Antifungal Drug Design Research Articles

Indole-derived chalcones as anti-dermatophyte agents: In vitro evaluation and in silico study

A series of indole-derived methoxylated chalcones were described as anti-dermatophyte agents. The in vitro antifungal susceptibility testing against different dermatophytes revealed that most of compounds had potent activity against the dermatophyte strains. In particular, the 4-ethoxy derivative 4d with MIC values of 0.25−2 μg/ml was the most potent compound against Trichophyton interdigitale, Trichophyton veruccosum and Microsporum fulvum. Moreover, the 4-butoxy analog 4i displaying MIC values in the range of 1−16 μg/ml had the highest inhibitory activity against Trichophyton mentagrophytes, Microsporum canis, and Arthroderma benhamiae. To predict whether the synthesized compounds interact with tubulin binding site of dermatophytes, the 3D-structure of target protein was modeled by homology modeling and then used for molecular docking and molecular dynamics (MD) simulation studies. Docking simulation revealed that the promising compound 4d can properly bind with tubulin. The molecular dynamics analysis showed that interactions of compound 4d with the active site of target protein have binding stability throughout MD simulation. The results of this study could utilize in the design of more effective antifungal drugs with tubulin inhibition mechanism against keratinophilic fungi.

Chemogenomic profiling to understand the antifungal action of a bioactive aurone compound.

Every year, more than 250,000 invasive candidiasis infections are reported with 50,000 deaths worldwide. The limited number of antifungal agents necessitates the need for alternative antifungals with potential novel targets. The 2-benzylidenebenzofuran-3-(2H)-ones have become an attractive scaffold for antifungal drug design. This study aimed to determine the antifungal activity of a synthetic aurone compound and characterize its mode of action. Using the broth microdilution method, aurone SH1009 exhibited inhibition against C. albicans, including resistant isolates, as well as C. glabrata, and C. tropicalis with IC50 values of 4-29 μM. Cytotoxicity assays using human THP-1, HepG2, and A549 human cell lines showed selective toxicity toward fungal cells. The mode of action for SH1009 was characterized using chemical-genetic interaction via haploinsufficiency (HIP) and homozygous (HOP) profiling of a uniquely barcoded Saccharomyces cerevisiae mutant collection. Approximately 5300 mutants were competitively treated with SH1009 followed by DNA extraction, amplification of unique barcodes, and quantification of each mutant using multiplexed next-generation sequencing. Barcode post-sequencing analysis revealed 238 sensitive and resistant mutants that significantly (FDR P values ≤ 0.05) responded to aurone SH1009. The enrichment analysis of KEGG pathways and gene ontology demonstrated the cell cycle pathway as the most significantly enriched pathway along with DNA replication, cell division, actin cytoskeleton organization, and endocytosis. Phenotypic studies of these significantly enriched responses were validated in C. albicans. Flow cytometric analysis of SH1009-treated C. albicans revealed a significant accumulation of cells in G1 phase, indicating cell cycle arrest. Fluorescence microscopy detected abnormally interrupted actin dynamics, resulting in enlarged, unbudded cells. RT-qPCR confirmed the effects of SH1009 in differentially expressed cell cycle, actin polymerization, and signal transduction genes. These findings indicate the target of SH1009 as a cell cycle-dependent organization of the actin cytoskeleton, suggesting a novel mode of action of the aurone compound as an antifungal inhibitor.

Crystal structures of Magnaporthe oryzae trehalose-6-phosphate synthase (MoTps1) suggest a model for catalytic process of Tps1.

