Aerosol and parenteral pneumocandins are effective in a rat model of pulmonary aspergillosis

Inhaled antifungal therapy: benefits, challenges, and clinical applications

INTRODUCTION

Disease due to pulmonary infection with Aspergillus, and other emerging opportunistic fungi remains a significant unmet need. Existing antifungal medicines are predominantly dosed either orally or systemically, but because of limited exposure to the lung lumen, adverse events, and problematic drug-drug interactions, inhaled treatment could provide an attractive option.

AREA COVERED

This review summarizes 1) the limitations of current antifungal therapy, 2) the beneficial effects of inhaled antifungal agents, 3) the clinical development of inhaled antifungal triazoles (repurposed with an innovative inhalation system or a novel inhaled agent) for the treatment of pulmonary fungal infections, and 4) the difficulties and challenges of inhaled antifungal agent development. Regrettably, details of novel inhaled devices or formulations were not covered.

EXPERT OPINION

Inhaled antifungal treatment could provide an attractive option by shifting the risk benefit ratio of treatment favorably. Preclinical and clinical studies with inhaled antifungal agents (off-label use) are encouraging so far. New inhaled antifungal triazoles are well tolerated in early clinical studies and warrant further clinical development. However, challenges remain and many unaddressed issues including required preclinical studies, appropriate clinical design, pharmacokinetics, delivery system(s) and regulatory process need to be resolved. Early communication with regulatory authorities is therefore recommended.

Inhaled Antifungal Agents for Treatment and Prophylaxis of Bronchopulmonary Invasive Mold Infections

Pulmonary mold infections are life-threatening diseases with high morbi-mortalities. Treatment is based on systemic antifungal agents belonging to the families of polyenes (amphotericin B) and triazoles. Despite this treatment, mortality remains high and the doses of systemic antifungals cannot be increased as they often lead to toxicity. The pulmonary aerosolization of antifungal agents can theoretically increase their concentration at the infectious site, which could improve their efficacy while limiting their systemic exposure and toxicity. However, clinical experience is poor and thus inhaled agent utilization remains unclear in term of indications, drugs, and devices. This comprehensive literature review aims to describe the pharmacokinetic behavior and the efficacy of inhaled antifungal drugs as prophylaxes and curative treatments both in animal models and humans.

Inhaled Antifungal Agents for the Treatment and Prophylaxis of Pulmonary Mycoses

Pulmonary mycoses are associated with high morbidity and mortality. The current standard treatment by systemic administration is limited by inadequate local bioavailability and systemic toxic effects. Aerosolisation of antifungals is an attractive approach to overcome these problems, but no inhaled antifungal formulation is currently available for the treatment of pulmonary mycoses. Hence, the development of respirable antifungals formulations is of interest and in high demand. In this review, the recent advances in the development of antifungal formulations for pulmonary delivery are discussed, including both nebulised and dry powder formulations. Although the clinical practices of nebulised parenteral amphotericin B and voriconazole formulations (off-label use) are reported to show promising therapeutic effects with few adverse effects, there is no consensus about the dosage regimen (e.g. the dose, frequency, and whether they are used as single or combination therapy). To maximise the benefits of nebulised antifungal therapy, it is important to establish standardised protocol that clearly defines the dose and specifies the device and the administration conditions. Dry powder formulations of antifungal agents such as itraconazole and voriconazole with favourable physicochemical and aerosol properties are developed using various powder engineering technologies, but it is important to consider their suitability for use in patients with compromised lung functions. In addition, more biological studies on the therapeutic efficacy and pharmacokinetic profile are needed to demonstrate their clinical potential.

In Vitro or In Vivo Models, the Next Frontier for Unraveling Interactions between Malassezia spp. and Hosts. How Much Do We Know?

Malassezia is a lipid-dependent genus of yeasts known for being an important part of the skin mycobiota. These yeasts have been associated with the development of skin disorders and cataloged as a causal agent of systemic infections under specific conditions, making them opportunistic pathogens. Little is known about the host-microbe interactions of Malassezia spp., and unraveling this implies the implementation of infection models. In this mini review, we present different models that have been implemented in fungal infections studies with greater attention to Malassezia spp. infections. These models range from in vitro (cell cultures and ex vivo tissue), to in vivo (murine models, rabbits, guinea pigs, insects, nematodes, and amoebas). We additionally highlight the alternative models that reduce the use of mammals as model organisms, which have been gaining importance in the study of fungal host-microbe interactions. This is due to the fact that these systems have been shown to have reliable results, which correlate with those obtained from mammalian models. Examples of alternative models are Caenorhabditis elegans, Drosophila melanogaster, Tenebrio molitor, and Galleria mellonella. These are invertebrates that have been implemented in the study of Malassezia spp. infections in order to identify differences in virulence between Malassezia species.

Animal Models of Aspergillosis

Aspergillosis is an airborne fungal disease caused by Aspergillus spp., a group of ubiquitous molds. This disease causes high morbidity and mortality in both humans and animals. The growing importance of this infection over recent decades has created a need for practical and reproducible models of aspergillosis. The use of laboratory animals provides a platform to understand fungal virulence and pathophysiology, assess diagnostic tools, and evaluate new antifungal drugs. In this review, we describe the fungus, various Aspergillus-related diseases in humans and animals and various experimental animal models. Overall, we highlight the advantages and limitations of the animal models, the experimental variables that can affect the course of the disease and the reproducibility of infection, and the critical need for standardization of the species, immunosuppressive drugs, route of infection, and diagnostic criteria to use.

