Comparison of the effect of temafloxacin, ciprofloxacin, or placebo on cerebral blood flow, glucose, and oxygen metabolism in healthy subjects by means of positron emission tomography

Toxicity of quinolone antimicrobial agents

An approach to minimization of toxicity of a new compound is to elucidate the mechanisms of toxicity of analogous compounds and to clarify their structure-toxicity relationships. A problem with this approach, however, is that such elucidation remains difficult. For quinolones, some improvements in this mechanistic approach have been achieved in the central nervous system (CNS), particularly with regard to their interaction with non-steroidal anti-inflammatory drugs (NSAIDs), and in genotoxicity and phototoxicity studies, particularly in comparison with other toxicities, such as to the cardiovascular, gastrointestinal, bone, reproductive, and developmental systems. This review concentrates on a description of the known effects of quinolones on various organ systems in experimental animals and humans. Given the logarithmic increase in the synthesis of new quinolones, it is questionable whether these drugs share similar safety and efficacy. Nevertheless, this mechanistic approach to the investigation and minimization of toxicity has produced satisfactory results to date and deserves to be continued.

Positron Emission Tomography: Clinical Applications and Pharmaceutical Care

Positron emission tomography {PET) is a nuclear medicine imaging technique which exploits the unique physical characteristics of radionuclides that decay by positron emission. These characteristics allow for in vivo quantitative measurement of three-dimensional distributions of radioactivity with a spatial resolution of 5 mm using current detector technology. In addition to these physical advantages, PET is the only imaging technique that can use the short-lived positron emitting radionuclides of the so-called “organic” elements: carbon (C-11), nitrogen (N-13), and oxygen (0–15). These elements are the building blocks of physiological compounds and can be used to study most enzymes, receptors, and other metabolically important compounds and their associated reactions. PET allows for the study of a variety of physiological and biochemical processes through the application of particular radiopharmaceuticals. PET has also been used to study the interaction of receptor-specific ligands in several receptor systems including dopaminergic, adrenergic, serotinergic, and opiod. C-11 and F-18 labeled receptor ligands have been used to study receptor selectivity and receptor concentrations in vivo. Recently, PET has been used to measure the pharmacokinetics of several novel antibiotics in humans allowing the direct measurement of tissue concentrations and correlation with classical pharmacokinetic parameters. This review discusses some of the current applications of PET in more detail.

Pharmacokinetics of [18F]fleroxacin in healthy human subjects studied by using positron emission tomography

Positron emission tomography (PET) with [18F]fleroxacin was used to study the pharmacokinetics of fleroxacin, a new broad-spectrum fluoroquinolone, in 12 healthy volunteers (9 men and 3 women). The subjects were infused with a standard therapeutic dose of fleroxacin (400 mg) supplemented with approximately 20 mCi of [18F]fleroxacin. Serial PET images were made and blood samples were collected for 8 h, starting at the initiation of the infusion. The subjects were then treated with unlabeled drug for 3 days (400 mg/day). On the fifth day, infusion of radiolabeled drug, PET imaging, and blood collection were repeated. In most organs, there was rapid accumulation of radiolabeled drug, with stable levels achieved within 1 h after completion of the infusion. Especially high peak concentrations (in micrograms per gram) were achieved in the kidney (> 34), liver (> 25), lung (> 20), myocardium (> 19), and spleen (> 18). Peak concentrations of drug more than two times the MIC for 90% of Enterobacteriaceae strains tested (> 10-fold for most organisms) were achieved in all tissues except the brain and remained above this level for more than 6 to 8 h. The plateau concentrations in tissues (2 to 8 h, in micrograms per gram +/- standard error of the mean) of drug were as follows: brain, 0.83 +/- 0.032; myocardium, 4.53 +/- 0.24; lung, 5.80 +/- 0.48; liver, 7.31 +/- 0.33; spleen, 6.00 +/- 0.47; bowel, 3.53 +/- 0.74; kidney, 8.85 +/- 0.64; bone, 2.87 +/- 0.29; muscle, 4.60 +/- 0.33; prostate, 4.65 +/- 0.48; uterus, 3.87 +/- 0.39; breast, 2.68 +/- 0.11; and blood, 2.35 +/- 0.09. Concentrations of fleroxacin in tissue were similar in males and females, before and after pretreatment with unlabeled drug.

