Pharmacodynamics of a new cephalosporin, PPI-0903 (TAK-599), active against methicillin-resistant Staphylococcus aureus in murine thigh and lung infection models: identification of an in vivo pharmacokinetic-pharmacodynamic target

PPI-0903 is a new cephalosporin with broad-spectrum activity, including beta-lactam-resistant Streptococcus pneumoniae and Staphylococcus aureus. We used the neutropenic murine thigh and lung infection models to examine the pharmacodynamic characteristics of PPI-0903. Serum drug levels following four fourfold-escalating single doses of PPI-0903 were measured by microbiologic assay. In vivo postantibiotic effects (PAEs) were determined after doses of 1.56, 6.25, 25, and 100 mg/kg of body weight in mice infected with S. pneumoniae ATCC 10813, S. aureus ATCC 29213, or Escherichia coli ATCC 25922. Dose fractionation studies over a 24-h dose range of 0.39 to 1,600 mg/kg were administered every 3, 6, 12, or 24 hours. Nonlinear regression analysis was used to determine which pharmacokinetic-pharmacodynamic (PK-PD) index (total and free 65% drug) best correlated with CFU/thigh at 24 h. Similar to other beta-lactam antibiotics, PPI-0903 produced short to modest in vivo PAEs with either S. pneumoniae or E. coli. The percent time that serum concentrations were above the MIC (%T>MIC) was the PK-PD index that best correlated with efficacy (R2=84 to 88% for the three organisms, compared with 9 to 41% for peak/MIC and 30 to 82% for the area under the concentration-time curve/MIC). In subsequent studies we used the neutropenic murine thigh infection model to determine if the magnitude of the free-drug %T>MIC needed for efficacy of PPI-0903 varied among pathogens (including resistant strains). Mice infected with one of five isolates of S. pneumoniae, four isolates of S. aureus, or four gram-negative bacilli were treated for 24 h with 0.10 to 400 mg/kg of PPI-0903 every 6 h. A sigmoid dose-response model was used to estimate the doses (mg/kg/24 h) required to achieve a net bacteriostatic affect over 24 h and to produce a reduction in the burden of organisms from the start of therapy by 1 and 2 log10 CFU/thigh. MICs ranged from 0.008 to 1 microg/ml. Mean free-drug %T>MICs+/-the standard deviation associated with the static effect endpoint for S. pneumoniae, S. aureus, and gram-negative isolates were 39+/-9, 26+/-8, and 32+/-6, respectively. Methicillin and penicillin resistance did not alter the magnitude of free-drug %T>MIC required for efficacy. The free-drug %T>MIC necessary for efficacy was slightly reduced in animals with normal neutrophil counts. Treatment effect was similar in both the thigh and lung infection models. The pharmacodynamic characteristics of PPI-0903 are similar to those of other compounds within the cephalosporin class.     

Activity and diffusion of tigecycline (GAR-936) in experimental enterococcal endocarditis

The activity of tigecycline (GAR-936), a novel glycylcycline, was investigated in vitro and in experimental endocarditis due to the susceptible Enterococcus faecalis JH2-2 strain, its VanA type transconjugant BM4316, and a clinical VanA type strain, E. faecium HB217 resistant to tetracycline. MICs of GAR-936 were 0.06 micro g/ml for the three strains. In vitro pharmacodynamic studies demonstrated a bacteriostatic effect of GAR-936 that was not enhanced by increasing concentrations to more than 1 micro g/ml and a postantibiotic effect ranging from 1 to 4.5 h for concentrations of 1- to 20-fold the MIC. Intravenous injection of [(14)C]GAR-936 to five rabbits with enterococcal endocarditis sacrificed 30 min, 4 h, or 12 h after the end of the infusion evidenced a lower clearance of GAR-936 from aortic vegetations than from serum and a homogeneous diffusion of GAR-936 into the vegetations. In rabbits with endocarditis, GAR-936 (14 mg/kg of body weight twice a day [b.i.d.]) given intravenously for 5 days was bacteriostatic against both strains of E. faecalis. Against E. faecium HB217, bacterial counts in vegetations significantly decreased during therapy (P < 0.01), and the effect was similar with GAR-936 at 14 mg/kg b.i.d., 14 mg/kg once a day (o.d.), and 7 mg/kg o.d., which provided concentrations in serum constantly above the MIC. Mean serum elimination half-life ranged from 3.3 to 3.6 h. No GAR-936-resistant mutants were selected in vivo with any regimen. We concluded that the combination of prolonged half-life, significant postantibiotic effect, and good and homogeneous diffusion into the vegetations may account for the in vivo activity of GAR-936 against enterococci susceptible or resistant to glycopeptides and tetracyclines, even when using a o.d. regimen in rabbits.     

