in special populations (critically ill and obese patients). PK principles determine whether an appropriate dose of antimicrobial reaches the intended pathogen(s). It involves the fundamental processes of absorption, distribution, metabolism and elimination, and is affected by the antimicrobial's physicochemical properties. Antimicrobial pharmacodynamics define the relationship between the drug concentration and its observed effect on the pathogen. The major indicator of the effect of the antibiotics is the minimum inhibitory concentration. The quantitative relationship between a PK and microbiological parameter is known as a PK/PD index, which describes the relationship between dose administered and the rate and extent of bacterial killing. Improvements in clinical outcomes have been observed when antimicrobial agents are dosed optimally to achieve their respective PK/PD targets. With the rising rates of antimicrobial resistance and a limited drug development pipeline, PK/PD concepts can foster more rational and individualised dosing regimens, improving outcomes while simultaneously limiting the toxicity of antimicrobials.

Triclosan Is an Aminoglycoside Adjuvant for Eradication of Pseudomonas aeruginosa Biofilms

One of the most important clinical obstacles in cystic fibrosis (CF) treatment is antibiotic treatment failure due to biofilms produced by Pseudomonas aeruginosa The ability of this pathogen to survive eradication by tobramycin and pathoadapt into a hyperbiofilm state leading to chronic infections is key to its success. Retrospective studies have demonstrated that preventing this pathoadaptation by improving eradication is essential to extend the lives of CF patients. To identify adjuvants that enhance tobramycin eradication of P. aeruginosa, we performed a high-throughput screen of 6,080 compounds from four drug-repurposing libraries. We identified that the Food and Drug Administration (FDA)-approved compound triclosan, in combination with tobramycin, resulted in a 100-fold reduction of viable cells within biofilms at 6 h, but neither compound alone had significant antimicrobial activity against biofilms. This synergistic treatment significantly accelerated the killing of biofilms compared to that with tobramycin treatment alone, and the combination was effective against 6/7 CF clinical isolates compared to tobramycin treatment alone, including a tobramycin-resistant strain. Further, triclosan and tobramycin killed persister cells, causing a 100-fold reduction by 8 h and complete eradication by 24 h. Triclosan also enhances tobramycin killing of multiple Burkholderia cenocepacia and Staphylococcus aureus clinical isolates grown as biofilms. Additionally, triclosan showed synergy with other aminoglycosides, such as gentamicin or streptomycin. Triclosan is a well-tolerated aminoglycoside adjuvant shown to be safe for human use that could improve the treatment of biofilm-based infections.

Extended-interval Dosing of Gentamicin in Premature Neonates Born at <32 Weeks' Gestation and >7 Days of age

PURPOSE

Extended-interval dosing (EID) regimens of gentamicin have been validated for treating confirmed or suspected early- and late-onset sepsis in preterm infants in the first week of life. Despite the marked changes in volume of distribution and renal clearance in preterm infants after the first few days of life, few studies have validated EID regimens of gentamicin in this population. The objective of the study was to evaluate an EID regimen of gentamicin in infants born at <32 weeks' gestational age and aged >7 days.

METHODS

This observational study of an EID regimen was conducted in 39 infants. Dosing interval was based on the serum drug concentration at 22 hours after the administration of the first dose of 5 mg/kg. Gentamicin peak (5-12 µg/mL) and trough (<2 µg/mL) levels were compared to those in a historical control group of 39 infants who received traditional-interval dosing (TID) of 2.5 mg/kg of gentamicin with dosing intervals of 8, 12, or 24 hours.

FINDINGS

There were no differences in birthweight, gestational age at birth, postmenstrual age, weight at the start of gentamicin administration, postnatal age, small for gestational age status, antenatal corticosteroid use, or postnatal indomethacin exposure between the 2 groups. In the EID group, dosing intervals were 24 hours in 30 infants, 36 hours in 6 infants, and 48 hours in 3 infants. Compared with the TID group (n = 39), the EID group had a significantly higher peak level (median, 9.0 vs 4.7 µg/mL) and a significantly lower trough level (median, 0.7 vs 1.1 µg/mL) (both, P < 0.001). On regression analysis, the postmenstrual age was correlated significantly with trough levels in the EID group. There was no adverse effect on renal function in either group. On follow-up, 1 infant in the EID group and 2 infants in the TID group had evidence of sensorineural hearing loss.

IMPLICATIONS

In infants born at <32 weeks' gestation and >7 days of age, an EID gentamicin regimen, with a dosing interval based on a single concentration measurement at 22 hours after the administration of the first dose, achieved therapeutic peak and trough levels and performed significantly better than did a TID regimen in reaching target peak and trough levels. Larger-scale trials are needed for assessing the clinical efficacy (treatment failure/success) of these regimens.

Resistance suppression by high-intensity, short-duration aminoglycoside exposure against hypermutable and non-hypermutable Pseudomonas aeruginosa

OBJECTIVES

Hypermutable bacteria are causing a drastic problem via their enhanced ability to become resistant. Our objectives were to compare bacterial killing and resistance emergence between differently shaped tobramycin concentration-time profiles at a given fAUC/MIC, and determine the tobramycin exposure durations that prevent resistance.

