The fractional maximal effect method: a new way to characterize the effect of antibiotic combinations and other nonlinear pharmacodynamic interactions

Influence of 6-aminonicotinamide (6AN) on Leishmania promastigotes evaluated by metabolomics: Beyond the pentose phosphate pathway

6-Aminonicotinamide (6AN) is an antimetabolite used to inhibit the NADPH-producing pentose phosphate pathway (PPP) in many cellular systems, making them more susceptible to oxidative stress. It is converted by a NAD(P)⁺ glycohydrolase to 6-aminoNAD and 6-aminoNADP, causing the accumulation of PPP intermediates, due to their inability to participate in redox reactions. Some parasites like Plasmodium falciparum and Coccidia are highly sensitive but not all cell types showed a strong responsiveness to 6AN, probably due to the different targeted pathway. For instance, in bacteria the main target is the Preiss-Handler salvage pathway for NAD⁺ biosynthesis. We were interested in testing 6AN on the kinetoplastid protozoan Leishmania as another model to clarify the mechanisms of action of 6AN, by using metabolomics. Leishmania promastigotes, the life-cycle stage residing in the sandfly, demonstrated a three order of magnitude higher EC50 (mM) compared to P. falciparum and mammalian cells (μM), although pre-treatment with 100 μM 6AN prior to sub-lethal oxidative challenge induced a supra-additive cell kill in L. infantum. By metabolomics, we did not detect 6ANAD/P suggesting that NAD⁺ glycohydrolases in Leishmania may not be highly efficient in catalysing transglycosidation as happens in other microorganisms. Contrariwise to the reported effect on 6AN-treated cancer cells, we did not detect 6-phosphogluconate (6 PG) accumulation, indicating that 6ANADP cannot bind with high affinity to the PPP enzyme 6 PG dehydrogenase. By contrast, 6AN caused a profound phosphoribosylpyrophosphate (PRPP) decrease and nucleobases accumulation confirming that PPP is somehow affected. More importantly, we found a decrease in nicotinate production, evidencing the interference with the Preiss-Handler salvage pathway for NAD⁺ biosynthesis, most probably by inhibiting the reaction catalysed by nicotinamidase. Therefore, our combined data from Leishmania strains, though confirming the interference with PPP, also showed that 6AN impairs the Preiss-Handler pathway, underlining the importance to develop compounds targeting this last route.

Pharmacodynamic interactions of amikacin with selected β-lactams and fluoroquinolones against canine Escherichia coli isolates

Knowledge of in vitro antimicrobial interactions can serve as a guide for clinical application of combination antimicrobial regimens. The aim of the present study was to determine the pharmacodynamic interactions of amikacin with either amoxicillin/clavulanic acid, ceftazidime, enrofloxacin or marbofloxacin against clinical canine Escherichia coli isolates. Bactericidal activity of individual antimicrobials was assessed by use of static kill curves. Interactions between amikacin and each of the β-lactams or fluoroquinolones were subsequently analyzed by employing the fractional maximal effect method. Amikacin, compared with all other agents, displayed the most rapid and extensive bacterial killing, the lowest level (with respect to MIC) at which half the maximal effect was observed and the most linear concentration-effect relationship. The combinations of amikacin with amoxicillin/clavulanic acid or ceftazidime were completely synergistic in four and three out of the five investigated isolates, respectively, with additivity being sporadically observed. On the other hand, the combinations of amikacin with enrofloxacin or marbofloxacin yielded a mosaic of interaction types with no discernible pattern or differentiation between fluoroquinolone-susceptible and resistant isolates; synergy was only infrequently observed, mainly at increased fluoroquinolone concentrations. In conclusion, the combinations of amikacin with the two β-lactams were found to be more promising, in terms of synergy achievement, compared with the respective combinations with the two fluoroquinolones.

Commercial Essential Oils as Potential Antimicrobials to Treat Skin Diseases

Essential oils are one of the most notorious natural products used for medical purposes. Combined with their popular use in dermatology, their availability, and the development of antimicrobial resistance, commercial essential oils are often an option for therapy. At least 90 essential oils can be identified as being recommended for dermatological use, with at least 1500 combinations. This review explores the fundamental knowledge available on the antimicrobial properties against pathogens responsible for dermatological infections and compares the scientific evidence to what is recommended for use in common layman's literature. Also included is a review of combinations with other essential oils and antimicrobials. The minimum inhibitory concentration dilution method is the preferred means of determining antimicrobial activity. While dermatological skin pathogens such as Staphylococcus aureus have been well studied, other pathogens such as Streptococcus pyogenes, Propionibacterium acnes, Haemophilus influenzae, and Brevibacterium species have been sorely neglected. Combination studies incorporating oil blends, as well as interactions with conventional antimicrobials, have shown that mostly synergy is reported. Very few viral studies of relevance to the skin have been made. Encouragement is made for further research into essential oil combinations with other essential oils, antimicrobials, and carrier oils.

