Comparative bactericidal activity of representative ß-lactams against Enterobacterales, Acinetobacter baumannii and Pseudomonas aeruginosa.

BACKGROUND: There is surprisingly little comparative published data on the bactericidal action of different sub-classes of ß-lactams against aerobic Gram-negative rods, and the assumption is that all behave in the same way. OBJECTIVES: To describe a systematic investigation of a representative penicillin, cephalosporin, monobactam and carbapenem against Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii and Pseudomonas aeruginosa. METHODS: Concentration-time-kill curves (TKC) were determined for three strains each of E. coli, K. pneumoniae, A. baumannii and P. aeruginosa. All strains were susceptible to the agents used. The antibiotics were piperacillin/tazobactam, ceftazidime, aztreonam and meropenem. The initial inoculum was 106 cfu/mL and TKC were determined over 48 h. The area-under-the-bacterial-kill curve to 24 h (AUBKC 24 log cfu·h/mL) and 48 h (AUBKC 48) were used to measure antibacterial effect (ABE). Population profiles before and after antibiotic exposure were recorded. RESULTS: Against E. coli and K. pneumoniae meropenem had a maximal ABE at ≥MIC × 1 concentrations while piperacillin/tazobactam and ceftazidime had maximal effect at ≥MIC × 4 and aztreonam at ≥MIC × 8 concentrations. Ceftazidime, aztreonam and meropenem had less ABE against K. pneumoniae than E. coli. Against P. aeruginosa, meropenem was most bactericidal, with a maximum ABE at 8×/16 × MIC. Other ß-lactams had notably less ABE. In contrast, against A. baumannii, ceftazidime and meropenem had the greatest ABE, with a maximal effect at ≥MIC × 4, concentration changes in population profiles were least apparent with E. coli. CONCLUSIONS: ß-Lactam sub-classes (penicillins, cephalosporins, monobactams and carbapenems) have different antibacterial effects against E. coli, K. pneumoniae, A. baumannii and P. aeruginosa. Extrapolation of in vitro pharmacodynamic findings from one species to another or one sub-class of ß-lactam to another is not justified.

Limited phylogenetic overlap between fluoroquinolone-resistant Escherichia coli isolated on dairy farms and those causing bacteriuria in humans living in the same geographical region.

BACKGROUND: Our primary aim was to test whether cattle-associated fluoroquinolone-resistant (FQ-R) Escherichia coli found on dairy farms are closely phylogenetically related to those causing bacteriuria in humans living in the same 50 × 50 km geographical region suggestive of farm-human sharing. Another aim was to identify risk factors for the presence of FQ-R E. coli on dairy farms. METHODS: FQ-R E. coli were isolated during 2017-18 from 42 dairy farms and from community urine samples. Forty-two cattle and 489 human urinary isolates were subjected to WGS, allowing phylogenetic comparisons. Risk factors were identified using a Bayesian regularization approach. RESULTS: Of 489 FQ-R human isolates, 255 were also third-generation-cephalosporin-resistant, with strong genetic linkage between aac(6')Ib-cr and blaCTX-M-15. We identified possible farm-human sharing for pairs of ST744 and ST162 isolates, but minimal core genome SNP distances were larger between farm-human pairs of ST744 and ST162 isolates (71 and 63 SNPs, respectively) than between pairs of isolates from different farms (7 and 3 SNPs, respectively). Total farm fluoroquinolone use showed a positive association with the odds of isolating FQ-R E. coli, while total dry cow therapy use showed a negative association. CONCLUSIONS: This work suggests that FQ-R E. coli found on dairy farms have a limited impact on community bacteriuria within the local human population. Reducing fluoroquinolone use may reduce the on-farm prevalence of FQ-R E. coli and this reduction may be greater when dry cow therapy is targeted to the ecology of resistant E. coli on the farm.

Characterization of cefotaxime-resistant urinary Escherichia coli from primary care in South-West England 2017-18.

