Effect of pH on the in vitro activities of amphotericin B, itraconazole, and flucytosine against Aspergillus isolates

Antifungal Encapsulated into Ligand-Functionalized Nanoparticles with High Specificity for Macrophages

Infectious diseases caused by intracellular microorganisms such as Histoplasma capsulatum represent a significant challenge worldwide. Drug encapsulation into functionalized nanoparticles (NPs) is a valuable alternative to improving drug solubility and bioavailability, preventing undesirable interactions and drug degradation, and reaching the specific therapeutic target with lower doses. This work reports on Itraconazole (ITZ) encapsulated into core-shell-like polymeric NPs and functionalized with anti-F4/80 antibodies for their targeted and controlled release into macrophages. Uptake assay on co-culture showed significant differences between the uptake of functionalized and bare NPs, higher with functionalized NPs. In vitro assays showed that F4/80-NPs with 0.007 µg/mL of encapsulated ITZ eliminated the H. capsulatum fungus in co-culture with macrophages effectively compared to the bare NPs, without any cytotoxic effect on macrophages after 24 h interaction. Furthermore, encapsulated ITZ modulated the gene expression of anti and pro-inflammatory cytokines (IL-1, INF-Y, IL-6 and IL-10) on macrophages. Additionally, the anti-F4/80 antibody-coating enhanced natural and adequate antifungal response in the cells, exerting a synergistic effect that prevented the growth of the fungus at the intracellular level. Functionalized NPs can potentially improve macrophage-targeted therapy, increasing NPs endocytosis and intracellular drug concentration.

Antifungal Susceptibility Testing: A Primer for Clinicians

Clinicians treating patients with fungal infections may turn to susceptibility testing to obtain information regarding the activity of different antifungals against a specific fungus that has been cultured. These results may then be used to make decisions regarding a patient's therapy. However, for many fungal species that are capable of causing invasive infections, clinical breakpoints have not been established. Thus, interpretations of susceptible or resistant cannot be provided by clinical laboratories, and this is especially true for many molds capable of causing severe mycoses. The purpose of this review is to provide an overview of susceptibility testing for clinicians, including the methods used to perform these assays, their limitations, how clinical breakpoints are established, and how the results may be put into context in the absence of interpretive criteria. Examples of when susceptibility testing is not warranted are also provided.

Molecular Mechanisms of 5-Fluorocytosine Resistance in Yeasts and Filamentous Fungi

Effective management and treatment of fungal diseases is hampered by poor diagnosis, limited options for antifungal therapy, and the emergence of antifungal drug resistance. An understanding of molecular mechanisms contributing to resistance is essential to optimize the efficacy of currently available antifungals. In this perspective, one of the oldest antifungals, 5-fluorocytosine (5-FC), has been the focus of recent studies applying advanced genomic and transcriptomic techniques to decipher the order of events at the molecular level that lead to resistance. These studies have highlighted the complexity of resistance and provided new insights that are reviewed in the present paper.

A buffered media system for yeast batch culture growth

Background

Fungi are premier hosts for the high-yield secretion of proteins for biomedical and industrial applications. The stability and activity of these secreted proteins is often dependent on the culture pH. As yeast acidifies the commonly used synthetic complete drop-out (SD) media that contains ammonium sulfate, the pH of the media needs to be buffered in order to maintain a desired extracellular pH during biomass production. At the same time, many buffering agents affect growth at the concentrations needed to support a stable pH. Although the standard for biotechnological research and development is shaken batch cultures or microtiter plate cultures that cannot be easily automatically pH-adjusted during growth, there is no comparative study that evaluates the buffering capacity and growth effects of different media types across pH-values in order to develop a pH-stable batch culture system.

Results

We systematically test the buffering capacity and growth effects of a citrate-phosphate buffer (CPB) from acidic to neutral pH across different media types. These media types differ in their nitrogen source (ammonium sulfate, urea or both). We find that the widely used synthetic drop-out media that uses ammonium sulfate as nitrogen source can only be effectively buffered at buffer concentrations that also affect growth. At lower concentrations, yeast biomass production still acidifies the media. When replacing the ammonium sulfate with urea, the media alkalizes. We then develop a medium combining ammonium sulfate and urea which can be buffered at low CPB concentrations that do not affect growth. In addition, we show that a buffer based on Tris/HCl is not effective in maintaining any of our media types at neutral pH even at relatively high concentrations.