Rodent Models of Invasive Aspergillosis due to Aspergillus fumigatus: Still a Long Path toward Standardization

Invasive aspergillosis has been studied in laboratory by the means of plethora of distinct animal models. They were developed to address pathophysiology, therapy, diagnosis, or miscellaneous other concerns associated. However, there are great discrepancies regarding all the experimental variables of animal models, and a thorough focus on them is needed. This systematic review completed a comprehensive bibliographic analysis specifically-based on the technical features of rodent models infected with Aspergillus fumigatus. Out the 800 articles reviewed, it was shown that mice remained the preferred model (85.8% of the referenced reports), above rats (10.8%), and guinea pigs (3.8%). Three quarters of the models involved immunocompromised status, mainly by steroids (44.4%) and/or alkylating drugs (42.9%), but only 27.7% were reported to receive antibiotic prophylaxis to prevent from bacterial infection. Injection of spores (30.0%) and inhalation/deposition into respiratory airways (66.9%) were the most used routes for experimental inoculation. Overall, more than 230 distinct A. fumigatus strains were used in models. Of all the published studies, 18.4% did not mention usage of any diagnostic tool, like histopathology or mycological culture, to control correct implementation of the disease and to measure outcome. In light of these findings, a consensus discussion should be engaged to establish a minimum standardization, although this may not be consistently suitable for addressing all the specific aspects of invasive aspergillosis.

Clinical pharmacokinetics of inhaled antimicrobials

Administration of inhaled antimicrobials affords the ability to achieve targeted drug delivery into the respiratory tract, rapid entry into the systemic circulation, high bioavailability and minimal metabolism. These unique pharmacokinetic characteristics make inhaled antimicrobial delivery attractive for the treatment of many pulmonary diseases. This review examines recent pharmacokinetic trials with inhaled antibacterials, antivirals and antifungals, with an emphasis on the clinical implications of these studies. The majority of these studies revealed evidence of high antimicrobial concentrations in the airway with limited systemic exposure, thereby reducing the risk of toxicity. Sputum pharmacokinetics varied widely, which makes it challenging to interpret the result of sputum pharmacokinetic studies. Many no vel inhaled antimicrobial therapies are currently under investigation that will require detailed pharmacokinetic studies, including combination inhaled antimicrobial therapies, inhaled nanoparticle formulations of several antibacterials, inhaled non-antimicrobial adjuvants, inhaled antiviral recombinant protein therapies and semi-synthetic inhaled antifungal agents. Additionally, the development of new inhaled delivery devices, particularly for mechanically ventilated patients, will result in a pressing need for additional pharmacokinetic studies to identify optimal dosing regimens.

Nebulizer delivery of micafungin aerosols

STUDY OBJECTIVES

To determine the optimal nebulization system for aerosolizing micafungin and to further assess the physiochemical properties of aerosolized micafungin.

DESIGN

In vitro experiment.

SETTING

University research center. NEBULIZERS: Pari LC Star, Hudson Updraft, Small Volume Nebulizer, and Aeroclipse II.

MEASUREMENTS AND MAIN RESULTS

Using a commercially available cascade impactor, the four nebulizers were tested for their ability to deliver micafungin to the lungs. Mass median aerodynamic diameter (MMAD) and fine particle fraction (FPF) percent less than 3.3 μm (FPF(3.3)) and less than 5.8 μm (FPF(5.8)) were determined during two sampling periods for each of three trials of all nebulizers. The mean ± standard error of the mean MMAD for the nebulizers ranged from 1.93 ± 0.09 to 2.49 ± 0.25 μm; FPF(3.3) and FPF(5.8) were approximately 50% and 90%, respectively, for all nebulizers.

CONCLUSION

Although all nebulizers appeared acceptable to deliver micafungin to the lungs, the Pari LC Star had the smallest MMAD and highest FPF(3.3) and FPF(5.8). These properties of the Pari LC Star should result in greater delivery of the aerosol to the lungs. Additional research on pulmonary delivery and clinical tolerability is warranted.

Aerosolized Delivery of Antifungal Agents

Pulmonary infections caused by Aspergillus species are associated with significant morbidity and mortality in immunocompromised patients. Although the treatment of pulmonary fungal infections requires the use of systemic agents, aerosolized delivery is an attractive option in prevention because the drug can concentrate locally at the site of infection with minimal systemic exposure. Current clinical evidence for the use of aerosolized delivery in preventing fungal infections is limited to amphotericin B products, although itraconazole, voriconazole, and caspofungin are under investigation. Based on conflicting results from clinical trials that evaluated various amphotericin B formulations, the routine use of aerosolized delivery cannot be recommended. Further research with well-designed clinical trials is necessary to elucidate the therapeutic role and risks associated with aerosolized delivery of antifungal agents. This article provides an overview of aerosolized delivery systems, the intrapulmonary pharmacokinetic properties of aerosolized antifungal agents, and key findings from clinical studies.

