Synthesis and biological activity of tripeptidyl polyoxins as antifungal agents

Total Synthesis of (+)-Polyoxamic Acid via Visible-Light-Mediated Photocatalytic β-Scission and 1,5-Hydrogen Atom Transfer of Glucose Derivative

A total synthesis of polyoxamic acid has been achieved. The key feature of the synthetic route is a visible-light-mediated β-scission and carbon-to-carbon 1,5-hydrogen atom transfer (1,5-HAT) to provide the functionalized alditol under mild conditions. This type of carbon-to-carbon 1,5-HAT initiated by C(sp³)-centered radicals has been scarcely reported. Furthermore, the reaction was adapted for flow chemistry, facilitating the total synthesis of polyoxamic acid.

Antifungal peptides produced by actinomycetes and their biological activities against plant diseases

Antibacterial peptides are a class of naturally occurring peptides produced by eukaryotes and prokaryotes. Some of them exhibit broad-spectrum antifungal activity. Antifungal peptides (AFPs) can be developed as antibiotic to control fungal infections in agriculture due to their different antifungal mechanisms. As actinomycetes are still one of the most important sources of novel antibiotics, in this review, the mechanisms of action of AFPs are explained. Characterization of several AFPs produced by actinomycetes and their biological activities against plant diseases are summarized. Furthermore, the pathway for total synthesis of naturally occurring cyclodepsipeptide, valinomycin, is proposed. Finally, the pathway for biosynthesis of kutzneride 2 is proposed and the structure-activity relationship of kutznerides is discussed.

Recent Status and Advancements in the Development of Antifungal Agents: Highlights on Plant and Marine Based Antifungals

The developing resistance in fungi has become a key challenge, which is being faced nowadays with the available antifungal agents in the market. Further search for novel compounds from different sources has been explored to meet this problem. The current review describes and highlights recent advancement in the antifungal drug aspects from plant and marine based sources. The current available antifungal agents act on specific targets on the fungal cell wall, like ergosterol synthesis, chitin biosynthesis, sphingolipid synthesis, glucan synthesis etc. We discuss some of the important anti-fungal agents like azole, polyene and allylamine classes that inhibit the ergosterol biosynthesis. Echinocandins inhibit β-1, 3 glucan synthesis in the fungal cell wall. The antifungals poloxins and nikkomycins inhibit fungal cell wall component chitin. Apart from these classes of drugs, several combinatorial therapies have been carried out to treat diseases due to fungal resistance. Recently, many antifungal agents derived from plant and marine sources showed potent activity. The renewed interest in plant and marine derived compounds for the fungal diseases created a new way to treat these resistant strains which are evident from the numerous literature publications in the recent years. Moreover, the compounds derived from both plant and marine sources showed promising results against fungal diseases. Altogether, this review article discusses the current antifungal agents and highlights the plant and marine based compounds as a potential promising antifungal agents.

Nanoparticle-Based Mycosis Vaccine

Many diseases that were considered major affliction of mankind in the past have been successfully eradicated with introduction of appropriate vaccine strategies. In order to expedite new challenges coming up to deal with various infectious diseases, nano-particulate-based subunit vaccines seem to be the demand of ordeal. The nano-vaccines can find better scope for the diseases that were not rampant in the semi-advanced world few years back. For example in present-day circumstances that corroborate with advancement in the field of medical sciences in terms of cancer chemotherapy, organ transplantation, therapy of autoimmune diseases, etc.; along with prevalence of altogether unheard diseases such as HIV infection, people are at risk of infliction with many more pathogens. In this regard, development of an effective prophylactic strategy against many opportunistic infections primarily caused by fungal pathogens needs better understanding of host pathogen relation and role of active immunity against pathogenic fungi. In the present study, we have tried to decipher effectiveness of a nano-sized vaccine delivery system in imparting protection against fungal pathogens.

