Joseph M. Blondeau

Mutant Prevention Concentrations of Fluoroquinolones for Clinical Isolates of Streptococcus pneumoniae

ABSTRACT The mutant prevention concentration (MPC) represents a threshold above which the selective proliferation of resistant mutants is expected to occur only rarely. A provisional MPC (MPCpr) was defined and measured for five fluoroquinolones with clinical isolates of Streptococcus pneumoniae. Based on their potential for restricting the selection of resistant mutants, the five fluoroquinolones, in descending order, were found to be moxifloxacin > trovafloxacin > gatifloxacin > grepafloxacin > levofloxacin. For several compounds, 90% of about 90 clinical isolates that lacked a known resistance mutation had a value of MPCpr that was close to or below the serum levels that could be attained with a dosing regimen recommended by the manufacturers. Since MPCpr overestimates MPC, these data identify moxifloxacin and gatifloxacin as good candidates for determining whether MPCpr can be used as a guide for choosing and eventually administering fluoroquinolones to significantly reduce the development of resistance.

Antimicrobial Use Guidelines for Treatment of Urinary Tract Disease in Dogs and Cats: Antimicrobial Guidelines Working Group of the International Society for Companion Animal Infectious Diseases

Urinary tract disease is a common reason for use (and likely misuse, improper use, and overuse) of antimicrobials in dogs and cats. There is a lack of comprehensive treatment guidelines such as those that are available for human medicine. Accordingly, guidelines for diagnosis and management of urinary tract infections were created by a Working Group of the International Society for Companion Animal Infectious Diseases. While objective data are currently limited, these guidelines provide information to assist in the diagnosis and management of upper and lower urinary tract infections in dogs and cats.

The role of PK/PD parameters to avoid selection and increase of resistance: Mutant prevention concentration

Summary The continuing escalation of antimicrobial resistant human pathogens and the limited number of new antimicrobial agents under development has dictated that our knowledge on the emergence of resistance and any potential strategies to slow the rate at which resistance occurs is of paramount importance. Investigations with fluoroquinolones resulted in the mutant prevention concentration (MPC) concept which represents a novel in vitro measurement of fluoroquinolone potency. In essence, the MPC defines the antimicrobial drug concentration threshold that would require an organism to simultaneously possess two resistance mutations for growth in the presence of the drug. An alternative definition is the drug concentration that prevents the growth of first-step resistant mutants or the minimal inhibitory concentration of the most resistant organism present in the heterogeneous bacterial population when tested against >109 organisms. From in vitro investigations, the new fluoroquinolones (gatifloxacin, gemifloxacin, moxifloxacin) were all found to have lower MPC values than did levofloxacin against clinical isolates of Streptococcus pneumoniae. Ciprofloxacin was found to have lower MPC values than levofloxacin against clinical isolates of Pseudomonas aeruginosa. When MPC data is applied to achievable and sustainable serum drug concentrations in the body, estimation of the time the serum drug concentration exceed both MIC and MPC values can be determined. This data along with kill data allows for an estimate of the amount of time drug concentration needs to exceed MIC/MPC values to not only result in significant kill but also to minimize resistance development. To date, MPC measurements have been determined in in vitro microbiological and pharmacological models and animal and human data are being investigated. The data summarized in this overview detail resistance issues for P. aeruginosa, S. pneumoniae and other pathogens. Also presented is a summary of the MPC concept and investigations completed to date. A brief summary of fluoroquinolone mechanisms of action and resistance is presented. Finally, some preliminary investigations with other classes of compounds are discussed. To date, very limited data is available to conclude if the MPC concept does or does not apply to other classes of antimicrobial agents.

