Correlation between bactericidal activity and postantibiotic effect for five antibiotics with different mechanisms of action

Antibiotics and ECMO in the Adult Population—Persistent Challenges and Practical Guides

Extracorporeal membrane oxygenation (ECMO) is an emerging treatment modality associated with a high frequency of antibiotic use. However, several covariables emerge during ECMO implementation, potentially jeopardizing the success of antimicrobial therapy. These variables include but are not limited to: the increased volume of distribution, altered clearance, and adsorption into circuit components, in addition to complex interactions of antibiotics in critical care illness. Furthermore, ECMO complicates the assessment of antibiotic effectiveness as fever, or other signs may not be easily detected, the immunogenicity of the circuit affects procalcitonin levels and other inflammatory markers while disrupting the immune system. We provided a review of pharmacokinetics and pharmacodynamics during ECMO, emphasizing practical application and review of patient-, illness-, and ECMO hardware-related factors.

Model-Based Exposure-Response Assessment for Spectinamide 1810 in a Mouse Model of Tuberculosis

Despite decades of research, tuberculosis remains a leading cause of death from a single infectious agent. Spectinamides are a promising novel class of antituberculosis agents, and the lead spectinamide 1810 has demonstrated excellent efficacy, safety, and drug-like properties in numerous in vitro and in vivo assessments in mouse models of tuberculosis. In the current dose ranging and dose fractionation study, we used 29 different combinations of dose level and dosing frequency to characterize the exposure-response relationship for spectinamide 1810 in a mouse model of Mycobacterium tuberculosis infection and in healthy animals. The obtained data on 1810 plasma concentrations and counts of CFU in lungs were analyzed using a population pharmacokinetic/pharmacodynamic (PK/PD) approach as well as classical anti-infective PK/PD indices. The analysis results indicate that there was no difference in the PK of 1810 in infected compared to healthy, uninfected animals. The PK/PD index analysis showed that bacterial killing of 1810 in mice was best predicted by the ratio of maximum free drug concentration to MIC (fC_max/MIC) and the ratio of the area under the free concentration-time curve to the MIC (fAUC/MIC) rather than the cumulative percentage of time that the free drug concentration is above the MIC (f%T_MIC). A novel PK/PD model with consideration of postantibiotic effect could adequately describe the exposure-response relationship for 1810 and supports the notion that the in vitro observed postantibiotic effect of this spectinamide also translates to the in vivo situation in mice. The obtained results and pharmacometric model for the exposure-response relationship of 1810 provide a rational basis for dose selection in future efficacy studies of this compound against M. tuberculosis.

Drug detoxification dynamics explain the postantibiotic effect

The postantibiotic effect (PAE) refers to the temporary suppression of bacterial growth following transient antibiotic treatment. This effect has been observed for decades for a wide variety of antibiotics and microbial species. However, despite empirical observations, a mechanistic understanding of this phenomenon is lacking. Using a combination of modeling and quantitative experiments, we show that the PAE can be explained by the temporal dynamics of drug detoxification in individual cells after an antibiotic is removed from the extracellular environment. These dynamics are dictated by both the export of the antibiotic and the intracellular titration of the antibiotic by its target. This mechanism is generally applicable for antibiotics with different modes of action. We further show that efflux inhibition is effective against certain antibiotic motifs, which may help explain mixed cotreatment success.

Bactericidal activity and post-antibiotic effect of ozenoxacin against Propionibacterium acnes

Ozenoxacin, a novel non-fluorinated topical quinolone, is used for the treatment of acne vulgaris in Japan. We investigated bactericidal activity and post-antibiotic effect (PAE) of ozenoxacin against Propionibacterium acnes, a major causative bacterium of acne vulgaris. The minimum inhibitory concentrations (MICs) of ozenoxacin against 3 levofloxacin-susceptible strains (MIC of levofloxacin; ≤4 μg/mL) and 3 levofloxacin-resistant strains (MIC of levofloxacin; ≥8 μg/mL) ranged from 0.03 to 0.06 μg/mL and from 0.25 to 0.5 μg/mL, respectively. These MICs of ozenoxacin were almost the same or lower than nadifloxacin and clindamycin. The minimum bactericidal concentrations (MBCs) of ozenoxacin against the levofloxacin-susceptible and -resistant strains were from 0.06 to 8 μg/mL and from 0.5 to 4 μg/mL, respectively. These MBCs were lower than those of nadifloxacin and clindamycin. In time-kill assay, ozenoxacin at 1/4, 1 and 4 times the respective MIC against both levofloxacin-susceptible and -resistant strains showed a concentration-dependent bactericidal activity. Ozenoxacin at 4 times the MICs against the levofloxacin-susceptible strains showed more potent and more rapid onset of bactericidal activity compared to nadifloxacin and clindamycin at 4 times the respective MICs. The PAEs of ozenoxacin at 4 times the MICs against the levofloxacin-susceptible strains were from 3.3 to 17.1 h, which were almost the same or longer than nadifloxacin and clindamycin. In contrast, the PAEs were hardly induced by any antimicrobial agents against the levofloxacin-resistant strains. The present findings suggest that ozenoxacin has a potent bactericidal activity against both levofloxacin-susceptible and -resistant P. acnes, and a long-lasting PAE against levofloxacin-susceptible P. acnes.

