Synergistic activities of fluconazole and voriconazole with terbinafine against four Candida species determined by checkerboard, time-kill, and Etest methods

In vitro effects of N-acetylcysteine in combination with antifungal agents against Malassezia pachydermatis isolated from canine otitis externa

BACKGROUND

Many clinicians prescribe antifungal agents to treat canine otitis externa (OE). However, studies evaluating the antifungal effects of N-acetylcysteine (NAC) and its combinations are limited.

HYPOTHESIS/OBJECTIVES

The aim of this study was to evaluate the antifungal effects of NAC alone and in combination with other antifungal agents against Malassezia pachydermatis isolated from canine OE.

MATERIALS AND METHODS

M. pachydermatis samples were collected from 13 dogs with OE. The final concentration of the inoculum suspensions of M. pachydermatis was 1-5 × 106 colony forming units/mL. The concentrations of the test compounds ketoconazole (KTZ), terbinafine (TER), nystatin (NYS) and NAC were 0.02-300 µg/mL, 0.04-80 µg/mL, 0.16-40 µg/mL and 1.25-20 mg/mL, respectively. The minimum inhibitory concentration (MIC) was measured to evaluate the susceptibility of the M. pachydermatis to KTZ, TER, NYS and NAC. The checkerboard testing method and fractional inhibitory concentration index were used to evaluate the effect of NAC in combination with KTZ, TER and NYS against M. pachydermatis.

RESULTS

The MIC90 values of M. pachydermatis were 4.6875-9.375 µg/mL, 1.25 µg/mL, 5-10 µg/mL and 10 mg/mL for KTZ, TER, NYS and NAC, respectively. The synergistic effects of KTZ, TER and NYS with NAC were identified in 0/13, 2/13 and 0/13 isolates, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

NAC had an antifungal effect against M. pachydermatis but did not exert synergistic effects when used with KTZ, TER and NYS. Thus, the use of NAC alone as a topical solution could be considered an effective treatment option for canine OE involving M. pachydermatis.

Visible-Light-Activated Molecular Machines Kill Fungi by Necrosis Following Mitochondrial Dysfunction and Calcium Overload

Invasive fungal infections are a growing public health threat. As fungi become increasingly resistant to existing drugs, new antifungals are urgently needed. Here, it is reported that 405-nm-visible-light-activated synthetic molecular machines (MMs) eliminate planktonic and biofilm fungal populations more effectively than conventional antifungals without resistance development. Mechanism-of-action studies show that MMs bind to fungal mitochondrial phospholipids. Upon visible light activation, rapid unidirectional drilling of MMs at ≈3 million cycles per second (MHz) results in mitochondrial dysfunction, calcium overload, and ultimately necrosis. Besides their direct antifungal effect, MMs synergize with conventional antifungals by impairing the activity of energy-dependent efflux pumps. Finally, MMs potentiate standard antifungals both in vivo and in an ex vivo porcine model of onychomycosis, reducing the fungal burden associated with infection.

A Secondary Metabolite Secreted by Penicillium citrinum Is Able to Enhance Parastagonospora nodorum Sensitivity to Tebuconazole and Azoxystrobin

Parastagonospora nodorum causes glume and leaf blotch of wheat, a harmful disease resulting in serious losses in grain yield. In many countries including Russia, fungicidal formulations based on triazoles and on triazoles combined with strobilurins are used to control this fungus. However, their prolonged application may promote the selection of fungicide-resistant strains of P. nodorum leading to significant attenuation or even loss of fungicidal effect. Chemosensitization of plant pathogenic fungi with natural compounds represents a promising strategy for mitigating fungicide resistance and other negative impacts of fungicides. In this work, we applied a chemosensitization approach towards P. nodorum strains non-resistant or resistant to tebuconazole or azoxystrobin using 6-demethylmevinolin (6-DMM), a metabolite of Penicillium citrinum. The resistant strains were obtained by the mutagenesis and subsequent culturing on agar media incorporated with increasing doses of Folicur® EC 250 (i.e., tebuconazole) or Quadris® SC 250 (i.e., azoxystrobin). Test strains m8-4 and kd-18, most resistant to tebuconazole and azoxystrobin, respectively, were selected for sensitization experiments. These experiments demonstrated that combining 6-DMM with Folicur® enhanced fungicidal effectiveness in vitro and in vivo in addition to attenuating the resistance of P. nodorum to tebuconazole in vitro. 6-DMM was also found to augment Quadris® efficacy towards kd-18 when applied on detached wheat leaves inoculated with this strain. Experiments on P. nodorum sensitization under greenhouse conditions included preventive (applying test compounds simultaneously with inoculation) or post-inoculation spraying of wheat seedlings with 6-DMM together with Folicur® at dose rates (DR) amounting to 10% and 20% of DR recommended for field application (RDR). Combined treatments were run in parallel with using the same DR of the fungicide and sensitizer, alone. A synergistic effect was observed in both preventive and post-inoculation treatments, when the sensitizer was co-applied with the fungicide at 10% of the RDR. In this case, disease reduction significantly exceeded the protective effect of Folicur® at 10% or 20% of the RDR, alone, and also a calculated additive effect. Collectively, our findings suggest that 6-DMM is promising as a putative component for formulations with triazole and strobilurin fungicides. Such new formulations would improve fungicide efficacy and, potentially, lower rates of fungicides needed for plant pathogen control.

Augmenting Azoles with Drug Synergy to Expand the Antifungal Toolbox

Fungal infections impact the lives of at least 12 million people every year, killing over 1.5 million. Wide-spread use of fungicides and prophylactic antifungal therapy have driven resistance in many serious fungal pathogens, and there is an urgent need to expand the current antifungal arsenal. Recent research has focused on improving azoles, our most successful class of antifungals, by looking for synergistic interactions with secondary compounds. Synergists can co-operate with azoles by targeting steps in related pathways, or they may act on mechanisms related to resistance such as active efflux or on totally disparate pathways or processes. A variety of sources of potential synergists have been explored, including pre-existing antimicrobials, pharmaceuticals approved for other uses, bioactive natural compounds and phytochemicals, and novel synthetic compounds. Synergy can successfully widen the antifungal spectrum, decrease inhibitory dosages, reduce toxicity, and prevent the development of resistance. This review highlights the diversity of mechanisms that have been exploited for the purposes of azole synergy and demonstrates that synergy remains a promising approach for meeting the urgent need for novel antifungal strategies.

