Amphotericin B: current understanding of mechanisms of action

Novel strategies to fight Candida species infection

In recent years, there has been a significant increase in the incidence of human fungal infections. The increase in cases of infection caused by Candida species, and the consequent excessive use of antimicrobials, has favored the emergence of resistance to conventional antifungal agents over the past decades. Consequently, Candida infections morbidity and mortality are also increasing. Therefore, new approaches are needed to improve the outcome of patients suffering from Candida infections, because it seems unlikely that the established standard treatments will drastically lower the morbidity of mucocutaneous Candida infections and the high mortality associated with invasive candidiasis. This review aims to present the last advances in the traditional antifungal therapy, and present an overview of novel strategies that are being explored for the treatment of Candida infections, with a special focus on combined antifungal agents, antifungal therapies with alternative compounds (plant extracts and essential oils), adjuvant immunotherapy, photodynamic therapy and laser therapy.

Quinacrine inhibits Candida albicans growth and filamentation at neutral pH

Candida albicans is a common cause of catheter-related bloodstream infections (CR-BSI), in part due to its strong propensity to form biofilms. Drug repurposing is an approach that might identify agents that are able to overcome antifungal drug resistance within biofilms. Quinacrine (QNC) is clinically active against the eukaryotic protozoan parasites Plasmodium and Giardia. We sought to investigate the antifungal activity of QNC against C. albicans biofilms. C. albicans biofilms were incubated with QNC at serially increasing concentrations (4 to 2,048 μg/ml) and assessed using a 2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide (XTT) assay in a static microplate model. Combinations of QNC and standard antifungals were assayed using biofilm checkerboard analyses. To define a mechanism of action, QNC was assessed for the inhibition of filamentation, effects on endocytosis, and pH-dependent activity. High-dose QNC was effective for the prevention and treatment of C. albicans biofilms in vitro. QNC with fluconazole had no interaction, while the combination of QNC and either caspofungin or amphotericin B demonstrated synergy. QNC was most active against planktonic growth at alkaline pH. QNC dramatically inhibited filamentation. QNC accumulated within vacuoles as expected and caused defects in endocytosis. A tetracycline-regulated VMA3 mutant lacking vacuolar ATPase (V-ATPase) function demonstrated increased susceptibility to QNC. These experiments indicate that QNC is active against C. albicans growth in a pH-dependent manner. Although QNC activity is not biofilm specific, QNC is effective in the prevention and treatment of biofilms. QNC antibiofilm activity likely occurs via several independent mechanisms: vacuolar alkalinization, inhibition of endocytosis, and impaired filamentation. Further investigation of QNC for the treatment and prevention of biofilm-related Candida CR-BSI is warranted.

Amphotericin B induces glial cell line-derived neurotrophic factor in the rat brain

Amphotericin B (AmB) is a polyene antifungal drug and is reported to be one of a few reagents having therapeutic effects on prion diseases, that is, a delay in the appearance of clinical signs and prolongation of the survival time in an animal model. In prion diseases, glial cells have been suggested to play important roles; however, the therapeutic mechanism of AmB on prion diseases remains elusive. We have previously reported that AmB changed the expression of neurotrophic factors in microglia and astrocytes (Motoyoshi et al., 2008, Neurochem. Int. 52, 1290–1296; Motoyoshi-Yamashiro et al., 2013, ibid. 63, 93–100). These results suggested that neurotrophic factors derived from glial cells might be involved in the therapeutic mechanism of AmB. In the present study, we examined immunohistochemically the effects of AmB on the expression of neurotrophic factors in the rat brain. We found that direct injection of AmB into the striatum significantly enhanced the expression of glial cell line-derived neurotrophic factor protein. Amphotericin B also increased the expressions of CD11b and glial fibrillary acidic protein, markers of microglia and astrocytes, respectively. Moreover, expressions of the two neurotrophic factors by AmB were co-localized with the expression of CD11b or glial fibrillary acidic protein. These results suggest that AmB in vivo might also activate glial cells and induce the production of neurotrophic factors protecting neurons in prion diseases.

Recent progress in the study of the interactions of amphotericin B with cholesterol and ergosterol in lipid environments

In the past decade substantial progress has been made in understanding the organization and biological activity of amphotericin B (AmB) in the presence of sterols in lipid environments. This review concentrates mainly on interactions of AmB with lipids and sterols, AmB channel formation in membranes, AmB aggregation, AmB modifications important for understanding its biological activity, and AmB models explaining its mechanism of action. Most of the reviewed studies concern monolayers at the water–gas interface, monolayers deposited on a solid substrate by use of the Langmuir–Blodgett technique, micelles, vesicles, and multi-bilayers. Liposomal AmB formulations and drug delivery are intentionally omitted, because several reviews dedicated to this subject are already available.

Pharmacokinetics and pharmacodynamics of antifungals in children: clinical implications

Invasive fungal disease (IFD) remains life threatening in premature infants and immunocompromised children despite the recent development of new antifungal agents. Optimal dosing of antifungals is one of the few factors clinicians can control to improve outcomes of IFD. However, dosing in children cannot be extrapolated from adult data because IFD pathophysiology, immune response, and drug disposition differ from adults. We critically examined the literature on pharmacokinetics (PK) and pharmacodynamics (PD) of antifungal agents and highlight recent developments in treating pediatric IFD. To match adult exposure in pediatric patients, dosing adjustment is necessary for almost all antifungals. In young infants, the maturation of renal and metabolic functions occurs rapidly and can significantly influence drug exposure. Fluconazole clearance doubles from birth to 28 days of life and, beyond the neonatal period, agents such as fluconazole, voriconazole, and micafungin require higher dosing than in adults because of faster clearance in children. As a result, dosing recommendations are specific to bracketed ranges of age. PD principles of antifungals mostly rely on in vitro and in vivo models but very few PD studies specifically address IFD in children. The exposure-response relationship may differ in younger children compared with adults, especially in infants with invasive candidiasis who are at higher risk of disseminated disease and meningoencephalitis, and by extension severe neurodevelopmental impairment. Micafungin is the only antifungal agent for which a specific target of exposure was proposed based on a neonatal hematogenous Candida meningoencephalitis animal model. In this review, we found that pediatric data on drug disposition of newer triazoles and echinocandins are lacking, dosing of older antifungals such as fluconazole and amphotericin B products still need optimization in young infants, and that target PK/PD indices need to be clinically validated for almost all antifungals in children. A better understanding of age-specific PK and PD of new antifungals in infants and children will help improve clinical outcomes of IFD by informing dosing and identifying future research areas.

