Activities of trovafloxacin, gatifloxacin, clinafloxacin, sparfloxacin, levofloxacin, and ciprofloxacin against penicillin-resistant Streptococcus pneumoniae in an in vitro infection model

Pharmacokinetics and Pharmacodynamics of the New Quinolones

In this review, the authors describe the pharmacokinetic (PK) and pharmacodynamic (PD) characteristics of the new quinolones (levofloxacin, gatifloxacin, moxifloxacin, gemifloxacin, and garenoxacin) and discuss their implications on adequate therapy of patients with respiratory infections. The newer quinolones display excellent bioavailability and have longer serum half-lives than ciprofloxacin. In addition, they have the ability to concentrate in respiratory tract tissues and fluids at levels that exceed serum-drug concentrations. Also, the newer quinolones exhibit broad-spectrum activity against both susceptible and resistant organisms. Those favorable PK/PD properties make the new quinolones an attractive therapeutic alternative to traditional agents for common respiratory infections. Understanding the PK/PD of quinolone antibiotics can facilitate selection of optimal regimens to hasten response, prevent treatment failures, and minimize the development of resistance.

A Review of New Fluoroquinolones : Focus on their Use in Respiratory Tract Infections

The new respiratory fluoroquinolones (gatifloxacin, gemifloxacin, levofloxacin, moxifloxacin, and on the horizon, garenoxacin) offer many improved qualities over older agents such as ciprofloxacin. These include retaining excellent activity against Gram-negative bacilli, with improved Gram-positive activity (including Streptococcus pneumoniae and Staphylococcus aureus). In addition, gatifloxacin, moxifloxacin and garenoxacin all demonstrate increased anaerobic activity (including activity against Bacteroides fragilis). The new fluoroquinolones possess greater bioavailability and longer serum half-lives compared with ciprofloxacin. The new fluoroquinolones allow for once-daily administration, which may improve patient adherence. The high bioavailability allows for rapid step down from intravenous administration to oral therapy, minimizing unnecessary hospitalization, which may decrease costs and improve quality of life of patients. Clinical trials involving the treatment of community-acquired respiratory infections (acute exacerbations of chronic bronchitis, acute sinusitis, and community-acquired pneumonia) demonstrate high bacterial eradication rates and clinical cure rates. In the treatment of community-acquired respiratory tract infections, the various new fluoroquinolones appear to be comparable to each other, but may be more effective than macrolide or cephalosporin-based regimens. However, additional data are required before it can be emphatically stated that the new fluoroquinolones as a class are responsible for better outcomes than comparators in community-acquired respiratory infections. Gemifloxacin (except for higher rates of hypersensitivity), levofloxacin, and moxifloxacin have relatively mild adverse effects that are more or less comparable to ciprofloxacin. In our opinion, gatifloxacin should not be used, due to glucose alterations which may be serious. Although all new fluoroquinolones react with metal ion-containing drugs (antacids), other drug interactions are relatively mild compared with ciprofloxacin. The new fluoroquinolones gatifloxacin, gemifloxacin, levofloxacin, and moxifloxacin have much to offer in terms of bacterial eradication, including activity against resistant respiratory pathogens such as penicillin-resistant, macrolide-resistant, and multidrug-resistant S. pneumoniae. However, ciprofloxacin-resistant organisms, including ciprofloxacin-resistant S. pneumoniae, are becoming more prevalent, thus prudent use must be exercised when prescribing these valuable agents.

In vitro pharmacodynamic models to determine the effect of antibacterial drugs

In vitro pharmacodynamic (PD) models are used to obtain useful quantitative information on the effect of either single drugs or drug combinations against bacteria. This review provides an overview of in vitro PD models and their experimental implementation. Models are categorized on the basis of whether the drug concentration remains constant or changes and whether there is a loss of bacteria from the system. Further subdifferentiation is based on whether bacterial loss involves dilution of the medium or is associated with dialysis or diffusion. For comprehension of the underlying principles, experimental settings are simplified and schematically illustrated, including the simulations of various in vivo routes of administration. The different model types are categorized and their (dis)advantages discussed. The application of in vitro models to special organs, infections and pathogens is comprehensively presented. Finally, the relevance and perspectives of in vitro investigations in drug discovery and clinical research are elucidated and discussed.

