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

Small molecules restore azole activity against drug-tolerant and drug-resistant Candida isolates

Each year, fungi cause more than 1.5 billion infections worldwide and have a devastating impact on human health, particularly in immunocompromised individuals or patients in intensive care units. The limited antifungal arsenal and emerging multidrug-resistant species necessitate the development of new therapies. One strategy for combating drug-resistant pathogens is the administration of molecules that restore fungal susceptibility to approved drugs. Accordingly, we carried out a screen to identify small molecules that could restore the susceptibility of pathogenic Candida species to azole antifungals. This screening effort led to the discovery of novel 1,4-benzodiazepines that restore fluconazole susceptibility in resistant isolates of Candida albicans, as evidenced by 100-1,000-fold potentiation of fluconazole activity. This potentiation effect was also observed in azole-tolerant strains of C. albicans and in other pathogenic Candida species. The 1,4-benzodiazepines selectively potentiated different azoles, but not other approved antifungals. A remarkable feature of the potentiation was that the combination of the compounds with fluconazole was fungicidal, whereas fluconazole alone is fungistatic. Interestingly, the potentiators were not toxic to C. albicans in the absence of fluconazole, but inhibited virulence-associated filamentation of the fungus. We found that the combination of the potentiators and fluconazole significantly enhanced host survival in a Galleria mellonella model of systemic fungal infection. Taken together, these observations validate a strategy wherein small molecules can restore the activity of highly used anti-infectives that have lost potency. IMPORTANCE In the last decade, we have been witnessing a higher incidence of fungal infections, due to an expansion of the fungal species capable of causing disease (e.g., Candida auris), as well as increased antifungal drug resistance. Among human fungal pathogens, Candida species are a leading cause of invasive infections and are associated with high mortality rates. Infections by these pathogens are commonly treated with azole antifungals, yet the expansion of drug-resistant isolates has reduced their clinical utility. In this work, we describe the discovery and characterization of small molecules that potentiate fluconazole and restore the susceptibility of azole-resistant and azole-tolerant Candida isolates. Interestingly, the potentiating 1,4-benzodiazepines were not toxic to fungal cells but inhibited their virulence-associated filamentous growth. Furthermore, combinations of the potentiators and fluconazole decreased fungal burdens and enhanced host survival in a Galleria mellonella model of systemic fungal infections. Accordingly, we propose the use of novel antifungal potentiators as a powerful strategy for addressing the growing resistance of fungi to clinically approved drugs.

Sources of Antifungal Drugs

Due to their eukaryotic heritage, the differences between a fungal pathogen's molecular makeup and its human host are small. Therefore, the discovery and subsequent development of novel antifungal drugs are extremely challenging. Nevertheless, since the 1940s, researchers have successfully uncovered potent candidates from natural or synthetic sources. Analogs and novel formulations of these drugs enhanced the pharmacological parameters and improved overall drug efficiency. These compounds ultimately became the founding members of novel drug classes and were successfully applied in clinical settings, offering valuable and efficient treatment of mycosis for decades. Currently, only five different antifungal drug classes exist, all characterized by a unique mode of action; these are polyenes, pyrimidine analogs, azoles, allylamines, and echinocandins. The latter, being the latest addition to the antifungal armamentarium, was introduced over two decades ago. As a result of this limited arsenal, antifungal resistance development has exponentially increased and, with it, a growing healthcare crisis. In this review, we discuss the original sources of antifungal compounds, either natural or synthetic. Additionally, we summarize the existing drug classes, potential novel candidates in the clinical pipeline, and emerging non-traditional treatment options.

