Predictors of effect of ampicillin-sulbactam against TEM-1 beta-lactamase-producing Escherichia coli in an in vitro dynamic model: enzyme activity versus MIC

Predicting the Effects of Carbapenem/Carbapenemase Inhibitor Combinations against KPC-Producing Klebsiella pneumoniae in Time-Kill Experiments: Alternative versus Traditional Approaches to MIC Determination

Traditionally, the antibacterial activity of β-lactam antibiotics in the presence of β-lactamase inhibitors is determined at the fixed inhibitor concentration. This traditional approach does not consider the ratio of antibiotic-to-inhibitor concentrations achieved in humans. To explore whether an alternative pharmacokinetically based approach to estimate MICs in combinations is predictive of antimicrobial efficacy, the effects of imipenem and doripenem alone and in combination with relebactam were studied in time-kill experiments against carbapenemase-producing Klebsiella pneumoniae. The carbapenem-to-relebactam concentration ratios in time-kill assays were equal to the therapeutic 24-h area under the concentration-time curve (AUC) ratios of the drugs (1.5/1). The simulated levels of carbapenem and relebactam were equal to their concentrations achieved in humans. When effects of combined regimens were plotted against respective C/MICs, a sigmoid relationship was obtained only with MICs determined by pharmacokinetically based method. The effectiveness of both carbapenems in the presence of relebactam was comparable by the results of time-kill experiments. These findings suggest that (1) antibiotic/inhibitor MICs determined at a pharmacokinetically based concentration ratio allow an adequate assessment of carbapenem susceptibility in carbapenemase-producing K. pneumoniae strains and can be used to predict antibacterial effects; (2) in time-kill experiments, the effects of imipenem and doripenem in the presence of relebactam are comparable.

Verification of a Novel Approach to Predicting Effects of Antibiotic Combinations: In Vitro Dynamic Model Study with Daptomycin and Gentamicin against Staphylococcus aureus

To explore whether susceptibility testing with antibiotic combinations at pharmacokinetically derived concentration ratios is predictive of the antimicrobial effect, a Staphylococcus aureus strain was exposed to daptomycin and gentamicin alone or in combination in multiple dosing experiments. The susceptibility of the S. aureus strain to daptomycin and gentamicin in combination was tested at concentration ratios equal to the ratios of 24 h areas under the concentration–time curve (AUC₂₄s) of antibiotics simulated in an in vitro dynamic model in five-day treatments. The MICs of daptomycin and gentamicin decreased in the presence of each other; this led to an increase in the antibiotic AUC₂₄/MIC ratios and the antibacterial effects. Effects of single and combined treatments were plotted against the AUC₂₄/MIC ratios of daptomycin or gentamicin, and a significant sigmoid relationship was obtained. Similarly, when the effects of single and combined treatments were related to the total exposure of both drugs (the sum of AUC₂₄/MIC ratios (∑AUC₂₄/MIC)), a significant sigmoid relationship was obtained. These findings suggest that (1) the effects of antibiotic combinations can be predicted by AUC₂₄/MICs using MICs of each antibacterial determined at pharmacokinetically derived concentration ratios; (2) ∑AUC₂₄/MIC is a reliable predictor of the antibacterial effects of antibiotic combinations.

Predicting the antistaphylococcal effects of daptomycin-rifampicin combinations in an in vitro dynamic model

To predict the effects of combined use of antibiotics on their pharmacodynamics, the susceptibility of Staphylococcus aureus to daptomycin-rifampicin combinations was tested at concentration ratios equal to the ratios of daptomycin and rifampicin 24-h areas under the concentration-time curve (AUC₂₄s) simulated in an in vitro dynamic model. In combination with rifampicin, daptomycin MICs decreased 2- to 31-fold, whereas rifampicin MICs were similar with or without daptomycin. The enhanced susceptibility of S. aureus to daptomycin combined with rifampicin resulted in both an increase of the actual AUC₂₄/MIC ratios and also more pronounced antibacterial effects compared with single treatments. The areas between the control growth and time-kill curves (ABBCs) determined in combined and single daptomycin treatments were plotted against AUC₂₄/MIC on the same graph (r² 0.90). These findings suggest that the effects of daptomycin-rifampicin combinations can be predicted by AUC₂₄/MICs of daptomycin using its MIC determined at pharmacokinetically derived daptomycin-to-rifampicin concentration ratios.

Fosfomycin efficacy and emergence of resistance among Enterobacteriaceae in an in vitro dynamic bladder infection model

Background

Urinary tract infections (UTIs) are among the most common bacterial infections and a frequent indication for antibiotic use. Fosfomycin, an important oral antibiotic for outpatient UTIs, remains a viable option for MDR uropathogens. We aimed to perform pharmacodynamic profiling simulating urinary concentrations to assess the adequacy of the current dosing regimen.

Methods

A dynamic in vitro bladder infection model was developed, replicating urinary fosfomycin concentrations after gastrointestinal absorption, systemic distribution and urinary elimination. Concentrations were measured by LC-MS/MS. Twenty-four Enterobacteriaceae strains (Escherichia coli, Klebsiella pneumoniae and Enterobacter cloacae; MIC range 0.25-64 mg/L) were examined. Pathogen kill and emergence of resistance was assessed over 72 h.

Results

Observed in vitro fosfomycin concentrations accurately simulated urinary fosfomycin exposures (Tmax 3.8 ± 0.5 h; Cmax 2630.1 ± 245.7 mg/L; AUC0-24 33 932.5 ± 1964.2 mg·h/L). Fifteen of 24 isolates regrew, with significant rises in fosfomycin MIC (total population MIC50 4 to 64 mg/L, MIC90 64 to > 1024 mg/L, P = 0.0039; resistant subpopulation MIC50 128 to > 1024 mg/L, MIC90 >1024 mg/L, P = 0.0020). E. coli and E. cloacae isolates were killed with pharmacokinetic/pharmacodynamic EI50 of fAUC0-24/MIC = 1922, fCmax/MIC = 149 and fTime>4×MIC = 44 h. In contrast, K. pneumoniae isolates were not reliably killed.