Pharmacodynamics of the new des-f(6)-quinolone garenoxacin in a murine thigh infection model

Garenoxacin is a new des-F(6)-quinolone with broad-spectrum activity against both gram-positive cocci and gram-negative bacilli. We used the neutropenic murine thigh infection model to characterize the time course of antimicrobial activity of garenoxacin and determine which pharmacokinetic-pharmacodynamic (PK-PD) parameter best correlated with efficacy. Serum drug levels following three fourfold-escalating single-dose levels of garenoxacin were measured by microbiologic assay. In vivo postantibiotic effects (PAEs) were determined after doses of 16 and 64 mg/kg of body weight. Mice had 10(6.5) to 10(6.7) CFU of Streptococcus pneumoniae strain ATCC 10813 or Staphylococcus aureus strain ATCC 33591 per thigh when they were treated for 24 h with garenoxacin at a dose of 4 to 128 mg/kg/day fractionated for 3-, 6-, 12-, and 24-hour dosing regimens. Nonlinear regression analysis was used to determine which PK-PD parameter best correlated with the measurement of CFU/thigh at 24 h. Pharmacokinetic studies yielded peak/dose values of 0.2 to 0.3, area under the concentration-time curve (AUC)/dose values of 0.1 to 0.5, and half-lives of 0.7 to 1.6 h. Garenoxacin produced in vivo PAEs of 1.4 to 8.2 h with S. pneumoniae ATCC 10813, 7.6 to >12.4 h with S. aureus ATCC 25923, and 0 to 1.5 h with Klebsiella pneumoniae ATCC 43816. The 24-h AUC/MIC ratio was the PK-PD parameter that best correlated with efficacy (R2=71 to 90% for the two organisms compared with 43 to 56% for the peak/MIC ratio and 47 to 75% for percent time above the MIC [% T>MIC]). In subsequent studies we used the neutropenic murine thigh infection model to determine if the magnitude of the AUC/MIC ratio needed for efficacy of garenoxacin varied among pathogens (including resistant strains). Mice had 10(5.9) to 10(7.2) CFU of 6 strains of S. aureus (2 methicillin resistant), 11 strains of S. pneumoniae (5 penicillin susceptible, 1 penicillin intermediate, and 5 penicillin resistant, and of the resistant strains, 3 were also ciprofloxacin resistant), and 4 gram-negative strains per thigh when treated for 24 h with 1 to 64 mg of garenoxacin per kg every 12 h. A sigmoid dose-response model was used to estimate the doses (mg/kg/24 h) required to achieve a net bacteriostatic effect over 24 h. MICs ranged from 0.008 to 4 microg/ml. The free drug 24-h AUC/MIC ratios for each static dose (2.8 to 128 mg/kg/day) varied from 8.2 to 145. The mean 24-h AUC/MIC ratios +/- standard deviations for S. pneumoniae, S. aureus, and gram-negative strains were 33 +/- 18, 81 +/- 37, and 33 +/- 30, respectively. Methicillin, penicillin, or ciprofloxacin resistance did not alter the magnitude of the AUC/MIC ratio required for efficacy.     

Antimicrobial activity and spectrum of the new glycylcycline, GAR-936 tested against 1,203 recent clinical bacterial isolates

The in vitro activity of GAR-936, a new semisynthetic glycylcycline, was evaluated in comparison with two tetracyclines and several other antimicrobial agents. A total of 1,203 recent clinical isolates were tested by reference broth or agar dilution methods. Among the members of the family Enterobacteriaceae, GAR-936 was generally two- to four-fold more active than minocycline, and two- to 16-fold more active than tetracycline. All enteric bacilli MIC90 results were < or = 4 microg/mL; the exception being Proteus mirabilis and indole-positive Proteae (> or = 8 microg/mL). GAR-936 demonstrated excellent activity against all gram-positive cocci with 90% of the penicillin-resistant Streptococcus pneumoniae isolates inhibited at 0.03 microg/ml, while the same isolates had a MIC90 of 8 and > 8 microg/mL for minocycline and tetracycline, respectively. All Enterococcus spp., including vancomycin-resistant isolates, were inhibited at 0.25 microg/mL of GAR-936 (MIC90, 0.12 or 0.25 microg/mL). Although GAR-936 (MIC50, 0.25 microg/mL) was two-fold less active than minocycline (MIC50, 0.12 microg/mL) against oxacillin-resistant Staphylococcus aureus, all isolates were inhibited at < or = 0.25 microg/mL. GAR-936 demonstrated good activity against nonfermentative bacteria such as Acinetobacter spp. (MIC90, 2 microg/ml) and Stenotrophomonas maltophilia (MIC90, 4 microg/mL), but the compound exhibited only modest activity against Pseudomonas aeruginosa (MIC50, 8 microg/mL). Haemophilus influenzae (MIC90, 1-2 microg/mL), Moraxella catarrhalis (MIC90, 0.12 microg/mL), and various Neisseria spp. (MIC90, 0.12-0.5 microg/mL) were susceptible to GAR-936. These results indicate that GAR-936 has potent in vitro activity against a wide range of clinically important pathogenic bacteria, and that several gram-positive and -negative isolates resistant to older tetracyclines and other drug classes remain susceptible to GAR-936, the newest glycylcycline candidate for clinical use.     