Techniques for the Assessment of In Vitro and In Vivo Antifungal Combinations

Systemic fungal infections are associated with high mortality rates despite adequate treatment. Moreover, acquired resistance to antifungals is increasing, which further complicates the therapeutic management. One strategy to overcome antifungal resistance is to use antifungal combinations. In vitro, several techniques are used to assess drug interactions, such as the broth microdilution checkerboard, agar-diffusion methods, and time-kill curves. Currently, the most widely used technique is the checkerboard method. The aim of all these techniques is to determine if the interaction between antifungal agents is synergistic, indifferent, or antagonistic. However, the interpretation of the results remains difficult. Several methods of analysis can be used, based on different theories. The most commonly used method is the calculation of the fractional inhibitory concentration index. Determination of the usefulness of combination treatments in patients needs well-conducted clinical trials, which are difficult. It is therefore important to study antifungal combinations in vivo, in experimental animal models of fungal infections. Although mammalian models have mostly been used, new alternative animal models in invertebrates look promising. To evaluate the antifungal efficacy, the most commonly used criteria are the mortality rate and the fungal load in the target organs.

The Antifungal and Synergistic Effect of Bisphosphonates in Cryptococcus

New treatment strategies are required for cryptococcosis, a leading mycosis in HIV-AIDS patients. Following the identification of Cryptococcus proteins differentially expressed in response to fluconazole, we targeted farnesyl pryrophosphate synthetase (FPPS), an enzyme in the squalene biosynthesis pathway, using nitrogenous bisphosphonates. We hypothesized that these would disrupt squalene synthesis and thereby produce synergy with fluconazole, which acts on a downstream pathway that requires squalene. The susceptibilities of 39 clinical isolates from 6 different species of Cryptococcus were assessed for bisphosphonates and fluconazole, used both independently and in combination. Effective fluconazole-bisphosphonate combinations were then assessed for fungicidal activity, efficacy against biofilms, and ability to resolve cryptococcosis in an invertebrate model. The nitrogenous bisphosphonates risedronate, alendronate, and zoledronate were antifungal against all strains tested. Zoledronate was the most effective (geometric mean MIC = 113.03 mg/liter; risedronate = 378.49 mg/liter; alendronate = 158.4 mg/liter) and was broadly synergistic when combined with fluconazole, with a fractional inhibitory concentration index (FICI) of ≤0.5 in 92% of isolates. Fluconazole and zoledronate in combination were fungicidal in a time-kill assay, inhibited Cryptococcus biofilms, prevented the development of fluconazole resistance, and resolved infection in a nematode model. Supplementation with squalene eliminated bisphosphonate-mediated synergy, demonstrating that synergy was due to the inhibition of squalene biosynthesis. This study demonstrates the utility of targeting squalene synthesis for improving the efficacy of azole-based antifungal drugs and suggests bisphosphonates are promising lead compounds for further antifungal development.

Assessment of the Sensitivity of Some Plant Pathogenic Fungi to 6-Demethylmevinolin, a Putative Natural Sensitizer Able to Help Overcoming the Fungicide Resistance of Plant Pathogens

Agricultural fungicides contaminate the environment and promote the spread of fungicide-resistant strains of pathogenic fungi. The enhancement of pathogen sensitivity to these pesticides using chemosensitizers allows the reducing of fungicide dosages without a decrease in their efficiency. Using Petri plate and microplate bioassays, 6-demethylmevinolin (6-DMM), a putative sensitizer of a microbial origin, was shown to affect both colony growth and conidial germination of Alternaria solani, A. alternata, Parastagonospora nodorum, Rhizoctonia solani, and four Fusarium species (F. avenaceum, F. culmorum, F. oxysporum, F. graminearum) forming a wheat root rot complex together with B. sorokiniana. Non- or marginally toxic 6-DMM concentrations suitable for sensitizing effect were determined by the probit analysis. The range of determined concentrations confirmed a possibility of using 6-DMM as a putative sensitizer for the whole complex of root rot agents, other cereal pathogens (A. alternata, P.nodorum), and some potato (R. solani, A. solani) and tomato (A. solani) pathogens. Despite the different sensitivities of the eight tested pathogens, 6-DMM lacked specificity to fungi and possessed a mild antimycotic activity that is typical of other known pathogen-sensitizing agents. The pilot evaluation of the 6-DMM sensitizing first confirmed a principal possibility of using it for the sensitization of B. sorokiniana and R. solani to triazole- and strobilurin-based fungicides, respectively.

Hevein-Like Antimicrobial Peptides Wamps: Structure-Function Relationship in Antifungal Activity and Sensitization of Plant Pathogenic Fungi to Tebuconazole by WAMP-2-Derived Peptides

Keywords: hevein-like antimicrobial peptides; antifungal activity; antifungal determinants; synergy; chemosensitization; tebuconazole; plant pathogenic fungi.

Beyond the Superficial: Disseminated Trichophyton rubrum Infection in a Kidney Transplant Recipient

Superficial dermatophyte infections are common in the general population and are readily treated with topical antifungals. Deeper invasion is rare, and dissemination to visceral organs is extremely uncommon. We describe a 66-year-old renal transplant recipient who developed disseminated Trichophyton rubrum infection while undergoing treatment for acute humoral rejection. The infection presented as a facial rash with subsequent dissemination to the lungs and chest wall. All sites of infection improved with combination administration of oral posaconazole and terbinafine. In this work, we review the available literature regarding management of disseminated Trichophyton infection and discuss therapeutic interventions for disseminated dermatophytosis in immunosuppressed hosts.

