Activity of cilofungin (LY 121019), a new lipopeptide antibiotic, on the cell wall and cytoplasmic membrane of Candida albicans. Structural modifications in scanning and transmission electron microscopy

Echinocandins: production and applications

The first echinocandin-type antimycotic (echinocandin B) was discovered in the 1970s. It was followed by the isolation of more than 20 natural echinocandins. These cyclic lipo-hexapeptides are biosynthesized on non-ribosomal peptide synthase complexes by different ascomycota fungi. They have a unique mechanism of action; as non-competitive inhibitors of β-1,3-glucan synthase complex they target the fungal cell wall. Results of the structure-activity relationship experiments let us develop semisynthetic derivatives with improved properties. Three cyclic lipohiexapeptides (caspofungin, micafungin and anidulafungin) are currently approved for use in clinics. As they show good fungicidal (Candida spp.) or fungistatic (Aspergillus spp.) activity against the most important human pathogenic fungi including azole-resistant strains, they are an important addition to the antifungal armamentarium. Some evidence of acquired resistance against echinocandins has been detected among Candida glabrata strains in recent years, which enhanced the importance of data collected on the mechanism of acquired resistance developing against the echinocandins. In this review, we show the structural diversity of natural echinocandins, and we summarize the emerging data on their mode of action, biosynthesis and industrial production. Their clinical significance as well as the mechanism of natural and acquired resistance is also discussed.

Aerosol and parenteral pneumocandins are effective in a rat model of pulmonary aspergillosis

The pneumocandins are semisynthetic analogs of echinocandin-like compounds that have shown efficacy in animal models of systemic candidiasis, disseminated aspergillosis, and pneumocystis pneumonia. However, the most common form of Aspergillus infection in susceptible patients is pulmonary aspergillosis, which was not directly tested in the mouse models used in the past. We have evaluated three pneumocandins, L-693,989, L-731,373, and L-733,560, in a rat model of pulmonary aspergillosis. Male Sprague-Dawley rats were treated for 2 weeks with cortisone and tetracycline and fed a low-protein diet before being inoculated via the trachea with 10(6) conidia of Aspergillus fumigatus H11-20. In the absence of drug treatment, the animals developed a progressive, rapidly fatal bronchopneumonia. All three pneumocandins at doses of 5 mg/kg (intraperitoneally [i.p.] every 12 h [q12h]) were effective in delaying mortality in this model. Survival at day 7 postinfection was 20% among controls (n = 10 for all groups), while it was 60, 80, and 90% in groups that were treated with L-693,989, L-731,373, and L-733,560, respectively. In another trial, survival at day 7 postinfection was 25% among controls (n = 8 for all groups); it was 87.5% in a group treated with amphotericin B (0.5 mg/kg i.p. q12h) and was 100% in a group treated with L-733,560 (0.625 mg/kg i.p. q12h). In a separate trial, aerosol L-693,989 administered 2 h before infection (5 mg/kg) delayed mortality. Eight of the 10 animals treated with aerosol L-693,989 survived for 7 days, whereas only 2 of 10 control animals survived. We conclude that the pneumocandins we tested were highly effective in an animal model of pulmonary aspergillosis.

Increased antifungal activity of L-733,560, a water-soluble, semisynthetic pneumocandin, is due to enhanced inhibition of cell wall synthesis

The pneumocandins are natural lipopeptide products of the echinocandin class which inhibit the synthesis of 1,3-beta-D-glucan in susceptible fungi. The lack of a corresponding pathway in mammalian hosts makes this mode of action an attractive one for treating systemic infections. Substitution by an aminoethyl ether at the hemiaminal and dehydration and reduction of the glutamine of pneumocandin B0 produced a semisynthetic compound (L-733,560) with intrinsic water solubility, significantly increased potency, and a broader antifungal spectrum. To evaluate the mechanism for the improved antifungal efficacy, we determined that L-733,560 was a more potent inhibitor of glucan synthase activity in vitro, did not affect the other membrane-bound enzymes tested, conferred susceptibility to lysis in the absence of osmotic support, and did not disrupt currents in liposomal bilayers or 86Rb+ fluxes from liposomes. In Aspergillus species L-733,560 also produced the same morphological alterations as pneumocandin B0. A stereoisomer of L-733,560 with poor antifungal activity was a weak inhibitor of glucan synthase. All of these results support the notion that the enhanced antifungal activity of L-733,560 is achieved by superior inhibition of glucan synthesis and not by nonspecific membrane effects or a second mode of action.