Antimicrobial activity, synergism and inhibition of germ tube formation by Crocus sativus-derived compounds against Candida spp

The limited arsenal of synthetic antifungal agents and the emergence of resistant Candida strains have prompted the researchers towards the investigation of naturally occurring compounds or their semisynthetic derivatives in order to propose new innovative hit compounds or new antifungal combinations endowed with reduced toxicity. We explored the anti-Candida effects, for the first time, of two bioactive compounds from Crocus sativus stigmas, namely crocin 1 and safranal, and some semisynthetic derivatives of safranal obtaining promising biological results in terms of minimum inhibitory concentration/minimum fungicidal concentration (MIC/MFC) values, synergism and reduction in the germ tube formation. Safranal and its thiosemicarbazone derivative 5 were shown to display good activity against Candida spp.

Antibacterial activity of essential oils mixture against PSA

Pseudomonas syringae pv. actinidiae (PSA) is the causal agent of bacterial canker of kiwifruit. It is very difficult to treat pandemic disease. The prolonged treatment with antibiotics, has resulted in failure and resistance and alternatives to conventional antimicrobial therapy are needed. The aim of our study was to analyse the phenotypic characteristics of PSA, identify new substances from natural source i.e. essential oils (EOs) able to contain the kiwifruit canker and investigate their potential use when utilised in combination. Specially, we investigated the morphological differences of PSA isolates by scanning electron microscope, and the synergic action of different EOs by time-kill and checkerboard methods. Our results demonstrated that PSA was able to produce extracellular polysaccharides when it was isolated from trunk, and, for the first time, that it was possible to kill PSA with a mixture of EOs after 1 h of exposition. We hypothesise on its potential use in agriculture.

Activities of antibiotic combinations against resistant strains of Pseudomonas aeruginosa in a model of infected THP-1 monocytes

Antibiotic combinations are often used for treating Pseudomonas aeruginosa infections but their efficacy toward intracellular bacteria has not been investigated so far. We have studied combinations of representatives of the main antipseudomonal classes (ciprofloxacin, meropenem, tobramycin, and colistin) against intracellular P. aeruginosa in a model of THP-1 monocytes in comparison with bacteria growing in broth, using the reference strain PAO1 and two clinical isolates (resistant to ciprofloxacin and meropenem, respectively). Interaction between drugs was assessed by checkerboard titration (extracellular model only), by kill curves, and by using the fractional maximal effect (FME) method, which allows studying the effects of combinations when dose-effect relationships are not linear. For drugs used alone, simple sigmoidal functions could be fitted to all concentration-effect relationships (extracellular and intracellular bacteria), with static concentrations close to (ciprofloxacin, colistin, and meropenem) or slightly higher than (tobramycin) the MIC and with maximal efficacy reaching the limit of detection in broth but only a 1 to 1.5 (colistin, meropenem, and tobramycin) to 2 to 3 (ciprofloxacin) log10 CFU decrease intracellularly. Extracellularly, all combinations proved additive by checkerboard titration but synergistic using the FME method and more bactericidal in kill curve assays. Intracellularly, all combinations proved additive only based on both FME and kill curve assays. Thus, although combinations appeared to modestly improve antibiotic activity against intracellular P. aeruginosa, they do not allow eradication of these persistent forms of infections. Combinations including ciprofloxacin were the most active (even against the ciprofloxacin-resistant strain), which is probably related to the fact this drug was the most effective alone intracellularly.

Meropenem/colistin synergy testing for multidrug-resistant Acinetobacter baumannii strains by a two-dimensional gradient technique applicable in routine microbiology

OBJECTIVES

Precise assessment of potential therapeutic synergy, antagonism or indifference between antimicrobial agents currently depends on time-consuming and hard-to-standardize in vitro chequerboard titration methods. We here present a method based on a novel two-dimensional antibiotic gradient technique named Xact™.

METHODS

We used a test comprising a combination of perpendicular gradients of meropenem and colistin in a single quadrant. We compared test outcomes with those obtained with classical chequerboard microbroth dilution testing in a study involving 27 unique strains of multidrug-resistant Acinetobacter baumannii from diverse origins.

RESULTS

We were able to demonstrate 92% concordance between the new technology and classical chequerboard titration using the A. baumannii collection. Two strains could not be analysed by Xact™ due to their out-of-range MIC of meropenem (>128 mg/L).

CONCLUSIONS

The new test was shown to be diagnostically useful, easy to implement and less labour intensive than the classical method.

Effects of Mentha suaveolens Essential Oil Alone or in Combination with Other Drugs in Candida albicans

Candidosis is the most important cause of fungal infections in humans. The yeast Candida albicans can form biofilms, and it is known that microbial biofilms play an important role in human diseases and are very difficult to treat. The prolonged treatment with drugs has often resulted in failure and resistance. Due to the emergence of multidrug resistance, alternatives to conventional antimicrobial therapy are needed. This study aims to analyse the effects induced by essential oil of Mentha suaveolens Ehrh (EOMS) on Candida albicans and its potential synergism when used in combination with conventional drugs. Morphological differences between control and EOMS treated yeast cells or biofilms were observed by scanning electron microscopy and transmission electron microscopy (SEM and TEM resp.,). In order to reveal the presence of cell cycle alterations, flow cytometry analysis was carried out as well. The synergic action of EOMS was studied with the checkerboard method, and the cellular damage induced by different treatments was analysed by TEM. The results obtained have demonstrated both the effects of EOMS on C. albicans yeast cells and biofilms and the synergism of EOMS when used in combination with conventional antifungal drugs as fluconazole (FLC) and micafungin (MCFG), and therefore we can hypothesize on its potential use in therapy. Further studies are necessary to know its mechanism of action.

