Correlation between in vitro and in vivo activity of antimicrobial agents against gram-negative bacilli in a murine infection model

An automated microscopy workflow to study Shigella-neutrophil interactions and antibiotic efficacy in vivo

Shigella are Gram-negative bacterial pathogens responsible for bacillary dysentery (also called shigellosis). The absence of a licensed vaccine and widespread emergence of antibiotic resistance has led the World Health Organisation (WHO) to highlight Shigella as a priority pathogen requiring urgent attention. Several infection models have been useful to explore the Shigella infection process; yet, we still lack information regarding events taking place in vivo. Here, using a Shigella-zebrafish infection model and high-content microscopy, we developed an automated microscopy workflow to non-invasively study fluorescently labelled bacteria and neutrophils in vivo. We applied our workflow to antibiotic-treated zebrafish, and demonstrate that antibiotics reduce bacterial burden and not neutrophil recruitment to the hindbrain ventricle. We discovered that nalidixic acid (a bactericidal antibiotic) can work with leukocytes in an additive manner to control Shigella flexneri infection and can also restrict dissemination of Shigella sonnei from the hindbrain ventricle. We envision that our automated microscopy workflow, applied here to study the interactions between Shigella and neutrophils as well as antibiotic efficacy in zebrafish, can be useful to innovate treatments for infection control in humans.

Squaraine Dyes Derived from Indolenine and Benzo[e]indole as Potential Fluorescent Probes for HSA Detection and Antifungal Agents

Four squaraine dyes derived from 2,3,3-trimethylindolenine and 1,1,2-trimethyl-1H-benzo[e]indole with different combinations of barbituric groups attach to the central ring, having ester groups and alkyl chains in the nitrogen atoms of heterocyclic rings were synthesized. These dyes were fully characterized and their photophysical behavior was studied in ethanol and phosphate-buffered saline solution. Absorption and emission bands between 631 and 712 nm were detected, with the formation of aggregates in aqueous media, which is typical of this class of dyes. Tests carried out with 1,3-diphenylisobenzofuran allowed us to verify the ability of the dyes to produce singlet oxygen. The interaction of synthesized dyes with human serum albumin (HSA) was also evaluated, being demonstrated a linear correlation between fluorescence intensity and protein concentration. The antifungal potential of the dyes against the yeast Saccharomyces cerevisiae was evaluated using a broth microdilution assay. In order to test the photosensitizing capacity of the synthesized dyes, tests were carried out in the dark and with irradiation, using a custom-built light-emitting diode that emits close to the absorption wavelength of the studied dyes. The results showed that the interaction of dyes with HSA and the antifungal activity depends on the different structural modifications of the dyes.

Understanding tolerance to cell wall-active antibiotics

Antibiotic tolerance-the ability of bacteria to survive for an extended time in the presence of bactericidal antibiotics-is an understudied contributor to antibiotic treatment failure. Herein, I review the manifestations, mechanisms, and clinical relevance of tolerance to cell wall-active (CWA) antibiotics, one of the most important groups of antibiotics at the forefront of clinical use. I discuss definitions of tolerance and assays for tolerance detection, comprehensively discuss the mechanism of action of β-lactams and other CWA antibiotics, and then provide an overview of how cells mitigate the potentially lethal effects of CWA antibiotic-induced cell damage to become tolerant. Lastly, I discuss evidence for a role of CWA antibiotic tolerance in clinical antibiotic treatment failure.

Aspirin Modulation of the Colorectal Cancer-Associated Microbe Fusobacterium nucleatum

Aspirin is a chemopreventive agent for colorectal adenoma and cancer (CRC) that, like many drugs inclusive of chemotherapeutics, has been investigated for its effects on bacterial growth and virulence gene expression. Given the evolving recognition of the roles for bacteria in CRC, in this work, we investigate the effects of aspirin with a focus on one oncomicrobe-Fusobacterium nucleatum We show that aspirin and its primary metabolite salicylic acid alter F. nucleatum strain Fn7-1 growth in culture and that aspirin can effectively kill both actively growing and stationary Fn7-1. We also demonstrate that, at levels that do not inhibit growth, aspirin influences Fn7-1 gene expression. To assess whether aspirin modulation of F. nucleatum may be relevant in vivo, we use the ApcMin/+ mouse intestinal tumor model in which Fn7-1 is orally inoculated daily to reveal that aspirin-supplemented chow is sufficient to inhibit F. nucleatum-potentiated colonic tumorigenesis. We expand our characterization of aspirin sensitivity across other F. nucleatum strains, including those isolated from human CRC tissues, as well as other CRC-associated microbes, enterotoxigenic Bacteroides fragilis, and colibactin-producing Escherichia coli Finally, we determine that individuals who use aspirin daily have lower fusobacterial abundance in colon adenoma tissues, as determined by quantitative PCR performed on adenoma DNA. Together, our data support that aspirin has direct antibiotic activity against F. nucleatum strains and suggest that consideration of the potential effects of aspirin on the microbiome holds promise in optimizing risk-benefit assessments for use of aspirin in CRC prevention and management.IMPORTANCE There is an increasing understanding of the clinical correlations and potential mechanistic roles of specific members of the gut and tumoral microbiota in colorectal cancer (CRC) initiation, progression, and survival. However, we have yet to parlay this knowledge into better CRC outcomes through microbially informed diagnostic, preventive, or therapeutic approaches. Here, we demonstrate that aspirin, an established CRC chemopreventive, exhibits specific effects on the CRC-associated Fusobacterium nucleatum in culture, an animal model of intestinal tumorigenesis, and in human colonic adenoma tissues. Our work proposes a potential role for aspirin in influencing CRC-associated bacteria to prevent colorectal adenomas and cancer, beyond aspirin's canonical anti-inflammatory role targeting host tissues. Future research, such as studies investigating the effects of aspirin on fusobacterial load in patients, will help further elucidate the prospect of using aspirin to modulate F. nucleatumin vivo for improving CRC outcomes.

Nanotechnology-Based Antimicrobial and Antiviral Surface Coating Strategies

Biocontamination of medical devices and implants is a growing issue that causes medical complications and increased expenses. In the fight against biocontamination, developing synthetic surfaces, which reduce the adhesion of microbes and provide biocidal activity or combinatory effects, has emerged as a major global strategy. Advances in nanotechnology and biological sciences have made it possible to design smart surfaces for decreasing infections. Nevertheless, the clinical performance of these surfaces is highly depending on the choice of material. This review focuses on the antimicrobial surfaces with functional material coatings, such as cationic polymers, metal coatings and antifouling micro-/nanostructures. One of the highlights of the review is providing insights into the virus-inactivating surface development, which might particularly be useful for controlling the currently confronted pandemic coronavirus disease 2019 (COVID-19). The nanotechnology-based strategies presented here might be beneficial to produce materials that reduce or prevent the transmission of airborne viral droplets, once applied to biomedical devices and protective equipment of medical workers. Overall, this review compiles existing studies in this broad field by focusing on the recent related developments, draws attention to the possible activity mechanisms, discusses the key challenges and provides future recommendations for developing new, efficient antimicrobial and antiviral surface coatings.

Therapeutic drug monitoring of β-lactam antibiotics in the ICU

INTRODUCTION

Individualizing antibiotic therapy is paramount to improve clinical outcomes while minimizing the risk of toxicity and antimicrobial therapy. β-lactam antibiotics are amongst the drugs most commonly prescribed in the Intensive Care Unit (ICU). The pharmacokinetics of β-lactam antibiotics are profoundly altered in critically ill patients, leading to the failure of standard drug dosing regimens to result in adequate drug concentrations. Therapeutic Drug Monitoring (TDM) of β-lactam antibiotics is a promising tool to help optimize β-lactam antibiotic therapy.