Antifungal Susceptibly Testing by Concentration Gradient Strip Etest Method for Fungal Isolates: A Review

Antifungal susceptibility testing is an important tool for managing patients with invasive fungal infections, as well as for epidemiological surveillance of emerging resistance. For routine testing in clinical microbiology laboratories, ready-to-use commercial methods are more practical than homemade reference techniques. Among commercially available methods, the concentration gradient Etest strip technique is widely used. It combines an agar-based diffusion method with a dilution method that determinates a minimal inhibitory concentration (MIC) in µg/mL. Many studies have evaluated the agreement between the gradient strip method and the reference methods for both yeasts and filamentous fungi. This agreement has been variable depending on the antifungal, the species, and the incubation time. It has also been shown that the gradient strip method could be a valuable alternative for detection of emerging resistance (non-wild-type isolates) as Etest epidemiological cutoff values have been recently defined for several drug-species combinations. Furthermore, the Etest could be useful for direct antifungal susceptibility testing on blood samples and basic research studies (e.g., the evaluation of the in vitro activity of antifungal combinations). This review summarizes the available data on the performance and potential use of the gradient strip method.

The Synergism of the Small Molecule ENOblock and Fluconazole Against Fluconazole-Resistant Candida albicans

Candida albicans is the most common opportunistic fungal pathogen which can cause life-threatening bloodstream infections known as candidaemia. It is very important to discover new drugs and targets for the treatment of candidaemia. In this study, we first investigated the combination antifungal effects of the small molecule ENOblock and fluconazole (FLC) against FLC-resistant C. albicans. A checkerboard microdilution assay showed that ENOblock has a significant synergistic effect in combination with FLC against FLC-resistant C. albicans. The time-kill curve further confirmed the synergism of this compound with FLC against FLC-resistant C. albicans. Moreover, we demonstrated the significant inhibitory effects of ENOblock alone and in combination with FLC against C. albicans hypha and biofilm formation. Furthermore, the XTT assay showed that ENOblock has relatively low toxicity to human umbilical vein endothelial cells. The in vivo antifungal efficacy of ENOblock was further assessed in a murine model of systemic C. albicans infection. Although ENOblock alone was not sufficient to treat C. albicans infection, the combination of FLC and ENOblock showed significant in vivo activity against FLC-resistant C. albicans. Finally, using surface plasmon resonance analysis as well as an inhibition assay, we determined that ENOblock directly interacted with CaEno1 and significantly inhibited the transglutaminase activity of this enzyme, which is involved in the growth and morphogenesis of C. albicans. In summary, these results demonstrate the synergistic effects of FLC and ENOblock against FLC-resistant C. albicans, and indicate that inhibition of the transglutaminase activity of CaEno1 by ENOblock might confer an advantage for the synergism of FLC and ENOblock, suggesting the potential of ENOblock as a new antifungal candidate.

Killing kinetics of anidulafungin, caspofungin and micafungin against Candida parapsilosis species complex: Evaluation of the fungicidal activity

BACKGROUND

Candida parapsilosis, Candida metapsilosis and Candida orthopsilosis are emerging as relevant causes of candidemia. Moreover, they show differences in their antifungal susceptibility and virulence. The echinocandins are different in terms of in vitro antifungal activity against Candida. Time-kill (TK) curves represent an excellent approach to evaluate the fungicidal activity of antifungal drugs.

AIMS

To compare the fungicidal activities of anidulafungin, caspofungin and micafungin against C. parapsilosis species complex by TK curves.

METHODS

Antifungal activities of three echinocandins against C. parapsilosis, C. metapsilosis and C. orthopsilosis were studied by TK curves. Drug concentrations assayed were 0.25, 2 and 8μg/ml. CFU/ml were determined at 0, 2, 4, 6, 24 and 48h.

RESULTS

Killing activities of echinocandins were species-, isolates- and concentration-dependent. Anidulafungin reached the fungicidad endpoint for 6 out of 7 isolates (86%); it required between 13.34 and 29.67h to reach this endpoint for the three species studied, but more than 48h were needed against one isolate of C. orthopsilosis (8μg/ml). Caspofungin fungicidal endpoint was only achieved with 8μg/ml against one isolate of C. metapsilosis after 30.12h (1 out of 7 isolates; 14%). Micafungin fungicidal endpoint was reached in 12.74-28.38h (8μg/ml) against one isolate each of C. parapsilosis and C. orthopsilosis, and against both C. metapsilosis isolates (4 out of 7 isolates; 57%).

CONCLUSIONS

C. metapsilosis was the most susceptible species to echinocandins, followed by C. orthopsilosis and C. parapsilosis. Anidulafungin was the most active echinocandin against C. parapsilosis complex.

In vitro interactions between IAP antagonist AT406 and azoles against planktonic cells and biofilms of pathogenic fungi Candida albicans and Exophiala dermatitidis

In vitro interactions of AT406, a novel IAP antagonist, and azoles including itraconazole, voriconazole, and fluconazole against planktonic cells and biofilms of Candida albicans and Exophiala dermatitidis were assessed via broth microdilution checkerboard technique. AT406 alone exhibited limited antifungal activity. However, synergistic effect between AT406 and fluconazole was observed against both planktonic cells and biofilms of C. albicans, including one fluconazole-resistant strain. Moreover, synergism was also demonstrated between AT406 and itraconazole against both planktonic cells and biofilms of E. dermatitidis. No interaction was observed between AT406 and voriconazole. No antagonism was observed in all combinations.

Pharmacokinetics of antifungal drugs: practical implications for optimized treatment of patients

Introduction

Because of the high mortality of invasive fungal infections (IFIs), appropriate exposure to antifungals appears to be crucial for therapeutic efficacy and safety.

Materials and methods

This review summarises published pharmacokinetic data on systemically administered antifungals focusing on co-morbidities, target-site penetration, and combination antifungal therapy.

Conclusions and discussion

Amphotericin B is eliminated unchanged via urine and faeces. Flucytosine and fluconazole display low protein binding and are eliminated by the kidney. Itraconazole, voriconazole, posaconazole and isavuconazole are metabolised in the liver. Azoles are substrates and inhibitors of cytochrome P450 (CYP) isoenzymes and are therefore involved in numerous drug–drug interactions. Anidulafungin is spontaneously degraded in the plasma. Caspofungin and micafungin undergo enzymatic metabolism in the liver, which is independent of CYP. Although several drug–drug interactions occur during caspofungin and micafungin treatment, echinocandins display a lower potential for drug–drug interactions. Flucytosine and azoles penetrate into most of relevant tissues. Amphotericin B accumulates in the liver and in the spleen. Its concentrations in lung and kidney are intermediate and relatively low myocardium and brain. Tissue distribution of echinocandins is similar to that of amphotericin. Combination antifungal therapy is established for cryptococcosis but controversial in other IFIs such as invasive aspergillosis and mucormycosis.