Water contamination reduces the tolerance of coral larvae to thermal stress

Coral reefs are highly susceptible to climate change, with elevated sea surface temperatures (SST) posing one of the main threats to coral survival. Successful recruitment of new colonies is important for the recovery of degraded reefs following mortality events. Coral larvae require relatively uncontaminated substratum on which to metamorphose into sessile polyps, and the increasing pollution of coastal waters therefore constitutes an additional threat to reef resilience. Here we develop and analyse a model of larval metamorphosis success for two common coral species to quantify the interactive effects of water pollution (copper contamination) and SST. We identify thresholds of temperature and pollution that prevent larval metamorphosis, and evaluate synergistic interactions between these stressors. Our analyses show that halving the concentration of Cu can protect corals from the negative effects of a 2-3°C increase in SST. These results demonstrate that effective mitigation of local impacts can reduce negative effects of global stressors.

Intracellular activity of antibiotics in a model of human THP-1 macrophages infected by a Staphylococcus aureus small-colony variant strain isolated from a cystic fibrosis patient: study of antibiotic combinations

In a companion paper (H. A. Nguyen et al., Antimicrob. Agents Chemother. 53:1434-1442, 2009), we showed that vancomycin, oxacillin, fusidic acid, clindamycin, linezolid, and daptomycin are poorly active against the intracellular form of a thymidine-dependent small-colony variant (SCV) strain isolated from a cystic fibrosis patient and that the activity of quinupristin-dalfopristin, moxifloxacin, rifampin, and oritavancin remains limited (2- to 3-log CFU reduction) compared to their extracellular activity. Antibiotic combination is a well-known strategy to improve antibacterial activity, which was examined here against an intracellular SCV strain using combinations with either rifampin or oritavancin. Time-kill curve analysis using either concentrations that caused a static effect for each antibiotic individually or concentrations corresponding to the maximum concentration in human serum showed largely divergent effects that were favorable when antibiotics were combined with rifampin at low concentrations only and with oritavancin at both low and high concentrations. The nature of the interaction between rifampin, oritavancin, and moxifloxacin was further examined using the fractional maximal effect method, which allows categorization of the effects of combinations when dose-effect relationships are not linear. Rifampin and oritavancin were synergistic at all concentration ratios investigated. Oritavancin and moxifloxacin were also synergistic but at high oritavancin concentrations only. Rifampin and moxifloxacin were additive. This approach may help in better assessing and improving the activity of antibiotics against intracellular SCV strains.

Synergism of Leu-Lys rich antimicrobial peptides and chloramphenicol against bacterial cells

To investigate the antibiotic activity and synergistic effect, analogues were designed to increase not only net positive charge by Lys-substitution but also hydrophobic helix region by Leu-substitution from CA (1-8)-MA (1-12) hybrid peptide (CA-MA). In particular, CA-MA analogue P5 (P5), designed by flexible region (GIG-->P)-substitution, Lys- (positions 4, 8, 14, 15) and Leu- (positions 5, 6, 12, 13, 16, 17, 20) substitutions, showed potent antibacterial activity in minimal inhibition concentration (MIC) and minimal bactericidal concentration (MBC) without having hemolytic activity. In addition, P5 and chloramphenicol has potent synergistic effect against tested cell lines. As determined by propidium iodide (PI) staining, flow cytometry showed that P5 plus chloramphenicol-treated cells had higher fluorescence intensity than untreated, P5- and chloramphenicol-treated cells. The effect on plasma membrane was examined by investigating the transmembrane potential depolarizing experiments of S. aureus with P5 and chloramphenicol. The result showed that the peptide exerts its antibacterial activity by acting on the plasma membrane. Furthermore, P5 caused significant morphological alterations of S. aureus, as shown by scanning electron microscopy. Our results suggest that peptide P5 is an excellent candidate as a lead compound for the development of novel anti-infective agents and synergistic effects with conventional antibiotic agents but lack hemolytic activity.