AREAS COVERED

The rationale behind TDM for β-lactam antibiotics is explained, as well as some more practical aspects such as when to sample, what concentrations to strive for and how to use it in clinical practice. We also discuss microbiological and analytical considerations, knowledge gaps, and future perspectives of β-lactam antibiotics TDM in ICU patients.

EXPERT OPINION

TDM of β-lactam antibiotics has been studied intensively in recent years. While TDM may not yet be widely available, and targets need to be further refined, TDM of β-lactam antibiotics will help to optimize antibiotic therapy in the critically ill patient, as an integrated part of an antimicrobial stewardship program.

Repeated determination of moxifloxacin concentrations in interstitial space fluid of muscle and subcutis in septic patients

BACKGROUND

For an effective antimicrobial treatment, it is crucial that antibiotics reach sufficient concentrations in plasma and tissue. Currently no data exist regarding moxifloxacin plasma concentrations and exposure levels in tissue under septic conditions.

OBJECTIVES

To determine the pharmacokinetics of moxifloxacin in plasma and interstitial space fluid over a prolonged period.

PATIENTS AND METHODS

Ten septic patients were treated with 400 mg of moxifloxacin once a day; on days 1, 3 and 5 of treatment plasma sampling and microdialysis in the subcutis and muscle of the upper thigh were performed to determine concentrations of moxifloxacin in different compartments. This trial was registered in the German Clinical Trials Register (DRKS, register number DRKS00012985).

RESULTS

Mean unbound fraction of moxifloxacin in plasma was 85.5±3.4%. On day 1, Cmax in subcutis and muscle was 2.8±1.8 and 2.5±1.3 mg/L, respectively, AUC was 24.8±15.1 and 21.3±10.5 mg·h/L, respectively, and fAUC0-24/MIC was 100.9±62.9 and 86.5±38.3 h, respectively. Cmax for unbound moxifloxacin in plasma was 3.5±0.9 mg/L, AUC was 23.5±7.5 mg·h/L and fAUC0-24/MIC was 91.6±24.8 h. Key pharmacokinetic parameters on days 3 and 5 showed no significant differences. Clearance was higher than in healthy adults, but tissue concentrations were comparable, most likely due to a lower protein binding.

CONCLUSIONS

Surprisingly, the first dose already achieved exposure comparable to steady-state conditions. The approved daily dose of 400 mg was adequate in our patient population. Thus, it seems that in septic patients a loading dose on the first day of treatment with moxifloxacin is not required.

Growth Rate of Escherichia coli During Human Urinary Tract Infection: Implications for Antibiotic Effect

Escherichia coli is the primary cause of urinary tract infection (UTI), which is one of the most frequent human infections. While much is understood about the virulence factors utilized by uropathogenic E. coli (UPEC), less is known about the bacterial growth dynamics taking place during infection. Bacterial growth is considered essential for successful host colonization and infection, and most antibiotics in clinical use depend on active bacterial growth to exert their effect. However, a means to measure the in situ bacterial growth rate during infection has been lacking. Due to faithful coordination between chromosome replication and cell growth and division in E. coli, chromosome replication provides a quantitative measure of the bacterial growth rate. In this study, we explored the potential for inferring in situ bacterial growth rate from a single urine sample in patients with E. coli bacteriuria by differential genome quantification (ori:ter) performed by quantitative PCR. We found active bacterial growth in almost all samples. However, this occurs with day-to-day and inter-patient variability. Our observations indicate that chromosome replication provides not only a robust measure of bacterial growth rate, but it can also be used as a means to evaluate antibiotic effect.

Comparative Activity of Ceftriaxone, Ciprofloxacin, and Gentamicin as a Function of Bacterial Growth Rate Probed by Escherichia coli Chromosome Replication in the Mouse Peritonitis Model

Commonly used antibiotics exert their effects predominantly on rapidly growing bacterial cells; yet, the growth dynamics taking place during infection in a complex host environment remain largely unknown. Hence, a means to measure in situ bacterial growth rate is essential to predict the outcome of antibacterial treatment. We have recently validated chromosome replication as a readout of in situ bacterial growth rate during Escherichia coli infection in the mouse peritonitis model. By the use of two complementary methods (quantitative PCR and fluorescence microscopy) for differential genome origin and terminus copy number quantification, we demonstrated the ability to track bacterial growth rate, both on a population average level and on a single-cell level, from one single biological specimen. Here, we asked whether the in situ growth rate predicts antibiotic treatment effect during infection in the same model. Parallel in vitro growth experiments were conducted as a proof of concept. Our data demonstrate that the activities of the commonly used antibiotics ceftriaxone and gentamicin correlated with pretreatment bacterial growth rate; both drugs performed better during rapid growth than during slow growth. Conversely, ciprofloxacin was less sensitive to bacterial growth rate, both in a homogenous in vitro bacterial population and in a more heterogeneous in vivo bacterial population. The method serves as a platform to test any antibiotic's dependency on active in situ bacterial growth. Improved insight into this relationship in vivo could ultimately prove helpful in evaluating future antibacterial strategies.

Identification of the In Vivo Pharmacokinetics and Pharmacodynamic Driver of Iclaprim

The neutropenic murine thigh infection model was used to define the pharmacokinetic/pharmacodynamic index linked to efficacy of iclaprim against Staphylococcus aureus ATCC 29213 and Staphylococcus pneumoniae ATCC 10813. The 24-h area under the curve (AUC)/MIC index was most closely linked to efficacy for S. aureus (R², 0.65), while both the 24-h AUC/MIC and the percentage of time that drug concentrations remain above the MIC (%T>MIC) were strongly associated with effect (R², 0.86 for both parameters) for S. pneumoniae.

Evaluation of Pharmacokinetic/Pharmacodynamic Model-Based Optimized Combination Regimens against Multidrug-Resistant Pseudomonas aeruginosa in a Murine Thigh Infection Model by Using Humanized Dosing Schemes

We previously optimized imipenem and tobramycin combination regimens against a double-resistant clinical Pseudomonas aeruginosa isolate by using in vitro infection models, mechanism-based pharmacokinetic/pharmacodynamic modeling (MBM), and Monte Carlo simulations. The current study aimed to evaluate these regimens in a neutropenic murine thigh infection model and to characterize the time course of bacterial killing and regrowth via MBM. We studied monotherapies and combinations of imipenem with tobramycin in vivo against the double-resistant clinical P. aeruginosa isolate by using humanized dosing schemes. Viable count profiles of total and resistant populations were quantified over 24 h. Tobramycin monotherapy (7 mg/kg every 24 h [q24h] as a 0.5-h infusion) was ineffective. Imipenem monotherapies (continuous infusion of 4 or 5 g/day with a 1-g loading dose) yielded 2.47 or 2.57 log₁₀ CFU/thigh killing at 6 h. At 24 h, imipenem at 4 g/day led to regrowth up to the initial inoculum (4.79 ± 0.26 log₁₀ CFU/thigh), whereas imipenem at 5 g/day displayed 1.75 log₁₀ killing versus the initial inoculum. The combinations (i.e., imipenem at 4 or 5 g/day plus tobramycin) provided a clear benefit, with bacterial killing of ≥2.51 or ≥1.50 log₁₀ CFU/thigh compared to the respective most active monotherapy at 24 h. No colonies were detected on 3×MIC agar plates for combinations, whereas increased resistance (at 3×MIC) emerged for monotherapies (except imipenem at 5 g/day). MBM suggested that tobramycin considerably enhanced the imipenem target site concentration up to 2.6-fold. The combination regimens, rationally optimized via a translational modeling approach, demonstrated substantially enhanced bacterial killing and suppression of regrowth in vivo against a double-resistant isolate and are therefore promising for future clinical evaluation.

