Activity of moxifloxacin, administered once a day, against Streptococcus pneumoniae in an in vitro pharmacodynamic model of infection

Actinomycetes from the Red Sea Sponge Coscinoderma mathewsi: Isolation, Diversity, and Potential for Bioactive Compounds Discovery

The diversity of actinomycetes associated with the marine sponge Coscinoderma mathewsi collected from Hurghada (Egypt) was studied. Twenty-three actinomycetes were separated and identified based on the 16S rDNA gene sequence analysis. Out of them, three isolates were classified as novel species of the genera Micromonospora, Nocardia, and Gordonia. Genome sequencing of actinomycete strains has revealed many silent biosynthetic gene clusters and has shown their exceptional capacity for the production of secondary metabolites, not observed under classical cultivation conditions. Therefore, the effect of mycolic-acid-containing bacteria or mycolic acid on the biosynthesis of cryptic natural products was investigated. Sponge-derived actinomycete Micromonospora sp. UA17 was co-cultured using liquid fermentation with two mycolic acid-containing actinomycetes (Gordonia sp. UA19 and Nocardia sp. UA 23), or supplemented with pure mycolic acid. LC-HRESIMS data were analyzed to compare natural production across all crude extracts. Micromonospora sp. UA17 was rich with isotetracenone, indolocarbazole, and anthracycline analogs. Some co-culture extracts showed metabolites such as a chlorocardicin, neocopiamycin A, and chicamycin B that were not found in the respective monocultures, suggesting a mycolic acid effect on the induction of cryptic natural product biosynthetic pathways. The antibacterial, antifungal, and antiparasitic activities for the different cultures extracts were also tested.

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.

Bacterial resistance studies using in vitro dynamic models: the predictive power of the mutant prevention and minimum inhibitory antibiotic concentrations

In light of the concept of the mutant selection window, i.e., the range between the MIC and the mutant prevention concentration (MPC), MPC-related pharmacokinetic indices should be more predictive of bacterial resistance than the respective MIC-related indices. However, experimental evidence of this hypothesis remains limited and contradictory. To examine the predictive power of the ratios of the area under the curve (AUC24) to the MPC and the MIC, the selection of ciprofloxacin-resistant mutants of four Escherichia coli strains with different MPC/MIC ratios was studied. Each organism was exposed to twice-daily ciprofloxacin for 3 days at AUC24/MIC ratios that provide peak antibiotic concentrations close to the MIC, between the MIC and the MPC, and above the MPC. Resistant E. coli was intensively enriched at AUC24/MPCs from 1 to 10 h (AUC24/MIC from 60 to 360 h) but not at the lower or higher AUC24/MPC and AUC24/MIC ratios. AUC24/MPC and AUC24/MIC relationships of the areas under the time courses of ciprofloxacin-resistant E. coli (AUBCM) were bell-shaped. A Gaussian-like function fits the AUBCM-AUC24/MPC and AUBCM-AUC24/MIC data combined for all organisms (r(2) = 0.69 and 0.86, respectively). The predicted anti-mutant AUC24/MPC ratio was 58 ± 35 h, and the respective AUC24/MIC ratio was 1,080 ± 416 h. Although AUC24/MPC was less predictive of strain-independent E. coli resistance than AUC24/MIC, the established anti-mutant AUC24/MPC ratio was closer to values reported for Staphylococcus aureus (60 to 69 h) than the respective AUC24/MIC ratio (1,080 versus 200 to 240 h). This implies that AUC24/MPC might be a better interspecies predictor of bacterial resistance than AUC24/MIC.

Comparative antibacterial effects of moxifloxacin and levofloxacin on Streptococcus pneumoniae strains with defined mechanisms of resistance: impact of bacterial inoculum

OBJECTIVES

We aim to further define the impact of the mechanism of fluoroquinolone resistance and inoculum load on the pharmacodynamic effects of levofloxacin and moxifloxacin on Streptococcus pneumoniae.

METHODS

The antibacterial effects of and emergence of resistance (EoR) to moxifloxacin (400 mg once daily) or levofloxacin (750 mg once daily or 500 mg twice daily) were compared using five S. pneumoniae strains containing no known resistance mechanisms, efflux resistance mechanisms, a parC mutation or parC and gyrA mutations, at high (10(8) cfu/mL) and low (10(6) cfu/mL) inocula. An in vitro pharmacokinetic model was used and simulations were performed over 96 h. After drug exposure, isolates were tested for the presence of efflux pumps and mutations in the quinolone resistance-determining regions.