AUC(0-t)/MIC is a continuous index of fluoroquinolone exposure and predictive of antibacterial response for Streptococcus pneumoniae in an in vitro infection model

OBJECTIVE: To conduct a comprehensive pharmacodynamic analysis of moxifloxacin and levofloxacin against Streptococcus pneumoniae in an in vitro infection model. METHODS: In dose escalation studies, single doses with peak concentrations equivalent to 1 x, 2 x, 4 x, 8 x, 16 x and 32 x MIC against two isolates of S. pneumoniae were studied over 24 h. Traditional pharmacodynamic indices, including peak concentration divided by MIC (peak/MIC), time of concentration above MIC (T > MIC) and AUC24/MIC, were estimated for all regimens. As a continuous index of fluoroquinolone exposure, AUC0-t/MIC was also calculated, as AUC from time 0 to 1, 2 and 6 h divided by MIC. Correlations between pharmacodynamic indices and antibacterial effects were examined using linear and non-linear methods. In validation experiments, the pharmacodynamic model was used to predict bacterial kill curves, produced by simulated clinical doses of moxifloxacin and levofloxacin against two other S. pneumoniae isolates. RESULTS: Peak/MIC was most predictive of early bacterial kill, whereas T > MIC was significantly associated with final bacterial counts at 24 h. Antibacterial effects were bacteriostatic when T > MIC was 48% and bactericidal when values exceeded 55%. AUC0-t/MIC was strongly associated with bacterial kill throughout the dosing interval. Bactericidal activity and bacterial eradication were associated with AUC0-t/MICs of 28 and 135, respectively. AUC0-t/MIC was also highly predictive of bacterial kill curves produced by simulated clinical doses of moxifloxacin and levofloxacin (precision 0.36 log10 cfu/mL, bias 0.02 log10 cfu/mL). CONCLUSION: This study demonstrated the novel application of AUC0-t/MIC as a continuous index of antibiotic activity, and provided extensive characterization of fluoroquinolone pharmacodynamics against S. pneumoniae.     

In vitro antimicrobial activity of GAR-936 tested against antibiotic-resistant gram-positive blood stream infection isolates and strains producing extended-spectrum beta-lactamases

GAR-936, a new, semisynthetic glycylcycline, has shown good antibacterial activity against a wide range of clinically important Gram-positive and –negative aerobic bacteria including Streptococcus pneumoniae, Hemophilus influenzae, Moraxella catarrhalis, Neisseria gonorrhoeae, most Enterobacteriaceae, Staphylococcus aureus and Enterococcus spp. The purpose of this study was to determine the activity of GAR-936 against a range of Gram-positive and –negative bloodstream isolates including many strains producing extended-spectrum β-lactamases (ESBLs). Six hundred four bloodstream isolates of Gram-positive cocci collected as part of the SENTRY surveillance program were selected for their geographic diversity. GAR-936 was also tested against an additional 176 Gram-negative cocci isolates (Klebsiella pneumoniae, 98 strains; Escherichia coli, 78 strains), 96 of which were ESBL-producers. Broth microdilution testing was used to determine the susceptibility of the selected organisms and a range of comparator antimicrobial agents whose choice was based on their activities against the selected pathogens and included a mix of both newer and older agents. Presence of an ESBL-producing strain was confirmed using the clavulanate test. GAR-936 demonstrated impressive activity against all 604 strains of Gram-positive cocci, with an MIC range of ≤0.015–1 μg/mL, and MIC₉₀s ranging from ≤0.05 μg/mL for S. pneumoniae to 0.25 for S. aureus and coagulase-negative staphylococci. MICs against the 176 Gram-negative isolates were higher (range 0.06–4 μg/mL), with MIC₉₀s of 0.25–1 μg/mL. The activity of GAR-936 was relatively unaffected by the presence of ESBLs. The activity of GAR-936 was particularly impressive against Gram-positive cocci when compared against the test results for vancomycin and the newer antimicrobial agents, linezolid and quinupristin/dalfopristin. The MIC₉₀s for GAR-936 were significantly lower than the comparator agents for all species tested with particularly impressive results against S. pneumoniae and enterococci.     

Pharmacodynamics of moxifloxacin and levofloxacin against Streptococcus pneumoniae, Staphylococcus aureus, Klebsiella pneumoniae and Escherichia coli: simulation of human plasma concentrations after intravenous dosage in an in vitro kinetic model