Diastereoselective synthesis of furanose and pyranose substituted glycine and alanine derivatives via proline-catalyzed asymmetric α-amination of aldehydes

A concise organocatalytic route toward the synthesis of furanose and pyranose substituted glycine and alanine derivatives is reported. These compounds are core structural units of some of the naturally available antibiotics and antifungal agents. Proline-catalyzed asymmetric α-amination of aldehydes derived from sugars is used as the key reaction to synthesize twelve sugar amino acid derivatives. The asymmetric transformations proceeded in good yields and with good to excellent diastereoselectivity. The application of the synthesized amino acids is demonstrated by synthesizing a tripeptide containing one of them.

Parallel solution-phase synthesis and general biological activity of a uridine antibiotic analog library

A small library of ninety four uridine antibiotic analogs was synthesized, under the Pilot Scale Library (PSL) Program of the NIH Roadmap initiative, from amine 2 and carboxylic acids 33 and 77 in solution-phase fashion. Diverse aldehyde, sulfonyl chloride, and carboxylic acid reactant sets were condensed to 2, leading after acid-mediated hydrolysis, to the targeted compounds 3–32 in good yields and high purity. Similarly, treatment of 33 with diverse amines and sulfonamides gave 34–75. The coupling of the amino terminus of d-phenylalanine methyl ester to the free 5′-carboxylic acid moiety of 33 followed by sodium hydroxide treatment led to carboxylic acid analog 77. Hydrolysis of this material gave analog 78. The intermediate 77 served as the precursor for the preparation of novel dipeptidyl uridine analogs 79–99 through peptide coupling reactions to diverse amine reactants. None of the described compounds show significant anticancer or antimalarial acivity. A number of samples exhibited a variety of promising inhibitory, agonist, antagonist, or activator properties with enzymes and receptors in primary screens supplied and reported through the NIH MLPCN program.

Phosphonoxins III: synthesis of α-aminophosphonate analogs of antifungal polyoxins with anti-Giardia activity

A synthesis of α-aminophosphonate analogs of polyoxins, termed phosphonoxin C1, C2, and C3, has been achieved. The key step was the addition of lithium dimethyl phosphite to the aldehyde of a protected threose derivative. α-Hydroxyphosphonate analogs C4 and C5 were also obtained by taking advantage of an unprecedented conversion of an azide to hydroxyl during treatment with hydrogen on palladium on carbon. The resulting phosphonoxin C5 inhibited the growth of an intestinal protozoan, Giardia lamblia, at low micromolar concentration.

Enhancement of the diversity of polyoxins by a thymine-7-hydroxylase homolog outside the polyoxin biosynthesis gene cluster

Polyoxins consist of 14 structurally variable components which differentiate at three branch sites of the carbon skeleton. Open reading frame (ORF) SAV_4805 of Streptomyces avermitilis, showing similarity to thymine-7-hydroxylase, was proved to enhance the diversity of polyoxins at the C-5 site of the 1-(5'-amino-5'-deoxy-β-d-allofuranuronosyl) pyrimidine moiety.

Syntheses of carbocyclic uracil polyoxin C analogs: application of Pd(0)/InI-allylation of 4-acetoxy-2-azetidinone

Carbocyclic uracil polyoxin C analogs are prepared from an acylnitroso-derived hetero Diels-Alder cycloadduct in fewer than nine steps. Pd(0)/InI-mediated allylation of 4-acetoxy-2-azetidinone is used to install the beta-amino acid side chain at the C-5' position of the carbocycle.

Studies on peptidyl nucleoside antibiotics: synthesis and antifungal evaluation of pyranosyl nucleoside analogs of nikkomycin

Background: The Streptomyces-derived nikkomycins are a unique class of peptidyl nucleoside natural products, with potent antifungal activity against a variety of pathogenic fungi. Results: In continuation of our structure–activity relationship studies on the nikkomycins, this paper describes the strategic design, synthesis and biological evaluation of a ‘doubly modified’ generation of nikkomycin analogs. The structural modifications included a ring-expanded carbohydrate core and a simplified peptidyl side chain. Biological screening of these novel analogs against clinical isolates of various human pathogenic fungi indicated that the described modifications of the structural features of nikkomycin could be a potentially beneficial strategy towards optimizing the antifungal potency of this class of peptidyl nucleoside antibiotics. Conclusion: Continued investigation of the pyranosyl nikkomycin analogs is warranted to fully explore and optimize the structural features of this novel lead for the desired development of a new class of therapeutically useful antifungal drugs.