OBJECTIVES: Third-generation cephalosporin-resistant Escherichia coli from community-acquired urinary tract infections are increasingly reported worldwide. We sought to determine and characterize the mechanisms of cefotaxime resistance employed by urinary E. coli obtained from primary care, over 12 months, in Bristol and surrounding counties in South-West England. METHODS: Cefalexin-resistant E. coli isolates were identified from GP-referred urine samples using disc susceptibility testing. Cefotaxime resistance was determined by subsequent plating onto MIC breakpoint plates. ß-Lactamase genes were detected by PCR. WGS was performed on 225 isolates and analyses were performed using the Center for Genomic Epidemiology platform. Patient information provided by the referring general practices was reviewed. RESULTS: Cefalexin-resistant E. coli (n=900) isolates were obtained from urines from 146 general practices. Following deduplication by patient approximately 69% (576/836) of isolates were cefotaxime resistant. WGS of 225 isolates identified that the most common cefotaxime-resistance mechanism was blaCTX-M carriage (185/225), followed by plasmid-mediated AmpCs (pAmpCs) (17/225), AmpC hyperproduction (13/225), ESBL blaSHV variants (6/225) or a combination of both blaCTX-M and pAmpC (4/225). Forty-four STs were identified, with ST131 representing 101/225 isolates, within which clade C2 was dominant (54/101). Ciprofloxacin resistance was observed in 128/225 (56.9%) of sequenced isolates, predominantly associated with fluoroquinolone-resistant clones ST131 and ST1193. CONCLUSIONS: Most cefalexin-resistant E. coli isolates were cefotaxime resistant, predominantly caused by blaCTX-M carriage. The correlation between cefotaxime resistance and ciprofloxacin resistance was largely attributable to the high-risk pandemic clones ST131 and ST1193. Localized epidemiological data provide greater resolution than regional data and can be valuable for informing treatment choices in the primary care setting.

Pharmacodynamics of plazomicin and a comparator aminoglycoside, amikacin, studied in an in vitro pharmacokinetic model of infection.

The new aminoglycoside plazomicin shows in vitro potency against multidrug-resistant Enterobacteriales. The exposure-response relationship of plazomicin and the comparator aminoglycoside amikacin was determined for Escherichia coli, while for Klebsiella pneumoniae only plazomicin was tested. An in vitro pharmacokinetic model was used. Five E. coli strains (two meropenem-resistant) and five K. pneumoniae strains (two meropenem-resistant) with plazomicin MICs of 0.5-4 mg/L were used. Antibacterial effect was assessed by changes in bacterial load and bacterial population profile. The correlation between change in initial inoculum after 24 h of drug exposure and the AUC/MIC ratio was good (plazomicin R2 ≥ 0.8302; amikacin R2 ≥ 0.9520). Escherichia coli plazomicin AUC/MIC ratios for 24-h static, -1, -2 and -3 log drop were 36.1 ± 18.4, 39.3 ± 20.9, 41.2 ± 21.9 and 44.8 ± 24.3, respectively, and for amikacin were 49.5 ± 12.7, 55.7 ± 14.8, 64.1 ± 19.2 and 73.3 ± 25.3. Klebsiella pneumoniae plazomicin AUC/MIC ratios for 24-h static, -1, -2 and -3 log drop were 34.0 ± 15.2, 46.8 ± 27.8, 67.4 ± 46.5 and 144.3 ±129.8. Plazomicin AUC/MIC ratios >66 and amikacin AUC/MIC ratios >57.7 were associated with suppression of E. coli growth on 4 × or 8 × MIC recovery plates. The equivalent plazomicin AUC/MIC to suppress resistance emergence with K. pneumoniae was >132. The plazomicin AUC/MIC for 24-h static effect and -1 log reduction in E. coli and K. pneumoniae bacterial load was in the range 30-60. Plazomicin AUC/MIC targets aligned with those of amikacin for E. coli.

Antibacterial effect of imipenem/relebactam on aerobic Gram-negative bacilli: in vitro simulations of 7 or 14 day human exposures.

OBJECTIVES: We assessed the antibacterial effect of human simulations of dosing with imipenem/relebactam (with or without amikacin) on Enterobacteriaceae or Pseudomonas aeruginosa over 7 or 14 day antibiotic exposures. METHODS: An in vitro pharmacokinetic model was used to assess changes in bacterial load and population profiles. RESULTS: Imipenem/relebactam produced an initial >4 log drop in viable counts followed by suppression for 7 days for Enterobacteriaceae whether the strain was WT, produced KPC enzymes or produced an AmpC enzyme with porin loss. Similarly, with the P. aeruginosa strains, there was an initial >4 log clearance over the first 24 h irrespective of whether the strain was WT, hyperexpressed AmpC or had OprD mutation with porin loss. However, with three of four strains there was modest regrowth over the 7 days. There were no changes in imipenem/relebactam MICs over the 7 days. Addition of amikacin in 7 day simulations resulted in more suppression of pseudomonal growth. In 14 day simulations with P. aeruginosa there was regrowth to 8 log10 by 14 days with imipenem/relebactam alone and associated increases in MICs. Addition of amikacin resulted in clearance from the model and prevented changes in population profiles. CONCLUSIONS: Imipenem/relebactam was highly effective at reducing the bacterial load of Enterobacteriaceae and there was no emergence of resistance. Against P. aeruginosa, the initial bacterial burden was also rapidly reduced, but there was subsequent regrowth, especially after 7 days of exposure. Addition of amikacin increased the clearance of P. aeruginosa and prevented emergence of resistance.