Current issues in the management of urinary tract infections: Extended-release ciprofloxacin as a novel treatment option

Symptomatic urinary tract infections (UTIs) are a major public health concern in the developed world, accounting for almost 8 million annual outpatient and emergency department visits in the US alone, while also representing one of the most common hospital-acquired infections. The vast majority of uncomplicated UTIs are caused by the Gram-negative bacillus Escherichia coli, with other pathogens including enterococci, Staphylococcus saprophyticus, Klebsiella spp. and Proteus mirabilis. Effective management of UTIs in both the inpatient and outpatient settings has been complicated by the fact that many uropathogenic strains have developed resistance to antimicrobials, including cotrimoxazole (trimethoprim/sulfamethoxazole), the current first-line treatment for uncomplicated UTIs in the US and many other countries. In some countries, other antimicrobial therapies, such as trimethoprim and nitrofurantoin, are also used for treatment of uncomplicated UTIs. Antimicrobial resistance has been associated with an increased rate of clinical failure, and reports from Canada and the US indicate that the prevalence of cotrimoxazole resistance exceeds 15% and can be as high as 25%.The emergence and dissemination of antimicrobial resistance can be reduced with the use of agents that have favourable pharmacokinetic/pharmacodynamic profiles and convenient dose administration regimens that facilitate patient adherence and, therefore, pathogen eradication. Fluoroquinolones have been used successfully to treat a wide range of community- and hospital-acquired infections, and the rates of fluoroquinolone resistance have remained low. Use of fluoroquinolones is recommended for uncomplicated UTIs in areas where the incidence of cotrimoxazole resistance exceeds 10%, as well as for the treatment of complicated UTIs and acute pyelonephritis.Ciprofloxacin is a widely used fluoroquinolone with high bactericidal activity against uropathogens and well established clinical efficacy in the treatment of UTIs. A new, extended-release formulation of ciprofloxacin (Cipro® XR) provides systemic drug exposure comparable with that achieved with twice-daily administration of conventional, immediate-release ciprofloxacin, while also attaining higher maximum plasma concentrations with less interpatient variability. Therapeutic drug concentrations with extended-release ciprofloxacin are established immediately after dose administration and maintained throughout the 24-hour dosage interval, permitting convenient, once-daily treatment. Clinical trial results confirm that extended-release ciprofloxacin is as safely used and effective as the conventional, immediate-release formulation of ciprofloxacin in patients with uncomplicated UTIs, complicated UTIs or acute uncomplicated pyelonephritis. These findings support the use of extended-release ciprofloxacin as a well tolerated, effective and convenient therapy for UTIs, which may improve patients’ adherence to therapy and, thereby, reduce the risk of infection recurrence and emergence of antimicrobial resistance.

Mutant Prevention Concentration of Gemifloxacin for Clinical Isolates of Streptococcus pneumoniae