Clinical utility of caspofungin eye drops in fungal keratitis

Treatment of fungal keratitis remains challenging. To date, only the polyenes and azoles are commonly used topically in the management of fungal keratitis. Natamycin, a polyene, is the only antifungal eye drop that is commercially available; the remainder are prepared in-house and are used in an 'off-label' manner. Failure of medical treatment for fungal keratitis is common, hence there is a need for more effective topical antifungal therapy. To increase the antifungal eye drop armamentarium, it is important to investigate the utility of other classes of antifungal agents for topical use. Caspofungin, an echinocandin antifungal agent, could potentially be used to address the existing shortcomings. However, little is known about the usefulness of topically administered caspofungin. This review will briefly explore the incidence, epidemiology and antifungal treatment of fungal keratitis. It will focus primarily on evidence related to the efficacy, safety and practicality of using caspofungin eye drops in fungal keratitis.

Combination therapy with tamoxifen and amphotericin B in experimental cutaneous leishmaniasis

Leishmaniasis chemotherapy remains very challenging. The high cost of active drugs, along with the severity of their side effects and the increasing failure rate of the current therapeutic schemes, calls for the discovery of new active drugs and schemes of treatment. The use of combination therapy has gained much attention in recent years as a possible strategy for overcoming the various shortcomings in the present arsenal. We recently described the effectiveness of tamoxifen in murine models of leishmaniasis, and here, we investigated the interactions between tamoxifen and amphotericin B, one of the most potent drugs used in leishmaniasis treatment. The in vitro interactions were indifferent for the association of tamoxifen and amphotericin B. The association was also assayed in vivo in Leishmania amazonensis-infected BALB/c mice and was found to yield at least additive effects at low doses of both drugs.

Neurotrophic natural products: chemistry and biology

Neurodegenerative diseases and spinal cord injury affect approximately 50 million people worldwide, bringing the total healthcare cost to over 600 billion dollars per year. Nervous system growth factors, that is, neurotrophins, are a potential solution to these disorders, since they could promote nerve regeneration. An average of 500 publications per year attests to the significance of neurotrophins in biomedical sciences and underlines their potential for therapeutic applications. Nonetheless, the poor pharmacokinetic profile of neurotrophins severely restricts their clinical use. On the other hand, small molecules that modulate neurotrophic activity offer a promising therapeutic approach against neurological disorders. Nature has provided an impressive array of natural products that have potent neurotrophic activities. This Review highlights the current synthetic strategies toward these compounds and summarizes their ability to induce neuronal growth and rehabilitation. It is anticipated that neurotrophic natural products could be used not only as starting points in drug design but also as tools to study the next frontier in biomedical sciences: the brain activity map project.

Amphotericin B topical microemulsion: formulation, characterization and evaluation

The present studies were designed to develop a microemulsion (ME) formulation of Amphotericin B (Amp B) for the treatment of invasive fungal infections. The oil phase was selected on the basis of drug solubility whereas the surfactant and co-surfactant were screened and selected on the basis of their oil solubilizing capacity as well as their efficiency to form ME. Pseudo-ternary phase diagrams were constructed and on the basis of ME existence ranges various formulations of Amp B were developed. The influence of surfactant and co-surfactant mass ratio (Smix) on the ME formation and permeation of ME through excised rat skin was studied. The optimized formulation (ME 7) consisting of 0.1% (w/w) Amp B, 5% (w/w) Isopropyl Myristate and 35% (w/w) Smix (3:1, Tween 80 and Propylene glycol), has shown a globule size of 84.20 ± 2.13 nm, a polydispersity index of 0.164 ± 0.031, pH 7.36 ± 0.02 and conductance of 229.3 ± 1.95 μS. ME 7 exhibited 2-fold higher drug permeation as compared to plain drug solution. Besides this, the formulation was also evaluated for drug content, stability, skin retention, skin sensitivity and anti-fungal activity. In vitro anti-fungal activity in Trichophyton rubrum fungal species have shown that ME7 has higher zone of inhibition and the formulation was found stable at 2-8°C and at room temperature (25 ± 2°C) for the period of three months. The results indicate that, the investigated ME may be used as a promising alternative for Amp B therapy.

ABC transporters coupled with the elevated ergosterol contents contribute to the azole resistance and amphotericin B susceptibility

Most screening approaches produce compounds that target survival genes and are likely to generate resistance over time. Simply having more drugs does not address the potential emergence of resistance caused by target mutation, drug efflux pumps over-expression, and so on. There is a great need to explore new strategies to treat fungal infections caused by drug-resistant pathogens. In this study, we found that azole-resistant Candida albicans with CaCDR1 and CaCDR2 over-expression is hypersensitive against amphotericin B (AmB) by our high throughput synergy screening (HTSS). In contrast, Δcdr1 and Δcdr2 knockout strains were resistant to AmB. Moreover, clinical isolates with increased expression of CaCDR1 and CaCDR2 demonstrated susceptibility to AmB, which can also synergize with the efflux pumps inducer fluphenazine (FPZ). Finally, the increased drug susceptibility to AmB in azole-resistant C. albicans with drug efflux pumps over-expression was consistent with the elevated expression of CaERG11 and its associated ergosterols in clinical isolates. Our data implies that the level of ergosterol contents determines the susceptibility to azoles and AmB in C. albicans. Deep understanding of the above mechanisms would offer new hope to treat drug-resistant C. albicans.