Antimicrobial blue light inactivation of pathogenic microbes: State of the art

As an innovative non-antibiotic approach, antimicrobial blue light in the spectrum of 400-470nm has demonstrated its intrinsic antimicrobial properties resulting from the presence of endogenous photosensitizing chromophores in pathogenic microbes and, subsequently, its promise as a counteracter of antibiotic resistance. Since we published our last review of antimicrobial blue light in 2012, there have been a substantial number of new studies reported in this area. Here we provide an updated overview of the findings from the new studies over the past 5 years, including the efficacy of antimicrobial blue light inactivation of different microbes, its mechanism of action, synergism of antimicrobial blue light with other angents, its effect on host cells and tissues, the potential development of resistance to antimicrobial blue light by microbes, and a novel interstitial delivery approach of antimicrobial blue light. The potential new applications of antimicrobial blue light are also discussed.

Pharmacokinetics and penetration of ceftazidime and avibactam into epithelial lining fluid in thigh- and lung-infected mice

Ceftazidime and the β-lactamase inhibitor avibactam constitute a new, potentially highly active combination in the battle against extended-spectrum-β-lactamase (ESBL)-producing bacteria. To determine possible clinical use, it is important to know the pharmacokinetic profiles of the compounds related to each other in plasma and the different compartments of infection in experimentally infected animals and in humans. We used a neutropenic murine thigh infection model and lung infection model to study pharmacokinetics in plasma and epithelial lining fluid (ELF). Mice were infected with ca. 10(6) CFU of Pseudomonas aeruginosa intramuscularly into the thigh or intranasally to cause pneumonia and were given 8 different (single) subcutaneous doses of ceftazidime and avibactam in various combined concentrations, ranging from 1 to 128 mg/kg of body weight in 2-fold increases. Concomitant samples of serum and bronchoalveolar lavage fluid were taken at up to 12 time points until 6 h after administration. Pharmacokinetics of both compounds were linear and dose proportional in plasma and ELF and were independent of the infection type, with estimated half-lives (standard deviations [SD]) in plasma of ceftazidime of 0.28 (0.02) h and of avibactam of 0.24 (0.04) h and volumes of distribution of 0.80 (0.14) and 1.18 (0.34) liters/kg. The ELF-plasma (area under the concentration-time curve [AUC]) ratios (standard errors [SE]) were 0.24 (0.03) for total ceftazidime and 0.27 (0.03) for unbound ceftazidime; for avibactam, the ratios were 0.20 (0.02) and 0.22 (0.02), respectively. No pharmacokinetic interaction between ceftazidime and avibactam was observed. Ceftazidime and avibactam showed linear plasma pharmacokinetics that were independent of the dose combinations used or the infection site in mice. Assuming pharmacokinetic similarity in humans, this indicates that similar dose ratios of ceftazidime and avibactam could be used for different types and sites of infection.

In vivo efficacy of anuran trypsin inhibitory peptides against staphylococcal skin infection and the impact of peptide cyclization

Staphylococcus aureus is a virulent pathogen that is responsible for a wide range of superficial and invasive infections. Its resistance to existing antimicrobial drugs is a global problem, and the development of novel antimicrobial agents is crucial. Antimicrobial peptides from natural resources offer potential as new treatments against staphylococcal infections. In the current study, we have examined the antimicrobial properties of peptides isolated from anuran skin secretions and cyclized synthetic analogues of these peptides. The structures of the peptides were elucidated by nuclear magnetic resonance (NMR) spectroscopy, revealing high structural and sequence similarity with each other and with sunflower trypsin inhibitor 1 (SFTI-1). SFTI-1 is an ultrastable cyclic peptide isolated from sunflower seeds that has subnanomolar trypsin inhibitory activity, and this scaffold offers pharmaceutically relevant characteristics. The five anuran peptides were nonhemolytic and noncytotoxic and had trypsin inhibitory activities similar to that of SFTI-1. They demonstrated weak in vitro inhibitory activities against S. aureus, but several had strong antibacterial activities against S. aureus in an in vivo murine wound infection model. pYR, an immunomodulatory peptide from Rana sevosa, was the most potent, with complete bacterial clearance at 3 mg · kg(-1). Cyclization of the peptides improved their stability but was associated with a concomitant decrease in antimicrobial activity. In summary, these anuran peptides are promising as novel therapeutic agents for treating infections from a clinically resistant pathogen.

Impact of MIC range for Pseudomonas aeruginosa and Streptococcus pneumoniae on the ceftolozane in vivo pharmacokinetic/pharmacodynamic target

Ceftolozane is a novel cephalosporin with activity against drug-resistant pathogens, including Pseudomonas aeruginosa and Streptococcus pneumoniae. The in vivo investigation reported here tested the limits of this drug against 20 P. aeruginosa and S. pneumoniae isolates across a wide MIC range and defined resistance mechanisms. The times above the MIC (T>MIC) targets for stasis and 1- and 2-log reductions were 31%, 39%, and 42% for P. aeruginosa and 18%, 24%, and 27% for S. pneumoniae, respectively. The 1-log endpoint was achieved for strains with MICs as high as 16 μg/ml.

In vivo activity of cefquinome against Escherichia coli in the thighs of neutropenic mice

Cefquinome is a cephalosporin with broad-spectrum antibacterial activity, including activity against enteric Gram-negative bacilli such as Escherichia coli. We utilized a neutropenic mouse model of colibacillosis to examine the pharmacodynamic (PD) characteristics of cefquinome, as measured by organism number in homogenized thigh cultures after 24 h of therapy. Serum drug levels following 4-fold-escalating single doses of cefquinome were measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The pharmacokinetic (PK) properties of cefquinome were linear over a dose range of 10 to 640 mg/kg of body weight. Serum half-lives ranged from 0.29 to 0.32 h. Dose fractionation studies over a 24-h dose range of 2.5 to 320 mg/kg were conducted every 3, 6, 12, or 24 h. Nonlinear regression analysis was used to determine which pharmacodynamic parameter best correlated with efficacy. The free percentage of the dosing interval that the serum levels exceed the MIC (fT>MIC) was the PK-PD index that best correlated with efficacy (R(2) = 73% for E. coli, compared with 13% for the maximum concentration of the free drug in serum [fCmax]/MIC and 45% for the free-drug area under the concentration-time curve from 0 to 24 h [fAUC0-24]/MIC). Subsequently, we employed a similar dosing strategy by using 4-fold-increasing total cefquinome doses administered every 4 h to treat animals infected with four additional E. coli isolates. A sigmoid maximum-effect (Emax) model was used to estimate the magnitudes of the %fT>MIC associated with net bacterial stasis, a 1-log10 CFU reduction from baseline, and a 2-log10 CFU reduction from baseline; the corresponding values were 28.01% ± 2.27%, 37.23% ± 4.05%, and 51.69% ± 9.72%. The potent bactericidal activity makes cefquinome an attractive option for the treatment of infections caused by E. coli.

