Translation elongation factor 2 is part of the target for a new family of antifungals

Sordarin - the antifungal antibiotic with a unique modus operandi

Fungal infections cause serious problems in many aspects of human life, in particular infections in immunocompromised patients represent serious problems. Current antifungal antibiotics target various metabolic pathways, predominantly the cell wall or cellular membrane. Numerous compounds are available to combat fungal infections, but their efficacy is far from being satisfactory and some of them display high toxicity. The emerging resistance represents a serious issue as well; hence, there is a considerable need for new anti-fungal compounds with lower toxicity and higher effectiveness. One of the unique antifungal antibiotics is sordarin, the only known compound that acts on the fungal translational machinery per se. Sordarin inhibits protein synthesis at the elongation step of the translational cycle, acting on eukaryotic translation elongation factor 2. In this review, we intend to deliver a robust scientific platform promoting the development of antifungal compounds, in particular focusing on the molecular action of sordarin.

Melleolides impact fungal translation via elongation factor 2

Melleolides from the honey mushroom Armillaria mellea represent a structurally diverse group of polyketide-sesquiterpene hybrids. Among various bioactivites, melleolides show antifungal effects against Aspergillus and other fungi. This bioactivity depends on a Δ2,4-double bond present in dihydroarmillylorsellinate (DAO) or arnamial, for example. Yet, the mode of action of Δ2,4-unsaturated, antifungal melleolides has been unknown. Here, we report on the molecular target of DAO in the fungus Aspergillus nidulans. Using a combination of synthetic chemistry to create a DAO-labelled probe, protein pulldown assays, MALDI-TOF-based peptide analysis and western blotting, we identify the eukaryotic translation elongation factor 2 (eEF2) as a binding partner of DAO. We confirm the inhibition of protein biosynthesis in vivo with an engineered A. nidulans strain producing the red fluorescent protein mCherry. Our work suggests a binding site dissimilar from that of the protein biosynthesis inhibitor sordarin, and highlights translational elongation as a valid antifungal drug target.

Systemic Antifungal Agents: Current Status and Projected Future Developments

By definition, an antifungal agent is a drug that selectively destroys fungal pathogens with minimal side effects to the host. Despite an increase in the prevalence of fungal infections particularly in immunocompromised patients, only a few classes of antifungal drugs are available for therapy, and they exhibit limited efficacy in the treatment of life-threatening infections. These drugs include polyenes, azoles, echinocandins, and nucleoside analogs. This chapter focuses on the currently available classes and representatives of systemic antifungal drugs in clinical use. We further discuss the unmet clinical needs in the antifungal research field; efforts in reformulation of available drugs such as Amphotericin B nanoparticles for oral drug delivery; development of new agents of known antifungal drug classes, such as albaconazole, SCY-078, and biafungin; and new drugs with novel targets for treatment of invasive fungal infections, including nikkomycin Z, sordarin derivatives, VT-1161 and VT-1129, F901318, VL-2397, and T-2307.

A novel multiple-stage antimalarial agent that inhibits protein synthesis

There is an urgent need for new drugs to treat malaria, with broad therapeutic potential and novel modes of action, to widen the scope of treatment and to overcome emerging drug resistance. Here we describe the discovery of DDD107498, a compound with a potent and novel spectrum of antimalarial activity against multiple life-cycle stages of the Plasmodium parasite, with good pharmacokinetic properties and an acceptable safety profile. DDD107498 demonstrates potential to address a variety of clinical needs, including single-dose treatment, transmission blocking and chemoprotection. DDD107498 was developed from a screening programme against blood-stage malaria parasites; its molecular target has been identified as translation elongation factor 2 (eEF2), which is responsible for the GTP-dependent translocation of the ribosome along messenger RNA, and is essential for protein synthesis. This discovery of eEF2 as a viable antimalarial drug target opens up new possibilities for drug discovery.

Structural insights into the mechanism of translational inhibition by the fungicide sordarin

The translational machinery has been found to be the target for a number of antibiotics. One such antibiotic sordarin selectively inhibits fungal translation by impairing the function of elongation factor 2 (eEF2) while being ineffective to higher eukaryotes. Surprisingly, sordarin is not even equally effective in impairing translation for all fungal species. The binding cavity of sordarin on eEF2 has been localized by X-ray crystallographic study and its unique specificity towards sordarin has been attributed to the species specific substitutions within a stretch of amino acids (sordarin specificity region, SSR) at the entrance of the cavity. In this study, we have analyzed the sordarin-binding cavity of eEF2 from different species both in isolated and ribosome-bound forms in order to decipher the mechanism of sordarin binding selectivity. Our results reveal that the molecular architecture as well as the microenvironment of the sordarin-binding cavity changes significantly from one species to another depending on the species specific substitutions within the cavity. Moreover, eEF2 binding to ribosome aggravates the effects of these substitutions. Thus, this study, while shedding light on the molecular mechanism underpinning the selective inhibitory effects of sordarin, will also be a helpful guide for future studies aiming at developing novel antifungal drugs with broader spectrum of activity.