Reactive oxygen species-inducing antifungal agents and their activity against fungal biofilms

Invasive fungal infections are associated with very high mortality rates ranging from 20–90% for opportunistic fungal pathogens such as Candida albicans, Cryptococcus neoformans and Aspergillus fumigatus. Fungal resistance to antimycotic treatment can be genotypic (due to resistant strains) as well as phenotypic (due to more resistant fungal lifestyles, such as biofilms). With regard to the latter, biofilms are considered to be critical in the development of invasive fungal infections. However, there are only very few antimycotics, such as miconazole (azoles), echinocandins and liposomal formulations of amphotericin B (polyenes), which are also effective against fungal biofilms. Interestingly, these antimycotics all induce reactive oxygen species (ROS) in fungal (biofilm) cells. This review provides an overview of the different classes of antimycotics and novel antifungal compounds that induce ROS in fungal planktonic and biofilm cells. Moreover, different strategies to further enhance the antibiofilm activity of such ROS-inducing antimycotics will be discussed.

The effect of antifungal combination on transcripts of a subset of drug-resistance genes in clinical isolates of Candida species induced biofilms

Biofilm formation is often associated with increased Candida resistance toward antifungal agents. Therefore, the current study aimed to assess the incidence of biofilm formation among Candida isolates and to investigate the effect of high doses of fluconazole {FLC}, voriconazole {VOC} and amphotericin B {AMB}, singly and in combination on mature biofilms. Moreover, it aimed to assess the expression of selected genes (CDR1, KRE1 and SKN1) responsible for Candida biofilm resistance. The study included 49 patients; samples were collected from the King Khalid Hospital, Riyadh, Saudi Arabia. Isolates were prepared for biofilm formation and quantification using 0.4% (w/v) crystal violet. Minimum Inhibitory concentration (MIC) and fractional inhibitory concentration (FIC) were conducted by the broth microdilution method. Biofilm eradication was evaluated using counting, XTT stain intensity and observed under the inverted microscope. Selected genes were evaluated in Candida biofilms under the effect of antifungal exposure using QPCR. The major isolates were Candida albicans (65.3%) followed by Candida tropicalis and Candida glabrata. 77.6% of the strains were biofilm formers. AMB showed susceptibility in 87.8% of isolates, followed by VOC (77.6%) and FLC (67.3%). MIC50 and MIC90 were (0.03, 0.125), (0.5, 8), (2, >128) μg/ml for AMB, VOC and FLC, respectively. 34.7% and 18.4% of the isolates were antagonistic to AMB/FLC and AMB/VOC, respectively. Mature biofilms of ten selected isolates were found resistant to FLC (1000 μg/ml). VOR and AMB concentration required to inhibit biofilm formation was 16-250 fold higher than the MIC for planktonic cells. Isolates showed significant reduction with antifungal combination when compared with the untreated controls (p value ⩽ 0.01), or using fluconazole alone (p value ⩽ 0.05). High doses of the antifungals were employed to assess the effect on the persisters' selected gene expression. Marked over expression of SKN1 and to a lesser extent KRE1 was noticed among the mature biofilms treated with AMB alone or in combination after 1 h of exposure, and SKN1 expression was even more sharply induced after 24 h. No statistically significant over expression of CDR1 was observed in biofilms after exposure to high doses of FLC, VOC or any of the combinations used.

In vitro analyses of mild heat stress in combination with antifungal agents against Aspergillus fumigatus biofilm

Aspergillus fumigatus biofilms still present a challenge for effective treatment in clinical settings. While mild heat stress has been introduced as a treatment for infectious diseases, the effectiveness of mild heat stress on A. fumigatus biofilm formation and antifungal susceptibility is still unknown. In the present study, confocal laser scanning microscopy (CLSM) was used to image and quantify Aspergillus fumigatus biofilm formation under three different regimens of continuous mild heat stress: at 37, 39, and 41°C. Furthermore, fungal growth has been investigated under the above conditions in combination with antifungal drugs (amphotericin B [AMB], micafungin [MCF], and voriconazole [VOC]) at early and late stages. CLSM analysis showed that higher temperatures induce earlier germination and greater hyphal elongation but poorer polar growth and reduced biofilm thickness. In the early stage of biofilm formation, the combination of treatment at 39 or 41°C with MCF or VOC produced no visible difference in biomass formation from similar treatments at 37°C with the same drug. Interestingly, AMB treatment at 37°C inhibited early stage biofilm formation to a much greater extent than at 39 and 41°C. At the late stage of biofilm formation, the mild heat treatments at 39 and 41°C with AMB, MCF, and VOC inhibited biomass formation compared to that at 37°C. The present data show that mild heat stress has a negative regulatory effect on biofilm formation in vitro, and antifungal drug improvement with mild heat treatment at late-stage biofilm formation provides useful indications of possible effective strategies for clinical management of aspergillosis.

Biomaterials surfaces capable of resisting fungal attachment and biofilm formation

Microbial attachment onto biomedical devices and implants leads to biofilm formation and infection; such biofilms can be bacterial, fungal, or mixed. In the past 15 years, there has been an increasing research effort into antimicrobial surfaces but the great majority of these publications present research on bacteria, with some reports also testing resistance to fungi. Very few studies have focused exclusively on antifungal surfaces. However, with increasing recognition of the importance of fungal infections to human health, particularly related to infections at biomaterials, it would seem that the interest in antifungal surfaces is disproportionately low. In studies of both bacteria and fungi, fungi tend to be the minor focus with hypothesized antibacterial mechanisms of action often generalized to also explain the antifungal effect. Yet bacteria and fungi represent two Distinct biological Domains and possess substantially different cellular physiology and structure. Thus it is questionable whether these generalizations are valid. Here we review the scientific literature focusing on surface coatings prepared with antifungal agents covalently attached to the biomaterial surface. We present a critical analysis of generalizations and their evidence. This review should be of interest to researchers of "antimicrobial" surfaces by addressing specific issues that are key to designing and understanding antifungal biomaterials surfaces and their putative mechanisms of action.

