Comprehensive stability analysis of 13 β-lactams and β-lactamase inhibitors in in vitro media, and novel supplement dosing strategy to mitigate thermal drug degradation

Non-clinical antibiotic development relies on in vitro susceptibility and infection model studies. Validating the achievement of the targeted drug concentrations is essential to avoid under-estimation of drug effects and over-estimation of resistance emergence. While certain β-lactams (e.g., imipenem) and β-lactamase inhibitors (BLIs; clavulanic acid) are believed to be relatively unstable, limited tangible data on their stability in commonly used in vitro media are known. We aimed to determine the thermal stability of 10 β-lactams and 3 BLIs via LC-MS/MS in cation-adjusted Mueller Hinton broth at 25 and 36°C as well as agar at 4 and 37°C, and in water at -20, 4, and 25°C. Supplement dosing algorithms were developed to achieve broth concentrations close to their target over 24 h. During incubation in broth (pH 7.25)/agar, degradation half-lives were 16.9/21.8 h for imipenem, 20.7/31.6 h for biapenem, 29.0 h for clavulanic acid (studied in broth only), 23.1/71.6 h for cefsulodin, 40.6/57.9 h for doripenem, 46.5/64.6 h for meropenem, 50.8/97.7 h for cefepime, 61.5/99.5 h for piperacillin, and >120 h for all other compounds. Broth stability decreased at higher pH. All drugs were ≥90% stable for 72 h in agar at 4°C. Degradation half-lives in water at 25°C were >200 h for all drugs except imipenem (14.7 h, at 1,000 mg/L) and doripenem (59.5 h). One imipenem supplement dose allowed concentrations to stay within ±31% of their target concentration. This study provides comprehensive stability data on β-lactams and BLIs in relevant in vitro media using LC-MS/MS. Future studies are warranted applying these data to antimicrobial susceptibility testing and assessing the impact of β-lactamase-related degradation.

Distinguishing Inducible and Non-Inducible Resistance to Colistin in Pseudomonas aeruginosa by Quantitative and Systems Pharmacology Modeling at Low and Standard Inocula

Colistin is a polymyxin and peptide antibiotic that can yield rapid bacterial killing, but also leads to resistance emergence. We aimed to develop a novel experimental and Quantitative and Systems Pharmacology approach to distinguish between inducible and non-inducible resistance. Viable count profiles for the total and less susceptible populations of Pseudomonas aeruginosa ATCC 27853 from static and dynamic in vitro infection models were simultaneously modeled. We studied low and normal initial inocula to distinguish between inducible and non-inducible resistance. A novel cutoff filter approach allowed us to describe the eradication and inter-conversion of bacterial populations. At all inocula, 4.84 mg/L of colistin (sulfate) yielded ≥4 log10 killing, followed by >4 log10 regrowth. A pre-existing, less susceptible population was present at standard but not at low inocula. Formation of a non-pre-existing, less susceptible population was most pronounced at intermediate colistin (sulfate) concentrations (0.9 to 5 mg/L). Both less susceptible populations inter-converted with the susceptible population. Simultaneously modeling of the total and less susceptible populations at low and standard inocula enabled us to identify the de novo formation of an inducible, less susceptible population. Inducible resistance at intermediate colistin concentrations highlights the importance of rapidly achieving efficacious polymyxin concentrations by front-loaded dosage regimens.

Penicillin-Binding Protein 5/6 Acting as a Decoy Target in Pseudomonas aeruginosa Identified by Whole-Cell Receptor Binding and Quantitative Systems Pharmacology

