Prevention of the selection of resistant Staphylococcus aureus by moxifloxacin plus doxycycline in an in vitro dynamic model: an additive effect of the combination

Drug Combinations to Prevent Antimicrobial Resistance: Various Correlations and Laws, and Their Verifications, Thus Proposing Some Principles and a Preliminary Scheme

Antimicrobial resistance (AMR) has been a serious threat to human health, and combination therapy is proved to be an economic and effective strategy for fighting the resistance. However, the abuse of drug combinations conversely accelerates the spread of AMR. In our previous work, we concluded that the mutant selection indexes (SIs) of one agent against a specific bacterial strain are closely related to the proportions of two agents in a drug combination. To discover probable correlations, predictors and laws for further proposing feasible principles and schemes guiding the AMR-preventing practice, here, three aspects were further explored. First, the power function (y = axb, a > 0) correlation between the SI (y) of one agent and the ratio (x) of two agents in a drug combination was further established based on the mathematical and statistical analyses for those experimental data, and two rules a1 × MIC1 = a2 × MIC2 and b1 + b2 = −1 were discovered from both equations of y = a1xb1 and y = a2xb2 respectively for two agents in drug combinations. Simultaneously, it was found that one agent with larger MPC alone for drug combinations showed greater potency for narrowing itself MSW and preventing the resistance. Second, a new concept, mutation-preventing selection index (MPSI) was proposed and used for evaluating the mutation-preventing potency difference of two agents in drug combination; a positive correlation between the MPSI and the mutant prevention concentration (MPC) or minimal inhibitory concentration (MIC) was subsequently established. Inspired by this, the significantly positive correlation, contrary to previous reports, between the MIC and the corresponding MPC of antimicrobial agents against pathogenic bacteria was established using 181 data pairs reported. These results together for the above three aspects indicate that the MPCs in alone and combination are very important indexes for drug combinations to predict the mutation-preventing effects and the trajectories of collateral sensitivity, and while the MPC of an agent can be roughly calculated from its corresponding MIC. Subsequently, the former conclusion was further verified and improved via antibiotic exposure to 43 groups designed as different drug concentrations and various proportions. The results further proposed that the C/MPC for the agent with larger proportion in drug combinations can be considered as a predictor and is the key to judge whether the resistance and the collateral sensitivity occur to two agents. Based on these above correlations, laws, and their verification experiments, some principles were proposed, and a diagram of the mutation-preventing effects and the resistant trajectories for drug combinations with different concentrations and ratios of two agents was presented. Simultaneously, the reciprocal of MPC alone (1/MPC), proposed as the stress factors of two agents in drug combinations, together with their SI in combination, is the key to predict the mutation-preventing potency and control the trajectories of collateral sensitivity. Finally, a preliminary scheme for antimicrobial combinations preventing AMR was further proposed for subsequent improvement research and clinic popularization, based on the above analyses and discussion. Moreover, some similar conclusions were speculated for triple or multiple drug combinations.

In vitro evaluation of antimicrobial resistance selection in Neisseria gonorrhoeae

Gonococcal infections represent an urgent public-health threat as >50% of cases caused by Neisseria gonorrhoeae strains display reduced susceptibility to at least one antimicrobial agent. We evaluated the pharmacodynamics of a number of antimicrobials against N. gonorrhoeae in order to assess the likelihood of mutant selection by these agents. The mutant prevention concentration (MPC) and mutant selection window (MSW) were determined for azithromycin, ceftriaxone, doxycycline, ertapenem, gentamicin, ciprofloxacin, levofloxacin and moxifloxacin against a wild-type strain of N. gonorrhoeae (ATCC 49226) and a gyrA mutant of ATCC 49226. Pharmacokinetic parameters, including peak concentration (C_max), half-life (t_1/2) and area under the plasma concentration-time curve over 24 h (AUC), associated with each agent were used to calculate the time within the MSW (T_MSW, percentage of the dosing interval that antimicrobial concentrations fall within the MSW), C_max/MPC ratio and AUC/MPC ratio for each antimicrobial agent. Concentrations of ceftriaxone (500 mg), ertapenem, ciprofloxacin, levofloxacin and moxifloxacin surpass the MPC for both strains. Results of pharmacodynamic analyses suggest that ertapenem, ciprofloxacin, levofloxacin and moxifloxacin may be most likely to prevent mutant selection in N. gonorrhoeae. Use of ceftriaxone, azithromycin, doxycycline or gentamicin for gonorrhoea is expected to lead to the ongoing emergence of resistance to these agents. There is a clear need to develop novel treatment regimens for gonococcal infections in order to limit the dissemination of resistance in N. gonorrhoeae.