Conclusions

Using dynamic in vitro simulations of urinary fosfomycin exposures, E. coli and E. cloacae isolates with MIC >16 mg/L, and all K. pneumoniae isolates, were not reliably killed. Emergence of resistance was significant. This challenges fosfomycin dosing and clinical breakpoints, and questions the utility of fosfomycin against K. pneumoniae. Further work on in vitro dose optimization is required.

Predicting effects of antibiotic combinations using MICs determined at pharmacokinetically derived concentration ratios: in vitro model studies with linezolid- and rifampicin-exposed Staphylococcus aureus

To predict the effects of combined use of antibiotics on their pharmacodynamics, the susceptibility of Staphylococcus aureus to linezolid-rifampicin combinations was tested at concentration ratios equal to the ratios of 24-area under the concentration-time curve (AUC₂₄) simulated in an in vitro dynamic model. The linezolid MICs in combination with rifampicin decreased 8- to 67-fold. The rifampicin MICs were similar with or without linezolid. The enhanced activity of linezolid combined with rifampicin increased the AUC₂₄/MIC ratios and provided more pronounced antibacterial effects compared with single treatments. The areas between the control growth and time-kill curves (ABBCs) determined in combined and single treatments with linezolid were plotted against AUC₂₄/MIC on the same graph (r² 0.94). These findings suggest that the effects of linezolid-rifampicin combinations can be predicted by AUC₂₄/MICs of linezolid using its MIC determined at pharmacokinetically derived linezolid-to-rifampicin concentration ratios.

Searching for the Optimal Predictor of Ciprofloxacin Resistance in Klebsiella pneumoniae by Using In Vitro Dynamic Models

There is growing evidence of applicability of the hypothesis of the mutant selection window (MSW), i.e., the range between the MIC and the mutant prevention concentration (MPC), within which the enrichment of resistant mutants is most probable. However, it is not clear if MPC-based pharmacokinetic variables are preferable to the respective MIC-based variables as interstrain predictors of resistance. To examine the predictive power of the ratios of the area under the curve (AUC24) to the MPC and to the MIC, the selection of ciprofloxacin-resistant mutants of three Klebsiella pneumoniae strains with different MPC/MIC ratios was studied. Each organism was exposed to twice-daily ciprofloxacin for 3 days at AUC24/MIC ratios that provide peak antibiotic concentrations close to the MIC, between the MIC and the MPC, and above the MPC. Resistant K. pneumoniae mutants were intensively enriched at an AUC24/MIC ratio of 60 to 360 h (AUC24/MPC ratio from 2.5 to 15 h) but not at the lower or higher AUC24/MIC and AUC24/MPC ratios, in accordance with the MSW hypothesis. AUC24/MPC and AUC24/MIC relationships with areas under the time courses of ciprofloxacin-resistant K. pneumoniae (AUBCM) were bell shaped. These relationships predict highly variable "antimutant" AUC24/MPC ratios (20 to 290 h) compared to AUC24/MIC ratios (1,310 to 2,610 h). These findings suggest that the potential of the AUC24/MPC ratio as an interstrain predictor of K. pneumoniae resistance is lower than that of the AUC24/MIC ratio.

Clinical characteristics of bloodstream infections due to ampicillin-sulbactam-resistant, non-extended- spectrum-beta-lactamase-producing Escherichia coli and the role of TEM-1 hyperproduction

Ampicillin-sulbactam is commonly used as an empirical therapy for invasive infections where Escherichia coli is a potential pathogen. We evaluated the clinical and microbiologic characteristics of bloodstream infection due to E. coli, with focus on cases that were nonsusceptible to ampicillin-sulbactam and not producing extended-spectrum β-lactamase (ESBL). Of a total of 357 unique bacteremic cases identified between 2005 and 2008, 111 (31.1%) were intermediate or resistant to ampicillin-sulbactam by disk testing. In multivariate analysis, a history of liver disease, organ transplant, peptic ulcer disease, and prior use of ampicillin-sulbactam were independent risk factors for bloodstream infection with ampicillin-sulbactam-nonsusceptible E. coli. Among cases that received ampicillin-sulbactam as an empirical therapy, an early clinical response was observed in 65% (22/34) of susceptible cases but in only 20% (1/5) of nonsusceptible cases. Among 50 ampicillin-sulbactam-resistant isolates examined, there was no clonal relatedness and no evidence of production of inhibitor-resistant TEM (IRT). Instead, the resistance was attributed to hyperproduction of TEM-1 β-lactamase in the majority of isolates. However, promoter sequences of bla(TEM-1) did not predict resistance to ampicillin-sulbactam. While the plasmid copy number did not differ between representative resistant and susceptible isolates, the relative expression of bla(TEM-1) was significantly higher in two of three resistant isolates than in three susceptible isolates. These results suggest high-level bla(TEM-1) expression as the predominant cause of ampicillin-sulbactam resistance and also the presence of yet-unidentified factors promoting overexpression of bla(TEM-1) in these isolates.

Influence of different resistance traits on the competitive growth of Haemophilus influenzae in antibiotic-free medium and selection of resistant populations by different {beta}-lactams: an in vitro pharmacodynamic approach

OBJECTIVES

The aim was to study the pharmacodynamics of cefditoren, amoxicillin/clavulanic acid and cefuroxime against mixed Haemophilus influenzae strains.

METHODS

Isolates [MICs (mg/L) of cefditoren, cefuroxime and amoxicillin/clavulanic acid] used were: one beta-lactamase-negative (beta(-); 0.015, 1 and 1), one beta-lactamase-positive (beta(+); 0.03, 4 and 8) and two strains exhibiting ftsI gene mutations [one beta(-) ampicillin-resistant (BLNAR; 0.015, 8 and 4) and one beta(+) amoxicillin/clavulanic acid-resistant (BLPACR; 0.03, 8 and 4)]. A computerized pharmacodynamic model simulating free antibiotic concentrations (calculated considering reported percentages of protein binding) of 400 mg twice-daily cefditoren, 500 mg twice-daily cefuroxime and 875/125 mg three times daily amoxicillin/clavulanic acid was used to explore antibacterial activity against initial mixed inocula with 25% of each strain. Areas under bacterial curves (AUBCs) from 0 to 24 h (log cfu.h/mL) were calculated and differences between values in antibiotic-free (AUBC(K)) and in antibiotic simulations determined (ABBC(0-24) = AUBC(K0-24)-AUBC(0-24)).