Antifungal Efficacy of an Intravenous Formulation Containing Monomeric Amphotericin B, 5-Fluorocytosine, and Saline for Sodium Supplementation

PURPOSE

Amphotericin B (AmB) and 5-fluorocytosine (5-FC) exhibit additive to synergistic activity against systemic mycoses. Incompatibility of prescribed formulations precludes concomitant IV administration, a route with distinct advantages. Previously, we used PEG-DSPE micelles to produce a reformulation of Fungizone (AmB-SD), AmB solubilized by sodium deoxycholate, called mAmB-90. Herein, we describe a second reformulation that facilitates co-delivery of mAmB-90 and 5-FC, and evaluate the effect of PEG-DSPE micelles on the combination's activity against Candida albicans.

METHODS

We assessed the effect of 5-FC addition on the stability, in vitro toxicity, and antifungal efficacy of mAmB-90. The aggregation state and particle size of mAmB-90 combined with 5-FC (FmAmB-90) was evaluated over 48 h. Hemolytic activity was measured in vitro. Antifungal activity was determined in vitro against C. albicans. The efficacy of monotherapy and combination treatment was evaluated in a neutropenic mouse model of disseminated candidiasis.

RESULTS

The aggregation state, particle size, and hemolytic activity of mAmB-90 were unaffected by 5-FC. While antifungal activity was similar in vitro, mAmB-90 alone and combined with 5-FC was more potent than AmB-SD in vivo.

CONCLUSIONS

Short-term stability and in vivo efficacy of our formulation suggest potential to simultaneously deliver AmB and 5-FC for potent antifungal efficacy.

NLLSS: Predicting Synergistic Drug Combinations Based on Semi-supervised Learning

Fungal infection has become one of the leading causes of hospital-acquired infections with high mortality rates. Furthermore, drug resistance is common for fungus-causing diseases. Synergistic drug combinations could provide an effective strategy to overcome drug resistance. Meanwhile, synergistic drug combinations can increase treatment efficacy and decrease drug dosage to avoid toxicity. Therefore, computational prediction of synergistic drug combinations for fungus-causing diseases becomes attractive. In this study, we proposed similar nature of drug combinations: principal drugs which obtain synergistic effect with similar adjuvant drugs are often similar and vice versa. Furthermore, we developed a novel algorithm termed Network-based Laplacian regularized Least Square Synergistic drug combination prediction (NLLSS) to predict potential synergistic drug combinations by integrating different kinds of information such as known synergistic drug combinations, drug-target interactions, and drug chemical structures. We applied NLLSS to predict antifungal synergistic drug combinations and showed that it achieved excellent performance both in terms of cross validation and independent prediction. Finally, we performed biological experiments for fungal pathogen Candida albicans to confirm 7 out of 13 predicted antifungal synergistic drug combinations. NLLSS provides an efficient strategy to identify potential synergistic antifungal combinations.

Drug combinations represent a promising strategy for overcoming fungal drug resistance and treating complex diseases. There is an urgent need to establish powerful computational methods for systematic prediction of synergistic drug combination on a large scale. Based on the assumption that principal drugs which obtain synergistic effect with similar adjuvant drugs are often similar and vice versa, NLLSS was developed to predict potential synergistic drug combinations by integrating known synergistic drug combinations, unlabeled drug combinations, drug-target interactions, and drug chemical structures. NLLSS has obtained the reliable performance in the cross validation and experimental validations, which indicated that NLLSS has an excellent performance of identifying potential synergistic drug combinations. Out of 13 predicted antifungal synergistic drug combinations, 7 candidates were experimentally confirmed. It is anticipated that NLLSS would be an important and useful resource by providing a new strategy to identify potential synergistic antifungal combinations, explore new indications of existing drugs, and provide useful insights into the underlying molecular mechanisms of synergistic drug combinations.

Synergistic anti-candidal activity and mode of action of Mentha piperita essential oil and its major components

CONTEXT

Mentha piperita L. (Lamiaceae) has been used in folk medicine since antiquity. Its essential oil (mint EO) and major bioactive components have antimicrobial properties but their mechanism of action is still not clear.

OBJECTIVE

The present work aims to elucidate M. piperita's anti-Candida activity and mode of action.

MATERIALS AND METHODS

Chemical constituents of mint EO were identified by GC-MS by injecting 0.1 ml sample in a splitless mode. MIC was determined by the broth dilution method. Synergy with fluconazole (FLC) was evaluated by checkerboard assay and FICI. Mid log phase cells harvested from YPD media were used for proton extrusion measurement and the rate of glucose-induced H(+) efflux gives PM-ATPase activity. Cell membrane integrity was estimated by total ergosterol content and scanning microscopy at respective MIC and sub-MIC values. In vitro hemolytic activity was performed to rule out possible cytotoxicity of the test compounds.

RESULTS

The MIC value of mint EO, carvone, menthol, and menthone was 225, 248, 500, and 4200 µg/ml, respectively. At their respective MICs, these compounds showed 47, 42, 35, and 29% decrease in PM-ATPase activity besides showing synergy with FLC. In case of FLC-resistant strains, the decrease in H(+) efflux was by 52, 48, 32, and 30%, a trend similar to the susceptible cases. Exposed Candida cells showed a 100% decrease in the ergosterol content, cell membrane breakage, and alterations in morphology.

DISCUSSION AND CONCLUSION

Our studies suggest that mint EO and its lead compounds exert antifungal activity by reducing ergosterol levels, inhibiting PM-ATPase leading to intracellular acidification, and ultimately cell death. Our results suggest that mint EO and its constituents are potential antifungal agents and need to be further investigated.