Substrate preference is altered by mutations in the fifth transmembrane domain of Ptr2p, the di/tri-peptide transporter of Saccharomyces cerevisiae

The integral membrane protein Ptr2p transports di/tri-peptides into the yeast Saccharomyces cerevisiae. The sequence FYXXINXG (FYING motif) in the 5th transmembrane domain (TM5) is invariably conserved among the members of the PTR (Peptide TRansport) family ranging from yeast to human. To test the role of TM5 in Ptr2p function, Ala-scanning mutagenesis of the 22 residues comprising TM5 was completed. All mutated transporters, with the exception of the Y248A mutant, were expressed as determined by immunoblots. In peptide-dependent growth assays, ten mutants of the non-FYING residues grew as well as wild-type Ptr2p on all twelve different peptides tested. All of the FYING motif mutants, except the non-expressed Y248A, plus seven other mutants in TM5 exhibited differential growth on peptides including Leu-Leu and Met-Met-Met indicating that these mutations conferred substrate preference. In assays measuring direct uptake of the radioactive peptides (3)H-Leu-Leu or (14)C-Met-Met-Met, the F, I and G mutants of the FYING motif did not demonstrate accumulation of these peptides over a ten minute interval. The mutation N252A of the FYING motif, along with L240A, M250A, and L258A, exhibited differential substrate preference for Met-Met-Met over Leu-Leu. Other mutations (T239A, Q241A, N242A, M245A, and A260) resulted in preference for Leu-Leu over Met-Met-Met. These data demonstrate that TM5, in particular its conserved FYING motif, is involved in substrate preference of Ptr2p.

Is a vaccine needed against Candida albicans?

The development of a useful Candida vaccine is a distinct possibility despite the fact that individuals with a lifetime of commensal sensitization do not develop sterile immunity to the organism. An effective Candida vaccine would be invaluable in preventing hematogenously disseminated candidiasis, as well as mucocutaneous disease. This review is a discussion of our current understanding of the interplay between commensal and pathogenic forms of Candida albicans and approaches toward active and passive immunoprevention against candidiasis.

Second-generation dimeric inhibitors of chitin synthase

Chitin synthase (CS) is essential for fungal cell wall biosynthesis and is an attractive medicinal target. Expanded results from our efforts to develop mechanism based inhibitors of CS are presented here. Specifically, we describe uridine dimers linked by tartrate amides as potential pyrophosphate mimics.

Total Synthesis of (+/-) Carbocyclic Polyoxin C and Its alpha-Epimer

Carbocyclic polyoxin C (2) and its alpha-epimer 3 were synthesized in racemic form in an efficient and diastereodivergent fashion from cis-4-(N-tert-butylcarbamoyl)cyclopent-2-en-1-ol (5a). This synthesis features a Pd(0)-catalyzed substitution reaction, a novel, mild reduction of an alpha-nitro ester to an amino acid ester, and an improved procedure for uracil ring formation.

Cloning of a Candida albicans peptide transport gene

A Candida albicans peptide transport gene, CaPTR2, was cloned from a C. albicans genomic library by functional complementation of a peptide transport deficient mutant (strain ptr2-2) of Saccharomyces cerevisiae. CaPTR2 restored peptide transport to transformants as determined by uptake of radiolabelled dileucine, growth on dipeptides as sources of required amino acids, and restoration of growth inhibition by toxic peptides. Plasmid curing experiments demonstrated that the peptide transport phenotype was plasmid borne. CaPTR2 was localized to chromosome R of C. albicans by contour-clamped homologous electric field gel chromosome blots. Deletion subclones and frameshift mutagenesis were used to narrow the peptide transport complementing region to a 5.1 kb DNA fragment. DNA sequencing of the complementing region identified an ORF of 1869 bp containing an 84 nucleotide intron. The deduced amino acid sequence predicts a protein of 70 kDa consisting of 623 amino acids with 12 hydrophobic segments. A high level of identity was found between the predicted protein and peptide transport proteins of S. cerevisiae and Arabidopsis thaliana. This study represents the first steps in the genetic characterization of peptide transport in C. albicans and initiates a molecular approach for the study of drug delivery against this pathogen.