RESULTS

In antibiotic-free medium, total population increased by 1.7 log(10) cfu/mL from 0 to 24 h: final composition approximately 90% beta(-), approximately 6.5% beta(+), approximately 2.5% BLNAR and approximately 1% BLPACR. At the end of antibiotic simulations, the predominant population was BLPACR followed by beta(+) after amoxicillin/clavulanic acid or BLNAR after cefuroxime exposures. ABBC(0-24) was higher (P < 0.01) for cefditoren compared with cefuroxime or amoxicillin/clavulanic acid whether considering total population (70.4 versus approximately 33), beta(+) (77.8 versus approximately 52), BLNAR (66.1 versus 18.6-30.4) or BLPACR (40.8 versus approximately 0).

CONCLUSIONS

Cefditoren offered higher antibacterial effect than cefuroxime and amoxicillin/clavulanic acid against a mixed population of H. influenzae strains due to its higher activity against beta-lactamase-producing strains and those carrying ftsI gene mutations.

Linezolid pharmacodynamics with Staphylococcus aureus in an in vitro dynamic model

To describe the relationship between the ratio of the 24-h area under the concentration-time curve (AUC(24)) to minimum inhibitory concentration (MIC) as well as the effect of linezolid on Staphylococcus aureus, the killing kinetics of three S. aureus strains was studied by in vitro simulation of 5-day antibiotic dosing over a wide range of AUC(24)/MIC ratios. Similarly susceptible meticillin-resistant S. aureus ATCC 43300 and S. aureus 479 and vancomycin-intermediate S. aureus ATCC 700699 (Mu50) at a starting inoculum of 10(8) colony-forming units (CFU)/mL were exposed to multiple-dose pharmacokinetics of twice-daily linezolid for 5 days. The simulated AUC(24)/MIC ratios varied from 30 h to 1200 h (S. aureus ATCC 43300), from 30h to 600 h (S. aureus 479) and from 50h to 400 h (S. aureus ATCC 700699). The cumulative antimicrobial effect was expressed by its intensity (I(E)) measured from the start of treatment to the time after the last antibiotic dose when numbers of antibiotic-exposed bacteria reached >or=10(8)CFU/mL. With each organism, bacterial re-growth followed a pronounced reduction of the starting inoculum that occurred at each simulated AUC(24)/MIC ratio except for the lowest value (30 h). This reduction was AUC(24)/MIC-dependent: the minimum numbers of surviving organisms decreased with increasing AUC(24)/MIC ratios. A sigmoid relationship was established between I(E) and the simulated AUC(24)/MIC ratio. This relationship was bacterial strain-independent; a logistic function fits the combined data with r(2)=0.95. The established AUC(24)/MIC-I(E) relationship is useful to predict the antistaphylococcal effects of linezolid at clinically attainable AUC(24)/MIC values.

Influence of inoculum size on the selection of resistant mutants of Escherichia coli in relation to mutant prevention concentrations of marbofloxacin

We demonstrate using an in vitro pharmacodynamic model that the likelihood of selection of Escherichia coli mutants resistant to a fluoroquinolone was increased when the initial size of the bacterial population, exposed to fluoroquinolone concentrations within the mutant selection window, was increased.

Concentration-response relationships as a basis for choice of the optimal endpoints of the antimicrobial effect: daptomycin and vancomycin pharmacodynamics with staphylococci in an in vitro dynamic model

The abilities of different indices of bacterial killing to ensure reasonable concentration-response relationships have been compared only in studies in vitro with fluoroquinolones. To ascertain the relevance of conclusions drawn in these studies to other antibiotic classes, five widely used indices that reflect the rate of initial killing (time to reduce the initial inoculum 10- and 100-fold-T(90%) and T(99%), respectively), the extent of killing (minimal number of surviving organisms-N(min)), and the entire antimicrobial effect (number of surviving organisms (N(t)) at the time t close to the end of the observation period (48 h in most experiments), and area between the control growth curve and the time-kill curve from zero point to t-ABBC) were examined with daptomycin (DAP)- and vancomycin (VAN)-exposed Staphylococcus aureus. To compare the pharmacodynamics of DAP and VAN and examine different parameters, killing kinetics of differentially susceptible S. aureus were studied over a wide range of ratios of area under the curve (AUC) to MIC. Killing kinetics of two clinical isolates, S. aureus 866 (MIC(DAP) 0.35 mg/L and MIC(VAN) 0.70 mg/L) and S. aureus 10 (MIC(DAP) 1.1mg/L and MIC(VAN) 1.3mg/L), were studied in an in vitro dynamic model that simulates human pharmacokinetics of DAP (as a single dose) and VAN (as two 12-h doses). Mono-exponential concentration decays were mimicked with half-lives of 9h (DAP) and 6h (VAN) at AUC/MIC ratios varying from 33 to 1150 h. T(90%), T(99%) and N(t) (at t=48 h) exhibited loose, if any, correlations with log AUC/MIC. Both ABBC (a direct measure of the antimicrobial effect) and N(min) (an inverse measure of the effect) correlated well with log AUC/MIC (r(2)=0.8-0.9). Based on the ABBC-log AUC/MIC relationships, the effects of DAP on S. aureus were predicted to be slightly greater than those of VAN at a given AUC/MIC ratio. The better abilities of ABBC and N(min) and the inability of T(90%) and T(99%) to provide reasonable AUC/MIC relationships with DAP and VAN support earlier findings reported with fluoroquinolones.

