Development of resistance to fleroxacin during therapy of experimental methicillin-susceptible Staphylococcus aureus endocarditis

Impact of antimicrobial dosing regimen on evolution of drug resistance in vivo: fluconazole and Candida albicans

Numerous factors have been theorized to affect the development of antimicrobial resistance, including those specific to the host, the organism, the environment, the drug, and the drug prescriber. One variable under the control of the prescriber is the drug dosing regimen. Dosing regimens can vary in dose level, dosing interval, and treatment duration. The current studies examined the relationships between antimicrobial dosing regimens and resistance development by use of an in vivo model. A murine model of systemic Candida albicans infection was used to examine resistance emergence during exposure to the triazole antifungal fluconazole. Data from this experimental model demonstrated that the more frequently administered dosing prevented selection of the isogenic resistant cell populations. Conversely, dosing regimens producing prolonged sub-MIC effects appeared to contribute to the outgrowth of isogenic resistant strains. The association between dosing and resistance emergence observed in the current investigation is disparate from that described for antimicrobial compounds with cidal killing characteristics. The inhibitory or static antimicrobial activity of the triazole compounds may explain these differences.

Efficacy of trovafloxacin against experimental Staphylococcus aureus endocarditis

Trovafloxacin is a new fluoronaphthyridone chemically and functionally related to members of the fluoroquinolone class of antimicrobial agents. The in vivo efficacy of the drug was compared with that of vancomycin by using the rabbit model of left-sided endocarditis. Rabbits infected with either a nafcillin-susceptible or -resistant test strain were treated with trovafloxacin (13.3 mg/kg of body weight every 12 h) or vancomycin (25 mg/kg of body weight every 8 h) for 4 days. In comparison with untreated controls, both antimicrobial agents effectively cleared bacteremia and significantly reduced bacterial counts in vegetations and tissues of animals infected with either test strain. No resistance to trovafloxacin emerged in test strains during therapy. We conclude that in this model trovafloxacin is as efficacious as vancomycin is and may serve as a viable alternative to vancomycin for use in humans with similar infections.

Pharmacodynamics of vancomycin alone and in combination with gentamicin at various dosing intervals against methicillin-resistant Staphylococcus aureus-infected fibrin-platelet clots in an in vitro infection model

We compared the pharmacodynamic activities of vancomycin with or without gentamicin in an in vitro infection model with methicilin-resistant Staphylococcus aureus-infected fibrin-platelet clots. Infected fibrin-platelet clots (FPCs) were prepared with human cryoprecipitate, human platelets, thrombin, and the organism (approximately 10[9] CFU of MRSA-494/g) and were suspended with monofilament line in an infection model capable of simulating human pharmacokinetics. Antibiotics were bolused to simulate vancomycin regimens of 2 g every 24 h (q24h), 1 g q12h, 500 mg q6h, and continuous infusion (steady-state concentration of 20 microg/ml) and gentamicin regimens of 1.5 mg/kg of body weight q12h and 5 mg/kg once daily (q.d.). Model experiments were performed in duplicate over 72 h. FPCs were removed from the models in quadruplicate at 0, 8, 24, 32, 48, 72 h, weighed, homogenized, diluted, and plated to determine colony counts. The inoculum density at 72 h was used to compare bactericidal activities between the regimens. All regimens containing vancomycin significantly decreased the bacterial inoculum compared to the growth control (P < 0.001). Vancomycin monotherapy regimens were similar in bacterial kill regardless of dosing frequency. The addition of gentamicin (either q12h or q.d.) significantly improved the bactericidal activity of the vancomycin q6h, q12h, and q24h regimens (P < 0.001). The greatest reduction in bacterial density at 72 h (P < 0.001) and the most rapid rate of kill (time to 99.9% killing) were achieved with the regimen consisting of 2 g of vancomycin q24h plus gentamicin (q.d. or q12h).

Efficacy of deoxycholate amphotericin B and unilamellar liposomal amphotericin B in prophylaxis of experimental Aspergillus fumigatus endocarditis

OBJECTIVE

To evaluate and compare in vivo the protective efficacy of unilamellar liposomal amphotericin B (L-AmB) with that of deoxycholate amphotericin B (D-AmB) in experimental endocarditis.

