Experimental models in antifungal chemotherapy

Animal models of invasive mycoses

Animal models of invasive fungal infections have been developed and are applied in a huge number of different animal species for a number of research purposes, for example, the study of pathogenesis, defense mechanisms, and therapeutic strategies. From the different models, which in most cases are based on the same fungal species and often the same strain, as in spontaneous human infections, fundamental results and knowledge of the diagnosis, progression, prophylaxis, and therapy have been achieved. However, in all models, one should be critical with respect to mimicking the disease entity of humans, which is often the focus of the research. In many of the models for instance, the time course is different to the one of humans, and in others, the propensity for localization and containment in specific organs does not parallel the situation in humans. Nevertheless, many animal models of invasive mycoses have proven valuable in a number of research areas. With regard to new generations of anti-mycotic drugs, the models play an essential role in demonstrating antifungal activity, as well as in demonstrating the absence of toxic side effects, a critical step which cannot be accomplished by in vitro studies.

Methodologies for in vitro and in vivo evaluation of efficacy of antifungal and antibiofilm agents and surface coatings against fungal biofilms

Unlike superficial fungal infections of the skin and nails, which are the most common fungal diseases in humans, invasive fungal infections carry high morbidity and mortality, particularly those associated with biofilm formation on indwelling medical devices. Therapeutic management of these complex diseases is often complicated by the rise in resistance to the commonly used antifungal agents. Therefore, the availability of accurate susceptibility testing methods for determining antifungal resistance, as well as discovery of novel antifungal and antibiofilm agents, are key priorities in medical mycology research. To direct advancements in this field, here we present an overview of the methods currently available for determining (i) the susceptibility or resistance of fungal isolates or biofilms to antifungal or antibiofilm compounds and compound combinations; (ii) the in vivo efficacy of antifungal and antibiofilm compounds and compound combinations; and (iii) the in vitro and in vivo performance of anti-infective coatings and materials to prevent fungal biofilm-based infections.

RNAseq analysis of Aspergillus fumigatus in blood reveals a just wait and see resting stage behavior

Background

Invasive aspergillosis is started after germination of Aspergillus fumigatus conidia that are inhaled by susceptible individuals. Fungal hyphae can grow in the lung through the epithelial tissue and disseminate hematogenously to invade into other organs. Low fungaemia indicates that fungal elements do not reside in the bloodstream for long.

Results

We analyzed whether blood represents a hostile environment to which the physiology of A. fumigatus has to adapt. An in vitro model of A. fumigatus infection was established by incubating mycelium in blood. Our model allowed to discern the changes of the gene expression profile of A. fumigatus at various stages of the infection. The majority of described virulence factors that are connected to pulmonary infections appeared not to be activated during the blood phase. Three active processes were identified that presumably help the fungus to survive the blood environment in an advanced phase of the infection: iron homeostasis, secondary metabolism, and the formation of detoxifying enzymes.

Conclusions

We propose that A. fumigatus is hardly able to propagate in blood. After an early stage of sensing the environment, virtually all uptake mechanisms and energy-consuming metabolic pathways are shut-down. The fungus appears to adapt by trans-differentiation into a resting mycelial stage. This might reflect the harsh conditions in blood where A. fumigatus cannot take up sufficient nutrients to establish self-defense mechanisms combined with significant growth.

Electronic supplementary material

The online version of this article (doi10.1186/s12864-015-1853-1) contains supplementary material, which is available to authorized users.

β-D-Glucan Assay in Diagnosis and Monitoring the Systemic Candidiasis in a Rat Model

BACKGROUND

Determination of β-D-Glucan (BDG) in the serum aids to diagnose the invasive fungal infections. The current study evaluated the diagnostic potential value of BDG assay in monitoring the disease in experimental systemic candidiasis in a rat model. The results can provide a useful preliminary data to improve this approach in developing countries.