Trehalose-6-phosphate (T6P) synthase (Tps1) catalyzes the formation of T6P from UDP-glucose (UDPG) (or GDPG, etc.) and glucose-6-phosphate (G6P), and structural basis of this process has not been well studied. MoTps1 (Magnaporthe oryzae Tps1) plays a critical role in carbon and nitrogen metabolism, but its structural information is unknown. Here we present the crystal structures of MoTps1 apo, binary (with UDPG) and ternary (with UDPG/G6P or UDP/T6P) complexes. MoTps1 consists of two modified Rossmann-fold domains and a catalytic center in-between. Unlike Escherichia coli OtsA (EcOtsA, the Tps1 of E. coli), MoTps1 exists as a mixture of monomer, dimer, and oligomer in solution. Inter-chain salt bridges, which are not fully conserved in EcOtsA, play primary roles in MoTps1 oligomerization. Binding of UDPG by MoTps1 C-terminal domain modifies the substrate pocket of MoTps1. In the MoTps1 ternary complex structure, UDP and T6P, the products of UDPG and G6P, are detected, and substantial conformational rearrangements of N-terminal domain, including structural reshuffling (β3-β4 loop to α0 helix) and movement of a 'shift region' towards the catalytic centre, are observed. These conformational changes render MoTps1 to a 'closed' state compared with its 'open' state in apo or UDPG complex structures. By solving the EcOtsA apo structure, we confirmed that similar ligand binding induced conformational changes also exist in EcOtsA, although no structural reshuffling involved. Based on our research and previous studies, we present a model for the catalytic process of Tps1. Our research provides novel information on MoTps1, Tps1 family, and structure-based antifungal drug design.

Actividad antifúngica in vitro del extracto crudo diluido del veneno de escorpión Hadruroides charcasus (Karsch 1879) frente a Candida albicans

The objective was to evaluate the in vitro antifungal activity of the diluted crude extract of the scorpion venom Hadruroides charcasus (Karsch 1879) against Candida albicans. It was an experimental study, 54 experimental units were used (9 poison concentrations, 2 strains and three repetitions). The venom of the scorpion was obtained by electrical stimulation. The double serial dilution method was determined at different concentrations of 100%, 50%, 25%, 12.5%, 6.25%, 3.125%, 1.56%, 0.78%, 0.39%. To evaluate the antifungal activity, the microdilution method was used to determine the Minimum Inhibitory Concentration (MIC). The results showed that by means of the microdilution method that for the strain n ° 1 presented a minimum inhibitory concentration (MIC) to 25% of the diluted raw venom, very different was for the strain n ° 2 did not present CMI. It was concluded that the findings of the current study show that the diluted crude extract of the scorpion venom H. charcasus possesses antifungal activity on strains of C. albicans, being possible to use it as a template for the design and development of new antifungal drugs.

Exploiting the vulnerable active site of a copper-only superoxide dismutase to disrupt fungal pathogenesis

Copper-only superoxide dismutases (SODs) represent a new class of SOD enzymes that are exclusively extracellular and unique to fungi and oomycetes. These SODs are essential for virulence of fungal pathogens in pulmonary and disseminated infections, and we show here an additional role for copper-only SODs in promoting survival of fungal biofilms. The opportunistic fungal pathogen Candida albicans expresses three copper-only SODs, and deletion of one of them, SOD5, eradicated candidal biofilms on venous catheters in a rodent model. Fungal copper-only SODs harbor an irregular active site that, unlike their Cu,Zn-SOD counterparts, contains a copper co-factor unusually open to solvent and lacks zinc for stabilizing copper binding, making fungal copper-only SODs highly vulnerable to metal chelators. We found that unlike mammalian Cu,Zn-SOD1, C. albicans SOD5 indeed rapidly loses its copper to metal chelators such as EDTA, and binding constants for Cu(II) predict that copper-only SOD5 has a much lower affinity for copper than does Cu,Zn-SOD1. We screened compounds with a variety of indications and identified several metal-binding compounds, including the ionophore pyrithione zinc (PZ), that effectively inhibit C. albicans SOD5 but not mammalian Cu,Zn-SOD1. We observed that PZ both acts as an ionophore that promotes uptake of toxic metals and inhibits copper-only SODs. The pros and cons of a vulnerable active site for copper-only SODs and the possible exploitation of this vulnerability in antifungal drug design are discussed.