Time-programmable drug dosing allows the manipulation, suppression and reversal of antibiotic drug resistance in vitro

Multi-drug strategies have been attempted to prolong the efficacy of existing antibiotics, but with limited success. Here we show that the evolution of multi-drug-resistant Escherichia coli can be manipulated in vitro by administering pairs of antibiotics and switching between them in ON/OFF manner. Using a multiplexed cell culture system, we find that switching between certain combinations of antibiotics completely suppresses the development of resistance to one of the antibiotics. Using this data, we develop a simple deterministic model, which allows us to predict the fate of multi-drug evolution in this system. Furthermore, we are able to reverse established drug resistance based on the model prediction by modulating antibiotic selection stresses. Our results support the idea that the development of antibiotic resistance may be potentially controlled via continuous switching of drugs.

First human systemic infection caused by Spiroplasma

Spiroplasma species are organisms that normally colonize plants and insects. We describe the first case of human systemic infection caused by Spiroplasma bacteria in a patient with hypogammaglobulinemia undergoing treatment with biological disease-modifying antirheumatic agents. Spiroplasma turonicum was identified through molecular methods in several blood cultures. The infection was successfully treated with doxycycline plus levofloxacin.

Comparative study of the mutant prevention concentrations of vancomycin alone and in combination with levofloxacin, rifampicin and fosfomycin against methicillin-resistant Staphylococcus epidermidis

No mutant-prevention concentration (MPC) with methicillin-resistant Staphylococcus epidermidis (MRSE) has been reported. The study aimed to evaluate the propensity of vancomycin individually and in combination to prevent MRSE from mutation. A total of 10 MRSE clinical isolates were included in the study. Susceptibility testing demonstrated that the susceptibility rates to vancomycin, rifampicin, levofloxacin and fosfomycin were 100, 100, 50 and 90%, respectively. The fractional inhibition concentration indices (FICI) for vancomycin combined with rifampicin, levofloxacin or fosfomycin were ≥1.5 but ≤2, ≥1.5 but ≤2, and >0.5 but ≤1.5, respectively, implying indifferent interactivity. The MPC with susceptible strains was determined to be the lowest antibiotic concentration inhibiting visible growth among 10(10) CFU on four agar plates (9 cm in diameter) after a 72-h incubation at 37°C. The MPCs were 16~32, >64, ≥64 and 4~16 μg ml(-1) for vancomycin, rifampicin, fosfomycin and levofloxacin, respectively. The vancomycin MPCs of combinations with fosfomycin (32 μg ml(-1)), levofloxacin (2 μg ml(-1)) and rifampicin (2 or 4 μg ml(-1)) were 1~4, 16~32 and 16~32 μg ml(-1), respectively. Against mutants selected by vancomycin, rifampicin, levofloxacin and fosfomycin individually, antibiotics had standard MICs of 2~4 μg ml(-1) for vancomycin, >64 μg ml(-1) for rifampicin, 4~8 μg ml(-1) for levofloxacin and 64 μg ml(-1) for fosfomycin. Thus single-step mutation can lead to resistance of MRSE to rifampicin, levofloxacin and fosfomycin, rather than non-susceptibility to vancomycin. Vancomycin-fosfomycin combination might be a superior alternative to vancomycin in blocking the growth of MRSE mutants, especially for high-organism-burden infections.

