Mode of action studies. Basis for the search of new antifungal drugs

Co-Delivery of Repurposing Itraconazole and VEGF siRNA by Composite Nanoparticulate System for Collaborative Anti-Angiogenesis and Anti-Tumor Efficacy against Breast Cancer

Combinations of two different therapeutic modalities of VEGF inhibitors against angiogenesis can cooperatively impede breast cancer tumor growth and enhance therapeutic efficacy. Itraconazole (ITZ) is a conventional antifungal drug with high safety; however, it has been repurposed to be a multi target anti-angiogenesis agent for cancer therapy in recent years. In the present study, composite nanoparticles co-loaded with ITZ and VEGF siRNA were prepared in order to investigate their anti-angiogenesis efficacy and synergistic anticancer effect against breast cancer. The nanoparticles had a suitable particle size (117.9 ± 10.3 nm) and weak positive surface charge (6.69 ± 2.46 mV), as well as good stability and drug release profile in vitro. Moreover, the nanoparticles successfully escaped from endosomes and realized cell apoptosis and cell proliferation inhibition in vitro. In vitro and in vivo experiments showed that the nanoparticles could induce the silencing of VEGF-related expressions as well as anti-angiogenesis efficacy, and the co-loaded ITZ-VEGF siRNA NPs could inhibit tumor growth effectively with low toxicity and side effects. Taken together, the as-prepared delivery vehicles are a simple and safe nano-platform that improves the antitumor efficacy of VEGF siRNA and ITZ, which allows the repositioning of the generic drug ITZ as a great candidate for antitumor therapy.

Roles for Structural Biology in the Discovery of Drugs and Agrochemicals Targeting Sterol 14α-Demethylases

Antifungal drugs and antifungal agrochemicals have significant limitations. These include several unintended consequences of their use including the growing importance of intrinsic and acquired resistance. These problems underpin an increasingly urgent need to improve the existing classes of antifungals and to discover novel antifungals. Structural insights into drug targets and their complexes with both substrates and inhibitory ligands increase opportunity for the discovery of more effective antifungals. Implementation of this promise, which requires multiple skill sets, is beginning to yield candidates from discovery programs that could more quickly find their place in the clinic. This review will describe how structural biology is providing information for the improvement and discovery of inhibitors targeting the essential fungal enzyme sterol 14α-demethylase.

Clade-Specific Sterol Metabolites in Dinoflagellate Endosymbionts Are Associated with Coral Bleaching in Response to Environmental Cues

Cnidarians cannot synthesize sterols (which play essential roles in growth and development) de novo but often use sterols acquired from endosymbiotic dinoflagellates. While sterol availability can impact the mutualistic interaction between coral host and algal symbiont, the biosynthetic pathways (in the dinoflagellate endosymbionts) and functional roles of sterols in these symbioses are poorly understood. In this study, we found that itraconazole, which perturbs sterol metabolism by inhibiting the sterol 14-demethylase CYP51 in dinoflagellates, induces bleaching of the anemone Heteractis crispa and that bleaching perturbs sterol metabolism of the dinoflagellate. While Symbiodiniaceae have clade-specific sterol metabolites, they share features of the common sterol biosynthetic pathway but with distinct architecture and substrate specificity features of participating enzymes. Tracking sterol profiles and transcripts of enzymes involved in sterol biosynthesis across time in response to different environmental cues revealed similarities and idiosyncratic features of sterol synthesis in the endosymbiont Breviolum minutum Exposure of algal cultures to high levels of light, heat, and acidification led to alterations in sterol synthesis, including blocks through downregulation of squalene synthase transcript levels accompanied by marked growth reductions.IMPORTANCE These results indicate that sterol metabolites in Symbiodiniaceae are clade specific, that their biosynthetic pathways share architectural and substrate specificity features with those of animals and plants, and that environmental stress-specific perturbation of sterol biosynthesis in dinoflagellates can impair a key mutualistic partnership for healthy reefs.