Electric field-induced effects on yeast cell wall permeabilization

The permeability of the yeast cells (Saccharomyces cerevisiae) to lipophilic tetraphenylphosphonium cations (TPP(+) ) after their treatment with single square-shaped strong electric field pulses was analyzed. Pulsed electric fields (PEF) with durations from 5 to 150 µs and strengths from 0 to 10 kV/cm were applied to a standard electroporation cuvette filled with the appropriate buffer. The TPP(+) absorption process was analyzed using an ion selective microelectrode (ISE) and the plasma membrane permeability was determined by measurements obtained using a calcein blue dye release assay. The viability of the yeast and the inactivation of the cells were determined using the optical absorbance method. The experimental data taken after yeasts were treated with PEF and incubated for 3 min showed an increased uptake of TPP(+) by the yeast. This process can be controlled by setting the amplitude and pulse duration of the applied PEF. The kinetics of the TPP(+) absorption process is described using the second order absolute rate equation. It was concluded that the changes of the charge on the yeast cell wall, which is the main barrier for TPP(+) , is due to the poration of the plasma membrane. The applicability of the TPP(+) absorption measurements for the analysis of yeast cells electroporation process is also discussed.

Fitness trade-offs restrict the evolution of resistance to amphotericin B

The rarity of clinical drug resistance to the antifungal amphotericin B is explained by the extreme costs that resistance mutations impose upon stress responses and virulence factors.

The evolution of drug resistance in microbial pathogens provides a paradigm for investigating evolutionary dynamics with important consequences for human health. Candida albicans, the leading fungal pathogen of humans, rapidly evolves resistance to two major antifungal classes, the triazoles and echinocandins. In contrast, resistance to the third major antifungal used in the clinic, amphotericin B (AmB), remains extremely rare despite 50 years of use as monotherapy. We sought to understand this long-standing evolutionary puzzle. We used whole genome sequencing of rare AmB-resistant clinical isolates as well as laboratory-evolved strains to identify and investigate mutations that confer AmB resistance in vitro. Resistance to AmB came at a great cost. Mutations that conferred resistance simultaneously created diverse stresses that required high levels of the molecular chaperone Hsp90 for survival, even in the absence of AmB. This requirement stemmed from severe internal stresses caused by the mutations, which drastically diminished tolerance to external stresses from the host. AmB-resistant mutants were hypersensitive to oxidative stress, febrile temperatures, and killing by neutrophils and also had defects in filamentation and tissue invasion. These strains were avirulent in a mouse infection model. Thus, the costs of evolving resistance to AmB limit the emergence of this phenotype in the clinic. Our work provides a vivid example of the ways in which conflicting selective pressures shape evolutionary trajectories and illustrates another mechanism by which the Hsp90 buffer potentiates the emergence of new phenotypes. Developing antibiotics that deliberately create such evolutionary constraints might offer a strategy for limiting the rapid emergence of drug resistance.

The evolution of drug resistance in human pathogens is considered an inevitable consequence of the selective pressures imposed by antimicrobial drugs. Yet resistance to one antifungal drug, amphotericin B (AmB), remains extremely rare despite decades of widespread use. Here we explore the biological mechanisms underlying this conundrum. By examining natural and experimental populations of Candida albicans, we identify multiple mutations that confer resistance to AmB in vitro. As with the evolution of resistance to other antifungals, we find that the chaperone protein Hsp90 is involved in enabling the evolution of resistance to AmB. We also discover, however, that mutations that confer AmB resistance impose massive costs on other aspects of fungal pathogenicity; strains that are resistant to AmB are hypersensitive to attack by the host immune system and are unable to invade and damage host tissue. Thus, the evolution of resistance to AmB is restricted by a tradeoff between tolerance of the drug and the ability to cause disease. We propose that developing new antibiotics for which resistance presents such dire tradeoffs may be a promising strategy to prevent the evolution of resistance.

Glycogen synthase kinase 3 beta (GSK3β) at the tip of neuronal development and regeneration

Gaining a basic understanding of the inhibitory molecules and the intracellular signaling involved in axon development and repulsion after neural lesions is of clear biomedical interest. In recent years, numerous studies have described new molecules and intracellular mechanisms that impair axonal outgrowth after injury. In this scenario, the role of glycogen synthase kinase 3 beta (GSK3β) in the axonal responses that occur after central nervous system (CNS) lesions began to be elucidated. GSK3β function in the nervous tissue is associated with neural development, neuron polarization, and, more recently, neurodegeneration. In fact, GSK3β has been considered as a putative therapeutic target for promoting functional recovery in injured or degenerative CNS. In this review, we summarize current understanding of the role of GSK3β during neuronal development and regeneration. In particular, we discuss GSK3β activity levels and their possible impact on cytoskeleton dynamics during both processes.

Properties and mechanisms of action of naturally occurring antifungal peptides

Antimicrobial peptides are a vital component of the innate immune system of all eukaryotic organisms and many of these peptides have potent antifungal activity. They have potential application in the control of fungal pathogens that are a serious threat to both human health and food security. Development of antifungal peptides as therapeutics requires an understanding of their mechanism of action on fungal cells. To date, most research on antimicrobial peptides has focused on their activity against bacteria. Several antimicrobial peptides specifically target fungal cells and are not active against bacteria. Others with broader specificity often have different mechanisms of action against bacteria and fungi. This review focuses on the mechanism of action of naturally occurring antifungal peptides from a diverse range of sources including plants, mammals, amphibians, insects, crabs, spiders, and fungi. While antimicrobial peptides were originally proposed to act via membrane permeabilization, the mechanism of antifungal activity for these peptides is generally more complex and often involves entry of the peptide into the cell.