RESULTS

A high inoculum diminished the antibacterial effect of moxifloxacin and levofloxacin. Levofloxacin at both dosages produced EoR with all strains. Levofloxacin regimens with AUC/MIC ratios <100 produced EoR. Moxifloxacin produced EoR with the parC strain only.

CONCLUSIONS

Levofloxacin dosing regimens with low AUC/MIC ratios select for efflux pump overexpression, leading to fluoroquinolone resistance. Levofloxacin dosing may select for gyrA mutations, inducing moxifloxacin resistance. These data confirm that a fluoroquinolone AUC/MIC ratio of >100 is required for prevention of EoR.

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.

New formulations of amoxicillin/clavulanic acid: a pharmacokinetic and pharmacodynamic review

The pharmacokinetic properties of amoxicillin and clavulanic acid when used alone or in combination are extensively reviewed and discussed in this article. The reported data support a nonlinear absorption process for amoxicillin. Saturable transport mechanisms, limited solubility and the existence of an absorption window are possibly involved in the gastrointestinal absorption of this antibacterial, all leading to a decrease in the peak plasma concentration (Cmax)/dose ratio, a prolongation of the time to reach Cmax, and broad variability for high doses of amoxicillin. Data available in the literature also suggest a possible interaction between amoxicillin and clavulanic acid that might decrease the absolute bioavailability of clavulanic acid. In the present review the intrinsic pharmacodynamics of each drug, together with the synergism produced by the amoxicillin/clavulanic acid association, are also reviewed and analysed. Not only beta-lactamase-producing strains, but also Streptococcus pneumoniae strains, seem to be more efficiently eradicated by the association of amoxicillin and clavulanic acid, and a relevant post-antibacterial effect and post-beta-lactamase inhibitor effect are likely to operate when amoxicillin is administered together with clavulanic acid. The principles of pharmacokinetic/pharmacodynamic analysis applied to amoxicillin are reviewed, with special emphasis being placed on the results obtained from in vitro studies and animal models regarding the new pharmacokinetically enhanced formulation. Theoretical considerations concerning the efficacy of this formulation provided by the application of pharmacokinetic/pharmacodynamic analysis to the scarce pharmacokinetic data available are also included. The broad pharmacokinetic variability of both amoxicillin and clavulanic acid, particularly when administered together and at high doses of amoxicillin, is highlighted and the interest in considering this aspect to improve predictions based on pharmacokinetic/pharmacodynamic analyses for the new formulations is indicated. Methodological recommendations such as the Monte Carlo simulation are proposed in order to obtain more realistic predictions in clinical practice.

Modeling in vivo pharmacokinetics and pharmacodynamics of moxifloxacin therapy for Mycobacterium tuberculosis infection by using a novel cartridge system

To study the efficacy of moxifloxacin treatment for tuberculosis, we utilized a novel cartridge system to simulate in vivo pharmacokinetics. We found this system to be a robust method for modeling in vivo pharmacokinetics and present data supporting the utility of intermittent moxifloxacin treatment as a component of antituberculosis chemotherapy.

Pharmacodynamics of ceftazidime plus the serine beta-lactamase inhibitor AM-112 against Escherichia coli containing TEM-1 and CTX-M-1 beta-lactamases

A strain of Escherichia coli containing TEM-1 and CTX-M-1 was tested in an in vitro pharmacokinetic model against ceftazidime with and without AM-112, a serine beta-lactamase inhibitor. Ceftazidime alone was less effective than ceftazidime plus AM-112, and a single dose was more effective than three fractionated doses.