Compounds active against cell walls of medically important fungi

A number of substances that directly or indirectly affect the cell walls of fungi have been identified. Those that actively interfere with the synthesis or degradation of polysaccharide components share the property of being produced by soil microbes as secondary metabolites. Compounds specifically interfering with chitin or beta-glucan synthesis have proven effective in studies of preclinical models of mycoses, though they appear to have a restricted spectrum of coverage. Semisynthetic derivatives of some of the natural products have offered improvements in activity, toxicology, or pharmacokinetic behavior. Compounds which act on the cell wall indirectly or by a secondary mechanism of action, such as the azoles, act against diverse fungi but are usually fungistatic in nature. Overall, these compounds are attractive candidates for further development.

Systemically administered antifungal agents. A review of their clinical pharmacology and therapeutic applications

Systemic antifungal agents express great diversity in their pharmacokinetic profiles, mechanisms of action, and toxicities. Understanding the diverse pharmacokinetic properties of systemic antifungals is critical to their appropriate application. Amphotericin B, drug of choice for most invasive mycoses, has unique pharmacokinetic properties, binding initially to serum lipoproteins and redistributing from blood to tissues. Dosing recommendations are based on the specific infection and the status of the host. Lipid formulations of amphotericin B may be able to attenuate some of its toxicities. Flucytosine is a water-soluble, fluorinated pyrimidine that possesses excellent bioavailability. It is administered only in combination with amphotericin B because of frequent development of secondary drug resistance, and is associated with dose-dependent bone marrow suppression. The antifungal azoles are relatively well tolerated, have broad spectrum antifungal activity, and are fungistatic in vitro. Ketoconazole and itraconazole are highly bound to plasma proteins, are extensively metabolised by the liver, and are relatively insoluble in aqueous solution. By comparison, fluconazole is only weakly bound to serum proteins, is relatively stable to metabolic conversion, and is water soluble. Fluconazole penetrates the cerebrospinal fluid well and is approved for primary and suppressive therapy of cryptococcal meningitis in AIDS patients. The echinocandins have a narrow spectrum of antifungal activity, being effective only against Candida spp.

Differential inhibition of chitin synthetases 1 and 2 from Saccharomyces cerevisiae by polyoxin D and nikkomycins

Polyoxin D, nikkomycin X, and nikkomycin Z are all competitive inhibitors of chitin synthetase 2 (Chs2), the essential enzyme for primary septum formation in Saccharomyces cerevisiae, and of Chs1, a repair enzyme. However, Chs2 is more resistant to these antibiotics than Chs1. When Co2+, the best stimulator of Chs2, was used in the assay for this enzyme, the differences in the Ki values for nikkomycins between the two isozymes reached 3 orders of magnitude. These results point to differences in the active sites of the two isozymes. Polyoxin D was much more effective than nikkomycin Z in inhibiting cell growth. This underlines the importance of the choice of enzyme and of assay conditions when cell wall-synthesizing enzymes are used in screens for possible antifungal agents.

The use of flow cytometry to monitor chitin synthesis in regenerating protoplasts of Candida albicans

Flow cytometry was used to monitor chitin synthesis in regenerating protoplasts of the yeast Candida albicans. Comparisons of cells stained with Calcofluor White, a fluorochrome with known affinity for chitin, and cells incubated in the presence of N-[3H]-acetylglucosamine, the precursor substrate for chitin, showed a linear relationship between fluorescence and incorporation of label over time. Changes in both the fluorescence and light scatter of regenerating protoplasts treated with inhibitors of fungal chitin synthase were also quantitated by flow cytometry.