Antibacterial effect of ceftolozane/tazobactam in combination with amikacin against aerobic Gram-negative bacilli studied in an in vitro pharmacokinetic model of infection.

Objectives: To use a pre-clinical infection model to assess the antibacterial effect of human simulations of dosing with ceftolozane/tazobactam (with or without amikacin) or meropenem against Enterobacteriaceae and Pseudomonas aeruginosa. Methods: An in vitro pharmacokinetic model was used to assess changes in bacterial load and profiles after exposure to mean human serum concentrations over 168 h. Changes in area under the bacterial kill curve (AUBKC; log cfu/mL·h) and growth on 4 × MIC recovery plates were the co-primary outcome measures. Results: Simulations of ceftolozane/tazobactam at 1 g/0.5 g or 2 g/1 g q8h or meropenem 2 g q8h all produced a >4 log reduction in bacterial load of Escherichia coli. Meropenem had smaller AUBKC values, indicating greater reduction in bacterial load than ceftolozane/tazobactam. Meropenem was also more effective than ceftolozane/tazobactam against Klebsiella pneumoniae strains. All regimens were equally effective in reducing P. aeruginosa bacterial load measured by AUBKC but growth on 4 × MIC recovery plates and changes in population profiles were only seen with meropenem. Addition of amikacin at 15 mg/kg q24h or 7.5 mg/kg q12h to 2 g/1 g of ceftolozane/tazobactam produced greater reductions in bacterial load but generated changes in amikacin population profiles with the 7.5 mg/kg q12h amikacin simulation. Conclusions: The doses of ceftolozane/tazobactam simulated were highly effective in reducing the bacterial load of E. coli (MIC ≤0.25 mg/L), but less so for K. pneumoniae (MIC 4 mg/L). For both species, meropenem produced an overall greater reduction in pathogen load. Ceftolozane/tazobactam and meropenem were equally effective as monotherapy against P. aeruginosa but emergence of resistance occurred with meropenem. Addition of amikacin to ceftolozane/tazobactam reduced the bacterial load of P. aeruginosa at the expense of emergence of resistance to amikacin.

Pharmacodynamics of inhaled amikacin (BAY 41-6551) studied in an in vitro pharmacokinetic model of infection.

Background: The pharmacodynamics of inhaled antimicrobials are poorly studied. Amikacin is being developed for inhalational therapy as BAY 41-6551. Objectives: We employed an in vitro pharmacokinetic model to study the pharmacokinetics/pharmacodynamics of amikacin. Methods: A dose-ranging design was used to establish fAUC/MIC and fCmax/MIC targets for static, -1 log drop and -2 log drop effects for strains of Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa. We then modelled epithelial lining fluid (ELF) concentration associated with inhaled amikacin (400 mg every 12 h), over 5 days using mean human concentrations. Results: The 24 h static effect fAUC/MIC targets and -1 log drop targets were 51.0 ±âŸ26.7 and 71.6 ±âŸ27.6 for all species of aerobic Gram-negative bacilli. fAUC/MIC targets for static effect, -1 log drop or -2 log drop were smaller than the 24 h values at 12 h and larger at 48 h. Emergence of resistance occurred maximally with E. coli in the fAUC/MIC range 12-60; K. pneumoniae 0-60 (48 h) and P. aeruginosa 12-80. When human ELF concentrations were modelled for strains with MIC ≤8 mg/L, there was rapid clearance and no regrowth. For strains with MIC ≥32 mg/L, there was initial clearance followed by regrowth. If MIC values were related to bacterial clearance then at least a static effect or -1 log drop in count would be expected for bacterial strains with MICs of ≤180 mg/L (static effect) or ≤148 mg/L (-1 log drop effect). Conclusions: An fAUC/MIC amikacin target of 50-80 is appropriate for aerobic Gram-negative bacilli and mean ELF concentrations of BAY 41-6551 would produce a static to -1 log clearance with strains up to 128 mg/L.

Pharmacodynamics of minocycline against Acinetobacter baumannii studied in a pharmacokinetic model of infection.

