Biochemical mode of action and enantiomeric selectivity of SDZ 89-485, a new triazole antimycotic

Post-antibiotic effect and post-expositional polyene antagonism of azole antifungal agents in Candida albicans: dependence on substance lipophilia

The lipophilic azoles itraconazole (ICZ), ketoconazole (KCZ) and miconazole (MCZ) have two things in common regarding their effect on Candida albicans. First, these azoles cause a growth inhibition that persists for at least 24 h after exposure (post-antibiotic effect), although this is only occasionally observed for ICZ. Secondly, these substances cause a decrease in the fungicidal activity of amphotericin B (AMB, 1 mg l-1) upon subsequent exposure to this drug. In contrast, fluconazole (FCZ) exhibits neither of these two effects. Further tests suggest that both of these phenomena observed may be related to the non-covalent binding of the three lipophilic azoles to lipophilic cytoplasmic components of yeast cells. With fluconazole, such bonds seem to be much weaker. The amount of relatively hydrophilic fluconazole that is bound non-specifically to the fungal cell is evidently too low to produce long-lasting post-exposure effects like those caused by lipophilic azoles.

Antagonistic interactions between azoles and amphotericin B with yeasts depend on azole lipophilia for special test conditions in vitro

The interactions of the azole antifungal agents fluconazole, itraconazole, ketoconazole, or miconazole with amphotericin B (AmB) in their effect on Candida albicans were investigated. These four azoles antagonized the fungistatic activity of AmB at sub-MICs if both substances acted simultaneously. This coincubation test was primarily developed to observe the azole-mediated demethylase inhibition quantitatively by bioassay. Interestingly, the occurrence of azole-AmB antagonism depended on azole lipophilia if specially selected test conditions were applied. By a consecutive incubation regimen, preincubation at high azole concentrations (1 to 50 micrograms/ml) and then subsequent incubation with AmB (1 microgram/ml), only preincubation with the three lipophilic azoles decreased the fungicidal activity of AmB but not that of FCZ. It was shown that yeasts absorb only lipophilic azoles to a remarkable extent. This fact might be responsible for the absence of antagonism of FCZ to AmB when yeasts were incubated consecutively. It can be concluded with caution that consecutive treatment of candidiasis with FCZ and AmB does not necessarily result in a clinically relevant antagonism.

[Post-antibiotic effect and post-exposure polyene antagonism of azole antimycotics in Candida albicans: dependency on lipophilia]

With the lipophilic azoles itraconazole (ICZ), ketoconazole (KCZ), and miconazole (MCZ) two effects, occurring in parallel, on Candida albicans were observed: Firstly, these azoles caused a growth inhibition which persisted for at least 24 hours (post-antibiotic effect, found regularly with KCZ and MCZ, with ICZ only occasionally). Furthermore, the fungicidal activity of amphotericin B (AMB, 1 mg/1) after exposure to the azoles was reduced. In contrast, to this, fluconazole (FCZ) produced neither of these effects. Additional experiments indicate that both actions of the three lipophilic azoles may be related to their noncovalent binding to lipophilic cytoplasmatic components of the yeast cells. In the case of fluconazol such bonds seem to be much weaker. Presumably, the amount of the relatively hydrophilic fluconazole, which will be bound to the cell, is too low as to produce long lasting post-exposure effects like those caused by the lipophilic azoles.

Terbinafine: mode of action and properties of the squalene epoxidase inhibition

Terbinafine (Lamisil) has primarily fungicidal action against many fungi as a result of its specific mechanism of squalene epoxidase inhibition. Treated fungi accumulate squalene while becoming deficient in ergosterol, an essential component of fungal cell membranes. The cidal action is closely associated with the development of high intracellular squalene concentrations, which are believed to interfere with fungal membrane function and cell wall synthesis. In the case of Candida albicans, growth inhibition with terbinafine appears to result from the ergosterol deficiency. The filamentous form of this fungus is more susceptible than the yeast form. Measurement of ergosterol biosynthesis by incorporation of radiolabelled precursors indicates a correlation between inhibition of growth and ergosterol biosynthesis in a range of pathogenic fungi. Terbinafine is a potent non-competitive inhibitor of squalene epoxidase from Candida (Ki = 30 nM). In contrast, inhibition of rat liver squalene epoxidase only occurs at higher drug concentrations (Ki = 77 microM), and is competitive with squalene. Thus, terbinafine has no effect on cholesterol biosynthesis in vivo. Squalene epoxidase is not an enzyme of the cytochrome P-450 type, thereby avoiding potential inhibition of this class of enzymes.

Interaction of terbinafine with human serum and serum proteins

The allylamine antimycotic terbinafine acts by inhibition of ergosterol biosynthesis at the level of squalene epoxidase. Using this mechanism in Candida parapsilosis cells, a functional assay was developed to investigate the effects of serum and serum proteins on the antifungal action of terbinafine and related drugs in vitro. Inhibition of ergosterol biosynthesis by terbinafine was antagonized by human serum in a dose-dependent non-saturable manner. The results were not affected by varying the period of pre-incubation of serum with the drug or with the fungal cells, or by performing the test in other species of Candida, Aspergillus and Trichophyton. Qualitatively similar effects were observed with the related allylamine compounds naftifine and SDZ 87-469, the extent of antagonism correlating with their lipophilicity. The effect appeared to be caused by non-specific binding of the drug to major serum components, including albumin and the lipoproteins (both LDL and HDL). Reduced bioavailability resulting from binding by serum may at least partly account for the low efficacy of terbinafine in experimental models of systemic infection, in contrast to its high efficacy in infections of the skin, nails and hair.