Antibacterial effects of amoxicillin-clavulanate against Streptococcus pneumoniae and Haemophilus influenzae strains for which MICs are high, in an in vitro pharmacokinetic model

The antibacterial effect of amoxicillin-clavulanate in two formulations, pharmacokinetically enhanced 16:1 amoxicillin-clavulanate twice a day (b.i.d.) and standard 7:1 amoxicillin-clavulanate b.i.d., were studied in an in vitro pharmacokinetic model of infection. Five strains of Streptococcus pneumoniae and two of Haemophilus influenzae, all associated with raised MICs (2 to 8 mg/liter), were used. The antibacterial effect was measured over 24 h by the area under the bacterial kill curve (AUBKC) and the log change in viable count at 24 h (Delta24). A high 10(8) CFU/ml and low 10(6) CFU/ml initial inocula were used. Employing the Delta24 effect measure, the time above MIC (T>MIC) 50% maximum effect (EC(50)) for S. pneumoniae was in the range 21 to 28% with an 80% maximal response of 41 to 51%, for the AUBKC measure, the value was 26 to 39%, irrespective of inoculum. For H. influenzae, the T>MIC EC(50) was 28 to 37%, and the 80% maximum response was 32 to 48% for the Delta24 measure and 20 to 48% for AUBKC. The maximum response occurred at a T>MIC of 50 to 60% for both species and inocula. The S. pneumoniae data were analyzed by analysis of variance to assess the effect of inoculum, formulation, and MIC on antibacterial effect. Standard and enhanced formulations had different effects depending on MIC, with the standard formulation less effective at higher amoxicillin-clavulanate MICs. This is explained by the greater T>MICs of the enhanced formulation. Although resistant to amoxicillin-clavulanate by conventional breakpoints, S. pneumoniae and H. influenzae strains for which MICs are 2 or 4 mg/liter may well respond to therapy with pharmacokinetically enhanced formulation amoxicillin-clavulanate.

Proof of concept: performance testing in models

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

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.

Mechanism of fluoroquinolone resistance is an important factor in determining the antimicrobial effect of gemifloxacin against Streptococcus pneumoniae in an in vitro pharmacokinetic model

Antibacterial effect and emergence of resistance to gemifloxacin and levofloxacin were studied in an in vitro pharmacokinetic model of infection. A panel of Streptococcus pneumoniae strains with known mechanisms of resistance were used; two strains had no known resistance mechanism, two had efflux pumps, three had gyrA plus parC mutations, and one had only a parC mutation. Gemifloxacin MICs were in the range of 0.016 to 0.25 mg/liter, and levofloxacin MICs ranged from 1 to 16 mg/liter. Antimicrobial effect was measured by area under the bacterial-kill curve up to 72 h, and emergence of resistance was determined by population analysis profile before and during drug exposure. The area under the curve (AUC)/MIC ratios for gemifloxacin and levofloxacin were 35 to 544 and 3 to 48, respectively. As expected on the basis of these AUC/MIC ratio differences, antibacterial effect was much greater for gemifloxacin than levofloxacin. In the gemifloxacin simulations, mechanism of resistance as well as MIC determined the antibacterial effect, as indicated by gemifloxacin's greater effect against efflux strains compared to those with gyrA or parC mutations despite similar MICs. This was not true of levofloxacin. Emergence of resistance was not easily demonstrated with either agent, and mechanism of resistance did not have any impact on it.

Evaluation of survival and pharmacodynamic relationships for five fluoroquinolones in a neutropenic murine model of pneumococcal lung infection

STUDY OBJECTIVE

To compare the antistreptococcal activity of five fluoroquinolone antibiotics, using a neutropenic murine model of pneumococcal pulmonary infection.

DESIGN

Animal experiment.

SETTING

University-affiliated research center.

ANIMALS

Neutropenic and control mice weighing 24-29 g.

INTERVENTION

After induction of neutropenia, renal failure, and infection with Streptococcus pneumoniae, the mice received one of five fluoroquinolones twice/day for 72 hours beginning 12 hours after infection. Dosages were selected to approximate 0.1 x AUC0-24 (area under the concentration-time curve from 0-24 hours) and AUC0-24 achieved in humans. Control mice received normal saline. Survival was assessed at regular intervals for up to 10 days. At least 10 mice were included in each cohort (range 10-34).