Use of Pharmacodynamic Principles to Optimise Dosage Regimens for Antibacterial Agents in the Elderly

Throughout most of the world we are witnessing an ever increasing number of aged people as a percentage of the general population. In the coming years, the unique spectrum of infections presented by an elderly population, particularly those in long-term care facilities, will challenge our ability to maintain an effective battery of antibacterials. The pharmacokinetic parameters of most antibacterial agents are altered when assessed in the elderly due in part to non-pathological physiological changes. The inability to clear a drug from the body due to declining lung, kidney/bladder, gastrointestinal and circulatory efficiency can cause accumulation in the body of drugs given in standard dosages. While this may have the potential benefit of achieving therapeutic concentrations at a lower dose, there is also a heightened risk of attaining toxic drug concentrations and an increased chance of unfavourable interactions with other medications. Pharmacodynamic issues in the elderly are related to problems that arise from treating elderly patients who may have a history of previous antibacterial treatment and exposure to resistant organisms from multiple hospitalisations. Furthermore, the elderly often acquire infections in tandem with other common disease states such as diabetes mellitus and heart disease. Thus, it is essential that optimised dosage strategies be designed specifically for this population using pharmacodynamic principles that take into account the unique circumstances of the elderly. Rational and effective dosage and administration strategies based on pharmacodynamic breakpoints and detailed understanding of the pharmacokinetics of antibacterials in the elderly increase the chances of achieving complete eradication of an infection in a timely manner. In addition, this strategy helps prevent selection of drug-resistant bacteria and minimises the toxic effects of antibacterial therapy in the elderly patient.

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.

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.

Comparative pharmacodynamics of the new fluoroquinolone ABT492 and levofloxacin with Streptococcus pneumoniae in an in vitro dynamic model

The kinetics of killing of Streptococcus pneumoniae exposed to ABT492 or levofloxacin were compared. S. pneumoniae ATCC 49619 and four ciprofloxacin-resistant clinical isolates, S. pneumoniae 1149, 391, 79 and 804, were exposed to ABT492 and levofloxacin as a single dose in a dynamic model that simulates human pharmacokinetics of the quinolones. With S. pneumoniae ATCC 49619 eight-fold ranging AUC/MIC ratios (60-500 h) were simulated for each quinolone. In addition, two larger AUC/MICs, i.e., 1080 and 2150 h for ABT492 and 1460 and 3660 h for levofloxacin which correspond to 100 and 200 mg doses of ABT492 and 200 and 500 mg doses of levofloxacin, respectively, were mimicked. Each ciprofloxacin-resistant organism was exposed to the clinical doses of ABT492 (400 mg) and levofloxacin (500 mg); the respective AUC/MIC ratios were from 580 to 3470 h and from 28 to 110 h. At comparable AUC/MICs (from 60 to 500 h), regrowth of S. pneumoniae ATCC 49619 followed initial killing, and the times to regrowth were longer with levofloxacin than ABT492. However, no regrowth of S. pneumoniae ATCC 49619 occurred at the higher AUC/MICs of ABT492 (1080 and 2150 h) and levofloxacin (1460 and 3660 h). Killing of S. pneumoniae 1149, 391 and 79 without bacterial regrowth, was provided by ABT492 (AUC/MIC 3470, 2310 and 1160 h, respectively) but not levofloxacin (AUC/MIC 55, 110 and 28 h, respectively). Regrowth of S. pneumoniae 804 was observed with both ABT492 and levofloxacin (AUC/MIC 580 and 55 h, respectively). Areas between the control growth curve and the time-kill curve (ABBCs) for ABT492 against S. pneumoniae 1149, 391 and 79 were 2.6-4.2 times larger than the respective ABBCs for levofloxacin, whereas similar ABBCs were found with S. pneumoniae 804 exposed to both quinolones. These findings predict significantly greater efficacy of ABT492 than levofloxacin at clinically achievable AUC/MIC ratios against ciprofloxacin-resistant S. pneumoniae and similar efficacies of the two quinolones against susceptible organisms.

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.

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.