Determination of the mutant selection window for clindamycin, doxycycline, linezolid, moxifloxacin and trimethoprim/sulfamethoxazole against community-associated meticillin-resistant Staphylococcus aureus (MRSA)

The risk that antimicrobials suitable for therapy of infections caused by community-associated meticillin-resistant Staphylococcus aureus (CA-MRSA) will allow the emergence of resistance has not been adequately studied. In this study, mutant prevention concentration (MPC) testing was used to investigate the propensity for future resistance induction in CA-MRSA by clindamycin, doxycycline, linezolid, moxifloxacin and trimethoprim/sulfamethoxazole. Four CA-MRSA isolates [two each of clones USA300 (10841 and NRS384) and USA400 (2833 and NRS123)] were tested as well as a single hospital-acquired strain (NRS385). A variable hierarchy of the tested antimicrobials with respect to the percentage of the dosage interval that concentrations fall within the mutant selection window (%T(MSW)) was determined, with all antimicrobials achieving %T(MSW) values >20%. Against the hospital-acquired strain, all antimicrobials except linezolid (%T(MSW)=74.10%) and moxifloxacin (%T(MSW)=21.65%) are predicted to attain concentrations below the minimum inhibitory concentration (MIC) and mutant selection window (MSW). No instance was observed in which the percentage of the dosage interval that concentrations exceed the MPC (%T>MPC) was found to be 100%. Therapeutic dosing of the tested agents is predicted to attain concentrations within the MSW to varying degrees in CA-MRSA. The risk that resistance to these antimicrobials will emerge in CA-MRSA with continued use warrants further investigation of their propensity to select resistant subpopulations.

Quinolones: action and resistance updated

The quinolones trap DNA gyrase and DNA topoisomerase IV on DNA as complexes in which the DNA is broken but constrained by protein. Early studies suggested that drug binding occurs largely along helix-4 of the GyrA (gyrase) and ParC (topoisomerase IV) proteins. However, recent X-ray crystallography shows drug intercalating between the -1 and +1 nucleotides of cut DNA, with only one end of the drug extending to helix-4. These two models may reflect distinct structural steps in complex formation. A consequence of drug-enzyme-DNA complex formation is reversible inhibition of DNA replication; cell death arises from subsequent events in which bacterial chromosomes are fragmented through two poorly understood pathways. In one pathway, chromosome fragmentation stimulates excessive accumulation of highly toxic reactive oxygen species that are responsible for cell death. Quinolone resistance arises stepwise through selective amplification of mutants when drug concentrations are above the MIC and below the MPC, as observed with static agar plate assays, dynamic in vitro systems, and experimental infection of rabbits. The gap between MIC and MPC can be narrowed by compound design that should restrict the emergence of resistance. Resistance is likely to become increasingly important, since three types of plasmid-borne resistance have been reported.

Antibiotic resistance--what's dosing got to do with it?

OBJECTIVE

This review seeks to identify original research articles that link antibiotic dosing and the development of antibiotic resistance for different antibiotic classes. Using this data, we seek to apply pharmacodynamic principles to assist clinical practice for suppressing the emergence of resistance. Concepts such as mutant selection window and mutant prevention concentration will be discussed.

DATA SOURCES

PubMed, EMBASE, and the Cochrane Controlled Trial Register.

STUDY SELECTION

All articles that related antibiotic doses and exposure to the formation of antibiotic resistance were reviewed.

DATA SYNTHESIS

The escalation of antibiotic resistance continues worldwide, most prominently in patients in intensive care units. Data are emerging from in vitro and in vivo studies that suggest that inappropriately low antibiotic dosing may be contributing to the increasing rate of antibiotic resistance. Fluoroquinolones have widely been researched and publications on other antibiotic classes are emerging. Developing dosing regimens that adhere to pharmacodynamic principles and maximize antibiotic exposure is essential to reduce the increasing rate of antibiotic resistance.

CONCLUSIONS

Antibiotic dosing must aim to address not only the bacteria isolated, but also the most resistant subpopulation in the colony, to prevent the advent of further resistant infections because of the inadvertent selection pressure of current dosing regimens. This may be achieved by maximizing antibiotic exposure by administering the highest recommended dose to the patient.