15N, 13C and 1H resonance assignments of FKBP12 proteins from the pathogenic fungi Mucor circinelloides and Aspergillus fumigatus

Invasive fungal infections are a leading cause of death in immunocompromised patients and remain difficult to treat since fungal pathogens, like mammals, are eukaryotes and share many orthologous proteins. As a result, current antifungal drugs have limited clinical value, are sometimes toxic, can adversely affect human reaction pathways and are increasingly ineffective due to emerging resistance. One potential antifungal drug, FK506, establishes a ternary complex between the phosphatase, calcineurin, and the 12-kDa peptidyl-prolyl isomerase FK506-binding protein, FKBP12. It has been well established that calcineurin, highly conserved from yeast to mammals, is necessary for invasive fungal disease and is inhibited when in complex with FK506/FKBP12. Unfortunately, FK506 is also immunosuppressive in humans, precluding its usage as an antifungal drug, especially in immunocompromised patients. Whereas the homology between human and fungal calcineurin proteins is > 80%, the human and fungal FKBP12s share 48–58% sequence identity, making them more amenable candidates for drug targeting efforts. Here we report the backbone and sidechain NMR assignments of recombinant FKBP12 proteins from the pathogenic fungi Mucor circinelloides and Aspergillus fumigatus in the apo form and compare these to the backbone assignments of the FK506 bound form. In addition, we report the backbone assignments of the apo and FK506 bound forms of the Homo sapiens FKBP12 protein for evaluation against the fungal forms. These data are the first steps towards defining, at a residue specific level, the impacts of FK506 binding to fungal and mammalian FKBP12 proteins. Our data highlight differences between the human and fungal FKBP12s that could lead to the design of more selective anti-fungal drugs.

New Benzodioxole-based Pyrazoline Derivatives: Synthesis and Anticandidal, In silico ADME, Molecular Docking Studies

Background: Azoles are commonly used in the treatment and prevention of fungal infections. They suppress fungal growth by acting on the heme group of lanosterol 14α-demethylase enzyme (CYP51), thus blocking the biosynthesis of ergosterol. </P><P> Objectives: Due to the importance of pyrazolines in the field of antifungal drug design, we aimed to design and synthesize new pyrazoline-based anticandidal agents. Methods: New pyrazoline derivatives were synthesized via the reaction of 1-(chloroacetyl)-3-(2- thienyl)-5-(1,3-benzodioxol-5-yl)-2-pyrazoline with aryl thiols. These compounds were evaluated for their in vitro antifungal effects on Candida species. Docking studies were performed to predict the affinity of the most effective anticandidal agents to substrate binding site of CYP51. Furthermore, MTT assay was performed to determine the cytotoxic effects of the compounds on NIH/3T3 mouse embryonic fibroblast cell line. A computational study for the prediction of ADME properties of all compounds was also carried out. Results: Compounds 5, 8, 10 and 12 were found as the most potent anticandidal agents against Candida albicans and Candida glabrata in this series with the same MIC values of ketoconazole and they also exhibited low toxicity against NIH/3T3 cells. Docking results indicated that all these compounds showed good binding affinity into the active site of CYP51. In particular, chloro substituted compounds 8 and 12 bind to CYP51 through direct coordination with the heme group. According to in silico studies, compound 8 only violated one parameter of Lipinski’s rule of five, making it a potential orally bioavailable agent. Conclusion: Compound 8 was defined as a promising candidate for further in vitro and in vivo studies.

Structural and In Vivo Studies on Trehalose-6-Phosphate Synthase from Pathogenic Fungi Provide Insights into Its Catalytic Mechanism, Biological Necessity, and Potential for Novel Antifungal Drug Design.