Animal models in drug development for MRSA

One of the foremost challenges of drug discovery in any therapeutic area is that of solidifying the correlation between in vitro activity and clinical efficacy. Between these is the confirmation that affecting a particular target in vivo will lead to a therapeutic benefit. In antibacterial drug discovery, there is a key advantage from the start, since the targets are bacteria-therefore, it is simple to ascertain in vitro whether a drug has the desired effect, i.e., bacterial cell inhibition or killing, and to understand the mechanism by which that occurs. The downstream criteria, whether a compound reaches the infection site and achieves appropriately high levels to affect bacterial viability, can be evaluated in animal models of infection. In this way animal models of infection can be a highly valuable and predictive bridge between in vitro drug discovery and early clinical evaluation.The Gram-positive pathogen Staphylococcus aureus causes a wide variety of infections in humans (Archer, Clin Infect Dis 26:1179-1181, 1998) and has been said to be able to infect every tissue type. Fortunately, over the years a great deal of effort has been expended toward developing infection models in rodents using this organism, with good success. This chapter will describe the advantages, methods, and outcome measurements of the rodent models most used in drug discovery for S. aureus. Mouse models will be the focus of this chapter, as they are the most economical and thus most commonly used, but a rat infection model is included as well.

Post-β-lactamase-inhibitor effect of tazobactam in combination with ceftolozane on extended-spectrum-β-lactamase-producing strains

The post-β-lactamase-inhibitor effect (PBLIE) of tazobactam combined with ceftolozane was evaluated by time-kill assays on two clinical Escherichia coli strains producing CTX-M-15 with or without TEM-1. The organisms were exposed (2 h) to 4 μg/ml/4 μg/ml of ceftolozane-tazobactam (4× MIC), 4 μg/ml of ceftolozane, and medium containing no drug, washed, and resuspended in medium alone or medium containing ceftolozane-tazobactam or ceftolozane. The PBLIE was determined as 1.3 to 2.1 h, and a postantibiotic effect was measured as 0.8 to 0.9 h.

In vivo activities of ceftolozane, a new cephalosporin, with and without tazobactam against Pseudomonas aeruginosa and Enterobacteriaceae, including strains with extended-spectrum β-lactamases, in the thighs of neutropenic mice

Ceftolozane is a new cephalosporin with potent activity against Pseudomonas aeruginosa and Enterobacteriaceae. A neutropenic murine thigh infection model was used to determine which pharmacokinetic/pharmacodynamic index and magnitude drives the efficacy of ceftolozane with Gram-negative bacilli, to compare the rates of in vivo killing of P. aeruginosa by ceftolozane and ceftazidime, and to determine the impact of different ratios of ceftolozane plus tazobactam on Enterobacteriaceae containing extended-spectrum β-lactamases (ESBLs). Neutropenic mice had 10(6.2-7.1) CFU/thigh when treated with ceftolozane for 24 h with (i) various doses (3.12 to 1,600 mg/kg) and dosage intervals (3, 6, 12, and 24 h) against two Enterobacteriaceae strains, (ii) 0.39 to 800 mg/kg every 6 h for four Enterobacteriaceae and four P. aeruginosa strains, and (iii) 400 or 800 mg/kg with 2:1. 4:1, and 8:1 ratios of tazobactam against five Enterobacteriaceae strains with ESBLs. The pharmacokinetics of ceftolozane at 25, 100, and 400 mg/kg were linear with peak/dose values of 1.0 to 1.4 and half-lives of 12 to 14 min. T>MIC was the primary index driving efficacy. For stasis (1 log kill), T>MIC was 26.3% ± 2.1% (31.6% ± 1.6%) for wild-type Enterobacteriaceae, 31.1% ± 4.9% (34.8% ± 4.4%) for Enterobacteriaceae with ESBLs, and 24.0% ± 3.3% (31.5% ± 3.9%) for P. aeruginosa. At 200 mg/kg every 3 h, the rate of in vivo killing of P. aeruginosa was faster with ceftolozane than with ceftazidime (-0.34 to -0.41 log10 CFU/thigh/h versus -0.21 to -0.24 log10 CFU/thigh/h). The 2:1 ratio of ceftolozane with tazobactam was the most potent combination studied. The T>MIC required for ceftolozane is less than with other cephalosporins and may be due to more rapid killing.

What does it take to synergistically combine sub-potent natural products into drug-level potent combinations?

There have been renewed interests in natural products as drug discovery sources. In particular, natural product combinations have been extensively studied, clinically tested, and widely used in traditional, folk and alternative medicines. But opinions about their therapeutic efficacies vary from placebo to synergistic effects. The important questions are whether synergistic effects can sufficiently elevate therapeutic potencies to drug levels, and by what mechanisms and at what odds such combinations can be assembled. We studied these questions by analyzing literature-reported cell-based potencies of 190 approved anticancer and antimicrobial drugs, 1378 anticancer and antimicrobial natural products, 99 natural product extracts, 124 synergistic natural product combinations, and 122 molecular interaction profiles of the 19 natural product combinations with collective potency enhanced to drug level or by >10-fold. Most of the evaluated natural products and combinations are sub-potent to drugs. Sub-potent natural products can be assembled into combinations of drug level potency at low probabilities by distinguished multi-target modes modulating primary targets, their regulators and effectors, and intracellular bioavailability of the active natural products.

In vivo pharmacodynamics of new lipopeptide MX-2401

MX-2401 is a novel lipopeptide (amphomycin analog) with a broad-spectrum bactericidal activity against Gram-positive organisms. We used murine thigh and lung infection models in neutropenic and normal mice to characterize the in vivo pharmacokinetic/pharmacodynamic (PK/PD) activities of MX-2401. The compound (2.5 to 40 mg/kg of body weight) demonstrated linear PK characterized by an area under the concentration-time curve (AUC) of 228 to 3,265 μg·h/ml and half-lives of 5.7 to 8.8 h. MICs ranged from 0.25 to 2 μg/ml. The in vivo postantibiotic effect was prolonged (8.5 h with Staphylococcus aureus and 10.3 to 12.3 with Streptococcus pneumoniae). MX-2401 exhibited dose-dependent in vivo activity against various strains of S. pneumoniae and S. aureus; penicillin and macrolide resistance in the pneumococci and methicillin resistance in the staphylococci had no impact on the antimicrobial activity of the drug. To determine which PK/PD index correlated best with MX-2401 activity, dose fractionation studies over a 72-hour period were performed. The maximum concentration of drug in serum divided by the MIC (C(max)/MIC) correlated best with the efficacy for both S. aureus and S. pneumoniae. Static doses required free-drug C(max)/MIC values of 0.683 to 1.06. Free-drug 72-h AUC/MIC values for the static dose were in the range of 7.49 to 32.3 and were less than expected. The drug showed modest enhancement in activity in the presence of white blood cells (1.7- to 3.4-fold). The potency of the drug in the lung was only marginally lower than in the thigh (1.3- to 1.9-fold). Based on its PK/PD profile, MX-2401 appears to be a promising new lipopeptide agent for treatment of infections by Gram-positive bacteria, including those induced by antibiotic-resistant pathogens.

Chronokinetics of ceftazidime after intramuscular administration to dogs

Ceftazidime, a third-generation cephalosporin, is widely used for the treatment of Pseudomonas aeruginosa infections. The aims of the present study were to characterize the pharmacokinetics of ceftazidime and to estimate the T > MIC against P. aeruginosa, after its intramuscular (i.m.) administration at two different dosing times (08:30 h and 20:30 h) to dogs, in order to determine whether time-of-day administration modifies ceftazidime pharmacokinetics and/or predicted clinical antipseudomonal efficacy. Six female healthy beagle dogs were administered ceftazidime pentahydrate by the intramuscular route in a single dose of 25 mg/kg at both 08:30 and 20:30 h, two weeks apart. Plasma ceftazidime concentrations were determined by microbiological assay. Pharmacokinetic parameters and time above the minimum inhibitory concentration (T > MIC) and 4xMIC for Pseudomonas aeruginosa were calculated from the disposition curve of each dog. No differences between the daytime and nighttime administrations were found for the main pharmacokinetic parameters, including C(max), t(max), t((1/2) lambda), AUC, and MRT; however, the high interindividual variability shown by these values and the small number of individuals may account for this lack of difference. Rate of absorption (k(a)) was significantly higher after the 20:30 h than 08:30 h administration. No significant differences between T > MIC were found when comparing the 08:30 h and 20:30 h administrations. Mean T > MIC values predicted a favorable bacteriostatic effect for all susceptible strains of P. aeruginosa for the 12 h dosing interval at both dosing times. Our results suggest that similar antipseudomonal activity may be expected when ceftazidime is administered at 8:30 and 20:30 h; however, as only two timepoints of drug administration were explored, we are unable to draw any conclusions for other treatment times during the 24 h.

