cMultidrug resistance in Gram-negative bacteria has become so threatening to human health that new antibacterial platforms are desperately needed to combat these deadly infections. The concept of siderophore conjugation, which facilitates compound uptake across the outer membrane by hijacking bacterial iron acquisition systems, has received significant attention in recent years. While standard in vitro MIC and resistance frequency methods demonstrate that these compounds are potent, broad-spectrum antibacterial agents whose activity should not be threatened by unacceptably high spontaneous resistance rates, recapitulation of these results in animal models can prove unreliable, partially because of the differences in iron availability in these different methods. Here, we describe the characterization of MB-1, a novel siderophore-conjugated monobactam that demonstrates excellent in vitro activity against Pseudomonas aeruginosa when tested using standard assay conditions. Unfortunately, the in vitro findings did not correlate with the in vivo results we obtained, as multiple strains were not effectively treated by MB-1 despite having low MICs. To address this, we also describe the development of new in vitro assays that were predictive of efficacy in mouse models, and we provide evidence that competition with native siderophores could contribute to the recalcitrance of some P. aeruginosa isolates in vivo.

The combination of ceftazidime and avibactam possesses potent activity against resistant Gram-negative pathogens, including Pseudomonas aeruginosa. We compared the efficacies of human simulated doses of ceftazidime and ceftazidime-avibactam using a hollow-fiber system and neutropenic and immunocompetent murine thigh infection models. Twenty-seven clinical P. aeruginosa isolates with ceftazidime MICs of 8 to 128 mg/liter and ceftazidime-avibactam MICs of 4 to 32 mg/liter were utilized in neutropenic mouse studies; 15 of the isolates were also evaluated in immunocompetent mice. Six isolates were studied in both the hollow-fiber system and the neutropenic mouse. In both systems, the free drug concentration-time profile seen in humans given 2 g of ceftazidime every 8 h (2-h infusion), with or without avibactam at 500 mg every 8 h (2-h infusion), was evaluated. In vivo activity was pharmacodynamically predictable based on the MIC. Ceftazidime decreased bacterial densities by >0.5 log unit for 10/27 isolates, while ceftazidime-avibactam did so for 22/27 isolates. In immunocompetent animals, enhancements in activity were seen for both drugs, with ceftazidime achieving reductions of >0.3 log unit for 10/15 isolates, whereas ceftazidimeavibactam did so against all 15 isolates. In vitro, ceftazidime resulted in regrowth by 24 h against all isolates, while ceftazidimeavibactam achieved stasis or better against 4/7 isolates. Mutants with elevated ceftazidime-avibactam MICs appeared after 24 h from 3/7 isolates studied in vitro; however, no resistant mutants were detected in vivo. Against this highly ceftazidime-nonsusceptible population of P. aeruginosa, treatment with human simulated doses of ceftazidime-avibactam resulted in pharmacodynamically predictable activity, particularly in vivo, against isolates with MICs of <16 mg/liter, and this represents a potential new option to combat these difficult-to-treat pathogens.A vibactam (formerly NXL104) is a novel non-␤-lactam ␤-lactamase inhibitor with activity against a wide variety of enzyme-mediated resistance mechanisms, including both class A and class C enzymes (25). Given the increasing prevalence of resistant Gram-negative pathogens and the high likelihood that portions of these resistance mechanisms are enzyme based, ␤-lactam combinations with avibactam represent an excellent opportunity to increase potency against these organisms, where so few options are currently available (12).One such combination that has received considerable interest is avibactam with ceftazidime. Recent in vitro studies evaluating this combination have shown significant potency increases compared with ceftazidime alone against a wide variety of Gram-negative pathogens (7,15,19,29). Endimiani and colleagues used this combination in mice to evaluate its efficacy against KPC-producing Klebsiella pneumoniae compared with ceftazidime alone (10). Using a set 4:1 ceftazidime-to-avibactam ratio, they found that while a number of ceftazidime-avibactam regimens within the dose range resulted in efficacy, ...