MATERIAL AND METHODS

In the rabbit model of experimental Aspergillus fumigatus endocarditis, two doses of each antifungal agent (1.5 mg/kg each) were administered intravenously at 4 hours and at 30 minutes before challenge with an inoculum of A. fumigatus. Three days later, the animals were sacrificed, and the aortic vegetations were analyzed.

RESULTS

All 19 animals that did not receive chemoprophylaxis acquired endocarditis. In contrast, endocarditis developed in 2 of 10 animals pretreated with D-AmB (P < 0.01) and 3 of 8 animals pretreated with L-AmB (P < 0.01). Both D-AmB and L-AmB prevented the development of endocarditis due to A. fumigatus and decreased the concentration of fungi in the aortic vegetations by more than 1 log10.

CONCLUSION

In the rabbit experimental model of Aspergillus endocarditis, D-AmB and L-AmB were equally effective in reducing the incidence of the infection and the tissue burden of fungi.

The population dynamics of antimicrobial chemotherapy

We present and analyze a series of mathematical models for the emergence of resistance during antibiotic treatment of an infected host. The models consider the population dynamics of antibiotic-sensitive and -resistant bacteria during the course of treatment and addresses the following problems: (i) the probability of obtaining a resistant mutant during the course of treatment as a function of antibiotic exposure; (ii) the conditions under which high, infrequent doses of an antibiotic are predicted to succeed in preventing the emergence of resistance; (iii) the conditions for the success of multiple drug treatment in suppressing the emergence of resistance and the relationship between antibiotic synergism and suppression of resistance; and (iv) the conditions under which nonadherence to the prescribed treatment regimen is predicted to result in treatment failure due to resistance. We analyze the predictions of the model for interpreting and extrapolating existing experimental studies of treatment efficacy and for optimizing treatment protocols to prevent the emergence of resistance.

Current perspectives on glycopeptide resistance

In the last 5 years, clinical isolates of gram-positive bacteria with intrinsic or acquired resistance to glycopeptide antibiotics have been encountered increasingly. In many of these isolates, resistance arises from an alteration of the antibiotic target site, with the terminal D-alanyl-D-alanine moiety of peptidoglycan precursors being replaced by groups that do not bind glycopeptides. Although the criteria for defining resistance have been revised frequently, the reliable detection of low-level glycopeptide resistance remains problematic and is influenced by the method chosen. Glycopeptide-resistant enterococci have emerged as a particular problem in hospitals, where in addition to sporadic cases, clusters of infections with evidence of interpatient spread have occurred. Studies using molecular typing methods have implicated colonization of patients, staff carriage, and environmental contamination in the dissemination of these bacteria. Choice of antimicrobial therapy for infections caused by glycopeptide-resistant bacteria may be complicated by resistance to other antibiotics. Severe therapeutic difficulties are being encountered among patients infected with enterococci, with some infections being untreatable with currently available antibiotics.

Fleroxacin. A review of its pharmacology and therapeutic efficacy in various infections

The fluoroquinolone antibacterial agent fleroxacin has a broad spectrum of in vitro activity which encompasses most Gram-negative species (particularly Enterobacteriaceae) and a number of Gram-positive organisms, including methicillin-sensitive staphylococci. It is available as oral and intravenous formulations. In clinical trials, fleroxacin has been evaluated in the treatment of uncomplicated urinary tract infections (single or multiple once-daily oral doses of 200 or 400mg), gonorrhoea and chancroid (single oral doses of 200 or 400mg), complicated urinary tract, nonpneumococcal lower respiratory tract and skin and soft tissue infections and typhoid fever (multiple once-daily oral or intravenous regimens, usually 400 mg/day), bacterial enteritis, and traveller's diarrhoea (single or multiple once-daily oral doses of 400mg). Bacteriological cure rates were generally around 90% or higher in complicated and uncomplicated urinary tract infections, uncomplicated gonorrhoea (approximately 100%), pyelonephritis, bacterial enteritis and typhoid fever, and exceeded 80% in lower respiratory tract, and skin and soft tissue infections and chancroid. These cure rates were similar to, or better than, those achieved with standard comparator antibacterial agents such as penicillins, cephalosporins, cotrimoxazole, or other quinolones. Fleroxacin 400mg once daily also achieved bacteriological cure in approximately 80% of patients with bone and joint infections in preliminary studies. In Japanese studies using a lower dosage of 200 or 300 mg/day, fleroxacin was reported to be bacteriologically effective in a range of infections, including urinary tract and upper and lower respiratory tract infections. Fleroxacin has a relatively long elimination half-life, which allows once-daily administration, and it appears to have less propensity for interactions with other medications in comparison to many other fluoroquinolones. Its tolerability profile is typical of this class of compound, with adverse events mostly relating to the gastrointestinal tract, CNS, and skin and appendages (including phototoxicity). Recent pooled tolerability data from worldwide clinical trials indicate that adverse events are reported by approximately 27% of patients receiving 200 mg/day orally or 400 mg/day orally or intravenously, and 17% of those receiving a single oral dose of 400mg. These exceed incidences reported for established fluoroquinolones, possibly indicating recent trends towards increased rates of reported adverse effects with these agents. However, in direct comparative studies with twice-daily fluoroquinolones, fleroxacin 400mg once daily produced a similar incidence of adverse effects to ofloxacin 800 mg/day and a slightly higher incidence than ciprofloxacin 1000 mg/day, while fleroxacin 200mg once daily produced a similar incidence to norfloxacin 800 mg/day.(ABSTRACT TRUNCATED AT 400 WORDS)