Potentiation of the Activity of Antibiotics against ATCC and MDR Bacterial Strains with (+)-α-Pinene and (-)-Borneol

The increasing rates of antimicrobial resistance have demanded the development of new drugs as conventional antibiotics have become significantly less effective. Evidence has identified a variety of phytocompounds with the potential to be used in the combat of infections caused by multidrug-resistant (MDR) bacteria. Considering the verification that terpenes are promising antibacterial compounds, the present research aimed to evaluate the antibacterial and antibiotic-modulating activity of (+)-α-pinene and (-)-borneol against MDR bacterial strains. The broth microdilution method was used to determine the minimum inhibitory concentration (MIC) of the compounds and antibiotics and further evaluate the intrinsic and associated antibiotic activity. These analyses revealed that (+)-α-pinene showed significant antibacterial activity only against E. coli (MIC = 512 μg.mL⁻¹), while no significant inhibition of S. aureus and P. aeruginosa growth was observed (MIC ≥ 1024 μg mL⁻¹). However, when combined with antibiotics, this compound induced a significant improvement in the activity of conventional antibiotics, as observed for ciprofloxacin, amikacin, and gentamicin against Staphylococcus aureus, as well as for amikacin and gentamicin against Escherichia coli, and amikacin against Pseudomonas aeruginosa. On the other hand, (-)-borneol was found to inhibit the growth of E. coli and enhance the antibiotic activity of ciprofloxacin and gentamicin against S. aureus. The present findings indicate that (+)-α-pinene and (-)-borneol are phytocompounds with the potential to be used in the combat of antibacterial resistance.

Antifungal Effect of Liposomal α-Bisabolol and When Associated with Fluconazole

Fungal pathologies caused by the genus Candida have increased in recent years due to the involvement of immunosuppressed people and the advance of resistance mechanisms acquired by these microorganisms. Liposomes are nanovesicles with lipid bilayers in which they store compounds. α-Bisabolol is a sesquiterpene with proven biological activities, and in this work it was tested alone in liposomes and in association with Fluconazole in vitro to evaluate the antifungal potential, Fluconazole optimization, and virulence inhibitory effect in vitro. Antifungal assays were performed against standard strains of Candida albicans, Candida tropicalis, and Candida krusei by microdilution to identify the IC50 values and to obtain the cell viability. The Minimum Fungicidal Concentration (MFC) was performed by subculturing on the solid medium, and at their subinhibitory concentration (Matrix Concentration (MC): 16,384 µg/mL) (MC/16), the compounds, both isolated and liposomal, were associated with fluconazole in order to verify the inhibitory effect of this junction. Tests to ascertain changes in morphology were performed in microculture chambers according to MC concentrations. Liposomes were characterized from the vesicle size, polydispersity index, average Zeta potential, and scanning electron microscopy. The IC50 value of the liposomal bisabolol associated with fluconazole (FCZ) was 2.5 µg/mL against all strains tested, revealing a potentiating effect. Liposomal bisabolol was able to potentiate the effect of fluconazole against the CA and CT strains by reducing its concentration and completely inhibiting fungal growth. α-Bisabolol in liposomal form inhibited the morphological transition in all strains tested at a concentration of MC/8. The liposomes were homogeneous, with vesicles with diameters of 203.8 nm for the liposomal bisabolol and a surface charge potential of −34.2 mV, conferring stability to the nanosystem. Through scanning microscopy, the spherical shapes of the vesicles were observed.

Nikkomycin Z-Ready to Meet the Promise?

Nikkomycin Z (NikZ) has fungicidal activity against some fungal species which currently requires patients to endure chronic therapy, sometimes for years. This review highlights reports of NikZ activity against fungal species for which current therapeutics are still inadequate, as a potential roadmap for continuing investigation. The possibility of faster and more complete clinical resolution by using NikZ has attracted scientific attention for decades. NikZ inhibits chitin structure formation, which is important for fungi, but not found in mammals. NikZ raised no safety concerns in a human Phase 1 trial or in extensive toxicology studies. NikZ showed strong clinical benefit in dogs with natural Coccidioides infection. NikZ has protected animals against fatal infections of Candida albicans. NikZ provides high protection in synergistic combination with several agent classes against Candida and Aspergillus species.