Self-aggregated nanoparticles based on amphiphilic poly(lactic acid)-grafted-chitosan copolymer for ocular delivery of amphotericin B

Background

The purpose of this study was to develop a self-aggregated nanoparticulate vehicle using an amphiphilic poly(lactic acid)-grafted-chitosan (PLA-g-CS) copolymer and to evaluate its potential for ocular delivery of amphotericin B.

Methods

A PLA-g-CS copolymer was synthesized via a “protection-graft-deprotection” procedure and its structure was confirmed by Fourier transform infrared spectroscopy, ¹H nuclear magnetic resonance, and X-ray diffraction spectra. Amphotericin B-loaded nanoparticles based on PLA-g-CS (AmB/PLA-g-CS) were prepared by the dialysis method and characterized for particle size, zeta potential, and encapsulation efficiency. Studies of these AmB/PLA-g-CS nanoparticles, including their mucoadhesive strength, drug release properties, antifungal activity, ocular irritation, ocular pharmacokinetics, and corneal penetration were performed in vitro and in vivo.

Results

Fourier transform infrared spectroscopy, ¹H nuclear magnetic resonance, and X-ray diffraction spectra showed that the PLA chains were successfully grafted onto chitosan molecules and that crystallization of chitosan was suppressed. The self-aggregated PLA-g-CS nanoparticles had a core-shell structure with an average particle size of approximately 200 nm and zeta potentials higher than 30 mV. Amphotericin B was incorporated into the hydrophobic core of the nanoparticles with high encapsulation efficiency. Sustained drug release from the nanoparticles was observed in vitro. The ocular irritation study showed no sign of irritation after instillation of the PLA-g-CS nanoparticles into rabbit eyes. The minimal inhibitory concentration of the AmB/PLA-g-CS nanoparticles showed antifungal activity similar to that of free amphotericin B against Candida albicans. The in vivo ocular pharmacokinetic study suggested that the PLA-g-CS nanoparticles have the advantage of prolonging residence time at the ocular surface. The corneal penetration study showed that the PLA-g-CS nanoparticles could penetrate into the cornea.

Conclusion

Our results suggest that this nanoparticulate vehicle based on a PLA-g-CS copolymer might be a promising system for effective ocular delivery of amphotericin B.

Cryptococcal meningitis: improving access to essential antifungal medicines in resource-poor countries

Cryptococcal meningitis is the leading cause of adult meningitis in sub-Saharan Africa, and contributes up to 20% of AIDS-related mortality in low-income and middle-income countries every year. Antifungal treatment for cryptococcal meningitis relies on three old, off-patent antifungal drugs: amphotericin B deoxycholate, flucytosine, and fluconazole. Widely accepted treatment guidelines recommend amphotericin B and flucytosine as first-line induction treatment for cryptococcal meningitis. However, flucytosine is unavailable in Africa and most of Asia, and safe amphotericin B administration requires patient hospitalisation and careful laboratory monitoring to identify and treat common side-effects. Therefore, fluconazole monotherapy is widely used in low-income and middle-income countries for induction therapy, but treatment is associated with significantly increased rates of mortality. We review the antifungal drugs used to treat cryptococcal meningitis with respect to clinical effectiveness and access issues specific to low-income and middle-income countries. Each drug poses unique access challenges: amphotericin B through cost, toxic effects, and insufficiently coordinated distribution; flucytosine through cost and scarcity of registration; and fluconazole through challenges in maintenance of local stocks--eg, sustainability of donations or insufficient generic supplies. We advocate ten steps that need to be taken to improve access to safe and effective antifungal therapy for cryptococcal meningitis.

Activation of cultured astrocytes by amphotericin B: stimulation of NO and cytokines production and changes in neurotrophic factors production

Amphotericin B (AmB) is a polyene antibiotic and reported to be one of a few reagents having therapeutic effects on prion diseases, such as the delay in the appearing of the clinical signs and the prolongation of the survival time. In prion diseases, glial cells have been suggested to play important roles by proliferating and producing various factors such as nitric oxide, proinflammatory cytokines, and neurotrophic factors. However, the therapeutic mechanism of AmB on prion diseases remains elusive. We have previously reported that AmB changed the expression of neurotoxic and neurotrophic factors in microglia (Motoyoshi et al., 2008, Neurochem. Int. 52, 1290-1296). In the present study, we examined the effects of AmB on cellular functions of rat cultured astrocytes. We found that AmB could activate astrocytes to produce nitric oxide via inducible nitric oxide synthase induction. AmB also induced mRNA expression of interleukin-1β and tumor necrosis factor-α, and productions of their proteins in astrocytes. Moreover, AmB changed levels of neurotrophic factor mRNAs and proteins. Among three neurotrophic factors examined here, neurotrophin-3 mRNA expression and its protein production in the cells were down-regulated by AmB stimulation. On the other hand, AmB significantly enhanced the amounts of glial cell line-derived neurotrophic factor and brain-derived neurotrophic factor proteins in the cells and the medium. These results suggest that AmB might show therapeutic effects on prion diseases by controlling the expression and production of such mediators in astrocytes.

Effect of amphotericin B on the metabolic profiles of Candida albicans

Amphotericin B (AmB) is a polyene antifungal drug widely used for systemic fungal infections. In this study, a metabonomic method using gas chromatography-mass spectrometry (GC/MS) was developed to characterize the metabolic profiles of Candida albicans cells exposed to AmB. Thirty-one differentially produced metabolites between AmB-treated and the control groups were identified, among which 10 metabolites were upregulated and 21 metabolites were downregulated. These differentially produced metabolites were mainly involved in polyamines synthesis, tricarboxylic acid (TCA) cycle, oxidative stress, glutathione metabolism, lipid synthesis and glycolysis. Further experiments showed that the polyamines including putrescine, spermidine, and spermine played an important role in the sensitivity of C. albicans cells upon AmB treatment, and combined use of AmB and inhibitors of polyamine biosynthesis pathway might be a potential antifungal strategy. This study provided a systemic view of the metabolic pattern in C. albicans upon exposure to AmB, which shed new light on the mechanisms of action of antifungal agents.