Conclusion

Here we show that the buffering of yeast batch cultures is not straight-forward and addition of a buffering agent to set a desired starting pH does not guarantee pH-maintenance during growth. In response, we present a buffered media system based on an ammonium sulfate/urea medium that enables relatively stable pH-maintenance across a wide pH-range without affecting growth. This buffering system is useful for protein-secretion-screenings, antifungal activity assays, as well as for other pH-dependent basic biology or biotechnology projects.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-021-02191-5.

Species Distribution and Comparison between EUCAST and Gradient Concentration Strips Methods for Antifungal Susceptibility Testing of 112 Aspergillus Section Nigri Isolates

Aspergillus niger, the third species responsible for invasive aspergillosis, has been considered as a homogeneous species until DNA-based identification uncovered many cryptic species. These species have been recently reclassified into the Aspergillus section Nigri However, little is yet known among the section Nigri about the species distribution and the antifungal susceptibility pattern of each cryptic species. A total of 112 clinical isolates collected from 5 teaching hospitals in France and phenotypically identified as A. niger were analyzed. Identification to the species level was carried out by nucleotide sequence analysis. The MICs of itraconazole, voriconazole, posaconazole, isavuconazole, and amphotericin B were determined by both the EUCAST and gradient concentration strip methods. Aspergillus tubingensis (n = 51, 45.5%) and Aspergillus welwitschiae (n = 50, 44.6%) were the most common species while A. niger accounted for only 6.3% (n = 7). The MICs of azole drugs were higher for A. tubingensis than for A. welwitschiae The MIC of amphotericin B was 2 mg/liter or less for all isolates. Importantly, MICs determined by EUCAST showed no correlation with those determined by the gradient concentration strip method, with the latter being lower than the former (Spearman's rank correlation tests ranging from 0.01 to 0.25 depending on the antifungal agent; P > 0.4). In conclusion, A. niger should be considered as a minority species in the section Nigri The differences in MICs between species for different azoles underline the importance of accurate identification. Significant divergences in the determination of MIC between EUCAST and the gradient concentration strip methods require further investigation.

Sterol-Response Pathways Mediate Alkaline Survival in Diverse Fungi

The ability for cells to maintain homeostasis in the presence of extracellular stress is essential for their survival. Stress adaptations are especially important for microbial pathogens to respond to rapidly changing conditions, such as those encountered during the transition from the environment to the infected host. Many fungal pathogens have acquired the ability to quickly adapt to changes in extracellular pH to promote their survival in the various microenvironments encountered during a host infection. For example, the fungus-specific Rim/Pal alkaline response pathway has been well characterized in many fungal pathogens, including Cryptococcus neoformans However, alternative mechanisms for sensing and responding to host pH have yet to be extensively studied. Recent observations from a genetic screen suggest that the C. neoformans sterol homeostasis pathway is required for growth at elevated pH. This work explores interactions among mechanisms of membrane homeostasis, alkaline pH tolerance, and Rim pathway activation. We find that the sterol homeostasis pathway is necessary for growth in an alkaline environment and that an elevated pH is sufficient to induce Sre1 activation. This pH-mediated activation of the Sre1 transcription factor is linked to the biosynthesis of ergosterol but is not dependent on Rim pathway signaling, suggesting that these two pathways are responding to alkaline pH independently. Furthermore, we discover that C. neoformans is more susceptible to membrane-targeting antifungals under alkaline conditions, highlighting the impact of microenvironmental pH on the treatment of invasive fungal infections. Together, these findings further connect membrane integrity and composition with the fungal pH response and pathogenesis.IMPORTANCE The work described here further elucidates how microorganisms sense and adapt to changes in their environment to establish infections in the human host. Specifically, we uncover a novel mechanism by which an opportunistic human fungal pathogen, Cryptococcus neoformans, responds to increases in extracellular pH in order to survive and thrive within the relatively alkaline environment of the human lung. This mechanism, which is intimately linked with fungal membrane sterol homeostasis, is independent of the previously well-studied alkaline response Rim pathway. Furthermore, this ergosterol-dependent alkaline pH response is present in Candida albicans, indicating that this mechanism spans diverse fungal species. These results are also relevant for novel antimicrobial drug development as we show that currently used ergosterol-targeting antifungals are more active in alkaline environments.