Pharmacokinetics and pharmacodynamics of intravenous levofloxacin in patients with early-onset ventilator-associated pneumonia

OBJECTIVE

To investigate the pharmacokinetics of levofloxacin and the pharmacokinetic-pharmacodynamic appropriateness of its total body exposure in patients in the intensive care unit (ICU) treated for early-onset ventilator-associated pneumonia (VAP) with intravenous levofloxacin 500mg twice daily.

DESIGN

Prospective non-blinded pharmacokinetic-pharmacodynamic study.

PARTICIPANTS

Ten critically ill adult patients with normal renal function.

METHODS

Blood and urine samples were collected at appropriate times during a 12-hour administration interval at steady state. Levofloxacin concentrations were determined by high-performance liquid chromatography. Clinical and microbiological outcomes were assessed.

RESULTS

Levofloxacin pharmacokinetics were only partially comparable with those obtained from literature data for healthy volunteers. Area under the concentration-time curve (AUC(tau)) over the 12-hour dosage interval was about 30-40% lower than in healthy volunteers (33.90 vs 49.60 mg. h/L). The reduced exposure may be due to a greater clearance of levofloxacin (0.204 vs 0.145 L/h/kg [3.40 vs 2.42 mL/min/kg]), leading to a shorter elimination half-life (5.2 vs 7.6 hours). Cumulative urinary excretion during the 12-hour dosage interval confirmed the greater excretion of unchanged drug in these patients compared with healthy subjects (76% vs 68%). Coadministered drugs used to treat underlying diseases (dopamine, furosemide, mannitol) may at least partially account for this enhanced elimination in critically ill patients. Intravenous levofloxacin 500mg twice daily ensured a median C(max)/MIC (maximum plasma concentration/minimum inhibitory concentration) ratio of 102 and a median 24-hour AUC/MIC ratio of 930 SIT(-1). h (inverse serum inhibitory titre integrated over time) against methicillin-sensitive Staphylococcus aureus and Haemophilus influenzae. The overall success rate of the assessable cases was 75% (6/8). Bacterial eradication was obtained in all of the assessable cases (8/8), but a superinfection (Acinetobacter anitratus,Pseudomonas aeruginosa) occurred in three cases.

CONCLUSIONS

The findings support the suitability of intravenous levofloxacin 500mg twice daily in the treatment of early-onset VAP in ICU patients with normal renal function. Levofloxacin may represent a valid alternative to non-pseudomonal beta-lactams or aminoglycosides in the empirical treatment of early-onset VAP. However, further larger studies are warranted to investigate its efficacy.

Linezolid, levofloxacin, and vancomycin against vancomycin-tolerant and fluoroquinolone-resistant Streptococcus pneumoniae in an in vitro pharmacodynamic model

STUDY OBJECTIVE

To compare the pharmacodynamic profiles of linezolid, levofloxacin, and vancomycin against clinical strains of Streptococcus pneumoniae, including vancomycin-tolerant and fluoroquinolone-resistant isolates.

DESIGN

In vitro pharmacodynamic model.

SETTING

Biosafety level 2, university research laboratory. BACTERIAL STRAINS: Ciprofloxacin-susceptible (79), ciprofloxacin-resistant (R921), and vancomycin-tolerant (P9802-020) clinical strains of S. pneumoniae.

INTERVENTION

An in vitro pharmacodynamic model was used to simulate standard dosing regimens of linezolid, levofloxacin, and vancomycin against the isolates 79, R921, and P9802-020.

MEASUREMENTS AND MAIN RESULTS

Bacterial density was profiled over 48 hours. Minimum inhibitory concentrations (MICs) for linezolid, levofloxacin, and vancomycin, respectively were 1, 1, 0.5 microg/ml for isolate 79; 1, 4, 0.5 microg/ml for R921; and 0.5, 0.5, 0.5 microg/ml for P9802-020. Vancomycin minimum bactericidal concentration (MBC) values varied across large ranges for the tested strains. Linezolid achieved 99.9% kill against 79 and R921 by 24 and 28 hours, respectively. Levofloxacin achieved 99.9% kill against 79 and P9802-020 by 28 and 4 hours, respectively. Vancomycin achieved 99.9% kill against 79 and R921 by 8 and 24 hours, respectively. Levofloxacin did not demonstrate activity against R921 at the 48-hour end point. Minimal kill (< 2 log) at 48 hours was noted for vancomycin and linezolid against P9802-020. Conclusion. Vancomycin tolerance appeared to be more reliably characterized by persistent viability in time-kill analyses than by MBC:MIC ratios. Vancomycin exhibited bactericidal activity against the non-vancomycin-tolerant strains of S. pneumoniae. Linezolid exhibited both bactericidal and bacteriostatic activity against all three strains tested, whereas levofloxacin demonstrated bactericidal activity against the fluoroquinolone-susceptible isolates. Further investigation of treatment alternatives for infections due to vancomycin-tolerant S. pneumoniae are needed.