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.

Pneumocystis carinii: a fungus resistant to antifungal therapies - mechanisms of action of antipneumocystis drugs

Pneumocystis carinii is a pathogen that causes a potentially lethal pneumonia in patients with AIDS and other immunodeficiency states. This review discusses the mechanisms of action of four classes of antipneumocystis agents: inhibitors of ergosterol synthesis and function, 1,3-beta-glucan synthase inhibitors, antifolates and DNA binding agents. Investigations of P. carinii's biologic pathways affected by the antipneumocystis actions of each of these classes of agents has generated important insights into the organism's basic biology and supports the organism's classification as a fungus. In addition, this review discusses some recent P. carinii research and its potential impact on drug development.

Suitability of Caspofungin for Aerosol Delivery

BACKGROUND

Aerosolized antifungal therapy is a promising route of drug delivery for pulmonary aspergillosis due to attainment of high localized concentrations. Caspofungin, a new antifungal agent with proven efficacy against invasive aspergillosis, has ideal potential for aerosolization.

STUDY OBJECTIVE

To examine in vitro the suitability of caspofungin for aerosol administration by characterizing factors that influence efficacy and airway tolerance of aerosol delivery: physicochemical properties, aerodynamics of drug particles, and efficiency of nebulizing systems.

DESIGN

Physicochemical characteristics of caspofungin solutions (10 mg/mL and 30 mg/mL) were analyzed: osmolality, pH, viscosity, and surface tension. A time-of-flight aerosol spectrometer API Aerosizer was used to determine aerosol particle size and distribution. Drug output was quantified by high-performance liquid chromatography assay. Nebulizer efficiency was measured by drug output and respirable fraction (percentage of aerosolized particles with a 1 to 5 mum aerodynamic diameter) and compared among three jet nebulizer/compressor systems: device 1, Micromist (Hudson RCI; Temecula, CA)/Pulmo-Aide (model 5650D; DeVilbiss; Somerset, PA); device 2, Sidestream MS 2400/Envoy model IRC 1192 (Invacare; Elyria, OH); and device 3, Pari LC Star/Proneb Ultra (Pari Respiratory Equipment; Midlothian, VA).

MEASUREMENTS AND RESULTS

Caspofungin requires 0.9% NaCl rather than sterile water as the diluent and addition of 0.3N NaOH buffer to adjust acidity of solutions (pH 6.17 to 6.26) in order to achieve optimal physicochemical properties for airway tolerability (osmolality, 150 to 550 milliosmol per kilogram; chloride ion, 31 to 300 mmol/L; and pH 7.4). The drug output rate increased with higher concentrations of drug solution: device 1, 4.0 mg/min vs 12.5 mg/min; device 2, 5.4 mg/min vs 14.7 mg/min; and device 3, 2.3 mg/min vs 12 mg/min, respectively. The percentage of particles within the respirable range varies depending on device and concentration of drug solutions (10 mg/mL vs 30 mg/mL): device 1, 85% vs 38%; device 2, 44% vs 57%; and device 3, 83% vs 93%, respectively.

CONCLUSION

Caspofungin solution with adjustments appears to have physicochemical and aerodynamic characteristics suitable for aerosolization when used with either the Pari LC Star/Proneb Ultra or Micromist/Pulmo-Aide devices. Further in vivo testing is warranted.

Pathophysiology of pulmonary aspergillosis

A description of the pathophysiology of aspergillosis is followed by a review of investigational considerations of animal models. Because a large body of invasive Aspergillus infection occurs as opportunistic infection, there is a large spectrum of the histopathological feature of lesions demonstrated at the site of infection. Histopathology of the lesions can be understood as a phenotypical representation of interaction between lowered defense mechanisms in the host and the virulence of invading fungi. Detailed observations with a consideration of previous pathological knowledge of infection and inflammation provide much important information useful in predicting the pathophysiology of the patient. Moreover, experimental studies can also provide much insight to elucidate pathogenesis of the infection that emerges from the clinical and pathological investigations. The importance of pathophysiology should be emphasized to understand the implications of radiographic images, clinical symptoms, and laboratory dates. By reviewing these, especially computed tomography (CT) images, we can see that they accurately mirror the histological features of the lesion that can be recognized as a phenotypical representation of pathophysiology of Aspergillus infection. This is also confirmed by the reports emphasizing the importance of CT scans to identify hallmark clinical signs and symptoms of the disease.

Experimental models of pulmonary infection

Experimental models of pulmonary infection are being discussed, focused on various aspects of good experimental design, such as choice of animal species and infecting strain, and route of infection/inoculation techniques (intranasal inoculation, aerosol inoculation, and direct instillation into the lower respiratory tract). In addition, parameters to monitor pulmonary infection are being reviewed such as general clinical signs, pulmonary-associated signs, complication of the pulmonary infection, mortality rate, and parameters after dissection of animals. Examples of pulmonary infection models caused by bacteria, fungi, viruses or parasites in experimental animals with intact or impaired host defense mechanisms are shortly summarized including key-references.