Comparative pharmacodynamics of the new fluoroquinolone ABT492 and levofloxacin with Streptococcus pneumoniae in an in vitro dynamic model

The kinetics of killing of Streptococcus pneumoniae exposed to ABT492 or levofloxacin were compared. S. pneumoniae ATCC 49619 and four ciprofloxacin-resistant clinical isolates, S. pneumoniae 1149, 391, 79 and 804, were exposed to ABT492 and levofloxacin as a single dose in a dynamic model that simulates human pharmacokinetics of the quinolones. With S. pneumoniae ATCC 49619 eight-fold ranging AUC/MIC ratios (60-500 h) were simulated for each quinolone. In addition, two larger AUC/MICs, i.e., 1080 and 2150 h for ABT492 and 1460 and 3660 h for levofloxacin which correspond to 100 and 200 mg doses of ABT492 and 200 and 500 mg doses of levofloxacin, respectively, were mimicked. Each ciprofloxacin-resistant organism was exposed to the clinical doses of ABT492 (400 mg) and levofloxacin (500 mg); the respective AUC/MIC ratios were from 580 to 3470 h and from 28 to 110 h. At comparable AUC/MICs (from 60 to 500 h), regrowth of S. pneumoniae ATCC 49619 followed initial killing, and the times to regrowth were longer with levofloxacin than ABT492. However, no regrowth of S. pneumoniae ATCC 49619 occurred at the higher AUC/MICs of ABT492 (1080 and 2150 h) and levofloxacin (1460 and 3660 h). Killing of S. pneumoniae 1149, 391 and 79 without bacterial regrowth, was provided by ABT492 (AUC/MIC 3470, 2310 and 1160 h, respectively) but not levofloxacin (AUC/MIC 55, 110 and 28 h, respectively). Regrowth of S. pneumoniae 804 was observed with both ABT492 and levofloxacin (AUC/MIC 580 and 55 h, respectively). Areas between the control growth curve and the time-kill curve (ABBCs) for ABT492 against S. pneumoniae 1149, 391 and 79 were 2.6-4.2 times larger than the respective ABBCs for levofloxacin, whereas similar ABBCs were found with S. pneumoniae 804 exposed to both quinolones. These findings predict significantly greater efficacy of ABT492 than levofloxacin at clinically achievable AUC/MIC ratios against ciprofloxacin-resistant S. pneumoniae and similar efficacies of the two quinolones against susceptible organisms.

Correlation between in vitro and in vivo activities of GM 237354, a new sordarin derivative, against Candida albicans in an in vitro pharmacokinetic-pharmacodynamic model and influence of protein binding

The antifungal effect of GM 237354, a sordarin derivative, was studied in an in vitro pharmacokinetic (PK)-pharmacodynamic dynamic system (bioreactor) which reproduces PK profiles observed in a previously described model of drug efficacy against murine systemic candidiasis. Immunocompetent mice infected intravenously with 10(5) CFU of Candida albicans were treated with GM 237354 at 2.5, 10, and 40 mg/kg of body weight every 8 h subcutaneously for 7 days. Free concentrations in serum were calculated by multiplying total concentrations measured in vivo by 0.05, the free fraction determined in vitro by equilibrium dialysis. In the bioreactor the inoculum was approximately 10(6) CFU/ml; and a one-compartment PK model was used to reproduce the PK profiles of free and total GM 237354 in serum obtained in mice, and clearance of C. albicans was measured over 48 h. A good correlation was observed when the in vivo fungal kidney burden and the area under the survival time curve were compared with the in vitro broth "burden," although only when free in vivo levels in serum were reproduced in vitro. GM 237354 displayed a 3-log decrease effect both in vivo and in vitro. The very few reports available on in vitro-in vivo correlations have been obtained with antibiotics. The good in vitro-in vivo correlation obtained with an antifungal agent shows that the in vitro dynamic system could constitute a powerful investigational tool prior to assessment of the efficacy of an anti-infective agent in animals and humans.

Principles and application of in-vitro drug pharmacokinetic simulation systems controlled by stepwise method

The principles and application of an in-vitro pharmacokinetic simulation system controlled by a stepwise method are described. In this system, drug concentrations in a culture medium are regulated by a periodic stepwise-adjustment method. The drug concentration is increased by adding a portion of concentrated drug solution, or decreased by draining a portion of the culture medium and the subsequent addition of drug-free medium. The drug concentration is adjusted each minute with peristaltic pumps controlled by a microcomputer. Concentrations of two drugs with independent pharmacokinetics can be simulated in the same medium. An algorithm for controlling drug concentrations by a microcomputer is presented in this report. This pharmacokinetic system was successfully applied to a urinary tract infection system, in which three infection models with different severity were reproduced in vitro. The in-vitro pharmacokinetic simulation system described here could be applicable not only for evaluating the bactericidal activities of antibacterial agents but also for evaluating other categories of drugs, such as antitumor agents, based on their pharmacokinetics.

Relationships of the area under the curve/MIC ratio to different integral endpoints of the antimicrobial effect: gemifloxacin pharmacodynamics in an in vitro dynamic model

Most integral endpoints of the antimicrobial effect are determined over an arbitrarily chosen time period, such as the dosing interval (tau), regardless of the actual effect duration. Unlike the tau-related endpoints, the intensity of the antimicrobial effect (I(E)) does consider its duration-from time zero to the time when bacterial counts on the regrowth curve achieve the same maximal numbers as in the absence of the antimicrobial. To examine the possible impact of this fundamental difference on the relationships of the antimicrobial effect to the ratio of the area under the concentration-time curve (AUC) to the MIC, a clinical isolate of Staphylococcus aureus was exposed to simulated gemifloxacin pharmacokinetics over a 40-fold range of AUC/MIC ratios, from 11 to 466 h. In each run, I(E) and four tau-related endpoints, including the area under the time-kill curve (AUBC), the area above the curve (AAC), the area between the control growth and time-kill curves (ABBC), and the ABBC related to the area under the control growth curve (AUGC), were calculated for tau = 24 h. Unlike the I(E), which displayed pseudolinear relationships with the AUC/MIC ratio; each tau-related endpoint showed a distinct saturation at potentially therapeutic AUC/MIC ratios (116 to 466 h) when the antimicrobial effect persisted longer than tau. This saturation results from the underestimation of the true effect and may be eliminated if ABBC, AAC, and AUBC (but not AUGC) are modified and determined in the same manner as the I(E) to consider the actual effect duration. These data suggest a marginal value of the tau-related endpoints as indices of the total antimicrobial effect. Since all of them respond to AUC/MIC ratio changes less than the I(E), the latter is preferable in comparative pharmacodynamic studies.