The β-lactam antibiotics have been successfully used for decades to combat susceptible Pseudomonas aeruginosa, which has a notoriously difficult to penetrate outer membrane (OM). However, there is a dearth of data on target site penetration and covalent binding of penicillin-binding proteins (PBP) for β-lactams and β-lactamase inhibitors in intact bacteria. We aimed to determine the time course of PBP binding in intact and lysed cells and estimate the target site penetration and PBP access for 15 compounds in P. aeruginosa PAO1. All β-lactams (at 2 × MIC) considerably bound PBPs 1 to 4 in lysed bacteria. However, PBP binding in intact bacteria was substantially attenuated for slow but not for rapid penetrating β-lactams. Imipenem yielded 1.5 ± 0.11 log10 killing at 1h compared to <0.5 log10 killing for all other drugs. Relative to imipenem, the rate of net influx and PBP access was ~ 2-fold slower for doripenem and meropenem, 7.6-fold for avibactam, 14-fold for ceftazidime, 45-fold for cefepime, 50-fold for sulbactam, 72-fold for ertapenem, ~ 249-fold for piperacillin and aztreonam, 358-fold for tazobactam, ~547-fold for carbenicillin and ticarcillin, and 1,019-fold for cefoxitin. At 2 × MIC, the extent of PBP5/6 binding was highly correlated (r2 = 0.96) with the rate of net influx and PBP access, suggesting that PBP5/6 acted as a decoy target that should be avoided by slowly penetrating, future β-lactams. This first comprehensive assessment of the time course of PBP binding in intact and lysed P. aeruginosa explained why only imipenem killed rapidly. The developed novel covalent binding assay in intact bacteria accounts for all expressed resistance mechanisms.

Penicillin-Binding Protein Occupancy Dataset for 18 β-Lactams and 4 β-Lactamase Inhibitors in Neisseria gonorrhoeae

The lack of effective first-line antibiotic treatments against Neisseria gonorrhoeae, and the worldwide dissemination of resistant strains, are the main drivers of a worsening global health crisis. β-lactam antibiotics have been the backbone of therapeutic armamentarium against gonococci. However, we are lacking critical insights to design rationally optimized therapies. In the present work, we generated the first PBP-binding data set on 18 currently available and clinically relevant β-lactams and 4 β-lactamase inhibitors in two N. gonorrhoeae ATCC type collection strains, 19424 and 49226 (PBP2 type XXII and A39T change in mtrR). PBP binding (IC50) was determined via the Bocillin FL binding assay in isolated membrane preparations. Three clusters of differential PBP IC50s were identified and were mostly consistent across both strains, but with quantitative differences. Carbapenems were coselective for PBP2 and PBP3 (0.01 to 0.03 mg/L). Third- and fourth-generation cephalosporins cefixime, cefotaxime, ceftazidime, cefepime, and ceftriaxone showed the lowest IC50 values for PBP2 (0.01 mg/L), whereas cefoxitin, ceftaroline, and ceftolozane required higher concentrations (0.04 to >2 mg/L). Aztreonam was selective for PBP2 in both strains (0.03 to 0.07 mg/L); amdinocillin bound this PBP at higher concentrations (1.33 to 2.94 mg/L). Penicillins specifically targeted PBP2 in strain ATCC 19424 (0.02 to 0.19 mg/L) and showed limited inhibition in strain ATCC 49226 (0.01 to >2 mg/L). Preferential PBP2 binding was observed by β-lactam-based β-lactamase inhibitors sulbactam and tazobactam (1.07 to 6.02 mg/L); meanwhile, diazabicyclooctane inhibitors relebactam and avibactam were selective for PBP3 (1.27 to 5.40 mg/L). This data set will set the bar for future studies that will help the rational use and translational development of antibiotics against multidrug-resistant (MDR) N. gonorrhoeae. IMPORTANCE The manuscript represents the first N. gonorrhoeae PBP-binding data set for 22 chemically different drugs in two type strains with different genetic background. We have identified three clusters of drugs according to their PBP binding IC50s and highlighted the binding differences across the two strains studied. With the currently available genomic information and the PBP-binding data, we have been able to correlate the target attainment differences and the mutations that affect the drug uptake with the MIC changes. The results of the current work will allow us to develop molecular tools of great practical use for the study and the design of new rationally designed therapies capable of combating the growing MDR gonococci threat.

In vitro evolution of cefepime/zidebactam (WCK 5222) resistance in Pseudomonas aeruginosa: dynamics, mechanisms, fitness trade-off and impact on in vivo efficacy

OBJECTIVES

To study the dynamics, mechanisms and fitness cost of resistance selection to cefepime, zidebactam and cefepime/zidebactam in Pseudomonas aeruginosa.