OBJECTIVES

The present study aimed to evaluate β-D-Glucan assay in diagnosis and monitoring the systemic candidiasis in a rat model.

MATERIALS AND METHODS

Twenty one rats were infected with 10(6) Candida albicans blastospore per rat. Twelve rats were considered as the negative controls (six immunocompromised rats without infection and six intact rats). During a week, every 24 hours the BDG sera level was determined by both Fungitell and Wako kits. To confirm the systemic infection in each rat, the suspensions of their internal organs were cultivated on agar plates and the number of colony forming units (CFU) of C. albicans was counted.

RESULTS

All the infected rats were positive with BDG tests. An increasing level of BDG was observed during early days after injection. The cutoff value for discrimination of BDG positive sera was obtained from the negative sera by the Fungitell kit. The sensitivity, specificity, positive and negative predictive values assessed for the Fungitell kit were 95%, 66.6%, 90.47% and 80%, respectively. These criteria for those of Wako were 90%, 83.3%, 94.7% and 71.4%, respectively.

CONCLUSIONS

While BDG assay seems to be a sensitive and specific adjunctive tool to diagnose and monitor the experimental systemic candidiasis, it seems that measuring the positive cutoff value in different laboratory conditions is necessary for favorable establishment of these tests.

In vitro and in vivo antifungal activities of selected Cameroonian dietary spices

Background

Spices and herbs have been used in food since ancient times to give taste and flavor and also as food preservatives and disease remedies. In Cameroon, the use of spices and other aromatic plants as food flavoring is an integral part of dietary behavior, but relatively little is known about their antifungal potential.

The present work was designed to assess the antifungal properties of extracts from spices used in Cameroonian dietary.

Methods

The in vitro antifungal activities of twenty three extracts from twenty one spices were assessed by the broth micro-dilution method against eight fungi. Also, the in vivo activity of Olax subscorpioidea extract (the most active extract) was evaluated in rat model of disseminated candidiasis due to Candida albicans by estimating the fungal burden in blood and kidney.

Results

Seven extracts (30%) exhibited moderate to significant antifungal activities, inhibiting the growth of the microorganisms at concentrations ranging from 0.048 to 0.39 mg/mL. Olax subscorpioidea extract exhibited the highest antifungal activity particularly against Candida albicans and Candida tropicalis (MIC of 0.097 mg/mL and 0.048 mg/mL respectively). Sixteen extracts (70%) were weakly active (MICs > 6.25 mg/mL). Oral administration of O. subscorpioidea extract at the dose 2 g/kg of body weight (bw) to artificially infected rats revealed a drop in the number of colony forming units per milliliter (cfu/mL) of Candida albicans cells in the blood below the detection limit (100 cfu/mL) while a modest decrease was observed in the kidney.

Conclusion

The present work shows that some of the spices studied possess interesting antifungal properties and could be used to treat candidiasis. Among the plant species tested, Olax subscorpioidea displayed the most promising result.

In vitro and in vivo antidermatophytic activity of the dichloromethane-methanol (1:1 v/v) extract from the stem bark of Polyscias fulva Hiern (Araliaceae)

Background

During the last decades, the number of people suffering from dermatophytoses has seriously increased, mainly due to the development of resistant strains of microorganisms to a range of formally efficient antibiotics. Polyscias fulva, a medium size tree which grows in the West Region of Cameroon is traditionally used for local application against dermatoses and orally against venereal infections. The dichloromethane-methanol (1:1 v/v) extract from the stem bark of Polyscias fulva was evaluated for its in vitro and in vivo antifungal activities.

Methods

The plant extract was prepared by maceration of its stem bark powder in CH₂Cl₂-MeOH (1:1 v/v). The extract obtained was successively partitioned in hexane, ethyl acetate and n-butanol. Phytochemical screening was performed using standard methods. In vitro antidermatophytic activity was assayed by the well diffusion and broth microdilution methods. The degree of dermal irritation of the crude extract was determined in guinea pigs using the occluded dermal irritation test method. The in vivo antidermatophytic activity of the extract-oil formulation (1.25, 2.5 and 5% w/w concentrations) was evaluated using Trichophyton mentagrophytes-induced dermatophytosis in a guinea pigs model.