Use of Modeling Techniques to Aid in Antibiotic Selection

Over the past decade, the use of modeling techniques in the development of novel antibiotics has been primarily associated with in vitro dynamic models. These models allow comparisons among different antibiotics by simulating human pharmacokinetics. Although dynamic models have been used extensively, their full potential has not been achieved because of inadequate experimental design and/or suboptimal quantitation of bacterial killing/regrowth curves inherent in many studies. These issues are discussed in this review, which is based on recent pharmacodynamic findings with novel fluoroquinolones.

A comparison of dynamic characteristics of fluconazole, itraconazole, and amphotericin B against Cryptococcus neoformans using time-kill methodology

This study evaluated the in vitro pharmacodynamics of fluconazole, itraconazole, and amphotericin B against Cryptococcus neoformans. MICs were determined for three clinical isolates according to NCCLS guidelines (M27). Time-kill studies were performed using antifungal concentrations of 0.25-32 x MIC and inocula of 10(3) and 10(5) CFU/ml. At predetermined time points over 72 hours, samples of each inoculum/drug combination were withdrawn and plated using a spiral plater. Colony counts were determined after incubation at 35 degrees C for 48 hours. Area under the kill curves (AUKCs) were plotted versus the AUC/MIC ratios. Inoculum effect was evaluated by calculating an estimated AUKC for the low inoculum then comparing it to the measured low inoculum using the unpaired Student's t-test. The MICs of fluconazole and itraconazole for isolate 97-1199, 97-1061, and 97-585 were 2, 4, 32 microg/ml and 0.03, 0.06, 0. 5 microg/ml, respectively. For amphotericin B, the MIC was 0. 25 microg/ml for each isolate. The triazoles demonstrated fungistatic activity against each isolate at both inocula with the exception of itraconazole against C. neoformans 97-585. Maximal suppression was noted at concentrations 8-16 x MIC correlating with an AUC/MIC of 192 for both inocula. Conversely, amphotericin B was fungicidal and displayed concentration-dependent activity against each isolate at both inocula. Maximal killing was observed at concentrations >4 x MIC for the low inoculum and >8 x MIC for the high inoculum for each isolate. No statistically significant differences were detected between the measured and estimated AUKCs for each antifungal agent. In conclusion, our results suggest that the triazoles were most effective against C. neoformans when concentrations were maintained at 8-16 x MIC. Amphotericin B, on the other hand, was concentration-dependent; thus, greater activity was exerted at higher concentrations.

Comparative pharmacodynamics of gatifloxacin and ciprofloxacin in an in vitro dynamic model: prediction of equiefficient doses and the breakpoints of the area under the curve/MIC ratio

To demonstrate the impact of the pharmacokinetics of gatifloxacin (GA) relative to those of ciprofloxacin (CI) on the antimicrobial effect (AME), the killing and regrowth kinetics of two differentially susceptible clinical isolates each of Staphylococcus aureus, Escherichia coli, and Klebsiella pneumoniae were studied. With each organism, a series of monoexponential pharmacokinetic profiles of GA (half-life [t(1/2)], 7 h) and CI (t(1/2) = 4 h) were simulated to mimic different single doses of GA and two 12-h doses of CI. The respective eightfold ranges of the ratios of the area under the concentration-time curve (AUC) to the MIC were 58 to 466 and 116 to 932 (microg. h/ml)/(microg/ml). The species- and strain-independent linear relationships observed between the intensity of AME (I(E)) and log AUC/MIC were not superimposed for GA and CI (r(2) = 0.99 in both cases). The predicted AUC/MIC ratio for GA that might be equivalent to a clinically relevant AUC/MIC breakpoint for CI was estimated to be 102 rather than 125 (microg. h/ml)/(microg/ml). The respective MIC breakpoints were 0.32 microg/ml (for a 400-mg dose of GA) and 0.18 microg/ml (for two 500-mg doses of CI). On the basis of the I(E)-log AUC/MIC relationships, equiefficient 24-h doses (D(24h)s) of GA and CI were calculated for hypothetical strains of S. aureus, E. coli, and K. pneumoniae for which the MICs were equal to the MICs at which 50% of isolates are inhibited. To provide an "acceptable" I(E) equal to 200 (log CFU/ml). h, i.e., the I(E) provided by AUC/MIC of 125 (microg. h/ml)/(microg/ml) for ciprofloxacin, the D(24h)s of GA for all three organisms were much lower (115, 30, and 60 mg) than the clinically proposed 400-mg dose. Although the usual dose of CI (two doses of 500 mg) would be in excess for E. coli and K. pneumoniae (D(24h) = two doses of 40 mg and two doses of 115 mg, respectively), even the highest clinical dose of CI (two doses of 750 mg) might be insufficient for S. aureus (D(24h), > two doses of 1,000 mg). The method of generalization of data obtained with specific organisms to other representatives of the same species described in the present report might be useful for prediction of the AMEs of new quinolones.

An evaluation of susceptibility testing methods for ampicillin-sulbactam using a panel of beta-lactamase-producing bacteria

Bacteria possessing TEM-1-like beta-lactamases are generally regarded as susceptible to ampicillin-sulbactam (SAM), while those harboring OXA-1 enzymes are considered resistant. The current study was undertaken to compare susceptibility testing using various combinations of ampicillin and sulbactam to improve clinical correlation. Members of the Enterobacteriaceae family harboring TEM-1, SHV-1 or OXA-1-like beta-lactamases were tested using the agar dilution method. A substantial proportion of strains harboring OXA-1-like beta-lactamases showed false susceptibility to SAM at the 1:1 ratio or fixed sulbactam concentration of 8 microg/ml. At a fixed sulbactam concentration of 4 microg/ml, the activity of ampicillin-sulbactam appeared to be reduced, with large numbers of TEM-1 producers becoming frankly resistant. Results obtained with the 2:1 ratio exhibited the closest correlation with that obtained by the currently recommended disk diffusion test. However, very major errors were still found between the disk diffusion test and agar dilution test, suggesting the necessity for consideration of a change in criteria for interpretation of disk diffusion test results. In conclusion, SAM susceptibility testing by agar dilution using other than a 2:1 ratio is not recommended and results should be interpreted with caution.