The coordination of prostaglandin E2 production by sphingosine-1-phosphate and ceramide-1-phosphate

The ability of pro-inflammatory cytokines such as interleukin-1beta (IL-1beta) to induce the major inflammatory mediator prostaglandin (PG) E(2) depends on the activation of two rate-limiting enzymes, phospholipase A(2) (PLA(2)) and cyclooxygenase 2 (COX-2). PLA(2) acts to generate arachidonic acid, which serves as the precursor substrate for COX-2 in the metabolic pathway leading to PGE(2) production. However, less is known about the mechanisms that coordinate the regulation of these two enzymes. We have provided prior evidence that sphingosine kinase 1 and its bioactive lipid product sphingosine-1-phosphate (S1P) mediate the effects of cytokines on COX-2 induction, whereas ceramide kinase and its distinct product, ceramide-1-phosphate (C1P), are required for the activation and translocation of cPLA(2) (FASEB J 17:1411-1421. 2003; J Biol Chem 278:38206-38213, 2003; J Biol Chem 279:11320-11326, 2004). Herein, we show that these two pathways are independent but coordinated, resulting in synergistic induction of PGE(2). Moreover, the combination of both S1P and C1P recapitulates the temporal and spatial activation of cPLA(2) and with COX-2 seen IL-1beta. Taken together, the results provide, for the first time, a mechanism that assures the coordinate expression and activation in time and space of COX-2 and cPLA(2), assuring maximal production of PGE(2).

Post-antibiotic effect induced by an antibiotic combination: influence of altered susceptibility to individual components

OBJECTIVES

The impact of altered susceptibility of an organism to the components of an antibiotic combination on post-antibiotic effect (PAE) was studied.

METHODS

The baseline PAEs expressed by Pseudomonas aeruginosa ATCC 27853 were recorded following 1 h of exposure to piperacillin and gentamicin, alone and in combination. Similar PAE assessments were made after resistance to the individual antibiotics was induced over 0.5-2x of their respective MIC.

RESULTS

Before any induction, the PAE produced by piperacillin alone was negligible and that by the combination was synergistic. After piperacillin resistance was induced, PAE exhibited by the beta-lactam remained negligible, and comparable PAEs were observed for gentamicin and the combination, suggesting an additive interaction with a dominant effect from gentamicin. When resistance was induced against gentamicin, progressively shorter PAE was expressed by the aminoglycoside alone and the combination at increasing levels of resistance. In addition, a measurable PAE was unexpectedly observed for piperacillin, whereas the interaction also became additive.

CONCLUSIONS

In summary, the PAE expressed by the test combination was highly dependent on the status of gentamicin resistance. The resistance profile exhibited by the organism against individual antibiotics of the combination showed an impact on the type of interaction expressed.

Post-antibiotic effect induced by an antibiotic combination: influence of mode, sequence and interval of exposure

OBJECTIVES

The effects of mode, sequence and interval of antibiotic exposure on the post-antibiotic effect (PAE) induced by rifampicin and tobramycin were studied using Escherichia coli ATCC 25922 as the test organism.

METHODS

In triplicate, baseline PAEs were evaluated by exposing E. coli to rifampicin and tobramycin individually and simultaneously for 1 h. PAEs were further assessed in a second study, with the organism exposed first to rifampicin for 1 h, followed by a second 1 h tobramycin exposure, commencing at the beginning, middle and end of the PAE phase induced by rifampicin. The third study was similar to the above, but with the sequence of the two antibiotics reversed, i.e. tobramycin then rifampicin.

RESULTS

The PAE produced by simultaneous exposure of the combination showed an apparent additive interaction (PAE: 5.0+/-0.3 h) when compared with the PAE of individual antibiotics (rifampicin alone: 3.0+/-0.1 h; tobramycin alone: 1.5+/-0.1 h). However, an antagonistic interaction was observed in the second study, with a more pronounced degree of antagonism at the beginning, dissipating towards the end of the previous rifampicin PAE (PAE at the beginning: 2.6+/-0.3 h; the middle: 1.5+/-0.2 h; and at the end: 1.7+/-0.3 h). By subtracting the residual contribution from the first rifampicin exposure, the net average PAEs attributed to the second tobramycin exposure actually increased, from -0.4 to 1.7 h from the beginning to the end of the rifampicin PAE. For the third study, an additive interaction was again observed when the organism was exposed to tobramycin first (PAE at the beginning: 4.7+/-0.4 h; the middle: 3.7+/-0.7 h; and at the end: 3.1+/-0.4 h). The timing of the second rifampicin exposure had no impact to the interaction; after correction, the net mean PAEs attributed to the second rifampicin exposure were maintained at 3.2, 3.2 and 3.1 h.

CONCLUSIONS

The present data suggest that the expression of interaction type on PAE by an antibiotic combination was dependent on the mode, sequence and interval of exposure. The impact of these variables should not be overlooked when clinical dosing regimens are optimized.

Theory of antimicrobial combinations: biocide mixtures - synergy or addition?

AIMS

To demonstrate the effect that non-linear dose responses have on the appearance of synergy in mixtures of antimicrobials.

METHODS AND RESULTS

A mathematical model, which allows the prediction of the efficacy of mixtures of antimicrobials with non-linear dose responses, was produced. The efficacy of antimicrobial mixtures that would be classified as synergistic by time-kill methodology was shown to be a natural consequence of combining antimicrobials with non-linear dose responses.

CONCLUSIONS

The effectiveness of admixtures of biocides and other antimicrobials with non-linear dose responses can be predicted. If the dose response (or dilution coefficient) of any biocidal component, in a mixture, is other than one, then the time-kill methodology used to ascertain the existence of synergy in antimicrobial combinations is flawed.