Differential internalization of amphotericin B--conjugated nanoparticles in human cells and the expression of heat shock protein 70

Although a variety of nanoparticles (NPs) functionalized with amphotericin B, an antifungal agent widely used in the clinic, have been studied in the last years their cytotoxicity profile remains elusive. Here we show that human endothelial cells take up high amounts of silica nanoparticles (SNPs) conjugated with amphotericin B (AmB) (SNP-AmB) (65.4 ± 12.4 pg of Si per cell) through macropinocytosis while human fibroblasts internalize relatively low amounts (2.3 ± 0.4 pg of Si per cell) because of their low capacity for macropinocytosis. We further show that concentrations of SNP-AmB and SNP up to 400 μg/mL do not substantially affect fibroblasts. In contrast, endothelial cells are sensitive to low concentrations of NPs (above 10 μg/mL), in particular to SNP-AmB. This is because of their capacity to internalize high concentration of NPs and high sensitivity of their membrane to the effects of AmB. Low-moderate concentrations of SNP-AmB (up to 100 μg/mL) induce the production of reactive oxygen species (ROS), LDH release, high expression of pro-inflammatory cytokines and chemokines (IL-8, IL-6, G-CSF, CCL4, IL-1β and CSF2) and high expression of heat shock proteins (HSPs) at gene and protein levels. High concentrations of SNP-AmB (above 100 μg/mL) disturb membrane integrity and kill rapidly human cells (60% after 5 h). This effect is higher in SNP-AmB than in SNP.

Candida infections and their prevention

Infections caused by Candida species have been increased dramatically worldwide due to the increase in immunocompromised patients. For the prevention and cure of candidiasis, several strategies have been adopted at clinical level. Candida infected patients are commonly treated with a variety of antifungal drugs such as fluconazole, amphotericin B, nystatin, and flucytosine. Moreover, early detection and speciation of the fungal agents will play a crucial role for administering appropriate drugs for antifungal therapy. Many modern technologies like MALDI-TOF-MS, real-time PCR, and DNA microarray are being applied for accurate and fast detection of the strains. However, during prolonged use of these drugs, many fungal pathogens become resistant and antifungal therapy suffers. In this regard, combination of two or more antifungal drugs is thought to be an alternative to counter the rising drug resistance. Also, many inhibitors of efflux pumps have been designed and tested in different models to effectively treat candidiasis. However, most of the synthetic drugs have side effects and biomedicines like antibodies and polysaccharide-peptide conjugates could be better alternatives and safe options to prevent and cure the diseases. Furthermore, availability of genome sequences of Candida   albicans and other non-albicans strains has made it feasible to analyze the genes for their roles in adherence, penetration, and establishment of diseases. Understanding the biology of Candida species by applying different modern and advanced technology will definitely help us in preventing and curing the diseases caused by fungal pathogens.

Determination of antifungal susceptibility patterns among the clinical isolates of Candida species

CONTEXT

Candida species are opportunistic yeasts that cause infections ranging from simple dermatosis to potentially life-threatening fungemia. The emergence of resistance to antifungal drugs has been increased in the past two decades.

AIM

the present study we determined to find out the susceptibility profiles of clinical isolates of Candida species against four antifungal drugs, including amphotericin B, ketoconazole, fluconazole and itraconazole.

MATERIALS AND METHODS

Antifungal susceptibility testing of the yeasts was done in accordance with the proposed guidelines for antifungal disk diffusion susceptibility testing of yeasts based on the CLSI document M44-A.

RESULTS

A total of 206 yeast isolates were assessed. Among the evaluated Candida species, the highest rates of resistance to ketoconazole were seen in Candida glabrata (16.6%) and Candida albicans (3.2%). Susceptibility and intermediate response to fluconazole were seen in 96.6% and 3.4% of the Candida isolates, respectively. A total of 19 (9.2%) yeast isolates showed petite phenomenon including 11 C. glabrata, 3 C. albicans, 2 Candida dubliniensis and one isolate of each Candida krusei and Candida parapsilosis.

CONCLUSION

The high number of petite mutation in the isolated yeasts should be seriously considered since it may be one of the reasons of antifungal treatment failure.

It only takes one to do many jobs: Amphotericin B as antifungal and immunomodulatory drug

“Amphotericin B acts through pore formation at the cell membrane after binding to ergosterol” is an accepted dogma about the action mechanism of this antifungal, and this sentence is widely found in the literature. But after 60 years of investigation, the action mechanism of Amphotericin B is not fully elucidated. Amphotericin B is a polyene substance that is one of the most effective drugs for the treatment of fungal and parasite infections. As stated above, the first mechanism of action described was pore formation after binding to the ergosterol present in the membrane. But it has also been demonstrated that AmB induces oxidative damage in the cells. Moreover, amphotericin B modulates the immune system, and this activity has been related to the protective effect of the molecule, but also to its toxicity in the host. This review tries to provide a general overview of the main aspects of this molecule, and highlight the multiple effects that this molecule has on both the fungal and host cells.

Apoptotic-like programed cell death in fungi: the benefits in filamentous species

Studies conducted in the early 1990s showed for the first time that Saccharomyces cerevisiae can undergo cell death with hallmarks of animal apoptosis. These findings came as a surprise, since suicide machinery was unexpected in unicellular organisms. Today, apoptosis in yeast is well-documented. Apoptotic death of yeast cells has been described under various conditions and S. cerevisiae homologs of human apoptotic genes have been identified and characterized. These studies also revealed fundamental differences between yeast and animal apoptosis; in S. cerevisiae apoptosis is mainly associated with aging and stress adaptation, unlike animal apoptosis, which is essential for proper development. Further, many apoptosis regulatory genes are either missing, or highly divergent in S. cerevisiae. Therefore, in this review we will use the term apoptosis-like programed cell death (PCD) instead of apoptosis. Despite these significant differences, S. cerevisiae has been instrumental in promoting the study of heterologous apoptotic proteins, particularly from human. Work in fungi other than S. cerevisiae revealed differences in the manifestation of PCD in single cell (yeasts) and multicellular (filamentous) species. Such differences may reflect the higher complexity level of filamentous species, and hence the involvement of PCD in a wider range of processes and life styles. It is also expected that differences might be found in the apoptosis apparatus of yeast and filamentous species. In this review we focus on aspects of PCD that are unique or can be better studied in filamentous species. We will highlight the similarities and differences of the PCD machinery between yeast and filamentous species and show the value of using S. cerevisiae along with filamentous species to study apoptosis.