Functional relationship between bacterial cell density and the efficacy of antibiotics

OBJECTIVES

To determine the functional relationship between the density of bacteria and the pharmacodynamics of antibiotics, and the potential consequences of this inoculum effect on the microbiological course of antibiotic treatment of Staphylococcus aureus infections.

METHODS

In vitro time-kill, MIC estimation and antibiotic bioassay experiments were performed with S. aureus ATCC 25923 to ascertain the functional relationship between rates of kill and the MICs of six classes of antibiotics and the density of bacteria exposed. The potential consequences of the observed inoculum effects on the microbiological course of antibiotic treatment are explored with a mathematical model.

RESULTS

Modest or substantial inoculum effects on efficacy were observed for all six antibiotics studied, such as density-dependent declines in the rate and extent of antibiotic-mediated killing and increases in MIC. Although these measures of antibiotic efficacy declined with inoculum, this density effect did not increase monotonically. At higher densities, the rate of kill of ciprofloxacin and oxacillin declined with the antibiotic concentration. For daptomycin and vancomycin, much of this inoculum effect is due to density-dependent reductions in the effective concentration of the antibiotic. For the other four antibiotics, this density effect is primarily associated with a decrease in per-cell antibiotic concentration. With parameters in the range estimated, our mathematical model predicts that the course of antibiotic treatment can be affected by cell density; treatment protocols based on conventional (density-independent) MICs can fail to clear higher density infections.

CONCLUSIONS

The MICs used for pharmacokinetic/pharmacodynamic indices should be functions of the anticipated densities of the infecting population.

Pharmacodynamics of doxycycline for chemoprophylaxis of Lyme disease: preliminary findings and possible implications for other antimicrobials

The purpose of this study was to begin to characterise the pharmacodynamic parameters of single-dose doxycycline for the prevention of Lyme disease following a tick bite. Based on limited data from published human and murine studies, it was found that there is a direct correlation between efficacy rate and the area under the time-concentration of free antibiotic curve divided by the minimum inhibitory concentration (fAUC/MIC) (R(2)=0.74, using Pearson correlation), but not the maximum concentration of free drug in serum divided by the MIC (fC(max)/MIC) or the time that the free drug concentration remains above the MIC (fT>MIC). To determine the possible implications of these findings for other antimicrobials, it was assumed that the pharmacodynamic properties of doxycycline would be pertinent to azithromycin, an antibiotic whose activity is known to correlate with AUC/MIC. By making such an extrapolation and using pharmacokinetic modelling with conservative assumptions on MIC values against Borrelia burgdorferi, it is hypothesised that a single 500 mg dose of azithromycin in humans should have comparable efficacy to doxycycline for the prevention of Lyme disease. Additional experimental studies are needed to clarify more precisely the pharmacodynamic properties of doxycycline and to validate the accuracy of this hypothesis.

Evaluating Empiric Treatment Options for Secondary Peritonitis Using Pharmacodynamic Profiling

BACKGROUND

Selecting an appropriate agent for empiric antibiotic therapy for secondary peritonitis is challenging. The pathogens responsible, aerobic gram-negative bacilli in particular, are becoming more resistant to antibiotics. The purpose of this study was to predict the ability of common antimicrobial regimens to achieve optimal pharmacodynamic exposure against aerobic bacteria implicated in secondary peritonitis, while considering current national resistance trends.

METHODS

Monte Carlo simulation was used to model pharmacodynamic endpoints and compare the cumulative fraction of response (CFR) for imipenem-cilastatin, meropenem, ertapenem, piperacillin/tazobactam, ceftazidime, ceftriaxone, ciprofloxacin, and levofloxacin against isolates of species associated with secondary peritonitis. Minimum inhibitory concentration (MIC) distributions for isolates collected in North America were obtained from the 2004 MYSTIC database. Pharmacokinetic parameters were derived from the literature; the endpoints evaluated included free drug time above the MIC (fT(>MIC)) and the area under the concentration-time curve to MIC ratio (AUC:MIC).

RESULTS

The simulation predicted that several compounds would have a superior probability of providing appropriate coverage of aerobic bacteria: Imipenem-cilastatin (98.6% CFR at 1 g q8h), meropenem (98.2% CFR at 1 g q8h), ertapenem (91.7% CFR at 1 g q24h), piperacillin/ tazobactam (93.7% CFR at 3.375 g q6h), ceftazidime (91.1% CFR at 2 g q8h), and cefepime (92.9% CFR at 1 g q12h and 95.8% CFR at 2 g q12h). Ceftriaxone, ciprofloxacin, and levofloxacin exhibited CFRs < 82%.

CONCLUSIONS

Considering contemporary susceptibility data for aerobic bacteria, monotherapy with any of the three carbapenems or piperacillin/tazobactam 3.375 g q6h would provide optimal exposure for the pathogens commonly encountered in secondary peritonitis. Cefepime (in combination with metronidazole to provide anti-anaerobic coverage) also would be an acceptable choice, as would ceftazidime given at 2 g q8h (again in combination with metronidazole). Despite the popularity of combination therapy based on ciprofloxacin, levofloxacin, or ceftriaxone with metronidazole, these choices appear to be inferior to the other options because of emerging antibiotic resistance, particularly in E. coli.

Activity of imipenem against VIM-1 metallo-β-lactamase-producing Klebsiella pneumoniae in the murine thigh infection model

The in-vivo activity of imipenem against VIM-1-producing Klebsiella pneumoniae (VPKP) was assessed in a thigh infection model in neutropenic mice. Animals were infected with three VPKP isolates (imipenem MICs 2, 4 and 32 mg/L, respectively) and a susceptible clinical isolate (MIC 0.125 mg/L) that did not produce any beta-lactamase with broad-spectrum activity. Bacterial density at the site of infection was determined after imipenem treatment (30 and 60 mg/kg every 2 h for 24 h). The log(10) reduction in CFU/thigh was greatest for the wild-type isolate, intermediate for the two imipenem-susceptible VPKP isolates, and lowest for the imipenem-resistant VPKP isolate. Whilst in-vivo imipenem activity appeared reduced against in-vitro susceptible VIM-1 producers compared with a VIM-1-negative control, an increased drug dosage could moderate this reduction.