Fluoroquinolone resistance is beginning to appear among isolates of Streptococcus pneumoniae (4, 7, 8, 10). We have argued that resistance arises as a consequence of dosing that places tissue concentrations between the MIC and the mutant prevention concentration (MPC), a new measure of activity related to the MIC of the least susceptible, single-step mutant (15, 16). If this is true, MPC can be used to identify fluoroquinolones that are least likely to selectively enrich resistant subpopulations. We previously estimated MPC for several fluoroquinolones with about 100 clinical isolates of S. pneumoniae obtained from the Royal University Hospital, Saskatoon, Canada (2). We now add gemifloxacin to the list of compounds compared and increase the number of isolates tested to 146 for all of the compounds. Table ​Table11 lists MICs and MPCs for gemifloxacin, moxifloxacin, gatifloxacin, and levofloxacin determined as described previously (2) using the same set of isolates for each compound. Fluoroquinolone-resistant isolates were excluded. Gemifloxacin had the lowest modal MPC (0.25 μg/ml), followed by moxifloxacin (0.5 μg/ml), gatifloxacin (1 μg/ml), and levofloxacin (2 μg/ml). The same rank order was observed when MPC was determined for 90% of the isolates. These data are consistent with gemifloxacin having more activity than the other compounds against resistant mutants (9, 14). When the MIC at which 90% of the susceptible isolates are inhibited (MIC90) was determined, gemifloxacin was also more active than moxifloxacin, gatifloxacin, and levofloxacin in these comparisons by 2, 3, and 4 dilutions, respectively. TABLE 1. Fluoroquinolone activity with clinical isolates of S. pneumoniaea Since the effectiveness of an antibacterial agent is likely to be a function of both activity (MIC and MPC) and pathogen exposure (5, 11), comparision of compounds requires consideration of drug pharmacokinetics in human tissues. From published values of concentrations in serum, we calculated the time above MPC for each compound when dosed as recommended by the manufacturer. Moxifloxacin is expected to have a concentration in serum above the MPC at which 90% of the isolates tested are prevented (MPC90) for 18 h. For gemifloxacin, gatifloxacin, and levofloxacin, those times are 4, 1 to 2, and 0 h, respectively. This suggests that moxifloxacin may be the most effective at restricting the development of resistance, even though gemifloxacin has the lowest MIC and MPC. Table ​Table11 also lists values of the area under the concentration-time curve from 0 to 24 h/MIC and the maximum concentration of drug in serum (Cmax)/MIC for recommended doses. For both parameters gemifloxacin exhibits higher values than moxifloxacin. If these two parameters are inversely related to the selection of resistant mutants (1, 6, 13), resistance should develop less often from treatment with gemifloxacin than with moxifloxacin. But time above MPC (Table ​(Table1)1) and low-concentration cycling (12) predict the opposite outcome. A clinical comparison of these two compounds may help distinguish between MPC-based ideas (15) and empirical pharmacodynamics (6, 13) for predicting the development of resistance. Such a comparison is important because neither method can be easily tested: MPC is an in vitro measure that does not take into account compartments in patients where drug concentrations and bacterial growth properties are poorly defined, and pharmacodynamic methods require examining very large numbers of patients to identify the point at which the overall prevalence of resistance does not increase.

MICs, MPCs and PK/PDs: a match (sometimes) made in hosts

There is little doubt that we have a global pandemic of antimicrobial-resistant microorganisms. Despite regional variations in resistance rates for various bacteria/drug combinations, the overall impact of this trend has impacted on individual patients and the economics of managing infections, as well as our approach to the empirical use of antimicrobial compounds for both inand outpatient management. Indeed, various treatment guidelines from expert working groups recommend different treatment options based on the likelihood of resistant pathogens. Currently, antibiotics are approved based on the demonstration of noninferiority of a new drug when compared with a standard antibiotic agent already approved for a specific indication. However, such trials may fail to take into account various microbiological or pharmacological parameters that could be used to determine optimal versus suboptimal dosing. As such, these parameters may not necessarily affect clinical outcome but may have a huge impact on the selection of drug-resistant pathogens.

Detection of Bordetella pertussis in a clinical laboratory by culture, polymerase chain reaction, and direct fluorescent antibody staining; accuracy, and cost

Control of Bordetella pertussis in the community is hampered by slow and insensitive diagnostic tests. We therefore examined the accuracy and cost of culture, direct fluorescent antibody (DFA) staining, and PCR in a routine clinical laboratory. Six hundred thirty seven nasopharyngeal swabs and aspirates in casamino acids transport medium were cultured, stained with polyclonal (Difco), and monoclonal (BL-5 and Accu-Mab) anti-B. pertussis reagents, and amplified by an IS481-specific PCR. PCR products were detected by a hybridization-enzyme immunoassay kit (Gen-eti-k DEIA, DiaSorin), with confirmation by a second PCR in a separate laboratory. Sensitivities and specificities of culture, polyclonal DFA, monoclonal DFA, and PCR were 36 and 100%, 11.4 and 94.6%, 8.3 and 98. 4%, and 95.0 and 99.3%, respectively, with a prevalence of 15.7%. The DFA tests were the most economical, and the PCR cost was 31% higher than culture. This study suggests that with minor improvements in economy, pertussis PCR can be implemented in a clinical laboratory with marked improvement in diagnostic accuracy.

Antimicrobial efficacy of gatifloxacin and moxifloxacin with and without benzalkonium chloride compared with ciprofloxacin and levofloxacin against methicillin-resistant Staphylococcus aureus.