METHODS

Static concentration time-kill studies over 24 h used Pseudomonas aeruginosa WT strains (ATCC 27853 and PAO1) and hypermutable PAOΔmutS. fAUC/MIC values of 36, 72 and 168 were assessed at initial inocula of 10⁶ and 10⁴ cfu/mL (all strains) and 10^1.2 cfu/mL (PAOΔmutS only) in duplicate. Tobramycin was added at 0 h and removed at 1, 4, 10 or 24 h. Proportions of resistant bacteria and MICs were determined at 24 h. Mechanism-based modelling was conducted.

RESULTS

For all strains, high tobramycin concentrations over 1 and 4 h resulted in more rapid and extensive initial killing compared with 10 and 24 h exposures at a given fAUC/MIC. No resistance emerged for 1 and 4 h durations of exposure, although extensive regrowth of susceptible bacteria occurred. The 24 h duration of exposure revealed less regrowth, but tobramycin-resistant populations had completely replaced susceptible bacteria by 24 h for the 10⁶ cfu/mL inoculum. The hypermutable PAOΔmutS showed the highest numbers of resistant bacteria. Total and resistant bacterial counts were described well by novel mechanism-based modelling.

CONCLUSIONS

Extensive resistance emerged for 10 and 24 h durations of exposure, but not for shorter durations. The tobramycin concentration-time profile shape is vital for resistance prevention and should aid the introduction of optimized combination regimens.

We Give Aminoglycoside Antibiotics at the End of Hemodialysis

We often administer dialyzable medications after dialysis to ensure that adequate concentrations are maintained in the body until the next session. In the case of aminoglycosides, we may have the opportunity to dose more aggressively predialysis, and then use the clearance of the drug by dialysis to limit toxicities. Predialysis aminoglycoside dosing is intriguing, although studies evaluating the safety and efficacy of this approach are necessary before it can be used routinely.

Evaluation of a Once/Day Tobramycin Regimen to Achieve Target Concentrations in Adult Patients with Cystic Fibrosis

STUDY OBJECTIVE

To evaluate the success of an initial tobramycin dosing regimen to achieve target peak and trough concentrations in adult patients with pulmonary exacerbations of cystic fibrosis (CF).

DESIGN

Retrospective single-center medical record review.

SETTING

Large tertiary care academic medical center.

PATIENTS

A total of 186 patient encounters where 112 patients with CF were treated for acute pulmonary exacerbations with 10 mg/kg/day of tobramycin between January 1, 2009, and December 5, 2014.

MEASUREMENTS AND MAIN RESULTS

Baseline demographics, clinical characteristics, tobramycin data, and pharmacokinetic variables were collected. The primary outcome evaluated the success of the initial tobramycin dosing regimen in attaining the target peak concentration. Secondary end points were achievement of the target trough concentration, achievement of combined peak and trough targets, and incidence of nephrotoxicity. Bivariate and multivariate analyses were performed to evaluate factors associated with achieving target concentrations. Of the 186 patient encounters, 41% achieved the target peak with the first dosing regimen, 62% achieved a target trough, and 23% achieved the target peak and trough. Nephrotoxicity occurred in 10% of patient encounters. A body mass index (BMI) of 18.5-24.9 kg/m(2) was associated with higher odds of meeting the target peak compared with a BMI lower than 18.5 kg/m(2) (odds ratio [OR] 24.5; 95% confidence interval [CI] 5.2-117.2). Conversely, a BMI of 18.5-24.9 kg/m(2) was associated with lower odds of attaining the target trough compared with a BMI lower than 18.5 kg/m(2) (OR 0.16; 95% CI 0.05-0.56). Higher volume of distribution and elimination rate constants (Kel ) were associated with significantly lower odds of achieving the target peak. In addition, higher Kel values were associated with significantly higher odds of achieving the target trough.

CONCLUSIONS

The current initial tobramycin regimen did not achieve target serum tobramycin concentrations reliably. Optimization of the initial CF tobramycin dosing regimen is warranted.

Pharmacometric Approaches to Personalize Use of Primarily Renally Eliminated Antibiotics in Preterm and Term Neonates

Sepsis remains a major cause of mortality and morbidity in neonates, and, as a consequence, antibiotics are the most frequently prescribed drugs in this vulnerable patient population. Growth and dynamic maturation processes during the first weeks of life result in large inter- and intrasubject variability in the pharmacokinetics (PK) and pharmacodynamics (PD) of antibiotics. In this review we (1) summarize the available population PK data and models for primarily renally eliminated antibiotics, (2) discuss quantitative approaches to account for effects of growth and maturation processes on drug exposure and response, (3) evaluate current dose recommendations, and (4) identify opportunities to further optimize and personalize dosing strategies of these antibiotics in preterm and term neonates. Although population PK models have been developed for several of these drugs, exposure-response relationships of primarily renally eliminated antibiotics in these fragile infants are not well understood, monitoring strategies remain inconsistent, and consensus on optimal, personalized dosing of these drugs in these patients is absent. Tailored PK/PD studies and models are useful to better understand relationships between drug exposures and microbiological or clinical outcomes. Pharmacometric modeling and simulation approaches facilitate quantitative evaluation and optimization of treatment strategies. National and international collaborations and platforms are essential to standardize and harmonize not only studies and models but also monitoring and dosing strategies. Simple bedside decision tools assist clinical pharmacologists and neonatologists in their efforts to fine-tune and personalize the use of primarily renally eliminated antibiotics in term and preterm neonates.