OBJECTIVES: To compare in an in vitro kinetic model the pharmacodynamics of moxifloxacin and levofloxacin with a concentration-time profile simulating the human free non-protein bound concentrations of 400 mg moxifloxacin intravenous (iv) once daily, 500 mg levofloxacin iv once daily and 750 mg levofloxacin iv once daily against strains of Streptococcus pneumoniae, Staphylococcus aureus, Klebsiella pneumoniae and Escherichia coli with variable susceptibility to fluoroquinolones. METHODS: The strains used in the study included S. pneumoniae ATCC 6306 (native strain), S. pneumoniae 19397 (double mutation; gyrA and parC), S. pneumoniae 4241 (single mutation; parC), S. aureus ATCC 13709 (native strain), S. aureus MB5 (single mutation; gyrA), E. coli M12 (single mutation; gyrA), E. coli ATCC 25922 (native strain) and K. pneumoniae ATCC 29655 (native strain). The strains were exposed to moxifloxacin and levofloxacin in an in vitro kinetic model simulating the free human serum concentration-time profile of moxifloxacin 400 mg once daily, levofloxacin 500 mg once daily and 750 mg once daily. Repeated samples were taken regularly during 24 h and viable counts were carried out. RESULTS AND CONCLUSIONS: A correlation was seen between both the area under the serum concentration curve and MIC (AUC/MIC) and the peak concentration/MIC (Cmax/MIC) versus area under the bactericidal killing curve (AUBKC) or Deltalog0-24 cfu/mL. Compiling all data, an AUC/MIC of approximately 100 and a Cmax/MIC of 10 gave a maximal bactericidal effect for both levofloxacin and moxifloxacin. In accordance with the results from others, our study indicated that a lower AUC/MIC was needed for S. pneumoniae in comparison with the Gram-negative bacteria studied. Moxifloxacin yielded higher AUC/MIC and Cmax/MIC against the investigated Gram-positive bacteria in comparison with levofloxacin 500 mg once daily and 750 mg once daily.     

Comparative in vitro activities of GAR-936 against aerobic and anaerobic animal and human bite wound pathogens

GAR-936 is a new semisynthetic glycylcycline with a broad antibacterial spectrum, including tetracycline-resistant strains. The in vitro activities of GAR-936, minocycline, doxycycline, tetracycline, moxifloxacin, penicillin G, and erythromycin were determined by agar dilution methods against 268 aerobic and 148 anaerobic strains of bacteria (including Pasteurella, Eikenella, Moraxella, Bergeyella, Neisseria, EF-4, Bacteroides, Prevotella, Porphyromonas, Fusobacterium, Staphylococcus, Streptococcus, Enterococcus, Corynebacterium, Propionibacterium, Peptostreptococcus, and Actinomyces) isolated from infected human and animal bite wounds in humans, including strains resistant to commonly used antimicrobials. GAR-936 was very active, with an MIC at which 90% of the strains are inhibited (MIC(90)) of < or =0.25 microg/ml, against all aerobic gram-positive and -negative strains, including tetracycline-resistant strains of Enterococcus, Streptococcus, and coagulase-negative staphylococci, except for Eikenella corrodens (MIC(90), < or =4 microg/ml). GAR-936 was also very active against all anaerobic species, including tetracycline-, doxycycline-, and minocycline-resistant strains of Prevotella spp., Porphyromonas spp., Bacteroides tectum, and Peptostreptococcus spp., with an MIC(90) of < or =0.25 microg/ml. Erythromycin- and moxifloxacin-resistant fusobacteria were susceptible to GAR-936, with an MIC(90) of 0.06 microg/ml.     

Disk diffusion susceptibility test development for the new glycylcycline, GAR-936

The in vitro activity of GAR-936, a new semisynthetic glycylcycline, was evaluated in comparison with two tetracyclines and the disk diffusion susceptibility test was assessed. Nearly 700 recent clinical isolates were tested by reference broth microdilution and disk diffusion (two disk contents) methods. Among the Enterobacteriaceae, GAR-936 was generally two- to 16-fold more active than other tetracyclines. All enteric bacilli MIC90 results were < or = 4 micrograms/mL; the exception being Proteus mirabilis and indole-positive Proteae (> or = 8 micrograms/mL). GAR-936 demonstrated excellent activity against all Gram-positive cocci with Enterococcus spp. (including vancomycin-resistant isolates) inhibited at 0.25 microgram/mL (GAR-936 MIC90, 0.12 or 0.25 microgram/mL) and the oxacillin-resistant Staphylococcus aureus strains were inhibited at < or = 0.25 microgram/mL. GAR-936 demonstrated good potency against several non-fermentative bacteria, but possessed limited activity against Pseudomonas aeruginosa (MIC50, 8 micrograms/mL). In vitro susceptibility test methods were developed (disk diffusion versus reference MIC results) and tentative breakpoints were proposed. Using susceptibility criteria of either < or = 2 or < or = 4 micrograms/mL, GAR-936 in vitro susceptibility tests demonstrated rare significant serious inter-method discords (< or = 1.2%) and an absolute agreement between test results of 92.3 to 96.2%. These results indicate that GAR-936 has potent in vitro activity against a wide range of clinically important rapidly growing pathogenic bacteria, and that this novel glycylcycline candidate for clinical use should be further developed.     

Optimal times above MICs of ceftibuten and cefaclor in experimental intra-abdominal infections.

