Vincent H. Tam

Detection and treatment options for Klebsiella pneumoniae carbapenemases (KPCs): an emerging cause of multidrug-resistant infection

Bacteria producing Klebsiella pneumoniae carbapenemases (KPCs) are rapidly emerging as a cause of multidrug-resistant infections worldwide. Bacterial isolates harbouring these enzymes are capable of hydrolysing a broad spectrum of beta-lactams including the penicillins, cephalosporins, carbapenems and monobactam. Detection of isolates harbouring carbapenemases can be inconsistent using automated systems, often requiring subsequent confirmatory tests. Phenotypic methods utilizing boronic acid disc tests have demonstrated promising results and appear practical for use in clinical microbiology laboratories. Treatment of infection caused by KPC bacteria is particularly worrisome as the carbapenems are often agents of the last resort for resistant Gram-negative infections. The optimal treatment of infections caused by KPC bacteria is not well established and clinical outcome data remain sparse. We reviewed the current literature regarding clinical outcomes following KPC infections, with a specific effort to summarize the clinical data available for specific antimicrobial agents. A total of 15 papers involving 55 unique patient cases were reviewed. While the total number of patients is relatively small, some useful insights could still be gathered to guide clinicians in the management of KPC infections. Tigecycline and the aminoglycosides were associated with positive outcomes in the majority of cases. Clinical success rates were low when the polymyxins were used as monotherapy, but were much higher when they were used in combination. Studies examining combination therapy and well-controlled clinical trials are needed to ascertain the optimal treatment of infections caused by KPC bacteria.

Application of a mathematical model to prevent in vivo amplification of antibiotic-resistant bacterial populations during therapy

The worldwide increase in the prevalence of multi-antibiotic-resistant bacteria has threatened the physicians ability to provide appropriate therapy for infections. The relationship between antimicrobial drug concentration and infecting pathogen population reduction is of primary interest. Using data derived from mice infected with the bacterium Pseudomonas aeruginosa and treated with a fluoroquinolone antibiotic, a mathematical model was developed that described relationships between antimicrobial drug exposures and changes in drug-susceptible and -resistant bacterial subpopulations at an infection site. Dosing regimens and consequent drug exposures that amplify or suppress the emergence of resistant bacterial subpopulations were identified and prospectively validated. Resistant clones selected in vivo by suboptimal regimens were characterized. No mutations were identified in the quinolone resistance-determining regions of gyrA/B or parC/E. However, all resistant clones demonstrated efflux pump overexpression. At base line, MexAB-OprM, MexCD-OprJ, and MexEF-OprN were represented in the drug-resistant population. After 28 hours of therapy, MexCD-OprJ became the predominant pump expressed in the resistant clones. The likelihood of achieving resistance-suppression exposure in humans with a clinically prescribed antibiotic dose was determined. The methods developed in this study provide insight regarding how mathematical models can be used to identify rational dosing regimens that suppress the amplification of the resistant mutant population.

Pharmacodynamics of Caspofungin in a Murine Model of Invasive Pulmonary Aspergillosis: Evidence of Concentration-Dependent Activity

BACKGROUND A paucity of data exists regarding the pharmacodynamics of caspofungin (CAS) during invasive pulmonary aspergillosis (IPA). We conducted a dosage-fractionation study to characterize the in vivo pharmacodynamics of CAS activity during IPA, using immunosuppressed mice inoculated intranasally with Aspergillus fumigatus. METHODS After single intraperitoneal doses (0.25, 1.0, and 4.0 mg/kg), plasma CAS concentrations were assayed by high-performance liquid chromatography. The pharmacokinetic data were analyzed by nonparametric population pharmacokinetic analysis. Three dosage groups (0.25, 1.0, and 4.0 mg/kg) fractionated into 3 different dosing intervals (q6, q24, or q48 h) were then used to evaluate the pharmacokinetic/pharmacodynamic effects (percentage of time greater than the minimum effective concentration [MEC], 96-h area under the plasma concentration curve:MEC ratio, and peak concentration in plasma [Cmax]:MEC ratio) at clinically achievable exposures. Mice were treated for 96 h and were then euthanized, and their lungs were harvested for analysis of pulmonary fungal burden by real-time quantitative polymerase chain reaction. RESULTS A concentration-dependent reduction in mean pulmonary fungal burden was evident in mice in the 1 mg/kg dosage-fractionation group, with significantly lower mean pulmonary fungal burden in mice dosed q48 h versus q6 h (P < .01). A paradoxical increase in pulmonary fungal burden was observed in the highest dosage-fractionation group. CONCLUSIONS CAS demonstrates concentration-dependent pharmacodynamics in the treatment of IPA. The Cmax : MEC ratio appears to be the parameter most closely associated with the reduction of pulmonary fungal burden.

