Bactericidal activity of enoxacin and lomefloxacin against Escherichia coli KL16

Suppression of Reactive Oxygen Species Accumulation Accounts for Paradoxical Bacterial Survival at High Quinolone Concentration

When bacterial cells are exposed to increasing concentrations of quinolone-class antibacterials, survival drops, reaches a minimum, and then recovers, sometimes to 100%. Despite decades of study, events underlying this paradoxical high-concentration survival remain obscure. Since reactive oxygen species (ROS) have been implicated in antimicrobial lethality, conditions generating paradoxical survival were examined for diminished ROS accumulation. Escherichia coli cultures were treated with various concentrations of nalidixic acid, followed by measurements of survival, rate of protein synthesis, and ROS accumulation. The last measurement used a dye (carboxy-H2DCFDA) that fluoresces in the presence of ROS; fluorescence was assessed by microscopy (individual cells) and flow cytometry (batch cultures). High, nonlethal concentrations of nalidixic acid induced lower levels of ROS than moderate, lethal concentrations. Sublethal doses of exogenous hydrogen peroxide became lethal and eliminated the nalidixic acid-associated paradoxical survival. Thus, quinolone-mediated lesions needed for ROS-executed killing persist at high, nonlethal quinolone concentrations, thereby implicating ROS as a key factor in cell death. Chloramphenicol suppressed nalidixic acid-induced ROS accumulation and blocked lethality, further supporting a role for ROS in killing. Nalidixic acid also inhibited protein synthesis, with extensive inhibition at high concentrations correlating with lower ROS accumulation and paradoxical survival. A catalase deficiency, which elevated ROS levels, overcame the inhibitory effect of chloramphenicol on nalidixic acid-mediated killing, emphasizing the importance of ROS. The data collectively indicate that ROS play a dominant role in the lethal action of narrow-spectrum quinolone-class compounds; a drop in ROS levels accounted for the quinolone tolerance observed at very high concentrations.

Bacterial Infections in the Elderly Patient: Focus on Sitafloxacin

Sitafloxacin (DU-6859a) is a new-generation oral fluoroquinolone with in vitro activity against a broad range of Gram-positive and -negative bacteria, including anaerobic bacteria, as well as against atypical bacterial pathogens. Particularly in Japan this antibiotic was approved in 2008 for treatment of a number of bacterial infections caused by Gram-positive cocci and Gram-negative cocci and rods, including anaerobia atypical bacterial pathogens. As compared to oral levofloxacin sitafloxacin was non-inferior in the treatment of community-acquired pneumonia and non-inferior in the treatment of complicated urinary tract infections, according to the results of randomized, double-blind, multicentre, non-inferiority trials. Non-comparative studies demonstrated the efficacy of oral sitafloxacin in otorhinolaryngological infections, urethritis in men, cervicitis in women and odontogenic infections. Most common adverse reactions were gastrointestinal disorders and laboratory abnormalities in patients receiving oral sitafloxacin; diarrhea and liver enzyme elevations were among the common. In the Japanese population sitafloxacin covers broad spectrum of bacteria as compared to carbapenems, whereas in the Caucasians its use is currently limited due to the potential for ultraviolet A phototoxicity. Sitafloxacin is a promising therapeutic agent which merits further investigation in randomized clinical trials of elderly patients.

Effect of N-1/c-8 ring fusion and C-7 ring structure on fluoroquinolone lethality

Quinolones rapidly kill bacteria by two mechanisms, one that requires protein synthesis and one that does not. The latter, which is measured as lethal action in the presence of the protein synthesis inhibitor chloramphenicol, is enhanced by N-1 cyclopropyl and C-8 methoxy substituents, as seen with the highly lethal compound PD161144. In some compounds, such as levofloxacin, the N-1 and C-8 substituents are fused. To assess the effect of ring fusion on killing, structural derivatives of levofloxacin and PD161144 differing at C-7 were synthesized and examined with Escherichia coli. A fused-ring derivative of PD161144 exhibited a striking absence of lethal activity in the presence of chloramphenicol. In general, ring fusion had little effect on lethal activity when protein synthesis was allowed, but fusion reduced lethal activity in the absence of protein synthesis to extents that depended on the C-7 ring structure. Additional fused-ring fluoroquinolones, pazufloxacin, marbofloxacin, and rufloxacin, also exhibited reduced activity in the presence of chloramphenicol. Energy minimization modeling revealed that steric interactions of the trans-oriented N-1 cyclopropyl and C-8 methoxy moieties skew the quinolone core, rigidly orient these groups perpendicular to core rings, and restrict the rotational freedom of C-7 rings. These features were not observed with fused-ring derivatives. Remarkably, structural effects on quinolone lethality were not explained by the recently described X-ray crystal structures of fluoroquinolone-topoisomerase IV-DNA complexes, suggesting the existence of an additional drug-binding state.