Combinatorial Activity of Flavonoids with Antibiotics Against Drug-Resistant Staphylococcus aureus

The use of resistance-modifying agents is a potential strategy that is used to prolong the effective life of antibiotics in the face of increasing antibiotic resistance. Since certain flavonoids are potent bacterial efflux pump inhibitors, we assessed morin, rutin, quercetin, hesperidin, and (+)-catechin for their combined activity with the antibiotics ciprofloxacin, tetracycline, erythromycin, oxacillin, and ampicillin against drug-resistant strains of Staphylococcus aureus, including methicillin-resistant S. aureus. Four established methods were used to determine the combined efficacy of each combination: microdilution checkerboard assays, time-kill determinations, the Etest, and dual disc-diffusion methods. The cytotoxicity of the flavonoids was additionally evaluated in a mouse fibroblast cell line. Quercetin and its isomer morin decreased by 3- to 16-fold the minimal inhibitory concentration of ciprofloxacin, tetracycline, and erythromycin against some S. aureus strains. Rutin, hesperidin, and (+)-catechin did not promote any potentiation of antibiotics. Despite the potential cytotoxicity of these phytochemicals at a high concentration (fibroblast IC50 of 41.8 and 67.5 mg/L, respectively), quercetin is commonly used as a supplement for several therapeutic purposes. All the methods, with exception of the time-kill assay, presented a high degree of congruence without any apparent strain specificity.

Possible mechanisms of the antifungal activity of fluconazole in combination with terbinafine against Candida albicans

CONTEXT

Candidiasis is a term describing infections by yeasts from the genus Candida, the majority Candida albicans. Treatment of such infections often requires antifungals such as the azoles, but increased use of these drugs has led to selection of yeasts with increased resistance to these drugs.

OBJECTIVE

Combination therapy would be one of the best strategies for the treatment of candidiasis due to increased resistance to azoles.

MATERIALS AND METHODS

The antifungal activities of fluconazole and terbinafine were evaluated in vitro alone and in combination using broth microdilution test and time kill study. Eventually the expression level of selected genes involved in ergosterol biosynthesis of Candida was evaluated using semi-quantitative RT-PCR.

RESULTS

The obtained results showed the significant MICs ranging from 0.25 to 8 µg/mL followed by FICs ranged from 0.37 to 1 in combination with fluconazole/terbinafine. Our findings have demonstrated that the combination of fluconazole and terbinafine could also significantly reduce the expression of ERG1, 3, and 11 in the cell membrane of Candida in all concentrations tested ranging from 1.73- to 6.99-fold.

DISCUSSION AND CONCLUSION

This study was undertaken with the ultimate goal of finding the probable targets of fluconazole/terbinafine in C. albicans by looking at its effects on cell membrane synthesis.

Evaluation of the best method to assess antibiotic potentiation by phytochemicals against Staphylococcus aureus

The increasing occurrence of bacterial resistance to antibiotics has now reached a critical level. Finding antibiotic coadjuvants capable to inhibit the bacterial resistance mechanisms would be a valuable mid-term solution, until new classes of antibiotics are discovered. Selected plant alkaloids were combined with 5 antibiotics against 10 Staphylococcus aureus strains, including strains expressing distinct efflux pumps and methicillin-resistant S. aureus strains. The efficacy of each combination was assessed using the microdilution checkerboard, time-kill, Etest, and disc diffusion methods. The cytotoxicity of the alkaloids was evaluated in a mouse fibroblast cell line. Potentiation was obtained in 6% of all 190 combinations, especially with the combination of: ciprofloxacin with reserpine (RES), pyrrolidine (PYR), and quinine (QUIN); tetracycline with RES; and erythromycin with PYR. The highest cytotoxicity values were found for QUIN (half maximal inhibitory concentration [IC50] = 25 ± 2.2 mg/L) and theophylline (IC50 = 100 ± 4.7 mg/L).

Unravelling the complex antimicrobial interactions of essential oils--the case of Thymus vulgaris (thyme)

Thymus vulgaris has gained tremendous popularity as an ornamental, culinary herb and its use in phytotherapy is well established and supported in the literature. The objective of this study was to explore possible interactions between selected molecules within Thymus vulgaris essential oil (TvEO) to gain a better understanding of how this complex essential oil exerts its antimicrobial activity. Evaluation of the antimicrobial efficacy and interactions were assessed on the essential oil and volatile constituents against various pathogens. Interactions between molecules at various ratios were graphically observed through the construction of isobolograms. Gas chromatography-mass spectrometry (GC-MS) analysis revealed 22 compounds which collectively represent >95% of the oil composition. Based on their minimum inhibitory concentration (MIC) values, they were categorised into weak (≥4 mg mL⁻¹), moderate (2-4 mg mL⁻¹) and noteworthy active (≤2 mg mL⁻¹) compounds. For the combination study, 21% synergistic, 42% additive, 36% indifferent and 1% antagonistic interactions were observed. Most of the interactions were observed between the weak and highly active molecules, and interestingly, no synergistic interaction was observed between the highly active compounds. Synergistic and additive interactions between the strong and weaker antimicrobial constituents present in TvEO enhance the antimicrobial efficacy of this commercially important essential oil.

Head-to-head comparison of inhibitory and fungicidal activities of fluconazole, itraconazole, voriconazole, posaconazole, and isavuconazole against clinical isolates of Trichosporon asahii

Treatment of disseminated Trichosporon infections still remains difficult. Amphotericin B frequently displays inadequate fungicidal activity and echinocandins have no meaningful antifungal effect against this genus. Triazoles are currently the drugs of choice for the treatment of Trichosporon infections. This study evaluates the inhibitory and fungicidal activities of five triazoles against 90 clinical isolates of Trichosporon asahii. MICs (μg/ml) were determined according to Clinical and Laboratory Standards Institute microdilution method M27-A3 at 24 and 48 h using two endpoints, MIC-2 and MIC-0 (the lowest concentrations that inhibited ∼50 and 100% of growth, respectively). Minimum fungicidal concentrations (MFCs; μg/ml) were determined by seeding 100 μl of all clear MIC wells (using an inoculum of 10(4) CFU/ml) onto Sabouraud dextrose agar. Time-kill curves were assayed against four clinical T. asahii isolates and the T. asahii ATCC 201110 strain. The MIC-2 (∼50% reduction in turbidity compared to the growth control well)/MIC-0 (complete inhibition of growth)/MFC values that inhibited 90% of isolates at 48 h were, respectively, 8/32/64 μg/ml for fluconazole, 1/2/8 μg/ml for itraconazole, 0.12/0.5/2 μg/ml for voriconazole, 0.5/2/4 μg/ml for posaconazole, and 0.25/1/4 μg/ml for isavuconazole. The MIC-0 endpoints yielded more consistent MIC results, which remained mostly unchanged when extending the incubation to 48 h (98 to 100% agreement with 24-h values) and are easier to interpret. Based on the time-kill experiments, none of the drugs reached the fungicidal endpoint (99.9% killing), killing activity being shown but at concentrations not reached in serum. Statistical analysis revealed that killing rates are dose and antifungal dependent. The lowest concentration at which killing activity begins was for voriconazole, and the highest was for fluconazole. These results suggest that azoles display fungistatic activity and lack fungicidal effect against T. asahii. By rank order, the most active triazole is voriconazole, followed by itraconazole ∼ posaconazole ∼ isavuconazole > fluconazole.