Correlating Drug-Target Kinetics and In vivo Pharmacodynamics: Long Residence Time Inhibitors of the FabI Enoyl-ACP Reductase

Drug-target kinetics enable time-dependent changes in target engagement to be quantified as a function of drug concentration. When coupled to drug pharmacokinetics (PK), drug-target kinetics can thus be used to predict in vivo pharmacodynamics (PD). Previously we described a mechanistic PK/PD model that successfully predicted the antibacterial activity of an LpxC inhibitor in a model of Pseudomonas aeruginosa infection. In the present work we demonstrate that the same approach can be used to predict the in vivo activity of an enoyl-ACP reductase (FabI) inhibitor in a model of methicillin-resistant Staphylococcus aureus (MRSA) infection. This is significant because the LpxC inhibitors are cidal, whereas the FabI inhibitors are static. In addition P. aeruginosa is a Gram-negative organism whereas MRSA is Gram-positive. Thus this study supports the general applicability of our modeling approach across antibacterial space.

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.

Postantibiotic effect of disinfection treatment by photolysis of hydrogen peroxide

The purpose of the present study was to evaluate the postantibiotic effect (PAE) of the disinfection treatment by photolysis of H2O2. Postantibiotic effect was induced in Staphylococcus aureus and Streptococcus salivarius by exposing the bacteria to H2O2 at concentrations of 250-1000 mmol/l, laser irradiation at a wavelength of 405 nm, and the combination of both (photolysis of H2O2) for 10-30 seconds. The photolysis of H2O2 induced significantly longer PAE than other treatments. The PAE was augmented dependently on not only the concentration of H2O2 but the laser irradiation time. Electron spin resonance analysis showed that the hydroxyl radical was also generated dependently on both the concentration of H2O2 and the laser irradiation time, suggesting that the hydroxyl radicals contribute to the PAE. These results suggest that the disinfection treatment by photolysis of H2O2 induces PAE in S. aureus and S. salivarius even though they were treated for only 10-30 seconds.

Adenylate kinase release as a high-throughput-screening-compatible reporter of bacterial lysis for identification of antibacterial agents

Adenylate kinase (AK) is a ubiquitous intracellular enzyme that is released into the extracellular space upon cell lysis. We have shown that AK release serves as a useful reporter of bactericidal agent activity and can be exploited for antimicrobial screening purposes. The AK assay exhibits improved sensitivity over that of growth-based assays and can detect agents that are active against bacteria in clinically relevant growth states that are difficult to screen using conventional approaches, such as small colony variants (SCV) and bacteria within established biofilms. The usefulness of the AK assay was validated by screening a library of off-patent drugs for agents that exhibit antimicrobial properties toward a variety of bacterial species, including Escherichia coli and all members of the "ESKAPE" pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species). The assay detected antibiotics within the library that were expected to be active against the organism screened. Moreover, 38 drugs with no previously reported antibacterial activity elicited AK release. Four of these were acquired, and all were verified to exhibit antimicrobial activity by standard susceptibility testing. Two of these molecules were further characterized. The antihistamine, terfenadine, was active against S. aureus planktonic, SCV population, and biofilm-associated cells. Tamoxifen, an estrogen receptor antagonist, was active toward E. faecium in vitro and also reduced E. faecium pathogenesis in a Galleria mellonella infection model. Our data demonstrate that the AK assay provides an attractive screening approach for identifying new antimicrobial agents. Further, terfenadine and tamoxifen may represent novel antimicrobial drug development scaffolds.

In vitro postantibiotic effect of colistin on multidrug-resistant Acinetobacter baumannii

Infections by multidrug-resistant Acinetobacter baumannii constitute an increasing threat for critically ill patients. Colistin is often the only antimicrobial retaining activity against these strains. The postantibiotic effect (PAE) of colistin was studied on 19 isolates of A. baumannii resistant to ampicillin/sulbactam, ciprofloxacin, and carbapenems with the viable count method. The mean PAEs of 1x MIC and 4x MIC concentrations of colistin on the tested isolates were 3.90 and 4.48 h, respectively, indicating that a modified dosage scheme with increased dosing intervals might retain activity whereas minimizing the incidence of adverse effects.