Cilofungin (LY121019), an antifungal agent with specific activity against Candida albicans and Candida tropicalis

Cilofungin (LY121019) is an antifungal agent that interferes with beta-glucan synthesis in the cells walls of fungi. The activity of this agent against 256 clinical isolates of yeasts was determined. It was found to be very active in vitro against Candida albicans (MIC for 90% of isolates [MIC90], less than or equal to 0.31 microgram/ml; minimal fungicidal concentration for 90% of isolates [MFC90], less than or equal to 0.31 micrograms/ml) and C. tropicalis (MIC90, less than or equal to 0.31 microgram/ml; MFC90, less than or equal to 0.31 microgram/ml) and moderately active against Torulopsis glabrata (MIC90 and MFC90, less than or equal to 20 micrograms/ml). All C. parapsilosis, Cryptococcus, and Saccharomyces cerevisiae strains were resistant. The activity of cilofungin was affected by medium and inoculum size. Antibiotic medium no. 3 was used as the standard medium. Isolates of C. albicans and C. tropicalis demonstrated a paradoxical effect in Sabouraud dextrose broth and yeast nitrogen base broth in that growth was partially inhibited at MICs equivalent to those in antibiotic medium no. 3, but growth continued, in many instances, throughout all concentrations tested. There was decreased activity of cilofungin with inocula greater than 10(5) CFU/ml. The temperature and duration of incubation did not affect its activity.

Chemochromatography: its use for the separation of nikkomycins X and Z

A novel mode of reversed-phase high-performance liquid chromatography in which the mobile phase reacts chemically with the compounds to be separated was developed. Nikkomycin X and nikkomycin Z, two natural isomeric nucleoside peptide antibiotics, move as a single peak on a C18 reversed-phase column using an aqueous trifluoroacetic acid mobile phase. Addition of sodium bisulfite (1.0%) to the mobile phase results in the formation of a polar bisulfite addition product with nikkomycin X, but not with nikkomycin Z, inside the HPLC column. This type of reactive chromatography, or chemochromatography, led to the analytical and preparative separation of nikkomycins X and Z which are normally very intractable to separation by conventional chromatographic techniques.

The genetics of medically important fungi

The present review is concerned with recent progress in basic genetic investigations with a variety of fungi which are pathogenic for man and animals. The principles and strategies involved in undertaking genetic investigations of sexual species and of asexual species are discussed. Progress in genetic analysis of Cryptococcus neoformans made possible by the discovery of its sexual phase is described in detail, as is progress in development of parasexual methods of analysis in Candida albicans. The genetic bases of virulence and drug resistance are discussed for those few species in which these phenotypes have been investigated. Suggestions for future research, including the application of recent advances in molecular biology to the study of pathogenic fungi, are presented.

Synergistic action of nikkomycins X and Z with papulacandin B on whole cells and regenerating protoplasts of Candida albicans

Combinations of nikkomycin X (NX) or nikkomycin Z (NZ), known inhibitors of chitin synthesis in fungi, together with papulacandin B (PB), an inhibitor of beta-glucan synthesis, were tested for synergistic activity against four isolates of Candida albicans by using the broth microdilution checkerboard technique and a method to assess the regeneration of cell wall material in protoplasts. The construction of isobolograms from the data generated by the checkerboard determinations revealed a synergistic effect for the two classes of compounds against all strains. The combination of NX and PB was more effective than the combination of NZ and PB, perhaps reflecting the lower Ki value of NX. While the presence of NX and NZ reduced chitin synthesis, as determined by staining with calcofluor white and assaying with a microfluorimeter, cells treated with PB demonstrated an increased synthesis of chitin. Protoplast regeneration experiments using similar concentrations of the two classes of compounds resulted in comparable findings. The combination of NX and PB resulted in a greater inhibition of chitin synthesis than did equivalent combinations of NZ and PB. These data suggest that combinations of agents active against cell wall synthesis in fungi may prove more useful as chemotherapeutic agents than such compounds used singly.

Hydrophobic polyoxins are resistant to intracellular degradation in Candida albicans