Antifungal activity of azole compounds CPA18 and CPA109 against azole-susceptible and -resistant strains of Candida albicans

OBJECTIVES

In this study we investigated the in vitro fungistatic and fungicidal activities of CPA18 and CPA109, two azole compounds with original structural features, alone and in combination with fluconazole against fluconazole-susceptible and -resistant Candida albicans strains.

METHODS

Antifungal activities were measured by MIC evaluation and time-kill studies. Azole binding analysis was performed by UV-Vis spectroscopy. Hyphal growth inhibition and filipin and propidium iodide staining assays were used for morphological analysis. An analysis of membrane lipids was also performed to gauge alterations in membrane composition and integrity. Synergism was calculated using fractional inhibitory concentration indices (FICIs). Evaluation of cytotoxicity towards murine macrophages was performed to verify selective antifungal activity.

RESULTS

Even though their binding affinity to C. albicans Erg11p is comparable to that of fluconazole, CPA compounds are active against resistant strains of C. albicans with a mutation in ERG11 sequences and/or overexpressing the ABC transporter genes CDR1 and CDR2, which encode ATP-dependent efflux pumps. Moreover, CPA18 is fungistatic, even against the two resistant strains, and was found to be synergistic with fluconazole. Differently from fluconazole and other related azoles, CPA compounds induced marked changes in membrane permeability and dramatic alterations in membrane lipid composition.

CONCLUSIONS

Our outcomes suggest that CPA compounds are able to overcome major mechanisms of resistance in C. albicans. Also, they are promising candidates for combination treatment that could reduce the toxicity caused by high fluconazole doses, particularly in immunocompromised patients.

Oral infections caused by yeasts in patients with head and neck cancer undergoing radiotherapy. Identification of the yeasts and evaluation of their antifungal susceptibility

Yeasts occur as part of the normal human microbiota. Nevertheless, some species are opportunistic, affecting immunocompromised patients such as those undergoing oncologic treatment.

OBJECTIVE

To detect the presence of yeasts in patients suffering from head and neck cancer who are receiving radiation therapy and display lesions in the oral cavity, compatible with candidiasis; and to evaluate the antifungal susceptibility of the isolates recovered.

METHODS

Sixty samples from patients were obtained by swabbing the oral mucosa. Identification of isolates were performed by classical taxonomic, morphological and biochemical methods as well as by using commercial identification kits. Susceptibility to antifungal drugs was determined by the agar diffusion method with Neosensitabs(®) disks.

RESULTS

Forty-six samples (77%) yielded positive findings, and species recovered were: Candida albicans (22 isolates), Candida tropicalis (13 isolates), Candida parapsilosis (six strains), Candida krusei (three strains), Candida dubliniensis and Saccharomyces cerevisiae (one each). All strains were susceptible to itraconazole, clotrimazole, voriconazole, nystatin and amphotericin B. On the other hand, 65% of strains were miconazole-susceptible while 35%, showed intermediate susceptibility. With regard to ketoconazole, only three strains (7%) corresponding to C. albicans (one isolate) and C. krusei (two isolates) displayed intermediate susceptibility. Only C. krusei strains were resistant to fluconazole while all the other species were susceptible. Eventually, only six isolates (13%) were susceptible to terbinafine while the remaining strains were resistant in vitro.

CONCLUSION

Early detection of etiological agents causing lesions, as well as the evaluation of their susceptibility to commonly used drugs, are crucial in order to choose the appropriate treatment that will minimize complications while improving the quality of patients' lives.

Voriconazole in the management of nosocomial invasive fungal infections

Voriconazole is a new triazole developed for the treatment of life-threatening fungal infections. The drug is available for both oral and intravenous administration; the oral formulation has excellent bioavailability. The side-effect profile of voriconazole presents an acceptable safety and tolerability spectrum: transient visual disturbances, liver enzyme abnormalities, and skin rashes are the most frequently reported side effects but rarely lead to discontinuation. The potential for drug–drug interactions is high, because of its extensive hepatic metabolism. Careful attention to dosage is required, and serum levels and the effects of interacting drugs should be monitored. Review of 25 470 isolates of yeasts and 3216 isolates of filamentous fungi showed voriconazole to have broad-spectrum activity against pathogenic yeasts including intrinsically fluconazole-resistant isolates such as Candida krusei, dimorphic fungi, and opportunistic moulds like Aspergillus spp, amphotericin-B-resistant Aspergillus terreus, Fusarium spp, and Scedosporium apiospermum. It displays excellent clinical efficacy in patients with fluconazole-resistant and -susceptible Candida infections, invasive bone and central nervous system aspergillosis, and various refractory fungal infections. Voriconazole has been approved by the US Food and Drug Administration and by the European Medicines Agency for the treatment of invasive aspergillosis, serious infections caused by Fusarium and S. apiospermum, fluconazole-resistant invasive Candida infections, and candidemia in nonneutropenic patients.