Activity-bioavailability balance in oral drug development for a selected group of 6-fluoroquinolones

A nomogram is proposed to select the best candidate in drug development studies with quinolones and is intended to substitute other possible models. The nomogram is referred to as an activity-bioavailability balance (ABB) because it includes the following two criteria: ABB = [(1/gm MIC drug candidate)/ (1/gm MIC ciprofloxacin)].[(F(calc) drug candidate)/( F(calc) ciproflaxacin)]. The in vitro activity of a group of 4'N-alkyl-ciprofloxacin derivatives was determined together with that of ciprofloxacin, initially against some reference strains and subsequently against 159 clinical isolates of eight selected species. The inverse of the geometric mean of the lowest concentration of drug at which the original inoculum was reduced to no more than two colonies (1/gm MIC), as an antimicrobial activity parameter, and the absolute oral bioavailability index (F(calc)), as predicted from in situ intestinal absorption rate constants, were used for calculation of the ABB values, which ranged from 0.1 to 17 for the species and compounds tested. Ciprofloxacin was the best candidate only against Escherichia coli, whereas 4'N-methyl- and/or 4'N-ethyl-ciprofloxacin showed better or much better ABB values than the model drug, and can be selected as potential drug candidates against the remaining clinical strains. The procedure described could be a useful technique for further drug development studies.

Bactericidal activity of levofloxacin and ciprofloxacin on clinical isolates of different phenotypes of Pseudomonas aeruginosa

Levofloxacin has been reported to have in vitro activity against both gram-positive and gram-negative bacteria. A recent survey carried out at our Institution showed clinical isolates of Pseudomonas aeruginosa to be more susceptible to levofloxacin than to ciprofloxacin. The in vitro activity of the two fluoroquinolones was evaluated further by looking at their bactericidal activity against two strains of each of the following antibio-phenotypes of P. aeruginosa: levofloxacin- and ciprofloxacin-susceptible, levofloxacin-susceptible/ciprofloxacin-resistant, levofloxacin-susceptible/ciprofloxacin-susceptible and ceftazidime-resistant, (National Committee for Clinical Laboratory Standards susceptibility breakpoints were used). MIC and MBC values were measured and time-kill experiments were carried out. Drugs were used at susceptibility or resistance breakpoint concentrations in the time-kill experiments and results were recorded over 12 h in an attempt to link in vitro results with the clinical situation The polypeptide profiles of outer membrane preparations of the six strains were examined by gel electrophoresis. Levofloxacin was shown to be more bactericidal than ciprofloxacin in the time-kill experiments. No differences were observed between the outer membrane proteins of the six strains. Levofloxacin showed greater bactericidal activity against P. aeruginosa clinical isolates than ciprofloxacin.

DNA gyrase, topoisomerase IV, and the 4-quinolones

For many years, DNA gyrase was thought to be responsible both for unlinking replicated daughter chromosomes and for controlling negative superhelical tension in bacterial DNA. However, in 1990 a homolog of gyrase, topoisomerase IV, that had a potent decatenating activity was discovered. It is now clear that topoisomerase IV, rather than gyrase, is responsible for decatenation of interlinked chromosomes. Moreover, topoisomerase IV is a target of the 4-quinolones, antibacterial agents that had previously been thought to target only gyrase. The key event in quinolone action is reversible trapping of gyrase-DNA and topoisomerase IV-DNA complexes. Complex formation with gyrase is followed by a rapid, reversible inhibition of DNA synthesis, cessation of growth, and induction of the SOS response. At higher drug concentrations, cell death occurs as double-strand DNA breaks are released from trapped gyrase and/or topoisomerase IV complexes. Repair of quinolone-induced DNA damage occurs largely via recombination pathways. In many gram-negative bacteria, resistance to moderate levels of quinolone arises from mutation of the gyrase A protein and resistance to high levels of quinolone arises from mutation of a second gyrase and/or topoisomerase IV site. For some gram-positive bacteria, the situation is reversed: primary resistance occurs through changes in topoisomerase IV while gyrase changes give additional resistance. Gyrase is also trapped on DNA by lethal gene products of certain large, low-copy-number plasmids. Thus, quinolone-topoisomerase biology is providing a model for understanding aspects of host-parasite interactions and providing ways to investigate manipulation of the bacterial chromosome by topoisomerases.