A review on comparative mechanistic studies of antimicrobial peptides against archaea

Archaea was until recently considered as a third domain of life in addition to bacteria and eukarya but recent studies support the existence of only two superphyla (bacteria and archaea). The fundamental differences between archaeal, bacterial, and eukaryal cells are probably the main reasons for the comparatively lower susceptibility of archaeal strains to current antimicrobial agents. The possible emerging pathogenicity of archaea and the role of archaeal methanogens in methane emissions, a potent greenhouse gas, has led many researchers to examine the sensitivity patterns of archaea and make attempts to find agents that have significant anti-archaeal activity. Even though antimicrobial peptides (AMPs) are well known with several published reviews concerning their mode of action against bacteria and eukarya, to our knowledge, to date no reviews are available that focus on the action of these peptides against archaea. Herein, we present a review on all the peptides that have been tested against archaea. In addition, in an attempt to shed more light on possible future work that needs to be performed we have included a brief overview of the chemical characteristics, spectrum of activity, and the known mechanism of action of each of these peptides against bacteria and/or fungi. We also discuss the nature of and key physiological differences between Archaea, Bacteria, and Eukarya that are relevant to the development of anti-archaeal peptides. Despite our relatively limited knowledge about archaea, available data suggest that AMPs have an even broader spectrum of activity than currently recognized.

Anti-Candida albicans natural products, sources of new antifungal drugs: A review

INTRODUCTION

Candida albicans is the most prevalent fungal pathogen in humans. Due to the development of drug resistance, there is today a need for new antifungal agents for the efficient management of C. albicans infections. Therefore, we reviewed antifungal activity, mechanisms of action, possible synergism with antifungal drugs of all natural substances experimented to be efficient against C. albicans for future.

METHODS

An extensive and systematic review of the literature was undertaken and all relevant abstracts and full-text articles analyzed and included in the review.

REVIEW

A total of 111 documents were published and highlighted 142 anti-C. albicans natural products. These products are mostly are reported in Asia (44.37%) and America (28.17%). According to in vitro model criteria, from the 142 natural substances, antifungal activity can be considered as important for 40 (28.20%) and moderate for 24 (16.90%). Sixteen products have their antifungal activity confirmed by in vivo gold standard experimentation. Microbial natural products, source of antifungals, have their antifungal mechanism well described in the literature: interaction with ergosterol (polyenes), inhibition 1,3-β-d-glucan synthase (Echinocandins), inhibition of the synthesis of cell wall components (chitin and mannoproteins), inhibition of sphingolipid synthesis (serine palmitoyltransferase, ceramide synthase, inositol phosphoceramide synthase) and inhibition of protein synthesis (sordarins). Natural products from plants mostly exert their antifungal effects by membrane-active mechanism. Some substances from arthropods are also explored to act on the fungal membrane. Interestingly, synergistic effects were found between different classes of natural products as well as between natural products and azoles.

CONCLUSION

Search for anti-C. albicans new drugs is promising since the list of natural substances, which disclose activity against this yeast is today long. Investigations must be pursued not only to found more new anti-Candida compounds from plants and organisms but also to carried out details on molecules from already known anti-Candida compounds and to more elucidate mechanisms of action.

Chitin and glucan, the yin and yang of the fungal cell wall, implications for antifungal drug discovery and therapy

The structural carbohydrate polymers glucan and chitin compliment and reinforce each other in a dynamic process to maintain the integrity and physical strength of the fungal cell wall. The assembly of chitin and glucan in the cell wall of the budding yeast Saccharomyces cerevisiae and the polymorphic human pathogen Candida albicans are essential processes that involve a range of fungal-specific enzymes and regulatory networks. The fungal cell wall is, therefore, an attractive target for novel therapies as host cells lack many cell wall-related proteins. The most recent class of antifungal drug approved for clinical use, the echinocandins, targets the synthesis of cell wall β(1-3)glucan. The echinocandins are effective at treating invasive and bloodstream Candida infections and are now widely used in the clinic. However, there have been sporadic reports of breakthrough infections in patients undergoing echinocandin therapy. The acquisition of point mutations in the FKS genes that encode the catalytic β(1-3)glucan synthase subunits, the target of the echinocandins, has emerged as a dominant resistance mechanism. Cells with elevated chitin levels are also less susceptible to echinocandins and in addition, treatment with sub-MIC echinocandin activates cell wall salvage pathways that increase chitin synthesis to compensate for reduced glucan production. The development of drugs targeting the cell wall has already proven to be beneficial in providing an alternative class of drug for use in the clinic. Other cell wall targets such as chitin synthesis still hold great potential for drug development but careful consideration should be given to the capacity of fungi to manipulate their walls in a dynamic response to cell wall perturbations.