Multiplex Genetic Engineering Exploiting Pyrimidine Salvage Pathway-Based Endogenous Counterselectable Markers

Selectable markers are indispensable for genetic engineering, yet their number and variety are limited. Most selection procedures for prototrophic cells rely on the introduction of antibiotic resistance genes. New minimally invasive tools are needed to facilitate sophisticated genetic manipulations. Here, we characterized three endogenous genes in the human fungal pathogen Aspergillus fumigatus for their potential as markers for targeted genomic insertions of DNAs of interest (DOIs). Since these genes are involved in uptake and metabolization of pyrimidines, resistance to the toxic effects of prodrugs 5-fluorocytosine and 5-fluorouracil can be used to select successfully integrated DOIs. We show that DOI integration, resulting in the inactivation of these genes, caused no adverse effects with respect to nutrient requirements, stress resistance, or virulence. Beside the individual use of markers for site-directed integration of reporter cassettes, including the 17-kb penicillin biosynthetic cluster, we demonstrate their sequential use by inserting three genes encoding fluorescent proteins into a single strain for simultaneous multicolor localization microscopy. In addition to A. fumigatus, we validated the applicability of this novel toolbox in Penicillium chrysogenum and Fusarium oxysporum Enabling multiple targeted insertions of DOIs without the necessity for exogenous markers, this technology has the potential to significantly advance genetic engineering.IMPORTANCE This work reports the discovery of a novel genetic toolbox comprising multiple, endogenous selectable markers for targeted genomic insertions of DNAs of interest (DOIs). Marker genes encode proteins involved in 5-fluorocytosine uptake and pyrimidine salvage activities mediating 5-fluorocytosine deamination as well as 5-fluorouracil phosphoribosylation. The requirement for their genomic replacement by DOIs to confer 5-fluorocytosine or 5-fluorouracil resistance for transformation selection enforces site-specific integrations. Due to the fact that the described markers are endogenously encoded, there is no necessity for the exogenous introduction of commonly employed markers such as auxotrophy-complementing genes or antibiotic resistance cassettes. Importantly, inactivation of the described marker genes had no adverse effects on nutrient requirements, growth, or virulence of the human pathogen Aspergillus fumigatus Given the limited number and distinct types of selectable markers available for the genetic manipulation of prototrophic strains such as wild-type strains, we anticipate that the proposed methodology will significantly advance genetic as well as metabolic engineering of fungal species.

Mechanistic Basis of pH-Dependent 5-Flucytosine Resistance in Aspergillus fumigatus

The antifungal drug 5-flucytosine (5FC), a derivative of the nucleobase cytosine, is licensed for the treatment of fungal diseases; however, it is rarely used as a monotherapeutic to treat Aspergillus infection. Despite being potent against other fungal pathogens, 5FC has limited activity against Aspergillus fumigatus when standard in vitro assays are used to determine susceptibility. However, in modified in vitro assays where the pH is set to pH 5, the activity of 5FC increases significantly. Here we provide evidence that fcyB, a gene that encodes a purine-cytosine permease orthologous to known 5FC importers, is downregulated at pH 7 and is the primary factor responsible for the low efficacy of 5FC at pH 7. We also uncover two transcriptional regulators that are responsible for the repression of fcyB and, consequently, mediators of 5FC resistance, the CCAAT binding complex (CBC) and the pH regulatory protein PacC. We propose that the activity of 5FC might be enhanced by the perturbation of factors that repress fcyB expression, such as PacC or other components of the pH-sensing machinery.