The duration of time that serum drug levels remain above the MIC (time above the MIC) for the pathogen has been shown to be the most significant parameter determining the efficacies of beta-lactam antibiotics. In the described study, we investigated the optimal time above the MIC of ceftibuten and cefaclor using a nonneutropenic mouse model of intra-abdominal infections caused by Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, and Streptococcus pneumoniae. The abilities of the drugs to protect mice against the organisms were determined in mouse protection tests, and the doses were fractionated to produce various dosing regimens with different times above the MIC. All drug-organism combinations showed a significant correlation (r > 0.9) between drug efficacy and the time above the MIC. Also, with ceftibuten treatment, the different dosing regimens that produced equal times above the MIC resulted in the same efficacy, whereas with cefaclor, an apparent dose-dependent effect was observed. These results showed that for a 100% recovery from K. pneumoniae and E. coli infections, the optimal times above the MIC with ceftibuten treatment were 2.2 and 1.6 h, respectively. Relatively high doses of both antibiotics were required to ensure recovery from S. pneumoniae infections. In vitro time-kill studies demonstrated that cefaclor exhibits a marked inoculum effect against the pathogens, and there was a concentration-dependent killing at a large inoculum size. On the other hand, ceftibuten showed no inoculum effect. It is suggested that optimization of both dose and time above the MIC appears to be necessary for the treatment of S. aureus infections with cefaclor, and this may apply to other beta-lactams tht exhibit marked inoculum effects.     

Pharmacodynamics of a new streptogramin, XRP 2868, in murine thigh and lung infection models

XRP 2868 is a new streptogramin antibiotic with broad-spectrum activity against gram-positive cocci. We used the neutropenic murine thigh and lung infection models to characterize the time course of antimicrobial activity of XRP 2868 and determine which pharmacokinetic/pharmacodynamic (PK/PD) parameter and magnitude best correlated with efficacy. Serum levels following four two- to fourfold-escalating single-dose levels of XRP 2868 were measured by liquid chromatography mass spectrometry assay. In vivo postantibiotic effects (PAEs) were determined after doses of 2.5, 10, and 40 mg/kg. Mice had 10(6.8) to 10(8.4) CFU/thigh of strains of Streptococcus pneumoniae ATCC 10813 or Staphylococcus aureus ATCC 29213 at the start of therapy when treated for 24 h with 2.5 to 640 mg/kg/day of XRP 2868 fractionated for 3-, 6-, 12-, and 24-h dosing regimens. Nonlinear regression analysis was used to determine which PK/PD parameter best correlated with CFU/thigh at 24 h. Pharmacokinetic studies exhibited peak dose values of 0.03 to 0.07, area under the concentration-time curve (AUC) dose values of 0.02 to 0.07, and half-lives of 0.35 to 1.27 h. XRP 2868 produced in vivo PAEs of 0.5 to 3.4 h with S. pneumoniae strain ATCC 10813 and -1.5 to 10.7 h with S. aureus strain ATCC 29213. The 24-h AUC/MIC was the PK/PD parameter that best correlated with efficacy. In subsequent studies, we used the neutropenic murine thigh infection model to determine if the magnitude of the AUC/MIC needed for the efficacy of XRP 2868 varied among pathogens (including resistant strains). Mice had 10(6.1) to 10(7.8) CFU/thigh of four isolates of S. aureus (three methicillin-susceptible and one methicillin-resistant strain) and nine isolates of S. pneumoniae (one penicillin-susceptible, four penicillin-intermediate, and four penicillin-resistant strains) when treated for 24 h with 0.16 to 640 mg/kg of XRP 2868 every 6 h. A sigmoid dose-response model was used to estimate the doses (mg/kg/24 h) required to achieve a net bacteriostatic affect over 24 h. MICs ranged from 0.06 to 0.25 microg/ml. The 24-h AUC/MICs for each static dose (20.7 to 252 mg/kg/day) varied from 3 to 70. Mean 24-h AUC/MICs +/- standard deviations (SDs) for S. pneumoniae and S. aureus isolates were 14 +/- 10 and 31 +/- 16, respectively. Beta-lactam and macrolide resistance did not alter the magnitude of AUC/MIC required for efficacy.     

Validation of cefditoren MIC quality control ranges by a multi-laboratory study (2001).

The potency of cefditoren has been well documented to include broad coverage against both Gram-positive and Gram-negative organisms including species causing community-acquired respiratory tract infections. The accuracy of in vitro test results has been questioned due to debate over inconsistencies in quality control (QC) recommendations published by the National Committee for Clinical Laboratory Standards (NCCLS). Previous studies have recommended conflicting and generally broad MIC dilution ranges for several commonly used QC organisms. This study used NCCLS recommended guidelines to re-evaluate the MIC ranges using a seven laboratory M23-A2 design. The results of this multi-laboratory investigation leads to the proposal of the following QC range recommendations for cefditoren: Streptococcus pneumoniae ATCC 49619 (0.016-0.06 microg/mL); Haemophilus influenzae ATCC 49247 (0.06-0.25 microg/mL); Escherichia coli ATCC 25922 (0.12-1 microg/mL); and Staphylococcus aureus ATCC 29213 (0.25-1 microg/mL). These proposed ranges validate those suggested previously for E. coli and the problematic H. influenzae, but a three log(2) dilution range may be sufficient for S. aureus and S. pneumoniae.     