Novel screening system revealed that intracellular cholesterol trafficking can be a good target for colon cancer prevention

In conventional research methods for cancer prevention, cell proliferation and apoptosis have been intensively targeted rather than the protection of normal or benign tumor cells from malignant transformation. In this study, we aimed to identify candidate colon cancer chemopreventive drugs based on the transcriptional activities of TCF/LEF, NF-κB and NRF2, that play important roles in the process of malignant transformation. We screened a “validated library” consisting of 1280 approved drugs to identify hit compounds that decreased TCF/LEF and NF-κB transcriptional activity and increased NRF2 transcriptional activity. Based on the evaluation of these 3 transcriptional activities, 8 compounds were identified as candidate chemopreventive drugs for colorectal cancer. One of those, itraconazole, is a clinically used anti-fungal drug and was examined in the Min mouse model of familial adenomatous polyposis. Treatment with itraconazole significantly suppressed intestinal polyp formation and the effects of itraconazole on transcriptional activities may be exerted partly through inhibition of intracellular cholesterol trafficking. This screen represents one of the first attempts to identify chemopreventive agents using integrated criteria consisting of the inhibition of TCF/LEF, NF-κB and induction of NRF2 transcriptional activity.

Methodology of Drought Stress Research: Experimental Setup and Physiological Characterization

Drought is one of the major stress factors affecting the growth and development of plants. In this context, drought-related losses of crop plant productivity impede sustainable agriculture all over the world. In general, plants respond to water deficits by multiple physiological and metabolic adaptations at the molecular, cellular, and organism levels. To understand the underlying mechanisms of drought tolerance, adequate stress models and arrays of reliable stress markers are required. Therefore, in this review we comprehensively address currently available models of drought stress, based on culturing plants in soil, hydroponically, or in agar culture, and critically discuss advantages and limitations of each design. We also address the methodology of drought stress characterization and discuss it in the context of real experimental approaches. Further, we highlight the trends of methodological developments in drought stress research, i.e., complementing conventional tests with quantification of phytohormones and reactive oxygen species (ROS), measuring antioxidant enzyme activities, and comprehensively profiling transcriptome, proteome, and metabolome.

Recent advances in drug repositioning for the discovery of new anticancer drugs

Drug repositioning (also referred to as drug repurposing), the process of finding new uses of existing drugs, has been gaining popularity in recent years. The availability of several established clinical drug libraries and rapid advances in disease biology, genomics and bioinformatics has accelerated the pace of both activity-based and in silico drug repositioning. Drug repositioning has attracted particular attention from the communities engaged in anticancer drug discovery due to the combination of great demand for new anticancer drugs and the availability of a wide variety of cell- and target-based screening assays. With the successful clinical introduction of a number of non-cancer drugs for cancer treatment, drug repositioning now became a powerful alternative strategy to discover and develop novel anticancer drug candidates from the existing drug space. In this review, recent successful examples of drug repositioning for anticancer drug discovery from non-cancer drugs will be discussed.

Ketoconazole modulates the infectivity of Ichthyophonus sp. (Mesomycetozoa) in vivo in experimentally injected European sea bass

In vitro studies have confirmed the inhibitory effect of the azol-derivative ketoconazole (KZ) on the growth of Ichthyophonus, an important pathogen causing epizootics in wild and cultured fish. We evaluated the effect of KZ in vivo in European sea bass Dicentrarchus labrax experimentally infected with the same Ichthyophonus isolate. Liposomes were used to vehiculate different doses of KZ to increase the effect on Ichthyophonus and lower the toxicity of the drug, and KZ toxicity was assessed in cultured sea bass juveniles. We also studied the effect of liposome-vehiculated KZ included in medicated food on ichthyophoniasis. KZ causes clear toxic effects in D. labrax juveniles at doses >80 mg kg-1, apparent in the reduced survival of fish and histological alterations to livers, kidneys and spleens. Fish injected with Ichthyophonus and treated with KZ dosages of ≤80 mg kg-1 d-1 presented lower ichthyophoniasis prevalence, fewer organs infected per fish, and fewer spores in the affected organs than the untreated fish. KZ seems to delay the onset of infection, but cannot stop further progression once established. However, this behaviour is not clearly reflected in the biometric and haematological data collected from these fish. We hypothesise that KZ's delaying effect would increase, if lower infective doses (more similar to natural situations) were used. The drug administration vehicle (liposomes vs. emulsions) did not affect the results. Our data confirm the potential utility of KZ in treating ichthyophoniasis and reveal its low toxicity for sea bass. Nevertheless, the optimal dose and appropriate application protocol remain to be determined.