Calcineurin inhibition or mutation enhances cell wall inhibitors against Aspergillus fumigatus

Calcineurin mutation or inhibition enhanced the antifungal morphological effect of cell wall inhibitors caspofungin or nikkomycin Z against Aspergillus fumigatus. Quantification of 1,3-beta-d-glucan revealed decreased amounts in the calcineurin A (DeltacnaA) mutant. Calcineurin can be an excellent adjunct therapeutic target in combination with other cell wall inhibitors against A. fumigatus.

Hydrophobic derivatives of 2-amino-2-deoxy-D-glucitol-6-phosphate: a new type of D-glucosamine-6-phosphate synthase inhibitors with antifungal action

Several N-acyl and ester derivatives of 2-amino-2-deoxy-D-glucitol-6-phosphate (ADGP) have been synthesised and tested as inhibitors of fungal enzymes involved in early steps of chitin biosynthesis and for antifungal activity. All the tested derivatives were found to be much poorer inhibitors of the enzyme, D-glucosamine-6-phosphate (GlcN-6-P) synthase, than the parent compound but some of them exhibited much better antifungal activity. MIC values for the investigated compounds ranged between 10 mg mL(-1), found for ADGP and 0.3 mg mL(-1) for the most active derivative, namely ADGP dimethyl ester. Increased affinity of ADGP derivatives to the artificial immobilised cell membrane was correlated with their enhanced ability to be taken up by fungal cells by free diffusion. It was found that some of the examined derivatives behaved as 'pro-drugs' and after internalisation were converted into ADGP in the cell-free extract. This conversion was relatively rapid for ADGP esters but very slow for N-acyl derivatives. Results of our studies demonstrate a possibility of design and preparation of GlcN-6-P synthase inhibitors exhibiting antifungal activity.

Microbial natural products as a source of antifungals

The vast number and variety of chemotherapeutic agents isolated from microbial natural products and used to treat bacterial infections have greatly contributed to the improvement of human health during the past century. However, only a limited number of antifungal agents (polyenes and azoles, plus the recently introduced caspofungin acetate) are currently available for the treatment of life-threatening fungal infections. Furthermore, the prevalence of systemic fungal infections has increased significantly during the past decade. For this reason, the development of new antifungal agents, preferably with novel mechanisms of action, is an urgent medical need. A selection of antifungal agents in early stages of development, produced by micro-organisms, is summarized in this review. The compounds are classified according to their mechanisms of action, covering inhibitors of the synthesis of cell wall components (glucan, chitin and mannoproteins), of sphingolipid synthesis (serine palmitoyltransferase, ceramide synthase, inositol phosphoceramide synthase and fatty acid elongation) and of protein synthesis (sordarins). In addition, some considerations related to the chemotaxonomy of the producing organisms and some issues relevant to antifungal drug discovery are also discussed.