Minocycline (MNO) is an old antibiotic that may have an important role in the treatment of multidrug-resistant Gram-negative bacterial infections as the burden of such infections increases. In this study, a single-compartment dilutional pharmacokinetic model was used to determine the relationship between MNO exposure and antibacterial effect, including the risk of resistance emergence, against strains of Acinetobacter baumannii. The mean ± standard deviation area under the unbound drug concentration-time curve to minimum inhibitory concentration ratio (fAUC/MIC) associated with a 24-h bacteriostatic effect was 16.4 ± 2.6 and with a -1 log reduction in bacterial load at 24 h was 23.3 ± 3.7. None of the strains reached a -2 log reduction over 48 h. Changes in population profiles were noted for two of the three strains studied, especially at fAUC/MIC ratios of >5-15. A reasonable translational pharmacodynamic target for MNO against A. baumannii could be an fAUC/MIC of 20-25. However, if maximum standard 24-h doses of intravenous MNO are used (400 mg/day), many strains would be exposed to MNO concentrations likely to change population profiles and associated with the emergence of resistance. Either MNO combination therapy or an increased MNO dose (>400 mg/day) should be considered when treating A. baumannii infections.

Differences in the pharmacodynamics of ceftaroline against different species of Enterobacteriaceae studied in an in vitro pharmacokinetic model of infection.

OBJECTIVES: Dose-ranging experiments were performed to study the pharmacodynamics of ceftaroline against Enterobacteriaceae. METHODS: A range of fT>MIC values (0%-100%) were simulated over 96 h using a single-compartment dilutional in vitro pharmacokinetic model using Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, Citrobacter koseri and Serratia marcescens (n = 16). Antibacterial effect was assessed by change in viable count and population profiles by growth on ceftaroline MIC ×2, ×4 and ×8 agar plates. The fT>MIC (%) was related to antibacterial effect using a sigmoid Emax model. RESULTS: The 24 h bacteriostatic effect fT>MIC was 39.7% ±â€Š15.7% and 43.2% ±â€Š15.6% for a -1 log drop for all strains. E. coli required lower exposures than K. pneumoniae, i.e. 24 h fT>MIC for a -3 log drop in viable count was 40.0% ±â€Š9.6% and 84.8% ±â€Š15.2% for K. pneumoniae. Similarly at 96 h, fT>MIC was >100% for K. pneumoniae (for four of five strains), 27.2%-66.2% for E. coli and 16.2%-86.6% for P. mirabilis. Strain-to-strain variation within species in the fT>MIC for static and cidal effect was marked; the 24 h bacteriostatic range was 14.1%-73.4% for P. mirabilis, 34.2%-44.6% for E. coli and 42.2%-62.5% for K. pneumoniae. Changes in ceftaroline population analysis profiles were observed with E. coli, K. pneumoniae and C. koseri, especially at fT>MIC values just below the bacteriostatic effect exposures. CONCLUSIONS: The pharmacodynamics of ceftaroline against the species within the Enterobacteriaceae group are different. K. pneumoniae requires higher drug exposures than E. coli, and P. mirabilis strains are highly variable, which may have important clinical correlates. Translational extrapolations from preclinical observations using E. coli to other Enterobacteriaceae species may not be optimal.

Pharmacodynamics of Ceftolozane plus Tazobactam Studied in an In Vitro Pharmacokinetic Model of Infection.

Ceftolozane plus tazobactam is an antipseudomonal cephalosporin combined with tazobactam, an established beta-lactamase inhibitor, and has in vitro potency against a range of clinically important ß-lactamase-producing bacteria, including most extended-spectrum-ß-lactamase (ESBL)-positive Enterobacteriaceae. The pharmacodynamics of ß-lactam-ß-lactamase inhibitor combinations presents a number of theoretical and practical challenges, including modeling different half-lives of the compounds. In this study, we studied the pharmacodynamics of ceftolozane plus tazobactam against Escherichia coli and Pseudomonas aeruginosa using an in vitro pharmacokinetic model of infection. Five strains of E. coli, including three clinical strains plus two CTX-M-15 (one high and one moderate) producers, and five strains of P. aeruginosa, including two with OprD overexpression and AmpC ß-lactamases, were employed. Ceftolozane MICs (E. coli, 0.12 to 0.25 mg/liter, and P. aeruginosa, 0.38 to 8 mg/liter) were determined in the presence of 4 mg/liter tazobactam. Dose ranging of ceftolozane (percentage of time in which the free-drug concentration exceeds the MIC [fT>MIC], 0 to 100%) plus tazobactam (human pharmacokinetics) was simulated every 8 hours, with half-lives (t1/2) of 2.5 and 1 h, respectively. Ceftolozane and tazobactam concentrations were confirmed by high-performance liquid chromatography (HPLC). The ceftolozane-plus-tazobactam fT>MIC values at 24 h for a static effect and a 1-log and 2-log drop in initial inoculum for E. coli were 27.8% ± 5.6%, 33.0% ± 5.6%, and 39.6% ± 8.5%, respectively. CTX-M-15 production did not affect the 24-h fT>MIC for E. coli strains. The ceftolozane-plus-tazobactam fT>MIC values for a 24-h static effect and a 1-log and 2-log drop for P. aeruginosa were 24.9% ± 3.0%, 26.6% ± 3.9%, and 31.2% ± 3.6%. Despite a wide range of absolute MICs, the killing remained predictable as long as the MICs were normalized to the corresponding fT>MIC. Emergence of resistance on 4× MIC plates and 8× MIC plates occurred maximally at an fT>MIC of 10 to 30% and increased as time of exposure increased. The fT>MIC for a static effect for ceftolozane plus tazobactam is less than that observed with other cephalosporins against E. coli and P. aeruginosa and is more similar to the fT>MIC reported for carbapenems.