Prediction of the effects of inoculum size on the antimicrobial action of trovafloxacin and ciprofloxacin against Staphylococcus aureus and Escherichia coli in an in vitro dynamic model

The effect of inoculum size (N0) on antimicrobial action has not been extensively studied in in vitro dynamic models. To investigate this effect and its predictability, killing and regrowth kinetics of Staphylococcus aureus and Escherichia coli exposed to monoexponentially decreasing concentrations of trovafloxacin (as a single dose) and ciprofloxacin (two doses at a 12-h interval) were compared at N0 = 10(6) and 10(9) CFU/ml (S. aureus) and at N0 = 10(6), 10(7), and 10(9) CFU/ml (E. coli). A series of pharmacokinetic profiles of trovafloxacin and ciprofloxacin with respective half-lives of 9.2 and 4 h were simulated at different ratios of area under the concentration-time curve (AUC) to MIC (in [micrograms x hours/milliliter]/[micrograms/milliliter]): 58 to 466 with trovafloxacin and 116 to 932 with ciprofloxacin for S. aureus and 58 to 233 and 116 to 466 for E. coli, respectively. Although the effect of N0 was more pronounced for E. coli than for S. aureus, only a minor increase in minimum numbers of surviving bacteria and an almost negligible delay in their regrowth were associated with an increase of the N0 for both organisms. The N0-induced reductions of the intensity of the antimicrobial effect (IE, area between control growth and the killing-regrowth curves) were also relatively small. However, the N0 effect could not be eliminated either by simple shifting of the time-kill curves obtained at higher N0s by the difference between the higher and lowest N0 or by operating with IEs determined within the N0-adopted upper limits of bacterial numbers (IE's). By using multivariate correlation and regression analyses, linear relationships between IE and log AUC/MIC and log N0 related to the respective mean values [(log AUC/MIC)average and (log N0)average] were established for both trovafloxacin and ciprofloxacin against each of the strains (r2 = 0.97 to 0.99). The antimicrobial effect may be accurately predicted at a given AUC/MIC of trovafloxacin or ciprofloxacin and at a given N0 based on the relationship IE = a + b [(log AUC/MIC)/(log AUC/MIC)average] - c [(log N0)/(log N0)average]. Moreover, the relative impacts of AUC/MIC and N0 on IE may be evaluated. Since the c/b ratios for trovafloxacin and ciprofloxacin against E. coli were much lower (0.3 to 0.4) than that for ampicillin-sulbactam as examined previously (1.9), the inoculum effect with the quinolones may be much less pronounced than with the beta-lactams. The described approach to the analysis of the inoculum effect in in vitro dynamic models might be useful in studies with other antibiotic classes.

A new approach to in vitro comparisons of antibiotics in dynamic models: equivalent area under the curve/MIC breakpoints and equiefficient doses of trovafloxacin and ciprofloxacin against bacteria of similar susceptibilities

Time-kill studies, even those performed with in vitro dynamic models, often do not provide definitive comparisons of different antimicrobial agents. Also, they do not allow determinations of equiefficient doses or predictions of area under the concentration-time curve (AUC)/MIC breakpoints that might be related to antimicrobial effects (AMEs). In the present study, a wide range of single doses of trovafloxacin (TR) and twice-daily doses of ciprofloxacin (CI) were mimicked in an in vitro dynamic model. The AMEs of TR and CI against gram-negative bacteria with similar susceptibilities to both drugs were related to AUC/MICs that varied over similar eight-fold ranges [from 54 to 432 and from 59 to 473 (microg . h/ml)/(microg/ml), respectively]. The observation periods were designed to include complete bacterial regrowth, and the AME was expressed by its intensity (the area between the control growth in the absence of antibiotics and the antibiotic-induced time-kill and regrowth curves up to the point where viable counts of regrowing bacteria equal those achieved in the absence of drug [IE]). In each experiment monoexponential pharmacokinetic profiles of TR and CI were simulated with half-lives of 9.2 and 4.0 h, respectively. Linear relationships between IE and log AUC/MIC were established for TR and CI against three bacteria: Escherichia coli (MIC of TR [MICTR] = 0.25 microg/ml; MIC of CI [MICCI] = 0.12 microg/ml), Pseudomonas aeruginosa (MICTR = 0.3 microg/ml; MICCI = 0.15 microg/ml), and Klebsiella pneumoniae (MICTR = 0.25 microg/ml; MICCI = 0.12 microg/ml). The slopes and intercepts of these relationships differed for TR and CI, and the IE-log AUC/MIC plots were not superimposed, although they were similar for all bacteria with a given antibiotic. By using the relationships between IE and log AUC/MIC, TR was more efficient than CI. The predicted value of the AUC/MIC breakpoint for TR [mean for all three bacteria, 63 (microg . h/ml)/(microg/ml)] was approximately twofold lower than that for CI. Based on the IE-log AUC/MIC relationships, the respective dose (D)-response relationships were reconstructed. Like the IE-log AUC/MIC relationships, the IE-log D plots showed TR to be more efficient than CI. Single doses of TR that are as efficient as two 500-mg doses of CI (500 mg given every 12 h) were similar for the three strains (199, 226, and 203 mg). This study suggests that in vitro evaluation of the relationships between IE and AUC/MIC or D might be a reliable basis for comparing different fluoroquinolones and that the results of such comparative studies may be highly dependent on their experimental design and datum quantitation.