MEASUREMENTS AND MAIN RESULTS

Ciprofloxacin, clinafloxacin, grepafloxacin, levofloxacin, and moxifloxacin were studied at subtherapeutic and therapeutic dosages against three quinolone-susceptible isolates of S. pneumoniae that lacked mutations in parC, parE, and gyrA. Pharmacokinetic profile of each agent and dosing regimen was determined. A composite survival curve for all fluoroquinolones and isolates was constructed. Relationships between survival rate at 72 hours and AUC:MIC (minimum inhibitory concentration), peak:MIC, time above the MIC (percentage of dosing interval) for total and free drug concentrations were fit by using a sigmoid maximal effect (Emax) model. Survival was significantly better in the higher dosage group than in the lower dosage group. Time above MIC did not display a correlation with outcome. The AUC:MIC showed a greater correlation with outcome (R2 = 0.56 total, 0.54 free) than did peak:MIC (R2 = 0.52 total, 0.51 free). With use of composite data, total AUC:MIC ratios associated with 50%, 90%, and 99% of Emax were 34:1, 56:1, and 95:1, respectively

CONCLUSIONS

In this model, efficacy was achieved with the fluoroquinolone antibiotics at dosages yielding AUC0-24 comparable to those obtained in humans. One pharmacodynamic parameter (i.e., AUC:MIC) may be applied to various fluoroquinolones and isolates of S. pneumoniae. The AUC:MIC was more predictive of outcome than was time above the MIC or peak:MIC.

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.

Expression of lux genes in a clinical isolate of Streptococcus pneumoniae: using bioluminescence to monitor gemifloxacin activity

A clinical isolate of Streptococcus pneumoniae was transformed with a plasmid containing the lux operon of Photorhabdus luminescens that had been modified to function in gram-positive bacteria. Cells containing this plasmid produced light stably and constitutively, without compromising the growth rate. Light output was correlated with measurements of optical density and viable counts during exponential growth and provided a sensitive, real-time measure of the pharmacodynamics of the fluoroquinolone gemifloxacin.

What in vitro models of infection can and cannot do

The science of pharmacodynamics analyzes the relationship between an antimicrobial's bactericidal effects and its pharmacokinetics. Ideally, randomized and well-controlled clinical trials are the best way to determine pharmacodynamic properties. However, in vitro models that recapitulate in vivo drug clearance profiles represent an increasingly important technology for carrying out pharmacodynamic studies in a more cost-effective, timely, and easily controlled fashion. Although in vitro pharmacodynamic models cannot incorporate all variables seen in vivo, they do provide valuable information for the drug development process and the determination of optimal dosing regimens.

Pharmacodynamics of gemifloxacin against Streptococcus pneumoniae in an in vitro pharmacokinetic model of infection

The pharmacodynamics of gemifloxacin against Streptococcus pneumoniae were investigated in a dilutional pharmacodynamic model of infection. Dose fractionation was used to simulate concentrations of gemifloxacin in human serum associated with 640 mg every 48 h (one dose), 320 mg every 24 h (two doses), and 160 mg every 12 h (four doses). Five strains of S. pneumoniae for which MICs were 0.016, 0.06, 0.1, 0.16, and 0.24 mg/liter were used to assess the antibacterial effect of gemifloxacin. An inoculum of 10(7) to 10(8) CFU/ml was used, and each experiment was performed at least in triplicate. The pharmacodynamic parameters (area under the concentration-time curve [AUC]/MIC, maximum concentration of drug in serum [C(max)]/MIC, and the time that the serum drug concentration remains higher than the MIC [T > MIC]) were related to antibacterial effect as measured by the area under the bacterial-kill curve from 0 to 48 h (AUBKC(48)) using an inhibitory sigmoid E(max) model. Weighted least-squares regression was used to predict the effect of the pharmacodynamic parameters on AUBKC(48), and Cox proportional-hazards regression was used to predict the effect of the three pharmacodynamic parameters on the time needed to kill 99.9% of the starting inoculum (T99.9). There was a clear relationship between strain susceptibility and clearance from the model. The simulations (160 mg every 12 h) were associated with slower initial clearance than were the other simulations; in contrast, bacterial regrowth occurred with the 640-mg simulation when MICs were > or =0.1 mg/liter. The percentage coefficient of variance was 19% for AUBKC(48), and the inhibitory sigmoid E(max) model best fit the relationship between AUBKC(48) and AUC/MIC. C(max)/MIC and T > MIC fit less well. The maximum response occurred at an AUC/MIC of >300 to 400. In weighted least-squares regression analysis, there was no evidence that C(max)/MIC was predictive of AUBKC(48), but both AUC/MIC and T > MIC were. A repeat analysis using only data for which the T > MIC was >75% and for which hence regrowth was minimized indicated that AUC/MIC alone was predictive of AUBKC(48). Initial univariate analysis indicated that all three pharmacodynamic parameters were predictive of T99.9, but in the multivariate model only C(max)/MIC reached significance. These data indicate that gemifloxacin is an effective antipneumococcal agent and that AUC/MIC is the best predictor of antibacterial effect as measured by AUBKC(48). However, C(max)/MIC is the best predictor of speed of kill, as measured by T99.9. T > MIC also has a role in determining AUBKC(48), especially when the dose spacing is considerable. Once-daily dosing seems most suitable for gemifloxacin.