ABSTRACTThe disaccharide trehalose is critical to the survival of pathogenic fungi in their human host. Trehalose-6-phosphate synthase (Tps1) catalyzes the first step of trehalose biosynthesis in fungi. Here, we report the first structures of eukaryotic Tps1s in complex with substrates or substrate analogues. The overall structures of Tps1 from Candida albicans and Aspergillus fumigatus are essentially identical and reveal N- and C-terminal Rossmann fold domains that form the glucose-6-phosphate and UDP-glucose substrate binding sites, respectively. These Tps1 structures with substrates or substrate analogues reveal key residues involved in recognition and catalysis. Disruption of these key residues severely impaired Tps1 enzymatic activity. Subsequent cellular analyses also highlight the enzymatic function of Tps1 in thermotolerance, yeast-hypha transition, and biofilm development. These results suggest that Tps1 enzymatic functionality is essential for the fungal stress response and virulence. Furthermore, structures of Tps1 in complex with the nonhydrolyzable inhibitor, validoxylamine A, visualize the transition state and support an internal return-like catalytic mechanism that is generalizable to other GT-B-fold retaining glycosyltransferases. Collectively, our results depict key Tps1-substrate interactions, unveil the enzymatic mechanism of these fungal proteins, and pave the way for high-throughput inhibitor screening buttressed and guided by the current structures and those of high-affinity ligand-Tps1 complexes.

Cryptococcus neoformans ADS lyase is an enzyme essential for virulence whose crystal structure reveals features exploitable in antifungal drug design

There is significant clinical need for new antifungal agents to manage infections with pathogenic species such as Cryptococcus neoformans Because the purine biosynthesis pathway is essential for many metabolic processes, such as synthesis of DNA and RNA and energy generation, it may represent a potential target for developing new antifungals. Within this pathway, the bifunctional enzyme adenylosuccinate (ADS) lyase plays a role in the formation of the key intermediates inosine monophosphate and AMP involved in the synthesis of ATP and GTP, prompting us to investigate ADS lyase in C. neoformans. Here, we report that ADE13 encodes ADS lyase in C. neoformans. We found that an ade13Δ mutant is an adenine auxotroph and is unable to successfully cause infections in a murine model of virulence. Plate assays revealed that production of a number of virulence factors essential for dissemination and survival of C. neoformans in a host environment was compromised even with the addition of exogenous adenine. Purified recombinant C. neoformans ADS lyase shows catalytic activity similar to its human counterpart, and its crystal structure, the first fungal ADS lyase structure determined, shows a high degree of structural similarity to that of human ADS lyase. Two potentially important amino acid differences are identified in the C. neoformans crystal structure, in particular a threonine residue that may serve as an additional point of binding for a fungal enzyme-specific inhibitor. Besides serving as an antimicrobial target, C. neoformans ADS lyase inhibitors may also serve as potential therapeutics for metabolic disease; rather than disrupt ADS lyase, compounds that improve the stability the enzyme may be used to treat ADS lyase deficiency disease.

Pharmacophore-Based Virtual Screening of Novel Inhibitors and Docking Analysis for CYP51A from Penicillium italicum.

Sterol 14α-demethylases from Cytochrome P450 family (CYP51s) are essential enzymes in sterol biosynthesis and well-known as the target of antifungal drugs. The 3D structure of CYP51A from Penicillium italicum (PiCYP51A) was constructed through homology modeling based on the crystal structure of human CYP51A (PDB: 3LD6). Molecular dynamics (MD) simulation was operated to relax the initial model and followed by quality assessment using PROCHECK program. On the basis of the docking information on the currently available CYP51s with the patent demethylase inhibitors (DMIs), pharmacophore-based virtual screening combined with docking analysis was performed to pick out twelve new compounds from ZINC database. Six hits revealed in the ligand database suggested potential ability to inhibit PiCYP51A. Compared to patent fungicide triazolone, the top three lead compounds had similar or higher affinity with the target enzyme, and accordingly, exhibited comparable or lower EC50 values to P. italicum isolates. The results could provide references for de novo antifungal drug design.