Activity of trovafloxacin (with or without ampicillin-sulbactam) against enterococci in an in vitro dynamic model of infection

Antibiotic-resistant enterococci are being increasingly identified as causal agents of infection. Trovafloxacin is a new fluoronaphthyridone with enhanced activity against gram-positive cocci and variable activity reported against Enterococcus spp. Twenty-one strains of vancomycin-resistant Enterococcus faecium and two strains of Enterococcus faecalis (one vancomycin resistant) were studied at an initial inoculum of 10(6) CFU/ml in time-kill assays with trovafloxacin (3 mg/liter), ampicillin-sulbactam (100/50 mg/liter), and the combination. Six strains of E. faecium (five vancomycin resistant) also were studied in an in vitro two-compartment dynamic model that mimics human pharmacokinetics with trovafloxacin simulated at 300 mg every 12 h (q12h), ampicillin-sulbactam at 2/1 g q6h, and the combination. Peripheral compartments were sampled q2h for 30 h for bacterial counts. Trovafloxacin MICs ranged from 0.5 to 32 mg/liter, and the nine strains of vancomycin-resistant E. faecium for which MICs were < or =2 mg/liter were more likely to show a reduction of 2 log units or more in viable counts in time-kill assays than were strains for which MICs were higher. Synergism with ampicillin-sulbactam was found for only one strain (trovafloxacin MIC, 16 mg/liter). Similar results were obtained in the pharmacokinetic model, with 2- to 4-log-unit reductions in viable bacteria for trovafloxacin-susceptible strains. Although no convincing evidence of synergism was found, ampicillin-sulbactam in combination minimized late bacterial regrowth of two trovafloxacin-susceptible strains. These data suggest that this high dose of trovafloxacin (with or without ampicillin-sulbactam) might be useful against strains of vancomycin-resistant E. faecium for which MICs were < or =2 mg/liter.

Parameters of bacterial killing and regrowth kinetics and antimicrobial effect examined in terms of area under the concentration-time curve relationships: action of ciprofloxacin against Escherichia coli in an in vitro dynamic model

Although many parameters have been described to quantitate the killing and regrowth of bacteria, substantial shortcomings are inherent in most of them, such as low sensitivity to pharmacokinetic determinants of the antimicrobial effect, an inability to predict a total effect, insufficient robustness, and uncertain interrelations between the parameters that prevent an ultimate determination of the effect. To examine different parameters, the kinetics of killing and regrowth of Escherichia coli (MIC, 0.013 microg/ml) were studied in vitro by simulating a series of ciprofloxacin monoexponential pharmacokinetic profiles. Initial ciprofloxacin concentrations varied from 0.02 to 19.2 microg/ml, whereas the half-life of 4 h was the same in all experiments. The following parameters were calculated and estimated: the time to reduce the initial inoculum (N0) 10-, 100-, and 1,000-fold (T90%, T99%, and T99.9%, respectively), the rate constant of bacterial elimination (k(elb)), the nadir level (Nmin) in the viable count (N)-versus-time (t) curve, the time to reach Nmin (t(min)), the numbers of bacteria that survived (Ntau) by the end of the observation period (tau), the area under the bacterial killing and regrowth curve (log N(A)-t curve) from the zero point (time zero) to tau (AUBC), the area above this curve (AAC), the area between the control growth curve (log N(C)-t curve) and the bacterial killing and regrowth curve (log N(A)-t curve) from the zero point to tau (ABBC) or to the time point when log N(A) reaches the maximal values observed in the log N(C)-t curve (I(E); intensity of the effect), and the time shift between the control growth and regrowth curves (T(E); duration of the effect). Being highly sensitive to the AUC, I(E), and T(E) showed the most regular AUC relationships: the effect expressed by I(E) or T(E) increased systematically when the AUC or initial concentration of ciprofloxacin rose. Other parameters, especially T90%, T99%, T99.9%, t(min), and log N0 - log Nmin = delta log Nmin, related to the AUC less regularly and were poorly sensitive to the AUC. T(E) proved to be the best predictor and t(min) proved to be the worst predictor of the total antimicrobial effect reflected by I(E). Distinct feedback relationships between the effect determination and the experimental design were demonstrated. It was shown that unjustified shortening of the observation period, i.e., cutting off the log N(A)-t curves, may lead to the degeneration of the AUC-response relationships, as expressed by log N0 - log Ntau = delta log Ntau, AUBC, AAC, or ABBC, to a point where it gives rise to the false idea of an AUC- or concentration-independent effect. Thus, use of I(E) and T(E) provides the most unbiased, robust, and comprehensive means of determining the antimicrobial effect.

Microbiologic and pharmacodynamic principals applied to the antimicrobial susceptibility testing of ampicillin/sulbactam: analysis of the correlations between in vitro test results and clinical response

The correlation between various ampicillin/sulbactam in vitro antimicrobial susceptibility test results and the clinical outcome of patients treated with this agent have been examined. A survey of over 29,000 clinical isolates of the family Enterobacteriaceae found that the proportion of susceptible pathogens as assessed by current susceptibility testing interpretive guidelines (NCCLS) for disk diffusion and dilution (MIC) assays was significantly less than the proportion of patients cured or clinically improved in ampicillin/sulbactam clinical trials. Also, the results of two NCCLS methods differ greatly in the perceived percentages of susceptible strains (63.9% versus 72.2%; unacceptable variation). Furthermore, the current interpretive criteria resulted in high false-susceptible (4.2%) and total (19.7%) error rates. When proposed interpretive guidelines were applied, approximately 73 to 87% of the Enterobacteriaceae strains were observed to be susceptible, the variation between methods was minimized, and the error rates were reduced. A retrospective analysis of data from clinical trials with ampicillin/sulbactam indicated that the proportion of patients who were cured or clinically improved and bacterially eradicated was not appreciably different in patients having baseline Enterobacteriaceae pathogens with MICs of 16 or 32 micrograms/ml (ampicillin MIC component) as compared to those with pathogens having MICs of < or = 8 micrograms/ml. Studies in animals, in vitro models, and pharmacokinetic considerations indicate that a change in the MIC breakpoint for ampicillin/sulbactam should be considered. The proposed interpretive guideline revisions for ampicillin/sulbactam susceptibility testing of the Enterobacteriaceae were 1) use current diagnostic reagents with criteria of < or = 16/8 micrograms/ml (> or = 14 mm) as susceptible and > or = 64/32 micrograms/ml (< or = 10 mm) as resistant; e.g., 75.9 to 76.0% spectrum and 1.3% false-susceptible error; 2) use alternative diagnostic reagents (1:1 ratio MIC; 20/20 micrograms disks) with criteria of < or = 8/8 micrograms/ml (> or = 18 mm) as susceptible and > or = 32/32 micrograms/ml (< or = 14 mm) as resistant; e.g., 73.3 to 76.9% spectrum and 1.8% false-susceptible error; or 3) use alternative diagnostic reagents with criteria of < or = 16/16 micrograms/ml (> or = 14 mm) as susceptible and > or = 64/64 micrograms/ml (< or = 10 mm) as resistant; e.g., 84.7 to 86.9% spectrum and 1.3% false-susceptible error. Data from a comprehensive in vitro survey of clinical isolates, retrospective analyses of clinical trials, and studies of animal models support the modification of contemporary interpretive guidelines for ampicillin/sulbactam antimicrobial susceptibility tests. The best short-term criteria would apply current in vitro diagnostic reagents and a modified susceptible breakpoint (< or = 16/8 micrograms/ml as susceptible; option 1 above) until new diagnostic reagents can be qualified by means of studies needed for quality assurance of standardized methods (NCCLS M23-A and FDA procedures). These changes would provide a better in vitro prediction of ampicillin/sulbactam efficacy in clinical practice.