Genetic modification of an echinocandin B-producing strain of Aspergillus nidulans to produce mutants blocked in sterigmatocystin biosynthesis

The production of echinocandin B (ECB), a lipopolypeptide used for chemical manufacture of the anti-Candida agent Cilofungin, was accomplished by fermentation using a strain of Aspergillus nidulans. In addition to ECB, this fermentation also produces a significant amount of sterigmatocystin (ST), a potent carcinogen structurally related to the aflatoxins. Mutants blocked in the ST biosynthetic pathway were created by genetic modification of the polyploid production strain C747. The following steps were involved: (i) reduction of the genotype to haploid by treatment with the spindle fiber poison methyl 1-(butylcarbamoyl)-2-benzimidazole carbamate (MBC), using colony morphology, conidia size, and the ability to obtain 5-fluoro-orotic acid (5-FOA)-resistant mutants as criteria for ploidy; (ii) mutagenesis of a haploid isolate using UV irradiation; and (iii) screening of mutants for inability to produce ST by thin layer chromatography. Six mutants blocked in ST production were isolated. All six remained capable of producing ECB equivalent in quantity to the haploid strain C747-GR14. One of the mutants was shown to be the result of a chromosomal translocation.

Morphological effects of lipopeptides against Aspergillus fumigatus correlate with activities against (1,3)-beta-D-glucan synthase

The lipopeptide antifungal agents, echinocandins, papulacandins, and pneumocandins, kill Candida albicans by inhibiting glucan synthesis. For this fungus, there is a good correlation of in vitro enzyme inhibition with in vitro assays of MICs. Semisynthetic lipopeptides such as cilofungin, LY303366, L-693,989, and L-733,560 have activity in vivo against Aspergillus infections but appear to be inactive in broth dilution in vitro tests (MICs, > 128 micrograms/ml). To understand how compounds which lack activity in vitro can have good in vivo activity, we monitored the effect of pneumocandins on the morphology of Aspergillus fumigatus and A, flavus strains by light microscopy and electron microscopy and related the changes in growth to inhibition of glucan synthesis. Pneumocandin B0 caused profound changes in hyphal growth; light micrographs showed abnormally swollen germ tubes, highly branched hyphal tips, and many cells with distended balloon shapes. Aspergillus electron micrographs confirmed that lipopeptides produce changes in cell walls; drug-treated germlings showed very stubby growth with thick walls and a conspicuous dark outer layer which was much thicker in the subapical regions. The rest of the hyphal tip ultrastructure was unaffected by the drug, indicating considerable specificity for the primary target. The drug-induced growth alteration produced very compact clumps in broth dilution wells, making it possible to score the morphological effect macroscopically. The morphological changes could be assayed quantitatively by using conventional broth microdilution susceptibility assay conditions. We defined the endpoint as the lowest concentration required to produce the morphological effect and called it the minimum effective concentration to distinguish it from the no-growth endpoints used in MIC determinations. The minimum effective concentration assay was related to inhibition of glucan synthase activity in vitro and may provide a starting point for development of susceptibility testing methods for lipopeptides.

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.