Insights into Newer Antimicrobial Agents Against Gram-negative Bacteria

Currently, drug resistance, especially against cephalosporins and carbapenems, among gram-negative bacteria is an important challenge, which is further enhanced by the limited availability of drugs against these bugs. There are certain antibiotics (colistin, fosfomycin, temocillin, and rifampicin) that have been revived from the past to tackle the menace of superbugs, including members of Enterobacteriaceae, Acinetobacter species, and Pseudomonas species. Very few newer antibiotics have been added to the pool of existing drugs. There are still many antibiotics that are passing through various phases of clinical trials. The initiative of Infectious Disease Society of America to develop 10 novel antibiotics against gram-negative bacilli by 2020 is a step to fill the gap of limited availability of drugs. This review aims to provide insights into the current and newer drugs in pipeline for the treatment of gram-negative bacteria and also discusses the major challenging issues for their management.

Dose Timing of Aminoglycosides in Hemodialysis Patients: A Pharmacology View

Aminoglycosides for patients undergoing intermittent hemodialysis (IHD) have traditionally been dosed at half the normal dose administered at the end of a hemodialysis session. Several investigations have suggested that administering higher doses preceding or with the initiation of dialysis would more readily optimize pharmacodynamic parameters. However, the selection of an optimal aminoglycoside dosing strategy in patients receiving IHD is complex and requires consideration of numerous factors, precluding a singular approach. By reviewing aminoglycoside pharmacokinetics, pharmacodynamics, risks for toxicity and resistance development, and practical considerations, we derive indication- and setting- specific recommendations. We identify some areas (definitive therapy of gram-negative infections in patients receiving predictable hemodialysis sessions, for example) where dosing preceding or with the initiation of dialysis is optimal and feasible, and others (gram-positive synergy, unstable patients with poor/unpredictable vascular access) where postdialysis dosing remains preferred. Finally, given the dearth of data exploring the pharmacodynamics and clinical outcomes of IHD patients receiving aminoglycoside therapy, we identify several key questions in need of investigation.

Population pharmacokinetic study of gentamicin in a large cohort of premature and term neonates

AIM

This study aims to investigate the clinical and demographic factors influencing gentamicin pharmacokinetics in a large cohort of unselected premature and term newborns and to evaluate optimal regimens in this population.

METHODS

All gentamicin concentration data, along with clinical and demographic characteristics, were retrieved from medical charts in a Neonatal Intensive Care Unit over 5 years within the frame of a routine therapeutic drug monitoring programme. Data were described using non-linear mixed-effects regression analysis ( nonmem®).

RESULTS

A total of 3039 gentamicin concentrations collected in 994 preterm and 455 term newborns were included in the analysis. A two compartment model best characterized gentamicin disposition. The average parameter estimates, for a median body weight of 2170 g, were clearance (CL) 0.089 l h(-1) (CV 28%), central volume of distribution (Vc ) 0.908 l (CV 18%), intercompartmental clearance (Q) 0.157 l h(-1) and peripheral volume of distribution (Vp ) 0.560 l. Body weight, gestational age and post-natal age positively influenced CL. Dopamine co-administration had a significant negative effect on CL, whereas the influence of indomethacin and furosemide was not significant. Both body weight and gestational age significantly influenced Vc . Model-based simulations confirmed that, compared with term neonates, preterm infants need higher doses, superior to 4 mg kg(-1) , at extended intervals to achieve adequate concentrations.

CONCLUSIONS

This observational study conducted in a large cohort of newborns confirms the importance of body weight and gestational age for dosage adjustment. The model will serve to set up dosing recommendations and elaborate a Bayesian tool for dosage individualization based on concentration monitoring.

Extended-interval gentamicin dosing in achieving therapeutic concentrations in malaysian neonates

OBJECTIVE

To evaluate the usefulness of extended-interval gentamicin dosing practiced in neonatal intensive care unit (NICU) and special care nursery (SCN) of a Malaysian hospital.