Coccidioidomycosis

The difficulties in managing this potentially horrific disease, with its myriad manifestations, are immense, because host factors dramatically impact outcome. Coccidioidomycosis should warrant great respect among clinicians, because, even with dramatic improvements in therapies, outcomes remain poor. Although there have been outstanding successes with these new therapies, tragic losses after years of immense patient suffering still occur. Coccidioidomycosis is a geographically restricted fungus but is one that inflicts tremendous suffering on affected patients. In addition, because of travel and the influx of susceptible hosts, dramatic increases in patients at risk for infection are seen throughout the southwest United States. The extended-spectrum azoles, such as posaconazole and voriconazole, may prove to be more efficacious in the treatment of coccidioidomycosis than prior agents, including amphotericin B, fluconazole, and itraconazole. Additional resources are needed to conduct randomised, controlled clinical trials for the treatment of this disease.

Effects of Y132H and F145L substitutions on the activity, azole resistance and spectral properties of Candida albicans sterol 14-demethylase P450 (CYP51): a live example showing the selection of altered P450 through interaction with environmental compounds

Three variants of Candida albicans CYP51 (sterol 14-demethylase P450) having Y132H and/or F145L substitutions were purified and characterized to reveal the effects of these amino acid substitutions on the enzymatic properties and azole resistance of the enzyme. Y132H and F145L substitutions modified the spectral properties of the enzyme, suggesting that they caused some structural change modifying the heme environments of CYP51. Y132H and F145L substitutions increased the resistance of the enzyme to azole compounds but considerably decreased the catalytic activity. This fact represents a trade-off between acquisition of azole resistance and maintenance of high activity in the CYP51 having Y132H and F145L substitutions. A fluconazole-resistant C. albicans strain DUMC136 isolated from patients receiving long-term azole treatment was a homozygote of the altered CYP51 having Y132H and F145L substitutions. However, neither of these substitutions was found in CYP51 of wild-type C. albicans so far studied. These facts suggest that the azole-resistant variant having Y132H and/or F145L substitutions might be selected only under azole-rich environments because of its azole resistance and impaired catalytic activity. This may be a live example showing one of the important processes of P450 diversification, the selection of altered P450 through the interaction with environmental compounds.

Characterization of mechanisms of fluconazole resistance in a Candida albicans isolate from a Japanese patient with chronic mucocutaneous candidiasis

We examined the mechanisms of fluconazole resistance in a fluconazole-resistant Candida albicans isolate from a Japanese patient with chronic mucocutaneous candidiasis. It was demonstrated that the highly resistant phenotype of this strain was associated with combined mechanisms of the energy-dependent reduced intracellular accumulation of fluconazole, presumably due to the increased expression of the ATP-binding cassette efflux pump CDR gene(s), and the reduced affinity of the target enzyme, Erg11p, to fluconazole. In particular, the reduced affinity of Erg11p was considered to contribute largely to the fluconazole resistance in the TIMM3209 strain. Biochemical studies indicated that the Erg11p from the TIMM3209 strain showed reduced susceptibility both to fluconazole and itraconazole of cell-free ergosterol biosynthesis, and cytochrome P-450 also showed reduced affinity to fluconazole in the carbon monoxidecytochrome P-450 complex formation assay. We identified two amino acid substitutions, Y132H and G448V, in Erg11p from the TIMM3209 strain. We found that the cytochrome P-450 from the TIMM3209 strain decayed during incubation at 37 C without fluconazole although it is unknown whether or not the phenomenon is linked to the resistant phenotype. These mutations are thought to confer the above-mentioned characteristics to Erg11p.

Hepatotoxicity of ketoconazole in Sprague-Dawley rats: glutathione depletion, flavin-containing monooxygenases-mediated bioactivation and hepatic covalent binding