Enoxacin: a reappraisal of its clinical efficacy in the treatment of genitourinary tract infections

Enoxacin is a 6-fluoronaphthyridinone antibacterial agent with good in vitro activity against Neisseria gonorrhoeae and most Gram-negative urinary tract pathogens. It is less active in vitro against Acinetobacter spp., Pseudomonas aeruginosa, and most Gram-positive bacteria, than against Gram-negative organisms. Enoxacin is rapidly absorbed, with a high oral bioavailability (87 to 91%). Of the absorbed dose, 44 to 56% is excreted unchanged in the urine, with peak urinary concentrations (>500 mg/L within 4 hours) remaining high (>100 mg/L) for up to 24 hours, sufficient to inhibit most urinary tract pathogens. Single (400 mg) and multiple oral dose regimens (100 to 600 mg twice or 3 times daily for 5 to 14 days) of enoxacin are as effective for the treatment of patients with complicated or uncomplicated urinary tract infections as other antibacterial agents such as amoxicillin, cefuroxime axetil, cotrimoxazole (trimethoprim-sulfamethoxazole) or trimethoprim. Noncomparative data suggest that enoxacin is also an effective agent for the treatment of prostatitis. Single 400 mgoral doses of enoxacin produce >/- 95% bacteriological cure rates in gonococcal infections, comparable to those produced by single intramuscular doses of ceftriaxone 250 mg. Perioperative doses of oral enoxacin 200 mg provide effective prophylaxis against postoperative bacteriuria after transurethral resection of the prostate. Concomitant administration of enoxacin with a number of commonly used therapeutic agents (e.g. antacids, methylxanthines, warfarin) affects the pharmacokinetic properties of either enoxacin or the coadministered agents. Enoxacin is reasonably well tolerated, with the incidence of adverse experiences ranging from 0 to 24%. Adverse events are mainly gastrointestinal, neurological or dermatological and resolve with minimal intervention. Overall, although enoxacin exhibits a number of clinical characteristics that are similar to those of other agents for the treatment of genitourinary tract infections, the advantages offered by this agent generally do not outweigh those of alternative fluoroquinolone agents. Thus, it is likely to prove to be yet another addition to the list of agents available for the management of these infections.

Levofloxacin. A review of its antibacterial activity, pharmacokinetics and therapeutic efficacy

Levofloxacin, an oral fluoroquinolone antibacterial agent, is the optical S-(-) isomer of ofloxacin. In vitro it is generally twice as potent as ofloxacin. Levofloxacin is active against most aerobic Gram-positive and Gram-negative organisms and demonstrates moderate activity against anaerobes. Drug penetration into body tissues and fluids is rapid and widespread after oral administration. In clinical trials conducted in Japan, oral levofloxacin has demonstrated antibacterial efficacy against a variety of infections, including upper and lower respiratory tract, genitourinary, obstetric, gynaecological and skin and soft tissues. In comparative trials with ofloxacin, levofloxacin, at half the daily dosage of ofloxacin, showed equivalent efficacy and a reduced incidence of adverse effects in the treatment of lower respiratory tract and complicated urinary tract infections. Levofloxacin has a tolerability profile similar to that of other oral fluoroquinolones, with gastrointestinal and central nervous system effects reported most commonly. Theophylline dosage adjustment does not appear to be necessary in patients receiving concomitant levofloxacin. Coadministration with antacids or with other drugs containing divalent or trivalent cations reduces levofloxacin absorption. Thus, levofloxacin has potential as a broad spectrum antibacterial drug in the treatment of a variety of infections. However, clinical trials recruiting non-Japanese patients are in progress and these results will form a basis on which future recommendations for the broader use of levofloxacin can be made.