Population pharmacokinetic model for gatifloxacin in pediatric patients

The broad spectrum of antimicrobial activity, oral bioavailability, extensive tissue distribution, and once-daily intravenous or oral dosing of gatifloxacin, an expanded-spectrum 8-methoxy fluoroquinolone, make it a potentially useful agent for the treatment of pediatric infections. A population pharmacokinetic model was developed to describe the pharmacokinetics of gatifloxacin in children. Data for analysis were obtained from a single-dose safety/pharmacokinetic study utilizing intensive blood sampling in patients aged 6 months to 16 years. Each subject received a single oral dose of gatifloxacin as a suspension, at doses of 5, 10, or 15 mg/kg of body weight. A total of 845 samples were obtained from 82 patients. A one-compartment model with first-order absorption and elimination was the most appropriate to describe the gatifloxacin concentrations. Covariate analysis using forward selection and backward elimination found that apparent clearance was related to body surface area, and apparent volume of distribution was related to body weight. No effect of age on drug clearance could be identified once clearance was corrected for body surface area. Based on pharmacokinetic simulations, the 10-mg/kg (maximum, 400 mg) once-daily dose of gatifloxacin is expected to provide drug exposure similar to that in healthy adults. The population pharmacokinetic model described herein will be used for Bayesian analyses of sparse pharmacokinetic sampling in phase II/III clinical trials and for Monte Carlo simulation experiments. The success of this strategy provides a model for future pediatric drug development programs.

Trough serum concentrations of β-lactam antibiotics in cancer patients: inappropriateness of conventional schedules to pharmacokinetic/pharmacodynamic properties of β-lactams

Serum concentrations of beta-lactams that continuously exceed the minimum inhibitory concentration may improve therapeutic outcomes for immunosuppressed patients. The trough serum levels of ceftazidime (CAZ), cefepime (FEP) or imipenem (IMP) were prospectively determined on days 1 and 3 of treatment in cancer patients. Seventy-eight episodes of suspected infection were analysed. Trough serum levels were higher than 4 mg/L in 62%, 24% and 0% of cases in the CAZ, FEP and IMP groups, respectively, and were higher than 20 mg/L in 24% of cases in the CAZ group compared with 0% both in the FEP and IMP groups. For suspected infectious episodes in cancer patients, the traditional intermittent regimen of beta-lactams does not appear to be appropriate for the pharmacokinetic/pharmacodynamic properties of these antibiotics.

Proof of concept: performance testing in models

Pharmacokinetic (PK) and pharmacodynamic (PD) principles that predict antimicrobial efficacy can be used to set targets for antimicrobial design and optimisation. Although current formulations of amoxicillin and amoxicillin/clavulanate have retained their efficacy against many, but not all, penicillin-nonsusceptible Streptococcus pneumoniae, additional coverage is required to address the growing problem of drug-resistant strains. Accordingly, two new oral formulations of amoxicillin/clavulanate, a paediatric formulation at 90/6.4 mg/kg/day and a pharmacokinetically enhanced formulation at 2000/125 mg twice daily for adults, were designed using PK/PD principles. These principles indicate that for amoxicillin and amoxicillin/clavulanate, a time above MIC of 35-40% of the dosing interval is predictive of high bacterial efficacy. In line with PK/PD predictions, simulation of human pharmacokinetics in in-vitro kinetic models and in a rat model of pneumonia, amoxicillin/clavulanate 2000/125 mg twice daily was highly effective against S. pneumoniae strains with amoxicillin MICs of 4 or 8 mg/L. Against strains with amoxicillin MICs of 4 mg/L, amoxicillin/clavulanate 2000/125 mg twice daily was significantly more effective than the conventional 875/125 mg twice daily formulation, azithromycin and levofloxacin, even though all levofloxacin MICs were < or = 1 mg/L. Following infection with S. pneumoniae strains with amoxicillin MICs of 8 mg/L, the amoxicillin/clavulanate 2000/125 mg twice daily formulation was more effective than the conventional amoxicillin/clavulanate formulations of 875/125 mg twice daily and three times daily and 1000/125 mg three times daily, and had similar or better efficacy than azithromycin and levofloxacin, depending on the strain. These data indicate the potential benefit of therapy with amoxicillin/clavulanate 2000/125 mg twice daily compared with conventional formulations and other marketed antimicrobials in the treatment of respiratory tract infection.

Optimization of dose and dose regimen of biapenem based on pharmacokinetic and pharmacodynamic analysis

Pharmacokinetic and pharmacodynamic (PK/PD) parameters, which are important indices of the therapeutic efficacy of antimicrobials, and the minimum inhibitory concentration (MIC) predictive of clinical efficacy at common clinical doses, were examined for biapenem (BIPM; 300 mg b.i.d.), imipenem/cilastatin (IPM/CS; 500 mg/500 mg b.i.d.), meropenem (MEPM; 500 mg b.i.d.), and ceftazidime (CAZ; 1000 mg b.i.d.), using a mouse model of thigh infection caused by Pseudomonas aeruginosa. The PK/PD parameter that most closely correlated with the therapeutic efficacy of all these antimicrobials was time above MIC (T > MIC). The values of T > MIC predictive of clinical efficacy against P. aeruginosa infection varied among antimicrobials and were >/=17%, >/=17%, >/=23%, and >/=33% for BIPM, IPM/CS, MEPM, and CAZ, respectively. From these values and the known plasma concentrations of the antimicrobials in humans after administration at the common clinical doses, the MIC for bacterial strains at which clinical efficacy can be expected was estimated to be </=4.4 microg/ml for BIPM, </=6.1 microg/ml for IPM/CS, </=2.2 microg/ml for MEPM, and </=13.6 microg/ml for CAZ. These MICs nearly coincided with the MIC(80) of the antimicrobials for 104 clinical isolates of P. aeruginosa strains. These results indicate that, even at a low dose, of 300 mg b.i.d., the clinical efficacy of BIPM against P. aeruginosa infection can be expected to be comparable to that of IPM/CS, MEPM, and CAZ.

Basic pharmacodynamics of antibacterials with clinical applications to the use of β-lactams, glycopeptides, and linezolid

Time above MIC for free drug concentrations is the important PK-PD parameter correlating with the efficacy of beta-lactam antibiotics. The duration of time plasma concentrations needed to exceed the MIC is relatively similar for most organisms except staphylococci. Neutrophils contribute very little to the overall activity of beta-lactams. The appearance of increasing antimicrobial resistance can challenge the efficacy of these drugs when concentrations do not exceed the MIC for 40% to 50% of the dosing interval. Time above MIC with oral amoxicillin and amoxicillin-clavulanate can be enhanced with high-dose formulations. Time above MIC with parenteral preparations can be enhanced by longer intravenous infusions or even continuous infusion. The 24-hour AUC-MIC is probably the important PK-PD parameter correlating with the efficacy of vancomycin and teicoplanin. It clearly is the important parameter for the efficacy of linezolid. Usual doses of these drugs generally provide adequate plasma concentrations to treat effectively infections in which plasma concentrations are predictive of tissue concentrations. Penetration of these drugs into respiratory secretions, such as ELF, is enhanced for linezolid and reduced for vancomycin. This may give linezolid an advantage over vancomycin in certain respiratory infections.

Effects of sulfamethizole and amdinocillin against Escherichia coli strains (with various susceptibilities) in an ascending urinary tract infection mouse model

Resistance to antibiotics used for the treatment of urinary tract infections (UTIs) is increasing worldwide. The impact of in vitro resistance on clinical outcome in UTIs requires further study, since most studies of both humans and animals have evaluated only the efficacy of antibiotics toward bacteria susceptible in vitro. We were interested in evaluating the relationship between the in vitro antibacterial effect and the in vivo efficacy after antibiotic treatment. We simulated a natural ascending UTI by use of the ascending UTI mouse model and used Escherichia coli strains with various susceptibilities to amdinocillin (mecillinam) and sulfamethizole. Mice were treated for 3 days with antibiotic doses approximating human urinary tract concentrations after a standard oral dose. For a susceptible strain (MIC, 0.5 micro g/ml) and a resistant strain (MIC, 128 micro g/ml), respectively, there were significant reductions in bacterial counts in the urine, bladder, and kidneys after treatment with amdinocillin, whereas for a strain for which the MIC was 16 micro g/ml, there was a significant reduction in bacterial counts in the kidneys only (P < 0.05). Treatment with sulfamethizole resulted in a significant reduction in bacterial counts in all samples from a susceptible strain (MIC, 128 micro g/ml) and a resistant strain (MIC, 512 micro g/ml). Infection with a sulII gene-positive strain (MIC, >2,048 micro g/ml) could not be treated with sulfamethizole, as no effect could be demonstrated in the urine, bladder, or kidneys. For amdinocillin, there was no clear-cut relationship between the in vitro susceptibility and the in vivo outcome, while for sulfamethizole, we found a relationship between the MIC for the strain and the effect in the urinary tract.