Results

Phytochemical screening indicated that, the crude extract, ethyl acetate, n-butanol and residue fractions contain in general saponins, tannins, alkaloids, anthraquinones and phenols while the hexane fraction contains only alkaloids. The ethyl-acetate, n-butanol and residue fractions displayed higher antifungal activities (MIC = 0.125-0.5 mg.mL^-1) against eight dermatophytes as compared to the crude extract (MIC = 0.5-1 mg.mL^-1). This latter appeared to have slight perceptible erythema effects on guinea pigs as the primary irritation index (PII) was calculated to be 0.54. In vivo, the antidermatophytic activities of the extract-oil formulations were dose-dependent. Griseofulvin-oil 5% at 0.01 g/kg and formulated extract-oil (5%) at 0.1 g/kg eradicated the microbial infection after thirteen and fourteen days of daily treatment respectively.

Conclusions

The results of preclinical in vitro and in vivo evaluations indicate that the extract-oil formulation at 5% may constitute an alternative means to alleviate fungal infections caused by dermatophytes.

A nebulized intra-tracheal rat model of invasive pulmonary aspergillosis

Animal models are particularly useful for the study of many infectious diseases, including those caused by fungi. Invasive pulmonary aspergillosis is most frequently studied in mouse models. We present here an animal model of this disease based on undernourished immunocompromised rats infected with Aspergillus fumigatus spores by intra-tracheal nebulisation.

Lessons from animal studies for the treatment of invasive human infections due to uncommon fungi

Clinical experience in the management of opportunistic infections, especially those caused by less common fungi, is, due to their rarity, very scarce; therefore, the most effective treatments remain unknown. The ever-increasing numbers of fungal infections due to opportunistic fungi have repeatedly proven the limitations of the antifungal armamentarium. Moreover, some of these fungi, such as Fusarium spp. or Scedosporium spp., are innately resistant to almost all the available antifungal drugs, which makes the development of new and effective therapies a high priority. Since it is difficult to conduct randomized clinical trials in these uncommon mycoses, the use of animal models is a good alternative for evaluating new therapies. This is an extensive review of the numerous studies that have used animal models for this purpose against a significant number of less common fungi. A table describing the different studies performed on the efficacy of the different drugs tested is included for each fungal species. In addition, there is a summary table showing the conclusions that can be derived from the analysis of the studies and listing the drugs that showed the best results. Considering the wide variability in the response to the antifungals that the different strains of a given species can show, the table highlights the drugs that showed positive results using at least two parameters for evaluating efficacy against at least two different strains without showing any negative results. These data can be very useful for guiding the treatment of rare infections when there is very little experience or when controversial results exist, or when treatment fails.

Differential mechanisms of resistance to sublethal systemic Aspergillus fumigatus infection in immunocompetent BALB/c and C57BL/6 mice