Comparison of real-time PCR, conventional PCR, and galactomannan antigen detection by enzyme-linked immunosorbent assay using bronchoalveolar lavage fluid samples from hematology patients for diagnosis of invasive pulmonary aspergillosis

An iCycler iQ real-time PCR assay targeting 18S rRNA Aspergillus-specific sequences was developed for the diagnosis of invasive pulmonary aspergillosis (IPA). Positive findings were obtained for 18 of 20 (90%) bronchoalveolar lavage (BAL) fluid specimens from patients with probable or confirmed IPA and were obtained for none of the 24 BAL samples from patients with no clinical evidence of aspergillosis. These results were concordant with those of a nested PCR assay, which detected 90% of the patients with IPA, while galactomannan ELISA revealed positivity for 100% of these patients, suggesting that combined use of methods might improve the diagnosis of IPA.

Multiple resistance mechanisms among Aspergillus fumigatus mutants with high-level resistance to itraconazole

A collection of Aspergillus fumigatus mutants highly resistant to itraconazole (RIT) at 100 micro g ml(-1) were selected in vitro (following UV irradiation as a preliminary step) to investigate mechanisms of drug resistance in this clinically important pathogen. Eight of the RIT mutants were found to have a mutation at Gly54 (G54E, -K, or -R) in the azole target gene CYP51A. Primers designed for highly conserved regions of multidrug resistance (MDR) pumps were used in reverse transcriptase PCR amplification reactions to identify novel genes encoding potential MDR efflux pumps in A. fumigatus. Two genes, AfuMDR3 and AfuMDR4, showed prominent changes in expression levels in many RIT mutants and were characterized in more detail. Analysis of the deduced amino acid sequence encoded by AfuMDR3 revealed high similarity to major facilitator superfamily transporters, while AfuMDR4 was a typical member of the ATP-binding cassette superfamily. Real-time quantitative PCR with molecular beacon probes was used to assess expression levels of AfuMDR3 and AfuMDR4. Most RIT mutants showed either constitutive high-level expression of both genes or induction of expression upon exposure to itraconazole. Our results suggest that overexpression of one or both of these newly identified drug efflux pump genes of A. fumigatus and/or selection of drug target site mutations are linked to high-level itraconazole resistance and are mechanistic considerations for the emergence of clinical resistance to itraconazole.

Overexpression of Sbe2p, a Golgi protein, results in resistance to caspofungin in Saccharomyces cerevisiae

Caspofungin inhibits the synthesis of 1, 3-beta-D-glucan, an essential cell wall target in fungi. Genetic studies in the model yeast Saccharomyces cerevisiae have shown that mutations in FKS1 and FKS2 genes result in caspofungin resistance. However, direct demonstration of the role of gene overexpression in caspofungin resistance has been lacking. We transformed wild-type S. cerevisiae with an S. cerevisiae URA3-based GAL1 cDNA library and selected transformants in glucose synthetic complete plates lacking uracil (glucose SC minus uracil plates). We then moved the transformants to galactose SC minus uracil plates containing caspofungin (1 microg/ml) and looked for caspofungin-resistant colonies. We retested the candidates (true positives were sensitive on glucose caspofungin and resistant on galactose caspofungin media, respectively). We identified 16 caspofungin-resistant candidates. Restriction analysis and hybridization confirmed that 15 of the 16 clones were identical. We sequenced one of the cDNA clones and found that it contained the cDNA for SBE2. SBE2 has been described in S. cerevisiae to encode a Golgi protein involved in the transport of cell wall components (B. Santos and M. Snyder, Mol. Biol. Cell, 11:435-452, 2000). The SBE2 cDNA plasmid conferred again galactose-dependent caspofungin resistance when transformed back into the wild-type S. cerevisiae. Finally, the SBE2 deletion mutant was hypersensitive to caspofungin. In conclusion, overexpression of Sbe2p under the regulated control of the GAL1 promoter results in caspofungin resistance in S. cerevisiae. This transport pathway may provide insight into the tolerance or lack of sensitivity to caspofungin of some pathogenic fungi.

Quantitative PCR assay to measure Aspergillus fumigatus burden in a murine model of disseminated aspergillosis: demonstration of efficacy of caspofungin acetate

Caspofungin acetate (MK-0991) is an antifungal antibiotic that inhibits the synthesis of 1,3-beta-D-glucan, an essential component of the cell wall of several pathogenic fungi. Caspofungin acetate was recently approved for the treatment of invasive aspergillosis in patients who are refractory to or intolerant of other therapies. The activity of 1,3-beta-D-glucan synthesis inhibitors against Aspergillus fumigatus has been evaluated in animal models of pulmonary or disseminated disease by using prolongation of survival or reduction in tissue CFU as assay endpoints. Because these methods suffer from limited sensitivity or poor correlation with fungal growth, we have developed a quantitative PCR-based (qPCR) (TaqMan) assay to monitor disease progression and measure drug efficacy. A. fumigatus added to naïve, uninfected kidneys as either ungerminated conidia or small germlings yielded a linear qPCR response over at least 4 orders of magnitude. In a murine model of disseminated aspergillosis, a burden of A. fumigatus was detected in each of five different organs at 4 days postinfection by the qPCR assay, and the mean fungal load in these organs was 1.2 to 3.5 log(10) units greater than mean values determined by CFU measurement. When used to monitor disease progression in infected mice, the qPCR assay detected an increase of nearly 4 log(10) conidial equivalents/g of kidney between days 1 and 4 following infection, with a peak fungal burden that coincided with the onset of significant mortality. Traditional CFU methodology detected only a marginal increase in fungal load in the same tissues. In contrast, when mice were infected with Candida albicans, which does not form true mycelia in tissues, quantitation of kidney burden by both qPCR and CFU assays was strongly correlated as the infection progressed. Finally, treatment of mice with induced disseminated aspergillosis with either caspofungin or amphotericin B reduced the A. fumigatus burden in infected kidneys to the limit of detection for the qPCR assay. Because of its much larger dynamic range, the qPCR assay is superior to traditional CFU determination for monitoring the progression of disseminated aspergillosis and evaluating the activity of antifungal antibiotics against A. fumigatus.