Deletion of the multiple-drug efflux pump AcrAB in Escherichia coli prolongs the postantibiotic effect

The mechanism of the postantibiotic effect (PAE) was examined in Escherichia coli. Drugs exhibited longer-lasting PAEs in an acrAB mutant, suggesting that intracellular drug concentrations influence the duration of the PAE. With specific assays for tetracycline and erythromycin, a direct link between intracellular persistence of antibiotics and maintenance of the PAE was established.

Post-antibiotic effect induced by an antibiotic combination: influence of mode, sequence and interval of exposure

OBJECTIVES

The effects of mode, sequence and interval of antibiotic exposure on the post-antibiotic effect (PAE) induced by rifampicin and tobramycin were studied using Escherichia coli ATCC 25922 as the test organism.

METHODS

In triplicate, baseline PAEs were evaluated by exposing E. coli to rifampicin and tobramycin individually and simultaneously for 1 h. PAEs were further assessed in a second study, with the organism exposed first to rifampicin for 1 h, followed by a second 1 h tobramycin exposure, commencing at the beginning, middle and end of the PAE phase induced by rifampicin. The third study was similar to the above, but with the sequence of the two antibiotics reversed, i.e. tobramycin then rifampicin.

RESULTS

The PAE produced by simultaneous exposure of the combination showed an apparent additive interaction (PAE: 5.0+/-0.3 h) when compared with the PAE of individual antibiotics (rifampicin alone: 3.0+/-0.1 h; tobramycin alone: 1.5+/-0.1 h). However, an antagonistic interaction was observed in the second study, with a more pronounced degree of antagonism at the beginning, dissipating towards the end of the previous rifampicin PAE (PAE at the beginning: 2.6+/-0.3 h; the middle: 1.5+/-0.2 h; and at the end: 1.7+/-0.3 h). By subtracting the residual contribution from the first rifampicin exposure, the net average PAEs attributed to the second tobramycin exposure actually increased, from -0.4 to 1.7 h from the beginning to the end of the rifampicin PAE. For the third study, an additive interaction was again observed when the organism was exposed to tobramycin first (PAE at the beginning: 4.7+/-0.4 h; the middle: 3.7+/-0.7 h; and at the end: 3.1+/-0.4 h). The timing of the second rifampicin exposure had no impact to the interaction; after correction, the net mean PAEs attributed to the second rifampicin exposure were maintained at 3.2, 3.2 and 3.1 h.

CONCLUSIONS

The present data suggest that the expression of interaction type on PAE by an antibiotic combination was dependent on the mode, sequence and interval of exposure. The impact of these variables should not be overlooked when clinical dosing regimens are optimized.

The integration of four major determinants of antibiotic action: bactericidal activity, postantibiotic effect, susceptibility, and pharmacokinetics

A functional pharmacokinetic/pharmacodynamic (PK/PD) index that could simultaneously describe three controlling PD variables, i.e., bactericidal activity, postantibiotic effect (PAE), and susceptibility, in relation to pharmacokinetics, was designed using an in vitro kinetic model. Tobramycin was tested against one standard and five clinical strains of Pseudomonas aeruginosa. The organisms showed minimum inhibitory concentrations (MICs) ranging between 1 and >1000 microg/ml. The model allowed antibiotic concentrations to be reduced exponentially from initial concentrations at fixed multiples of MIC. Antibiotic removal was performed when the decreasing concentrations hit the MIC of individual strain to provide a wide range of AUC(>MIC) within an identical frame of AUC(>MIC)/MIC (AUIC) values. Viable counts were measured at antibiotic addition and before/after its removal for bactericidal activity and PAE assessments. A linear relationship was observed between PAE and bactericidal rate constants, though the pattern varied among different strains. Characterization of the exposure (AUC(>MIC))-effect relationships using the Emax model revealed that the less susceptible strains displayed lower Emax and higher EC50 for both antimicrobial effects. By employing the AUIC as a common frame of reference, regression analysis showed a significant linear correlation (p < 0.05) between the mean PAE and bactericidal rate data and, thereby simultaneously defining the four contributing factors of the PK/PD system. It appears that the AUIC, by conveying the pharmacokinetic and susceptibility information, could serve as a PK/PD index in bridging the interdependency of PAE and bactericidal activity. More importantly, the collective assessment of these four factors would allow more optimal evaluation of dosage regimens.