Pharmacodynamics of ceftaroline against Staphylococcus aureus studied in an in vitro pharmacokinetic model of infection.

An in vitro single-compartment dilutional pharmacokinetic model was used to study the pharmacodynamics of ceftaroline against Staphylococcus aureus (both methicillin-susceptible S. aureus [MSSA] and methicillin-resistant S. aureus [MRSA]). Mean serum free concentrations of ceftaroline (the active metabolite of the prodrug ceftaroline fosamil) dosed in humans at 600 mg every 12 h (q12h) were simulated, and activities against 12 S. aureus strains (3 MSSA strains and 9 MRSA strains, 3 of which had a vancomycin-intermediate phenotype) were determined. Ceftaroline produced 2.5- to 4.0-log10-unit reductions in viable counts by 24 h with all strains and a 0.5- to 4.0-log-unit drop in counts at 96 h. The antibacterial effect could not be related to the strain MIC across the ceftaroline MIC range from 0.12 to 2.0 µg/ml. In dose-ranging studies, the cumulative percentage of a 24-h period that the free drug concentration exceeded the MIC under steady-state pharmacokinetic conditions (fT(MIC)) of 24.5% ± 8.9% was associated with a 24-h bacteriostatic effect, one of 27.8% ± 9.5% was associated with a -1-log-unit drop, and one of 32.1% ± 8.1% was associated with a -2-log-unit drop. The MSSA and MRSA strains had similar fT(MIC) values. fT(MIC) values increased with increasing duration of exposure up to 96 h. Changes in ceftaroline population analysis profiles were related to fT(MIC). fT(MIC)s of <50% were associated with growth on 4× MIC recovery plates at 96 h of drug exposure. These data support the use of ceftaroline fosamil at doses of 600 mg q12h to treat S. aureus strains with MICs of ≤ 2 µg/ml. An fT(MIC) of 25 to 30% would make a suitable pharmacodynamic index target, but fTMIC values of ≥ 50% are needed to suppress the emergence of resistance and require clinical evaluation.

Pharmacodynamics of the antibacterial effect of and emergence of resistance to doripenem in Pseudomonas aeruginosa and Acinetobacter baumannii in an in vitro pharmacokinetic model.