1. This study has examined ketoconazole (KT)-induced hepatotoxicity in vivo and in vitro, using male Sprague-Dawley rats with [(3)H]KT (1.5 micro Ci mg(-1)) at 40 and 90 mg KT kg(-1) doses. Blood and liver samples were collected from 0 to 24 h for alanine aminotransaminase (ALT), glutathione (GSH) and covalent binding analyses. 2. Covalent binding occurred as early as 0.5 h, peaked at 2 h (0.026 +/- 0.01 nmol KT mg(-1) protein) and 8 h (0.088 +/- 0.04 nmol KT mg(-1) protein) for 40 and 90 mg KT kg(-1) doses, respectively. ALT levels increased at 0.5 h for the 40 and 90 mg KT kg(-1) doses (44.3 and 56.4 U ml(-1), respectively) relative to control, 22.7 U ml(-1). At 24 h, the 90 mg KT kg(-1) dose reduced hepatic GSH levels from 9.92 +/- 1.1 to 4.76 +/- 0.3 nmol GSH mg(-1) protein. 3. The role of the flavin-containing monooxygenases (FMO) utilized Sprague-Dawley microsomes with 1, 10 and 100 micro M [(3)H]KT. Maximum covalent binding occurring at 100 micro M KT. Heat inactivation of microsomal FMO significantly decreased covalent binding by 75%, whereas 1 mM GSH significantly reduced covalent binding by 65%. 4. Thus, KT-induced hepatotoxicity is dose- and time-dependent and appears to be FMO mediated, in part, to metabolites that may react with protein and, possibly, GSH.

Antifungal agents of use in animal health--chemical, biochemical and pharmacological aspects

A limited number of antifungal agents is licensed for use in animals, however, many of those available for the treatment of mycoses in humans are used by veterinary practitioners. This review includes chemical aspects, spectra of activity, mechanisms of action and resistance, adverse reactions and drug interactions of the antifungals in current use.

A novel mechanism of fluconazole resistance associated with fluconazole sequestration in Candida albicans isolates from a myelofibrosis patient

A series of 10 strains of Candida albicans, from TIMM 3309 to TIMM 3318, were repeatedly isolated in one myelofibrosis-complicated patient with recurrent candidemia. The latter five isolates, from TIMM 3314 to TIMM 3318, became suddenly resistant to fluconazole during the 10 to 16 weeks after antimycotic therapy. We investigated the resistant mechanism of fluconazole using one susceptible isolate and two of the five resistant isolates in the series. The ergosterol synthesis by cell-free extracts from the two resistant isolates was less susceptible to fluconazole partly as a result of a decreased affinity of cytochrome P-450. Unexpectedly, these two resistant isolates showed higher levels of an intracellular accumulation of [H]fluconazole than the susceptible isolate and the control strain of C. albicans ATCC 10231. In the resistant isolate, TIMM 3318, most intracellular incorporated fluconazole was distributed in the 12,000 X g pellet (P-120) fraction by centrifugation unlike the two susceptible strains. An observation of the ultrastructure of TIMM 3318 showed the most notable alteration to be the characteristic appearance of numerous vesicular vacuoles (diameter, 150 to 400 nm); these vacuoles were not observed, however, in either of the susceptible strains. A direct observation of the subcellular fraction prepared from TIMM 3318 by the electron microscopy negative-staining method suggests that most of the vesicular vacuoles were recovered in the P-120 fraction. These results suggest that fluconazole sequestration caused by vesicular vacuoles of the resistant isolate might act as a novel mechanism of fluconazole resistance besides the decreased affinity of cytochrome P-450.

Ultrastructural and biochemical alterations induced by 22,26-azasterol, a delta(24(25))-sterol methyltransferase inhibitor, on promastigote and amastigote forms of Leishmania amazonensis

We report on the antiproliferative effects and the ultrastructural and biochemical alterations induced in vitro by 22,26-azasterol, a sterol Delta(24(25))-methyltransferase (24-SMT) inhibitor, on Leishmania amazonensis. When promastigotes and amastigotes were exposed to 100 nM 22,26-azasterol, complete growth arrest and cell lysis ensued after 72 (promastigotes) or 120 (amastigotes) h. Exposure of parasites to this azasterol led to the complete depletion of parasite endogenous sterols (episterol and 5-dehydroepisterol) and their replacement by 24-desalkyl sterols (zymosterol, cholesta-5,7,24-trien-3beta-ol, and cholesta-7,24-dien-3beta-ol), while 14-methyl-zymosterol and 4,14-dimethyl-zymosterol accumulated as a result of simultaneous incubation of the parasites with 22,26-azasterol and ketoconazole, a known inhibitor of the parasite's sterol C14-demethylase. These results confirmed that 24-SMT is the primary site of action of the azasterol. Profound changes were also observed in the phospholipid compositions of treated cells, in which a twofold reduction in the content of phosphatidylserine was observed; this was accompanied by a concomitant increase in the content of phosphatidylinositol. Transmission electron microscopy showed that 22,26-azasterol induced marked morphological changes, including mitochondrial swelling, invaginations of the inner mitochondrial membrane, and the appearance of large bodies containing concentric membranes. Other modifications included increases in the numbers of acidocalcisomes, megasomes, and lipid inclusions and the appearance of typical autophagic structures and cell body protrusions toward the flagellar pocket. We conclude that the dramatic alteration of the lipid composition of the parasite's membranes induced by the drug underlies the ultrastructural alterations that lead to the loss of cell viability and that 24-SMT inhibitors could be useful as selective antileishmanial agents.