What do we really know about antibiotic pharmacodynamics?

Antibiotic pharmacodynamics is an evolving science that focuses on the relationship between drug concentration and pharmacologic effect, which is an antibiotic-induced bacterial death that also can manifest as an adverse drug reaction. The pharmacologic action of antibiotics usually can be described as concentration dependent or independent, although such classifications are highly reliant on the specific antibiotic and bacterial pathogen being studied. Quantitative pharmacodynamic parameters, such as ratio of the area under the concentration-time curve during a 24-hour dosing period to minimum inhibitory concentration (AUC0-24:MIC), ratio of maximum serum antibiotic concentration to MIC (Cmax:MIC), and duration of time that antibiotic concentrations exceed MIC (T>MIC), have been proposed as likely predictors of clinical and microbiologic success or failure for different pairings of antibiotic and bacteria. Thus far, most pharmacodynamic data reported have focused on fluoroquinolones, but work has been conducted on vancomycin, beta-lactams, macrolides, aminoglycosides, and other antibiotics. Despite the development of a number of different pharmacodynamic modeling systems, remarkable agreement exists between in vitro, animal, and limited human data. Although still somewhat premature and requiring additional clinical validation, antibiotic pharmacodynamics will likely advance on four fronts: the science should prove to be extremely useful and represent a cost-effective and efficient method to help develop new antibiotics; formulary committees will likely use pharmacodynamic parameters to assist in differentiating antibiotics of the same chemical class in making antibiotic formulary selections; pharmacodynamic principles will likely be used to design optimal antibiotic strategies for patients with severe infections; and limited data to date suggest that the application of pharmacodynamic concepts may limit or prevent the development of antibiotic resistance. The study of antibiotic pharmacodynamics appears to hold great promise and will likely become a routine part of our daily clinical practices.

Pharmacodynamics of fluoroquinolones against Streptococcus pneumoniae in patients with community-acquired respiratory tract infections

Fluoroquinolone antibiotic agents have demonstrated efficacy in the treatment of respiratory tract infections. This analysis was designed to examine the relationship between drug exposure, as measured by the free-drug area under the concentration-time curve at 24 h (AUC(24))/MIC ratio, and clinical and microbiological responses in patients with community-acquired respiratory tract infections involving Streptococcus pneumoniae. The study population included 58 adult patients (34 males, 24 females) who were enrolled in either of two phase III, randomized, multicenter, double-blind studies of levofloxacin versus gatifloxacin for the treatment of community-acquired pneumonia or acute exacerbation of chronic bronchitis. Clearance equations from previously published population pharmacokinetic models were used in conjunction with dose and adjusted for protein binding to estimate individual patient free-drug AUC(24)s. In vitro susceptibility was determined in a central laboratory by broth microdilution in accordance with NCCLS guidelines. Pharmacodynamic analyses were performed on data from all evaluable patients with documented S. pneumoniae infection using univariate and multivariable logistic regression; pharmacodynamic breakpoints were estimated using Classification and Regression Tree analysis. A statistically significant (P = 0.013) relationship between microbiological response and the free-drug AUC(24)/MIC ratio was detected. At a free-drug AUC(24)/MIC ratio of <33.7, the probability of a microbiological response was 64%, and at a free-drug AUC(24)/MIC ratio of >33.7, it was 100% (P < 0.01). These findings may provide a minimum target free-drug AUC(24)/MIC ratio for the treatment of infections involving S. pneumoniae with fluoroquinolone antibiotics and provide a paradigm for the selection of fluoroquinolones to be brought forward from drug discovery into clinical development and dose selection for clinical trials. Further, when target free-drug AUC(24)/MIC ratios are used in conjunction with stochastic modeling techniques, these findings may be used to support susceptibility breakpoints for fluoroquinolone antibiotics and S. pneumoniae.

Animal pharmacokinetics and interspecies scaling of sordarin derivatives following intravenous administration

Sordarin derivatives constitute a new group of synthetic antifungal agents that selectively inhibit fungal protein synthesis. They have demonstrated in vitro activity against the most important fungal pathogens, both yeast and filamentous. This new family of compounds has also shown in vivo activity against murine Candida albicans, Histoplasma capsulatum, and Coccidioides immitis experimental infections, as well as against Pneumocystis carinii pneumonia in rats. After intravenous dosing in animals, both the area under the concentration-time curve and the elimination half-life were highest in Cynomolgus monkeys, followed by those in rats, mice, and rabbits. The volume of distribution at steady state for sordarin derivatives was similar in all species tested. The clearance in rats and mice was higher than for other species. GM 237354, a sordarin derivative, was characterized by high serum protein binding in mouse, rat, and monkey serum (unbound fraction, < or =5%). An indirect evaluation of the effect of liver function upon the metabolism of this class of compounds has been made in animals with impaired liver function such as Gunn rats, as well as in allometric studies that showed better correlations of half-life to liver blood flow than to animal body weight. Linearity of the main pharmacokinetic parameters was demonstrated after intravenous dosing of the representative compound GM 193663 at 10 and 20 mg/kg of body weight in rats. Allometry was used to determine whether human pharmacokinetic parameters can be predicted from animal data by regression analysis against body weight and liver blood flow. All these results have demonstrated that the human pharmacokinetics of sordarin derivatives can be forecast from animal data.

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.

Pharmacodynamic effects of subinhibitory antibiotic concentrations

The pharmacodynamics of antibiotics have become increasingly important for the determination of optimal dosing regimens. Studies over the past decade have demonstrated marked differences in the time course of antimicrobial activity for different classes of antibiotics both in vitro, in animals and in human trials. One of the explanations for the success of intermittent dosing regimens has been the delay in regrowth after the concentration has fallen under the MIC, the so called postantibiotic effect (PAE). In addition to the PAE, the success of discontinuous dosing regimens may be attributed to both the function of a normal host defence and to the effects of subinhibitory antibiotic concentrations (sub-MICs). It has been shown that there is a difference between the effects of sub-MICs following a suprainhibitory dose (postantibiotic sub-MIC effect; PA SME) and the effects of sub-MICs (SME) alone. It seems that the PA SME is more clinically relevant compared with the PAE, since exposure to suprainhibitory concentrations will always be followed by sub-MICs in vivo. A long PA SME could indicate that longer dosing intervals may be used for that antibiotic /bacterial combination and together with the known effects of sub-MICs on bacterial virulence and the influence of the immune system, it may explain the efficacy of antibiotics with short half-lives even of they are given infrequently.

Use of preclinical data for selection of a phase II/III dose for evernimicin and identification of a preclinical MIC breakpoint

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.