An in vitro dilutional pharmacokinetic model of infection was used to study the pharmacodynamics of doripenem in terms of the ability to kill Pseudomonas aeruginosa or Acinetobacter baumannii and also changes in their population profiles. In dose-ranging studies, the cumulative percentages of a 24-h period that the drug concentration exceeds the MIC under steady-state pharmacokinetic conditions (T(MIC)s) required for doripenem to produce a 24-h bacteriostatic effect and a -2-log-unit reduction in viable count were 25% ± 11% and 35% ± 13%, respectively, for P. aeruginosa (MIC range, 0.24 to 3 mg/liter) and 20% ± 11% and 33% ± 12%, respectively, for Acinetobacter spp. (MIC range, 0.45 to 3.0 mg/liter). A T(MIC) of >40 to 50% produced a maximum response with both species at 24 h or 48 h of exposure. After 24 h of exposure to doripenem at a T(MIC) in the range of 12.5 to 37.5%, P. aeruginosa and A. baumannii population profiles revealed mutants able to grow on 4× MIC-containing medium; such changes were further amplified by 48 h of exposure. Dose-fractionation experiments targeting T(MIC)s of 12.5%, 25%, or 37.5% as six exposures, two exposures, or a single exposure over 48 h with a single strain of P. aeruginosa indicated that changes in population profiles were greatest with multiple exposures at T(MIC) targets of 12.5 or 25%. In contrast, multiple exposures at 37.5% T(MIC) most effectively suppressed total bacterial counts and changes in population profiles. Simulations of human doses of doripenem of 500 mg, 1,000 mg, 2,000 mg, and 3,000 mg every 8 h over 96 h showed marked initial killing up to 6 h but growback thereafter. Changes in population profiles occurred only in the regimen of 500 mg every 8 h against P. aeruginosa but occurred with all dose regimens for A. baumannii strains. A doripenem T(MIC) of ≥40 to 50% is maximally effective in killing P. aeruginosa or A. baumannii and suppressing changes in population profiles in short-term experiments for up to 48 h; however, a T(MIC) of 12.5 to 25% amplifies population changes, especially with exposures every 8 h. In longer-term experiments, up to 96 h, even doripenem doses of 4 to 6 times those used in human studies proved incapable of pathogen eradication and prevention of changes in population profiles. The association of a T(MIC) of 25 to 37.5% with changes in population profiles has implications in terms of future clinical breakpoint setting.

Reducing the risk of surgical site infection: a case controlled study of contamination of theatre clothing.

Surgical site infections are one of the most important causes of healthcare associated infections (HCAI), accounting for 20% of all HCAIs. Surgical site infections affect 1% of joint replacement operations. This study was designed to assess whether theatre clothing is contaminated more inside or outside the theatre suite. Petri dishes filled with horse blood agar were pressed on theatre clothes at 0, 2, 4, 6 and 8 hours to sample bacterial contamination in 20 doctors whilst working in and outside the theatre suite. The results showed that there was greater bacterial contamination when outside the theatre suite at 2 hours. There were no differences in the amount of contamination at 4, 6 and 8 hours. This study suggests that the level of contamination of theatre clothes is similar both inside and outside the theatre setting.

Pharmacodynamics of telavancin studied in an in vitro pharmacokinetic model of infection.

The antibacterial effects of telavancin, vancomycin, and teicoplanin against six Staphylococcus aureus strains (1 methicillin-susceptible S. aureus [MSSA] strain, 4 methicillin-resistant S. aureus [MRSA] strains, and 1 vancomycin-intermediate S. aureus [VISA] strain) and three Enterococcus sp. strains (1 Enterococcus faecalis strain, 1 Enterococcus faecium strain, and 1 vancomycin-resistant E. faecium [VREF] strain) were compared using an in vitro pharmacokinetic model of infection. Analyzing the data from all five vancomycin-susceptible S. aureus (VSSA) strains or all 4 MRSA strains showed that telavancin was superior in its antibacterial effect as measured by the area under the bacterial kill curve at 24 h (AUBKC(24)) and 48 h (AUBKC(48)) in comparison to vancomycin or teicoplanin (P < 0.05). Telavancin was also superior to vancomycin and teicoplanin in terms of its greater early killing effect (P < 0.05). Against the three Enterococcus spp. tested, telavancin was superior to vancomycin in terms of its AUBKC(24), AUBKC(48), and greater early bactericidal effect (P < 0.05). Dose-ranging studies were performed to provide free-drug area under the concentration-time curve over 24 h in the steady state divided by the MIC (fAUC/MIC) exposures from 0 to 1,617 (7 to 14 exposures per strain) for 5 VSSA, 4 VISA, and the 3 Enterococcus strains. The fAUC/MIC values for a 24-h bacteriostatic effect and a 1-log-unit drop in the viable count were 43.1 ± 38.4 and 50.0 ± 39.0 for VSSA, 3.2 ± 1.3 and 4.3 ± 1.3 for VISA, and 15.1 ± 8.8 and 40.1 ± 29.4 for the Enterococcus spp., respectively. The reason for the paradoxically low fAUC/MIC values for VISA strains is unknown. There was emergence of resistance to telavancin in the dose-ranging studies, as indicated by subpopulations able to grow on plates containing 2× MIC telavancin concentrations compared to the preexposure population analysis profiles. Changes in population analysis profiles were less likely with enterococci than with S. aureus, and the greatest risk of changed profiles occurred for both species at fAUC/MIC ratios of 1 to 10. Maintaining a fAUC/MIC ratio of >50 reduced the risk of subpopulations able to grow on antibiotic-containing media emerging. These data help explain the clinical effectiveness of telavancin against MRSA and indicate that telavancin may have clinically useful activity against Enterococcus spp., and perhaps also VISA, at human doses of 10 mg/kg of body weight/day. In addition, they support a clinical breakpoint of sensitive at ≤1 mg/liter for both S. aureus and Enterococcus spp.