Evaluation of 8-Hydroxyquinoline Derivatives as Hits for Antifungal Drug Design.

Clioquinol is an 8-hydroxyquinoline derivative that was widely used from the 1950s to 1970s as an oral antiparasitic agent. In 1970, the oral forms were withdrawn from the market due to reports of toxicity, but topical formulations for antifungal treatment remained available. Thus, the purpose of this study was to evaluate the toxicity, anti-Candida and antidermatophyte activity and to determine pharmacodynamic characteristics of clioquinol and other 8-hydroxyquinoline derivatives (8-hydroxy-5-quinolinesulfonic acid and 8-hydroxy-7-iodo-5-quinolinesulfonic acid). Antifungal activity was tested by broth microdilution and the fungicidal or fungistatic effect was checked by a time-kill assay. Permeation and histopathological evaluation were performed in Franz diffusion cells with ear skin of pigs and examined under light microscopy. An HET-CAM test was used to determine the potential irritancy. The three compounds were active against all isolates showing anti-Candida and antidermatophyte activity, with MIC ranges of 0.031-2μg/ml, 1-512μg/ml, and 2-1024μg/ml for clioquinol, 8-hydroxy-5-quinolinesulfonic acid, and 8-hydroxy-7-iodo-5-quinolinesulfonic acid, respectively. All compounds showed fungistatic effect for Candida, 8-hydroxy-5-quinolinesulfonic acid, and 8-hydroxy-7-iodo-5-quinolinesulfonic acid showed a fungicidal effect for M. canis and T. mentagrophytes, and clioquinol showed a fungicidal effect only for T. mentagrophytes. Furthermore, they presented a fungicidal effect depending on the time and concentration. The absence of lesions was observed in histopathological evaluation and no compound was irritating. Moreover, clioquinol and 8-hydroxy-5-quinolinesulfonic acid accumulated in the epithelial tissue, and 8-hydroxy-7-iodo-5-quinolinesulfonic acid had a high degree of permeation. In conclusion, 8-hydroxyquinoline derivatives showed antifungal activity and 8-hydroxy-5-quinolinesulfonic acid demonstrated the potential for antifungal drug design.

Nystatin Modulates Genes in Immunity and Wingless Signaling Pathways in Cow Blood

Nystatin is an antifungal agent isolated from bacteria found in the dairy cow environment. It disrupts small platforms in the cell membrane, composed of sphingolipids and cholesterol known as lipid rafts. Pathogen recognition receptors (PRR) may be embedded in these lipid rafts. This study was conducted to evaluate the in vitro effects of the lipid raft inhibitor Nystatin, on the expression of genes in the innate and adaptive immunity and wnt signaling pathway in cow peripheral blood. Blood collected from four adult female Holstein-Friesian cows (n=4) was treated with 100ng/mL of Nystatin in vitro. Samples treated with Phosphate Buffer Saline served as control. Total protein concentration and prostaglandin E2in plasma were determined. Total RNA was isolated from cells and was used for cDNA synthesis. The effect of Nystatin on the expression of 84 genes on the cow Wingless signaling pathway and human innate and adaptive immunity arrays were assessed in cow blood using real-time PCR. Fold change in transcript abundance was calculated using Livak’s method. Nystatin was found to modulate transcription and translation of genes involved in homeostasis and immunity in cow blood. It also increased the concentration of total plasma protein and PGE2in cow blood and may thus have had a pro-inflammatory effect. This study provides evidence for the association between lipid raft inhibition and alterations in the wingless signaling pathway in ruminant blood. Furthermore, the results presented may inform antifungal drug design and use in cows.