Rapamycin and less immunosuppressive analogs are toxic to Candida albicans and Cryptococcus neoformans via FKBP12-dependent inhibition of TOR

Candida albicans and Cryptococcus neoformans cause both superficial and disseminated infections in humans. Current antifungal therapies for deep-seated infections are limited to amphotericin B, flucytosine, and azoles. A limitation is that commonly used azoles are fungistatic in vitro and in vivo. Our studies address the mechanisms of antifungal activity of the immunosuppressive drug rapamycin (sirolimus) and its analogs with decreased immunosuppressive activity. C. albicans rbp1/rbp1 mutant strains lacking a homolog of the FK506-rapamycin target protein FKBP12 were found to be viable and resistant to rapamycin and its analogs. Rapamycin and analogs promoted FKBP12 binding to the wild-type Tor1 kinase but not to a rapamycin-resistant Tor1 mutant kinase (S1972R). FKBP12 and TOR mutations conferred resistance to rapamycin and its analogs in C. albicans, C. neoformans, and Saccharomyces cerevisiae. Our findings demonstrate the antifungal activity of rapamycin and rapamycin analogs is mediated via conserved complexes with FKBP12 and Tor kinase homologs in divergent yeasts. Taken together with our observations that rapamycin and its analogs are fungicidal and that spontaneous drug resistance occurs at a low rate, these mechanistic findings support continued investigation of rapamycin analogs as novel antifungal agents.

The interaction of human monocytes, monocyte-derived macrophages, and polymorphonuclear neutrophils with caspofungin (MK-0991), an echinocandin, for antifungal activity against Aspergillus fumigatus

The collaboration between human effector cells and caspofungin (MK-0991), a 1,3-beta-D glucan synthase inhibitor, was studied for antifungal activity against Aspergillus fumigatus. Caspofungin was co-cultured for 24h with either human monocytes (Monos), monocyte-derived macrophages (MDM), or polymorphonuclear neutrophils (PMN) against germlings of A. fumigatus and antifungal activity assessed using the XTT metabolic assay. Caspofungin at 0.1 micorg/ml and 0.05 microg/ml or Monos alone against germlings caused significant inhibition. Microscopically this was correlated with less growth and stunted malformed hyphae. The addition of caspofungin at 0.1 microg/ml and 0.05 microg/ml to the monocyte cultures increased antifungal activity. The inhibition of the combination was significantly greater than drug alone (P <.01) and Monos alone (P <.01). MDM against Aspergillus germlings inhibited hyphal growth. The combination of caspofungin at 0.1 microg/ml and 0.05 microg/ml to the macrophage cultures increased antifungal activity. The growth inhibition by the combination was significantly greater than drug alone (P <.01) and MDM alone (P <.01). There was no significant interference with or enhancement of PMNs and caspofungin. These data support the activity of caspofungin against A. fumigatus in vitro, and indicates a cooperative activity with human effector cells. This suggests caspofungin in vivo would have increased efficacy as it combines with host defenses against A. fumigatus.

The role of murine models in the development of antifungal therapy for systemic mycoses

Animal testing is crucial to the development of new antifungal compounds. This review describes the role that murine and other animal models have played in the development of three classes of antifungal agents: the polyenes, the triazoles and the echinocandins and the ways in which these models have been either the positive link in the path from in vitro studies to the patient, or have foreclosed later clinical evaluation. Efficacy studies in particular mycoses are discussed, as well as studies designed to determine whether combinations of antifungal drugs may have value over single agents. Copyright 2000 Harcourt Publishers Ltd.