Synergy, pharmacodynamics, and time-sequenced ultrastructural changes of the interaction between nikkomycin Z and the echinocandin FK463 against Aspergillus fumigatus

We investigated the potential synergy between two cell wall-active agents, the echinocandin FK463 (FK) and the chitin synthase inhibitor nikkomycin Z (NZ), against 16 isolates of filamentous fungi. Susceptibility testing was performed with a broth macrodilution procedure by NCCLS methods. The median minimal effective concentration (MEC) of FK against all Aspergillus species was 0.25 microg/ml (range, 0.05 to 0.5 microg/ml). For Fusarium solani and Rhizopus oryzae, MECs of FK were >512 microg/ml. The median MEC of NZ against Aspergillus fumigatus was 32 microg/ml (range, 8 to 64 microg/ml), and that against R. oryzae was 0.5 microg/ml (range, 0.06 to 2 microg/ml); however, for the other Aspergillus species, as well as F. solani, MECs were >512 microg/ml. A checkerboard inhibitory assay demonstrated synergy against A. fumigatus (median fractional inhibitory concentration index = 0.312 [range, 0.15 to 0.475]). The effect was additive to indifferent against R. oryzae and indifferent against other Aspergillus spp. and F. solani. We further investigated the pharmacodynamics of hyphal damage by MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay and examined the time-sequenced changes in hyphal ultrastructure. Significant synergistic hyphal damage was demonstrated with the combination of NZ (2 to 32 microg/ml) and FK (0.03 to 0.5 microg/ml) over a wide range of concentrations (P < 0.001). The synergistic effect was most pronounced after 12 h of incubation and was sustained through 24 h. Time-sequenced light and electron microscopic studies demonstrated that structural alterations of hyphae were profound, with marked transformation of hyphae to blastospore-like structures, in the presence of FK plus NZ, while fungi treated with a single drug showed partial recovery at 24 h. The methods used in this study may be applicable to elucidating the activity and interaction of other cell wall-active agents. In summary, these two cell wall-targeted antifungal agents, FK and NZ, showed marked time-dependent in vitro synergistic activity against A. fumigatus.

Mechanism of antifungal action of kanosamine

The antibiotic kanosamine inhibited growth of Saccharomyces cerevisiae and a range of human pathogenic fungi, including Candida albicans. Kanosamine was transported into C. albicans cells by the glucose transport system and subsequently phosphorylated. The product of its intracellular metabolism, kanosamine-6-phosphate, was an inhibitor of the enzyme glucosamine-6-phosphate synthase. Inhibition was competitive in respect to one of the substrates, D-fructose-6-phosphate, with Ki = 5.9 mM, and was non-competitive in respect to the second substrate, L-glutamine. On the other hand, kanosamine-6-phosphate had no effect on the enzyme catalysing the next metabolic step, namely glucosamine-6-phosphate N-acetylase. The action of kanosamine on C. albicans cells resulted in profound morphological changes, inhibition of septum formation and cell agglutination. Experiments with S. cerevisiae mutants showed that the presence of the Cdr1p drug efflux pump did not affect the antifungal activity of kanosamine.

New drugs and novel targets for treatment of invasive fungal infections in patients with cancer

Invasive fungal infections have emerged as important causes of morbidity and mortality in profoundly immunocompromised patients with cancer. Current treatment strategies for these infections are limited by antifungal resistance, toxicity, drug interactions, and expense. In order to overcome these limitations, new antifungal compounds are being developed, which may improve our therapeutic armamentarium for prevention and treatment of invasive mycoses in high-risk patients with neoplastic diseases.

Inhibitors of the fungal cell wall. Synthesis of 4-aryl-4-N-arylamine-1-butenes and related compounds with inhibitory activities on beta(1-3) glucan and chitin synthases

As part of our project devoted to the search for antifungal agents, which act via a selective mode of action, we synthesized a series of new 4-aryl- or 4-alkyl-N-arylamine-1-butenes and transformed some of them into 2-substituted 4-methyl-tetrahydroquinolines and quinolines by using a novel three-step synthesis. Results obtained in agar dilution assays have shown that 4-aryl homoallylamines not possessing halogen in their structures, tetrahydroquinolines and quinolines, display a range of antifungal properties in particular against Epidermophyton floccosum and Microsporum canis. Regarding the mode of action, all active compounds showed in vitro inhibitory activities against beta(1-3) glucan-synthase and mainly against chitin-synthase. These enzymes catalyze the synthesis of beta(1-3) glucan and chitin, respectively, major polymers of the fungal cell wall. Since fungal but not mammalian cells are encased in a cell wall, its inhibition may represent a useful mode of action for these antifungal compounds.