Structures of trehalose-6-phosphate phosphatase from pathogenic fungi reveal the mechanisms of substrate recognition and catalysis

Trehalose is a disaccharide essential for the survival and virulence of pathogenic fungi. The biosynthesis of trehalose requires trehalose-6-phosphate synthase, Tps1, and trehalose-6-phosphate phosphatase, Tps2. Here, we report the structures of the N-terminal domain of Tps2 (Tps2NTD) from Candida albicans, a transition-state complex of the Tps2 C-terminal trehalose-6-phosphate phosphatase domain (Tps2PD) bound to BeF3 and trehalose, and catalytically dead Tps2PD(D24N) from Cryptococcus neoformans bound to trehalose-6-phosphate (T6P). The Tps2NTD closely resembles the structure of Tps1 but lacks any catalytic activity. The Tps2PD-BeF3-trehalose and Tps2PD(D24N)-T6P complex structures reveal a "closed" conformation that is effected by extensive interactions between each trehalose hydroxyl group and residues of the cap and core domains of the protein, thereby providing exquisite substrate specificity. Disruption of any of the direct substrate-protein residue interactions leads to significant or complete loss of phosphatase activity. Notably, the Tps2PD-BeF3-trehalose complex structure captures an aspartyl-BeF3 covalent adduct, which closely mimics the proposed aspartyl-phosphate intermediate of the phosphatase catalytic cycle. Structures of substrate-free Tps2PD reveal an "open" conformation whereby the cap and core domains separate and visualize the striking conformational changes effected by substrate binding and product release and the role of two hinge regions centered at approximately residues 102-103 and 184-188. Significantly, tps2Δ, tps2NTDΔ, and tps2D705N strains are unable to grow at elevated temperatures. Combined, these studies provide a deeper understanding of the substrate recognition and catalytic mechanism of Tps2 and provide a structural basis for the future design of novel antifungal compounds against a target found in three major fungal pathogens.

The Aspergillus fumigatus Damage Resistance Protein Family Coordinately Regulates Ergosterol Biosynthesis and Azole Susceptibility.

ABSTRACTErgosterol is a major and specific component of the fungal plasma membrane, and thus, the cytochrome P450 enzymes (Erg proteins) that catalyze ergosterol synthesis have been selected as valuable targets of azole antifungals. However, the opportunistic pathogen Aspergillus fumigatus has developed worldwide resistance to azoles largely through mutations in the cytochrome P450 enzyme Cyp51 (Erg11). In this study, we demonstrate that a cytochrome b5-like heme-binding damage resistance protein (Dap) family, comprised of DapA, DapB, and DapC, coordinately regulates the functionality of cytochrome P450 enzymes Erg5 and Erg11 and oppositely affects susceptibility to azoles. The expression of all three genes is induced in an azole concentration-dependent way, and the decreased susceptibility to azoles requires DapA stabilization of cytochrome P450 protein activity. In contrast, overexpression of DapB and DapC causes dysfunction of Erg5 and Erg11, resulting in abnormal accumulation of sterol intermediates and further accentuating the sensitivity of ΔdapA strains to azoles. The results of exogenous-hemin rescue and heme-binding-site mutagenesis experiments demonstrate that the heme binding of DapA contributes the decreased azole susceptibility, while DapB and -C are capable of reducing the activities of Erg5 and Erg11 through depletion of heme. In vivo data demonstrate that inactivated DapA combined with activated DapB yields an A. fumigatus mutant that is easily treatable with azoles in an immunocompromised mouse model of invasive pulmonary aspergillosis. Compared to the single Dap proteins found in Saccharomyces cerevisiae and Schizosaccharomyces pombe, we suggest that this complex Dap family regulatory system emerged during the evolution of fungi as an adaptive means to regulate ergosterol synthesis in response to environmental stimuli.