Abstract We compared the antimicrobial activity of gatifloxacin and moxifloxacin with and without benzalkonium chloride (BAK) against clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA). Minimum inhibitory concentrations (MICs) against clinical isolates of MRSA were evaluated. Approximately 105 CFU/ml of methicillinresistant S. aureus was added to Mueller-Hinton broth containing two-fold concentration increments of drug. For the evaluation of gatifloxacin with BAK, 50 μg/ml of BAK were added to the first well of the plate with gatifloxacin or moxifloxacin and then serially diluted. The combination of gatifloxacin or moxifloxacin with BAK was more active than either fluoroquinolone without BAK. The MICs ranged from ≤0.008 μg/ml to 0.125 μg/ml for gatifloxacin plus BAK, from 0.063 μg/ml to ≥8 μg/ml with unpreserved gatifloxacin from <0.004 to 0.25 for moxifloxacin plus BAK, and from ≤0.016 μg/ml to 16.0 μg/ml with unpreserved moxifloxacin. The combinations of gatifloxacin or moxifloxacin and BAK were highly active against MRSA in vitro, providing MICs that were approximately 2- to 500-fold lower than the MICs provided by either gatifloxacin or moxifloxacin without BAK.

Moxifloxacin: a review of the microbiological, pharmacological, clinical and safety features

Antimicrobial agents are used to treat patients with infectious diseases. Initial antimicrobial compounds originated from natural sources and were generally deemed to be narrow in spectrum. Today, we are in the era of designer drugs that have been specifically developed with current issues in infectious diseases in mind. For example, some new compounds require once daily dosing, have minimal side effects, are active against resistant pathogens and, for some, have a lower propensity for selecting for antimicrobial resistance during patient therapy.

Low Correlation between MIC and Mutant Prevention Concentration

Soon after the term mutant prevention concentration (MPC) was coined to define the MIC of the least susceptible mutant subpopulation of a microbial culture (2), we noticed that MPCs and MICs correlated poorly (r2 = 0.39) for a set of closely related fluoroquinolones when determined with Mycobacterium smegmatis (9). Subsequently, isolated examples were described in which correlation was low for a variety of fluoroquinolones with strains of Escherichia coli, Salmonella enterica, and Staphylococcus aureus (5, 8, 10), and a set of 20 clinical isolates of E. coli showed a low correlation (r2 = 0.58) for ciprofloxacin (6). To determine whether a low correlation between MICs and MPCs is likely to be a general phenomenon, we calculated the correlation coefficients for several quinolones with five bacterial species and for three macrolides with Streptococcus pneumoniae using data from published and unpublished studies of clinical isolates. As shown in Table ​Table1,1, r2,determined by linear regression, was below 0.5 for fluoroquinolones with E. coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, S. aureus, and S. pneumoniae (an exception was levofloxacin with K. pneumoniae [r2 = 0.7]). Values of r2 were slightly above 0.5 for three macrolides with S. pneumoniae (Table ​(Table11). TABLE 1. Relationship between MICs and MPCs Low correlations between MICs and MPCs with clinical isolates are likely to require a complex explanation. These isolates probably contain mutant subpopulations that vary considerably in relative abundance and drug susceptibility, which will contribute to a wide variation in MICs when the mutants are abundant enough to be scored. The isolates may also contain many different multistep mutants (1) which may or may not represent the least susceptible subpopulations that determine MPCs. Added complexity derives from some resistance mutations having a much larger effect on MPCs than on MICs (4). Indeed, isolates with the same MIC were found to have values of MPC that ranged over 5 twofold dilutions. A consequence of a low correlation between MICs and MPCs is that MPCs cannot be estimated accurately from MICs on an individual patient basis. Thus, using antimutant strategies for individual patients will require measurement of the MPC. Likewise, empirical estimates of antimutant activity that are keyed to MIC-based pharmacokinetic-pharmacodynamic indices, such as area under the concentration-time curve at 24 h/MIC, will tend to exhibit more patient-to-patient variability than indices using MPCs.