Two novel polyoxins, N-epsilon-(octanoyl)-lysyl-uracil polyoxin C (Oct-Lys-UPOC) and N-gamma-(octyl)-glutaminyluracil polyoxin C (Oct-Gln-UPOC), were synthesized by reacting uracil polyoxin C with the appropriate amino acid p-nitrophenyl ester. Oct-Lys-UPOC and Oct-Gln-UPOC were strong inhibitors (Kis = 1.7 X 10(-6)M) of chitin synthetase from Candida albicans membrane preparations. In a permeabilized-cell assay, Oct-Gln-UPOC had a 10-fold-lower inhibitory activity toward chitin synthetase than did the Oct-Lys-UPOC analog. Both compounds were resistant to hydrolysis by a cell extract of C. albicans H317; however, Oct-Gln-UPOC was hydrolyzed with a half-life of 23 min by a permeabilized-cell preparation. Oct-Lys-UPOC was resistant to hydrolysis by permeabilized cells. Oct-Gln-UPOC and Oct-Lys-UPOC did not compete with the transport of peptides or uridine into the cell. At concentrations up to 2 mM these two new polyoxins were ineffective in the inhibition of cell growth or reduction of cell viability, but they induced aberrant morphologies in C. albicans at a concentration of 0.25 mM. These data suggest that polyoxins containing hydrophobic amino acids retain strong chitin synthetase inhibitory activity and are resistant to cellular hydrolysis. They provide the first example of effective synthetic chitin synthetase inhibitors which are stable inside C. albicans.

Multiplicity of peptide permeases in Candida albicans: evidence from novel chromophoric peptides

Evidence is presented for the presence of multiple peptide permeases in the eucaryotic organism Candida albicans. Instrumental in these studies were the peptides L-alanyl-L-2-thiophenylglycine (Ala-alpha-TPG) and L-alanyl-L-2-thiophenylglycyl-L-alanine (Ala-alpha-TPG-Ala), which contain thiophenol attached to the alpha-carbon of glycine. Subsequent to transport into the fungal cell, enzymatic hydrolysis of these peptides resulted in the release of free thiophenol, which was quantified by using Ellman reagent. Thiophenol release was shown to be directly correlated to peptide transport and hydrolysis, with transport being the rate-limiting step in intact cells. These peptides, whose uptake showed Michaelis-Menten kinetics, have been used to determine peptide uptake in C. albicans. In addition, we found that the intracellular peptidases can readily be assayed in permeabilized cells and that bestatin, an aminopeptidase inhibitor, inhibits all detectable peptidase activity. C. albicans 124 was able to transport and hydrolyze both Ala-alpha-TPG and Ala-alpha-TPG-Ala, whereas the mutant (124NIK5) was able to transport only the tripeptide. The intracellular peptidases of this mutant were unaffected. In wild-type C. albicans 124, oligopeptides were able to compete with uptake of Ala-alpha-TPG-Ala to a far greater extent than with that of Ala-alpha-TPG; dipeptides inhibited uptake of both Ala-alpha-TPG and Ala-alpha-TPG-Ala. These results provide complementary evidence for the existence of distinct transport systems.

Chitin synthase in Candida albicans: comparison of digitonin-permeabilized cells and spheroplast membranes

The treatment of Candida albicans (yeast form) with digitonin or dimethyl sulfoxide permeabilized cells and caused the activation of chitin synthase in situ. Endogenous activation was completely prevented by the sulfhydryl reagents N-ethylmaleimide, p-chloromercuribenzoate, and 5,5'-dithiobis(2-nitrobenzoic acid); partially prevented by the protease inhibitors antipain, leupeptin, and N alpha-tosyl-L-lysyl chloromethyl ketone; and also partially prevented by EDTA. Thus, a clostripain-like protease may be involved in the endogenous activation phenomenon. The pH activity profile, cofactor requirements, and kinetic parameters of the endogenously activated chitin synthase were identical to those of the trypsin-activated enzyme in protoplast membranes.

Sensitivity to nikkomycin Z in Candida albicans: role of peptide permeases

The uptake of tritiated nikkomycin Z, a potent inhibitor of chitin synthetase, is mediated by a peptide transport system in Candida albicans. Kinetic transport assays with radioactive di- and tripeptides and competition studies suggest that two distinct systems operate in this yeast. Nikkomycin Z was transported through one of these systems, common to di- and tripeptides. A peptide transport-deficient mutant was isolated on the basis of its resistance to nikkomycin Z. The mutant lost most of its capacity to take up dipeptides but simultaneously increased its ability to transport tripeptides. These results indicate that C. albicans handles peptides through multiple transport systems and adjusts their expression to environmental conditions.