Pharmacokinetics of doxycycline in sheep after intravenous and oral administration

The pharmacokinetics of doxycycline were investigated in sheep after oral (PO) and intravenous (IV) administration. The IV data were best described using a 2- (n = 5) or 3- (n = 6) compartmental open model. Mean pharmacokinetic parameters obtained using a 2-compartmental model included a volume of distribution at steady-state (V(ss)) of 1.759+/-0.3149L/kg, a total clearance (Cl) of 3.045+/-0.5264mL/kg/min and an elimination half-life (t(1/2beta)) of 7.027+/-1.128h. Comparative values obtained from the 3-compartmental mean values were: V(ss) of 1.801+/-0.3429L/kg, a Cl of 2.634+/-0.6376mL/kg/min and a t(1/2beta) of 12.11+/-2.060h. Mean residence time (MRT(0-infinity)) was 11.18+/-3.152h. After PO administration, the data were best described by a 2-compartment open model. The pharmacokinetic parameter mean values were: maximum plasma concentration (C(max)), 2.130+/-0.950microg/mL; time to reach C(max) (t(max)), 3.595+/-3.348h, and absorption half-life (t(1)/(2k)(01)), 36.28+/-14.57h. Non-compartmental parameter values were: C(max), 2.182+/-0.9117microg/mL; t(max), 3.432+/-3.307h; F, 35.77+/-10.20%, and mean absorption time (MAT(0-infinity)), 25.55+/-15.27h. These results suggest that PO administration of doxycycline could be useful as an antimicrobial drug in sheep.

Pharmacodynamics of doxycycline for chemoprophylaxis of Lyme disease: preliminary findings and possible implications for other antimicrobials

The purpose of this study was to begin to characterise the pharmacodynamic parameters of single-dose doxycycline for the prevention of Lyme disease following a tick bite. Based on limited data from published human and murine studies, it was found that there is a direct correlation between efficacy rate and the area under the time-concentration of free antibiotic curve divided by the minimum inhibitory concentration (fAUC/MIC) (R(2)=0.74, using Pearson correlation), but not the maximum concentration of free drug in serum divided by the MIC (fC(max)/MIC) or the time that the free drug concentration remains above the MIC (fT>MIC). To determine the possible implications of these findings for other antimicrobials, it was assumed that the pharmacodynamic properties of doxycycline would be pertinent to azithromycin, an antibiotic whose activity is known to correlate with AUC/MIC. By making such an extrapolation and using pharmacokinetic modelling with conservative assumptions on MIC values against Borrelia burgdorferi, it is hypothesised that a single 500 mg dose of azithromycin in humans should have comparable efficacy to doxycycline for the prevention of Lyme disease. Additional experimental studies are needed to clarify more precisely the pharmacodynamic properties of doxycycline and to validate the accuracy of this hypothesis.

Antibacterial Effects of Moxifloxacin and Levofloxacin Simulating Epithelial Lining Fluid Concentrations against Community-Acquired Methicillin-Resistant Staphylococcus aureus

BACKGROUND AND OBJECTIVE

Current North American guidelines advocate the use of respiratory fluoroquinolones for the empirical management of community-acquired pneumonia (CAP). While community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) has emerged as a pathogen frequently encountered in skin and skin structure infections, it has also now been recognised as a causative pathogen in CAP. Since fluoroquinolones may be used empirically to treat unsuspected CA-MRSA pneumonia, the objective of this study was to evaluate the antibacterial properties of levofloxacin and moxifloxacin using human simulated drug exposures in epithelial lining fluid (ELF).

METHODS

An in vitro model was used to simulate the ELF concentrations, previously determined in older adults receiving multiple doses, of levofloxacin 500 mg once daily and moxifloxacin 400mg once daily. Four CA-MRSA isolates were studied at a starting inoculum of 10(6) colony-forming units (CFU)/mL; selected isolates were also studied at 10(8) CFU/mL. Bacterial density and resistance were quantitatively assessed over 48 hours. Drug exposure (area under the concentration-time curve [AUC]) was confirmed using validated drug assays.