Comparison of the echinocandin caspofungin with amphotericin B for treatment of histoplasmosis following pulmonary challenge in a murine model

Twenty clinical isolates of Histoplasma capsulatum were tested for their in vitro susceptibilities to caspofungin in comparison to those to amphotericin B by following National Committee for Clinical Laboratory Standards guidelines for yeasts. The mean MICs were 16.6 microgram/ml (range, 8 to 32 microgram/ml) for caspofungin and 0.56 microgram/ml (range, 0.5 to 1.0 microgram/ml) for amphotericin B. Survival experiments used a 10(5) dose in a pulmonary challenge model with B6C3F(1) mice. All mice that received amphotericin B at 2 mg/kg of body weight every other day (q.o.d.), 30% of mice that received caspofungin at 8 mg/kg/day, and 20% of mice that received caspofungin at 4 mg/kg/day survived to day 15, while mice that received caspofungin at 2 mg/kg/day and all control mice that received the vehicle died by day 14. Amphotericin B at 2 mg/kg q.o.d. markedly reduced the fungal burden in the lungs and spleens, as measured by Histoplasma antigen detection techniques and quantitative cultures, for each comparison. Caspofungin at 10 mg/kg twice a day (b.i.d.) did not reduce the fungal burden, as measured by antigen detection techniques, but slightly reduced the levels of fungi in both the lungs and spleens, as determined by quantitative cultures. Caspofungin at 5 mg/kg b.i.d. did not affect fungal burden. Overall, caspofungin had only a slight effect on survival or fungal burden.

Efficacy of FK463, a (1,3)-beta-D-glucan synthase inhibitor, in disseminated azole-resistant candida albicans infection in mice

The efficacy of FK463, a new (1,3)-beta-D-glucan synthase inhibitor, against azole-resistant Candida albicans strains has been studied. The MIC of FK463 was lower than those of azoles and amphotericin B against CDR1-expressing C26 and CaMDR-expressing C40 strains. All mice treated with FK463 (1 mg/kg) survived disseminated murine candidiasis. The fungal burden in the kidney after 6 days was markedly reduced after therapy with FK463 and amphotericin B sodium deoxycholate, and plasma (1,3)-beta-D-glucan concentration was found to be lower in FK463-treated mice. In our study, FK463 was found to be a potent antifungal agent against disseminated infection with azole-resistant C. albicans.

Antifungal peptides: potential candidates for the treatment of fungal infections

Many diversely produced natural peptides, as well as those produced semisynthetically and synthetically, have been found to inhibit the growth or even be lethal to a wide range of fungi. Some of these have the potential to aid mankind in combating mycoses caused by emerging pathogens or as a result of the increasing number of antibiotic-resistant fungi. Antifungal peptides may also assist in non-medical fields such as agriculture. For example, introduction by transgenic research of antifungal peptides could improve crop production yields by increasing host resistance to fungal invasion. The aim of this review is to provide information on research on these important peptides.

In vitro activities of a new lipopeptide antifungal agent, FK463, against a variety of clinically important fungi

The in vitro antifungal activity and spectrum of FK463 were compared with those of amphotericin B, fluconazole, and itraconazole by using a broth microdilution method specified by National Committee for Clinical Laboratory Standards document M27-A (National Committee for Clinical Laboratory Standards, Wayne, Pa., 1997). FK463 exhibited broad-spectrum activity against clinically important pathogens including Candida species (MIC range, <==0.0039 to 2 microg/ml) and Aspergillus species (MIC range, <==0.0039 to 0.0313 microg/ml), and its MICs for such fungi were lower than those of the other antifungal agents tested. FK463 was also potently active against azole-resistant Candida albicans as well as azole-susceptible strains, and there was no cross-resistance with azoles. FK463 showed fungicidal activity against C. albicans, i.e., a 99% reduction in viability after a 24-h exposure at concentrations above 0.0156 microg/ml. The minimum fungicidal concentration (MFC) assays indicated that FK463 was fungicidal against most isolates of Candida species. In contrast, the MFCs of FK463 for A. fumigatus isolates were much higher than the MICs, indicating that its action is fungistatic against this species. FK463 had no activity against Cryptococcus neoformans, Trichosporon species, or Fusarium solani. Neither the test medium (kind and pH) nor the inoculum size greatly affected the MICs of FK463, while the addition of 4% human serum albumin increased the MICs for Candida species and A. fumigatus more than 32 times. Results from preclinical in vitro evaluations performed thus far indicate that FK463 should be a potent parenteral antifungal agent.

Efficacy of NND-502, a novel imidazole antimycotic agent, in experimental models of Candida albicans and Aspergillus fumigatus infections

In vitro and in vivo anti-Candida albicans and anti-Aspergillus fumigatus activities of NND-502, a new imidazole-antimycotic, were compared with those of fluconazole (FCZ), itraconazole (ITZ) and/or amphotericin B (AmB). NND-502 exhibited strong in vitro antifungal activity against both fungal species; its MIC against C. albicans was 1-4 times lower than that of FCZ, and its MIC against A. fumigatus was at least 60-2000 times lower than that of ITZ and AmB. In vivo antifungal treatments with each drug were initiated 1 h after inoculation in the experimental models, so that antifungal potential reflected prophylactic activity rather than therapeutic activity. The oral regimen of NND-502 in a murine model of systemic C. albicans infection was much less effective than that of FCZ. In vivo anti-A. fumigatus activity of oral NND-502 in a murine model of systemic infection was apparently superior to that of FCZ and ITZ in terms of prolonging survival. In addition to the murine model of systemic aspergillosis, intravenous NND-502 was shown to be highly effective in a rat model of pulmonary aspergillosis compared with intravenous AmB; 90% of animals survived at a dose of 2.5 mg/kg per day of NND-502 while only 30% of animals escaped death when 5 mg/kg per day of AmB was used. This potent efficacy of NND-502 was also confirmed in a sublethal challenge study in which the administration of the agent at a dose as low as 1.25 mg/kg per day resulted in the significant reduction of organisms in the lung; no comparable effect of AmB was found.