METHODS

Cross-sectional observational study with pharmacokinetic analysis of all patients aged ≤28 days who received gentamicin treatment in NICU/SCN. Subjects received dosing according to a regimen modified from an Australian-based pediatric guideline. During a study period of 3 months, subjects were evaluated for gestational age, body weight, serum creatinine concentration, gentamicin dose/interval, serum peak and trough concentrations, and pharmacokinetic parameters. Descriptive percentages were used to determine the overall dosing accuracy, while analysis of variance (ANOVA) was conducted to compare the accuracy rates among different gestational ages. Pharmacokinetic profile among different gestational age and body weight groups were compared by using ANOVA.

RESULTS

Of the 113 subjects included, 82.3% (n = 93) achieved therapeutic concentrations at the first drug-monitoring assessment. There was no significant difference found between the percentage of term neonates who achieved therapeutic concentrations and the premature group (87.1% vs. 74.4%), p = 0.085. A total of 112 subjects (99.1%) achieved desired therapeutic trough concentration of <2 mg/L. Mean gentamicin peak concentration was 8.52 mg/L (95% confidence interval [Cl], 8.13-8.90 mg/L) and trough concentration was 0.54 mg/L (95% CI, 0.48-0.60 mg/L). Mean volume of distribution, half-life, and elimination rate were 0.65 L/kg (95% CI, 0.62-0.68 L/kg), 6.96 hours (95% CI, 6.52-7.40 hours), and 0.11 hour(-1) (95% CI, 0.10-0.11 hour(-1)), respectively.

CONCLUSION

The larger percentage of subjects attaining therapeutic range with extended-interval gentamicin dosing suggests that this regimen is appropriate and can be safely used among Malaysian neonates.

Stochastic process pharmacodynamics: dose timing in neonatal gentamicin therapy as an example

We consider dosing regimens designed to cure patients by eradicating colony forming units (CFU) such as bacteria. In the field of "population" pharmaco-kinetics/dynamics (PK/PD), inter-individual variability (IIV) of patients is estimated using model parameter statistical distributions. We consider a more probabilistic approach to IIV called stochastic process theory, motivated by the fact that tumor treatment planning uses both approaches. Stochastic process PD can supply additional insights and suggest different dosing regimens due to its emphasis on the probability of complete CFU eradication and its predictions on "pure chance" fluctuations of CFU number per patient when treatment has reduced this integer to less than ~100. To exemplify the contrast between stochastic process PD models and standard deterministic PD models, which track only average CFU number, we analyze, neglecting immune responses, neonatal intravenous gentamicin dosing regimens directed against Escherichia coli. Our stochastic calculations predict that the first dose is crucial for CFU eradication. For example, a single 6 mg/kg dose is predicted to have a higher eradication probability than four daily 4 mg/kg doses. We conclude: (1) neonatal gentamicin dosing regimens with larger first doses but smaller total doses deserve investigation; (2) in general, if standard PK/PD models predict average CFU number drops substantially below 100, the models should be modified to incorporate stochastic effects more accurately, and will then usually make more favorable, or less unfavorable, predictions for front boosting ("hit hard early"). Various caveats against over-interpreting the calculations are given.

Pharmacokinetic/pharmacodynamic modelling approaches in paediatric infectious diseases and immunology

Pharmacokinetic/pharmacodynamic (PKPD) modelling is used to describe and quantify dose-concentration-effect relationships. Within paediatric studies in infectious diseases and immunology these methods are often applied to developing guidance on appropriate dosing. In this paper, an introduction to the field of PKPD modelling is given, followed by a review of the PKPD studies that have been undertaken in paediatric infectious diseases and immunology. The main focus is on identifying the methodological approaches used to define the PKPD relationship in these studies. The major findings were that most studies of infectious diseases have developed a PK model and then used simulations to define a dose recommendation based on a pre-defined PD target, which may have been defined in adults or in vitro. For immunological studies much of the modelling has focused on either PK or PD, and since multiple drugs are usually used, delineating the relative contributions of each is challenging. The use of dynamical modelling of in vitro antibacterial studies, and paediatric HIV mechanistic PD models linked with the PK of all drugs, are emerging methods that should enhance PKPD-based recommendations in the future.

Combination therapy for carbapenem-resistant Gram-negative bacteria

The emergence of resistant to carbapenems Gram-negative bacteria (CR GNB) has severely challenged antimicrobial therapy. Many CR GNB isolates are only susceptible to polymyxins; however, therapy with polymyxins and other potentially active antibiotics presents some drawbacks, which have discouraged their use in monotherapy. In this context, along with strong pre-clinical evidence of benefit in combining antimicrobials against CR GNB, the clinical use of combination therapy has been raised as an interesting strategy to overcome these potential limitations of a single agent. Polymyxins, tigecycline and even carbapenems are usually the cornerstone agents in combination schemes. Optimization of the probability to attain the pharmacokinetic/pharmacodynamic targets by both cornerstone drug and adjuvant drug is of paramount importance to achieve better clinical and microbiological outcomes. Clinical evidence of the major drugs utilized in combination schemes and how they should be prescribed considering pharmacokinetic/pharmacodynamic characteristics against CR GNB will be reviewed in this article.