In vitro antifungal activity of nikkomycin Z in combination with fluconazole or itraconazole

Nikkomycins are nucleoside-peptide antibiotics produced by Streptomyces species with antifungal activities through the inhibition of chitin synthesis. We investigated the antifungal activities of nikkomycin Z alone and in combination with fluconazole and itraconazole. Checkerboard synergy studies were carried out by a macrobroth dilution procedure with RPMI 1640 medium at pH 6.0. At least 10 strains of the following fungi were tested: Candida albicans, other Candida spp., Cryptococcus neoformans, Coccidioides immitis, Aspergillus spp., and dematiacious fungi (including Exophiala jeanselmei, Exophiala spinifera, Bipolaris spicifera, Wangiella dermatitidis, Ochroconis humicola, Phaeoannellomyces werneckii, and Cladophialophora bantiana), and 2 strains each of Fusarium, Scedosporium, Paecilomyces, Penicillium, and Trichoderma spp. A total of 110 isolates were examined. Inocula of fungal elements were standardized by hemacytometer counting or spectrophotometrically. MICs and minimum lethal concentrations (MLCs) were determined visually by comparison of growth in drug-treated tubes with growth in drug-free control tubes. Additive and synergistic interactions between nikkomycin and either fluconazole or itraconazole were observed against C. albicans, Candida parapsilosis, Cryptococcus neoformans, and Coccidioides immitis. Marked synergism was also observed between nikkomycin and itraconazole against Aspergillus fumigatus and Aspergillus flavus. No antagonistic interaction between the drugs was observed with any of the strains tested.

Adhesion of Candida albicans to epithelial cells--effect of nikkomycin

This study investigated the effect of the chitin synthetase inhibitors, the nikkomycins (NZ and NZ+NX), on Candida albicans adhesion to buccal epithelial cells (BECs) in vitro. The effect was expressed in reduced chitin synthetase activity and chitin content of fungal cells. In vitro adhesion assays to BECs of Candida exposed to NZ and NZ+NX revealed reduced adhesion values. Light, scanning and transmission electron microscopy (SEM, TEM) of NZ-treated and untreated micro-organisms showed changed fungal morphology and reduced adherence of the treated yeasts. Scanning electron microscopy of NZ-treated C. albicans labelled with gold-conjugated wheatgerm agglutinin (WGA) revealed less labelling than in the untreated organisms. A close contact between the fungus and the epithelial cell at a site with intense WGA-gold labelling was noted in TEM experiments. The data point to the involvement of chitin in the adhesion of C. albicans to epithelial cells.

Use of decimal assay for additivity to demonstrate synergy in pair combinations of econazole, nikkomycin Z, and ibuprofen against Candida albicans in vitro

Interactions between six compounds (econazole, miconazole, amphotericin B, nystatin, nikkomycin Z, and ibuprofen) were investigated for their antifungal activities against Candida albicans by using pair combinations in an in vitro decimal assay for additivity based on disk diffusion. Additive interactions were observed between miconazole and econazole, amphotericin B and nystatin, and amphotericin B and ibuprofen, while an antagonistic interaction was observed between econazole and amphotericin B. Synergistic interactions were recorded for the combinations of econazole and ibuprofen, econazole and nikkomycin Z, and ibuprofen and nikkomycin Z.