METHODS

WT P. aeruginosa PAO1 and its ΔmutS derivative (PAOMS) were exposed to stepwise increasing concentrations of cefepime, zidebactam and cefepime/zidebactam. Selected mutants were characterized for change in susceptibility profiles, acquired mutations, fitness, virulence and in vivo susceptibility to cefepime/zidebactam. Mutations were identified through WGS. In vitro fitness was assessed by measuring growth in minimal medium and human serum-supplemented Mueller-Hinton broth. Virulence was determined in Caenorhabditis elegans and neutropenic mice lung infection models. In vivo susceptibility to a human-simulated regimen (HSR) of cefepime/zidebactam was studied in neutropenic mice lung infection.

RESULTS

Resistance development was lower for the cefepime/zidebactam combination than for the individual components and high-level resistance was only achieved for PAOMS. Cefepime resistance development was associated with mutations leading to the hyperexpression of AmpC or MexXY-OprM, combined with PBP3 mutations and/or large chromosomal deletions involving galU. Zidebactam resistance was mainly associated with mutations in PBP2. On the other hand, resistance to cefepime/zidebactam required multiple mutations in genes encoding MexAB-OprM and its regulators, as well as PBP2 and PBP3. Cumulatively, these mutations inflicted significant fitness cost and cefepime/zidebactam-resistant mutants (MIC = 16-64 mg/L) remained susceptible in vivo to the HSR.

CONCLUSIONS

Development of cefepime/zidebactam resistance in P. aeruginosa required multiple simultaneous mutations that were associated with a significant impairment of fitness and virulence.

Effective inhibition of PBPs by cefepime and zidebactam in the presence of VIM-1 drives potent bactericidal activity against MBL-expressing Pseudomonas aeruginosa

OBJECTIVES

The combination of cefepime and the novel β-lactam enhancer zidebactam (WCK 5222) is under development for the treatment of difficult-to-treat Gram-negative infections. Against MBL-producing pathogens, cefepime and zidebactam induce cell elongation and spheroplast formation, indicating PBP3 and PBP2 dysfunction, respectively, having a potent bactericidal effect as a combination. The objective of the present study was to determine the mechanistic basis of the bactericidal effect of cefepime/zidebactam on MBL-expressing pathogens.

METHODS

Pseudomonal PBP-binding affinities of cefepime, zidebactam and imipenem were assessed at different timepoints and also in the presence of purified VIM-1 using a Bocillin FL competition assay. The antibacterial activity of cefepime/zidebactam against three VIM-expressing Pseudomonas aeruginosa isolates was assessed by time-kill and neutropenic mouse lung/thigh infection studies.

RESULTS

Amidst cefepime-hydrolysing concentrations of VIM-1, substantial cefepime binding to target PBPs was observed. High-affinity binding of zidebactam to PBP2 remained unaltered in the presence of VIM-1; however, MBL addition significantly affected imipenem PBP2 binding. Furthermore, the rate of cefepime binding to the primary target PBP3 was found to be higher compared with the imipenem PBP2 binding rate. Finally, complementary PBP inhibition by cefepime/zidebactam resulted in enhanced bactericidal activity in time-kill and neutropenic mouse lung/thigh infection studies against VIM-6-, VIM-10- and VIM-11-expressing P. aeruginosa, thus revealing the mechanistic basis of β-lactam enhancer action.

CONCLUSIONS

For the first time ever (to the best of our knowledge), this study demonstrates that in the presence of VIM-1 MBL, β-lactamase-labile cefepime and β-lactamase-stable zidebactam produce effective inhibition of respective target PBPs. For cefepime, this seems to be a result of a faster rate of PBP binding, which helps it overcome β-lactamase-mediated hydrolysis.

In vitro dynamics and mechanisms of resistance development to imipenem and imipenem/relebactam in Pseudomonas aeruginosa

OBJECTIVES

We analysed the dynamics and mechanisms of resistance development to imipenem alone or combined with relebactam in Pseudomonas aeruginosa WT (PAO1) and mutator (PAOMS; ΔmutS) strains.