Multilaboratory testing of two-drug combinations of antifungals against Candida albicans, Candida glabrata, and Candida parapsilosis

There are few multilaboratory studies of antifungal combination testing to suggest a format for use in clinical laboratories. In the present study, eight laboratories tested quality control (QC) strain Candida parapsilosis ATCC 22019 and clinical isolates Candida albicans 20533.043, C. albicans 20464.007, Candida glabrata 20205.075, and C. parapsilosis 20580.070. The clinical isolates had relatively high azole and echinocandin MICs. A modified CLSI M27-A3 protocol was used, with 96-well custom-made plates containing checkerboard pairwise combinations of amphotericin B (AMB), anidulafungin (AND), caspofungin (CSP), micafungin (MCF), posaconazole (PSC), and voriconazole (VRC). The endpoints were scored visually and on a spectrophotometer or enzyme-linked immunosorbent assay (ELISA) reader for 50% growth reduction (50% inhibitory concentration [IC(50)]). Combination IC(50)s were used to calculate summation fractional inhibitory concentration indices (FICIs) (ΣFIC) based on the Lowe additivity formula. The results revealed that the IC(50)s of all drug combinations were lower or equal to the IC(50) of individual drugs in the combination. A majority of the ΣFIC values were indifferent (ΣFIC = 0.51 to 2.0), but no antagonism was observed (ΣFIC ≥ 4). Synergistic combinations (ΣFIC ≤ 0.5) were found for AMB-PSC against C. glabrata and for AMB-AND and AMB-CSP against C. parapsilosis by both visual and spectrophotometric readings. Additional synergistic interactions were revealed by either of the two endpoints for AMB-AND, AMB-CSP, AMB-MCF, AMB-PSC, AMB-VRC, AND-PSC, CSP-MCF, and CSP-PSC. The percent agreements among participating laboratories ranged from 37.5% (lowest) for AND-CSP and POS-VOR to 87.5% (highest) for AMB-MCF and AND-CSP. Median ΣFIC values showed a wide dispersion, and interlaboratory agreements were less than 85% in most instances. Additional studies are needed to improve the interlaboratory reproducibility of antifungal combination testing.

In vitro synergy of eugenol and methyleugenol with fluconazole against clinical Candida isolates

The species Candida is a group of opportunistic pathogenic commensals in immune-compromised patients. Treatment of Candida infections is becoming increasingly difficult due to antifungal drug resistance, especially with fluconazole (FLC), which is a commonly used azole. In the present study the in vitro antifungal activity of eugenol (EUG) and methyleugenol (MEUG) alone and in combination against 64 FLC-sensitive and 34 FLC-resistant clinical Candida isolates is highlighted. All the strains were susceptible to both the naturally occurring phenyl propanoids. The nature of the interaction was studied from fractional inhibitory concentration indices (FICIs) for both EUG plus FLC, and MEUG plus FLC combinations calculated from chequerboard microdilution assays. FICI values depicted a high synergism of FLC with both compounds, which was greatest with MEUG. FLC-resistant Candida isolates showed high sensitivity to both compounds. No antagonistic activity was seen in the strains tested in the present study. From these results we suggest that EUG and MEUG have great potential as antifungals, and that FLC can be supplemented with EUG and MEUG to treat FLC-resistant Candida infections.

Anticandidal effect of Ocimum sanctum essential oil and its synergy with fluconazole and ketoconazole

Holy basil, Ocimum sanctum (L.) is time-honored for its medicinal properties; however its antimicrobial characteristics are used only in 'Ayurvedic medicines'. Attention has been drawn to antifungal activity and a possible synergistic antifungal effect of Ocimum sanctum essential oil (OSEO) and established azole antimycotics-fluconazole and ketoconazole. To put forward this approach, antifungal activity has been assessed in seventy four fluconazole-sensitive and sixteen fluconazole-resistant Candida isolates. Hemolytic activity on human erythrocytes was also studied to rule out the possibility of allied additional cytotoxicity. The observed selectively fungicidal characteristics signify a promising candidature of O. sanctum essential oil as an antifungal agent in combinational treatments for candidosis.

Use of Antifungal Combination Therapy: Agents, Order, and Timing

Given the substantial morbidity and mortality related to invasive fungal infections, treatment with a combination of antifungal agents is often considered. A growing body of literature from in vitro studies, animal models, and clinical experience provides data evaluating this approach. This review describes combination antifungal strategies for the management of cryptococcal meningitis, invasive candidiasis, invasive aspergillosis, and rare mold infections. The potential effects that sequencing and timing have on the efficacy of such approaches are discussed, with a focus on recent clinical data in this arena.

In Candida albicans, resistance to flucytosine and terbinafine is linked to MAT locus homozygosity and multilocus sequence typing clade 1

A panel of 637 isolates of Candida albicans that had been typed by multilocus sequence typing (MLST) and tested for susceptibility to amphotericin B, caspofungin, fluconazole, flucytosine, itraconazole, ketoconazole, miconazole, terbinafine and voriconazole was the material for a statistical analysis of possible associations between antifungal susceptibility and other properties. For terbinafine and flucytosine, the greatest proportion of low-susceptibility isolates, judged by two resistance breakpoints, was found in MLST clade 1 and among isolates homozygous at the MAT locus, although only three isolates showed cross-resistance to the two agents. Most instances of low susceptibility to azoles, flucytosine and terbinafine were among oropharyngeal isolates from HIV-positive individuals. Statistically significant correlations were found between terbinafine and azole minimal inhibitory concentrations (MICs), while correlations between flucytosine MICs and azole MICs were less strong. It is concluded that a common regulatory mechanism may operate to generate resistance to the two classes of agent that inhibit ergosterol biosynthesis, terbinafine and the azoles, but that flucytosine resistance, although still commonly associated with MAT homozygosity, is differently regulated.