Studies of systemic and pulmonary Aspergillus fumigatus infection demonstrated differential susceptibility of inbred mice of various genetic background to lethal outcome, with an opposite pattern of Th1 cytokine interferon-γ (IFN-γ) and Th2 cytokine interleukin-4 (IL-4) in susceptible vs resistant mice. We have shown recently reciprocal IFN-γ and IL-4 expression in spleens of Th1-prone C57BL/6 mice in sublethal systemic aspergillosis. In this study, resistance to systemic (i.v.) A. fumigatus infection was investigated in Th2-prone BALB/c mice by survival rate at different fungal inocula, efficiency of reduction of visceral organ and spleen fungal burden at sublethal conidia dose and splenic immune response to this dose and compared to C57BL/6 mice. No strain differences in survival were noted at three A. fumigatus doses, with similar extent and dynamics of fungal eradication from all organs following sublethal conidia dose injection. Progressive decrease in spleen fungal burden was associated with different dynamics and quality of changes in spleen activity of BALB/c and C57BL/6 mice. Increased spleen mass and cellularity was noted in both strains, with higher values in BALB/c mice at some time points what might be ascribed to peripheral blood cell recruitment, as well as hematopoietic activity and red pulp upgrowth. Infection tipped the balance towards pro-inflammatory antifungal splenic response by a highly increasing IFN-γ and without changing the IL-4 expression in BALB/c mice, in contrast to down-regulating anti-inflammatory (IL-4) and a moderately increasing IFN-γ response in C57BL/6 mice. Jointly, stimulation of IL-17 expression noted in both strains provided an optimal inflammatory milieu in the spleen of infected mice that might have contributed to efficient removal of conidia.

The utility of a nebulised intra-tracheal rat model of invasive pulmonary aspergillosis

Invasive pulmonary aspergillosis (IPA) is of particular concern to immunodeficient patients, whose mortality rates may exceed 80%. The development of an animal model that faithfully reproduces the pathophysiology of IPA would improve the studies on diagnostic and therapeutic modes, and the use of rats as a possible model for IPA seems to have been largely overlooked. Such a model could be established with the MicroSprayer IA-1B. Male Sprague-Dawley rats (6-8 weeks old) were rendered immunodeficient by cyclophosphamide injections and a protein-deficient diet. On day D0, they were anaesthetised by inhalation of 5% isoflurane and infected by the intra-tracheal aerosolization of 100 microl of an Aspergillus fumigatus spore suspension through a MicroSprayer IA-1B. This inoculation process was simple and rapid, with no deaths observed during or immediately after the procedure. The rats regained consciousness within 1 min. Follow-up data including those for clinical factors (weight changes, mortality rate), biological factors (Aspergillus antigens) and histological factors were consistent with previous studies. The advantages of this model include the ease of animal manipulation, the reproducibility of infection and the potential for repeated blood sampling.

Animal models: an important tool in mycology

Animal models of fungal infections are, and will remain, a key tool in the advancement of the medical mycology. Many different types of animal models of fungal infection have been developed, with murine models the most frequently used, for studies of pathogenesis, virulence, immunology, diagnosis, and therapy. The ability to control numerous variables in performing the model allows us to mimic human disease states and quantitatively monitor the course of the disease. However, no single model can answer all questions and different animal species or different routes of infection can show somewhat different results. Thus, the choice of which animal model to use must be made carefully, addressing issues of the type of human disease to mimic, the parameters to follow and collection of the appropriate data to answer those questions being asked. This review addresses a variety of uses for animal models in medical mycology. It focuses on the most clinically important diseases affecting humans and cites various examples of the different types of studies that have been performed. Overall, animal models of fungal infection will continue to be valuable tools in addressing questions concerning fungal infections and contribute to our deeper understanding of how these infections occur, progress and can be controlled and eliminated.

Development of an orogastrointestinal mucosal model of candidiasis with dissemination to visceral organs

Studies were done to develop a murine model that mimics the pattern of mucosal candidiasis followed by disseminated disease seen in patients given cytotoxic chemotherapy. Developmental studies showed that suppression of mice with 5-fluorouracil beginning 3 days prior to infection and given every 7 days thereafter necessitated antibacterial treatment but resulted in a reproducible model. Candida albicans given in the drinking water resulted in oral infection by day 3 that significantly increased from days 10 to 15 and mucosal infection with 4 to 7 log(10) Candida CFU in the esophagus, stomach, small intestine, and cecum. Dissemination to livers occurred and was 100% on days 5 to 15; fewer animals had kidney infection. The median kidney or liver CFU were 2 or 3 log(10) CFU, respectively, on day 15; despite this, mortality was low through 21 days of infection. As a demonstration of the utility of the model to test antifungal activity, daily treatment with 10 or 50 mg/kg itraconazole significantly reduced dissemination to the liver and kidneys and reduced tongue CFU compared to controls. Overall, these studies indicate that a nonlethal model of oral and gastrointestinal mucosal candidiasis with dissemination can be established in mice. Drug efficacy in treating localized infection and in preventing or treating disseminated infection can be studied.