Thiazole compounds with activity against Cryptococcus gattii and Cryptococcus neoformans in vitro

Human cryptococcosis can occur as a primary or opportunistic infection and develop as an acute, subacute, or chronic, systemic infection involving different host organs. We evaluated the antifungal activity of thirteen compounds against Cryptococcus gattii and Cryptococcus neoformans in vitro, by assessing the toxicity of the compounds showing the greatest antifungal activity in VERO cells and murine macrophages. From these results, four compounds were considered promising for further studies because they displayed low cytotoxicity and significant antifungal activity. The heterocyclic compounds 1b, 1c, 1d, and 1m have antifungal activity levels between that of amphotericin B and fluconazole in vitro. The death curve of Cryptococcus spp. treated with these four compounds was similar to the curve obtained for amphotericin B, in that we observed a significant reduction in cell viability within the first 24 h of treatment. Additionally, we found that there was no effect when these compounds were combined with amphotericin and fluconazole, except for 1c, which antagonized the effect of amphotericin B against C. gattii, also reflected in the reduction of the post-antifungal effect (PAFE); however, this interaction did not alter the ergosterol content. The results shown in this paper reveal the discovery of novel thiazole compounds, which are easy to synthesize, and with potentially exhibit antifungal activity, and display low cytotoxicity in normal mammalian cells. These compounds can be used as prototypes for the design of new antifungal drugs against C. gattii and C. neoformans.

Structural Analysis of a Fungal Methionine Synthase with Substrates and Inhibitors

The cobalamin-independent methionine synthase from Candida albicans, known as Met6p, is a 90-kDa enzyme that consists of two (βα)8 barrels. The active site is located between the two domains and has binding sites for a zinc ion and substrates l-homocysteine and 5-methyl-tetrahydrofolate-glutamate3. Met6p catalyzes transfer of the methyl group of 5-methyl-tetrahydrofolate-glutamate3 to the l-homocysteine thiolate to generate methionine. Met6p is essential for fungal growth, and we currently pursue it as an antifungal drug design target. Here we report the binding of l-homocysteine, methionine, and several folate analogs. We show that binding of l-homocysteine or methionine results in conformational rearrangements at the amino acid binding pocket, moving the catalytic zinc into position to activate the thiol group. We also map the folate binding pocket and identify specific binding residues, like Asn126, whose mutation eliminates catalytic activity. We also report the development of a robust fluorescence-based activity assay suitable for high-throughput screening. We use this assay and an X-ray structure to characterize methotrexate as a weak inhibitor of fungal Met6p.

Crystal structures of the fungal pathogen Aspergillus fumigatus protein farnesyltransferase complexed with substrates and inhibitors reveal features for antifungal drug design

Species of the fungal genus Aspergillus are significant human and agricultural pathogens that are often refractory to existing antifungal treatments. Protein farnesyltransferase (FTase), a critical enzyme in eukaryotes, is an attractive potential target for antifungal drug discovery. We report high-resolution structures of A. fumigatus FTase (AfFTase) in complex with substrates and inhibitors. Comparison of structures with farnesyldiphosphate (FPP) bound in the absence or presence of peptide substrate, corresponding to successive steps in ordered substrate binding, revealed that the second substrate-binding step is accompanied by motions of a loop in the catalytic site. Re-examination of other FTase structures showed that this motion is conserved. The substrate- and product-binding clefts in the AfFTase active site are wider than in human FTase (hFTase). Widening is a consequence of small shifts in the α-helices that comprise the majority of the FTase structure, which in turn arise from sequence variation in the hydrophobic core of the protein. These structural effects are key features that distinguish fungal FTases from hFTase. Their variation results in differences in steady-state enzyme kinetics and inhibitor interactions and presents opportunities for developing selective anti-fungal drugs by exploiting size differences in the active sites. We illustrate the latter by comparing the interaction of ED5 and Tipifarnib with hFTase and AfFTase. In AfFTase, the wider groove enables ED5 to bind in the presence of FPP, whereas in hFTase it binds only in the absence of substrate. Tipifarnib binds similarly to both enzymes but makes less extensive contacts in AfFTase with consequently weaker binding.