Relationships between antimicrobial effect and area under the concentration-time curve as a basis for comparison of modes of antibiotic administration: meropenem bolus injections versus continuous infusions

In comparative studies of different modes of administration (MAs) simulated in in vitro dynamic models, only one dose of antibiotic is usually mimicked. Such an experimental design can provide a prediction of the antimicrobial effect (AME) of a given combination of drug, clinical isolate, and infection site, but may be inappropriate for accurate comparison of MAs. An alternative design providing comparison of different MAs with various antibiotic doses in a wide range and with evaluation of the respective relationships between AME and the AUC was proposed and examined. Two series of meropenem pharmacokinetic profiles, i.e., monoexponentially decreasing concentrations (bolus doses) and constant concentrations (6-h continuous infusion), were in vitro simulated. The simulated initial concentrations (Co[from 0.62 to 48 micrograms/ml]) and steady-state concentrations (Css[from 0.016 to 8 micrograms/ml]) were chosen to provide similar AUC for 0 to 6 h (AUC0-6) ranges for both MAs (from 0.070 to 50.0 micrograms.h/ml and from 0.09 to 48.0 micrograms.h/ml, respectively). The AME of meropenem on Staphylococcus aureus ATCC 25923 (MIC, 0.06 micrograms/ml) was determined at each time (t) point as a difference (E) between the logarithms of viable counts (N) in the control cultures without antibiotic (NC) and in cultures exposed to antibiotics (NA). Time courses of E observed at different Co of Css levels were compared in terms of the areas under the E-t curves (ABBCt). The finite values of the ABBCt observed by the end of the 6 -h observation period, which are equivalent to the area between bacterial count-time curves observed in the absence and presence of antibiotic (ABBC), were plotted versus the respective AUCs produced by each of the MAs. The ABBC versus AUC curves had a similar pattern: a plateau achieved at high AUCs followed by a steep rise in ABBC at relatively low AUCs was inherent in both of the MAs. The superiority of bolus dosing over the infusions could be documented only for meropenem concentrations below the MIC. At higher Co or Css (i.e., at an AUC of > or = 0.4 micrograms.h/ml), the ABBC versus AUC curves plotted for each of the MAs could practically be superimposed. On the whole, both MAs appeared to be equiefficient in terms of the ABBC. These results suggest that AUC analysis of the AME may be a useful tool for comparing different MAs. Such comparative studies should be designed in a manner that provides the use of similar AUC ranges, since the AUC may be considered as a common pharmacokinetic denominator in comparing one MA or dosing regimen to another.

Net effect of inoculum size on antimicrobial action of ampicillin-sulbactam: studies using an in vitro dynamic model

To examine the predictable effect of inoculum size on the kinetics of the antimicrobial action of ampicillin-sulbactam, five TEM-1 beta-lactamase-producing Escherichia coli strains were studied in an in vitro dynamic model at two different initial inocula (N0S). All bacteria were exposed to ampicillin-sulbactam in a simulated system reflecting the pharmacokinetic profiles in human tissue after the administration of a single intravenous dose of ampicillin (2 g) plus sulbactam (1 g). Each strain was studied at low (4.0 to 5.2 log CFU/ml) and high (5.0 to 7.1 log CFU/ml) N0S. Despite pronounced differences in susceptibilities, the patterns of the killing curves observed with a given strain at different N0S were similar. As expected, viable bacterial counts increased with inoculum size. Striking visual contrasts in the respective curves for each organism were reflected by the area under the bacterial count-time curve (AUBC) but not by the difference between the N0 and the lowest bacterial counts (Nmin) at the nadir of the killing curve: the N0-associated changes in the AUBC on average were 75%, versus 2.5% for log N0--logNmin. To examine qualitative differences in antimicrobial effects at different N0S (i.e., the net effect of the inoculum), the difference in the high and low N0S was subtracted from each point on the killing curve obtained at the higher N0 for each strain. These adjusted curves were virtually superimposable on the observed killing curves obtained at the lower N0. Moreover, by using adjusted data, the AUBC values were similar at the two inocula, although slight (average, 11%) but systematic increases in the AUBC occurred at high N0S. Thus, there was only a weak net effect of inoculum size on the antibacterial effect of ampicillin-sulbactam. Due to similar slopes of the AUBC-log N0 plots, the antibacterial action at different N0S may be easily predicted by an approximate equation; the predicted AUBCs were unbiased and well correlated with the observed AUBCs (r = 0.997). Compiled data obtained with normalized AUBCs for different strains at different N0S yielded a positive correlation (r = 0.963) between the N0-normalized AUBC and the MIC of ampicillin-sulbactam. The adjustment and normalization procedure described might be a useful tool for revealing the net effect of the inoculum and to predict the inoculum effect if there are no qualitative differences in antimicrobial action at different inocula.