In vitro models, in vivo models, and pharmacokinetics: what can we learn from in vitro models?

In vitro pharmacokinetic models of infection can make an important contribution to the study of the pharmacodynamic properties of an antibacterial agent. In conjunction with animal and human pharmacodynamic evaluations, they provide data to allow for the optimization of drug dosing regimens. In vitro models can be used simply to describe the effect of a drug on a bacterial population as well as to provide data for more-analytical studies, including hypothesis testing. Analytical study designs provide information on the pharmacodynamic parameter best related to the chosen outcome, as well as its magnitude. Factors such as the characteristics of the model (method of drug removal, inoculum density, and growth phase), doses simulated, species and susceptibility range of bacteria, and methods and analytical tools used to measure antibacterial effect will have an effect on the conclusions drawn. In vitro models have an important future role in ensuring antibiotic efficacy and in reducing the risks of resistance.

Determination of moxifloxacin in growth media by high-performance liquid chromatography

A direct injection high-performance liquid chromatographic method with column switching has been developed to determine moxifloxacin in Mueller-Hinton broth. A LiChrocart 4-4 pre-column filled with a LiChrospher 100 RP 18, 5 microm and a 150 x 4.6 mm I.D. column packed with a Supelcozil ABZ+ Plus were used and led to a retention time of 5.70 min. Fluorescence detection allowed one to reach a quantification limit of 0.05 microg/ml with a 100-microl sample size. The standard curves were linear from 0.05 to 3.2 microg/ml. Intra- and inter-day imprecisions within the linearity range were < or =4.76 and < or =5.75%, respectively. The mean relative errors for the same day and the day-to-day inaccuracies ranged from -2.93 to +4.50% and from -1.10 to +6.00%, respectively. The method was demonstrated to be useful for pharmacokinetic-pharmacodynamic studies of moxifloxacin in an in vitro model.

What have we learned from pharmacokinetic and pharmacodynamic theories?

Pharmacokinetic characteristics and pharmacodynamic properties dictate antimicrobial response and, along with natural immune responses, clinical outcomes. As new agents are developed with long half-lives, we will lose the ability to differentiate between concentration-dependent and time-dependent properties. The area under the inhibitory concentration curve (AUIC) defines drug regimens as a ratio of drug exposure to minimum inhibitory concentration (MIC) and allows them to be compared with each other. With AUIC and agents with long half-lives, these comparisons are possible regardless of chemical classification or concentration or time-dependent activity. Historical examples of reduced drug exposure from decreased doses (i.e., cefaclor, clarithromycin, and ciprofloxacin), and thus low AUIC values, directly correlate with drug resistance. In the face of rising MICs (as is occurring worldwide with Streptococcus pneumoniae), close attention to appropriate dosing and concentration above the MIC may delay and potentially even prevent antibiotic resistance. Creating selective pressure on reliable antibiotics by inappropriately reducing their doses will undoubtedly challenge these agents and may destroy entire drug classes with similar mechanisms of action or resistance.

Comparative bactericidal activities of ciprofloxacin, clinafloxacin, grepafloxacin, levofloxacin, moxifloxacin, and trovafloxacin against Streptococcus pneumoniae in a dynamic in vitro model