Doripenem is a new broad-spectrum carbapenem with activity against a range of gram-negative pathogens, including nonfermenting bacteria such as Pseudomonas aeruginosa. The objective of this study was to evaluate simulated human exposures to doripenem using a neutropenic murine thigh infection model against 24 clinical P. aeruginosa isolates with a wide range of MICs. Dosing regimens in mice were designed to approximate the free time above MIC (fT>MIC) observed with 500 mg doripenem every 8 h given as either a 1-h or 4-h intravenous infusion in humans. Maximal antibacterial killing was associated with doripenem exposures of >40% fT>MIC; bacteriostatic effects were noted at Ϸ20% fT>MIC. The simulated 1-h infusion provided bactericidal effects for isolates with MICs of <2 g/ml, while variable killing was noted for isolates with MICs of 4 to 8 g/ml and regrowth for isolates with an MIC of 16 g/ml. The 4-h infusion regimen displayed similar killing for isolates with MICs of <2 g/ml and enhanced activity for two of the four isolates with an MIC of 4 g/ml. Given that the 4-h regimen yields negligible fT>MIC for MICs of >8 g/ml, regrowth was generally observed. Simulated doses of 500 mg doripenem every 8 h infused over 1 h demonstrated antibacterial killing for P. aeruginosa isolates with MICs of 0.125 to 8 g/ml. Exposures of >40% fT>MIC resulted in the most pronounced bactericidal effects, while killing was variable for 20 to 30% fT>MIC. Infusing doses over 4 h enhanced efficacy against selected pseudomonal isolates with an MIC of 4 g/ml.

AFN-1252, a new antimicrobial agent, specifically and potently inhibits fatty acid synthesis in Staphylococcus aureus. We characterized in vivo pharmacokinetic and pharmacodynamic profiles of AFN-1252 administered orally to neutropenic mice inoculated in thighs (∼106 CFU) with methicillin-susceptible S. aureus (MSSA) ATCC 29213. Efficacy was also assessed in mice inoculated with MSSA, hospital-acquired Methicillin-resistant Staphylococcus aureus (HA-MRSA) or community-acquired (CA)-MRSA, and administered AFN-1252 or linezolid orally. Bacterial density was determined after 24 hours and efficacy defined as the change in CFU/thigh versus untreated controls at time 0. With MSSA, antibacterial reductions of ≧1 log were observed at ≧20 mg/kg doses, with ƒAUC/minimum inhibitory concentration (MIC) best describing the pharmacodynamic profile of AFN-1252. The 80, 50 and 5% maximum effects were observed with ƒAUC/MIC values of 22·3, 17·0, and 9·6, respectively. Similar values were obtained for CA-MRSA and HA-MRSA. AFN-1252 was 4–40 fold more effective than linezolid against CA-MRSA and HA-MRSA. These data demonstrate the excellent in vivo potency of AFN-1252 against phenotypically diverse S. aureus.

Cefepime was evaluated in vivo against two inoculum sizes of four strains of Escherichia coli that produced extended-spectrum beta-lactamases (ESBLs) in a murine neutropenic thigh infection model to characterize the pharmacodynamic activity of cefepime in the presence of ESBL-producing bacteria and to evaluate if differences in lengths of cefepime exposure are required with various inocula. Three strains possessed a single enzyme each: TEM-10, TEM-12, and TEM-26. The fourth strain possessed two TEM-derived ESBLs and a third uncharacterized enzyme. Two non-ESBL-producing E. coli strains were included for comparison. Mice received various doses of cefepime to achieve a spectrum of percentages of time the drug was above the MIC (%T>MICs) for each isolate at both inocula. No significant difference in cefepime exposure was required to achieve similar bactericidal effects for ESBL-and non-ESBL-producing isolates when the starting inoculum was 10 5 CFU of E. coli per thigh. The increased MICs observed in vitro for the ESBL-producing strains at 10 7 CFU/ml did not predict the amount of exposure required to achieve a comparable level of bactericidal activity in vivo at the corresponding starting inoculum of 10 7 CFU/thigh. Compared to the cefepime exposure in tests with the lower inoculum (10 5 CFU/thigh), less exposure was required when the starting inoculum was 10 7 CFU/thigh (%T>MIC, 6% versus 26%), such that similar doses (in milligrams per kilogram of body weight) produced similar bactericidal effects with both inocula of ESBL-producing isolates. Equivalent exposures of cefepime produced similar effects against the microorganisms regardless of the presence of ESBL production. Pharmacodynamic profiling undertaken with conventional cefepime MIC determinations predicted in vivo microbial outcomes at both inoculum sizes for the ESBL-producing isolates evaluated in this study. These data support the use of conventional MIC determinations in the pharmacodynamic assessment of cefepime.