Postantibiotic effect and delay of regrowth in strains carrying mutations that save proteins or RNA

The postantibiotic effect (PAE) values found for proteinase-defective (Lon(-)) Escherichia coli and RNase-defective E. coli exposed to antibiotics were reduced (31 to 60% and 35 to 50%, respectively) in comparison with the control (AB1157), and in the recA13 mutant these values were about 0.4 h with all drugs. Nalidixic acid, under anaerobic conditions, induced no PAE (0 to 0.1 h) in AB1157. A delay in regrowth (0.2 to 0.26 h) was noted with dnaA46(Ts), gyrA43(Ts), and gyrB41(Ts) mutants cultured for 2 h at 43 degrees C. These findings suggest that when proteins and RNA are saved, the cell rapidly resumes the original growth rate.

Macrolides: pharmacokinetics and pharmacodynamics

Three pharmacokinetic/pharmacodynamic parameters--(i) the peak concentration to the minimum inhibitory concentration ratio (C(max)/MIC); (ii) the area under the concentration-time curve to MIC ratio (AUC(24h)/MIC); and (iii) the time the concentration exceeds the MIC (T>MIC)--are important predictors of the clinical efficacy of antibiotics. For antibiotics with pronounced concentration-dependent killing, such as the fluoroquinolones or the aminoglycosides, C(max)/MIC and AUC(24)/MIC are the main factors that establish efficacy. Antibiotics with a weak, or no, concentration dependency generally have their efficacy linked to T>MIC, and these include the beta-lactams and the conventional macrolides. Antibiotics with weak concentration-dependent effects, but with prolonged persistent effects, such as tetracyclines and azithromycin, have their activity mostly related to the AUC(24)/MIC. By applying these concepts to current antibiotics, and also to the development of novel agents, it is possible to optimise their dosages and administration schedules. This will maximise therapeutic efficacy, may prevent or delay the emergence of bacterial resistance to antibiotics, and can certainly minimise side-effects.

Investigation of Smith's quinolone killing mechanisms during the PAE of ciprofloxacin on Escherichia coli

Quinolone antibacterials interact with the DNA-DNA gyrase complex, but subsequent events that lead to cell death are unresolved. Three distinct mechanisms of quinolone lethality have been identified by Smith and co-workers: Mechanism A, which requires RNA and protein synthesis and cell division for expression; Mechanism B, which remains active when these functions are precluded; and Mechanism C, which is active on non-dividing cells. Exposure to 4x MIC ciprofloxacin (Cip) in nutrient broth (NB) for 3 h reduced the viability of Escherichia coli AB1157 to 0.25%. Addition of rifampicin (Rif) or chloramphenicol (Cm), to inhibit RNA or protein synthesis, respectively, increased survival 70-fold. Treatment of cells with Cip in phosphate-buffered saline (PBS), to inhibit cell division, increased survival 20-fold. No further cell death occurred once the various drug combinations or PBS had been washed out and cells resuspended in drug-free nutrient broth. These latter conditions allow expression of the post-antibiotic effect (PAE). PAE was lengthened in cells exposed to Cip in the presence of Rif or Cm, probably as a result of delay in the initiation of inducible DNA repair.

Postantibiotic effects and postantibiotic sub-MIC effects of amoxicillin on Streptococcus gordonii and Streptococcus sanguis

Amoxicillin is one of the most frequently recommended antibiotics for prophylaxis of infective endocarditis in dental/oral procedures. In this study, the postantibiotic effect (PAE), postantibiotic sub-MIC (PASME) and sub-MIC effect (SME) of amoxicillin on oral streptococci, Streptococcus gordonii and Streptococcus sanguis, which are two of the major etiological agents in infective endocarditis, were investigated. The PAE was induced by 10 x MIC of amoxicillin for 2 h and the antibiotic was eliminated by washing. The PASMEs were studied by addition of 0.1, 0.2 and 0.3 x MICs during the postantibiotic phase of the bacteria, and the SMEs were studied by exposing bacteria to amoxicillin at the sub-MICs only. The PAE of amoxicillin was 2.0 h with S. gordonii DL1 and 0.7 h with S. sanguis MPC1. The PASME and SME of amoxicillin were observed both for S. gordonii DL1 and for S. sanguis MPC1. However, the durations of effects for S. sanguis MPC1 were shorter than those for S. gordonii DL1. The PASME values for both strains increased as the concentration of amoxicillin increased. The PASME values for both strains were substantially longer than the SME values. The present study illustrates the existence of PAE, PASME and SME for amoxicillin against S. gordonii and S. sanguis, thereby extending the pharmacodynamic advantages of amoxicillin for these bacteria in the prophylaxis procedures of infective endocarditis.