Animal models testing monotherapy versus combination antifungal therapy: lessons learned and future directions

PURPOSE OF REVIEW

The continued rise in serious fungal infections and rises in therapy failure dictate that more efficacious therapies be developed. Combination therapy using available drugs is an attractive choice, yet primarily only anecdotal clinical data are available. We review here data from animal models as an indicator of future potential.

RECENT FINDINGS

The primary data are from murine studies and we will briefly review chemotherapeutic combination studies, some showing benefit over monotherapy and some showing no benefit over monotherapy. In addition, we will address the potential of immunotherapy in combination with conventional therapy.

SUMMARY

The data derived from animal model studies of antifungal drug efficacy have proven to be predictive of clinical utility. Studies on combination therapy will prove useful to the clinician in evaluating courses of treatment, especially where clinical-trial data are not available or probable in the future.

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.

The contribution of animal models of aspergillosis to understanding pathogenesis, therapy and virulence

Animal models of aspergillosis have been used extensively to study various aspects of pathogenesis, innate and acquired host-response, disease transmission and therapy. Several different animal models of aspergillosis have been developed. Because aspergillosis is an important pulmonary disease in birds, avian models have been used successfully to study preventative vaccines. Studies done to emulate human disease have relied on models using common laboratory animal species. Guinea pig models have primarily been used in therapy studies of invasive pulmonary aspergillosis (IPA). Rabbits have been used to study IPA and systemic disease, as well as fungal keratitis. Rodent, particularly mouse, models of aspergillosis predominate as the choice for most investigators. The availability of genetically defined strains of mice, immunological reagents, cost and ease of handling are factors. Both normal and immunosuppressed animals are used routinely. These models have been used to determine efficacy of experimental therapeutics, comparative virulence of different isolates of Aspergillus, genes involved in virulence, and susceptibility to infection with Aspergillus. Mice with genetic immunological deficiency and cytokine gene-specific knockout mice facilitate studies of the roles cells, and cytokines and chemokines, play in host-resistance to Aspergillus. Overall, these models have been critical to the advancement of therapy, and our current understanding of pathogenesis and host-resistance.

Animal models of zygomycosis--Absidia, Rhizopus, Rhizomucor, and Cunninghamella

Infections caused by zygomycetes, which have been increasing in recent years, are known for their difficulty of diagnosis and treatment. Because little is known about this fungus and its infection, vigorous research is now in serious demand. As in many other systemic mycoses, animal model studies are essential in the investigation of zygomycosis, particularly for the study of pathogenesis, diagnosis and treatment. Unfortunately, such studies have been limited when compared with those of aspergillosis. To help investigating the disease, here in this review article, the profile of human zygomycosis is briefly described, followed by a review of the heretofore used animal models of zygomycosis. Among clinically important zygomycetes causing human infection, animal models are available for Absidia corymbifera, Rhizopus oryzae, R. microsporus var. rhizopodiformis, Rhizomucor pusillus and Cunninghamella bertholletiae. Mice are the most commonly used animals, but models using guinea pigs and rabbits are also available. Pretreatment of animals with cyclophosphamide, corticosteroid, alloxan or streptozocine is frequently done to create an immunocompromised state. Treatment with desferrioxamine, an iron chelator, is also used to make animal models. In terms of the route of infection, the airborne route is used for pathophysiological studies in pulmonary infection models, but sometimes intravenous injection is preferred, particularly for antifungal drug studies. When pathophysiological analysis is the purpose of the study, the animals must be cautiously examined both histopathologically and mycologically. For the most part, zygomycosis model studies can be performed in a similar manner to those of aspergillosis. However, Aspergillus spp. and zygomycetes are completely different fungi, and researchers should be aware of the specific, critical aspects when handling zygomycosis models, such as homogenization of infected organs and staining of pathological samples.