Pharmacodynamics in the study of drug resistance and establishing in vitro susceptibility breakpoints: ready for prime time

Considerable advancements have been made in providing informative, relevant interpretations of the results of antimicrobial susceptibility tests to clinicians, clinical microbiologists, epidemiologists, and researchers. At the same time, the science of pharmacokinetics has flourished, and the importance of drug exposure in vivo on outcome is now recognized by researchers and clinicians alike. More recently, pharmacokinetic and quantitative measures of antimicrobial susceptibility have been integrated using pharmacokinetic-pharmacodynamic (PK-PD) models to forecast clinical and microbiological outcomes. Stochastic methods utilizing patient population pharmacokinetics, target organism minimum inhibitory concentration (MIC) distributions, and PK-PD targets from non-clinical models of infection or clinical data have established a new paradigm for determining in vitro susceptibility breakpoints and selection of empirical therapy in clinical practice. Given the increasing problem of antimicrobial resistance, these new tools are valuable additions for clinicians, researchers, and regulatory authorities.

Pharmacokinetics-pharmacodynamics of a sordarin derivative (GM 237354) in a murine model of lethal candidiasis

Sordarins are a new class of antifungal agents which selectively inhibit fungal protein synthesis (FPS) by impairing the function of elongation factor 2. The present study investigates possible correlations between sordarin pharmacokinetic (PK) properties and therapeutic efficacy, based on a murine model of invasive systemic candidiasis, and provides a rationale for dose selection in the first study of efficacy in humans. A significant correlation between PK parameters and the in vivo activity of GM 237354, taken as a representative FPS inhibitor, was demonstrated in a murine model of lethal systemic candidiasis. Area under the concentration-time curve (AUC) and maximum concentration of drug in serum (C(max)) over 24 h were determined after a single GM 237354 subcutaneous (s.c.) dose (50 mg/kg of body weight) in healthy animals (no significant PK changes with infection were observed for other sordarin derivatives). These results have been used to simulate PK profiles obtained after several doses and/or schedules in animal therapy. A PK-pharmacodynamic (PD) parameter such as the time that serum drug concentrations remain above the MIC (t > MIC) was also determined. Treatment efficacies were evaluated in terms of the area under the survival time curve (AUSTC), using Kaplan-Meier survival analysis and in terms of kidney fungal burden (log CFU/gram) after s.c. doses of 2.5, 5, 10, 20, and 40 mg/kg every 4, 8, or 12 h (corresponding to total daily doses of 5 to 240 mg/kg). The results show all treatments to significantly prolong survival versus that of infected and nontreated controls (P < 0.05). Relationships between simulated PK and PK-PD parameters and efficacy were explored. A good correlation independent of the dosing interval was observed with AUC (but not C(max) or t > MIC) and both AUSTC and kidney burden. Following repeated dosing every 8 h, AUC(50) (AUC at which 50% of the maximum therapeutic efficacy is obtained) was estimated as 21.7 and 37.1 microg. h/ml (total concentrations) for AUSTC and kidney burden using a sigmoid E(max) and an inhibitory sigmoid E(max) PK-PD model, respectively. For an efficacy target of 90% survival, AUC was predicted as 67 microg. h/ml. We conclude that the PK-PD approach is useful for evaluating relationships between PK parameters and efficacy in antifungal research. Moreover, the results obtained with this approach could be successfully applied to clinical studies.

Comparison of in-vitro pharmacodynamics of once and twice daily ciprofloxacin

The pharmacodynamics of ciprofloxacin were explored in an in-vitro continuous bacterial culture model of infection, by simulating two oral dosing regimens; 0.5 g 12-hourly (bd) and 1 g 24-hourly (od). Three strains of Escherichia coli (ciprofloxacin MICs 0.03, 0.5 and 2 mg/L); two strains of Pseudomonas aeruginosa (MICs 0.09 and 1.5 mg/L), two strains of Staphylococcus aureus (MICs 0.12 and 1 mg/L) and two strains of Streptococcus pneumoniae (MICs 0.5 and 2 mg/L) were used. Three pharmacodynamic parameters, T > MIC, C(max)/MIC and AUC/MIC (T = time, C(max) = peak serum concentration, AUC = area under the curve), were compared with area under the bacterial-kill curve (AUBKC) (after transformation of the AUBKC) using a simple E(max) or sigmoidal E(max) model. AUBKC was taken to be the main antibacterial effect measure. The models were compared by inspection of residuals and Akaike information criterion. E(max) models adequately described the relationship between AUC/MIC and AUBKC and between C(max)/MIC and AUBKC, but not between T> MIC and AUBKC. All three pharmacodynamic parameters are related to each other but multiple regression analysis indicated that AUC/MIC was the best individual predictor of AUBKC. Despite this, comparison of od and bd regimens indicates some advantage to od in terms of early antibacterial effect. Serum concentration-time curve shape has some importance in determining antibacterial effect. These data indicate that for ciprofloxacin AUC/MIC ratio is not the sole determinant of antibacterial effect.

Protective effect of trovafloxacin, ciprofloxacin and ampicillin against Streptococcus pneumoniae in a murine sepsis model

Trovafloxacin is a new fluoroquinolone that has potent microbiological activity against the pneumococcus, including penicillin-resistant strains. To evaluate the protective effect of trovafloxacin, ciprofloxacin and ampicillin against penicillin-susceptible, -intermediate and -resistant strains of Streptococcus pneumoniae, an intraperitoneal, immunocompetent mouse model of sepsis was used. The minimum lethal dose (MLD) for each isolate was determined in duplicate. A single sc dose of each antibiotic was administered over a wide range of doses 1 h after the ip inoculation of the test isolate at the MLD. The assessment of the protective dose for 50% of the population (PD50) for each antimicrobial/bacteria combination was performed in triplicate and the PD50 value was calculated at the end of 5 days. Results showed that trovafloxacin provided PD50 values that were significantly lower than those of ciprofloxacin for all isolates. For the penicillin-susceptible and -intermediate isolates, the PD50 values of ampicillin were significantly lower than those for either of the fluoroquinolones studied; however, trovafloxacin was statistically superior to both ciprofloxacin and ampicillin against the penicillin-resistant strain. Therefore, regardless of penicillin susceptibility, trovafloxacin has potent activity against Streptococcus pneumoniae and may be a viable alternative for the treatment of penicillin-resistant isolates.

In vitro and in vivo antibacterial activities of CS-834, a new oral carbapenem

CS-834 is a prodrug of the carbapenem R-95867, developed by Sankyo Co., Ltd., Tokyo, Japan. To investigate the possibility that CS-834 may be the first carbapenem usable in an oral dosage form, its in vitro antibacterial activity (as R-95867) and in vivo antibacterial activity were compared with those of cefpodoxime proxetil, cefditoren pivoxil, cefdinir, ofloxacin, imipenem, and amoxicillin. R-95867 had high levels of activity against methicillin-susceptible staphylococci and streptococci, including penicillin-resistant Streptococcus pneumoniae, as well as Neisseria gonorrhoeae, Moraxella catarrhalis, the members of the family Enterobacteriaceae (with the exception of Serratia marcescens), Haemophilus influenzae, and Bordetella pertussis; for all these strains, the MICs at which 90% of tested strains are inhibited (MIC90s) were 1.0 microg/ml or less. Against methicillin-resistant staphylococci, enterococci, Serratia marcescens, Burkholderia cepacia, Stenotrophomonas maltophilia, and Acinetobacter calcoaceticus, R-95867 showed activity comparable to or slightly less than that of imipenem, with MIC90s ranging from 2 to >128 microg/ml. The in vivo efficacy of oral CS-834 against experimental mouse septicemia caused by gram-positive and gram-negative bacteria was better than that of comparative drugs. In murine respiratory infection models, the efficacy of CS-834 reflected not only its potent in vitro activity but also the high levels present in the lungs.