Amikacin Antimicrobial Susceptibility Discs

Abstract

Background

The purpose of this study was to define the potency of amikacin and comparator agents against a collection of blood and respiratory nosocomial isolates implicated in ICU based pulmonary infections gathered from US hospitals.

Methods

Minimum inhibitory concentrations of amikacin, aztreonam, cefepime, ceftazidime, ceftolozane/tazobactam, ceftriaxone, ciprofloxacin, imipenem, meropenem, piperacillin/tazobactam and tobramycin were tested against 2460 Gram-negative isolates. Amikacin had 96 % susceptibility against the combined E. coli and K. pneumoniae isolates and 95 % susceptibility against P. aeruginosa.

Results

Ninety-six percent of all of isolates tested were susceptible (i.e., MICs ≤16 mg/L) to amikacin by current laboratory standards which demonstrates a high level of activity to combat infections caused by these organisms including ESBL, MDR, β-lactam and fluoroquinolone resistant strains. Moreover, 99 % of all organisms had amikacin MICs ≤64 mg/L.

Conclusions

Overall, these data highlight the continued potency of amikacin and suggest that the achievable lung concentrations of approximately 5000 mg/L with the administration of the amikacin by inhalation (Amikacin Inhale, BAY41-6551) will exceed the MICs typically observed for P. aeruginosa, E. coli and K. pneumoniae in the hospital setting.

Background

The management of hospital-acquired pneumonia (HAP) and ventilator-associated pneumonia (VAP) has been made increasingly difficult due to the emergence of resistance and the potential for reduced antibiotic lung penetration in the intubated patient. HAP continues to be the second most common cause of nosocomial infections in the United States and is associated with increases in hospital length of stay, healthcare costs and represents a major cause of mortality especially in critically ill patients [1, 2]. VAP, a subset of HAP, that occurs in mechanically ventilated patients more than 48 h after tracheal intubation and occurs in 9–40 % of mechanically ventilated patient’s making it among the most frequent infections in the ICU [1, 3].

Gram-negative bacteria are responsible for a substantial proportion of HAP and VAP infections. Pseudomonas aeruginosa, along with the Enterobacteriacae, Escherichia coli and Klebsiella pneumoniae are amongst the most common etiological organisms representing approximately two-thirds of causative agents [4, 5]. Nosocomial pneumonia caused by P. aeruginosa, E. coli and K. pneumoniae continues to pose significant challenges in US hospitals due to their prevalence and the acquisition of numerous antimicrobial resistance mechanisms. As a result, the selection of empirical antibiotic therapy in patients with nosocomial respiratory tract infections has become increasingly challenging as the number of potentially effective agents has been reduced due to evolving resistance. Yet more challenging is delivering sufficiently high antibiotic concentrations to the lung as many parenteral therapies have poor or variable penetration. The delivery of antibiotics directly to the site of infection presents a unique clinical opportunity to enhance patient outcomes by achieving high local concentrations that overcome resistance while minimizing the potential for toxicity associated with systemic administration.

Amikacin Inhale (BAY41-6551) is a reformulated solution of amikacin (AMK) combined with a drug-delivery module that is currently under phase III study as an adjunctive therapy for the treatment of Gram-negative pneumonia in intubated and mechanical ventilated patients. In vitro pharmacodynamic models evaluating the achievable epithelial lining fluid (ELF) concentrations after the administration of AMK inhalation against Gram-negative organisms demonstrated rapid and sustainable bactericidal killing of AMK both alone and in combination with systemic exposures of meropenem when AMK MICs were ≤256 mg/L [6]. Our objective was to define the potency and MIC distribution of AMK against a US collection of P. aeruginosa, E. coli, K. pneumoniae nosocomial isolates and relate these data to achievable lung concentrations of the compound when delivered via the aerosol route.

Methods

Fifty US hospitals, 41 teaching and 9 community provided non-duplicate nosocomial blood and respiratory isolates of E. coli, K. pneumoniae and P. aeruginosa from adult inpatients. Additionally, five of these hospitals also provided S. maltophilia respiratory isolates. Organisms were identified at each participating site using the standard methods. The isolates were transferred to trypticase soy agar slants for shipping to the Center for Anti-Infective Research and Development, Hartford Hospital, Hartford, CT, USA. Collection occurred from 2013 into 2014.

Clinical Laboratory Standards Institute (CLSI) defined broth microdilution methods were employed to determine minimum inhibitory concentration (MICs) for AMK, aztreonam (ATM), cefepime (FEP), ceftazidime (CAZ), ceftolozane/tazobactam (C/T), ceftriaxone (CRO), ciprofloxacin (CIP), imipenem (IPM), meropenem (MEM), piperacillin/tazobactam (TZP) and tobramycin (TOB) [7]. Antibiotics were purchased from Sigma (St. Louis, MO) except for C/T which was provided by Cubist Pharmaceuticals. Quality control was performed on each batch of MIC testing using E. coli 25922 and P. aeruginosa 27853 as defined by CLSI. All transfer and colony counts were performed on trypticase soy agar plates containing 5 % blood. CLSI and FDA breakpoints were used to define susceptibility. For C/T the FDA breakpoints of 2 mg/L for Enterobacteriaceae and 4 mg/L for P. aeruginosa were utilized [8]. Isolates that were non-susceptible to AMK (i.e., ≥32 mg/L) by current laboratory definitions were repeated and confirmed.

Pseudomonas aeruginosa were identified as multidrug resistant (MDR) if they displayed resistance to 3 or more classes as represented by the following phenotypic resistance profiles: CIP (MIC ≥4 mg/L), IPM (MIC ≥8 mg/L), CAZ (MIC ≥32 mg/L), TZP (MIC ≥128 mg/L) and TOB (MIC ≥16 mg/L) [9].

Escherichia coli and K. pneumoniae were tested for extended spectrum β-lactamases (ESBL) production if they had an MIC of ≥1 to 2 mg/L of the following: ATM, CRO or CAZ. CLSI defined ESBL confirmation studies were then undertaken using additional MIC testing with CAZ, CAZ with clavulanate, cefotaxime and cefotaxime with clavulanate [7].

Isolates testing non-susceptible to ertapenem, imipenem, or meropenem were evaluated for carbapenemase production using the CarbaNP test [10].