Pharmacodynamics of gatifloxacin against Streptococcus pneumoniae in an in vitro pharmacokinetic model: impact of area under the curve/MIC ratios on eradication

PK/PD Analysis by Nonlinear Mixed-Effects Modeling of a Marbofloxacin Dose Regimen for Treatment of Goat Mastitis Produced by Coagulase-Negative Staphylococci

Simple Summary

Coagulase-negative staphylococci are main pathogens that produce goat mastitis. Marbofloxacin is a third-generation fluoroquinolone approved to treat mastitis in animals. Since the efficacy of an antimicrobial is related with its concentration in the site of infection, and the latter depends of dose and biological processes that determine the distribution of the antimicrobial in different tissues and secretions, the objectives of this study were to evaluate the efficacy of a dose regimen of marbofloxacin (10 mg/kg/24 h) administered intramuscularly for five days in goats with mastitis induced by coagulase-negative staphylococci, by an evaluation of the concentrations of marbofloxacin achieved in blood and milk over time (called pharmacokinetics), and characterizing the concentration–effect relationship of marbofloxacin against coagulase-negative staphylococci in Mueller Hinton broth and goat milk, by time kill assays, in order to determine the concentrations of marbofloxacin related with an adequate bacterial count reduction (measured by efficacy index AUC/MIC). The proposed dose regimen was adequate for the treatment of goat mastitis produced by coagulase-negative staphylococci, resulting in a microbiological and clinical cure of all animals. The animal model used in this study provided important pharmacokinetic information about the effect of the infection on the pharmacokinetics of marbofloxacin. Pharmacodynamic modeling showed that marbofloxacin concentrations needed for antimicrobial efficacy were higher in goat milk compared with Mueller Hinton broth. Bacterial resistance to antimicrobials is a serious problem, since marbofloxacin is considered a critically important antimicrobial, and its rational and prudent use could extend its utility over time.

Abstract

Coagulase-negative staphylococci are main pathogens that produce goat mastitis. Marbofloxacin is a third-generation fluoroquinolone approved for treat mastitis in animals. The objectives of this study were: (i) to determine the pharmacokinetics of marbofloxacin (10 mg/kg/24 h) in serum and milk administered intramuscularly for five days in goats with mastitis induced by coagulase-negative staphylococci; (ii) to characterize the concentration–effect relationship of marbofloxacin against coagulase-negative staphylococci in Mueller Hinton broth and goat milk; (iii) to determine AUC/MIC cutoff values of marbofloxacin, and (iv) to perform a PK/PD analysis to evaluate the efficacy of the dose regimen for the treatment of goat mastitis produced by coagulase-negative staphylococci. Marbofloxacin presented context-sensitive pharmacokinetics, influenced by the evolution of the disease, which decreased marbofloxacin disposition in serum and milk. Marbofloxacin showed a median (95% CI) fAUC/MIC values for MIC of 0.4 and 0.8 µg/mL of 26.66 (22.26–36.64) and 32.28 (26.57–48.35) related with −2 log₁₀CFU/mL reduction; and 32.26 (24.81–81.50) and 41.39 (29.38–128.01) for −3 log₁₀CFU/mL reduction in Mueller Hinton broth. For milk, −2 log₁₀CFU/mL reduction was achieved with 41.48 (35.29–58.73) and 51.91 (39.09–131.63), and −3 log₁₀CFU/mL reduction with 51.04 (41.6–82.1) and 65.65 (46.68–210.16). The proposed dose regimen was adequate for the treatment of goat mastitis produced by coagulase-negative staphylococci, resulting in microbiological and clinical cure of all animals. The animal model used in this study provided important pharmacokinetic information about the effect of the infection on the pharmacokinetics of marbofloxacin. Pharmacodynamic modeling showed that fAUC/MIC cutoff values were higher in goat milk compared with Mueller Hinton broth.

Combatting the Rising Tide of Antimicrobial Resistance: Pharmacokinetic/Pharmacodynamic Dosing Strategies for Maximal Precision

OBJECTIVE

Antimicrobial pharmacokinetics/pharmacodynamics (PK/PD) principles and PK/PD models have been essential in characterizing the mechanism of antibiotic bacterial killing and determining the most optimal dosing regimen that maximizes clinical outcomes. This review summarized the fundamentals of antimicrobial PK/PD and the various types of PK/PD experiments that shaped the utilization and dosing strategies of antibiotics today.

METHODS

Multiple databases - including PubMed, Scopus, and EMBASE - were searched for published articles that involved PK/PD modelling and precision dosing. Data from in vitro, in vivo and mechanistic PK/PD models were reviewed as a basis for compiling studies that guide dosing regimens used in clinical trials.

RESULTS

Literature regarding the utilization of exposure-response analyses, mathematical modelling and simulations that were summarized are able to provide a better understanding of antibiotic pharmacodynamics that influence translational drug development. Optimal pharmacokinetic sampling of antibiotics from patients can lead to personalized dosing regimens that attain target concentrations while minimizing toxicity. Thus the development of a fully integrated mechanistic model based on systems pharmacology can continually adapt to data generated from clinical responses, which can provide the framework for individualized dosing regimens.

CONCLUSIONS

The promise of what PK/PD can provide through precision dosing for antibiotics has not been fully realized in the clinical setting. Antimicrobial resistance, which has emerged as a significant public health threat, has forced clinicians to empirically utilize therapies. Future research focused on implementation and translation of PK/PD-based approaches integrating novel approaches that combine knowledge of combination therapies, systems pharmacology and resistance mechanisms are necessary. To fully realize maximally precise therapeutics, optimal PK/PD strategies are critical to maximize antimicrobial efficacy against extremely-drug-resistant organisms, while minimizing toxicity.

Pharmacokinetics, milk penetration and PK/PD analysis by Monte Carlo simulation of marbofloxacin, after intravenous and intramuscular administration to lactating goats

The main objectives of this study were (i) to evaluate the serum pharmacokinetic behaviour and milk penetration of marbofloxacin (MFX; 5 mg/kg), after intravenous (IV) and intramuscular (IM) administration in lactating goats and simulate a multidose regimen on steady-state conditions, (ii) to determine the minimum inhibitory concentration (MIC) and mutant prevention concentration (MPC) of coagulase negative staphylococci (CNS) isolated from caprine mastitis in Córdoba, Argentina and (iii) to make a PK/PD analysis by Monte Carlo simulation from steady-state pharmacokinetic parameters of MFX by IV and IM routes to evaluate the efficacy and risk of the emergence of resistance. The study was carried out with six healthy, female, adult Anglo Nubian lactating goats. Marbofloxacin was administered at 5 mg/kg bw by IV and IM route. Serum and milk concentrations of MFX were determined with HPLC/uv. From 106 regional strains of CNS isolated from caprine mastitis in herds from Córdoba, Argentina, MICs and MPCs were determined. MIC₉₀ and MPC₉₀ were 0.4 and 6.4 μg/ml, respectively. MIC and MPC-based PK/PD analysis by Monte Carlo simulation indicates that IV and IM administration of MFX in lactating goats may not be adequate to recommend it as an empirical therapy against CNS, because the most exigent endpoints were not reached. Moreover, this dose regimen could increase the probability of selecting mutants and resulting in emergence of resistance. Based on the results of Monte Carlo simulation, the optimal dose of MFX to achieve an adequate antimicrobial efficacy should be 10 mg/kg, but it is important take into account that fluoroquinolones are substrates of efflux pumps, and this fact may determine that assumption of linear pharmacokinetics at high doses of MFX may be incorrect.