Analysis of macromolecular biosynthesis to define the quinolone-induced postantibiotic effect in Escherichia coli

Quinolones inhibit DNA gyrase, and the major effects of this inhibition are on replication and transcription of DNA. The postantibiotic effect (PAE) refers to continued inhibition of cell division, in terms of the viable count, following transient exposure to an antibiotic. Previous work has shown that quinolone-treated cells have not fully recovered by the time the classically defined PAE has ended. We describe the PAE of the quinolones CI-960, enoxacin, and ciprofloxacin on macromolecular biosynthesis in the clinical isolate Escherichia coli J96 in an attempt to relate the PAE to the time that it actually takes for the cells to recover fully. DNA synthesis was inhibited immediately upon exposure to these quinolones at 0.5x or 0.75x the MIC. This inhibition continued for several hours following quinolone removal. The effects of these quinolones on RNA and protein synthesis varied; enoxacin treatment at 0.5x the MIC resulted in an increase of over 60% in both RNA and protein synthesis per unit of cell mass, while ciprofloxacin and CI-960 at that level had no significant effects on either RNA or protein synthesis. The effects of enoxacin and ciprofloxacin on bacterial protein profiles were also distinguishable, and these changes corresponded to their PAE on DNA synthesis. Throughout the study, all measures of the physiological status of the cells returned to normal by the time DNA synthesis per unit of cell mass did so. These results suggest that DNA synthesis per unit of cell mass provides an accurate measure of the time required for quinolone-treated cells to recover fully.

The mode of action of quinolones: the paradox in activity of low and high concentrations and activity in the anaerobic environment

All 4-quinolones that have been examined display rapid bactericidal activity which is biphasic. At concentrations above the MIC, the lethality of the drugs increases until a concentration known as the optimum bactericidal concentration (OBC) beyond which the bactericidal activity then declines. The biphasic response appears to be due to the inhibition of RNA synthesis at concentrations above the OBC, as RNA synthesis is required for the full bactericidal activity of the 4-quinolones. However, differences in the biphasic response are observed as some fluoroquinolones are still able to kill bacteria in the absence of bacterial protein or RNA synthesis, thus reducing the inhibition of bactericidal activity at concentrations above the OBC. It has been proposed that this ability to kill bacteria in the absence of protein or RNA synthesis is due to the possession of an additional bactericidal mechanism by these fluoroquinolones. Oxygen also appears to be essential for the lethality of the clinically available 4-quinolones although it is not required for the drugs to inhibit bacterial multiplication. Therefore these drugs are not bactericidal under anaerobic conditions.

Mode of action of the new quinolones: new data

New details of the molecular interactions of quinolones with their target DNA gyrase and DNA have come from the nucleotide sequences of the gyrA genes from resistant mutants of Escherichia coli and wild-type strains of other bacteria and studies of gyrase A tryptic fragments, all suggesting the importance of an amino-terminal domain in quinolone action. Alterations in DNA supertwisting were also associated with altered quinolone susceptibility, possibly by indirect effects on DNA gyrase expression. Specific binding of relevant concentrations of norfloxacin to a complex of DNA gyrase and DNA in the presence of ATP, the cooperativity of DNA binding, and the crystalline structure of nalidixic acid have led to a model in which quinolones bind cooperatively to a pocket of single-strand DNA created by DNA gyrase. Quinolones vary in their relative activity against DNA gyrase and its eukaryotic homolog topoisomerase II, and in some assays increased action against the eukaryotic enzyme was associated with genotoxicity. Inhibition of bacterial DNA synthesis by quinolones may correlate with MICs in some species, but comparisons of drug accumulation and inhibition of DNA synthesis in permeabilized cells among species have been difficult to interpret. The specific factors necessary for bacterial killing by quinolones in addition to interaction with DNA gyrase have remained elusive, but include oxygen and new protein synthesis. The coordinate expression of the SOS proteins appears not to be necessary for quinolone lethality. Two independent mutants with selective reduced killing by quinolones and beta-lactams indicate overlap in the pathways of bactericidal activity of these classes of agents with distinct targets.

Correlation between pharmacokinetics, pharmacodynamics and efficacy of antibacterial agents in animal models

On the basis of a review of published literature it is demonstrated that pharmacokinetic parameters of antibacterial agents correlate well with therapeutic efficacy in animal models, provided pharmacodynamic parameters are taken into account. The time that serum levels exceed the MIC is the most significant parameter determining efficacy of beta-lactams, whereas the efficacy of aminoglycosides is dependent on serum concentrations and the area under the curve. The efficacy of quinolones tends to be correlated to the doses administered or drug levels achieved. However, specific pharmacodynamic properties contribute significantly to the therapeutic efficacy of a few quinolones only whereas other quinolones lack these specific pharmacodynamic attributes. Thus, the correlation of pharmacokinetic parameters with therapeutic efficacy provides important basic concepts for the design of preclinical and clinical studies in the course of which additional pharmacodynamic properties will become apparent.