Identification of the FKS1 gene of Candida albicans as the essential target of 1,3-beta-D-glucan synthase inhibitors

Pneumocandins and echinocandins are fungicidal antibiotics, currently in clinical development, that inhibit 1,3-beta-D-glucan synthase (GS) in several human fungal pathogens. We have identified a gene from the diploid organism Candida albicans that encodes a target of these inhibitors. A 2.1-kb portion of this gene, designated CaFKS1, has significant homology to the Saccharomyces cerevisiae FKS1 and FKS2 genes, which encode partially functionally redundant subunits of GS. To evaluate the role of CaFkslp in susceptibility to echinocandins, we disrupted CaFKS1 on one homolog each of the spontaneous pneumocandin-resistant C. albicans mutants CAI4R1, NR2, NR3, and NR4. These mutants had been selected previously on agar plates containing the pneumocandin L-733,560. The clones derived from this transformation were either resistant (Ech[r]) or fully sensitive (Ech[s]) to inhibition by L-733,560 in both liquid broth microdilution and in vitro GS assays. The site of plasmid insertion in the transformants was mapped by Southern blot analysis, using restriction site polymorphisms in the CaFKS1 gene to distinguish between the two alleles (designated CaFKS1h and CaFKS1b). For strains CAI4R1 and NR2, the CaFKS1b allele was disrupted in each Ech(r) transformant; for strain NR4, CaFKS1h was disrupted in each Ech(r) transformant. We conclude that (i) strains CAI4R1, NR2, and NR4 are heterozygous for a dominant or semidominant pneumocandin resistance mutation at CaFKS1, (ii) drug resistance mutations can occur in either CaFKS1 allele, and (iii) CaFks1p is a target of the echinocandins. For transformants of strain NR3, all the clones we analyzed were uniformly Ech(r), and only the CaFKS1h allele, either in disrupted or wild-type form, was detected on genomic Southern blots. We believe gene conversion at the CaFKS1 locus may have produced two Cafks1h alleles that each contain an Ech(r) mutation. Transformants derived from the mutants were analyzed for susceptibility to pneumocandin treatment in a mouse model of disseminated candidiasis. Strains heterozygous for the resistant allele (i.e., C. albicans CAI4R1, NR2, and NR4) were moderately resistant to treatment, while strains without a functional Ech(s) allele (i.e., strain NR3 and derivatives of strain CAI4R1 with the disruption plasmid integrated in the Ech[s] allele) displayed strong in vivo echinocandin resistance. Finally, we were unable to inactivate both alleles at CaFKS1 by two-step integrative disruption, suggesting that CaFks1p is likely to be an essential protein in C. albicans.

Comparison of the in vitro activities of the echinocandin LY303366, the pneumocandin MK-0991, and fluconazole against Candida species and Cryptococcus neoformans

Two new glucan synthesis inhibitors, the echinocandin LY303366 and the pneumocandin MK-0991 (formerly L-743,872), were studied for their antifungal activities in vitro in relation to each other and in relation to the activity of the triazole fluconazole. Systematic analysis of broth macrodilution testing by varying the starting inoculum size, medium composition, medium pH, temperature of incubation, length of incubation, or selection of endpoints failed to identify significant differences in antifungal activity for either LY303366 or MK-0991 in comparison to the activity under standard test conditions specified for other antifungal agents in National Committee for Clinical Laboratory Standards (NCCLS) document M27A. Under standardized conditions, both drugs exhibited prominent activity against Candida species including Candida glabrata and Candida krusei but showed little activity against Cryptococcus neoformans. This spectrum of activity differed from that of fluconazole, which exhibited marginal activity against C. glabrata and C. krusei but prominent activity against other Candida species and C. neoformans. For individual strains, broth microdilution MICs of LY303366 and MK-0991 were similar to but frequently higher than broth macrodilution results. In contrast, fluconazole broth microdilution MICs were often lower than broth microdilution results. We conclude that the test conditions specified in NCCLS document M27A are applicable to these two new glucan synthesis inhibitors and that systematic differences between broth microdilution procedures and the broth macrodilution reference standard will need to be addressed before the two test methods can be used interchangeably.

Activity of voriconazole (UK-109,496) against clinical isolates of Aspergillus species and its effectiveness in an experimental model of invasive pulmonary aspergillosis

Voriconazole, a new azole antifungal agent, showed potent activity against clinical isolates of Aspergillus spp. in vitro. For A. fumigatus, the MIC range was < 0.03 to 0.5 microgram/ml and the MIC at which 90% of isolates are inhibited was 0.25 microgram/ml. In an experimental model of invasive pulmonary aspergillosis which mimics infection in humans, oral voriconazole at dosages of 30 mg/kg of body weight per day significantly delayed or prevented mortality.