Guidelines for aminoglycoside use and applicability to geriatric patients

OBJECTIVES

The authors had for objective to evaluate the applicability of AFSSAPS guidelines for aminoglycoside use to geriatric patients.

METHODS

Theoretical doses and dosing regimens allowing reaching target concentrations in this population were calculated by applying a pharmacokinetic model to 30 geriatric patients treated by amikacin.

RESULTS

The dose allowing reaching a maximum concentration of 60 mg/L was 1.217 mg on average. The time required to reach a blood concentration lower than or equal to 2.5mg/L was 62.5±70.4 hours. Forty-six percent of patients had a trough concentration greater than 2.5 mg/L, 48 hours after administration. For these patients, the time between critical minimum inhibitory concentration (MIC) and toxicity threshold concentration was 21.9±14.9 hours.

CONCLUSION

Reaching a target concentration can be problematic in geriatric patients. It is frequently necessary to use dosing intervals greater than 48 hours. The effectiveness and safety of these regimens remain uncertain.

Dosing of Gentamicin in Patients With End-Stage Renal Disease Receiving Hemodialysis

The aim of this study was to evaluate dosing schedules of gentamicin in patients with end-stage renal disease and receiving hemodialysis. Forty-six patients were recruited who received gentamicin while on hemodialysis. Each patient provided approximately 4 blood samples at various times before and after dialysis for analysis of plasma gentamicin concentrations. A population pharmacokinetic model was constructed using NONMEM (version 5). The clearance of gentamicin during dialysis was 4.69 L/h and between dialysis was 0.453 L/h. The clearance between dialysis was best described by residual creatinine clearance (as calculated using the Cockcroft and Gault equation), which probably reflects both lean mass and residual clearance mechanisms. Simulation from the final population model showed that predialysis dosing has a higher probability of achieving target maximum concentration (Cmax) concentrations (> 8 mg/L) within acceptable exposure limits (area under the concentration-time curve [AUC] values > 70 and < 120 mg x h/L per 24 hours) than postdialysis dosing.

Development of a Semimechanistic Model to Describe the Pharmacokinetics of Gentamicin in Patients Receiving Hemodialysis

The aim of this study was to ascertain the most suitable dosing schedule for gentamicin in patients receiving hemodialysis. We developed a model to describe the concentration-time course of gentamicin in patients receiving hemodialysis. Using the model, an optimal dosing schedule was evaluated. Various dosing regimens were compared in their ability to achieve maximum concentration (C(max), > or = 8 mg/L) and area under the concentration time-curve (AUC > or = 70 mg x h/L and < or = 120 mg x h/L per 24 hours). The model was evaluated by comparing model predictions against real data collected retrospectively. Simulations from the model confirmed the benefits of predialysis dosing. The mean optimal dose was 230 mg administered immediately before dialysis. The model was found to have good predictive performance when simulated data were compared to data observed in real patients. In summary, a model was developed that describes gentamicin pharmacokinetics in patients receiving hemodialysis. Predialysis dosing provided a superior pharmacokinetic profile than did postdialysis dosing.

Extended interval dosing of gentamicin in premature neonates ≤ 28-week gestation

AIM

To evaluate an extended interval dosing (EID) regimen of gentamicin in neonates ≤28-week gestation.

METHODS

In 2008, an EID regimen for gentamicin was introduced for all neonates admitted to the NICU in Calgary. The dosing interval was based on a 22 h level after the first dose of 5mg/kg. We conducted an observational study in 33 infants ≤28-week gestation on the EID regimen from the first day of life and compared gentamicin peak and trough levels with a historical control of 34 infants who received gentamicin in a dose of 2.5 mg/kg every 24 h (TID, traditional interval dosing).

RESULTS

In the EID group, based on the 22 h level, dosing interval was 36 h in 20 neonates and 48 h in 13 neonates. All neonates, except one, achieved therapeutic peak and trough levels. Compared to the TID group, the EID group had higher peak levels (median 9.8 μg/mL vs. 4.6 μg/mL, p < 0.001) with no difference in trough levels. With target peak levels of 5-12 μg/mL and trough levels of <2 μg/mL, a higher proportion of neonates in the TID group would need dose adjustment.

CONCLUSION

In neonates ≤ 28-week gestation, an EID regimen from day one of life, using a single level 22 h after the first dose for dosing interval, achieves therapeutic peak and trough levels and more optimum peak levels as compared to a TID regimen.