Pharmacokinetics and pharmacodynamics of levofloxacin in critically ill patients with ventilator-associated pneumonia

The pharmacokinetics of levofloxacin and outcome of levofloxacin therapy in critically ill patients with ventilator-associated pneumonia (VAP) were assessed. Further theoretical considerations regarding the pharmacokinetic/pharmacodynamic (PK/PD) appropriateness of levofloxacin therapy were made. Twelve patients completed the study, all of whom were treated with a standard intravenous levofloxacin regimen (2x500 mg on Day 1, then 1x500 mg daily). The maximum free plasma levofloxacin concentration (fC(max,ss)) and the area under the free concentration-time curve (fAUC) were 8.13+/-1.64 mg/L and 49.63+/-15.60 mgh/L, respectively. Optimal PK/PD target parameters were achieved in 10 patients; clinical success was attained in 11 of the 12 patients who completed the study. Bacterial eradication was obtained in 9 of the 11 cases with microbiologically confirmed bacteriological aetiology. Intravenous levofloxacin therapy (500 mg/day) was proven to be an effective regimen in this limited number of patients with VAP. However, theoretical considerations based on PK/PD indices predict that, with the current susceptibility breakpoint of 2mg/L, even higher levofloxacin doses (e.g. 1000 mg) could result in treatment failures in infections caused by pathogens labelled as levofloxacin-susceptible in the microbiology report.

Pharmacodynamic assessment based on mutant prevention concentrations of fluoroquinolones to prevent the emergence of resistant mutants of Streptococcus pneumoniae

The objective of this study was to investigate the relationship between pharmacokinetic and pharmacodynamic parameters, on the basis of the mutant prevention concentration (MPC) concept, and the emergence of resistant mutants of Streptococcus pneumoniae to fluoroquinolone antibacterials. Some clinical isolates with various MIC and MPC values of moxifloxacin and levofloxacin were exposed under conditions simulating the time-concentration curves observed when moxifloxacin (400 or 80 mg, once a day) or levofloxacin (200 mg, twice a day) was orally administered by using an in vitro pharmacodynamic model. The decrease in susceptibility was evaluated by altering the population analysis profiles after moxifloxacin or levofloxacin treatment for 72 h. When the area under the concentration-time curve from 0 to 24 h (AUC(0-24))/MPC and peak concentration (C(max))/MPC were above 13.41 and 1.20, respectively, complete eradication occurred and no decrease in susceptibility was observed. On the other hand, when AUC(0-24)/MPC and C(max)/MPC were below 0.84 and 0.08, respectively, the susceptibility decreased. However, the time inside the mutant selective window and the time above the MPC did not show any correlation with the decrease in susceptibility. These results suggest that AUC(0-24)/MPC and C(max)/MPC are important parameters for predicting the emergence of resistant mutants and that higher values indicate greater effectiveness.

The pharmacokinetic and pharmacodynamic profile of tigecycline

Tigecycline, a first-in-class expanded-spectrum antimicrobial agent, has demonstrated efficacy in the treatment of complicated intra-abdominal and skin and skin-structure infections. This new antibiotic is available as an intravenous formulation and exhibits linear pharmacokinetics. It is rapidly distributed and has a large volume of distribution, indicating extensive tissue penetration. After a 100-milligram loading dose, followed by 50 milligrams every 12 h, the steady-state maximum concentration in serum after a 1-h infusion is approximately 0.6 microg/mL, the 24-h steady-state area under the concentration-time curve is approximately 5-6 microg.h/mL, and the terminal elimination half-life is approximately 40 h. The major route of elimination of tigecycline is through the feces, primarily as unchanged drug. The pharmacokinetic profile is not affected by severe or end-stage renal disease, nor is it significantly altered by hemodialysis. The pharmacokinetics of tigecycline are also not affected by food, although tolerability is increased if the drug is administered following a meal.

Clinical implications of pharmacokinetics and pharmacodynamics of fluoroquinolones

This review summarizes key data illustrating the clinical importance of pharmacodynamics, particularly among the fluoroquinolone family of antibacterials. Antibacterials are often divided into 2 groups--either time-dependent or concentration-dependent agents--on the basis of their mechanism of killing. Fluoroquinolones are concentration-dependent agents, and the parameter that correlates most closely with clinical and/or bacteriological success is the ratio of the area under plasma concentration curve (AUC) to the minimum inhibitory concentration (MIC). The AUC : MIC threshold may vary by organism. For example, a ratio of at least 30 is often cited as optimal to achieve success against Streptococcus pneumoniae, whereas higher ratios (>100) are considered to be optimal for the treatment of infections due to gram-negative bacilli. Data are cited to suggest that the minimum ratio necessary to prevent the selection of resistant mutants may, in fact, be somewhat higher. Maximizing the AUC : MIC through the use of potent therapy may offer an opportunity to limit the development of resistance to fluoroquinolones.

Pharmacokinetic-pharmacodynamic relationships describing the efficacy of oritavancin in patients with Staphylococcus aureus bacteremia

Bloodstream infections due to antimicrobial-resistant Staphylococcus aureus occur with increasing frequency and represent an important cause of morbidity and mortality. To date, the evaluation of pharmacokinetic-pharmacodynamic relationships for efficacy among patients with bacteremia has been limited. The objectives of these analyses were to evaluate relationships between microbiological and clinical responses for patients with S. aureus bacteremia and exposures for oritavancin, a novel bactericidal glycopeptide in development. Bayesian oritavancin exposure predictions, following treatment with 5, 6.5, 8, or 10 mg/kg of body weight/day, were derived using a validated population pharmacokinetic model for 55 patients with S. aureus bacteremia. Using classification and regression tree analysis, a breakpoint of the percentage of the dosing interval duration for which free-drug concentrations were above the MIC (free-drug % time > MIC) of 22% was identified for microbiological response; the probabilities of success greater than or equal to and less than this value were 93% and 76%, respectively. Using logistic regression, a relationship was found between microbiological response and free-drug % time > MIC (odds ratio = 4.42, P = 0.09, and odds ratio = 8.84, P = 0.05, when one patient, a medical outlier, was excluded). A similar relationship was found for clinical response. These results will be valuable in supporting dose selection of oritavancin for patients with S. aureus bacteremia.