Impact of multidrug-resistant Pseudomonas aeruginosa infection on patient outcomes

Rates of antibiotic resistance in Pseudomonas aeruginosa are increasing worldwide. The multidrug-resistant (MDR) phenotype in P. aeruginosa could be mediated by several mechanisms including multidrug efflux systems, enzyme production, outer membrane protein (porin) loss and target mutations. Currently, no international consensus on the definition of multidrug resistance exists, making direct comparison of the literature difficult. Inappropriate empirical therapy has been associated with increased mortality in P. aeruginosa infections; delays in starting appropriate therapy may contribute to increased length of hospital stay and persistence of infection. In addition, worse clinical outcomes may be associated with MDR infections owing to limited effective antimicrobial options. This article aims to summarize the contemporary literature on patient outcomes following infections caused by drug-resistant P. aeruginosa. The impact of antimicrobial therapy on patient outcomes, mortality and morbidity; and the economic impact of MDR P. aeruginosa infections will be examined.

Nebulized Colistin in the Treatment of Pneumonia Due to Multidrug-Resistant Acinetobacter baumannii and Pseudomonas aeruginosa

Twenty-one patients with multidrug-resistant (MDR) Acinetobacter baumannii and Pseudomonas aeruginosa pneumonia were treated with nebulized polymyxin E (colistin). Overall clinical and microbiological response rates were 57.1% and 85.7%, respectively. Nebulized colistin may be reasonably efficacious and safe for treatment of MDR pneumonia. Its role in therapy warrants further investigation in comparative studies.

Pharmacodynamics of Polymyxin B against Pseudomonas aeruginosa

ABSTRACT Despite limited data, polymyxin B (PB) is increasingly used clinically as the last therapeutic option for multidrug-resistant (MDR) gram-negative bacterial infections. We examined the in vitro pharmacodynamics of PB against four strains of Pseudomonas aeruginosa. Clonal relatedness of the strains was assessed by random amplification of polymorphic DNA. Time-kill studies over 24 h were performed with approximately 105 and 107 CFU/ml of bacteria, using PB at 0, 0.25, 0.5, 1, 2, 4, 8, and 16× MIC. Dose fractionation studies were performed using an in vitro hollow-fiber infection model (HFIM) against a wild-type and a MDR strain. Approximately 105 CFU/ml of bacteria were exposed to placebo and three regimens (every 8 h [q8h], q12h, and q24h) simulating the steady-state unbound PB pharmacokinetics resulting from a daily dose of 2.5 mg/kg of body weight and 20 mg/kg (8 times the clinical dose). Samples were obtained over 4 days to quantify PB concentrations, total bacterial population, and subpopulation with reduced PB susceptibility (>3× MIC). The bactericidal activity of PB was concentration dependent, but killing was significantly reduced with a high inoculum. In HFIM studies, a significant reduction in bacterial load was seen at 4 h in all active regimens, but selective amplification of the resistant subpopulation(s) was apparent at 24 h with the clinical dose (both strains). Regrowth was eventually observed in all dosing regimens with the MDR strain, but its occurrence was prevented in the wild-type strain by using 8 times the clinical dose (regardless of dosing intervals). Our results suggested that the bactericidal activity of PB was concentration dependent and appeared to be related to the ratio of the area under the concentration-time curve to the MIC.

Framework for optimisation of the clinical use of colistin and polymyxin B: the Prato polymyxin consensus.

In the face of diminishing therapeutic options for the treatment of infections caused by multidrug-resistant, Gram-negative bacteria, clinicians are increasingly using colistin and polymyxin B. These antibiotics became available clinically in the 1950s, when understanding of antimicrobial pharmacology and regulatory requirements for approval of drugs was substantially less than today. At the 1st International Conference on Polymyxins in Prato, Italy, 2013, participants discussed a set of key objectives that were developed to explore the factors affecting the safe and effective use of polymyxins, identify the gaps in knowledge, and set priorities for future research. Participants identified several factors that affect the optimum use of polymyxins, including: confusion caused by several different conventions used to describe doses of colistin; an absence of appropriate pharmacopoeial standards for polymyxins; outdated and diverse product information; and uncertainties about susceptibility testing and breakpoints. High-priority areas for research included: better definition of the effectiveness of polymyxin-based combination therapy compared with monotherapy via well designed, randomised controlled trials; examination of the relative merits of colistin versus polymyxin B for various types of infection; investigation of pharmacokinetics in special patient populations; and definition of the role of nebulised polymyxins alone or in combination with intravenous polymyxins for the treatment of pneumonia. The key areas identified provide a roadmap for action regarding the continued use of polymyxins, and are intended to help with the effective and safe use of these important, last-line antibiotics.