Characterization of the recombinant Candida albicans β-1,2-mannosyltransferase that initiates the β-mannosylation of cell wall phosphopeptidomannan

The presence of β-mannosides in their cell walls confers specific features on the pathogenic yeasts Candida albicans and Candida glabrata compared with non-pathogenic yeasts. In the present study, we investigated the enzymatic properties of Bmt1 (β-mannosyltransferase 1), a member of the recently identified β-mannosyltransferase family, from C. albicans. A recombinant soluble enzyme lacking the N-terminal region was expressed as a secreted protein from the methylotrophic yeast Pichia pastoris. In parallel, functionalized natural oligosaccharides isolated from Saccharomyces cerevisiae and a C. albicans mutant strain, as well as synthetic α-oligomannosides, were prepared and used as potential acceptor substrates. Bmt1p preferentially utilizes substrates containing linear chains of α-1,2-linked mannotriose or mannotetraose. The recombinant enzyme consecuti-vely transfers two mannosyl units on to these acceptors, leading to the production of α-mannosidase-resistant oligomannosides. NMR experiments further confirmed the presence of a terminal βMan (β-1,2-linked mannose) unit in the first enzyme product. In the future, a better understanding of specific β-1,2-mannosyltransferase molecular requirements will help the design of new potential antifungal drugs.

In vitro activity of the protegrin IB-367 alone and in combination compared with conventional antifungal agents against dermatophytes.

The occurrence of resistance or side effects in patients receiving antifungal agents leads to failure in the treatment of mycosis. The aim of this experimental study was to investigate the in vitro effects of IB-367 alone and in combination with three standard antifungal drugs, fluconazole (FLU), itraconazole (ITRA) and terbinafine (TERB), against 20 clinical isolates of dermatophytes belonging to three species. Minimum inhibitory concentrations (MICs), minimal fungicidal concentrations (MFCs), synergy test, time-kill curves, fungal biomass (FB) and hyphal damage using 2,3-bis-(2-methoxy-4-nitro-5-sulfenylamino carbonil)-2H-tetrazolium hydroxide assay (XTT) were performed to study the efficacy of IB-367. In this study, we observed that TERB and ITRA had MICs lower values for all the strains compared to IB-367 and FLU. Synergy was found in 35%, 30% and 25% of IB-367/FLU, IB-367/ITRA and IB-367/TERB interactions respectively. IB-367 exerted a fungicidal activity against Trichophyton mentagrophytes, T. rubrum and Microsporum canis at concentrations starting from 1x MIC. At a concentration of 5x MIC, IB-367 showed the highest rates of hyphae damage for M. canis 53% and T. mentagrophytes 50%; against the same isolates it caused a reduction of 1log of the total viable count cell hyphae damage. We propose IB-367 as a promising candidate for the future design of antifungal drugs.

CoMFA studies and in vitro evaluation of some 3-substituted benzylthio quinolinium salts as anticryptococcal agents

The 3-dimensional quantitative structure–activity relationship (3D-QSAR) molecular modeling technique or comparative molecular field analysis (CoMFA) has been used to design analogs of the natural product cryptolepine (1). Twenty-three compounds with their in vitro biological activities (IC50 values) against Crytococcus neoformans were used to generate the training set database of compounds for the CoMFA studies. The cross-validated q2, noncross-validated r2, and partial least squares (PLS) analysis results were used to predict the biological activity of 11 newly designed test set compounds. The best CoMFA model produced a q2 of 0.815 and an r2 of 0.976 indicating high statistical significance as a predictive model. The steric and electrostatic contributions from the contour map were interpreted from the color-coded contour plots generated from the PLS model and the active structural components for potency against C. neoformans were determined and validated in the test set compounds. The 3-substituted benzylthio quinolinium salts (4) that make up the test set were synthesized and evaluated based on the predicted activity from the CoMFA model and the results produced a good correlation between the predicted and experimental activity (R=0.82). Thus, CoMFA has served as an effective tool to aid the design of new analogs and in this case, it has aided the identification of compounds equipotent with amphotericin B, the gold standard in antifungal drug design.