The role of pharmacodynamic research in the assessment and development of new antibacterial drugs

Antibacterial resistance continues to increase world wide, with some bacterial pathogens exhibiting resistance to virtually all available drugs. As the plague of antibacterial resistance continues to grow and create serious therapeutic problems, it is essential that the development of new antibacterial agents continue. Pharmacodynamic research plays an important role in the development of new antibacterial agents, as pharmacodynamic data can help define the clinical potential of a new drug and identify the strengths and weaknesses in comparison to other drugs already on the market. Furthermore, pharmacodynamic experiments can help focus the clinical phases of drug development by providing key information on the pharmacodynamic parameters that influence efficacy and the pharmacodynamic targets that should be achieved to optimize clinical success. Characterization of these pharmacodynamic properties for a new drug in development can help direct the design of the best dose and dosing strategy for clinical trials. This review will focus on the tools, methods, and strategies used to characterize the pharmacodynamics of antibacterial agents and aide in their development for clinical use.

Pharmacokinetic/pharmacodynamic profile for tigecycline-a new glycylcycline antimicrobial agent

Tigecycline is a new first-in-class glycylcycline antimicrobial agent with expanded broad-spectrum activity against both Gram-negative and Gram-positive aerobes and anaerobes, as well as atypical bacterial species. The spectrum of activity extends to clinically relevant susceptible and multidrug-resistant strains of Staphylococcus aureus, Streptococcus pneumoniae, vancomycin-resistant enterococci, and Enterobacteriaceae, including extended-spectrum beta-lactamase-producing strains. Tigecycline is administered as an intravenous formulation and has been studied in the treatment of serious polymicrobial infections, including complicated skin and skin-structure infections and intra-abdominal infections. Pharmacokinetic analysis of data from phase 1 trials of healthy subjects indicate that tigecycline has a large volume of distribution, signifying extensive tissue penetration, and a long terminal elimination half-life (approximately 40 h), easily allowing for twice-daily dose administration. Tigecycline penetrates well into blister fluid, which supports the positive findings of phase 2 and 3 studies of the efficacy of tigecycline in the treatment of serious skin and skin-structure infections. Metabolic studies in humans have revealed that tigecycline undergoes very limited metabolism and the primary route of elimination of unchanged drug is through the feces, with glucuronidation and renal elimination as secondary routes. A preliminary pharmacokinetic (PK)/pharmacodynamic analysis in experimental animal models of infection indicates that the efficacy of tigecycline is probably best predicted by the ratio of the area under the concentration-time curve to the minimum inhibitory concentration. The expanded in vitro activity against a broad range of bacteria, including resistant pathogens, and favorable PK profile of tigecycline suggest that this novel antimicrobial agent should offer clinicians an option for the treatment of patients with serious bacterial infections.

Pharmacodynamic evaluation of tosufloxacin against Streptococcus pneumoniae in an in vitro model simulating serum concentration

We compared the antibacterial effects and the emergence of resistance to tosufloxacin or levofloxacin for Streptococcus pneumoniae by simulating the serum concentration according to the Japanese clinical regimens using an in vitro pharmacokinetic-pharmacodynamic model. For quinolone-susceptible strain ATCC49619, tosufloxacin showed bactericidal activity, given that both the AUC(0-24h)/MIC ratios at the dosage of 150 mg t.i.d. and 300 mg b.i.d. of tosufloxacin tosilate were 138 and 193, and the C(max)/MIC ranges were 7.93-10.2 and 15.9-17.6, respectively, which were greater than those of levofloxacin (100 mg t.i.d. and 200 mg b.i.d.). The greater area above the killing curves (AAKCs) or shorter time to achieve 99.9% killing (99.9% KT) in both models of tosufloxacin than those of levofloxacin was related to their larger AUC(0-24h)/MIC and C(max)/MIC. Exposure of only 100 mg t.i.d. of levofloxacin led to outgrowth of the parC mutants, which were twofold less susceptible to levofloxacin than the parent strain. Neither of the tosufloxacin tosilate regimens resulted in isolation of resistant mutants of this strain. For the parC mutant strain D-3197, both the AUC(0-24h)/MIC and C(max)/MIC ratios of tosufloxacin were greater than those of levofloxacin, which resulted in comparable or better bactericidal activity as compared to those of levofloxacin. However, both fluoroquinolones and both regimens led to outgrowth of resistant mutants, which possessed a mutation in gyrA in addition to parC. In conclusion, tosufloxacin is superior to levofloxacin in bactericidal activity against S. pneumoniae in the Japanese clinical regimens, especially in the quinolone-susceptible strain, without emergence of resistant subpopulations.

Application of Pharmacokinetics and Pharmacodynamics to Antimicrobial Therapy of Community-Acquired Respiratory Tract Infections

To achieve bacteriologic and clinical success, sufficient concentrations of antimicrobial at the site of infection must be maintained for an adequate period of time. These dynamics are determined by combining drug pharmacokinetic and pharmacodynamic (PK/PD) data with minimum inhibitory concentrations. Bacteriologically confirmed failures have been reported in otitis media and, with a lesser degree of evidence, in pneumococcal pneumonia with a variety of agents that include beta-lactams, macrolides and fluoroquinolones. These failures have been shown to be due to infection by resistant pathogens or suboptimal therapy. However, no clinical failure has been reported during therapy for bacteremic pneumococcal pneumonia with adequate doses of beta-lactams. The failures reported with macrolides or fluoroquinolones have been due to either preexisting resistance to these agents that cannot be overcome by increasing the dose of the antimicrobial or, more rarely, the emergence of resistance during therapy. In this review, we offer an overview of the most important attributes of the main antimicrobials that are currently used in the treatment of community-acquired respiratory tract infections from a PK/PD perspective.

Phomoxins B and C: polyketides from an endophytic fungus of the genus Eupenicillium

Chemical investigations of the culture broth from an endophytic fungus Eupenicillium sp. have afforded two natural products phomoxins B (1) and C (2) as well as the previously reported fungal metabolite eupenoxide (3). Compounds 1 and 2 both contain a cyclic carbonate moiety that is rare among natural products. This paper reports the full spectroscopic characterisation of phomoxins B (1) and C (2) by NMR, UV, IR and MS data. All compounds were inactive against a panel of nosocomial microbes.