Pharmacokinetic-pharmacodynamic model for gentamicin and its adaptive resistance with predictions of dosing schedules in newborn infants

Gentamicin is commonly used in the management of neonatal infections. Development of adaptive resistance is typical for aminoglycosides and reduces the antibacterial effect. There is, however, a lack of understanding of how this phenomenon influences the effect of different dosing schedules. The aim was to develop a pharmacokinetic-pharmacodynamic (PKPD) model that describes the time course of the bactericidal activity of gentamicin and its adaptive resistance and to investigate different dosing schedules in preterm and term newborn infants based on the developed model. In vitro time-kill curve experiments were conducted on a strain of Escherichia coli (MIC of 2 mg/liter). The gentamicin exposure was either constant (0.125 to 16 mg/liter) or dynamic (simulated concentration-time profiles in a kinetic system with peak concentrations of 2.0, 3.9, 7.8, and 16 mg/liter given as single doses or as repeated doses every 6, 12, or 24 h). Semimechanistic PKPD models were fitted to the bacterial counts in the NONMEM (nonlinear mixed effects modeling) program. A model with compartments for growing and resting bacteria, with a function allowing the maximal bacterial killing of gentamicin to reduce with exposure, characterized both the fast bactericidal effect and the adaptive resistance. Despite a lower peak concentration, preterm neonates were predicted to have a higher bacterial killing effect than term neonates for the same per-kg dose because of gentamicin's longer half-life. The model supported an extended dosing interval of gentamicin in preterm neonates, and for all neonates, dosing intervals of 36 to 48 h were as effective as a 24-h dosing interval for the same total dose.

Current use for old antibacterial agents: polymyxins, rifamycins, and aminoglycosides

This article reviews three classes of antibacterial agents that are uncommonly used in bacterial infections and therefore can be thought of as special-use agents. The polymyxins are reserved for gram-negative bacilli that are resistant to virtually all other classes of drugs. Rifampin is used therapeutically, occasionally as a companion drug in treatment of refractory gram-positive coccal infections, especially those involving foreign bodies. Rifaximin is a new rifamycin that is a strict enteric antibiotic approved for treatment of traveler's diarrhea and is showing promise as a possible agent for refractory Clostridium difficile infections. The aminoglycosides are used mainly as companion drugs for the treatment of resistant gram-negative bacillary infections and for gram-positive coccal endocarditis.

Adaptive resistance to cationic compounds in Pseudomonas aeruginosa

Adaptive resistance is an autoregulated phenomenon characterised by induction of resistance in the presence of drug and reversal to the sensitive phenotype in its absence. This type of resistance is well documented for polycationic antibiotics, including aminoglycosides and polymyxins, in Pseudomonas aeruginosa and other aerobic Gram-negative bacilli. It is not caused by selection of resistant mutants but rather by phenotypic alterations in order to survive the lethal drug effect. Adaptive resistance to aminoglycosides is mainly mediated by the MexXY-OprM efflux pump that is rapidly upregulated in bacteria surviving the first exposure to aminoglycosides and is downregulated when bacteria are no longer in contact with the drug. A two-component regulatory system designated ParR-ParS plays a major role in adaptive resistance induced by cationic peptides. In the presence of cationic peptides, ParR-ParS activates the lipopolysaccharide modification operon (arnBCADTEF) leading to increased resistance in polymyxins and aminoglycosides. The bactericidal kinetics related to adaptive resistance have important clinical implications and provide a rationale for administering cationic antibiotics in larger initial and longer interval bolus dosing. A better understanding of this phenomenon and the molecular mechanisms responsible will be essential not only for optimum use of cationic antibiotics but also for developing new agents with ability to counteract the detrimental effects of adaptive resistance and thus enhance the therapeutic efficacy of polycationic compounds.

Interpreting culture and susceptibility data in critical care: perks and pitfalls

PROBLEM

The need for immediate, effective antimicrobial therapy in the critical care patient must be tempered by approaches which simultaneously minimize emergence of antimicrobial resistance. Ideally, therapy will successfully resolve clinical signs of infection, while eradicating infecting pathogens such that the risk of resistance is avoided. Increasing limitations associated with empirical antimicrobial choices direct the need for culture and susceptibility data as a basis of therapy. Even so, such in vitro data should be utilized within its limitations.

OBJECTIVES

To demonstrate the attributes and limitations of patient and population culture and susceptibility (pharmacodynamic) data in the selection of antimicrobial drugs and to demonstrate the design of individualized dosing regimens based on integration of pharmacodynamic (PD) and pharmacokinetic (PK) data.

DIAGNOSIS

Limitations in culture and susceptibility testing begin with sample collection and continue through drug selection and dose design. Among the challenges in interpretation is discrimination between pathogens and commensals. Properly collected samples are critical for generation of data relevant to the patient's infection. Data are presented as minimum inhibitory concentrations (MICs). The MIC facilitate selection of the most appropriate drug, particularly when considered in the context of antimicrobial concentrations achieved in the patient at a chosen dose. Integration of MIC data with key PK data yields the C(max):MIC important to efficacy of concentration-dependent drugs and T>MIC, which guides use of time-dependent drugs. These indices are then used to design dosing regimens that are more likely to kill all infecting pathogens. In the absence of patient MIC data, population data (eg, MIC(90)) may serve as a reasonable surrogate.

CONCLUSIONS

Properly collected, performed, and interpreted culture and susceptibility data are increasingly important in the selection of and design of dosing regimens for antimicrobial drugs. Integration of PK and PD data as modified by host and microbial factors supports a hit hard, exit fast approach to therapy that will facilitate efficacy while minimizing resistance.