Yeast ribosomal protein L40 assembles late into precursor 60 S ribosomes and is required for their cytoplasmic maturation

Most ribosomal proteins play important roles in ribosome biogenesis and function. Here, we have examined the contribution of the essential ribosomal protein L40 in these processes in the yeast Saccharomyces cerevisiae. Deletion of either the RPL40A or RPL40B gene and in vivo depletion of L40 impair 60 S ribosomal subunit biogenesis. Polysome profile analyses reveal the accumulation of half-mers and a moderate reduction in free 60 S ribosomal subunits. Pulse-chase, Northern blotting, and primer extension analyses in the L40-depleted strain clearly indicate that L40 is not strictly required for the precursor rRNA (pre-rRNA) processing reactions but contributes to optimal 27 SB pre-rRNA maturation. Moreover, depletion of L40 hinders the nucleo-cytoplasmic export of pre-60 S ribosomal particles. Importantly, all these defects most likely appear as the direct consequence of impaired Nmd3 and Rlp24 release from cytoplasmic pre-60 S ribosomal subunits and their inefficient recycling back into the nucle(ol)us. In agreement, we show that hemagglutinin epitope-tagged L40A assembles in the cytoplasm into almost mature pre-60 S ribosomal particles. Finally, we have identified that the hemagglutinin epitope-tagged L40A confers resistance to sordarin, a translation inhibitor that impairs the function of eukaryotic elongation factor 2, whereas the rpl40a and rpl40b null mutants are hypersensitive to this antibiotic. We conclude that L40 is assembled at a very late stage into pre-60 S ribosomal subunits and that its incorporation into 60 S ribosomal subunits is a prerequisite for subunit joining and may ensure proper functioning of the translocation process.

Microbial interactions and differential protein expression in Staphylococcus aureus -Candida albicans dual-species biofilms

The fungal species Candida albicans and the bacterial species Staphylococcus aureus are responsible for a majority of hospital-acquired infections and often coinfect critically ill patients as complicating polymicrobial biofilms. To investigate biofilm structure during polymicrobial growth, dual-species biofilms were imaged with confocal scanning laser microscopy. Analyses revealed a unique biofilm architecture where S. aureus commonly associated with the hyphal elements of C. albicans. This physical interaction may provide staphylococci with an invasion strategy because candidal hyphae can penetrate through epithelial layers. To further understand the molecular mechanisms possibly responsible for previously demonstrated amplified virulence during coinfection, protein expression studies were undertaken. Differential in-gel electrophoresis identified a total of 27 proteins to be significantly differentially produced by these organisms during coculture biofilm growth. Among the upregulated staphylococcal proteins was l-lactate dehydrogenase 1, which confers resistance to host-derived oxidative stressors. Among the downregulated proteins was the global transcriptional repressor of virulence factors, CodY. These findings demonstrate that the hyphae-mediated enhanced pathogenesis of S. aureus may not only be due to physical interactions but can also be attributed to the differential regulation of specific virulence factors induced during polymicrobial growth. Further characterization of the intricate interaction between these pathogens at the molecular level is warranted, as it may aid in the design of novel therapeutic strategies aimed at combating fungal–bacterial polymicrobial infection.

Stm1p alters the ribosome association of eukaryotic elongation factor 3 and affects translation elongation

Stm1p is a Saccharomyces cerevisiae protein that is primarily associated with cytosolic 80S ribosomes and polysomes. Several lines of evidence suggest that Stm1p plays a role in translation under nutrient stress conditions, although its mechanism of action is not yet known. In this study, we show that yeast lacking Stm1p (stm1Delta) are hypersensitive to the translation inhibitor anisomycin, which affects the peptidyl transferase reaction in translation elongation, but show little hypersensitivity to other translation inhibitors such as paromomycin and hygromycin B, which affect translation fidelity. Ribosomes isolated from stm1Delta yeast have intrinsically elevated levels of eukaryotic elongation factor 3 (eEF3) associated with them. Overexpression of eEF3 in cells lacking Stm1p results in a growth defect phenotype and increased anisomycin sensitivity. In addition, ribosomes with increased levels of Stm1p exhibit decreased association with eEF3. Taken together, our data indicate that Stm1p plays a complementary role to eEF3 in translation.

Recent approaches to antifungal therapy for invasive mycoses

Invasive fungal infections with primary and opportunistic mycoses have become increasingly common in recent years and pose a major diagnostic and therapeutic challenge. They represent a major area of concern in today's medical fraternity. The occurrence of invasive fungal diseases, particularly in AIDS and other immunocompromised patients, is life-threatening and increases the economic burden. Apart from the previously known polyenes and imidazole-based azoles, newly discovered triazoles and echinocandins are more effective in terms of specificity, yet some immunosuppressed hosts are difficult to treat. The main reasons for this include antifungal resistance, toxicity, lack of rapid and microbe-specific diagnoses, poor penetration of drugs into sanctuary sites, and lack of oral or intravenous preparations. In addition to combination antifungal therapy, other novel antimycotic treatments such as calcineurin signaling pathway blockers and vaccines have recently emerged. This review briefly summarizes recent developments in the pharmacotherapeutic treatment of invasive fungal infections.

A molecular barcoded yeast ORF library enables mode-of-action analysis of bioactive compounds

We present a yeast chemical-genomics approach designed to identify genes that when mutated confer drug resistance, thereby providing insight about the modes of action of compounds. We developed a molecular barcoded yeast open reading frame (MoBY-ORF) library in which each gene, controlled by its native promoter and terminator, is cloned into a centromere-based vector along with two unique oligonucleotide barcodes. The MoBY-ORF resource has numerous genetic and chemical-genetic applications, but here we focus on cloning wild-type versions of mutant drug-resistance genes using a complementation strategy and on simultaneously assaying the fitness of all transformants with barcode microarrays. The complementation cloning was validated by mutation detection using whole-genome yeast tiling microarrays, which identified unique polymorphisms associated with a drug-resistant mutant. We used the MoBY-ORF library to identify the genetic basis of several drug-resistant mutants and in this analysis discovered a new class of sterol-binding compounds.