RESULTS

At a standard 10(6) starting inoculum, sustained bacterial kill (3.6-4.5 log) with both fluoroquinolones was noted for CA-MRSA isolates 27 and 44 (AUC/minimum inhibitory concentration [MIC] = 383-3923). Despite an MIC of 8 microg/mL (AUC/MIC = 25) for isolate 3, levofloxacin displayed a 2.8 log kill, while moxifloxacin (MIC 1 microg/mL) sustained a 4.5 log kill (AUC/MIC = 207) over 48 hours. Against isolate 59, levofloxacin displayed no antibacterial effect (AUC/MIC = 3), while moxifloxacin with an MIC of 8 microg/mL (AUC/MIC = 31) killed 4.6 log. At a high inoculum (10(8)), both fluoroquinolones showed 5.2-5.6 log kill for the susceptible isolate (44), while moxifloxacin showed no antibacterial activity against isolate 59. Drug exposure (AUC/MIC) appeared to correlate well (r(2) = 0.99) with the change in log CFU/mL. Maximal activity was observed for both drugs at an AUC/MIC of approximately 30.

CONCLUSION

When evaluated at human simulated ELF concentrations, both levofloxacin and moxifloxacin appeared to demonstrate sustained antibacterial activity for CA-MRSA isolates with MICs <or=8 microg/mL at a starting inoculum of 10(6). Use of a high inoculum (10(8)) appeared to compromise the antimicrobial activity of the fluoroquinolones when the MIC was 8 microg/mL, but did not mitigate antibacterial kill for susceptible isolates.

Mutant prevention concentrations of levofloxacin alone and in combination with azithromycin, ceftazidime, colistin (Polymyxin E), meropenem, piperacillin-tazobactam, and tobramycin against Pseudomonas aeruginosa

The mutant prevention concentrations (MPCs) of levofloxacin alone and in combination with ceftazidime, colistin (polymyxin E), meropenem, piperacillin-tazobactam, and tobramycin were established against Pseudomonas aeruginosa. Antibiotic combinations using levofloxacin with any antibiotic with individual activity against P. aeruginosa resulted in synergistic lowering (an at-least-fourfold reduction) of the isolate's MPC.

Community-associated methicillin-resistant Staphylococcus aureus: reconsideration of therapeutic options

Methicillin resistance, long recognized as characteristic of nosocomial Staphylococcus aureus, has increasingly been identified in community-acquired strains in the past 15 years. The genotypes of community-associated methicillin-resistant S. aureus (MRSA) are different from nosocomial strains, and unlike nosocomial strains, they have a distinctive methicillin-resistance chromosomal cassette (designated type IV), are usually susceptible to multiple classes of antimicrobials other than beta-lactams, carry a distinctive virulence factor (the Panton-Valentine leukocidin), cause mainly skin and soft tissue infection and less frequently, necrotizing pneumonia, and involve predominantly children and young adults. Outbreaks have been reported in certain segments of the population (eg, football players, wrestlers, prison inmates, and native people) that often do not have the established risk factors for MRSA. However, these strains have also caused infections likely acquired in an institutional health care setting. Delay in starting appropriate antibiotic therapy for severe infections caused by MRSA can be life-threatening. This requires a reconsideration of the empiric choice of an anti-staphylococcal beta-lactam for seriously ill patients with suspected community-associated S. aureus infections.

Optimal pharmacological therapy for community-acquired pneumonia: the role of dual antibacterial therapy

The optimal pharmacological therapy of community-acquired pneumonia (CAP) is one of the most ardently debated issues in medicine. Presently, most guidelines recommend either a fluoroquinolone alone or dual therapy with a third-generation cephalosporin plus a macrolide in patients hospitalised with CAP, but few provide clinicians with specific considerations for selecting from these agents. Despite a similar spectrum of activity and favourable resistance patterns (for fluoroquinolones and third-generation cephalosporins) against CAP pathogens, there is emerging evidence that dual therapy may be superior to monotherapy in certain populations.In patients with non-severe CAP, the evidence supports the use of either monotherapy or dual therapy in most patients; however, patients with severe CAP or bacteraemic pneumococcal CAP experience improved survival when treated with dual therapy. It is unclear from this evidence if any specific combination of agents is the most effective, but the combination of a third-generation cephalosporin plus a macrolide is the most extensively studied. Dual therapy was superior to monotherapy irrespective of the susceptibility of the aetiological pathogen, thus insufficient antimicrobial spectrum does not explain the disparity. The most likely explanation for improved outcomes with dual therapy is the combined effect of optimised antimicrobial spectrum (including atypicals), decreased impact of resistance to a single agent and the immunomodulatory effects of macrolides. Increasing resistance in patients with non-severe CAP warrants the consideration of dual therapy and perhaps a reappraisal of agents usually reserved for second-line therapy, including doxycycline, in these populations as well. In light of the available evidence, dual therapy should be strongly considered in all patients with severe CAP, especially when complicated by pneumococcal bacteraemia.