Lipopeptide inhibitors of fungal glucan synthase

The echinocandins and pneumocandins are lipopeptide antifungal agents that inhibit the synthesis of 1,3-beta-D-glucan, an essential cell wall homopolysaccharide found in many pathogenic fungi. Compounds with this fungal-specific target have several attractive features: lack of mechanism-based toxicity, potential for fungicidal activity and activity against strains with intrinsic or acquired resistance mechanisms for existing antimycotics. Semi-synthetic analogues of naturally occurring lipopeptides are currently in clinical trials with the aim of treating systemic candidiasis and aspergillosis. Thus a fuller understanding of the target enzyme and its inhibition by these compounds should be useful for epidemiological and other clinical studies. Although it has been long known that lipopeptides inhibit fungal glucan synthase activity both in cell extracts and in whole cells, the genetic and biochemical identification of the proteins involved has been accomplished only recently. We now know that in Saccharomyces cerevisiae, glucan synthase is a heteromeric enzyme complex comprising one large integral membrane protein (specified by either FKS1 or by FKS2) and one small subunit more loosely associated with the membrane (specified by RHO1). Additional components may also be involved. The heteromeric enzyme complex containing Fks1p constitutes the majority of the activity found in vegetatively growing cells in this organism. The FKS2 gene product is needed for sporulation. Lipopeptides affect the function of the Fksp component from either FKS gene. The current model for interaction and regulation of these components in S. cerevisiae and the application to Candida albicans and other pathogenic fungi are discussed in this review.

Isolation of a gene involved in 1,3-beta-glucan synthesis in Aspergillus nidulans and purification of the corresponding protein

Saccharomyces cerevisiae has two highly homologous genes, FKS1 and FKS2, which encode interchangeable putative catalytic subunits of 1,3-beta-glucan synthase (GS), an enzyme that synthesizes an essential polymer of the fungal cell wall. To determine if GS in Aspergillus species is similar, an FKS homolog, fksA, was cloned from Aspergillus nidulans by cross-hybridization, and the corresponding protein was purified. Sequence analysis revealed a 5,716-nucleotide coding region interrupted by two 56-bp introns. The fksA gene encodes a predicted peptide of 229 kDa, FksAp, that shows a remarkable degree of conservation in size, charge, amino acid identity, and predicted membrane topology with the S. cerevisiae FKS proteins (Fksps). FksAp exhibits 64 and 65% identity to Fks1p and Fks2p, respectively, and 79% similarity. Hydropathy analysis of FksAp suggests an integral membrane protein with 16 transmembrane helices that coincide with the transmembrane helices of the Saccharomyces Fksps. The sizes of the nontransmembrane domains are strikingly similar to those of Fks1p. The region of FksAp most homologous to the Saccharomyces FKS polypeptides is a large hydrophilic domain of 578 amino acids that is predicted to be cytoplasmic. This domain is 86% identical to the corresponding region of Fks1p and is a good candidate for the location of the catalytic site. Antibodies raised against a peptide derived from the FksAp sequence recognize a protein of approximately 200 kDa in crude membranes and detergent-solubilized active extracts. This protein is enriched approximately 300-fold in GS purified by product entrapment. Purified anti-FksAp immunoglobulin G immunodepletes nearly all of the GS activity in crude or purified extracts when Staphylococcus aureus cells are used to precipitate the antibodies, although it does not inhibit enzymatic activity when added to extracts. The purified GS is inhibited by echinocandins with a sensitivity equal to that displayed by whole cells. Thus, the product of fksA is important for the activity of highly purified preparations of GS, either as the catalytic subunit itself or as an associated copurifying subunit that mediates susceptibility of enzymatic activity to echinocandin inhibition.

Isolation and sequence analysis of the cDNA encoding delta 1-pyrroline-5-carboxylate reductase from Zalerion arboricola

A cDNA encoding delta 1-pyrroline-5-carboxylate reductase (P5CR) was isolated from the pneumocandin (Pmo)-producing fungus, Zalerion arboricola (Za), by complementation of a P5CR-deficient mutant (pro3) of Saccharomyces cerevisiae (Sc). The cloned cDNA was placed under control of the Sc galactokinase (GAL1) promoter and restored P5CR activity to the pro3 mutant. Sequence analysis revealed that the Za P5CR-encoding cDNA encodes an approx. 35 kDa protein with substantial amino acid (aa) identity to P5CR from another filamentous fungus, Neurospora crassa (Nc). Za P5CR exhibits a moderate degree of aa identity to P5CR from plants, bacteria, human and Sc. This is the first gene to be isolated from Za.

In vitro activities of semisynthetic pneumocandin L-733,560 against fluconazole-resistant and -susceptible Candida albicans isolates

Lipopeptide L-733,560 is a water-soluble derivative of pneumocandin B0 that exhibits enhanced anti-Candida activity. We investigated the in vitro activity of L-733,560 compared with those of amphotericin B, flucytosine, and itraconazole, against fluconazole-resistant (n = 44) and fluconazole-susceptible (n = 46) Candida albicans isolates. Tests were performed with a photometer-read broth microdilution method with RPMI-2% glucose and National Committee for Clinical Laboratory Standards reference strains. Except for those of itraconazole, MICs were not significantly different between the two groups of isolates, as expected for agents with different mechanisms of action. L-733,560 was the most active agent against C.albicans, with MICs for 50 and 90% of the strains tested of 0.01 and 0.06 microgram/ml, respectively.