Comparison of gatifloxacin and levofloxacin administered at various dosing regimens to hospitalised patients with community-acquired pneumonia: pharmacodynamic target attainment study using North American surveillance data for Streptococcus pneumoniae

This work aimed at determining the target attainment potential of gatifloxacin and levofloxacin in specific age-related patient populations such as elderly (> or =65 years) versus younger (<65 years) hospitalised patients with community-acquired pneumonia (CAP). Previously described population pharmacokinetic models of gatifloxacin and levofloxacin administration in patients with serious CAP were utilised to simulate gatifloxacin and levofloxacin pharmacokinetics. Pharmacokinetic simulations and susceptibility data for Streptococcus pneumoniae from the ongoing national surveillance study, Canadian Respiratory Organism Susceptibility Study (CROSS), were then used to produce pharmacodynamic indices of free-drug area under the curve over 24h relative to the minimum inhibitory concentration (free-drug AUC(0-24)/MIC(all)). Monte Carlo simulations were then used to analyse target attainment both of gatifloxacin and levofloxacin to achieve free-drug AUC(0-24)/MIC(all)> or =30 against S. pneumoniae in patients with CAP. Dosing regimens for gatifloxacin were 400 mg once daily (qd) administered to younger patients (<65 years) and gatifloxacin 200 mg qd to elderly patients (> or =65 years). Dosing regimens for levofloxacin were simulated as 500 mg, 750 mg and 1000 mg qd administered to elderly patients as well as younger patients. Monte Carlo simulations using gatifloxacin 400mg against S. pneumoniae yielded probabilities of achieving free-drug AUC(0-24)/MIC(all) of 30 of 96.6% for all patients, 92.3% for younger patients and 97.7% for elderly patients. When administered to elderly patients, a reduced dose of gatifloxacin 200mg qd could achieve a target attainment potential of 91.4%. Monte Carlo simulation using levofloxacin 500 mg qd yielded probabilities of achieving free-drug AUC(0-24)/MIC(all) of 30 of 92.3% for all patients, 95.7% for elderly patients compared with 72.7% for younger patients. Using levofloxacin 750 mg and 1000 mg qd had probabilities of achieving free-drug AUC(0-24)/MIC(all) of 30 of 97.0% and 98.3%, 98.1% and 99.2%, and 90.1% and 95.2% for all patients, elderly patients and younger patients, respectively. The probability of achieving free-drug AUC(0-24)/MIC(all) of 100 was low both with gatifloxacin and levofloxacin, except in the case of elderly patients receiving levofloxacin in a dose of 1000 mg qd (78.5%). We conclude that gatifloxacin and levofloxacin pharmacokinetics in elderly patients with CAP are markedly different from those of younger patients. Higher gatifloxacin/levofloxacin AUC and longer half-life (t(1/2)) values in elderly patients with CAP compared with younger patients provide better pharmacodynamic parameters (free-drug AUC(0-24)/MIC) leading to a higher probability of pharmacodynamic target attainment and improved bacteriological outcome against S. pneumoniae. Gatifloxacin 400mg qd results in a high probability of target attainment and improved bacteriological outcome against S. pneumoniae both in young and elderly CAP patients. However, gatifloxacin administered at a lowered dose of 200 mg qd in elderly patients could still be successful in producing a favourable antibacterial effect. Levofloxacin administered at a dose of 750 mg qd results in a high probability of target attainment and improved bacteriological outcome against S. pneumoniae in all patients with CAP.

Appropriate outpatient treatment of acute bacterial exacerbations of chronic bronchitis

Acute exacerbations of chronic bronchitis (AECB), which are characteristic of chronic obstructive pulmonary disease (COPD), contribute to morbidity and decreased quality of life for patients with COPD. A significant proportion of these exacerbations are due to bacterial infections. The Council for Appropriate and Rational Antibiotic Therapy (CARAT) criteria provide guidance for choosing the optimal drug at its optimal dose and duration for antimicrobial treatment of AECB due to bacterial infection. Evidence-based guidelines recommend stratifying patients according to risk factors to improve selection of targeted antimicrobial therapy. With increasing rates of resistance to some antimicrobials, resistance is also an important consideration for reducing treatment failures and decreasing the need for further pharmacologic treatment. Fluoroquinolones are recommended as first-line therapy for patients with chronic bronchitis who have risk factors; gatifloxacin, gemifloxacin, and levofloxacin are highly active against commonly encountered pathogens. Safety profiles are an important consideration because adverse events and poor tolerability can reduce patient adherence rates, which in turn can lead to poorer outcomes. Safety profiles also become an important consideration as shorter-course, higher-dose therapies become more prevalent. First-line therapy with a well-tolerated antibiotic that is active against the predominant pathogens, combined with low resistance rates and a convenient once-a-day dosing regimen, may reduce overall costs. Fluoroquinolones exhibit low resistance, good activity levels, and high respiratory penetration, and they are particularly well suited for shorter-course, higher-dose regimens in selected patients. Shorter-course, higher dose regimens, in turn, may be more effective, cost-efficient, and appropriate for controlling the rise of resistance than standard regimens.

Treatment of rhinosinusitis in the outpatient setting

Rhinosinusitis is one of the most common respiratory tract conditions seen by primary care physicians. Each year approximately 20 million cases of acute bacterial rhinosinusitis (ABRS) occur in the United States. Since diagnosis of ABRS relies on clinical evaluation, treatments are usually empirical and include an antibiotic treatment that covers the common bacteria associated with ABRS infection, Streptococcus pneumoniae and Haemophilus influenzae. The Council for Appropriate and Rational Antibiotic Therapy (CARAT) recommends that antimicrobial therapy for rhinosinusitis should combine high susceptibility, clinical effectiveness, safety, and tolerability. The most efficacious antibiotics for ABRS include the respiratory fluoroquinolones gatifloxacin, levofloxacin, and moxifloxacin, as well as ceftriaxone and amoxicillin-clavulanate. The use of fluoroquinolones or high-dose amoxicillin-clavulanate is recommended for patients with mild disease who have had recent antimicrobial therapy or for patients with moderate disease. These drugs are generally well tolerated with mild adverse effects. Resistance to fluoroquinolones in S pneumoniae and H influenzae has remained low in spite of their increased use. Recent studies indicate that short-course, high-dose treatment regimens may reduce total drug use, improve tolerability and adherence, prevent increases in resistance, and increase efficacy. The use of fluoroquinolones or amoxicillin-clavulanate in a short-course, high-dose regimen may represent an exciting new protocol in the treatment of rhinosinusitis.

Gatifloxacin: a review of its use in the treatment of bacterial infections in the US

Gatifloxacin (Tequin) is an 8-methoxy fluoroquinolone approved in the US for use in the treatment of community-acquired pneumonia (CAP), acute exacerbations of chronic bronchitis (AECB), acute sinusitis, uncomplicated and complicated urinary tract infections (UTIs), pyelonephritis, gonorrhoea and uncomplicated skin and skin structure infections. Gatifloxacin has a broad spectrum of antibacterial activity in vitro and good clinical and bacteriological efficacy in patients with indicated infections following once-daily administration by the intravenous or oral routes. It is generally well tolerated; the most common adverse events are associated with the gastrointestinal tract and CNS. Recent approvals for the use of gatifloxacin in the treatment of CAP due to multidrug-resistant Streptococcus pneumoniae (MDRSP) and in uncomplicated skin and skin structure infections extend the role of this drug in the treatment of bacterial infections in the US.