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.

Activities of moxifloxacin against, and emergence of resistance in, Streptococcus pneumoniae and Pseudomonas aeruginosa in an in vitro pharmacokinetic model

The pharmacodynamics of moxifloxacin against Streptococcus pneumoniae and Pseudomonas aeruginosa were investigated in a pharmacokinetic infection model. Three strains of S. pneumoniae, moxifloxacin, and two strains of P. aeruginosa were used. Antibacterial effect and emergence of resistance were measured for both species over a 72-h period using an initial inoculum of about 10(8) CFU/ml. At equivalent area under the curve (AUC)/MIC ratios, S. pneumoniae was cleared from the model while P. aeruginosa was not. For S. pneumoniae, the area under the bacterial kill curve up to 72 h could be related to AUC/MIC ratio using an inhibitory maximum effect (E(max)) model (concentration required for 50% E(max) [EC(50)], 45 +/- 22; r(2), 0.97). For P. aeruginosa even at the highest AUC/MIC ratio (427), bacterial clearance was insufficient for the EC(50) to be calculated. Emergence of resistance occurred with P. aeruginosa but not to any significant extent with S. pneumoniae. Emergence of resistance in P. aeruginosa as measured by population analysis profile (PAP-AUC) was dependent on drug exposure and time of exposure. In weighted least-squares regression analysis AUC/MIC ratio was predictive of PAP-AUC. When emergence of resistance was measured by the time for the colony counts on media containing antibiotic to increase by 2 logs, again AUC/MIC was the best predictor of emergence of resistance. However, for both experiments using S. pneumoniae and P. aeruginosa the correlation between all the pharmacodynamic parameters was high. These data indicate that for a given fluoroquinolone the magnitude of the AUC/MIC ratio for antibacterial effect is dependent on the bacterial species. Emergence of resistance is dependent on (i) species, (ii) duration of drug exposure, and (iii) drug exposure. A single AUC/MIC ratio magnitude is not adequate to predict antibacterial effect or emergence of resistance for all bacterial species.

Development of novel antibacterial peptides that kill resistant isolates

The rapid emergence of bacterial strains that are resistant to current antibiotics requires the development of novel types of antimicrobial compounds. Proline-rich cationic antibacterial peptides such as pyrrhocoricin kill responsive bacteria by binding to the 70 kDa heat shock protein DnaK and inhibiting protein folding. We designed and synthesized multiply protected dimeric analogs of pyrrhocoricin and optimized the in vitro antibacterial efficacy assays for peptide antibiotics. Pyrrhocoricin and the designed dimers killed beta-lactam, tetracycline- or aminoglycoside-resistant strains of Escherichia coli, Salmonella typhimurium, Klebsiella pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis in the submicromolar or low micromolar concentration range. One of the peptides also killed Pseudomonas aeruginosa. The designed dimers showed improved stability in mammalian sera compared to the native analog. In a murine H. influenzae lung infection model, a single dose of a dimeric pyrrhocoricin analog reduced the bacteria in the bronchoalveolar lavage when delivered intranasally. The solid-phase synthesis was optimized for large-scale laboratory preparations.