In vitro antifungal activities and in vivo efficacies of 1,3-beta-D-glucan synthesis inhibitors L-671,329, L-646,991, tetrahydroechinocandin B, and L-687,781, a papulacandin

The in vivo anti-Candida activities of 1,3-beta-D-glucan synthesis inhibitors L-671,329, L-646,991 (cilofungin), L-687,901 (tetrahydroechinocandin B), and L-687,781 (a papulacandin analog) were evaluated by utilizing a murine model of disseminated candidiasis that has enhanced susceptibility to Candida albicans but increased sensitivity for discriminating antifungal efficacy. DBA/2 mice were challenged intravenously with 1 x 10(4) to 5 x 10(4) CFU of C. albicans MY1055 per mouse. Compounds were administered intraperitoneally at concentrations ranging from 1.25 to 10 mg/kg of body weight twice daily for 4 days. At 6 h and 1, 2, 3, 4, 7, and 9 days after challenge, five mice per group were sacrificed and their kidneys were homogenized and plated for enumeration of Candida organisms (CFU per gram). Progressiveness of response trends and no-statistical-significance-of-trend doses were derived to rank compound efficacy. 1,3-beta-D-Glucan synthesis 50% inhibitory concentrations were determined by using a C. albicans (MY1208) membrane glucan assay. Candida and Cryptococcus neoformans MICs and minimal fungicidal concentrations were determined by broth microdilution. L-671,329, L-646,991, L-687,901, and L-687,781 showed similar 1,3-beta-D-glucan activities, with 50% inhibitory concentrations of 0.64, 1.30, 0.85, and 0.16 micrograms/ml, respectively. Data from in vitro antifungal susceptibility studies showed that L-671,329, L-646,991, and L-687,901 had similar MICs ranging from 0.5 to 1.0 micrograms/ml, while L-687,781 showed slightly higher MICs of 1.0 to 2.0 micrograms/ml for C. albicans MY1055. Lipopeptide compounds were ineffective against C. neoformans strains. Results from in vivo experiments comparing significant trend and progressiveness in response analyses indicated that L-671,329 and L-646,991 were equipotent but slightly less active than L-687-901, while L-687,781 was ineffective at 10 mg/kg. Fungicidal activities of L-671,329, L-646,991, and L-687,901 were observed in vivo, with significant reduction in Candida CFU per gram of kidneys compared with those in sham-treated mice at doses of > or = 2.5 mg/kg evident as early as 1 day after challenge.

Effect of cilofungin on phagocytosis and intracellular killing of Candida albicans by human neutrophils

The effect of a beta glucan synthase inhibitor, cilofungin, amphotericin B and 5-fluorocytosine on opsonization, phagocytosis and intracellular killing of Candida albicans blastospores by glass-adherent human neutrophils was determined by microscopy and dye exclusion staining. Pretreatment of blastospores for 2, 4, 6 and 18 h with 1.25 mg/l cilofungin resulted in 24-28% of neutrophils ingesting compared to 24% with no drug present. There was no significant difference in the phagocytic index with increased incubation time. Percentage ingestion and phagocytic index values were not significantly different when blastospores were pretreated for 2 h in the presence of 3, 7, 15, 31 and 62 mg/l cilofungin. Comparable concentrations of amphotericin B and 5-fluorocytosine gave similar results. In the absence of cilofungin, intracellular killing after 2 h was 54%. In the presence of 1 and 10 mg/l cilofungin, 85-90% of intracellular blastospores were killed. Therapeutic levels of amphotericin B or 5-fluorocytosine did not enhance intracellular killing. These data demonstrate the potentiation of intracellular killing of Candida albicans by neutrophils in the presence of cilofungin.

Stimulation of human neutrophil phagocytosis of Candida albicans by a beta-glucan synthase inhibitor, cilofungin

The effect of a lipopeptide antifungal agent, cilofungin, on serum opsonization and phagocytosis of Candida albicans yeast phase cells in human neutrophil monolayer assays was investigated. Simultaneous addition of fungicidal concentrations of cilofungin did not enhance or inhibit phagocytosis of C. albicans. Pretreatment of Candida blastospores with cilofungin in the absence of serum complement for 1 h did not affect phagocytosis. However, pretreatment of blastospores with cilofungin and complement promoted a significant increase in ingestion. Pretreatment of neutrophils with cilofungin in serum-free media did not affect neutrophil viability. In contrast, pre-exposure of neutrophils to cilofungin in the presence of complement inhibited ingestion of blastospores.