Pharmacokinetics of gatifloxacin in infants and children

Gatifloxacin is an 8-methoxy fluoroquinolone effective against a broad spectrum of pathogens common in pediatric infections. The safety and pharmacokinetics of a single dose of gatifloxacin were studied in pediatric patients from 6 months to 16 years of age. Seventy-six pediatric patients (average age, 6.7 +/- 5.0 years) were administered a single oral dose of gatifloxacin suspension (5, 10, or 15 mg/kg of body weight; 600-mg maximum) in a dose-escalating manner. Subjects were stratified by age into 4 groups. An additional 12 children, greater than 6 years of age, received gatifloxacin as the tablet formulation at a dose of approximately 10 mg/kg. Gatifloxacin's apparent clearance and half-life were 5.5 +/- 2.1 ml/min/kg and 5.1 +/- 1.4 h. The maximum concentration of drug in plasma and area under the concentration-time curve (AUC) increased in a manner approximately proportional to the dose. At the 10-mg/kg dose, the bioavailability was similar between the suspension and tablet formulation. The apparent oral clearance of gatifloxacin, normalized for body weight, exhibited a small but statistically significant decrease with increasing age. In all subjects receiving gatifloxacin at 10 mg/kg, the AUC exceeded 20 microg . h/ml (estimated free AUC/MIC ratio of > or =34 for MIC of < or =0.5 microg/ml). These data suggest that gatifloxacin at a dose of 10 mg/kg every 24 h will achieve therapeutic concentrations in plasma in infants and children.

Assessment of pharmacokinetic-pharmacodynamic target attainment of gemifloxacin against Streptococcus pneumoniae

The treatment of community-acquired respiratory tract infections has been complicated by the emergence of multidrug-resistant Streptococcus pneumoniae. Although traditionally rare, a growing concern for fluoroquinolone-resistant pneumococci has surfaced. More pharmacodynamically potent antimicrobial agents are clearly needed, as the use of such agents may further optimize clinical and microbiological outcomes for patients and slow the emergence of fluoroquinolone resistance. For fluoroquinolones, the ratio of the 24-h area under the concentration-time curve of the agent to the minimum inhibitory concentration of the agent against the pathogen for the fraction of unbound drug is the major pharmacokinetic-pharmacodynamic (PK-PD) measure correlating with efficacy in nonclinical models and infected patients. A 2500-patient Monte Carlo simulation, utilizing a patient-population pharmacokinetic model derived from phase 3 registration trials and the minimum inhibitory concentration distribution for gemifloxacin against 3117 clinical strains of S. pneumoniae, was carried out to estimate the probability of gemifloxacin attaining exposures associated with efficacy. The overall probability PK-PD target attainment for gemifloxacin was greater than 0.99. Gemifloxacin is among the most pharmacodynamically potent fluoroquinolones and is more potent than ciprofloxacin, ofloxacin, and levofloxacin. Preferential use of pharmacodynamically potent agents over other alternatives may lead to improved clinical outcomes and decreased selection of fluoroquinolone-resistant pneumococci.

The use of fluoroquinolones in the treatment of skin infections

Fluoroquinolones have been studied for both uncomplicated and complicated skin and skin structure infections. Their broad spectrum, rapid bactericidal activity, extensive tissue penetration, excellent bioavailability and ease of administration have made these drugs a common choice for many infectious diseases, including skin infections. Extensive research has shown the fluoroquinolones to be as effective as beta-lactam antibiotics in managing a spectrum of diseases including erysipelas, cellulitis, impetigo, surgical wounds and diabetic foot infections. However, resistance to the fluoroquinolones has increased among the staphylococci, streptococci, Enterobacteriaceae and other important Gram-negative bacilli. Resistance has been linked directly to the widespread use of these compounds. Despite their appeal in the treatment of both uncomplicated and complicated skin infections, the fluoroquinolones should be reserved as alternatives to beta-lactams and other antibiotics or as empirical therapy in complicated infections until pathogens have been identified and drug regimens can be focused.

Pharmacodynamic target attainment analysis against Streptococcus pneumoniae using levofloxacin 500 mg, 750 mg and 1000 mg once daily in plasma (P) and epithelial lining fluid (ELF) of hospitalized patients with community acquired pneumonia (CAP)

The pharmacokinetics and pharmacodynamics of levofloxacin in patients with respiratory infections such as community-acquired pneumonia (CAP) are poorly documented. This work aimed at assessing the pharmacodynamic target attainment against Streptococcus pneumoniae using levofloxacin 500 mg, 750 mg and 1000 mg administered once daily in plasma (P) and epithelial lining fluid (ELF) of hospitalized patients with community acquired pneumonia. The pharmacokinetics of levofloxacin in elderly (>/=65 years) compared with younger patients (<65 years) hospitalized with CAP were simulated. Susceptibility data with S. pneumoniae from our ongoing national surveillance study (Canadian Respiratory Organism Susceptibility Study-CROSS) were then used to produce pharmacodynamic indices of AUC(0-24)/MIC(all.) Monte Carlo simulations were then used to analyse target attainment of levofloxacin using doses of 500 mg, 750 mg and 1000 mg once daily to achieve free drug AUC(0-24)/MIC(all) >/= 30-100 versus S. pneumoniae in patients with CAP. Pharmacokinetics of levofloxacin simulated after 500 mg, 750 mg and 1000 mg once daily dosing resulted in levofloxacin volume of distribution: elderly patients = younger patients, while levofloxacin clearance was: elderly patients < younger patients. Levofloxacin t(1/2) values were longer in elderly patients (9.8 +/- 2.5h) than younger patients with CAP (7.4 +/- 2.5h). Free levofloxacin AUC(0-24) as well as AUC(0-24)/MIC(all) for S. pneumoniae were higher in elderly patients than younger patients. Monte Carlo simulation using levofloxacin 500 mg yielded probabilities of achieving free-drug AUC(0-24)/MIC(all) of 30 in P and ELF (95.7% and 98.1%) in elderly and younger patients (72.7% and 80.6%) respectively. Levofloxacin 750 mg and 1000 mg once daily had probability of achieving free-drug AUC(0-24)/MIC(all) of 30 in P/ELF of 98.1%/98.6% and 99.2%/99.0%, respectively, in elderly patients compared with 89.9%/94.1% and 95.2%/96.5%, respectively, for younger patients. Probability of achieving of AUC(0-24)/MIC(all) of 100 in P or ELF was very low in both patient populations at different doses except in the case of elderly patients receiving levofloxacin in a dose of 1000 mg once daily P/ELF of 78.5%/87.0%. We conclude that levofloxacin pharmacokinetics in elderly patients with CAP are markedly different from those of younger patients. Levofloxacin 750 mg OD provides high probabilities of achieving free-drug AUC(0-24)/MIC(all) of 30 in both plasma and epithelial lining fluid in patients with CAP including younger patients. Levofloxacin 500 mg OD provides high probabilities of achieving free-drug AUC(0-24)/MIC(all) of 30 in elderly patients with CAP, although we favour the 750 mg dosing in these patients as well. Levofloxacin 750 mg OD results in high probability of pharmacodynamic target attainment and improved bacteriological outcome against S. pneumoniae in patients with CAP.