SIGNIFICANCE AND IMPACT OF THE STUDY

The kinetic model developed allows the prediction of the efficacy of antimicrobial combinations. Combinations of known antimicrobials, which reduce the time taken to achieve a specified level of microbial inactivation, can be easily assessed once the kinetic profile of each component has been obtained. Most patented cases of antimicrobial synergy have not taken into account the possible effect of non-linear dose responses of the component materials. That much of the earlier literature can now be predicted, suggests that future cases will require more thorough proof of the alleged synergy.

An approach for the evaluation of synergy between antimicrobials

Synergy can be assessed by a variety of microbiological techniques. Mathematical modelling of synergy data can accomplish similar results and may provide more information than currently available techniques. In this study a combination of 1/4 MIC of aztreonam (AZTR) and 1 MIC of ciprofloxacin (CIPX) showed synergy against Pseudomonas aeruginosa in killing curve studies (based on the two log different criteria) at the end of 24 h. However, re-growth was always observed even when bactericidal concentrations of antibiotics were present. The surviving organisms showed a two-fold increase of MIC to CIPX after the studies and this resistance was not reversible. A mathematical model to quantitatively describe this observation is proposed.

Comparison of methods of interpretation of checkerboard synergy testing

Four different methods for interpreting the results of checkerboard synergy testing were compared by applying each to a set of synergy study data. Statistically significant differences in synergy were detected among methods (% synergy ranged from 10 to 83%). As interpretations were found to vary widely based upon method, one should be aware of this in interpreting the relevant literature.

Fractional maximal effect method for in vitro synergy between amoxicillin and ceftriaxone and between vancomycin and ceftriaxone against Enterococcus faecalis and penicillin-resistant Streptococcus pneumoniae

In the present study we assessed the use of a new in vitro testing method and graphical representation of the results to investigate the potential effectiveness of combinations of amoxicillin (AMZ) plus ceftriaxone (CRO) and of CRO plus vancomycin (VAN) against strains of Streptococcus pneumoniae highly resistant to penicillin and cephalosporins (PRP strains). We used the fractional maximal effect (FME) method of time-kill curves to calculate adequate concentrations of the drugs to be tested rather than relying on arbitrary choices. The concentrations obtained, each of which corresponded to a fraction of the maximal effect, were tested alone and in combination with the bacterial strains in a broth medium. Synergy was defined as a ratio of observed effect/theoretical effect, called FME, of greater than 1, additivity was defined as an FME equal to 1, and antagonism was defined as an observed effect lower than the best effect of one of the antibiotics used alone. The area between antagonism and additivity is the indifference zone. The well-known synergy between amoxicillin and gentamicin against a reference strain of Enterococcus faecalis was confirmed, with a best FME equal to 1.07. Two strains of PRP, strains PRP-1 and PRP-2, were studied. The MICs for PRP-1 and PRP-2 were as follows: penicillin, 4 and 16 microg/ml, respectively; AMZ, 2 and 8 microg/ml, respectively, CRO, 1 and 4 microg/ml, respectively; and VAN, 0.5 and 0.25 microg/ml, respectively. For PRP-1 the best FME for the combination AMZ-CRO was 1.22 with drug concentrations of 1.68 mg/liter for AMZ and 0.17 mg/liter for CRO; the best FME for the combination VAN-CRO was 1.75 with VAN at 0.57 mg/liter and CRO at 0.17 mg/liter. For PRP-2 the best FME obtained for the combination AMZ-CRO was 1.05 with drug concentrations of 11.28 mg/liter for AMZ and 0.64 mg/liter for CRO; the best FME obtained for the combination VAN-CRO was 1.35 with VAN at 0.25 mg/liter and CRO at 1.49 mg/liter. These results demonstrated the synergy of both combinations, AMZ-CRO and VAN-CRO, against PRP strains at drug concentrations achievable in humans. Consequently, either of the combinations can be proposed for use for the treatment of PRP infections.

Antibiotic synergy and antagonism

The field of synergistic combinations of antibiotics is extremely broad and mostly has been explored in vitro. Some fixed combinations were successfully developed commercially. A few combinations were tested in animal models, and a smaller number was studied in human patients. Any practitioner in infectious diseases has some individual cases, published or unpublished, which add some evidence to the role of synergistic combinations in difficult therapy problems--either on the side of the patient when immunosuppressed or on the side of the bacterial strain, when multiply resistant. MRSA, VISA, E. faecium resistant to penicillin G and highly resistant to aminoglysocides and to vancomycin, P. aeruginosa resistant to ceftazidime and imipenem, and Acinetobacter baumani resistant to imipenem are some of the bacterial strains dangerous for the patient and the hospital, which trigger the imagination of the microbiologist and physician to find a satisfactory treatment. On the side of the drug industry, the increasing knowledge of resistance mechanisms and of synergistic mechanisms may open some new approach, such as efflux inhibitors, a membrane-active compound that can be combined with a partner antibiotic. Antagonism between antibiotics would be worthwhile to study because it likely contributes to the disadvantages of the inappropriate use of antimicrobial combinations.