FR290581, a novel sordarin derivative: synthesis and antifungal activity

Sordarin is a unique natural product antifungal agent that is an inhibitor of elongation factor 2. To improve biological activity, we synthesized various compounds by novel modification of the aglycone, sordaricin. As a result, we have discovered the novel sordarin derivative FR290581. This compound exhibited superior activity and a good pharmacokinetic profile, and also displayed good in vivo activity against Candida albicans.

A chemical genomic screen in Saccharomyces cerevisiae reveals a role for diphthamidation of translation elongation factor 2 in inhibition of protein synthesis by sordarin

Sordarin and its derivatives are antifungal compounds of potential clinical interest. Despite the highly conserved nature of the fungal and mammalian protein synthesis machineries, sordarin is a selective inhibitor of protein synthesis in fungal organisms. In cells sensitive to sordarin, its mode of action is through preventing the release of translation elongation factor 2 (eEF2) during the translocation step, thus blocking protein synthesis. To further investigate the cellular components required for the effects of sordarin in fungal cells, we have used the haploid deletion collection of Saccharomyces cerevisiae to systematically identify genes whose deletion confers sensitivity or resistance to the compound. Our results indicate that genes in a number of cellular pathways previously unknown to play a role in sordarin response are involved in its growth effects on fungal cells and reveal a specific requirement for the diphthamidation pathway of cells in causing eEF2 to be sensitive to the effects of sordarin on protein synthesis. Our results underscore the importance of the powerful genomic tools developed in yeast (Saccharomyces cerevisiae) to more comprehensively understanding the cellular mechanisms involved in the response to therapeutic agents.

An Avenue to the Sordarin Core Adaptable to Analog Synthesis

We describe a synthesis of ketones 3 (X = O-R or CN; Y = H or alkyl), which are useful building blocks for the preparation of analogs of the potent antifungal agent sordarin, 1. Congeners of 1 constructed from 3 should permit detailed SAR investigations of the terpenoid core of the natural product.

Improvement of sordarin production through process optimization: combining traditional approaches with DOE

BMS-353645, also known as sordarin, was of interest based on its activity against pathogenic fungi. The objective of these studies was to provide high quality starting substrate for chemical modification aimed at further improving biological activity, with particular interest in the inhibition of Aspergillus. In the work presented here, Design of Experiments, or DOE, was successfully combined with traditional approaches to significantly improve sordarin yields in fermentation flasks. Overall, yields were increased 25-fold from <100 microg/g to as high as 2,609 microg/g in flasks through the use of various medium and conduction changes supplemented with DOE. The improved process was then successfully scaled to pilot plant tanks with the best batch producing 2,389 microg/g sordarin at the 250-l scale.

Sordarin derivatives induce a novel conformation of the yeast ribosome translocation factor eEF2

The sordarins are fungal specific inhibitors of the translation factor eEF2, which catalyzes the translocation of tRNA and mRNA after peptide bond formation. We have determined the crystal structures of eEF2 in complex with two novel sordarin derivatives. In both structures, the three domains of eEF2 that form the ligand-binding pocket are oriented in a different manner relative to the rest of eEF2 compared with our previous structure of eEF2 in complex with the parent natural product sordarin. Yeast eEF2 is also shown to bind adenylic nucleotides, which can be displaced by sordarin, suggesting that ADP or ATP also bind to the three C-terminal domains of eEF2. Fusidic acid is a universal inhibitor of translation that targets EF-G or eEF2 and is widely used as an antibiotic against Gram-positive bacteria. Based on mutations conferring resistance to fusidic acid, cryo-EM reconstructions, and x-ray structures of eEF2, EF-G, and an EF-G homolog, we suggest that the conformation of EF-G stalled on the 70 S ribosome by fusidic acid is similar to that of eEF2 trapped on the 80 S ribosome by sordarin.

Current state of three-dimensional characterisation of antifungal targets and its use for molecular modelling in drug design

The alarming rise in life-threatening systemic fungal infections due to the emergence of drug-resistant fungal strains had produced an increased demand for new antimycotics, especially those targeting novel antifungal structures. Drug discovery has developed from screening natural products and chemical synthesis to a modern approach, namely structure-based drug design. Whilst many antifungal agents currently in use were discovered more than 30 years ago, characterisation of various drug targets has only been achieved recently, contributing immensely to understanding the structure-activity relationships of antifungals and their targets. Three-dimensional characterisation has become a well established tool for modern antifungal drug research and should play an important role in investigations for new antifungal agents.