Population pharmacokinetics and pharmacodynamics of garenoxacin in patients with community-acquired respiratory tract infections

Garenoxacin (T-3811ME, BMS-284756) is a novel, broad-spectrum des-F(6) quinolone currently under study for the treatment of community-acquired respiratory tract infections. This analysis assessed garenoxacin population pharmacokinetics and exposure-response relationships for safety (adverse effects [AE]) and antimicrobial activity (clinical cure and bacteriologic eradication of Streptococcus pneumoniae and the grouping of Haemophilus influenzae, Haemophilus parainfluenzae, and Moraxella catarrhalis). Data were obtained from three phase II clinical trials of garenoxacin administered orally as 400 mg once daily for 5 to 10 days for the treatment of community-acquired pneumonia, acute exacerbation of chronic bronchitis, and sinusitis. Samples were taken from each patient before drug administration, 2 h following administration of the first dose, and on the day 3 to 5 visit. Individual Bayesian estimates of the fu (fraction unbound), the Cmax, and the fu for the area under the concentration-time curve from 0 to 24 h (fu AUC(0-24)) were calculated as measurements of drug exposure by using an ex vivo assessment of average protein binding. Regression analysis was performed to examine the following relationships: treatment-emergent AE incidence and AUC(0-24), Cmax, or patient factors; clinical response or bacterial eradication and drug exposure (fu Cmax/MIC, fu AUC(0-24)/MIC, and other exposure covariates); or disease and patient factors. Garenoxacin pharmacokinetics were described by a one-compartment model with first-order absorption and elimination. Clearance was dependent on creatinine clearance, ideal body weight, age, obesity, and concomitant use of pseudoephedrine. The volume of distribution was dependent on weight and gender. Patients with mild or moderate renal dysfunction had, on average, approximately a 16 or 26% decrease in clearance, respectively, compared to patients of the same gender and obesity classification with normal renal function. AE occurrence was not related to garenoxacin exposure. Overall, clinical cure and bacterial eradication rates were 91 and 90%, respectively, for S. pneumoniae and 93 and 92%, respectively, for the grouping of H. influenzae, H. parainfluenzae, and M. catarrhalis. The fu AUC(0-24)/MIC ratios were high (>90% were >200), and none of the pharmacokinetic-pharmacodynamic exposure measurements indexed to the MIC or other factors were significant predictors of clinical or bacteriologic response. Garenoxacin clearance was primarily related to creatinine clearance and ideal body weight. Although garenoxacin exposure was approximately 25% higher for patients with moderate renal dysfunction, this increase does not appear to be clinically significant as exposures in this patient population were not significant predictors of AE occurrence. Garenoxacin exposures were at the upper end of the exposure-response curves for measurements of antimicrobial activity, suggesting that 400 mg of garenoxacin once daily is a safe and adequate dose for the treatment of the specified community-acquired respiratory tract infections.

Relationship between increased levofloxacin use and decreased susceptibility of Streptococcus pneumoniae in the United States

Increasing reports of fluoroquinolone-non-susceptible Streptococcus pneumoniae are of clinical concern. We examined the relationship between outpatient fluoroquinolone use and susceptibility of community-acquired S. pneumoniae isolates. Using multivariable general linear modeling, US SENTRY Antimicrobial Surveillance Program and Intercontinental Medical Statistics data (1997-2002) were analyzed to determine the influence of selected patient-, institution-, and geographic region-specific factors, including local fluoroquinolone usage, on the minimum inhibitory concentration (MIC) of levofloxacin against S. pneumoniae. Levofloxacin MIC50, MIC90, and MIC range (n = 384 from 26 hospitals) were 1, 1, and < or =0.5 to >4 microg/mL, respectively. Variables associated with changes in geometric mean MIC included geographical region (P < 0.0001), medical service (P = 0.0002), study year (P = 0.0006), primary diagnosis group (P = 0.02), and 2 interactions (duration of hospital stay before isolate collection by bed capacity, P = 0.06, and levofloxacin use by geographical region, P = 0.08; P < 0.001 when study year was removed from the model). MIC increased with levofloxacin use across all geographical regions, with increases of 54% and 126% in the southwest and west, respectively. In contrast to other fluoroquinolones, increased levofloxacin use, along with other variables, was associated with decreased pneumococcal susceptibility. Given the US environment of increasing pneumococcal resistance, these data may be useful in better understanding factors related to emergence of fluoroquinolone resistance.

Clinical effect of gatifloxacin, 200mg daily, on bacterial community-acquired pneumonia in the elderly

The clinical effect of gatifloxacin (GFLX) at 200 mg daily (half of common dosage) on bacterial community-acquired pneumonia in the elderly was examined. Subjects were 30 patients with bacterial community-acquired pneumonia aged 65 years or more (mild, 21 patients; moderate, 9 patients), and GFLX at 100 mg per dose was administered twice daily for 4-14 days. The results included clinical effectiveness in 21 of 21 mildly affected patients (efficacy rate, 100%), in 8 of 9 moderately affected patients (efficacy rate, 88.9%), and 29 of a total of 30 patients (efficacy rate, 96.7%). With regard to bacteriological effect, 28 of 29 strains were eradicated (eradication rate, 96.6%). Abnormal laboratory findings included mild elevations in GPT, GOT, and ALP in only 1 patient. Based on these findings, we concluded that administration of GFLX at 200 mg daily is recommended for bacterial community-acquired pneumonia in the elderly.

