One of the most challenging issues in the design of phase II/III clinical trials of antimicrobial agents is dose selection. The choice is often based on preclinical data from pharmacokinetic (PK) studies with animals and healthy volunteers but is rarely linked directly to the target organisms except by the MIC, an in vitro measure of antimicrobial activity with many limitations. It is the thesis of this paper that rational dose-selection decisions can be made on the basis of the pharmacodynamics (PDs) of the test agent predicted by a mathematical model which uses four data sets: (i) the distribution of MICs for clinical isolates, (ii) the distribution of the values of the PK parameters for the test drug in the population, (iii) the PD target(s) developed from animal models of infection, and (iv) the protein binding characteristics of the test drug. In performing this study with the new anti-infective agent evernimicin, we collected a large number (n ‫؍‬ 4,543) of recent clinical isolates of gram-positive pathogens (Streptococcus pneumoniae, Enterococcus faecalis and Enterococcus faecium, and Staphylococcus aureus) and determined the MICs using E-test methods (AB Biodisk, Stockholm, Sweden) for susceptibility to evernimicin. Population PK data were collected from healthy volunteers (n ‫؍‬ 40) and patients with hypoalbuminemia (n ‫؍‬ 12), and the data were analyzed by using NPEM III. PD targets were developed with a neutropenic murine thigh infection model with three target pathogens: S. pneumoniae (n ‫؍‬ 5), E. faecalis (n ‫؍‬ 2), and S. aureus (n ‫؍‬ 4). Drug exposure or the ratio of the area under the concentration-time curve/MIC (AUC/MIC) was found to be the best predictor of microbiological efficacy. There were three possible microbiological results: stasis of the initial inoculum at 24 h (10 7 CFU), log killing (pathogen dependent, ranging from 1 to 3 log 10 ), or 90% maximal killing effect (90% E max ). The levels of protein binding in humans and mice were similar. The PK and PD of 6 and 9 mg of evernimicin per kg of body weight were compared; the population values for the model parameters and population covariance matrix were used to generate five Monte Carlo simulations with 200 subjects each. The fractional probability of attaining the three PD targets was calculated for each dose and for each of the three pathogens. All differences in the fractional probability of attaining the target AUC/MIC in this PD model were significant. For S. pneumoniae, the probability of attaining all three PD targets was high for both doses. For S. aureus and enterococci, there were increasing differences between the 6-and 9-mg/kg evernimicin doses for reaching the 2 log killing (S. aureus), 1 log killing (enterococci), or 90% E max AUC/MIC targets. This same approach may also be used to set preliminary in vitro MIC breakpoints.The drug development process traditionally follows the initial "first-in-human" pharmacokinetic (PK) studies with phase II dose-finding studies. Such studies are often relatively small and provide litt...

Background: For its low cost and ease of handling, the mouse remains the preferred experimental animal for preclinical tests. To avoid the interaction of the animal immune system, in vivo antibiotic pharmacodynamic studies often employ cyclophosphamide (CPM) to induce neutropenia. Although high doses (350-450 mg/kg) are still used and their effects on mouse leukocytes have been described, a lower dose (250 mg/kg) is widely preferred today, but the characteristics and applicability of this approach in outbred mice have not been determined.