Nikkomycin Z supersensitivity of an echinocandin-resistant mutant of Saccharomyces cerevisiae

Echinocandins and nikkomycins are antibiotics that inhibit the synthesis of the essential cell wall polysaccharide polymers 1,3-beta-glucan and chitin, respectively. Some 40 echinocandin-resistant Saccharomyces cerevisiae mutants were isolated and assigned to five complementation groups. Four complementation groups contained mutants with 38 recessive mutations. The fifth complementation group comprised mutants with one dominant mutation, etg1-3 (strain MS10), and one semidominant mutation, etg1-4 (strain MS14). MS10 and MS14 are resistant to the semisynthetic pneumocandin B, L-733,560, and to aculeacin A but not to papulacandin. In addition, microsomal membranes of both mutant strains contain 1,3-beta-glucan synthase activity that is resistant to L-733,560 but not to papulacandin. Furthermore, MS14 is also supersensitive to nikkomycin Z. The echinocandin resistance and the nikkomycin Z supersensitivity of MS14 cosegregated in genetic crosses. The wild-type gene (designated ETG1 [C. Douglas, J. A. Marrinan, and M. B. Kurtz, J. Bacteriol. 176:5686-5696, 1994, and C. Douglas, F. Foor, J. A. Marrinan, N. Morin, J. B. Nielsen, A. Dahl, P. Mazur, W. Baginsky, W. Li, M. El-Sherbeini, J. A. Clemas, S. Mandala, B. R. Frommer, and M. B. Kurtz, Proc. Natl. Acad. Sci. USA, in press]) was isolated from a genomic library in the plasmid YCp50 by functional complementation of the nikkomycin Z supersensitivity phenotype. The cloned DNA also partially complements the echinocandin resistance phenotype, indicating that the two phenotypes are due to single mutations. The existence of a single mutation, in MS14, simultaneously affecting sensitivity to a glucan synthase inhibitor and a chitin synthase inhibitor implies a possible interaction between the two polymers at the cell surface.

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.

Chitin biosynthesis in Candida albicans grown in vitro and in vivo and its inhibition by nikkomycin Z

An N-acetyl-D-[14C]glucosamine radiolabel incorporation assay has been used to monitor chitin biosynthesis in whole cells of Candida albicans both in vitro and in vivo in two different mouse infection models, one using the peritoneal cavity as a chamber in which to add and retrieve cells and the other using infected kidneys. Specific labeling of chitin in alkali-insoluble material was confirmed by chitinase digestion, analysis of acid hydrolysates, and the use of nikkomycin Z as a probe. Nikkomycin Z was shown to strongly inhibit chitin biosynthesis in C. albicans grown in vitro and in vivo in both models. This demonstrates that nikkomycin Z-susceptible chitin synthase activity is present in C. albicans when the fungus is in its pathogenic state in vivo. The limited use of nikkomycin as a therapeutic agent is discussed.

Positive interaction of nikkomycins and azoles against Candida albicans in vitro and in vivo

Nikkomycins X and Z (NZ), competitive inhibitors of fungal chitin synthetase, were combined with azoles in a series of in vitro checkerboard assays to test for synergism against Candida spp. All combinations of nikkomycins and azoles tested resulted in marked synergistic activity against an isolate of Candida albicans, with fractional inhibitory concentration indices ranging from 0.016 to 0.28. No synergistic effect was demonstrable with isolates of C. tropicalis, C. parapsilosis, or C. krusei, though results for the latter two were suggestive of an additive effect. In survival models of mice infected intravenously with C. albicans, NZ administered singly in doses ranging from 5 to 50 mg/kg of body weight twice a day was able to delay the onset of mortality but showed no dose-response effect. The combination of NZ and the azole R 3783 administered orally in a ratio of 8:1 to 40:1 or greater (wt/wt) enhanced survival better than did the drugs given individually, but this effect was less evident for combinations involving fluconazole. In short-term organ load assays with outbred mice infected intravenously with C. albicans, high ratios of NZ to R 3783 reduced the CFU per gram in kidneys more significantly than did the drugs individually. Statistically significant reductions were not seen for short-term fungal burden assays using combinations of NZ and fluconazole in outbred mice or in inbred mice more susceptible to candidiasis. In a model of rat vaginal candidiasis, the combination of NZ and R 3783 administered either orally or vaginally was more effective than the drugs used singly. Thus, under certain conditions, combination therapy with nikkomycin and select azoles may offer promise for an increased therapeutic effect in candidiasis.

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.