Mutant selection window in levofloxacin and moxifloxacin treatments of experimental pneumococcal pneumonia in a rabbit model of human therapy

For some pneumococci the fluoroquinolone MICs are low but the mutant prevention concentrations (MPCs) are high; this difference defines in vitro the mutant selection window (MSW). We investigated in vivo the bacterial reduction and the occurrence of resistant mutants with moxifloxacin (MFX; 400 mg once daily) or levofloxacin (LVX; 500 mg twice daily) in treatments similar to those in humans with experimental pneumonia due to pneumococci (expPP) exhibiting various MICs and MPCs. The MIC/MPC for MFX and LVX and genotypes were as follows: strain 16089, 0.125/0.125 and 0.5/0.5 (wild type); strain MS1A, 0.25/0.25 and 1/2 (efflux); strain MS2A, 0.25/4 and 1.75/28 (parC79); strain MR3B4, 0.25/4 and 2/32 (parC79); strain M16, 0.5/2 and 8/32 (parC83); strain Gyr-1207, 1.5/3 and 8/16 (gyrA); and strain MQ3A, 4/4 and 16/64 (parC and gyrA). Both drugs were efficient with wild type-expPP, but only MFX was efficient with efflux-expPP. No bacterial reduction was observed for parC-expPPs due to mutants observed in 18 to 100% of animals, depending on the strain and the drug tested. These mutants showed unbound area under the concentration-time curve and MICs of from 50 to 164 for MFX. The in vivo pharmacodynamic boundaries of the MSW were different for MFX and LVX. We conclude that, after LVX or MFX treatment, mutants occur in vivo if there is a preexisting parC mutation, since the drug concentrations fall below the MPCs of these strains. Since the MPC determination cannot be routinely determined, these phenotypes or genotypes should be detected by simple tests to guide the therapeutic options.

Application of pharmacokinetics and pharmacodynamics to antimicrobial therapy of respiratory tract infections

The pharmacologic field that studies antimicrobial pharmacokinetics and pharmacodynamics (PK/PD) has had a major impact on the choice and dosing regimens used for many antibiotics especially those used in the treatment of respiratory tract infections. PK/PD parameters are particularly important in light of increasing antimicrobial resistance. Drug pharmacokinetic features, such as serum concentrations over time and area under the concentration-time curve, when integrated with minimum inhibitory concentration (MIC) values of antibiotics against pathogens, can predict the probability of bacterial eradication and clinical success. These pharmacokinetic and pharmacodynamic relationships also are important in preventing the selection and spread of resistant strains and have led to the description of the mutation prevention concentration, which is the lowest concentration of antimicrobial that prevents selection of resistant bacteria from high bacterial inocula. b-lactams are time-dependent agents without significant post-antibiotic effects, resulting in bacterial eradication when unbound serum concentrations exceed MICs of these agents against infecting pathogens for >40% to 50% of the dosing interval. Macrolides, azaolides, and lincosamides are time-dependent agents with prolonged post-antibiotic effects, and fluoroquinolones are concentration-dependent agents, resulting in both cases in bacterial eradication when unbound serum area-under-the-curve to MIC ratios exceed 25 to 30. These observations have led to changes in recommended antimicrobial dosing against respiratory pathogens and are used to assess the role of current agents, develop new formulations, and assess potency of new antimicrobials.

Pharmacokinetic and pharmacodynamic issues in the treatment of bacterial infectious diseases

This review outlines some of the many factors a clinician must consider when selecting an antimicrobial dosing regimen for the treatment of infection. Integration of the principles of antimicrobial pharmacology and the pharmacokinetic parameters of an individual patient provides the most comprehensive assessment of the interactions between pathogen, host, and antibiotic. For each class of agent, appreciation of the different approaches to maximize microbial killing will allow for optimal clinical efficacy and reduction in risk of development of resistance while avoiding excessive exposure and minimizing risk of toxicity. Disease states with special considerations for antimicrobial use are reviewed, as are situations in which pathophysiologic changes may alter the pharmacokinetic handling of antimicrobial agents.

Clinical pharmacodynamics of quinolones

An understanding of fundamental PK-PD principles forms the basis for the rational use of antimicrobial agents. For quinolones, the fAUC24:MIC ratio is predictive of efficacy in animal and in vitro infection models, and in infected patients. The magnitude of the fAUC24:MIC ratio predictive of efficacy in animal and in vitro infection models has been shown to be concordant with those obtained from human data. By accounting for PK and microbiologic variability together with PK-PD targets associated with efficacy or resistance suppression by Monte Carlo simulation, it is possible to discriminate between therapeutic regimens and select those regimens likely to be of greater benefit to patients. The maturation of antimicrobial PK-PD as a scientific discipline continues to accelerate and currently impacts clinical practice, drug development, and regulatory decision making.

The effect of pharmacokinetic/pharmacodynamic (PK/PD) parameters of gatifloxacin on its bactericidal activity and resistance selectivity against clinical isolates of Streptococcus pneumoniae

The impact of the pharmacokinetic/pharmacodynamic (PK/PD) parameters (the 24h area under the concentration-time curve [AUC24h]/minimum inhibitory concentration [MIC] and maximum concentration in serum [Cmax]/MIC ratio) after single oral dosing of gatifloxacin on its bactericidal activity and resistance selectivity against quinolone-susceptible clinical isolates of Streptococcus pneumoniae J-69 was investigated using an in vitro PK model. The MICs of gatifloxacin, levofloxacin, and ciprofloxacin were 0.25, 1, and 1 micro g/ml, respectively. When the range of AUC24h/MIC ratios was varied from 9.0 to 36 with a constant Cmax/MIC ratio of 3.4, the bactericidal activity was correlated with the AUC24h/MIC ratios. Eradication was observed at an AUC24h/MIC ratio of 36. On the other hand, the resistance selectivity was associated with the Cmax/MIC ratio. Mutant strains were selected at a Cmax/MIC ratio of 0.84, but not 1.7 with a constant AUC24h/MIC ratio of 9.0. These results suggested that an AUC24h/MIC ratio of > or =36 and a Cmax/MIC ratio of > or =1.7 might be possible benchmarks to show enough bacterial eradication and prevention of emergence of resistant strains to gatifloxacin, respectively. When the serum concentrations after clinical oral dosing of gatifloxacin (200 mg b.i.d.), levofloxacin (100 mg t.i.d.), and ciprofloxacin (200 mg t.i.d.) were simulated, the bactericidal activity of gatifloxacin was higher than those of levofloxacin and ciprofloxacin. Moreover, no resistant strain was obtained by the exposure to gatifloxacin and levofloxacin, whereas ciprofloxacin selected resistant strains. The clinically relevant oral dosage of gatifloxacin was anticipated to result in a high AUC24h/MIC90 ratio of 81 and a Cmax/MIC90 ratio of 4.4, suggesting that this agent is clinically effective in the treatment of pneumococcal infections.