Characterization of the 26S rRNA-binding domain in Saccharomyces cerevisiae ribosomal stalk phosphoprotein P0

The stalk is a universal structure of the large ribosomal subunit involved in the function of translation factors. The bacterial stalk is highly stable but its stability is notably reduced in eukaryotes, favouring a translation regulatory activity of this ribosomal domain, which has not been reported in prokaryotes. The RNA-binding protein P0 plays a key role in determining the eukaryotic stalk activities, and characterization of the P0/RNA interaction is essential to understand the evolutionary process. Using a series of Saccharomyces cerevisiae-truncated proteins, a direct involvement of two N-terminal regions, I3-M58 and K81-V121, in the interaction of P0 with the ribosome has been shown. Two other conserved regions, R122-T149 and G162-T182, affect P0 interaction with other stalk components and the sensitivity to sordarin anti-fungals but are not essential for RNA binding. Moreover, P0 and a P0 fragment comprising only the first 121 amino acids show a similar in vitro affinity for the highly conserved 26S rRNA binding site. A protein chimera containing the first 165 amino acids of L10, the P0 bacterial counterpart, is able to complement the absence of P0 and also shows the same P0 RNA binding characteristics. Altogether, the results indicate that the affinity of the stalk RNA-binding protein for its substrate has been highly conserved, and changes in the stability of the interaction of P0 with the ribosome, which are essential for the new eukaryotic functions, result from the evolution of the overall stalk structure.

Pharmacological agents in development for invasive aspergillosis

The urgent medical need for new potent antifungal agents in the management of invasive aspergillosis (IA) has resulted in the development of several compounds which may be of value in the future for the treatment or prophylaxis of IA. In the past years, several novel types of drugs have been discovered and developed, some of which are already in late-stage clinical trials and ready to enter the market. This paper discusses the antifungal agents, classified by their mode of action, that are currently available and the agents which are still in development for treatment or prevention of IA.

New agents for the treatment of systemic fungal infections – current status

Systemic antifungal chemotherapy is enjoying its most dynamic era. More antifungal agents are under development than ever before, including agents in entirely new classes. Major goals of current investigations are to identify compounds with a wide spectrum of activity, minimal toxicity and a high degree of target specificity. The antifungal drugs in development include new azoles {voriconazole, posaconazole (formerly SCH-56592), ravuconazole (formerly BMS-207147)}, lipid formulations of amphotericin B, a lipid formulation of nystatin, echinocandins {anidulafungin (formerly, LY-303366, VER-002), caspofungin (formerly MK-991), micafungin (formerly FK-463)}, antifungal peptides other than echinocandins, and sordarin derivatives. This discussion reviews the currently available antifungal agents and summarises the developmental issues that surround these new systemic antifungal drugs.

Ribosomal P0 protein domain involved in selectivity of antifungal sordarin derivatives

The ribosomal stalk protein P0 is involved in the susceptibility to the antifungal sordarin derivatives, as reported for a number of Saccharomyces cerevisiae resistant mutants. Mammals and some lower eukaryotes are naturally resistant to these compounds. It is shown here that expression in S. cerevisiae of the ribosomal protein P0 from Homo sapiens and from other sordarin-resistant organisms results in a decrease in the sensitivity of the cells to an agent of this class. To further characterize the P0 region responsible for inducing sordarin resistance, a series of protein chimeras containing complementary regions of the human and yeast P0 proteins were constructed and expressed in yeast. The chimeras complement the absence of the native yeast P0 except in chimeras containing the human P0 carboxyl-terminal domain. Resistance to sordarins was found to be associated with the presence of an HsP0 amino acid sequence comprising P118 to F138, which unexpectedly led to higher resistance than the presence of the complete human P0. A comparison of the corresponding region in P0 from yeast and sordarin-insensitive organisms, followed by site-directed mutagenesis, indicates that residues in positions 119, 124, and 126 have an important role in determining resistance to sordarins. Moreover, since sordarins block the eukaryotic elongation factor 2 (EF2) function, the P0 region affecting sordarin susceptibility must correspond to EF2-interacting domains of the ribosomal stalk protein, which affects the drug-binding site in the elongation factor.

Domain movements of elongation factor eEF2 and the eukaryotic 80S ribosome facilitate tRNA translocation

An 11.7-A-resolution cryo-EM map of the yeast 80S.eEF2 complex in the presence of the antibiotic sordarin was interpreted in molecular terms, revealing large conformational changes within eEF2 and the 80S ribosome, including a rearrangement of the functionally important ribosomal intersubunit bridges. Sordarin positions domain III of eEF2 so that it can interact with the sarcin-ricin loop of 25S rRNA and protein rpS23 (S12p). This particular conformation explains the inhibitory action of sordarin and suggests that eEF2 is stalled on the 80S ribosome in a conformation that has similarities with the GTPase activation state. A ratchet-like subunit rearrangement (RSR) occurs in the 80S.eEF2.sordarin complex that, in contrast to Escherichia coli 70S ribosomes, is also present in vacant 80S ribosomes. A model is suggested, according to which the RSR is part of a mechanism for moving the tRNAs during the translocation reaction.

Model systems in drug discovery: chemical genetics meets genomics

Animal model systems are an intricate part of the discovery and development of new medicines. The sequencing of not only the human genome but also those of the various pathogenic bacteria, the nematode Caenorhabditis elegans, the fruitfly Drosophila, and the mouse has enabled the discovery of new drug targets to push forward at an unprecedented pace. The knowledge and tools in these "model" systems are allowing researchers to carry out experiments more efficiently and are uncovering previously hidden biological connections. While the history of bacteria, yeast, and mice in drug discovery are long, their roles are ever evolving. In contrast, the history of Drosophila and C. elegans at pharmaceutical companies is short. We will briefly review the historic role of each model organism in drug discovery and then update the readers as to the abilities and liabilities of each model within the context of drug development.