Several new quinolones that exhibit enhanced in vitro activity against Streptococcus pneumoniae have been developed. Using a dynamic in vitro model, we generated time-kill data for ciprofloxacin, clinafloxacin, grepafloxacin, levofloxacin, moxifloxacin, and trovafloxacin against three isolates of quinolone-susceptible S. pneumoniae. Three pharmacokinetic profiles were simulated for each of the study agents (0.1, 1, and 10 times the area under the concentration-time curve [AUC]). Target 24-h AUCs were based upon human pharmacokinetic data resulting from the maximal daily doses of each agent. Ciprofloxacin was the least active agent against all three isolates. With regimens that simulated the human 24-h AUC, ciprofloxacin resulted in an initial, modest decline in the numbers of CFU per milliliter; however, by 48 h the numbers of CFU per milliliter returned to or exceeded the starting inoculum. At the AUC, levofloxacin resulted in variable bacteriostatic and bactericidal activities against the isolates. The remaining agents yielded bactericidal (99.9% reduction) activity by 48 h with regimens that simulated the AUC. At 0.1 time the AUC ciprofloxacin and levofloxacin produced no inhibitory effect, grepafloxacin exhibited bacteriostatic activity, trovafloxacin had mixed static and cidal activities, and clinafloxacin and moxifloxacin caused significant reductions in the numbers of CFU per milliliter by 48 h. All six agents produced cidal activity at 10 times the AUC. In this dynamic in vitro model of infection, the quinolones demonstrated various degrees of activity against S. pneumoniae. The rank order of activity, with respect to bactericidal effect, was ciprofloxacin (least active) << levofloxacin < grepafloxacin, trovafloxacin < clinafloxacin and moxifloxacin (most active). The rank order of the agents with respect to the selection of resistance was ciprofloxacin (most likely) > grepafloxacin, moxifloxacin, and trovafloxacin > levofloxacin > clinafloxacin.

Relationships of the area under the curve/MIC ratio to different integral endpoints of the antimicrobial effect: gemifloxacin pharmacodynamics in an in vitro dynamic model

Most integral endpoints of the antimicrobial effect are determined over an arbitrarily chosen time period, such as the dosing interval (tau), regardless of the actual effect duration. Unlike the tau-related endpoints, the intensity of the antimicrobial effect (I(E)) does consider its duration-from time zero to the time when bacterial counts on the regrowth curve achieve the same maximal numbers as in the absence of the antimicrobial. To examine the possible impact of this fundamental difference on the relationships of the antimicrobial effect to the ratio of the area under the concentration-time curve (AUC) to the MIC, a clinical isolate of Staphylococcus aureus was exposed to simulated gemifloxacin pharmacokinetics over a 40-fold range of AUC/MIC ratios, from 11 to 466 h. In each run, I(E) and four tau-related endpoints, including the area under the time-kill curve (AUBC), the area above the curve (AAC), the area between the control growth and time-kill curves (ABBC), and the ABBC related to the area under the control growth curve (AUGC), were calculated for tau = 24 h. Unlike the I(E), which displayed pseudolinear relationships with the AUC/MIC ratio; each tau-related endpoint showed a distinct saturation at potentially therapeutic AUC/MIC ratios (116 to 466 h) when the antimicrobial effect persisted longer than tau. This saturation results from the underestimation of the true effect and may be eliminated if ABBC, AAC, and AUBC (but not AUGC) are modified and determined in the same manner as the I(E) to consider the actual effect duration. These data suggest a marginal value of the tau-related endpoints as indices of the total antimicrobial effect. Since all of them respond to AUC/MIC ratio changes less than the I(E), the latter is preferable in comparative pharmacodynamic studies.

Influence of Pharmacokinetic and Pharmacodynamic Principles on Antibiotic Selection

When evaluating the efficacy of antibiotics for the treatment of respiratory tract infections, such as community acquired pneumonia and acute exacerbations of chronic bronchitis, assessment of clinical cure may not be the most relevant parameter, as it may not be related to microbiological eradication or to the minimum inhibitory concentration (MIC) of the infecting pathogen. It is more relevant to study the efficacy of the antibiotic in eradicating the bacterial pathogen, because this is frequently related to both the MIC of the pathogen and the antibiotic dosage regimen. Pharmacodynamics correlates the concentration of antibiotic in the blood or at the infection site with its biological effect against the organism (bacteriological eradication). For beta-lactams, the pharmacodynamic parameter that best correlates with eradication is time (T) above MIC (T > MIC); for aminoglycosides and fluoroquinolones, it is the area under the curve at 24 hours (AUC(24))-to-MIC ratio (AUC(24)/MIC). Knowledge of pharmacodynamics allows optimum use of antibiotics; in vitro models, animal models, and retrospective and prospective clinical trials have shown that the use of such knowledge optimizes bacteriological eradication and enhances patient outcome. In the future, pharmacodynamic studies will be used not only to assess optimal ways for antibiotics to eradicate resistant pathogens, but also to investigate the ability of antibiotics to prevent the development of resistance on therapy and to eradicate pathogens from colonizing sites.