Bacterial strain-to-strain variation in pharmacodynamic index magnitude, a hitherto unconsidered factor in establishing antibiotic clinical breakpoints.

Antibiotic pharmacodynamic modeling allows variations in pathogen susceptibility and human pharmacokinetics to be accounted for when considering antibiotic doses, potential bacterial pathogen targets for therapy, and clinical susceptibility breakpoints. Variation in the pharmacodynamic index (area-under-the-concentration curve to 24 h [AUC(24)]/MIC; maximum serum concentration of drug in the serum/MIC; time the serum concentration remains higher than the MIC [T > MIC]) is not usually considered. In an in vitro pharmacokinetic model of infection using a dose-ranging design, we established the relationship between AUC(24)/MIC and the antibacterial effect for moxifloxacin against 10 strains of Staphylococcus aureus. The distributions of AUC(24)/MIC targets for 24-h bacteriostatic effect and 1-log, 2-log, and 3-log drops in bacterial counts were used to calculate potential clinical breakpoint values, and these were compared with those obtained by the more conventional approach of taking a single AUC(24)/MIC target. Consideration of the AUC(24)/MIC as a distribution rather than a single value resulted in a lower clinical breakpoint.

In vitro activities of three new dihydrofolate reductase inhibitors against clinical isolates of gram-positive bacteria.

BAL0030543, BAL0030544, and BAL0030545 are dihydrophthalazine inhibitors with in vitro potency against gram-positive pathogens. The MIC(50)s for methicillin (meticillin)-sensitive Staphylococcus aureus, methicillin-resistant Staphylococcus aureus, hetero-vancomycin-resistant Staphylococcus aureus, and vancomycin-resistant Staphylococcus aureus (VISA) range from 0.015 to 0.25 microg/ml (MIC(90)s < or = 0.5 microg/ml). MIC(50)s for beta-hemolytic streptococci range from 0.03 to 0.06 microg/ml, MIC(50)s for Streptococcus pneumoniae range from 0.06 to 0.12 microg/ml, MIC(50)s for Listeria monocytogenes range from 0.015 to 0.06 microg/ml, and MIC(50)s for Streptococcus mitis are < or = 0.015 microg/ml. These three dihydrophthalazine antifolates have improved potency compared to that of trimethoprim and activity against gram-positive pathogens resistant to other drug classes. (This work was presented in part at the 48th Interscience Conference on Antimicrobial Agents and Chemotherapy, Washington, DC, 2008.).

Comparative antibacterial effects of daptomycin, vancomycin and teicoplanin studied in an in vitro pharmacokinetic model of infection.

OBJECTIVES: To compare the antibacterial effects (ABEs) of the free (f) drugs daptomycin, vancomycin and teicoplanin against methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant S. aureus (VRSA), using high and low inocula in a pharmacokinetic in vitro model. To determine the daptomycin fAUC/MIC ratio for a static effect and 3 log reduction in viable count and relate this target to the clinical breakpoint. METHODS: Five clinical MRSA isolates held at Southmead Hospital were used (SMH 15841, SMH 40289, SMH 40275, SMH 33922 and SMH 33024) together with a VRSA isolate (SMH 19898); inocula of 10(6) and 10(8) cfu/mL were used. Daptomycin (6 mg/kg once daily), vancomycin (1 g twice daily) and teicoplanin (400 mg once daily) regimens were simulated. ABEs were measured using the 24 h area-under-the-bacterial kill curve (AUBKC) and log change in viable count at 24 h (Delta24). For daptomycin, dose escalation was used to determine the relationship between ABE and AUC/MIC. RESULTS: Daptomycin was bactericidal against the MRSA strains. Daptomycin and vancomycin were active against the VRSA strain; teicoplanin had a static effect. The higher inoculum reduced the ABEs. Analysis of variance (ANOVA) indicated that daptomycin had a superior ABE to teicoplanin and vancomycin. Daptomycin fAUC/MIC was related to AUBKC and Delta24; the fAUC/MIC ratios for a static effect and 1 log and 3 log drop were 37.2 +/- 16.5, 40.6 +/- 17.8 and 49.8 +/- 19.2, respectively. CONCLUSIONS: These data define the fAUC/MIC sizes for daptomycin for bacteriostatic and bactericidal ABEs and indicate that a 6 mg/kg dose of daptomycin is superior to vancomycin and teicoplanin against MRSA and VRSA strains.