Pharmacodynamics of glycopeptides in the mouse peritonitis model of Streptococcus pneumoniae or Staphylococcus aureus infection

The emergence of resistance to various antibiotics in pneumococci leaves the glycopeptides as the only antibiotics against which pneumococci have no resistance mechanism. This situation has led to a renewed interest in the use of glycopeptides. It has not yet been possible to conclude which one or more of the pharmacokinetic or pharmacodynamic (PK/PD) parameters are the most important and best predictors for the effects of treatment with glycopeptides in animal models or in humans. We used the mouse peritonitis model with immunocompetent mice and with Staphylococcus aureus and Streptococcus pneumoniae as infective organisms. A wide spectrum of different treatment regimens with vancomycin and teicoplanin was tested to study the pharmacodynamics of these drugs. In studies in which the single dose that protected 50% of lethally infected mice (ED(50)) was given as one dose or was divided into two doses, survival was significantly decreased when the dose was divided. The only statistically significant correlations between the percentage of survival of the mice after 6 days and each of the PK/PD parameters were for peak concentration (C(max))/MIC and S. aureus and for the free fraction of C(max) (C(max-free))/MIC and S. pneumoniae. For S. pneumoniae, the ED(50) for different dosing regimens increased with the number of doses given; e.g., the single-dose ED(50)s for vancomycin and teicoplanin were 0.65 and 0. 45 mg/kg, respectively, but the ED(50)s for dosing regimens with 2-h doses given for 48 h were 6.79 and 5.67 mg/kg, respectively. In experiments with 39 different vancomycin dosing regimens and 40 different teicoplanin dosing regimens against S. pneumoniae, the different PK/PD parameters were analyzed using logistic regression. The C(max-free)/MIC was one of two parameters that best explained the effect for both drugs; for vancomycin, the other important parameter was the AUC/MIC, and for teicoplanin, the other parameter was the time the free fraction of the drug is above the MIC. The effect analyzed as a function of C(max-free)/MIC disclosed thresholds with shifts from almost no effect to full effect at ratios of five to six for vancomycin and two to three for teicoplanin.     

Susceptibilities of Mycoplasma hominis, M. pneumoniae, and Ureaplasma urealyticum to GAR-936, dalfopristin, dirithromycin, evernimicin, gatifloxacin, linezolid, moxifloxacin, quinupristin-dalfopristin, and telithromycin compared to their susceptibilities to reference macrolides, tetracyclines, and quinolones

The susceptibilities of Mycoplasma hominis, Mycoplasma pneumoniae, and Ureaplasma urealyticum to eight new antimicrobial agents were determined by agar dilution. M. pneumoniae was susceptible to the new glycylcycline GAR-936 at 0.12 microg/ml and evernimicin at 4 microg/ml, but it was resistant to linezolid. It was most susceptible to dirithromycin, quinupristin-dalfopristin, telithromycin, reference macrolides, and josamycin. M. hominis was susceptible to linezolid, evernimicin, and GAR-936. It was resistant to macrolides and the ketolide telithromycin but susceptible to quinupristin-dalfopristin and josamycin. U. urealyticum was susceptible to evernimicin (8 to 16 microg/ml) and resistant to linezolid. It was less susceptible to GAR-936 (4.0 microg/ml) than to tetracycline (0.5 microg/ml). Telithromycin and quinupristin-dalfopristin were the most active agents against ureaplasmas (0.06 microg/ml). The new quinolone gatifloxacin was active against M. pneumoniae and M. hominis at 0.12 to 0.25 microg/ml and active against ureaplasmas at 1.0 microg/ml. The MICs of macrolides were markedly affected by pH, with an 8- to 32-fold increase in the susceptibility of M. pneumoniae as the pH increased from 6.9 to 7.8. A similar increase in susceptibility with increasing pH was also observed with ureaplasmas. Tetracyclines showed a fourfold increase of activity as the pH decreased 1 U, whereas GAR-936 showed a fourfold decrease in activity with a decrease in pH.     

Therapeutic efficacy of GAR-936, a novel glycylcycline, in a rat model of experimental endocarditis

GAR-936, a novel glycylcycline, was investigated with a rat model of experimental endocarditis. It was compared with vancomycin against both vancomycin-susceptible and -resistant Enterococcus faecalis and methicillin-resistant Staphylococcus aureus. GAR-936 exhibited the lowest MICs (2 log(10) CFU, compared to those in untreated controls, for both vancomycin-susceptible and -resistant (VanA and VanB) E. faecalis strains and >4 log(10) CFU for a methicillin-resistant S. aureus isolate. The glycylcycline was more efficacious at a lower administered dose in the rat model of endocarditis than was vancomycin. The efficacy of GAR-936 in this model was apparently not enhanced by a factor in rat serum, as was observed for vancomycin with a time-kill curve. The results of this study demonstrate the therapeutic potential of GAR-936 for the treatment of enterococcal and staphylococcal infections and warrant further investigation.     