New pharmacodynamic parameters for antimicrobial agents

The application of pharmacodynamic theories to antimicrobial chemotherapy has greatly improved the prediction of the time course of activity expressed by antibiotics. Being a major component of the antibiotic-bacterium interaction system, pharmacodynamics, when properly integrated with the pharmacokinetics established for the antibiotic, allow better evaluation of the dosage regimen in conjunction with its clinical response. Before this approach becomes effective, detailed background information on the complex antibiotic-bacterium interactions have to be secured. To achieve this, proper characterization of a time-kill curve is a prerequisite. The use of susceptibility endpoints such as the MIC with respect to the antibiotic concentrations achievable in vivo represent the conventional approach to clinical dosing of antimicrobial agents, i.e. by maintaining concentrations above the MIC. Recently, a number of surrogate markers have been proposed by combining suitable pharmacokinetic parameters and susceptibility data, e.g. peak/MIC ratio, AUC>MIC, time above MIC, AUIC etc. to enhance the prediction of clinical outcomes. Attempts have been made to apply these pharmacokinetic/pharmacodynamic markers to antibiotics of the same class as well as to antibiotics from different classes. This review aims to discuss the various microbial dynamic responses in relation to antibiotic exposure and the development of different pharmacokinetic/pharmacodynamic markers for use in current antimicrobial chemotherapy.

Achieving an optimal outcome in the treatment of infections. The role of clinical pharmacokinetics and pharmacodynamics of antimicrobials

Over the past few decades, the importance of applying pharmacokinetic principles to the design of drug regimens has been increasingly recognised by clinicians. From the perspective of antimicrobial chemotherapy, an improvement in clinical outcome and/or a reduction in toxicity are of primary interest. Before application of these pharmacokinetic theories can be effective, the interrelationships between antimicrobial, pathogen and host factors must be clearly defined. Information regarding the pharmacokinetics of the antimicrobial and the quantification of pathogen susceptibility is required. Even though susceptibility end-points such as minimum inhibitory concentration (MIC) and minimum bactericidal concentration are widely employed, they do not provide any information on dynamic changes of bacterial densities. In this regard, time-kill studies can provide more basic knowledge of the complex bacterial responses to the antimicrobial. Better prediction of these responses can be afforded by the use of mathematical models. More recently, various surrogate end-points employing a combination of suitable pharmacokinetic parameters and susceptibility data, for example the ratio of peak concentration to MIC, the area under the concentration-time curve above the MIC (AUC > MIC), the time above the MIC, or the area under the inhibitory curve (AUIC), have been suggested for better prediction of the activity of different classes of antimicrobials. To allow more extensive investigations of the contribution of pharmacokinetics to the pharmacodynamics of antimicrobials, various in vitro kinetic models have been developed. However, certain limitations exist, and it is necessary to avoid over-interpretation of the data generated by these models. Two important microbial dynamic responses, postantibiotic effect and resistance selection, must be further explored before the full impact of pharmacokinetics on antimicrobial chemotherapy can be depicted. The present paper aims at discussing all the relevant factors and provides some pertinent information on the use of pharmacokinetic-pharmacodynamic principles in antimicrobial therapy.

Application of logistic growth model to pharmacodynamic analysis of in vitro bactericidal kinetics

A new pharmacodynamic model for the analysis of in vitro bactericidal kinetics was developed based on the logistic growth model, with the bacterial phases divided into two compartments. The model equations are expressed as nonlinear simultaneous differential equations, and the Runge-Kutta-Gill method was adopted to numerically solve the equations in both the simulation and the least squares curve-fitting procedures. The model can describe the initial killing and the regrowth phases and can explain the nonlinear dependence of the killing rate on the drug concentration. The model can also explain the plateau in the bacterial growth curve that is often observed in in vitro experiments. The model was applied to analysis of the in vitro time-killing data of beta-lactam antibiotics, S-4661, meropenem, imipenem, cefpirome, and ceftazidim against three types of bacteria, Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus. The results of curve-fitting using the least squares program MULTI (Runge) showed good fits for all types of drugs and bacteria. The relationship between the characteristics of the drug-bacteria interactions and the estimated pharmacodynamic parameters is discussed.

Antibiotic exposure and its relationship to postantibiotic effect and bactericidal activity: constant versus exponentially decreasing tobramycin concentrations against Pseudomonas aeruginosa

In vitro postantibiotic effects (PAEs) exhibited by a standard strain of Pseudomonas aeruginosa following exposure to tobramycin at constant concentrations were compared to those at exponentially decreasing concentrations. Exposure to a constant concentration showed more extensive bacterial killing and resulted in longer PAEs at comparable areas under the concentration-time curves above the MIC. This phenomenon suggests a significant contribution of pharmacokinetics to antimicrobial pharmacodynamics.