Microbiologic effectiveness of time- or concentration-based dosing strategies in Streptococcus pneumoniae

This in vitro study evaluated the pharmacodynamic performance of levofloxacin using different dosing strategies against both a levofloxacin-sensitive (MIC = 1 mg/liter) and -resistant (MIC = 16 mg/liter) strain of Streptococcus pneumoniae. The strain was genotypically characterized by a mutation in gyrA and two mutations in parE; resistance was shown not to be efflux-mediated. The purpose of this study was to determine if simulated levofloxacin dosing strategies focused either on time or concentration would affect microbiologic outcome. Differing peak concentration/MIC ratios (1,2, and 10), T>MIC (3.6,9.6,15.6, and 24 h corresponding to 15, 40, 65, and 100% of the 24-h dosing interval), and AUC/MIC ratios (13-180) were generated by varying dosing strategies. Initial bacterial inocula were decreased by 99.9% in each experiment conducted. Despite the wide variation in exposure levels, in terms of AUC/MIC, Cp-max/MIC, and T>MIC, the kill portions of the bacterial density curves were super-imposable between all permutations of antibiotic exposure. However, there appeared to be an AUC/MIC breakpoint (35-40) defining bacterial regrowth. Over a 10-fold concentration range, levofloxacin appeared to kill S. pneumoniae in a concentration-independent fashion. When given in concentrations suitable to achieve specified pharmacodynamic endpoints (AUC/MIC >/=35), levofloxacin demonstrated the ability to eradicate both a levofloxacin-resistant and levofloxacin-sensitive strain of S. pneumoniae in the in vitro model.

IV-to-oral switch therapy for community-acquired pneumonia requiring hospitalization: focus on gatifloxacin

The majority of the 1.1 million patients hospitalized for community-acquired pneumonia (CAP) in the United States begin therapy with an intravenous antibiotic. A switch to oral therapy as soon as patients are clinically stable reduces the length of hospitalization and associated costs. Fluoroquinolones are appropriate candidates for switch therapy. Gatifloxacin is an excellent choice when a fluoroquinolone is being considered for sequential switch therapy in the treatment of CAP requiring hospitalization.

Developments in PK/PD: optimising efficacy and prevention of resistance. A critical review of PK/PD in in vitro models

In vitro pharmacokinetic models are excellent tools with which to study an antibacterial's pharmacodynamics (pD), being flexible, adaptable, low cost, and correlating well with animal and human systems. They can be used to perform simple descriptive studies on antibacterial effect, determine the dominant pD factor and its magnitude for antibacterial effect, and finally be used to assess the effect of dosing on emergence of resistance. A wide range of model designs are used and some standardisation maybe of value in the near future, however it is clear that in vitro models in conjunction with animal studies and human trials offer an excellent way of studying drug dosing to optimise outcomes.

A critical review of the fluoroquinolones: focus on respiratory infections

The new fluoroquinolones (clinafloxacin, gatifloxacin, gemifloxacin, grepafloxacin, levofloxacin, moxifloxacin, sitafloxacin, sparfloxacin and trovafloxacin) offer excellent activity against Gram-negative bacilli and improved Gram-positive activity (e.g. against Streptococcus pneumoniae and Staphylococcus aureus) over ciprofloxacin. Ciprofloxacin still maintains the best in vitro activity against Pseudomonas aeruginosa. Clinafloxacin, gatifloxacin, moxifloxacin, sitafloxacin, sparfloxacin and trovafloxacin display improved activity against anaerobes (e.g. Bacteroides fragilis) versus ciprofloxacin. All of the new fluoroquinolones display excellent bioavailability and have longer serum half-lives than ciprofloxacin allowing for once daily dose administration. Clinical trials comparing the new fluoroquinolones to each other or to standard therapy have demonstrated good efficacy in a variety of community-acquired respiratory infections (e.g. pneumonia, acute exacerbations of chronic bronchitis and acute sinusitis). Limited data suggest that the new fluoroquinolones as a class may lead to better outcomes in community-acquired pneumonia and acute exacerbations of chronic bronchitis versus comparators. Several of these agents have either been withdrawn from the market, had their use severely restricted because of adverse effects (clinafloxacin because of phototoxicity and hypoglycaemia; grepafloxacin because of prolongation of the QTc and resultant torsades de pointes; sparfloxacin because of phototoxicity; and trovafloxacin because of hepatotoxicity), or were discontinued during developmental phases. The remaining fluoroquinolones such as gatifloxacin, gemifloxacin, levofloxacin and moxifloxacin have adverse effect profiles similar to ciprofloxacin. Extensive post-marketing safety surveillance data (as are available with ciprofloxacin and levofloxacin) are required for all new fluoroquinolones before safety can be definitively established. Drug interactions are limited; however, all fluoroquinolones interact with metal ion containing drugs (eg. antacids). The new fluoroquinolones (gatifloxacin, gemifloxacin, levofloxacin and moxifloxacin) offer several advantages over ciprofloxacin and are emerging as important therapeutic agents in the treatment of community-acquired respiratory infections. Their broad spectrum of activity which includes respiratory pathogens such as penicillin and macrolide resistant S. pneumoniae, favourable pharmacokinetic parameters, good bacteriological and clinical efficacy will lead to growing use of these agents in the treatment of community-acquired pneumonia, acute exacerbations of chronic bronchitis and acute sinusitis. These agents may result in cost savings especially in situations where, because of their potent broad-spectrum activity and excellent bioavailability, they may be used orally in place of intravenous antibacterials. Prudent use of the new fluoroquinolones will be required to minimise the development of resistance to these agents.