Identification of yeast genes that confer resistance to chitosan oligosaccharide (COS) using chemogenomics

Background

Chitosan oligosaccharide (COS), a deacetylated derivative of chitin, is an abundant, and renewable natural polymer. COS has higher antimicrobial properties than chitosan and is presumed to act by disrupting/permeabilizing the cell membranes of bacteria, yeast and fungi. COS is relatively non-toxic to mammals. By identifying the molecular and genetic targets of COS, we hope to gain a better understanding of the antifungal mode of action of COS.

Results

Three different chemogenomic fitness assays, haploinsufficiency (HIP), homozygous deletion (HOP), and multicopy suppression (MSP) profiling were combined with a transcriptomic analysis to gain insight in to the mode of action and mechanisms of resistance to chitosan oligosaccharides. The fitness assays identified 39 yeast deletion strains sensitive to COS and 21 suppressors of COS sensitivity. The genes identified are involved in processes such as RNA biology (transcription, translation and regulatory mechanisms), membrane functions (e.g. signalling, transport and targeting), membrane structural components, cell division, and proteasome processes. The transcriptomes of control wild type and 5 suppressor strains overexpressing ARL1, BCK2, ERG24, MSG5, or RBA50, were analyzed in the presence and absence of COS. Some of the up-regulated transcripts in the suppressor overexpressing strains exposed to COS included genes involved in transcription, cell cycle, stress response and the Ras signal transduction pathway. Down-regulated transcripts included those encoding protein folding components and respiratory chain proteins. The COS-induced transcriptional response is distinct from previously described environmental stress responses (i.e. thermal, salt, osmotic and oxidative stress) and pre-treatment with these well characterized environmental stressors provided little or any resistance to COS.

Conclusions

Overexpression of the ARL1 gene, a member of the Ras superfamily that regulates membrane trafficking, provides protection against COS-induced cell membrane permeability and damage. We found that the ARL1 COS-resistant over-expression strain was as sensitive to Amphotericin B, Fluconazole and Terbinafine as the wild type cells and that when COS and Fluconazole are used in combination they act in a synergistic fashion. The gene targets of COS identified in this study indicate that COS’s mechanism of action is different from other commonly studied fungicides that target membranes, suggesting that COS may be an effective fungicide for drug-resistant fungal pathogens.

Effect of amphotericin B nanodisks on plant fungal diseases

BACKGROUND

The development of water-soluble nanodevices extends the potential use of compounds developed for other purposes (e.g. antifungal drugs or antibiotics) for applications in agriculture. For example, the broad-spectrum, water-insoluble, macrolide polyene antibiotic amphotericin B (AMB) could be used to inhibit phytopathogenic fungi. A new formulation embedding AMB in nanodisks (NDs) enhances antibiotic solubility and confers protection against environmental damage. In the present study, AMB-NDs were tested for efficacy against several phytopathogenic fungi in vitro and on infected living plants (chickpea and wheat).

RESULTS

Compared with AMB in dimethylsulfoxide (DMSO), AMB-NDs increased the sensitivity of several fungal species to this antimycotic in vitro. Sensitivity varied with fungal species as well as with the forma specialis. Phytophthora cinnamomi, previously reported as insensitive to other polyene antimycotics, remained unaffected at the doses examined. Some effect against disease symptoms were obtained with AMB-NDs against fusarium wilt in chickpea, whereas the results were highly variable in wheat, depending on both the species and treatment regimen.

CONCLUSION

The results confirm that formulation of AMB into ND increases its effectiveness against phytopathogenic fungi in vitro, opening the possibility for its use on infected plants in the field.

Mechanism of amphotericin B resistance in clinical isolates of Leishmania donovani

The clinical value of amphotericin B, the mainstay therapy for visceral leishmaniasis in sodium antimony gluconate-nonresponsive zones of Bihar, India, is now threatened by the emergence of acquired drug resistance, and a comprehensive understanding of the underlying mechanisms is the need of the hour. We have selected an amphotericin B-resistant clinical isolate which demonstrated 8-fold-higher 50% lethal doses (LD(50)) than an amphotericin B-sensitive strain to explore the mechanism of amphotericin B resistance. Fluorimetric analysis demonstrated lower anisotropy in the motion of the diphenylhexatriene fluorescent probe in the resistant strain, which indicated a higher fluidity of the membrane for the resistant strain than for the sensitive strain. The expression patterns of the two transcripts of S-adenosyl-l-methionine:C-24-Δ-sterol methyltransferase and the absence of ergosterol, replaced by cholesta-5,7,24-trien-3β-ol in the membrane of the resistant parasite, indicate a decreased amphotericin B affinity, which is evidenced by decreased amphotericin B uptake. The expression level of MDR1 is found to be higher in the resistant strain, suggesting a higher rate of efflux of amphotericin B. The resistant parasite also possesses an upregulated tryparedoxin cascade and a more-reduced intracellular thiol level, which helps in better scavenging of reactive oxygen species produced by amphotericin B. The resistance to amphotericin B was partially reverted by the thiol metabolic pathway and ABC transporter inhibitors. Thus, it can be concluded that altered membrane composition, ATP-binding cassette transporters, and an upregulated thiol metabolic pathway have a role in conferring amphotericin B resistance in clinical isolates of Leishmania donovani.