Current use for old antibacterial agents: polymyxins, rifamycins, and aminoglycosides

This article reviews three classes of antibacterial agents that are uncommonly used in bacterial infections and therefore can be thought of as special-use agents. The polymyxins are reserved for gram-negative bacilli that are resistant to virtually all other classes of drugs. Rifampin is used therapeutically, occasionally as a companion drug in treatment of refractory gram-positive coccal infections, especially those involving foreign bodies. Rifaximin is a new rifamycin that is a strict enteric antibiotic approved for treatment of traveler's diarrhea and is showing promise as a possible agent for refractory Clostridium difficile infections. The aminoglycosides are used mainly as companion drugs for the treatment of resistant gram-negative bacillary infections and for gram-positive coccal endocarditis.

Observational trial of a 48-hour gentamicin dosing regimen derived from Monte Carlo simulations in infants born at less than 28 weeks' gestation

OBJECTIVE

To develop and validate a 48-hour gentamicin dosing regimen for infants born at <28 weeks' gestation.

STUDY DESIGN

Using previously published pharmacokinetic data, we performed Monte Carlo simulations for several candidate gentamicin dosing regimens. On the basis of these simulations, we changed dosing for infants born at <28 weeks to 4.5 mg/kg every 48 hours. We then conducted an observational study of 30 infants on this new regimen and compared serum gentamicin levels with 60 historical control subjects who received 2.5 mg/kg every 24 hours.

RESULTS

Infants in the 48-hour group achieved higher gentamicin peaks (mean 9.43 microg/mL vs 6.0 microg/mL, P < .001) and lower gentamicin troughs (mean 1.08 microg/mL vs 1.54 microg/mL, P < .001) compared with the 24-hour group. Seven percent of the 48-hour group infants had a gentamicin peak <6 microg/mL versus 43% in the 24-hour group. With a goal for peaks of 6 to 12 microg/mL and for troughs of <1.5 microg/mL, infants in the 48-hour group required fewer adjustments of their dosing regimens compared with the 24-hour group (26.7% vs 78.3%).

CONCLUSIONS

Gentamicin given every 48 hours to infants born at <28 weeks achieves optimal blood concentrations more frequently than does once-daily dosing. Monte Carlo simulations on the basis of pharmacokinetic modeling are useful to optimize drug dosing in premature infants.

Low-oxygen-recovery assay for high-throughput screening of compounds against nonreplicating Mycobacterium tuberculosis

Screening for new antimicrobial agents is routinely conducted only against actively replicating bacteria. However, it is now widely accepted that a physiological state of nonreplicating persistence (NRP) is responsible for antimicrobial tolerance in many bacterial infections. In tuberculosis, the key to shortening the 6-month regimen lies in targeting this NRP subpopulation. Therefore, a high-throughput, luminescence-based low-oxygen-recovery assay (LORA) was developed to screen antimicrobial agents against NRP Mycobacterium tuberculosis. M. tuberculosis H37Rv containing a plasmid with an acetamidase promoter driving a bacterial luciferase gene was adapted to low oxygen conditions by extended culture in a fermentor with a 0.5 headspace ratio. The MICs of 31 established antimicrobial agents were determined in microplate cultures maintained under anaerobic conditions for 10 days and, for comparative purposes, under aerobic conditions for 7 days. Cultures exposed to drugs under anaerobic conditions followed by 28 h of "recovery" under ambient oxygen produced a luminescent signal that was, for most compounds, proportional to the number of CFU determined prior to the recovery phase. No agents targeting the cell wall were active against NRP M. tuberculosis, whereas drugs hitting other cellular targets had a range of activities. The calculated Z' factor was in the range of 0.58 to 0.84, indicating the suitability of the use of LORA for high-throughput assays. This LORA is sufficiently robust for use for primary high-throughput screening of compounds against NRP M. tuberculosis.

Principles of Antimicrobial Therapy

The most compelling reason for practicing judicious antimicrobial use is to facilitate therapeutic success. The definition of therapeutic success has changed in recent years, however; not only does success include eradication of infection, but it must now include avoidance of resistance. If the goal of antimicrobial therapy is to achieve sufficient concentrations at the site of infection such that the infecting organism is killed, therapy should be successful.

Breakpoints for predicting Pseudomonas aeruginosa susceptibility to inhaled tobramycin in cystic fibrosis patients: use of high-range Etest strips