Genetic Diversity and In Vitro Antifungal Susceptibility of 200 Clinical and Environmental Aspergillus flavus Isolates

Aspergillus flavus has been frequently reported as the leading cause of invasive aspergillosis in certain tropical and subtropical countries. Two hundred A. flavus strains originating from clinical and environmental sources and collected between 2008 and 2015 were phylogenetically identified at the species level by analyzing partial β-tubulin and calmodulin genes. In vitro antifungal susceptibility testing was performed against antifungals using the European Committee on Antimicrobial Susceptibility Testing (EUCAST) broth microdilution method. In addition, genotyping was performed using a short-tandem-repeat (STR) assay of a panel of six microsatellite markers (A. flavus 2A, 2B, 2C, 3A, 3B, and 3C), in order to determine the genetic variation and the potential relationship between clinical and environmental isolates. The geometric means of the minimum inhibitory concentrations/minimum effective concentrations (MICs/MECs) of the antifungals across all isolates were (in increasing order): posaconazole, 0.13 mg/liter; anidulafungin, 0.16 mg/liter; itraconazole, 0.29 mg/liter; caspofungin, 0.42 mg/liter; voriconazole, 0.64 mg/liter; isavuconazole, 1.10 mg/liter; amphotericin B, 3.35 mg/liter; and flucytosine, 62.97 mg/liter. All of the clinical isolates were genetically different. However, an identical microsatellite genotype was found between a clinical isolate and two environmental strains. In conclusion, posaconazole and anidulafungin showed the greatest in vitro activity among systemic azoles and echinocandins, respectively. However, the majority of the A. flavus isolates showed reduced susceptibility to amphotericin B. Antifungal susceptibility of A. flavus was not linked with the clinical or environmental source of isolation. Microsatellite genotyping may suggest an association between clinical and environmental strains, although this requires further investigation.

Amphotericin B-copper(II) complex as a potential agent with higher antifungal activity against Candida albicans

Amphotericin B (AmB) is a polyene antibiotic produced by Streptomyces nodosus used for more than 50 years in the treatment of acute systemic fungal infections. It exhibits a broad spectrum of activity against fungal and protozoan pathogens with relatively rare resistance. The aim of this study was to prepare and evaluate the utility of the AmB-Cu(2+) complex as a potential compound with a high fungicidal activity at lower concentrations, compared with conventional AmB. It was hypothesized that insertion of copper ions into fungal cell membranes, together with the AmB-Cu(2+) complex bypassing the natural homeostatic mechanisms of this element, may contribute to the increased fungicidal activity of AmB. The analysis of results indicates the increased antifungal activity of the AmB-Cu(2+) complex against Candida albicans in comparison with the pure AmB and Fungizone. Additionally, it was stated that the increased antifungal activity of the AmB-Cu(2+) complex is not the sum of the toxic effects of AmB and Cu(2+) ions, but is a result of the unique structure of this compound.

Pharmacodynamics and dose-response relationships of liposomal amphotericin B against different azole-resistant Aspergillus fumigatus isolates in a murine model of disseminated aspergillosis

The management of invasive aspergillosis (IA) has become more complicated due to the emergence of acquired azole resistance in Aspergillus fumigatus, which is associated with treatment failure and a mortality rate of 88%. Treatment with liposomal amphotericin B (L-AmB) may be a useful alternative to improve therapeutic outcome in azole-resistant IA. Four clinical A. fumigatus isolates obtained from patients with proven IA were studied in a nonneutropenic murine model of infection: a wild-type isolate without mutations in the cyp51A gene and three azole-resistant isolates harboring a single mutation at codon 220 (M220I) and tandem repeat mutations (a 34-bp tandem repeat mutation in the promoter region of the cyp51A gene in combination with substitutions at codon L98 [TR(34)/L98H] and a 46-bp tandem repeat mutation in the promoter region of the cyp51A gene in combination with mutation at codons Y121 and T289 [TR(46)/Y121F/T289A]), respectively. Female CD-1 mice were infected intravenously 24 h prior to the start of therapy. Groups of 11 mice were treated at days 1, 2, and 5 postchallenge with increasing 4-fold doses of L-AmB ranging from 0.004 to 16 mg/kg/day and observed for 14 days. Survival for all 4 isolates at day 14 was significantly better than that of controls. A dose-response relationship was observed independent of the azole resistance mechanism. The Hill-type model with a variable slope fitted the relationship between the dose and 14-day survival well for all isolates, with R(2) values of 0.95 (wild-type), 0.97 (M220I), 0.85 (TR(34)/L98H), and 0.94 (TR(46)/Y121F/T289A), respectively. Multiple logistic regression analysis confirmed that there was no significant difference between groups. The results of these experiments indicate that L-AmB was able to prolong survival in vivo in disseminated IA independent of the presence of an azole resistance mechanism in a dose-dependent manner, and therefore, they support a role for L-AmB in the treatment of azole-resistant IA.