The in vivo pharmacodynamic activities of two glycylcyclines (GAR-936 and WAY 152,288) were assessed in an experimental murine thigh infection model in neutropenic mice. Mice were infected with one of several strains of Streptococcus pneumoniae, Staphylococcus aureus, Escherichia coli, or Klebsiella pneumoniae. Most infections were treated with a twice-daily dosing schedule, with administration of 0.75 to 192 mg of GAR-936 or WAY 152,288 per kg of body weight. A maximum-effect dose-response model was used to calculate the dose that produced a net bacteriostatic effect over 24 h of therapy. This dose was called the bacteriostatic dose. More extensive dosing studies were performed with S. pneumoniae 1199, E. coli ATCC 25922, and K. pneumoniae ATCC 43816, with doses being given as one, two, four, or eight equal doses over a period of 24 h. The dosing schedules were designed in order to minimize the interrelationship between the various pharmacokinetic and pharmacodynamic parameters studied. These parameters were time above 0.03 to 32 times the MIC, area under the concentration-time curve (AUC), and maximum concentration of drug in serum (C max ). The bacteriostatic dose remained essentially the same, irrespective of the dosing frequency, for S. pneumoniae 1199 (0.3 to 0.9 mg/kg/day). For E. coli ATCC 25922 and K. pneumoniae ATCC 43816, however, more frequent dosing led to lower bacteriostatic doses. Pharmacokinetic studies demonstrated dose-dependent elimination half-lives of 1.05 to 2.34 and 1.65 to 3.36 h and serum protein bindings of 59 and 71% for GAR-936 and WAY 152,288, respectively. GAR-936 and WAY 152,288 were similarly effective against the microorganisms studied, with small differences in maximum effect and 50% effective dose. The glycylcyclines were also similarly effective against tetracycline-sensitive and tetracycline-resistant bacteria. Time above a certain factor (range, 0.5 to 4 times) of the MIC was a better predictor of in vivo efficacy than C max or AUC for most organism-drug combinations. The results demonstrate that in order to achieve 80% maximum efficacy, the concentration of unbound drug in serum should be maintained above the MIC for at least 50% of the time for GAR-936 and for at least 75% of the time for WAY 152,288. The results of these experiments will aid in the rational design of dose-finding studies for these glycylcyclines in humans. 288 are members of the class of glycylcyclines, a new group of antibiotics derived from minocycline. These drugs have potent activity against a variety of tetracycline-sensitive and tetracycline-resistant bacteria (1-3, 5, 8, 10, 14, 15).The objective of the present study was to determine the effects of various dosing regimens on the in vivo efficacy of GAR-936 and WAY 152,288 and identify which pharmacokinetic or pharmacodynamic parameter best correlated with efficacy. The in vivo antibacterial activities of GAR-936 and WAY 152,288 against several isolates of common human pathogens (Streptococcus pneumoniae, Staphylococcus aureus, Escherichia coli, ...

Generic versions of intravenous antibioticsare not required to demonstrate therapeutic equivalence with the innovator because therapeutic equivalence is assumed from pharmaceutical equivalence. To test such assumptions, we studied three generic versions of vancomycin in simultaneous experiments with the innovator and determined the concentration and potency of the active pharmaceutical ingredient by microbiological assay, single-dose pharmacokinetics in infected mice, antibacterial effect by broth microdilution and time-kill curves (TKC), and pharmacodynamics against two wild-type strains of Staphylococcus aureus by using the neutropenic mouse thigh infection model. The main outcome measure was the comparison of magnitudes and patterns of in vivo efficacy between generic products and the innovator. Except for one product exhibiting slightly greater concentration, vancomycin generics were undistinguishable from the innovator based on concentration and potency, protein binding, in vitro antibacterial effect determined by minimal inhibitory or bactericidal concentrations and TKC, and serum pharmacokinetics. Despite such similarities, all generic products failed in vivo to kill S. aureus, while the innovator displayed the expected bactericidal efficacy: maximum antibacterial effect (E max ) (95% confidence interval [CI]) was 2.04 (1.89 to 2.19), 2.59 (2.21 to 2.98), and 3.48 (2.92 to 4.04) versus 5.65 (5.52 to 5.78) log 10 CFU/g for three generics and the innovator product, respectively (P < 0.0001, any comparison). Nonlinear regression analysis suggests that generic versions of vancomycin contain inhibitory and stimulatory principles within their formulations that cause agonistic-antagonistic actions responsible for in vivo failure. In conclusion, pharmaceutical equivalence does not imply therapeutic equivalence for vancomycin.

Recent studies have reported that Staphylococcus aureus small colony variants (SCVs) can cause highly persistent infections in humans and in cultured endothelial cells. To understand the process by which SCVs of S. aureus appear in subjects who have not received antibiotic treatment, bovine endothelial cells were coincubated with a wild S. aureus strain for 72 h in the presence of lysostaphin. Intracellular bacteria were harvested and screened for stable SCVs. Intracellular bacteria developed the SCV phenotype at a greater rate than control bacteria not exposed to endothelial cells: The intracellular induction rate was approximately 10(-3) versus a spontaneous rate of <10(-7). This observation suggest that SCVs are induced by the intracellular milieu and suggest a possible mechanism for the intriguing pathophysiology of tissue persistence of staphylococci.