Pharmacokinetics of 18F-labeled fleroxacin in rabbits with Escherichia coli infections, studied with positron emission tomography

18F-labeled fleroxacin was used to measure the pharmacokinetics of fleroxacin in healthy and infected animals by positron emission tomography (PET) and tissue radioactivity measurements. In all experiments, a pharmacological dose of unlabeled drug (10 mg/kg) was coinjected with the tracer. The pharmacokinetics of [18F]fleroxacin was measured in groups of healthy mice (n = six per group) at 10, 30, 60, and 120 min after injection and in groups of rats with Escherichia coli thigh infections (n = six per group) at 60 and 120 min after injection by radioactivity measurements in excised tissues. In healthy rabbits (n = 4) and in rabbits with E. coli thigh infections (n = 4), tissue concentrations of drug were determined by serial PET imaging over 2 h; after the final image was acquired, animals were sacrificed and concentrations measured by PET were compared with the results of tissue radioactivity measurements. In all three species, there was rapid equilibration of [18F]fleroxacin to significant concentrations in most peripheral organs; low concentrations of drug were detected in the brain. Accumulations of radiolabeled drug in infected and healthy thigh muscles were similar. Peak concentrations of drug of more than three times the MIC for 90% of members of the family Enterobacteriaceae (greater than 100-fold for most organisms) were achieved in all tissues except brain and remained above this level for more than 2 h. Especially high peak concentrations were achieved in the kidney (greater than 75 micrograms/g), liver (greater than 50 micrograms/g), blood (greater than 25 micrograms/g), and bone and lung (greater than 10 micrograms/g). Since the MICs for 90% of all Enterobacteriaceae are <2 micrograms/ml, fleroxacin should be particularly useful in treating gram-negative infections affecting these tissues. In contrast, the low concentration of drug delivered to the brain should limit the toxicity of the drug for the central nervous system.

Comparison of organ-specific toxicity of temafloxacin in animals and humans

This article summarizes animal studies conducted to determine the toxic and mutagenic potential of temafloxacin. The four target tissues of potential concern with fluoroquinolone use are the kidney, the eye, the weight-bearing joints of young animals, and the central nervous system. Based on the results of these studies in rats and dogs, it appears unlikely that crystalluria or nephrotoxicity will occur in humans who receive temafloxacin. Pre-marketing clinical trials in humans (n = 5,308) correlate well with chronic toxicity animal studies, reporting no crystalluria or clinically significant nephrotoxicity. Reversible electroretinographic (ERG) changes in dog studies were demonstrated only with the administration of high temafloxacin dosages. A Phase I study evaluating the safety of temafloxacin at 600 mg b.i.d. for 14 days in human subjects reported no significant changes in ophthalmologic parameters. Evidence of cartilaginous joint damage was observed in puppies receiving oral temafloxacin, in young dogs receiving intravenous temafloxacin, and in a single dog receiving a lethal dosage in a dose range-finding study. However, these toxic findings were not evident in any dogs in the subacute or chronic oral toxicity studies or in a longer duration intravenous study. Although limited evidence would suggest that young children may not be at risk, thorough clinical investigations of quinolones in these patients have only recently been initiated. Signs of central nervous system toxicity caused by temafloxacin were absent in two rodent studies, during which clonic convulsions were induced by concomitant use of fenbufen plus enoxacin or ciprofloxacin, and in human subjects evaluated by positron emission tomography. Temafloxacin, contrary to most other quinolones, was considered nonmutagenic in all mutagenicity tests conducted. In reproductive studies, temafloxacin was not uniquely toxic to the developing conceptus in the laboratory rat, mouse, rabbit, or primate. Based on these animal studies, temafloxacin appears to be non-mutagenic and to have a low potential for producing renal or ocular toxicity; however, like other quinolones, it should not be routinely used in children or pregnant women because of evidence of cartilage damage reported in young dogs. Premarketing clinical trials to date confirm the safety of temafloxacin use in adults.

Assessment of temafloxacin neurotoxicity in rodents

Temafloxacin hydrochloride (HCl) was studied in fenbufen-treated mice and for its effects on the locomotor activity of rats. Mice were observed for convulsions for 90 minutes after the administration of temafloxacin HCl (100 mg/kg orally [p.o.]), ciprofloxacin HCl (100 mg/kg p.o.), enoxacin (100 mg/kg p.o.), or oxolinic acid (100 mg/kg p.o.). The quinolones were coadministered with either saline (0.2 mL p.o.) or fenbufen (100 mg/kg p.o.), a nonsteroidal antiinflammatory compound. No convulsions occurred following the administration of the quinolones with saline. Clonic convulsions occurred in 90% of mice receiving enoxacin and fenbufen and in 20% of mice receiving ciprofloxacin HCl and fenbufen. No convulsions occurred in mice receiving temafloxacin HCl or oxolinic acid with fenbufen. At doses of 30, 100, and 300 mg/kg p.o., temafloxacin HCl and enoxacin had no effect on locomotor activity of rats, but oxolinic acid markedly stimulated locomotor activity. The lack of central nervous system (CNS) stimulation and the absence of interaction with fenbufen suggest that temafloxacin may have less potential for adverse CNS effects than other quinolone compounds.