Simultaneous pharmacodynamic analysis of the lag and bactericidal phases exhibited by beta-lactams against Escherichia coli

Antibiotic-bacterium interactions are complex in nature. In many cases, bacterial killing does not commence immediately after the addition of an antibiotic, and a lag period is observed. Antibiotic permeation and/or the intermediate steps that exist between antibiotic-receptor binding and expression of cell death are two major possible causes for such lag period. This study was primarily designed to determine the relationship, if any, between antibiotic concentrations and the lag periods by a modeling approach. Short-term time-kill studies were conducted for amoxicillin, ampicillin, penicillin-G, oxacillin, and dicloxacillin against Escherichia coli. In conjunction with the use of a saturable rate model to describe the concentration-dependent killing process, a first-order induction (initiation) rate constant was used to characterize the delay in bacterial killing during the lag period. For all of the beta-lactams tested, parameters describing the bactericidal effect suggest that amoxicillin and ampicillin were much more potent than oxacillin and dicloxacillin. The induction rate constant estimates for both ampicillin and amoxicillin were found to relate linearly to concentrations. Nevertheless, these induction rate constant estimates were lower for penicillin-G, oxacillin, and dicloxacillin and increased nonlinearly with concentrations until an apparent plateau was observed. These findings support the hypothesis that the permeation process is potentially a rate-limiting step for the rapid bactericidal beta-lactams such as ampicillin and amoxicillin. However, as suggested by previous observations of the various morphological changes induced by beta-lactams, the contribution of the steps following antibiotic-receptor complex formation to the lag period might be significant for the less bactericidal antibiotics such as oxacillin and dicloxacillin. Findings from the present modeling approach can potentially be used to guide future bench experimentation.

Performance of the fractional maximal effect method: comparative interaction studies of ciprofloxacin and protein synthesis inhibitors

The performance of the recently developed Fractional Maximal Effect (FME) method was evaluated along with the conventional checkerboard technique and time-kill method. Ciprofloxacin in combination with tobramycin was tested against Escherichia coli and Pseudomonas aeruginosa and in combination with tetracycline, chloramphenicol, erythromycin against Escherichia coli. Two combinations, amoxicillin-tetracycline (antagonistic) and tobramycin-ticarcillin (synergistic), were included as reference interactions. The FME method unequivocally showed an antagonistic interaction between ciprofloxacin and all the protein synthesis inhibitors (PSIs) tested, while the other two methods yielded variable results. At a total FME (TFME) level of 1, the FME method demonstrated a similar degree of antagonism against ciprofloxacin by tetracycline, chloramphenicol, and erythromycin, and much lower by tobramycin. Internal consistency of the FME operation was demonstrated by the identical conclusions obtained at both TFME levels of 0.5 and 1. The FME method appears to be a practical alternative for resolving the inconsistencies observed in conventional methods of antibiotic combination testing.

In vitro pharmacodynamics of piperacillin, piperacillin-tazobactam, and ciprofloxacin alone and in combination against Staphylococcus aureus, Klebsiella pneumoniae, Enterobacter cloacae, and Pseudomonas aeruginosa

The time-kill curve methodology was used to determine the pharmacodynamics of piperacillin, ciprofloxacin, piperacillin-tazobactam and the combinations piperacillin-ciprofloxacin and ciprofloxacin-piperacillin-tazobactam. Kill curve studies were performed for piperacillin, ciprofloxacin, and piperacillin-tazobactam at concentrations of 0.25 to 50 times the MICs for 13 strains of bacteria: four Pseudomonas aeruginosa, three Enterobacter cloacae, three Klebsiella pneumoniae, and three Staphylococcus aureus isolates (tazobactam concentrations of 0.5, 4, and 12 micrograms/ml). By using a sigmoid Emax model and nonlinear least squares regression, the 50% lethal concentrations and the maximum lethal rates of each agent were determined for each bacterial strain. For piperacillin-ciprofloxacin and ciprofloxacin-piperacillin-tazobactam, kill curve studies were performed with concentrations obtained by the fractional maximal effect method (R. C. Li, J. J. Schentag, and D. E. Nix, Antimicrob. Agents Chemother. 37:523-531, 1993) and from individual 50% lethal concentrations and maximum lethal rates. Ciprofloxacin-piperacillin-tazobactam was evaluated only against the four P. aeruginosa strains. Interactions between piperacillin and ciprofloxacin were generally additive. At physiologically relevant concentrations of piperacillin and ciprofloxacin, ciprofloxacin had the highest rates of killing against K. pneumoniae. Piperacillin-tazobactam (12 micrograms/ml) had the highest rate of killing against E. cloacae. Piperacillin-ciprofloxacin with relatively higher ciprofloxacin concentrations had the greatest killing rates against S. aureus. This combination had significantly higher killing rates than piperacillin (P < 0.002). For all the bacterial strains tested, killing rates by ciprofloxacin were significantly higher than those by piperacillin-tazobactam (4 and 12 micrograms/ml had significantly higher killing rates than piperacillin alone (P < 0.02 and P < 0.004, respectively). The effect of the combination of piperacillin-ciprofloxacin, in which piperacillin concentrations were relatively higher, was not statistically different from that of piperacillin alone (p > or = 0.71). The combination of ciprofloxacin-piperacillin-tazobactam achieved greater killing than other combinations or monotherapies against P. aeruginosa. The reduction in the initial inoculum was 1 to 4 logs greater with ciprofloxacin-piperacillin-tazobactam at 4 and 12 micrograms/ml than with any other agent or combination of agents. On the basis of the additive effects prevalently demonstrated in the in vitro study, the combinations of piperacillin-ciprofloxacin and piperacillin-tazobactam are rational therapeutic options. Greater killing of P. aeruginosa was demonstrated with ciprofloxacin-piperacillin--tazobactam. Since treatment failure of P. aeruginosa pneumonia is a significant problem, clinical studies are warranted.