Treatment approaches for community-acquired methicillin-resistant Staphylococcus aureus infections

PURPOSE OF REVIEW

This review addresses therapeutic approaches to community-acquired methicillin-resistant Staphylococcus aureus (MRSA) infections, focusing on recently published data in the English-language medical literature dating from June 2004 to July 2005.

RECENT FINDINGS

During the reviewed time period, the overall understanding of the epidemiology and virulence of community-acquired MRSA has continued to advance. This same period has also seen numerous works dealing with the newer and emerging anti-staphylococcal antimicrobial agents. Important clinical trials involving linezolid, daptomycin, tigecycline, dalbavancin, and telavancin have been completed. At the same time, owing to the cost of newer agents and the broad susceptibility pattern of community-acquired MRSA, many older antimicrobial agents (long-acting tetracyclines, fluoroquinolones, rifampin, trimethoprim-sulfamethoxazole, and clindamycin) have also been reexamined.

SUMMARY

As data accumulates, the newer antimicrobial agents active against community-acquired MRSA continue to demonstrate their value. Despite the appeal and abundant publications involving newer agents, older antimicrobials certainly retain a therapeutic role. Considerable work needs to be done prospectively evaluating older agents for community-acquired MRSA disease. The most important therapeutic intervention for the majority of community-acquired MRSA infections is adequate drainage of purulent fluid collections. Antimicrobial selection for community-acquired MRSA infections should be governed by disease severity, susceptibility patterns (especially based on timely clinical specimen culture), clinical response to therapy, and cost.

Mutant prevention concentrations of ciprofloxacin for urinary tract infection isolates of Escherichia coli

OBJECTIVES

To measure the mutant prevention concentration (MPC) of ciprofloxacin for a set of urinary tract infection (UTI) Escherichia coli isolates with different levels of susceptibility and determine whether MPC can be predicted from MIC.

METHODS

MPC was defined as the lowest ciprofloxacin concentration that prevented the growth of resistant colonies when 10(10) bacteria were spread on solid medium and incubated for 96 h at 37 degrees C. MIC was measured by Etest. Bacteria surviving (persisting) at MPC were isolated and quantified from agar plugs taken after 96 h. The genes hipA and hipB were amplified by PCR from persisters and sequenced.

RESULTS

Isolates with MICs above the NCCLS breakpoint for ciprofloxacin resistance (4 mg/L) typically have MPCs greater than 32 mg/L. Isolates with MICs below the breakpoint for ciprofloxacin susceptibility (1 mg/L) have MPCs up to 5 mg/L. MPC/MIC is approximately 16 for most susceptible isolates but there are several notable exceptions (MPC/MIC > 100). Resistant colonies arising one dilution step below MPC often had MIC > MPC. In every case tested, a proportion of cells survived (persisted), but did not grow into colonies, at MPC, without any increase in MIC.

CONCLUSIONS

MPCs were determined for all ciprofloxacin-susceptible isolates. MPC is not accurately predicted from MIC. Colonies selected below MPC frequently have MIC > MPC, suggesting multiple mutations. A small fraction of cells from all strains tested survived for 96 h at MPC, without any associated increase in MIC. These survivors/persisters are not hipAB mutants.

Methicillin-Resistant Staphylococcus aureus: An Evolutionary, Epidemiologic, and Therapeutic Odyssey

Methicillin-resistant Staphylococcus aureus, first identified just over 4 decades ago, has undergone rapid evolutionary changes and epidemiologic expansion. It has spread beyond the confines of health care facilities, emerging anew in the community, where it is rapidly becoming a dominant pathogen. This has led to an important change in the choice of antibiotics in the management of community-acquired infections and has also led to the development of novel antimicrobials.