Role of IL-10 in invasive aspergillosis: increased resistance of IL-10 gene knockout mice to lethal systemic aspergillosis

IL-10 is associated with a Th2 response, down-regulation of a Th1 response and macrophage activation. We assessed the role of IL-10 during systemic infection with Aspergillus fumigatus. Systemic aspergillosis was established in female C56B1/6 IL-10(-/-) (KO) and wild-type (WT) C57B1/6 mice by i.v. administration of 1 x 10(5)-6 x 10(5) conidia of A. fumigatus. In two experiments, KO survived longer than did WT (P < 0.001). Determination of fungal burdens in the kidneys and brain showed that KO carried significantly lower burdens in both organs than did WT on day 3 (P < 0.001). Semiquantitative histological analyses showed fewer inflammatory foci/mm2 in brain and kidneys of KO than WT (P < 0.03 and < 0.001, respectively) and that extent of infection and associated tissue injury were greater in WT. Although beneficial in some bacterial infections, exogenous IL-10 has been shown deleterious in models of fungal infection. Our data indicate IL-10 is deleterious during systemic aspergillosis infection, increasing the host susceptibility to lethal infection. We speculate this might be related to greater Th2 or lesser Th1 responses, or down-regulation of macrophage responses, in WT compared with KO.

Antifungal pharmacodynamics: review of the literature and clinical applications

Invasive fungal infections are seen with growing frequency, likely due to increases in numbers of patients at risk of infection. Optimal selection and dosing of antifungal agents are important, as these infections are often refractory to available therapy. In contrast to antibacterials, studies examining the pharmacodynamic properties of antifungals and their application in treating invasive disease often are lacking. Agents administered for invasive infections are amphotericin B, flucytosine, and azole antifungals. Several drugs are under investigation, such as posiconazole, voriconazole, and the echinocandins, and preliminary pharmacodynamic data likely will help shape dosing regimens. Clinical trials that investigated dosage and administration, as well as the potential benefits of combination and sequential therapy, are addressed. In addition, antifungal susceptibility and animal models of infection are discussed.

Murine models of blastomycosis, coccidioidomycosis, and histoplasmosis

Animal models have contributed much to the knowledge of fungal infections and their corresponding therapeutic treatments. This is true for animal models of the primary fungal pathogens, Blastomyces dermatitidis, Coccidioides immitis, and Histoplasma capsulatum. This review gives a brief background of human diseases associated with these organisms and describes the development, details, and utility of murine models of blastomycosis, as well as coccidioidomycosis and histoplasmosis.

Aspergillus fumigatus and aspergillosis

Aspergillus fumigatus is one of the most ubiquitous of the airborne saprophytic fungi. Humans and animals constantly inhale numerous conidia of this fungus. The conidia are normally eliminated in the immunocompetent host by innate immune mechanisms, and aspergilloma and allergic bronchopulmonary aspergillosis, uncommon clinical syndromes, are the only infections observed in such hosts. Thus, A. fumigatus was considered for years to be a weak pathogen. With increases in the number of immunosuppressed patients, however, there has been a dramatic increase in severe and usually fatal invasive aspergillosis, now the most common mold infection worldwide. In this review, the focus is on the biology of A. fumigatus and the diseases it causes. Included are discussions of (i) genomic and molecular characterization of the organism, (ii) clinical and laboratory methods available for the diagnosis of aspergillosis in immunocompetent and immunocompromised hosts, (iii) identification of host and fungal factors that play a role in the establishment of the fungus in vivo, and (iv) problems associated with antifungal therapy.