In vitro pharmacodynamic activity of gatifloxacin, gemifloxacin, moxifloxacin and levofloxacin against Streptococcus pneumoniae containing specific mutations in DNA gyrase and topoisomerase IV

An in vitro pharmacodynatnic modeling apparatus (PDMA) generated specific bacterial kill profiles for single-dose regimens of gatifloxacin (GT), gemifloxacin (GM), moxifloxacin (MX) and levofloxacin (LV) against isolates of Streptococcus pneumoniae with specific QRDR profiles: SP-WT (no modifications); SP-C (changes in parC); and SP-AC (changes in both parC and gyrA). No differences in 3-log reduction time or total log reduction were observed among the four agents for SP-WT; however, LV failed to achieve a 3-log reduction in SP-C and SP-AC, and total log reduction after 12 hrs was minimal compared to the other agents. GM and MX required less time for 3-log reduction of SP-AC compared to GT, but total log reductions in SP-AC were similar among the three newer quinolone agents (GM > MX > GT). The study isolates with QRDR modifications greatly reduced LV activity. GM and MX maintained the greatest degree of activity against all study isolates and their activity was not adversely influenced by the genetic modifications in SP-C and SP-AC. The dual targeting characteristic of GM was also assessed, but did not offer significant advantages relative to MX and GT.

Impact of pharmacodynamics on breakpoint selection for susceptibility testing

In their report, Ericsson and Sherris stated that "adequately described sensitivity categorization schemes have been based on four main concepts" and went on to describe the advantages and inherent limitations of those four criteria. These limitations are very comparable with what is increasingly viewed as limitations to the current breakpoint systems because those four concepts have always been tried to be brought together in one breakpoint system. In line of this development, the European Committee on Antimicrobial Susceptibility Testing has recently recognized that clinical breakpoints based on PK-PD relationships confer a different meaning to resistance than early detection of microorganisms that do not belong to the natural bacterial population, but somehow have acquired resistance mechanisms and have introduced a wild-type cutoff breakpoint conveying a different meaning than clinical breakpoint. Similarly, there is increasing worry on the setting of clinical breakpoints based on frequency distributions because they do not bear an optimal relationship between dose and bacteriologic or clinical effect. This is especially apparent for older drugs because PK-PD was not used until a few years ago. Because dose-effect relationships have now been reasonably well established for most drugs, these should now be used to reappraise current clinical breakpoints.

[Antibacterial property and clinical effect of gatifloxacin, a novel quinolone antibacterial agent]

Gatifloxacin, a novel 8-methoxyquinolone, was approved in April 2002 and launched in June 2002. Gatifloxacin shows a broad spectrum of antibacterial activity against Gram-negative, Gram-positive, anaerobic, and atypical pathogens. The activity is higher than those of other quinolones against RTI pathogens of S. pneumoniae including the penicillin-resistant strains, H. influenzae, Mycoplasma, and Chlamydia. This drug strongly inhibits the type II topoisomerase, DNA gyrase, and topoisomerase IV of S. pneumoniae and S. aureus to nearly the same extent, leading to the potent activity and low resistance. After an oral administration in humans, gatifloxacin is well absorbed and distributed, and the majority is excreted in the urine as the unchanged form. Its serum half-life is 7-8 h. The clinical effectiveness was observed for various infectious diseases including RTI and UTI. The bacterial eradication rate is 94.1% for Gram-positives, 90.7% for Gram-negatives, and 97.7% for anaerobes. In particular, gatifloxacin showed a high eradication rate of 98.7% for S. pneumoniae. The total cure rate and eradication rate of gatifloxacin in clinical studies are 91.1% and 93.3%, respectively, indicating that the potent activity and good PK profile account for its clinical efficacy.

Bactericidal activities of methoxyfluoroquinolones gatifloxacin and moxifloxacin against aerobic and anaerobic respiratory pathogens in serum

Gatifloxacin (Bristol-Myers Squibb) and moxifloxacin (Bayer) are new methoxyfluoroquinolones with broad-spectrum activity against aerobic and anaerobic pathogens of the respiratory tract. In this investigation, we analyzed the bactericidal activity in serum over time of these antimicrobials against three aerobic (Streptococcus pneumoniae, Haemophilus influenzae, and Staphylococcus aureus) and four anaerobic (Peptostreptococcus micros, Peptostreptococcus magnus, Fusobacterium nucleatum, and Prevotella melaninogenica) bacteria associated with respiratory tract infections. Serum samples were obtained from 11 healthy male subjects following a single 400-mg oral dose of gatifloxacin and moxifloxacin. These samples were collected prior to and at 2, 6, 12, and 24 h after the dose of each drug. Gatifloxacin exhibited bactericidal activity for a median of 12 h against Streptococcus pneumoniae (MIC = 0.5 micro g/ml), Peptostreptococcus micros (MIC = 0.25 micro g/ml), and F. nucleatum (MIC = 0.5 micro g/ml) and 24 h against H. influenzae (MIC = 0.03 micro g/ml), Staphylococcus aureus (MIC = 0.125 micro g/ml), Peptostreptococcus magnus (MIC = 0.125 micro g/ml), and Prevotella melaninogenica (MIC = 0.5 micro g/ml). Moxifloxacin exhibited bactericidal activity for a median of 24 h against Streptococcus pneumoniae (MIC = 0.125 micro g/ml), H. influenzae (MIC = 0.015 micro g/ml), Staphylococcus aureus (MIC = 0.06 micro g/ml), F. nucleatum (MIC = 0.5 micro g/ml), Prevotella melaninogenica (MIC =0.5 micro g/ml), Peptostreptococcus magnus (MIC = 0.125 micro g/ml), and Peptostreptococcus micros (MIC = 0.25 micro g/ml). The results from this pharmacodynamic study suggest that these fluoroquinolones would have prolonged killing activity against these organisms in vivo and may have clinical utility in the treatment of mixed aerobic-anaerobic respiratory tract infections.

Pharmacodynamics of 750 mg and 500 mg doses of levofloxacin against ciprofloxacin-resistant strains of Streptococcus pneumoniae

An in vitro pharmacokinetic model (IVPM) was used to evaluate the pharmacodynamics of the 750 mg and 500 mg doses of levofloxacin against 4 ciprofloxacin-nonsusceptible Streptococcus pneumoniae. Levofloxacin MICs ranged from 1.4 to 3.2 micro g/ml. Log-phase cultures (5 x 10(7) cfu/ml) were inoculated into the IVPM and exposed to the peak free-drug concentrations of levofloxacin achieved in human serum with each dose. Levofloxacin was dosed at 0 and 24 h, elimination pharmacokinetics were simulated, and viable counts were measured over 30 h. The 750 mg dose was rapidly bactericidal against all 4 strains, achieving eradication within 30 h. Against strains with levofloxacin MICs of 1.4 and 1.8 micro g/ml, the 500 mg dose exhibited pharmacodynamics similar to the 750 mg dose. In contrast, against strains with levofloxacin MICs of 2.6 and 3.2 micro g/ml, viable counts never fell below 10(4) cfu/ml. The rapid killing and eradication of these pneumococci by the 750 mg dose warrant the clinical evaluation of this new dose in the treatment of pneumococcal infections.