Current status of antifungal susceptibility testing methods

Antifungal susceptibility testing is a very dynamic field of medical mycology. Standardization of in vitro susceptibility tests by the Clinical and Laboratory Standards Institute (CLSI) and the European Committee for Antimicrobial Susceptibility Testing (EUCAST), and current availability of reference methods constituted the major remarkable steps in the field. Based on the established minimum inhibitory concentration (MIC) breakpoints, it is now possible to determine the susceptibilities of Candida strains to fluconazole, itraconazole, voriconazole, and flucytosine. Moreover, utility of fluconazole antifungal susceptibility tests as an adjunct in optimizing treatment of candidiasis has now been validated. While the MIC breakpoints and clinical significance of susceptibility testing for the remaining fungi and antifungal drugs remain yet unclear, modifications of the available methods as well as other methodologies are being intensively studied to overcome the present drawbacks and limitations. Among the other methods under investigation are Etest, colorimetric microdilution, agar dilution, determination of fungicidal activity, flow cytometry, and ergosterol quantitation. Etest offers the advantage of practical application and favorable agreement rates with the reference methods that are frequently above acceptable limits. However, MIC breakpoints for Etest remain to be evaluated and established. Development of commercially available, standardized colorimetric panels that are based on CLSI method parameters has added more to the antifungal susceptibility testing armamentarium. Flow cytometry, on the other hand, appears to offer rapid susceptibility testing but requires specified equipment and further evaluation for reproducibility and standardization. Ergosterol quantitation is another novel approach, which appears potentially beneficial particularly in discrimination of azole-resistant isolates from heavy trailers. The method is yet investigational and requires to be further studied. Developments in methodology and applications of antifungal susceptibility testing will hopefully provide enhanced utility in clinical guidance of antifungal therapy. However, and particularly in immunosuppressed host, in vitro susceptibility is and will remain only one of several factors that influence clinical outcome.

Multilaboratory testing of antifungal combinations against a quality control isolate of Candida krusei

Candida krusei ATCC 6258 was tested by eight laboratories using 96-well plates containing checkerboard pairwise combinations of amphotericin B (AMB), posaconazole (PSC), caspofungin (CSP), and voriconazole (VRC). The methodology led to reproducible results across the laboratories. All drug combinations yielded MICs lower than the MICs of any two drugs tested singly, and combinations of AMB, PSC, CSP, and VRC were indifferent (no antagonism) by summations of fractional inhibitory concentration.

[In vitro antifungal activity of voriconazole: New data after the first years of clinical experience]

Voriconazole has been developed to meet the increasing need for new and useful antifungal agents for the treatment of invasive mycoses. This review describes the spectrum of voriconazole antifungal activity based on data from in vitro studies published during the last three years. This survey demonstrates that voriconazole has a broad antifungal spectrum against the most common fungal pathogens being its action fungistatic for Candida and fungicidal for Aspergillus and other filamentous fungi. Overall, more than 95% of all Candida isolates tested are susceptible to voriconazole and less than 3% are resistant. Similar or even better activity rates have been described for Aspergillus, Cryptococcus and most of yeasts and moulds of medical importance. We also discuss the limitations related to the azole cross-resistance observed in some Candida glabrata isolates, the poor activity of voriconazole against Scedosporium prolificans, its activity against fungal biofilms and the great potential usefulness of combination of voriconazole with other antifungal drugs.

Measurement of voriconazole activity against Candida albicans, C. glabrata, and C. parapsilosis isolates using time-kill methods validated by high-performance liquid chromatography

We developed a high-performance liquid chromatography (HPLC) assay to validate time-kill and postantifungal-effect (PAFE) experiments for voriconazole against Candida albicans, Candida glabrata, and Candida parapsilosis isolates. Voriconazole exerted prolonged fungistatic activity but no PAFE at concentrations achievable in human sera. HPLC confirmed that experiments were conducted at the desired steady-state voriconazole concentrations.

Voriconazole : a review of its use in the management of invasive fungal infections

Voriconazole (VFEND), a synthetic second-generation, broad-spectrum triazole derivative of fluconazole, inhibits the cytochrome P450 (CYP)-dependent enzyme 14-alpha-sterol demethylase, thereby disrupting the cell membrane and halting fungal growth. In the US, intravenous and/or oral voriconazole is recommended in adults for the treatment of invasive aspergillosis, candidaemia in non-neutropenic patients, disseminated infections caused by Candida spp., oesophageal candidiasis, and in patients with scedosporiosis and fusariosis who are refractory to or intolerant of other antifungal therapy. In Europe, intravenous and/or oral voriconazole is recommended in adults and paediatric patients of at least 2 years of age for the treatment of invasive aspergillosis, candidaemia in non-neutropenic patients, fluconazole-resistant serious invasive Candida spp. infections, scedosporiosis and fusariosis. In large randomised trials, voriconazole was an effective and generally well tolerated primary treatment for candidiasis and invasive aspergillosis in adults and adolescents. More limited data also support the use of voriconazole for the treatment of invasive fungal infections in children, in those with rare fungal infections, such as Fusarium spp. or Scedosporium spp., and in those refractory to or intolerant of other standard antifungal therapies. The availability of both parenteral and oral formulations and the almost complete absorption of the drug after oral administration provide for ease of use and potential cost savings, and ensure that therapeutic plasma concentrations are maintained when switching from intravenous to oral therapy. On the other hand, the numerous drug interactions associated with voriconazole may limit its usefulness in some patients. Further clinical experience will help to more fully determine the position of voriconazole in relation to other licensed antifungal agents. In the meantime, voriconazole is a valuable emerging option for the treatment of invasive aspergillosis and rare fungal infections, including Fusarium spp. and Scedosporium spp. infections, and provides an alternative option for the treatment of candidiasis, particularly where the causative organism is inherently resistant to other licensed antifungal agents.

Combination antifungal therapy: the new frontier

The past decade has seen a significant increase in the incidence of invasive fungal infections. The antifungal armamentarium for the treatment of serious fungal infections remains limited. A possible approach to overcoming antifungal drug resistance and high mortality rates seen in severe fungal infections is to combine two or three classes of antifungals, especially if the drugs have different mechanisms of action. Combinations of new agents along with more traditional antifungals have now been shown to possess some synergistic or at least additive activity against many fungi in in vitro and animal studies. On the other hand, caution is still needed since some antifungal combinations have also demonstrated antagonistic activity. Well-controlled clinical trials are still required to define the most efficacious antifungal regimen. Furthermore, these trials should also evaluate the side-effect potential of combination regimens and the pharmacoeconomic impact these regimens may have.

Spondylodiscitis caused by Candida krusei: case report and susceptibility patterns

A 62-year-old man with amphotericin B-resistant Candida krusei spondylodiscitis, following an episode of candidemia caused by the same strain, was successfully treated with caspofungin plus voriconazole. Amphotericin B fungicidal concentrations were better predictors of the clinical outcome than were MICs. This is the first case of C. krusei spondylodiscitis reported in the literature.