Identification of a broad-spectrum azasordarin with improved pharmacokinetic properties

The synthesis and antifungal activity of 5'- and 5'-6'-substituted azasordarin derivatives are described. Modification of the 5'-position led to the discovery of the spirocyclopentyl analogue 7g, which is the first azasordarin to register single-digit MIC values versus Aspergillus spp. Further investigation identified the 5'-i-Pr derivative 7b, which displays superior pharmacokinetic properties compared to other azasordarins.

Sordaricin antifungal agents

Compounds based on sordaricin were prepared via organometallic addition onto a fully protected sordaricin aldehyde. The fungal growth inhibition profiles for these compounds were established and the results are presented here. The synthesis of homologated sordaricin as well as ether and ester derivatives is presented, and structural rearrangement products upon oxidation. These compounds were evaluated as agents to inhibit fungal growth.

Core-modified sordaricin derivatives: synthesis and antifungal activity

Core-modified sordaricin derivatives were prepared via biotransformation followed by chemical modification and tested for antifungal activity. The antifungal activity proved to be very sensitive to modifications in the sterics and/or lipophilicity of the diterpene skeleton. Introduction of polar groups such as hydroxyl in the diterpene core results in loss of potency while small and lipophilic groups such as fluorine and the 7,8-olefin are well tolerated.

Sordarin oxazepine derivatives as potent antifungal agents

The synthesis and biological activity of sordarin oxazepine derivatives are described. The key step features a regioselective oxidation of an unprotected triol followed by double reductive amination to afford the ring-closed products. The spectrum of antifungal activity for these novel derivatives includes coverage of Candida albicans, Candida glabrata, and Cryptococcus neoformans.

Translation elongation-3-like factors: are they rational antifungal targets?

The occurrence of fungal infection has escalated significantly in recent years and is expected to continue to increase for the foreseeable future. Unfortunately, only a limited number of antifungal drugs are currently available partially due to a lack of suitable targets. The most commonly used antifungals target the same molecule in the cell membrane and, while efficacious, are either extremely toxic or susceptible to resistance. This article examines elongation factor-3, which is unique to fungi and essential for fungal cell survival and, thus, an attractive antifungal target. A search for inhibitors of this 'perfect target' led to identification of compounds (sordarins) which inhibited elongation factor-2, a protein with a mammalian homologue. Molecular analysis demonstrated why sordarins can specifically act against fungal elongation factor-2. This data questions the validity of pursuing genes as targets only if they are unique to fungi. Proteins that are homologous to elongation factor-3 are also discussed. The advances in molecular techniques and bioinformatics will allow the re-evaluation of targets previously thought to be unattractive. In addition, molecular genetics provides new and novel information on cellular processes that can potentially introduce new targets.

Therapeutic efficacies of GW471552 and GW471558, two new azasordarin derivatives, against pneumocystosis in two immunosuppressed-rat models

Two new azasordarins, GW471552 and GW471558, were studied in vivo for treatment of Pneumocystis carinii pneumonia. In the Wistar rat spontaneous pneumonia model, both azasordarins significantly reduced the number of P. carinii cysts per gram of lung homogenate when administered at 1 mg/kg of body weight twice a day for 10 days. In a nude rat inoculation model, both compounds showed therapeutic efficacy at 0.25 mg/kg twice a day for 10 days.

Pharmacotherapy of Candida bloodstream infections: new treatment options, new era

Evolving medical practices and the widespread use of fluconazole have clearly affected the spectrum of invasive mycoses now encountered by clinicians. The proportion of infections due to azole-resistant Candida species and invasive moulds has increased steadily over the last decade, creating a need for broad-spectrum antifungal agents with safety profiles similar to fluconazole. Efforts to address this need have lead to the reformulation of older, broad-spectrum antifungals and the development of new agents with enhanced activity against non-C. albicans and Aspergillus species. This review highlights pharmacodynamic, pharmacokinetic, safety and cost considerations for current and emerging antifungal therapies to be used in the treatment of bloodstream candidiasis.

Resistance of human fungal pathogens to antifungal drugs

Resistance mechanisms can be engaged in clinically relevant fungal pathogens under different conditions when exposed to antifungal drugs. Over past years, active research was undertaken in the understanding of the molecular basis of antifungal drug resistance in these pathogens, and especially against the class of azole antifungals. The isolation of various alleles of the gene encoding the target of azoles has enabled correlation of the appearance of resistance with distinct mutations. Resistance mechanisms to azoles also converge to the upregulation of multidrug transporter genes, whose products have the capacity to extrude from cells several chemically unrelated antifungal agents and toxic compounds. Genome-wide studies of azole-resistant isolates are now permitting a more comprehensive analysis of the impact of resistance on gene expression, and may deliver new clues to their mechanisms. Several laboratories are also exploring, as well as possible alternative resistance pathways, the role of biofilm formation by several fungal species in the development of resistance to various antifungals, including azoles.

Synthesis and evaluation of N-substituted 1,4-oxazepanyl Sordaricins as selective fungal EF-2 inhibitors

Sordaricin analogues possessing 6-methoxy-7-methyl-1,4-oxazepane moiety instead of the sugar part were synthesized and evaluated. It was found that N-substituents on the oxazepane ring had influence on biological activity. In particular, N-(2-methylpropenyl) derivative 12p exhibited potent in vitro antifungal activity. Furthermore, 12p maintained significant activity (MIC 0.25 microg/mL) against Candida albicans SANK51486 even in the presence of 20% horse serum.