In vitro activities of metronidazole and its hydroxy metabolite against Bacteroides spp

Metronidazole is metabolized to two major oxidative products: an acid metabolite and a hydroxy metabolite. While the activity of the acid metabolite is negligible, the activity of the hydroxy metabolite is approximately 65% of the activity of the parent drug. Pharmacokinetic studies of metronidazole and its hydroxy metabolite have shown that the MICs of both compounds remain above the MICs for most anaerobic organisms over an 8-h dosing interval. By a checkerboard assay, the combined activities of metronidazole and the hydroxy metabolite were examined against 4 quality control strains of Bacteroides species. Macrobroth tube dilutions were set up with Wilkins-Chalgren broth. Serial twofold dilutions of each agent were performed to achieve final concentrations ranging from 0.06 to 4.0 micrograms/ml. The MICs for Bacteroides fragilis and B. distasonis were 1.0 microgram/ml for both parent drug and metabolite. For B. thetaiotamicron and B. ovatus, the MICs of metronidazole and the hydroxy metabolite were 1.0 and 2.0 micrograms/ml, respectively. Synergy was determined by calculating the fractional inhibitory concentration (FIC) index. The interpretative criteria for the FIC index were as follows: synergy, FIC < or = 0.5; partial synergy, 0.51 to 0.75; indifference, FIC 0.76 to 4.0; and antagonism, FIC > 4.0. Partial synergy was observed for the four anaerobes tested, with FIC indices ranging from 0.63 to 0.75. On the basis of this data, in vitro susceptibilities to agents such as metronidazole may ultimately require reevaluation to account for active metabolites.

Pharmacodynamic modeling of bacterial kinetics: beta-lactam antibiotics against Escherichia coli

A simple pharmacodynamic model has been developed to describe the bacterial kinetics exhibited by beta-lactam antibiotics. In contrast with previous models that only characterized the early killing phase of a time-kill curve, the present model is capable of simultaneously describing both the killing and regrowth phases. The model relied on the use of both first-order bactericidal and resistance formation rate constants to accurately define the time-dependent changes in the bacterial populations of an antibiotic-treated culture. The concentration dependency of the bactericidal rate constant was further delineated using a saturable-receptor model. Furthermore, an exponential decrease in the resistance formation rate with increasing antibiotic concentrations was demonstrated. The evolving pharmacodynamic model was also explored via computer simulations by perturbing the two governing rate constants. The model was subsequently applied to the description of time-kill data for amoxicillin, penicillin G, and cephalexin against Escherichia coli. The description of amdinocillin's action against E. coli was not as comprehensive because of the existence of a second killing phase. However, this model can be applicable to many classes of antibiotics that display the usual killing and regrowth phases in time-kill studies. The pharmacodynamic model can potentially improve the prediction of bacterial killing and regrowth and foster an improved understanding of complex antimicrobial pharmacodynamics.

In vitro synergism of ceftriaxone combined with aminoglycosides against Pseudomonas aeruginosa

The antipseudomonal activities of ceftriaxone (CEF) or ceftazidime (CAZ), each combined with tobramycin (TOB) or netilmicin (NET), against 90 clinically significant Pseudomonas aeruginosa isolates were examined both by checkerboard and time-kill assays. As expected, susceptibility testing of single antibiotics by agar dilution demonstrated good activity for CAZ (89% susceptible), TOB (94%), and NET (58%), but poor activity for CEF (15%). Checkerboard studies revealed striking synergy (FIC indices < or = 0.5) for CEF, however, in combination with either TOB (72%) or NET (81%), compared with CAZ-TOB (44%) or CAZ-NET (60%) (P < 0.01, respectively). No antagonism (FIC indices > or = 4) was found in any of these combinations. The MIC90s of CEF, CAZ, or aminoglycosides in the combinations were reduced at least fourfold: CEF, from > 128 to 32 mg/liter; CAZ, from 16 to 4 mg/liter; TOB, from 4 to 0.5 mg/liter; and NET, from 32 to 4 mg/liter. With CEF and NET, the percentage of strains sensitive to < or = 8 mg/liter of both drugs alone and in combination increased from 9% to 69%. Results of the time-kill assay for CEF-NET agreed reasonably well with the checkerboard method (Spearman rank correlation coefficient, 0.40, P < or = 0.01), and generated a bactericidal outcome in 60% (24 of the 40 isolates), when tested with combinations at 1/4 MBC of either antibiotic alone. Importantly, concentrations of CEF and aminoglycoside combinations that demonstrated synergy by either checkerboard or time-kill assays were achievable in serum clinically. These data suggest a unique interaction of CEF-aminoglycoside combinations against P. aeruginosa.(ABSTRACT TRUNCATED AT 250 WORDS)