Cryptic species and azole resistance in the Aspergillus niger complex

Aspergillus niger is a common clinical isolate. Multiple species comprise the Aspergillus section Nigri and are separable using sequence data. The antifungal susceptibility of these cryptic species is not known. We determined the azole MICs of 50 black aspergilli, 45 from clinical specimens, using modified EUCAST (mEUCAST) and Etest methods. Phylogenetic trees were prepared using the internal transcribed spacer, beta-tubulin, and calmodulin sequences to identify strains to species level and the results were compared with those obtained with cyp51A sequences. We attempted to correlate cyp51A mutations with azole resistance. Etest MICs were significantly different from mEUCAST MICs (P < 0.001), with geometric means of 0.77 and 2.79 mg/liter, respectively. Twenty-six of 50 (52%) isolates were itraconazole resistant by mEUCAST (MICs > 8 mg/liter), with limited cross-resistance to other azoles. Using combined beta-tubulin/calmodulin sequences, the 45 clinical isolates grouped into 5 clades, A. awamori (55.6%), A. tubingensis (17.8%), A. niger (13.3%), A. acidus (6.7%), and an unknown group (6.7%), none of which were morphologically distinguishable. Itraconazole resistance was found in 36% of the isolates in the A. awamori group, 90% of the A. tubingensis group, 33% of the A. niger group, 100% of the A. acidus group, and 67% of the unknown group. These data suggest that cyp51A mutations in section Nigri may not play as important a role in azole resistance as in A. fumigatus, although some mutations (G427S, K97T) warrant further study. Numerous cryptic species are found in clinical isolates of the Aspergillus section Nigri and are best reported as "A. niger complex" by clinical laboratories. Itraconazole resistance was common in this data set, but azole cross-resistance was unusual. The mechanism of resistance remains obscure.

Evaluating the resistance to posaconazole by E-test and CLSI broth microdilution methodologies of Candida spp. and pathogenic moulds

E-test methodology was compared with Clinical and Laboratory Standards Institute (CLSI) broth microdilution, particularly concerning the detection of resistance to posaconazole among clinical fungal isolates. The susceptibility of a large set of fungal strains (n = 300) was evaluated following 24 and 48 h in two different culture media (RPMI 1640 and Sabouraud agar). Fungal strains were highly susceptible to posaconazole; however, few less susceptible strains were found, mostly regarding Candida albicans, Candida glabrata, Acremonium sp., Cladosporium sp. and Scedosporium apiospermum. Broth microdilution and E-test methods provided similar results for posaconazole-susceptible strains, while the less susceptible fungal strains (10.3% of the strains showed MIC > or =2 microg/mL) resulted in higher discrepancies between the two methodologies, particularly concerning Candida spp. E-test susceptibility values were critically affected by the pH of the culture media. Sabouraud medium provided similar susceptibility results for moulds to those for RPMI, soon after 24 h. Posaconazole resistance was rare in this study, but routine susceptibility methods, such as the E-test, should be able to detect fungal strains with reduced susceptibility. E-test methodology still needs improvements to recognise accurately strains less susceptible to posaconazole.

In vitro activities at pH 5.0 and pH 7.0 and in vivo efficacy of flucytosine against Aspergillus fumigatus

The antifungal agent flucytosine was found to be active in vitro against Aspergillus fumigatus isolates when the MIC was determined at pH 5.0 instead of pH 7.0. The in vitro MIC at pH 5.0 corresponded to the in vivo efficacy of flucytosine monotherapy in a murine model of invasive aspergillosis.

Combination antifungals: an update

Invasive fungal infections are an important cause of morbidity and mortality in specific patient populations. There has been an impressive increase in the antifungal armamentarium, yet optimal therapies for many invasive fungal infections remain unknown. Genomic sequencing of a number of pathogenic fungi will pave the way to discovering additional newer targets for antifungal drug design. These new discoveries, plus the existing repertoire of antifungal agents, create the need to effectively model single and combination antifungal agents. Future therapies may also include the use of cell-stress pathway inhibitors in combination with existing antifungal agents. This review focuses on combination antifungal therapy against Cryptococcus neoformans, Candida and Aspergillus species. Combination therapy is only supported by randomized clinical trials for cryptococcal meningitis. We review data from in vitro and animal model studies as well as insights from clinical trials to discuss current thoughts and highlight the gaps in our knowledge surrounding combination antifungal therapy.