In vivo pharmacodynamic activity of daptomycin

Daptomycin is a lipopeptide antibiotic with activity against a wide range of gram-positive bacteria. We used the neutropenic murine thigh model to characterize the pharmacodynamics of daptomycin. ICR/Swiss mice were rendered neutropenic with cyclophosphamide; and the thigh muscles of the mice were infected with strains of Staphylococcus aureus, Streptococcus pneumoniae, and Enterococcus faecium. Animals were treated by subcutaneous injection of daptomycin at doses of 0.20 to 400 mg/kg of body weight/day divided into one, two, four, or eight doses over 24 h. Daptomycin exhibited linear pharmacokinetics, with an area under the concentration-time curve (AUC) from time zero to infinity/dose of 9.4 and a half-life of 0.9 to 1.4 h. The level of protein binding was 90%. Free daptomycin exhibited concentration-dependent killing and produced in vivo postantibiotic effects (PAEs) of 4.8 to 10.8 h. Nonlinear regression analysis was used to determine which pharmacokinetic (PK) or pharmacodynamic (PD) parameter was important for efficacy by using free drug concentrations. The peak concentration/MIC (peak/MIC) ratio and 24-h AUC/MIC ratio were the PK and PD parameters that best correlated with in vivo efficacy (R(2) = 83 to 87% for peak/MIC and R(2) = 86% for the AUC/MIC ratio, whereas R(2) = 47 to 50% for the time that the concentration was greater than the MIC) against standard strains of S. aureus and S. pneumoniae. The peak/MIC ratios required for a bacteriostatic effect ranged from 12 to 36 for S. pneumoniae, 59 to 94 for S. aureus, and 0.14 to 0.25 for E. faecium. The AUC/MIC ratios needed for a bacteriostatic effect ranged from 75 to 237 for S. pneumoniae, 388 to 537 for S. aureus, and 0.94 to 1.67 for E. faecium. The free daptomycin concentrations needed to average from one to two times the MIC over 24 h to produce a bacteriostatic effect and two to four times the MIC over 24 h to produce greater than 99% killing. The long PAE and potent bactericidal activity make daptomycin an attractive option for the treatment of infections caused by gram-positive bacteria.     

In vitro activity of GAR-936 against vancomycin-resistant enterococci, methicillin-resistant Staphylococcus aureus and penicillin-resistant Streptococcus pneumoniae

We report the activity of the new glycylcycline antimicrobial agent GAR-936 against 37 clinical isolates of vancomycin-resistant enterococci (including organisms carrying the vanA, vanB, vanC-1, and vanC-2/3 genes), 26 clinical isolates of methicillin-resistant S. aureus and 30 clinical isolates of high-level penicillin-resistant S. pneumoniae. All isolates of vancomycin-resistant enterococci, methicillin-resistant S. aureus, and penicillin-resistant S. pneumoniae were inhibited by < or = 1, < or = 2, or < or = 0.25 microg/ml of GAR-936, respectively. Time kill experiments using vancomycin-resistant enterococci did not demonstrate synergy or antagonism between 2 microg/ml of GAR-936 and 0.25 microg/ml of quinupristin/dalfopristin.     

In vitro activity of para-guanidinoethylcalix[4]arene against susceptible and antibiotic-resistant Gram-negative and Gram-positive bacteria

OBJECTIVES: Emergence of multidrug-resistant bacteria has encouraged vigorous efforts to develop antimicrobial agents with new mechanisms of action. In this study, the in vitro antibacterial activity of para-guanidinoethylcalix[4]arene was evaluated and compared with that of its constitutive monomer, para-guanidinoethylphenol. Hexamidine, a widely used antiseptic, and synthalin A, an old antidiabetic and anti-trypanosomal compound, were chosen as references. METHODS: MIC and MBC were determined for five reference strains (Escherichia coli ATCC 25922, Staphylococcus aureus ATCC 25923 and ATCC 29213, Enterococcus faecalis ATCC 29212 and Pseudomonas aeruginosa ATCC 27853), as well as five antibiotic-resistant clinical isolates. Toxicity on MRC-5 and HaCaT eukaryotic cell lines was also evaluated by MTT and Neutral Red assays. RESULTS: No antibacterial activity was observed for para-guanidinoethylphenol (MIC >or= 512 mg/L) and synthalin A (MIC >or= 64 mg/L). Conversely, para-guanidinoethylcalix[4]arene and hexamidine: (i) showed a broad antibacterial spectrum, both on Gram-positive and on Gram-negative bacteria (MIC = 4 mg/L against E. coli and 8 mg/L against S. aureus for para-guanidinoethylcalix[4]arene), to a lesser degree against E. faecalis and P. aeruginosa (MIC = 32 mg/L); (ii) were bacteriostatic (MBC >or= 256 mg/L); and (iii) MICs and MBCs obtained for clinical isolates were similar to those obtained with reference strains. Both compounds, the monomer and the calixarene, showed no apparent cytotoxicity, whereas hexamidine and synthalin A had significant toxic effects that increased with time and concentration and in a range of 100-1000 times that for calixarene. CONCLUSIONS: In conclusion, results confirm para-guanidinoethylcalix[4]arene as a broad-spectrum new agent or an auxiliary in antimicrobial chemotherapy.     