METHODS

PAO1 or PAOMS strains were incubated for 24 h in Mueller-Hinton Broth with 0.125-64 mg/L of imipenem ± relebactam 4 mg/L. Tubes from the highest antibiotic concentration showing growth were reinoculated in fresh medium containing concentrations up to 64 mg/L of imipenem ± relebactam for 7 days. Two colonies per strain, replicate experiment and antibiotic from early (Day 1) and late (Day 7) cultures were characterized by determining the susceptibility profiles, WGS and determination of the expression of ampC and efflux-pump-coding genes. Virulence was studied in a Caenorhabditis elegans infection model.

RESULTS

Relebactam reduced imipenem resistance development for both strains, although resistance emerged much faster for PAOMS. WGS indicated that imipenem resistance was associated with mutations in the porin OprD and regulators of ampC, while the mutations in imipenem/relebactam-resistant mutants were located in oprD and regulatoras of MexAB-OprM. High-level imipenem/relebactam resistance was only documented in the PAOMS strain and was associated with an additional specific (T680A) mutation located in the catalytic pocket of ponA (PBP1a) and with reduced virulence in the C. elegans model.

CONCLUSIONS

Imipenem/relebactam could be a useful alternative for the treatment of MDR P. aeruginosa infections, potentially reducing resistance development during treatment. Moreover, this work deciphers the potential resistance mechanisms that may emerge upon the introduction of this novel combination into clinical practice.

Combating Multidrug-Resistant Bacteria by Integrating a Novel Target Site Penetration and Receptor Binding Assay Platform Into Translational Modeling

Multidrug-resistant bacteria are causing a serious global health crisis. A dramatic decline in antibiotic discovery and development investment by pharmaceutical industry over the last decades has slowed the adoption of new technologies. It is imperative that we create new mechanistic insights based on latest technologies, and use translational strategies to optimize patient therapy. Although drug development has relied on minimal inhibitory concentration testing and established in vitro and mouse infection models, the limited understanding of outer membrane permeability in Gram-negative bacteria presents major challenges. Our team has developed a platform using the latest technologies to characterize target site penetration and receptor binding in intact bacteria that inform translational modeling and guide new discovery. Enhanced assays can quantify the outer membrane permeability of β-lactam antibiotics and β-lactamase inhibitors using multiplex liquid chromatography tandem mass spectrometry. While β-lactam antibiotics are known to bind to multiple different penicillin-binding proteins (PBPs), their binding profiles are almost always studied in lysed bacteria. Novel assays for PBP binding in the periplasm of intact bacteria were developed and proteins identified via proteomics. To characterize bacterial morphology changes in response to PBP binding, high-throughput flow cytometry and time-lapse confocal microscopy with fluorescent probes provide unprecedented mechanistic insights. Moreover, novel assays to quantify cytosolic receptor binding and intracellular drug concentrations inform target site occupancy. These mechanistic data are integrated by quantitative and systems pharmacology modeling to maximize bacterial killing and minimize resistance in in vitro and mouse infection models. This translational approach holds promise to identify antibiotic combination dosing strategies for patients with serious infections.

Novel Cassette Assay To Quantify the Outer Membrane Permeability of Five β-Lactams Simultaneously in Carbapenem-Resistant Klebsiella pneumoniae and Enterobacter cloacae