New pharmacodynamic parameters for antimicrobial agents

The application of pharmacodynamic theories to antimicrobial chemotherapy has greatly improved the prediction of the time course of activity expressed by antibiotics. Being a major component of the antibiotic-bacterium interaction system, pharmacodynamics, when properly integrated with the pharmacokinetics established for the antibiotic, allow better evaluation of the dosage regimen in conjunction with its clinical response. Before this approach becomes effective, detailed background information on the complex antibiotic-bacterium interactions have to be secured. To achieve this, proper characterization of a time-kill curve is a prerequisite. The use of susceptibility endpoints such as the MIC with respect to the antibiotic concentrations achievable in vivo represent the conventional approach to clinical dosing of antimicrobial agents, i.e. by maintaining concentrations above the MIC. Recently, a number of surrogate markers have been proposed by combining suitable pharmacokinetic parameters and susceptibility data, e.g. peak/MIC ratio, AUC>MIC, time above MIC, AUIC etc. to enhance the prediction of clinical outcomes. Attempts have been made to apply these pharmacokinetic/pharmacodynamic markers to antibiotics of the same class as well as to antibiotics from different classes. This review aims to discuss the various microbial dynamic responses in relation to antibiotic exposure and the development of different pharmacokinetic/pharmacodynamic markers for use in current antimicrobial chemotherapy.

Achieving an optimal outcome in the treatment of infections. The role of clinical pharmacokinetics and pharmacodynamics of antimicrobials

Over the past few decades, the importance of applying pharmacokinetic principles to the design of drug regimens has been increasingly recognised by clinicians. From the perspective of antimicrobial chemotherapy, an improvement in clinical outcome and/or a reduction in toxicity are of primary interest. Before application of these pharmacokinetic theories can be effective, the interrelationships between antimicrobial, pathogen and host factors must be clearly defined. Information regarding the pharmacokinetics of the antimicrobial and the quantification of pathogen susceptibility is required. Even though susceptibility end-points such as minimum inhibitory concentration (MIC) and minimum bactericidal concentration are widely employed, they do not provide any information on dynamic changes of bacterial densities. In this regard, time-kill studies can provide more basic knowledge of the complex bacterial responses to the antimicrobial. Better prediction of these responses can be afforded by the use of mathematical models. More recently, various surrogate end-points employing a combination of suitable pharmacokinetic parameters and susceptibility data, for example the ratio of peak concentration to MIC, the area under the concentration-time curve above the MIC (AUC > MIC), the time above the MIC, or the area under the inhibitory curve (AUIC), have been suggested for better prediction of the activity of different classes of antimicrobials. To allow more extensive investigations of the contribution of pharmacokinetics to the pharmacodynamics of antimicrobials, various in vitro kinetic models have been developed. However, certain limitations exist, and it is necessary to avoid over-interpretation of the data generated by these models. Two important microbial dynamic responses, postantibiotic effect and resistance selection, must be further explored before the full impact of pharmacokinetics on antimicrobial chemotherapy can be depicted. The present paper aims at discussing all the relevant factors and provides some pertinent information on the use of pharmacokinetic-pharmacodynamic principles in antimicrobial therapy.

Molecular investigation of the postantibiotic effects of clarithromycin and erythromycin on Staphylococcus aureus cells

The kinetics of recovery after inhibition of growth by erythromycin and clarithromycin were examined in Staphylococcus aureus cells. After inhibition for one mass doubling by 0.5 microg of the antibiotics/ml, a postantibiotic effect (PAE) of 3 and 4 h duration was observed for the two drugs before growth resumed. Cell viability was reduced by 25% with erythromycin and 45% with clarithromycin compared with control cells. Erythromycin and clarithromycin treatment reduced the number of 50S ribosomal subunits to 24 and 13% of the number found in untreated cells. 30S subunit formation was not affected. Ninety minutes was required for resynthesis to give the control level of 50S particles. Protein synthesis rates were diminished for up to 4 h after the removal of the macrolides. This continuing inhibition of translation was the result of prolonged binding of the antibiotics to the 50S subunit as measured by 14C-erythromycin binding to ribosomes in treated cells. The limiting factors in recovery from macrolide inhibition in these cells, reflected as a PAE, are the time required for the synthesis of new 50S subunits and the slow loss of the antibiotics from ribosomes in inhibited cells.