Pretreatment with an ethanolic extract of Taiwanofungus camphoratus (Antrodia camphorata) enhances the cytotoxic effects of amphotericin B

Taiwanofungus camphoratus, a well-known Chinese medicine used in Taiwan, possesses several pharmacological functions, including anticancer effects. In the present study, we aimed to investigate a novel anticancer effect by pretreating cancer cells with an ethanolic extract of T. camphoratus (TCEE) followed by the administration of an antifungal agent amphotericin B (AmB). Both TCEE and AmB showed significant dose-dependent cytotoxicity in HT29 cells. Pretreatment with a nontoxic dose of TCEE enhanced the cytotoxicity of AmB. Furthermore, significant apoptotic cell death was found in cells treated with TCEE and AmB. A combination treatment with AmB plus TCEE resulted in a significant repression of tumor growth in HT29 xenografts. Collectively, our results indicated that combined treatment with AmB and TCEE effectively induced apoptosis and inhibited tumor growth. In the future, TCEE may serve as a potential complementary and alternative medicine to treat patients suffering from colorectal cancer.

Antimycotic-antibiotic amphotericin B promotes influenza virus replication in cell culture

In general, antibiotics are not rated as substances that inhibit or support influenza virus replication. We describe here the enhancing effect of the polyene antibiotic amphotericin B (AmB) on influenza virus growth in Vero cells. We show that isolation rates of influenza A and B viruses from clinical samples can be dramatically enhanced by adding AmB to the culture medium. We demonstrate that AmB promotes the viral uptake and endocytic processing of the virus particles. This effect is specific for Vero and human nasal epithelial cells and was not observed in Madin-Darby canine kidney cells. The effect of AmB was subtype specific and more prominent for human seasonal influenza strains but absent for H5N1 human viruses. The AmB-enhancing effect seemed to be solely due to the viral hemagglutinin function. Our results indicate that the use of AmB may facilitate influenza virus isolation and production in Vero cells.

One-pot synthesis of a new antifungal polymerisable monomer and its characterisation by coordination-ion spray mass spectrometry

We have investigated the synthesis of a new antifungal agent with a polymerisable moiety for the prevention of denture stomatisis. Nystatin (antifungal polyene) is modified in one step by reaction with isocyanatoethylmethacrylate to afford a new polymerisable antifungal agent in good yield (90%). In order to prove the monografting of the acrylate derivative and to localise the new group in the skeleton of the molecule, a rapid and efficient analytical method involving electrospray ionisation mass spectrometry (ESI-MS) was developed for the study. In view of the structures of such antifungal agents, their complexation with metal cations was investigated by Coordination-Ion Spray Mass Spectrometry (CIS-MS). This mass spectrometry study covers two aspects: improving the MS signal to overcome the low ionisation efficiency in ESI-MS and exploring the complexation behaviour of the induced structure to optimise the antifungal properties.

Current concepts in antifungal pharmacology

The introduction of new antifungal agents (eg, echinocandins, second-generation triazoles) in the past decade has transformed the management of invasive mycoses to the point that drug toxicity is no longer the major limiting factor in treatment. Yet, many of these newer antifungal agents have important limitations in their spectrum of activity, pharmacokinetics, and unique predisposition for pharmacokinetic drug-drug interactions and unusual toxicities associated with long-term use. This article reviews key pharmacological aspects of systemic antifungal agents as well as evolving strategies, such as pharmacokinetic-pharmacodynamic optimization and therapeutic drug monitoring, to improve the safety and efficacy of systemic antifungal therapy.

Probable Mechanism(s) of Antifungal Activity of SJA-95, a Heptaene Polyene Antibiotic

A new strain, streptomyces sp. S. 24 was isolated from a soil sample collected from Japan. The strain produced heptaene polyene antibiotic, SJA-95, in submerged culture and found to elicit promising antifungal activity against yeasts, filamentous fungi and clinical isolates, both in vitro and in vivo. Experimental studies were carried out using biological methods to understand the probable mechanism(s) of antifungal activity of SJA-95. Our experimental findings suggest that SJA-95 binds more avidly to ergosterol, the sterol in fungal cell membranes, than to cholesterol found in mammalian cell membranes. Such preferential binding of SJA-95 to ergosterol might help to establish its usefulness as a chemotherapeutic agent with lesser adverse reactions.

Amphotericin B induces trehalose synthesis and simultaneously activates an antioxidant enzymatic response in Candida albicans

BACKGROUND

Enzymes involved in trehalose metabolism have been proposed as potential targets for new antifungals. To analyse this proposal, the susceptibility to Amphotericin B (AmB) of the C. albicans trehalose-deficient mutant tps1Δ/tps1Δ, was examined.

METHODS

Determination of endogenous trehalose and antioxidant enzymatic activities as well as RT-PCR analysis in cells subjected to AmB treatments was performed.

RESULTS

Exponential tps1Δ null cultures showed high degree of cell killing upon exposure to increasing AmB doses respect to CAI.4 parental strain. Reintroduction of the TPS1 gene restored the percentage of cell viability. AmB induced significant synthesis of endogenous trehalose in parental cells, due to the transitory accumulation of TPS1 mRNA or to the moderate activation of trehalose synthase (Tps1p) with the simultaneous deactivation of neutral trehalase (Ntc1p). Since tps1Δ/tps1Δ mutant cells are highly susceptible to acute oxidative stress, the putative antioxidant response to AmB was also measured. A conspicuous activation of catalase and glutathione reductase (GR), but not of superoxide dismutase (SOD), was observed when the two cell types were exposed to high concentrations of AmB (5μg/ml). However, no significant differences were detected between parental and tps1Δ null strains as regards the level of activities.

CONCLUSIONS

The protective intracellular accumulation of trehalose together with the induction of antioxidant enzymatic defences are worthy mechanisms involved in the resistance of C. albicans to the fungicidal action of AmB.

GENERAL SIGNIFICANCE

The potential usefulness of trehalose synthesis proteins as an interesting antifungal target is reinforced. More importantly, AmB elicits a complex defensive response in C. albicans.