Bactericidal activities of teicoplanin, vancomycin, and gentamicin alone and in combination against Staphylococcus aureus in an in vitro pharmacodynamic model of endocarditis

We adapted an in vitro pharmacodynamic model of infection to incorporate simulated endocardial vegetations. The bactericidal activities of teicoplanin, vancomycin, gentamicin, and various combinations of these drugs were studied against a strain of methicillin-susceptible Staphylococcus aureus obtained from a patient being treated for endocarditis at Detroit Receiving Hospital. Bacteria were grown overnight, concentrated, and added to a mixture of cryoprecipitate (80%) and thrombin (10%) to achieve approximately 5 x 10(9) CFU/g. Fibrin clots (8 to 10) were suspended into the model, removed at 24, 48, and 72 h in duplicate, weighed, and homogenized in 1.25% trypsin. Control experiments were conducted to characterize the growth kinetics. The following antibiotics were administered to simulate the pharmacokinetics of the drugs in humans: teicoplanin at 3 and 15 mg/kg of body weight, vancomycin at 15 mg/kg, and gentamicin at 1 mg/kg. Fibrin clot samples used to detect resistance were plated on antibiotic-containing tryptic soy agar plates. For the teicoplanin and vancomycin regimens, protein binding to cryoprecipitate, thrombin, and fibrin clot was determined to be 32, 43, and 50% and 26, 28, and 29%, respectively. In comparison with no treatment, vancomycin or teicoplanin at 15 mg/kg or either of these regimens combined with gentamicin significantly reduced bacterial counts (P < 0.0001). Monotherapy with teicoplanin at 3 mg/kg or gentamicin resulted in no killing activity. Combination treatment with teicoplanin at 3 mg/kg and gentamicin resulted in the killing of approximately 2 log10 CFU/g by 72 h and the development of resistance to gentamicin. The results obtained with the in vitro model of endocarditis are similar to the results reported by several investigators with the rabbit model of infective endocarditis. This unique infection model is useful for designing initial drug dosage regimens and may be predictive of drug efficacy against infective endocarditis.

Quinolone resistance in Pseudomonas aeruginosa and Staphylococcus aureus. Development during therapy and clinical significance

This review focuses on published information on the experimental as well as clinical data on the emergence of quinolone resistant isolates. In the course of clinical use of fluoroquinolones, only a sporadic emergence of quinolone resistance has been noted. The resistant organisms emerged particularly in certain clinical settings where large numbers of organisms frequently causing chronic infections are present and/or in loci where quinolone concentrations may not be optimal. In terms of occurrence in individuals, quinolone resistance has emerged most frequently in hospitalized and nursing-home patients with identifiable risk factors. Epidemiological studies revealed that in nearly all the cases studied one or one predominating quinolone resistant clone was selected that was horizontally transmitted. Thus, the emergence of quinolone resistance is not due to an independent selection of resistant strains in a number of patients, but to the clonal spread of one strain once it has acquired quinolone resistance. Therefore, the rate of quinolone resistance is very likely to be lower than reported.