Inhaled administration of tobramycin assures high concentrations in cystic fibrotic lungs, improving the therapeutic ratio over that of parenteral tobramycin levels, particularly against Pseudomonas aeruginosa. Conventional Clinical and Laboratory Standards Institute (CLSI; formerly National Committee for Clinical Laboratory Standards) breakpoints only consider parenteral levels and do not take into account these high antimicrobial concentrations. The Spanish Antibiogram Committee (The MENSURA Group) has tentatively defined specific breakpoint values for inhaled tobramycin when testing P. aeruginosa isolates from cystic fibrosis (CF) patients (susceptible, < or =64 microg/ml; resistant, > or =128 microg/ml). The antimicrobial susceptibilities of 206 prospectively collected CF P. aeruginosa isolates were determined by the reference agar dilution method. For tobramycin, the performance of high range tobramycin Etest strips (AB Biodisk, Solna, Sweden) and conventional tobramycin disks were assessed with the same collection. Applying MENSURA proposed breakpoints, 95.1% of the strains were categorized as susceptible to tobramycin, either using agar dilution or Etest high-range strips (99% categorical agreement between both methods). With CLSI breakpoints, susceptibility rates decreased to 79.1 and 81.1% for agar dilution and Etest strips, respectively (83.5% categorical agreement). Minor, major, and very major errors for Etest strips (CLSI criteria) were 13.6, 1.2, and 14.8%, respectively. Upon applying the new proposed criteria for inhaled tobramycin, only one major and one very major error were observed with Etest strips. Whenever inhaled tobramycin is considered for therapy, we suggest that P. aeruginosa strains from CF patients categorized as intermediate or resistant to tobramycin according to the CLSI criteria should be retested with high-range Etest strips and recategorized using MENSURA interpretive criteria. CLSI breakpoints should still be followed when intravenous tobramycin is used in CF patients, particularly during the course of exacerbations.

Aminoglycoside extended interval dosing in neonates is safe and effective: a meta-analysis

OBJECTIVES

To review the evidence from controlled clinical trials of neonates given equal daily aminoglycoside doses as extended interval dosing (dosage interval typically 24 hours in term and 36-48 hours in immature neonates) compared with traditional dosing (dosage interval typically 8-12 hours in term and 12-24 hours in immature neonates).

DESIGN

Systematic review and meta-analysis of controlled trials found in electronic databases, trial registers, and references in reviews and selected trials.

SETTINGS

The selected trials were blinded and assessed for methodological quality. Each trial's own predefined criteria for treatment failure, nephrotoxicity, ototoxicity, and therapeutic serum drug concentrations were used.

SUBJECTS

Controlled trials of neonatal aminoglycoside treatment in which equal aminoglycoside daily doses were given at traditional and extended dosage intervals.

MAIN OUTCOME MEASURES

Serum drug concentrations outside the therapeutic range. Treatment failure and toxicity.

RESULTS

Sixteen trials involving 823 neonates met the inclusion criteria for the systematic review. Twelve trials involving 698 neonates were included in the meta-analysis of the pharmacokinetics. Compared with traditional dosing, extended interval dosing was associated with a significantly lower risk of both peak (summary risk ratio 0.50, 95% confidence interval 0.26 to 0.94) and trough (0.36, 0.25 to 0.56) serum drug concentrations outside the therapeutic range. Accurate information on treatment failure was obtained in nine trials involving 555 neonates. One trial reported treatment failure. In this trial two neonates in the traditional dosing group did not respond to treatment within 72 hours. Nephrotoxicity was investigated in 589 neonates in 12 trials and ototoxicity in 210 neonates in four trials, with no significant differences between the two dosing regimens.

CONCLUSIONS

Extended interval dosing of aminoglycosides in neonates is safe and effective, with a reduced risk of serum drug concentrations outside the therapeutic range.

Phenotypic tolerance: antibiotic enrichment of noninherited resistance in bacterial populations

When growing bacteria are exposed to bactericidal concentrations of antibiotics, the sensitivity of the bacteria to the antibiotic commonly decreases with time, and substantial fractions of the bacteria survive. Using Escherichia coli CAB1 and antibiotics of five different classes (ampicillin, ciprofloxacin, rifampin, streptomycin, and tetracycline), we examine the details of this phenomenon and, with the aid of mathematical models, develop and explore the properties and predictions of three hypotheses that can account for this phenomenon: (i) antibiotic decay, (ii) inherited resistance, and (iii) phenotypic tolerance. Our experiments cause us to reject the first two hypotheses and provide evidence that this phenomenon can be accounted for by the antibiotic-mediated enrichment of subpopulations physiologically tolerant to but genetically susceptible to these antibiotics, phenotypic tolerance. We demonstrate that tolerant subpopulations generated by exposure to one concentration of an antibiotic are also tolerant to higher concentrations of the same antibiotic and can be tolerant to antibiotics of the other four types. Using a mathematical model, we explore the effects of phenotypic tolerance to the microbiological outcome of antibiotic treatment and demonstrate, a priori, that it can have a profound effect on the rate of clearance of the bacteria and under some conditions can prevent clearance that would be achieved in the absence of tolerance.

Once-daily aminoglycoside administration

Once-daily aminoglycoside dosing is effective in treating gram-negative infections, including those associated with bones and joints.

Current use for old antibacterial agents: polymyxins, rifampin, and aminoglycosides

This article discusses three classes of antibacterial agents that are uncommonly used in bacterial infections (other than mycobacterial infections) and can be thought of as special-use agents. These are the polymyxins, rifampin, and the aminoglycosides.