Pharmacodynamics of minocycline against Staphylococcus aureus in an in vitro pharmacokinetic model.

Free drug serum concentrations of minocycline associated with the doses given to humans (100 mg every 12 hours for 24 hours) were simulated in an in vitro hollow-fiber pharmacokinetic model. Four strains of methicillin (meticillin)-resistant Staphylococcus aureus (MRSA), United Kingdom EMRSA 15 and 16 plus a pair of blood culture isolates before and after long-term minocycline treatment, were employed. The minocycline MICs for these four strains were 0.04 mg/liter, 0.19 mg/liter, 0.06 mg/liter, and 0.75 mg/liter. The antibacterial effect (ABE) of minocycline was measured using the area under the bacterial kill curve to 24 h (AUBKC) and the log change in viable count at 24 h (d24). The ABEs of minocycline with and without the addition of rifampin (rifampicin) were compared to those of vancomycin, and dose escalation and fractionation were used to determine the dominant pharmacodynamic index and its size. Minocycline alone produced a 1.5- to 2.0-log(10)-unit reduction in viable count for the strains with MICs of <0.2 mg/liter, while the addition of rifampin increased the ABE for these strains (P < 0.05). Vancomycin simulations produced a reduction in viable counts of 2.8 to 4.5 log units at 24 h, which was equivalent to the minocycline-plus-rifampin combination. Free area under the concentration-time curve (AUC)/MIC was best related to AUBKC or d24 using a sigmoid maximal effect (Emax) model with r(2) of 0.92 and 0.87, respectively, and the AUC/MIC ratios for no change and -1-log-unit, -2-log-unit, and -3-log-unit drop at 24 h were 33.9, 75.9, 1,350, and >2,000, respectively. Fractionation of the dose at free AUC/MICs associated with human doses showed no difference between once, twice, or three times a day dosing. In contrast, fractionation of the dose at a free AUC associated with a static effect indicated that once daily dosing was superior. These data show that minocycline is an AUC/MIC-driven agent at human exposures and that the addition of rifampin may offer benefit in terms of MRSA killing.

Pharmacodynamics of the antibacterial effect and emergence of resistance to tomopenem, formerly RO4908463/CS-023, in an in vitro pharmacokinetic model of Staphylococcus aureus infection.

The antibacterial effects (ABE) of tomopenem (formerly RO4908463/CS-023) against seven Staphylococcus aureus strains (methicillin-resistant S. aureus [MRSA] strain tomopenem MICs, 0.5 to 16 mg/liter; methicillin-sensitive S. aureus [MSSA] strain tomopenem MIC, 0.06 mg/liter) were studied in an in vitro pharmacokinetic model. Initially, two human doses were simulated, 750 mg every 8 hours (8hly) and 1,500 mg 8hly intravenously, using S. aureus at a standard inoculum of 10(6) CFU/ml. There was a rapid clearance of bacteria from the model by 12 h after drug exposure with most strains. Clearance was not related to the tomopenem MIC. The ABE of these two tomopenem dose regimens were also tested at a high inoculum, 10(8) CFU/ml; in all simulations, there was a >4-log drop in viable count at 24 h. Strains were not cleared from the model at 10(8) CFU/ml, in contrast to what was seen for the standard inoculum. When the ABE of tomopenem at 750 mg 8hly was compared to those of vancomycin, tomopenem was seen to have a superior effect, as measured by the area under the bacterial kill curve at 24 h (AUBKC24) and 48 h (P < 0.05). Dose ranging studies were performed to provide time-above-MIC (T>MIC) drug exposures of 0 to 100% (8 to 10 doses per strain) with five MRSA/MSSA strains. The T>MIC for a 24-h bacteriostatic effect was 8% +/- 5% (range, 1.3% to 15.4%); the T>MIC for a 4-log drop in viable count was 32% +/- 18% (range, 12.8% to 36.2%). The T>MIC for a 90% maximum response using AUBKC24 as ABE was 24.9% +/- 15.7%. Inoculum had little impact on T>MIC exposures for ABE. There was emergence of resistance to tomopenem in the dose ranging studies, with increased growth of subpopulations on plates containing tomopenem at 2x and 4x the MIC compared to what was seen for preexposure population analysis at T>MICs of <20%. The pharmacodynamics of tomopenem against S. aureus is similar to those of other members of the carbapenem class, with the exception that MRSA is included. These data indicate that tomopenem will have clinically useful activity against MRSA at T>MICs achievable in humans.