Modeling the combination of amphotericin B, micafungin, and nikkomycin Z against Aspergillus fumigatus in vitro using a novel response surface paradigm

Response surface methods for the study of multiple-agent interaction allow one to model all of the information present in full concentration-effect data sets and to visualize and quantify local regions of synergy, additivity, and antagonism. In randomized wells of 96-well plates, Aspergillus fumigatus was exposed to various combinations of amphotericin B, micafungin, and nikkomycin Z. The experimental design was comprised of 91 different fixed-ratio mixtures, all performed in quintuplicate. After 24 h of drug exposure, drug effect on fungal viability was assessed using the tetrazolium salt 2,3-bis {2-methoxy-4-nitro-5-[(sulfenylamino) carbonyl]-2H-tetrazolium-hydroxide} (XTT) assay. First, we modeled each fixed-ratio combination alone using the four-parameter Hill concentration-effect model. Then, we modeled each parameter, including the 50% inhibitory concentration (IC(50)) effect, versus the proportion of each agent using constrained polynomials. Finally, we modeled the three-agent response surface overall. The overall four-dimensional response surface was complex, but it can be explained in detail both analytically and graphically. The grand model that fit the best included complex polynomial equations for the slope parameter m and the combination index (equivalent to the IC(50) for a fixed-ratio concentration, but with concentrations normalized by the respective IC(50)s of the drugs alone). There was a large region of synergy, mostly at the nikkomycin Z/micafungin edge of the ternary plots for equal normalized proportions of each drug and extending into the center of the plots. Applying this response surface method to a huge data set for a three-antifungal-agent combination is novel. This new paradigm has the potential to significantly advance the field of combination antifungal pharmacology.

Determination of antifungal activities in serum samples from mice treated with different antifungal drugs allows detection of an active metabolite of itraconazole

To establish an in vitro method of predicting in vivo efficacy of antifungal drugs against Candida albicans and Aspergillus fumigatus, the antifungal activities of fluconazole, itraconazole, and amphotericin B were determined in mouse serum. The minimum inhibitory concentration (MIC) of each drug was measured using mouse serum as a diluent. For C. albicans, the assay endpoint of azoles was defined as inhibition of mycelial extension (mMIC) and for A. fumigatus, as no growth (MIC). The MICs of amphotericin B for both pathogens were defined as the MIC at which no mycelial growth occurred. Serum MIC or mMIC determinations were then used to estimate the concentration of the drugs in serum of mice treated with antifungal drugs by multiplying the antifungal titer of the serum samples by the serum (m)MIC. The serum drug concentrations were also determined by HPLC. The serum concentrations estimated microbiologically showed good agreement with those determined by HPLC, except for itraconazole. Analysis of the serum samples from itraconazole-treated mice by a sensitive bioautography revealed the presence of additional spots, not seen in control samples of itraconazole. The bioautography assay demonstrated that the additional material detected in serum from mice treated with itraconazole was an active metabolite of itraconazole. The data showed that the apparent reduction in the itraconazole serum concentration as determined by HPLC was the result of the formation of an active metabolite, and that the use of a microbiological method to measure serum concentrations of drugs can provide a method for prediction of in vivo efficacy of antifungal drugs.

Effect of amphotericin B and micafungin combination on survival, histopathology, and fungal burden in experimental aspergillosis in the p47phox-/- mouse model of chronic granulomatous disease