Oxime derivatives of sordaricin as potent antifungal agents

Oxime derivatives of the sordarin aglycone have been identified as potent antifungal agents. The in vitro spectrum of activity includes coverage against Candida albicans and Candida glabrata with MICs as low as 0.06 microg/mL. The antifungal activity was established to be exquisitely sensitive to the spatial orientation of the lipophilic side chains.

Synthesis of Sordaricin analogues as potent antifungal agents against Candida albicans

Sordaricin derivatives possessing a cyclohexane ring appendage attached via an ether, thioether, amine, oxime, ester or amide linkage were synthesized and their antifungal activity was evaluated in vitro. Compounds containing a thioether bond or an oxime bond as a linkage exhibited potent MICs (< or = 0.125 microg/mL) against four Candida albicans strains including azole-low-susceptible strains. They were also active (MIC < or = 0.125 microg/mL) against Candida glabrata. Their in vivo efficacy was confirmed in a murine intravenous infection model with Candida albicans.

Applications of yeast in drug discovery

The yeast Saccharomyces cerevisiae is perhaps the best-studied eukaryotic organism. Its experimental tractability, combined with the remarkable conservation of gene function throughout evolution, makes yeast the ideal model genetic organism. Yeast is a non-pathogenic model of fungal pathogens used to identify antifungal targets suitable for drug development and to elucidate mechanisms of action of antifungal agents. As a model of fundamental cellular processes and metabolic pathways of the human, yeast has improved our understanding and facilitated the molecular analysis of many disease genes. The completion of the Saccharomyces genome sequence helped launch the post-genomic era, focusing on functional analyses of whole genomes. Yeast paved the way for the systematic analysis of large and complex genomes by serving as a test bed for novel experimental approaches and technologies, tools that are fast becoming the standard in drug discovery research

Role of the ribosomal stalk components in the resistance of Aspergillus fumigatus to the sordarin antifungals

Aspergillus fumigatus, an important human nosocomial pathogen, is resistant to sordarin derivatives, a new family of antifungals that inhibit protein synthesis by interaction with the EF-2-ribosomal stalk complex. To explore the role of the A. fumigatus ribosome in the resistance mechanism, the fungal stalk proteins were biochemically and genetically characterized and expressed in the sensitive Saccharomyces cerevisiae. Two acidic phosphoproteins homologous to the 12 kDa P1 and P2 proteins described in other organisms were found together with the 34 kDa P0 protein, the third stalk component. The genes encoding each fungal stalk protein were expressed in mutant S. cerevisiae strains lacking the equivalent proteins. Both AfP1 and AfP2 proteins interact with their yeast counterparts of the opposite type and bind to the ribosomal particles in the presence of either the S. cerevisiae or the A. fumigatus P0 protein. The A. fumigatus acidic phosphoproteins did not alter the yeast ribosome sordarin sensitivity. On the contrary, the presence of the fungal P0 induces in vivo and in vitro resistance to sordarin derivatives when present in the yeast ribosome. The mutations A117-->E, P122-->R and G124-->V in A. fumigatus P0 reduce the resistance capacity of the protein. An S. cerevisiae strain with the complete ribosomal stalk of A. fumigatus was obtained, which could be useful for the screening of new antifungals against this pathogenic fungus.

Antifungal activities and cytotoxicity studies of six new azasordarins

GW 471552, GW 471558, GW 479821, GW 515716, GW 570009, and GW 587270 are members of a new family of sordarin derivatives called azasordarins. The in vitro activities of these compounds were evaluated against clinical isolates of yeasts, including Candida albicans, Candida non-albicans, and Cryptococcus neoformans strains. Activities against Pneumocystis carinii, Aspergillus spp., less common molds, and dermatophytes were also investigated. Azasordarin derivatives displayed significant activities against the most clinically important Candida species, with the exception of C. krusei. Against C. albicans, including fluconazole-resistant strains, MICs at which 90% of the isolates tested are inhibited (MIC(90)s) were 0.002 microg/ml with GW 479821, 0.015 microg/ml with GW 515716 and GW 587270, and 0.06 microg/ml with GW 471552, GW 471558, and GW 570009. The MIC(90)s of GW 471552, GW 471558, GW 479821, GW 515716, GW 570009, and GW 587270 were 0.12, 0.12, 0.03, 0.06, 0.12, and 0.06 microg/ml, respectively, against C. tropicalis and 4, 0.25, 0.06, 0.25, 0.5, and 0.5 microg/ml, respectively, against C. glabrata. In addition, some azasordarin derivatives (GW 479821, GW 515716, GW 570009, and GW 58720) were active against C. parapsilosis, with MIC(90)s of 2, 4, 4, and 1 microg/ml, respectively. The compounds were extremely potent against P. carinii, showing 50% inhibitory concentrations of <or=0.001 microg/ml. However Cryptococcus neoformans was resistant to all compounds tested (MIC > 16 microg/ml). These azasordarin derivatives also showed significant activity against emerging fungal pathogens, which affect immunocompromised patients, such as Rhizopus arrhizus, Blastoschizomyces capitatus, and Geotrichum clavatum. Against these organisms, the MICs of GW 587270 ranged from 0.12 to 1 microg/ml, those of GW 479821 and GW 515716 ranged from 0.12 to 2 microg/ml, and those of GW 570009 ranged from 0.12 to 4 microg/ml. Against Fusarium oxysporum, Scedosporium apiospermum, Absidia corymbifera, Cunninghamella bertholletiae, and dermatophytes, GW 587270 was the most active compound, with MICs ranging from 4 to 16 microg/ml. Against Aspergillus spp., the MICs of the compounds tested were higher than 16 microg/ml. The in vitro selectivity of azasordarins was investigated by cytotoxicity studies performed with five cell lines and primary hepatocytes. Concentrations of compound required to achieve 50% inhibition of the parameter considered (Tox(50)s) of GW 570009, GW 587270, GW 479281, and GW 515716 in the cell lines ranged from 60 to 96, 49 to 62, 24 to 36, and 16 to 38 microg/ml, respectively. The cytotoxicity values of GW 471552 and GW 471558 were >100 microg/ml for all cell lines tested. Tox(50)s on hepatocytes were in the following order: GW 471558 > GW 471552 > GW 570009 > GW 587270 > GW 515716 > GW 479821, with values ranging from higher than 100 microg/ml to 23 microg/ml. The cytotoxicity results obtained with fully metabolizing rat hepatocytes were in total agreement with those obtained with cell lines. In summary, the in vitro activities against important pathogenic fungi and the selectivity demonstrated in mammalian cell lines justify additional studies to determine the clinical usefulness of azasordarins.