Optimization of Meropenem Minimum Concentration/MIC Ratio To Suppress In Vitro Resistance of Pseudomonas aeruginosa

ABSTRACT Suppression of resistance in a dense Pseudomonas aeruginosa population has previously been shown with optimized quinolone exposures. However, the relevance to β-lactams is unknown. We investigated the bactericidal activity of meropenem and its propensity to suppress P. aeruginosa resistance in an in vitro hollow-fiber infection model (HFIM). Two isogenic strains of P. aeruginosa (wild type and an AmpC stably derepressed mutant [MIC = 1 mg/liter]) were used. An HFIM inoculated with approximately 1 × 108 CFU/ml of bacteria was subjected to various meropenem exposures. Maintenance doses were given every 8 h to simulate the maximum concentration achieved after a 1-g dose in all regimens, but escalating unbound minimum concentrations (Cmins) were simulated with different clearances. Serial samples were obtained over 5 days to quantify the meropenem concentrations, the total bacterial population, and subpopulations with reduced susceptibilities to meropenem (>3× the MIC). For both strains, a significant bacterial burden reduction was seen with all regimens at 24 h. Regrowth was apparent after 3 days, with the Cmin/MIC ratio being ≤1.7 (time above the MIC, 100%). Selective amplification of subpopulations with reduced susceptibilities to meropenem was suppressed with a Cmin/MIC of ≥6.2 or by adding tobramycin to meropenem (Cmin/MIC = 1.7). Investigations that were longer than 24 h and that used high inocula may be necessary to fully evaluate the relationship between drug exposures and the likelihood of resistance suppression. These results suggest that the Cmin/MIC of meropenem can be optimized to suppress the emergence of non-plasmid-mediated P. aeruginosa resistance. Our in vitro data support the use of an extended duration of meropenem infusion for the treatment of severe nosocomial infections in combination with an aminoglycoside.

Prevalence, Resistance Mechanisms, and Susceptibility of Multidrug-Resistant Bloodstream Isolates of Pseudomonas aeruginosa

ABSTRACT Pseudomonas aeruginosa is an important pathogen commonly implicated in nosocomial infections. The occurrence of multidrug-resistant (MDR) P. aeruginosa strains is increasing worldwide and limiting our therapeutic options. The MDR phenotype can be mediated by a variety of resistance mechanisms, and the corresponding relative biofitness is not well established. We examined the prevalence, resistance mechanisms, and susceptibility of MDR P. aeruginosa isolates (resistant to ≥3 classes of antipseudomonal agents [penicillins/cephalosporins, carbapenems, quinolones, and aminoglycosides]) obtained from a large, university-affiliated hospital. Among 235 nonrepeat bloodstream isolates screened between 2005 and 2007, 33 isolates (from 20 unique patients) were found to be MDR (crude prevalence rate, 14%). All isolates were resistant to carbapenems and quinolones, 91% were resistant to penicillins/cephalosporins, and 21% were resistant to the aminoglycosides. By using the first available isolate for each bacteremia episode (n = 18), 13 distinct clones were revealed by repetitive-element-based PCR. Western blotting revealed eight isolates (44%) to have MexB overexpression. Production of a carbapenemase (VIM-2) was found in one isolate, and mutations in gyrA (T83I) and parC (S87L) were commonly found. Growth rates of most MDR isolates were similar to that of the wild type, and two isolates (11%) were found to be hypermutable. All available isolates were susceptible to polymyxin B, and only one isolate was nonsusceptible to colistin (MIC, 3 mg/liter), but all isolates were nonsusceptible to doripenem (MIC, >2 mg/liter). Understanding and continuous monitoring of the prevalence and resistance mechanisms of MDR P. aeruginosa would enable us to formulate rational treatment strategies to combat nosocomial infections.

Risk factors for nephrotoxicity associated with continuous vancomycin infusion in outpatient parenteral antibiotic therapy

OBJECTIVES Continuous vancomycin infusion is increasingly used for outpatient management of infections, but the relationship between vancomycin and nephrotoxicity is controversial. We investigated the risk factors associated with nephrotoxicity in this setting. METHODS A retrospective cohort study of patients receiving continuous vancomycin infusion as outpatient parenteral antibiotic therapy (OPAT) was performed. The likelihood of developing nephrotoxicity (> or =50% increase in serum creatinine from baseline) was evaluated in relation to demographic variables, underlying co-morbidities, infectious disease diagnoses, concomitant drug exposures and vancomycin concentration. Logistic regression was used to determine the association of various variables. Classification and regression tree analysis was used to determine the most significant breakpoint for continuous variables. RESULTS We examined 102 adult patients between January 2004 and June 2007. The mean +/- SD age, baseline serum creatinine and steady-state vancomycin concentration were 48.2 +/- 17.6 years, 78.0 +/- 32.5 micromol/L and 15.5 +/- 10.8 mg/L, respectively. The majority of the patients (66.7%) were treated for bone and joint infection. The cumulative incidence of nephrotoxicity was 15.7%. Nephrotoxicity was found to be associated with hypertension [odds ratio (OR) 5.302 (95% confidence interval (CI) 1.159-24.246), P = 0.031], exposure to aminoglycosides [OR 6.594 (95% CI 1.026-42.385), P = 0.047], loop diuretics [OR 8.123 (95% CI 1.449-45.528), P = 0.017], and steady-state vancomycin concentration > or =28 mg/L [OR 21.236 (95% CI 2.687-167.857), P = 0.004]. CONCLUSIONS We have identified independent risk factors for nephrotoxicity in patients receiving continuous infusion vancomycin in OPAT. A serum steady-state vancomycin concentration > or =28 mg/L markedly increases the risk.