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.

Activities of newer fluoroquinolones against ciprofloxacin-resistant Streptococcus pneumoniae

The incidence of ciprofloxacin resistance in Streptococcus pneumoniae is low but steadily increasing, which raises concerns regarding the clinical impact of potential cross-resistance with newer fluoroquinolones. To investigate this problem, we utilized an in vitro pharmacodynamic model and compared the activities of gatifloxacin, grepafloxacin, levofloxacin, moxifloxacin, and trovafloxacin to that of ciprofloxacin against two laboratory-derived, ciprofloxacin-resistant derivatives of S. pneumoniae (strains R919 and R921). Ciprofloxacin resistance in these strains involved the activity of a multidrug efflux pump and possibly, for R919, a mutation resulting in an amino acid substitution in GyrA. Gatifloxacin, grepafloxacin, levofloxacin, moxifloxacin, and trovafloxacin achieved 99.9% killing of both R919 and R921 in < or =28 h. With respect to levofloxacin, significant regrowth of both mutants was observed at 48 h (P < 0.05). For gatifloxacin, grepafloxacin, moxifloxacin, and trovafloxacin, regrowth was minimal at 48 h, with each maintaining 99.9% killing against both mutants. No killing of either R919 or R921 was observed with exposure to ciprofloxacin. During model experiments, resistance to gatifloxacin, grepafloxacin, moxifloxacin, and trovafloxacin did not develop but the MICs of ciprofloxacin and levofloxacin increased 1 to 2 dilutions for both R919 and R921. Although specific area under the concentration-time curve from 0 to 24 h (AUC(0--24))/MIC and maximum concentration of drug in serum (C(max))/MIC ratios have not been defined for the fluoroquinolones with respect to gram-positive organisms, our study revealed that significant regrowth and/or resistance was associated with AUC(0-24)/MIC ratios of < or =31.7 and C(max)/MIC ratios of < or =3.1. It is evident that the newer fluoroquinolones tested possess improved activity against S. pneumoniae, including strains for which ciprofloxacin MICs were elevated.

Multicenter evaluation of the efficacy and safety of gatifloxacin in Mexican adult outpatients with respiratory tract infections

Respiratory tract infections (RTIs), the most common indication for outpatient antimicrobial therapy, impose a heavy medical and societal burden and present a difficult therapeutic challenge in the face of increasing pathogen resistance worldwide. Gatifloxacin is a new broad-spectrum fluoroquinolone with excellent activity against prevalent respiratory bacteria, including penicillin-resistant Streptococcus pneumoniae and atypical pathogens. A multicenter, open-label, noncomparative surveillance study carried out in Mexico evaluated the safety and efficacy of oral gatifloxacin 400 mg once daily in 17,923 adult outpatients with community-acquired pneumonia (CAP) (n = 3322), acute exacerbations of chronic bronchitis (AECB) (n = 5885), and acute bacterial sinusitis (n = 8716). Voluntary, unpaid physician participation contributed to an unbiased study design. Physician-assessed global rate of cure or improvement was 96.3%; efficacy was 95.8% in CAP, 96.1% in AECB, and 96.4% in sinusitis. The incidences of relapse (1.5%) and therapeutic failure (0.7%) were low. The most commonly reported adverse events, nausea (2.76%), headache (2.20%), and dizziness (1.33%), were generally mild and self-limited. Oral gatifloxacin 400 mg once daily is effective and safe for patients with CAP, AECB, and acute sinusitis.