Identification of a putative sordarin binding site in Candida albicans elongation factor 2 by photoaffinity labeling

Candida albicans EF-2 binds sordarin to a single class of binding sites with K(d) = 1.26 microm. Equimolar mixtures of EF-2 and ribosomes, in the presence of a non-hydrolyzable GTP analog, reveal two classes of high affinity sordarin binding sites with K(d) = 0.7 and 41.5 nm, probably due to the existence of two ribosome populations. Photoaffinity labeling of C. albicans EF-2 in the absence of ribosomes has been performed with [(14)C]GM258383, a photoactivatable sordarin derivative. Labeling is saturable and can be considered specific, because it can be prevented with another sordarin analog. The fragment Gln(224)-Lys(232) has been identified as the modified peptide within the EF-2 sequence, Lys(228) being the residue to which the photoprobe was linked. This fragment is included within the G"-subdomain of EF-2. These results are discussed in the light of the high sordarin specificity toward fungal systems.

Species-specific inhibition of fungal protein synthesis by sordarin: identification of a sordarin-specificity region in eukaryotic elongation factor 2

The sordarin class of natural products selectively inhibits fungal protein synthesis by impairing the function of eukaryotic elongation factor 2 (eEF2). Mutations in Saccharomyces cerevisiae eEF2 or the ribosomal stalk protein rpP0 can confer resistance to sordarin, although eEF2 is the major determinant of sordarin specificity. It has been shown previously that sordarin specifically binds S. cerevisiae eEF2 while there is no detectable binding to eEF2 from plants or mammals, despite the high level of amino acid sequence conservation among these proteins. In both whole-cell assays and in vitro translation assays, the efficacy of sordarin varies among different species of pathogenic fungi. To investigate the basis of sordarin's fungal selectivity, eEF2 has been cloned and characterized from several sordarin-sensitive and -insensitive fungal species. Results from in vivo expression of Candida species eEF2s in S. cerevisiae and in vitro translation and growth inhibition assays using hybrid S. cerevisiae eEF2 proteins demonstrate that three amino acid residues within eEF2 account for the selectivity of this class of compounds. It is also shown that the corresponding residues at these positions in human eEF2 are sufficient to confer sordarin insensitivity to S. cerevisiae identical to that observed with mammalian eEF2.

Activities of sordarins in experimental models of candidiasis, aspergillosis, and pneumocystosis

Sordarin derivatives represent a new class of antifungal agents that act as potent inhibitors of fungal protein synthesis and possess a broad spectrum of activity. The in vivo activity of GM193663 and GM237354 was studied in mouse models of disseminated candidiasis and aspergillosis and in a rat model of pneumocystosis. The pharmacokinetic behavior of both sordarin derivatives was studied in mice and rats. In all studies, compounds were administered by the subcutaneous route. After a subcutaneous dose of 50 mg/kg of body weight to mice, the maximum level in serum, area under the concentration-time curve, half-life, and clearance for GM193663 and GM237354 were 51.8 and 23 microg/ml, 79.5 and 46 microg. h/ml, 0.8 and 0.85 h, and 21 and 25 ml/h, respectively. Systemic candidiasis and aspergillosis were established in CD-1 male mice infected with Candida albicans or Aspergillus fumigatus. For systemic candidiasis, compounds were given three times per day for seven consecutive days at 15, 30, 60, or 120 mg/kg/day. GM193663 and GM237354 showed dose-related efficacy against C. albicans, with 50% effective doses, 1 month after infection, of 25.2 and 10.7 mg/kg/dose, respectively. In experimental infections with A. fumigatus, GM237354 was given three times per day at 30, 60, or 120 mg/kg/day for five consecutive days. Animals treated with GM237354 demonstrated irregular responses. The survival of animals treated with GM237354 was 0, 30, and 0% at 30, 60, and 120 mg/kg/day, respectively. The therapeutic efficacy of GM193663 and GM237354 against Pneumocystis carinii was studied in an experimental P. carinii pneumonia (PCP) rat model. After a subcutaneous dose of 10 mg/kg given to rats, the maximum level in serum, area under the concentration-time curve, half-life, and clearance for GM193663 and GM237354 were 6.6 and 7.2 microg/ml, 8.5 and 11.8 microg. h/ml, 0.7 and 0.8 h, and 230 and 133 ml/h, respectively. To induce spontaneous PCP, rats were chronically immunosuppressed with dexamethasone. Infected animals were treated twice daily for 10 days at 0.2, 2, or 10 mg/kg/day. The therapeutic effect was estimated by the reduction in the number of cysts in the lungs of treated versus untreated animals. GM193663 and GM237354 significantly reduced the mean (+/- standard deviation) log number of cysts from 7.6 +/- 0.2 in the untreated group to 4.7 +/- 0.2 and 4.6 +/- 0.1, respectively, when the drugs were administered at a dose of 2 mg/kg/day. The log number of cysts was also reduced in infected animals given lower doses of the compounds (0.2 mg/kg/day). In summary, GM193663 and GM237354 are new sordarin derivatives that may potentially play a major role in the treatment of candidiasis and PCP. Further testing with Aspergillus in other animal models is warranted.