Poor penetration through the outer membrane (OM) of Gram-negative bacteria is a major barrier of antibiotic development. While β-lactam antibiotics are commonly used against Klebsiella pneumoniae and Enterobacter cloacae, there are limited data on OM permeability especially in K. pneumoniae Here, we developed a novel cassette assay, which can simultaneously quantify the OM permeability to five β-lactams in carbapenem-resistant K. pneumoniae and E. cloacae Both clinical isolates harbored a blaKPC-2 and several other β-lactamases. The OM permeability of each antibiotic was studied separately ("discrete assay") and simultaneously ("cassette assay") by determining the degradation of extracellular β-lactam concentrations via multiplex liquid chromatography-tandem mass spectrometry analyses. Our K. pneumoniae isolate was polymyxin resistant, whereas the E. cloacae was polymyxin susceptible. Imipenem penetrated the OM at least 7-fold faster than meropenem for both isolates. Imipenem penetrated E. cloacae at least 258-fold faster and K. pneumoniae 150-fold faster compared to aztreonam, cefepime, and ceftazidime. For our β-lactams, OM permeability was substantially higher in the E. cloacae compared to the K. pneumoniae isolate (except for aztreonam). This correlated with a higher OmpC porin production in E. cloacae, as determined by proteomics. The cassette and discrete assays showed comparable results, suggesting limited or no competition during influx through OM porins. This cassette assay allowed us, for the first time, to efficiently quantify the OM permeability of multiple β-lactams in carbapenem-resistant K. pneumoniae and E. cloacae Characterizing the OM permeability presents a critical contribution to combating the antimicrobial resistance crisis and enables us to rationally optimize the use of β-lactam antibiotics.IMPORTANCE Antimicrobial resistance is causing a global human health crisis and is affecting all antibiotic classes. While β-lactams have been commonly used against susceptible isolates of Klebsiella pneumoniae and Enterobacter cloacae, carbapenem-resistant isolates are spreading worldwide and pose substantial clinical challenges. Rapid penetration of β-lactams leads to high drug concentrations at their periplasmic target sites, allowing β-lactams to more completely inactivate their target receptors. Despite this, there are limited tangible data on the permeability of β-lactams through the outer membranes of many Gram-negative pathogens. This study presents a novel, cassette assay, which can simultaneously characterize the permeability of five β-lactams in multidrug-resistant clinical isolates. We show that carbapenems, and especially imipenem, penetrate the outer membrane of K. pneumoniae and E. cloacae substantially faster than noncarbapenem β-lactams. The ability to efficiently characterize the outer membrane permeability is critical to optimize the use of β-lactams and combat carbapenem-resistant isolates.

First Penicillin-Binding Protein Occupancy Patterns of β-Lactams and β-Lactamase Inhibitors in Klebsiella pneumoniae

Penicillin-binding proteins (PBPs) are the high-affinity target sites of all β-lactam antibiotics in bacteria. It is well known that each β-lactam covalently binds to and thereby inactivates different PBPs with various affinities. Despite β-lactams serving as the cornerstone of our therapeutic armamentarium against Klebsiella pneumoniae, PBP binding data are missing for this pathogen. We aimed to generate the first PBP binding data on 13 chemically diverse and clinically relevant β-lactams and β-lactamase inhibitors in K. pneumoniae PBP binding was determined using isolated membrane fractions from K. pneumoniae strains ATCC 43816 and ATCC 13883. Binding reactions were conducted using β-lactam concentrations from 0.0075 to 256 mg/liter (or 128 mg/liter). After β-lactam exposure, unbound PBPs were labeled by Bocillin FL. Binding affinities (50% inhibitory concentrations [IC50]) were reported as the β-lactam concentrations that half-maximally inhibited Bocillin FL binding. PBP occupancy patterns by β-lactams were consistent across both strains. Carbapenems bound to all PBPs, with PBP2 and PBP4 as the highest-affinity targets (IC50, <0.0075 mg/liter). Preferential PBP2 binding was observed by mecillinam (amdinocillin; IC50, <0.0075 mg/liter) and avibactam (IC50, 2 mg/liter). Aztreonam showed high affinity for PBP3 (IC50, 0.06 to 0.12 mg/liter). Ceftazidime bound PBP3 at low concentrations (IC50, 0.06 to 0.25 mg/liter) and PBP1a/b at higher concentrations (4 mg/liter), whereas cefepime bound PBPs 1 to 4 at more even concentrations (IC50, 0.015 to 2 mg/liter). These PBP binding data on a comprehensive set of 13 clinically relevant β-lactams and β-lactamase inhibitors in K. pneumoniae enable, for the first time, the rational design and optimization of double β-lactam and β-lactam-β-lactamase inhibitor combinations.