Mutant prevention concentration of nalidixic acid, ciprofloxacin, clinafloxacin, levofloxacin, norfloxacin, ofloxacin, sparfloxacin or trovafloxacin for Escherichia coli under different growth conditions

OBJECTIVES: We used two different strains of Escherichia coli, E.coli ATCC 25922 and a recent urinary isolate from a clinical sample, to investigate in vitro how the MIC and mutant prevention concentration (MPC) are affected by different temperatures (37 or 20 degrees C) or oxygen tension (aerobic or anaerobic atmosphere; MIC, MIC(an); MPC, MPC(an)).Materials and methods: MIC and MPC for E.coli ATCC 25922 and the clinical isolate were determined on agar containing ciprofloxacin or levofloxacin, and for the ATCC strain on agar supplemented with nalidixic acid, norfloxacin, ofloxacin, sparfloxacin, trovafloxacin or clinafloxacin. RESULTS: Results for the ATCC strain and the clinical strain for ciprofloxacin or levofloxacin were similar. The MPC values for E.coli ATCC 25922 were 2 x MIC (trovafloxacin), 4 x MIC (ciprofloxacin, norfloxacin, ofloxacin), 8 x MIC (clinafloxacin, levofloxacin), 16 x MIC (sparfloxacin) and 32 x MIC (nalidixic acid) at 37 degrees C and under aerobic conditions. Generally, a 37 degrees C aerobic atmosphere was associated with the highest MPC values. As an exception, both the MIC and the MPC of ciprofloxacin were higher under anaerobic versus aerobic conditions (MIC(an) approximately 8 x MIC; MPC(an) = 4 x MPC) for both E.coli isolates. Irrespective of the quinolone or growth conditions, the MIC for mutants was 1-256 x wild-type MIC. Calculated from published serum half-lives and the MPC values from this study, a putative selection period, in which resistant mutants might be selected, was calculated to be 14 h for nalidixic acid, 16 h for norfloxacin and ciprofloxacin, 28 h for ofloxacin, 30 h for trovafloxacin, 35 h for levofloxacin, 40 h for clinafloxacin, and 120 h for sparfloxacin. CONCLUSIONS: As calculated from our model in respect to the length of the selection period, long serum half-lives of recently developed compounds could not be compensated for by a more favourable activity in terms of MPC. Higher concentrations of ciprofloxacin may be required under an anaerobic atmosphere to prevent the emergence of resistant mutants among 10(10) cfu.     

Pharmacodynamic activity of garenoxacin against ciprofloxacin-resistant Streptococcus pneumoniae

BACKGROUND: The pharmacodynamic parameter that best correlates with bacteriological eradication for fluoroquinolones is the free (f) area under the 24 h serum concentration curve (AUC24) to MIC (fAUC24/MIC) ratio. This study assessed garenoxacin fAUC24/MIC against ciprofloxacin-resistant Streptococcus pneumoniae using an in vitro pharmacodynamic model. METHODS: A total of 14 S. pneumoniae including 1 fluoroquinolone-susceptible and 13 ciprofloxacin-resistant S. pneumoniae (ParC, efflux, ParC with efflux, and ParC and GyrA) were studied. The quinolone-resistance determining regions (QRDRs) of parC and gyrA were sequenced and efflux was assessed using a reserpine assay. S. pneumoniae with garenoxacin MICs (mg/L) [number of strains] studied were: 0.03 [1], 0.06 [2], 0.12 [2], 0.25 [2], 0.5 [3], 1 [2] and 2 [2]. The in vitro pharmacodynamic model was inoculated with 1 x 10(6) cfu/mL and garenoxacin was dosed once daily at 0 and 24 h to simulate fAUC24 and t1/2 obtained after standard oral doses in healthy volunteers (400 mg once daily, free AUC24 20 mg.h/L, t1/2 16 h). Sampling was performed over 48 h to assess viable growth. RESULTS: Garenoxacin fAUC24/MIC achieved in the model ranged from 12 to 800. Garenoxacin fAUC24/MIC 200-800 was bactericidal (> or = 3 log(10) killing) at 6, 24 and 48 h against ciprofloxacin-resistant S. pneumoniae mutants including ParC mutants only, efflux mutants only and ParC/efflux mutants. Garenoxacin fAUC24/MIC 48-96 was bactericidal (> or = 3 log(10) killing) at 24 and 48 h against all ciprofloxacin-resistant S. pneumoniae mutants. Garenoxacin fAUC24/MIC < or = 24 (against ParC and GyrA mutants) resulted in a bacteriostatic effect with regrowth at 24 and 48 h. CONCLUSIONS: Garenoxacin was bactericidal against ciprofloxacin-resistant S. pneumoniae at fAUC24/MIC > or = 48. Garenoxacin fAUC24/MIC < or = 24 resulted in a bacteriostatic effect with regrowth at 24 and 48 h.