Chronic granulomatous disease (CGD) is an inherited disorder of the NADPH oxidase characterized by recurrent life-threatening bacterial and fungal infections. We characterized the effects of single and combination antifungal therapy on survival, histopathology, and laboratory markers of fungal burden in experimental aspergillosis in the p47phox-/- knockout mouse model of CGD. CGD mice were highly susceptible to intratracheal Aspergillus fumigatus challenge, whereas wild-type mice were resistant. CGD mice were challenged intratracheally with a lethal inoculum (1.25 x 10(4) CFU/mouse) of A. fumigatus and received one of the following regimens daily from day 0 to 4 after challenge (n = 19 to 20 per treatment group): (i) vehicle, (ii) amphotericin B (intraperitoneal; 1 mg/kg of body weight), (iii) micafungin (intravenous; 10 mg/kg), or (iv) amphotericin B plus micafungin. The rank order of therapeutic efficacy based on prolonged survival, from highest to lowest, was as follows: amphotericin B plus micafungin, amphotericin B alone, micafungin alone, and the vehicle. Lung histology showed pyogranulomatous lesions and invasive hyphae, but without hyphal angioinvasion or coagulative necrosis. Treatment with micafungin alone or combined with amphotericin B produced swelling of invasive hyphae that was not present in mice treated with the vehicle or amphotericin B alone. Assessment of lung fungal burden by quantitative PCR showed no significant difference between treatment groups. Serum galactomannan levels were at background despite documentation of invasive aspergillosis by histology. Our findings showed the superior efficacy of the amphotericin B and micafungin combination compared to either agent alone after A. fumigatus challenge and also demonstrated unique features of CGD mice as a model for experimental aspergillosis.

Efficacy and pharmacodynamics of flucytosine monotherapy in a nonneutropenic murine model of invasive aspergillosis

The therapeutic efficacy of flucytosine (5FC) monotherapy and the pharmacodynamic index predictive of efficacy were evaluated in a nonneutropenic mouse model of acute invasive aspergillosis. Mice were infected intravenously with an Aspergillus fumigatus isolate (the median MICs of 5FC were 128 mug/ml under the standard condition, 0.5 microg/ml at pH 6.0, and 0.031 microg/ml at pH 5.0) 2 h prior to the start of therapy and were treated for 7 days with different 5FC dosing regimens. The total doses ranged from 50 to 800 mg/kg of body weight/day and were administered at 6-, 12-, and 24-h intervals. The efficacy was assessed by means of survival. The survival rates of the treatment groups ranged from 40 to 90%, while the survival rate of the control group was 20%. The efficacy found depended primarily on the total daily dose. However, the power of our sample size may have been too low to exclude an effect of dose fractionation. The pharmacodynamic index that most strongly correlated with the efficacy was the area under the serum concentration-time curve and MIC ratio (R(2) = 0.86). We conclude that 5FC monotherapy is efficacious in a murine Aspergillus fumigatus infection model.

Relationship between in vitro activities of amphotericin B and flucytosine and pH for clinical yeast and mold isolates

In this study, we investigated the pH dependency of the in vitro activities of amphotericin B (AMB) and flucytosine (5FC) against Candida spp., Cryptococcus neoformans, Aspergillus fumigatus, Rhizopus spp., and Scedosporium prolificans in RPMI 1640 buffered with citrate buffer (pH 4.0, 5.0, 5.4, and 6.0), citrate-phosphate buffer (pH 5.4, 6.0, 6.4, and 7.0), and 3-[N-morpholino]propanesulfonic acid (MOPS) (pH 6.4, 7.0, 7.4, and 7.9). For 5FC, no significant differences were found between MICs obtained with the different buffers, while for AMB, significant differences were found. The MICs obtained with citrate-phosphate buffer were approximately 1 twofold-dilution step higher than the MICs obtained with MOPS. We demonstrated that the in vitro activities of AMB and 5FC against yeast and mold isolates were pH dependent. The in vitro activity of AMB decreased when the pH was lowered, while the in vitro activity of 5FC increased. The effect of the pH on the in vitro activities was dependent not only on the antifungal agent tested but also on the microorganism. For AMB, there was a nonlinear relationship (median r(2), 0.864) for Candida spp., C. neoformans, A. fumigatus, and Rhizopus spp. over the pH range tested. The mean MICs ranged from 0.5 to 2.52 microg/ml at pH 7.0 and from 20.16 to 32 microg/ml at pH 5.0. For S. prolificans, there was no relationship. For 5FC, there was a linear relationship for Candida spp. (median r(2), 0.767) and a nonlinear relationship for C. neoformans and A. fumigatus (median r(2), 0.882) over the pH range tested. The mean MIC values ranged from 0.125 to 1,024 microg/ml at pH 7.0 and from 0.02 to 4 microg/ml at pH 5.0. For Rhizopus spp. and S. prolificans, the relationship could not be determined, since the MIC was >1,024 microg/ml over a pH range of 4.0 to 7.9.