Possible land management uses of common cypress to reduce wildfire initiation risk: a laboratory study

Accurate determination of flammability is required in order to improve knowledge about vegetation fire risk. Study of the flammability of different plant species is essential for the Mediterranean area, where most ecosystems are adapted to natural fire but vulnerable to recurrent human-induced fires, which are the main cause of forest degradation. However, the methods used to evaluate vegetation flammability have not yet been standardized. Cupressus sempervirens is a native or naturalized forest tree species in the Mediterranean area that is able to tolerate prolonged drought and high temperatures. The aim of this study was to characterize the flammability of C. sempervirens var. horizontalis at particle level by using different bench-scale calorimetry techniques (mass loss calorimeter, epiradiator and oxygen bomb) to determine the main flammability descriptors (ignitability, sustainability, combustibility and consumability) in live crown and litter samples. Our findings indicate that this variety of cypress is relatively resistant to ignition because of the high ash content, the high critical heat flux, the high time to ignition displayed by both crown and litter samples and the ability of the leaves to maintain a high water content during the summer. We also discuss the possibility of exploiting some morphological, functional and ecological traits of the species to construct a barrier system (with selected varieties of cypress) as a promising complementary land management tool to reduce the fire spread and intensity in a Mediterranean context.

Influence of high mutation rates on the mechanisms and dynamics of in vitro and in vivo resistance development to single or combined antipseudomonal agents

We studied the mechanisms and dynamics of the development of resistance to ceftazidime (CAZ) alone or combined with tobramycin (TOB) or ciprofloxacin (CIP) in vitro and in vivo (using a mouse model of lung infection with human antibiotic regimens). Pseudomonas aeruginosa strain PAO1 and its hypermutable derivative PAODeltamutS were used, and the results were compared with those previously obtained with CIP, TOB, and CIP plus TOB (CIP-TOB) under the same conditions. An important (200-fold) amplification of the number of resistant mutant cells was documented for PAODeltamutS-infected mice that were under CAZ treatment compared to the number for mice that received placebo, whereas the median number of resistant mutant cells was below the detection limits for mice infected by PAO1. These results were intermediate between the high amplification with CIP (50,000-fold) and the low amplification with TOB (10-fold). All CAZ-resistant single mutant cells selected in vitro or in vivo hyperproduced AmpC. On the other hand, the three combinations studied were found to be highly effective in the prevention of in vivo resistance development in mice infected with PAODeltamutS, although the highest therapeutic efficacy (in terms of mortality and total bacterial load reduction) compared to those of the individual regimens was obtained with CIP-TOB and the lowest was with CAZ-CIP. Nevertheless, mutant cells that were resistant to the three combinations tested were readily selected in vitro for PAODeltamutS (mutation rates from 1.2 x 10(-9) to 5.8 x 10(-11)) but not for PAO1, highlighting the potential risk for antimicrobial resistance development associated with the presence of hypermutable strains, even when combined therapy was used. All five independent CAZ-TOB-resistant PAODeltamutS double mutants studied presented the same resistance mechanism (AmpC hyperproduction plus an aminoglycoside resistance mechanism not related to MexXY), whereas four different combinations of resistance mechanisms were documented for the five CAZ-CIP-resistant double mutants.

Transcervical falloposcopy: preliminary experience

The new technique of endoluminal tubal exploration was evaluated by performing transcervical falloposcopy instead of chromoperturbation under control of concurrent laparoscopy. In this feasibility study, catheterization was performed with the use of either a transhysteroscopic or a free-hand tubal cannulation technique. A total of 66 patients were investigated for primary or secondary infertility with proximal and/or distal suspected tubal defects on the basis of prior hysterosalpingography; three patients were investigated for unruptured tubal pregnancy; two patients were investigated to localize the tip of the tubal embryo transfer catheter. Transcervical catheterization was successful in 110 of the 130 tubes (84.6%). Successful and informative falloposcopy was achieved in 30% of the 110 cannulated tubes. The transcervical free-hand cannulation technique was as effective as the transhysteroscopic approach. Recanalization of at least one tube was achieved in 83% of women with proximal obstruction. Tubal cannulation by the tubal embryo transfer catheter was confirmed by falloposcopy in the two cases where free-hand catheterization was used. This study confirms that it is possible to visualize the tubal lumen and demonstrates that the free-hand cannulation technique is a simple and effective alternative to the transhysteroscopic approach. However, further progress in catheter technology has to be achieved in order to perform regularly successful transcervical falloposcopy in damaged tubes.