Conformational changes induced in the Saccharomyces cerevisiae GTPase-associated rRNA by ribosomal stalk components and a translocation inhibitor

The yeast ribosomal GTPase associated center is made of parts of the 26S rRNA domains II and VI, and a number of proteins including P0, P1alpha, P1beta, P2alpha, P2beta and L12. Mapping of the rRNA neighborhood of the proteins was performed by footprinting in ribosomes from yeast strains lacking different GTPase components. The absence of protein P0 dramatically increases the sensitivity of the defective ribosome to degradation hampering the RNA footprinting. In ribosomes lacking the P1/P2 complex, protection of a number of nucleotides is detected around positions 840, 880, 1100, 1220-1280 and 1350 in domain II as well as in several positions in the domain VI alpha-sarcin region. The protection pattern resembles the one reported for the interaction of elongation factors in bacterial systems. The results exclude a direct interaction of these proteins with the rRNA and are compatible with an increase in the ribosome affinity for EF-2 in the absence of the acidic P proteins. Interestingly, a sordarin derivative inhibitor of EF-2 causes an opposite effect, increasing the reactivity in positions protected by the absence of P1/P2. Similarly, a deficiency in protein L12 exposes nucleotides G1235, G1242, A1262, A1269, A1270 and A1272 to chemical modification, thus situating the protein binding site in the most conserved part of the 26S rRNA, equivalent to the bacterial protein L11 binding site.

Efficacies of sordarin derivatives GM193663, GM211676, and GM237354 in a murine model of systemic coccidioidomycosis. p6

Sordarin derivatives (Glaxo Wellcome) are a new class of compounds that selectively inhibit fungal protein synthesis and have a broad spectrum of activity. Systemic coccidioidomycosis was established in female CD-1 mice infected with Coccidioides immitis, and therapy was begun on day 4 with either GM193663, GM211676, GM237354, fluconazole, or no treatment; compounds were given twice daily orally for 19 days at 20 or 100 mg/kg/day. The serum pharmacokinetics of the compounds were studied in uninfected mice. The MICs of GM193663, GM211676, and GM237354 for C. immitis were 1.56, 0.39, and 0.39 microgram/ml, respectively, and the minimum fungicidal concentrations were 6.25, 3.13, and 0.39 microgram/ml, respectively. Peak serum levels (sampled at 1 to 2 h) after a single 50-mg/kg dose were 9.8 microgram/ml for GM193663, 13 microgram/ml for GM211676, and 6.0 microgram/ml for GM237354. No accumulation occurred after 19 days of dosing, and peak levels were lower at 3.2 microgram/ml for GM193663, 4.0 microgram/ml for GM211676, and <2.5 microgram/ml for GM237354. We estimate that the t(1/2) for each compound in serum is <2 h. In vivo, all compounds showed dose-responsive efficacy, significantly prolonging survival over the control groups (100% lethal dose); 80 to 100% of the mice given the 100-mg/kg doses of fluconazole or a GM drug survived. All 100-mg/kg/day regimens were equivalent. At 20 mg/kg/day, GM211676 was equivalent to 100 mg of fluconazole/kg/day, indicating that GM211676 was approximately 5-fold more efficacious. No mice surviving the 49 days of the experiment were free of infection. All drugs dose responsively reduced the fungal burden in the spleen, liver, and lungs, and GM237354 at 100 mg/kg/day was superior to all of the other regimens in the reduction of burden in all organs. C. immitis was susceptible both in vitro and in vivo to the GM compounds, which were found to be equivalent or superior to fluconazole. These results are encouraging, indicating that further testing in other models of fungal disease is warranted.

Three-dimensional cryo-electron microscopy localization of EF2 in the Saccharomyces cerevisiae 80S ribosome at 17.5 A resolution

Using a sordarin derivative, an antifungal drug, it was possible to determine the structure of a eukaryotic ribosome small middle dotEF2 complex at 17.5 A resolution by three-dimensional (3D) cryo-electron microscopy. EF2 is directly visible in the 3D map and the overall arrangement of the complex from Saccharomyces cerevisiae corresponds to that previously seen in Escherichia coli. However, pronounced differences were found in two prominent regions. First, in the yeast system the interaction between the elongation factor and the stalk region of the large subunit is much more extensive. Secondly, domain IV of EF2 contains additional mass that appears to interact with the head of the 40S subunit and the region of the main bridge of the 60S subunit. The shape and position of domain IV of EF2 suggest that it might interact directly with P-site-bound tRNA.