Itraconazole

Itraconazole is a broad spectrum triazole antifungal agent. It has favourable pharmacodynamic and pharmacokinetic profiles and is available as both oral and i.v. formulations. Over the last two decades, clinical and animal infection studies have demonstrated the efficacy of itraconazole in a wide range of superficial fungal infections including difficult-to-treat dermatophytoses and onychomycoses. Furthermore, shortened treatment regimens have proven to be effective, ranging from 1-day treatment for vaginal candidosis to 1-week pulse therapy per month, for 2-4 months, in onychomycosis and follicular dermatophytosis. Clinical experience with itraconazole in the treatment of deep mycoses is less comprehensive. However, results in systemic candidosis, sporotrichosis, blastomycosis, paracoccidioiodomycosis, certain types of histoplasmosis and aspergillosis are extremely encouraging. Itraconazole is less effective in the treatment of chromomycosis and coccidioidomycosis. Nevertheless, considering the refractory nature of these diseases, itraconazole has proven to be a valuable addition to the antifungal drugs currently available for treatment. Itraconazole has been well-tolerated with doses of up to 400 mg/day being generally free of serious adverse effects. However, a potential for drug interactions exists, mediated through the cytochrome P450 enzyme 3A4 system, which should be considered when itraconazole is used as part of a multi-drug regimen.

5-Fluorocytosine antagonizes the action of sterol biosynthesis inhibitors in Candida glabrata

The concentration-dependent antagonistic interaction between 5-fluorocytosine and a sterol biosynthesis inhibitor (SBI) was studied using intact cells and cell-free extracts of Candida glabrata. 5-Fluorocytosine promoted incorporation of radioactivity into 4-desmethylsterols (P < 0.01), and enhanced the relative and absolute increases of ergosterol (P < 0.05) in C. glabrata incubated aerobically with an SBI (miconazole or amorolfine). Further aerobic incubation of C. glabrata with combinations of a nucleic acid or protein synthesis inhibitor (rifampicin or chlortetracycline) and an SBI (miconazole) promoted a similar increase in ergosterol biosynthesis. In contrast, 5-fluorocytosine reduced the incorporation of radioactivity into 4,4-dimethylsterols (P < 0.01), but had no obvious effect on the absolute ergosterol level in C. glabrata incubated statically with miconazole. In cell-free extracts of cultures previously incubated with 5-fluorocytosine, ergosterol synthesis was less sensitive to the action of miconazole. Antagonism between 5-fluorocytosine and the SBI is thus mediated by a reversal of inhibition of intracellular ergosterol synthesis. The possible mechanisms underlying antagonism between 5-fluorocytosine and SBIs that inhibit different sites of the sterol biosynthesis pathway, as well as its clinical relevance to combination therapy, are discussed.

Safety and efficacy of intermittent therapy with itraconazole in finger- and toenail onychomycosis: a multicentre trial

The efficacy and safety of intermittent itraconazole therapy were investigated in patients with onychomycosis. Patients were divided into two groups according to site and extent of infection. Group A comprised 635 patients with toenail onychomycosis (at least one nail with > or = 20% involvement; n = 560) or fingernail onychomycosis (at least one nail with > 75% involvement; n = 63) or both (n = 12). These patients received itraconazole 400 mg day-1 for 1 week per month for 3 months. Group B comprised 48 patients with fingernail onychomycosis (at least one nail with > or = 20% involvement but no nail with > 75% involvement) who received itraconazole 400 mg day-1 for 1 week per month for 2 months. Patients were followed for a further 18 weeks without treatment, and received another treatment cycle if not cured or markedly improved 6 weeks after the end of the last cycle. An additional cycle was administered to 76 patients with fingernail onychomycosis (group A, n = 43; group B, n = 28) and to 316 patients with toenail onychomycosis. Clinical response rates and mycological cure rates at study end point were 89.0% and 68.4% respectively for toenails, 91.4% and 85.3% respectively for group A fingernails and 84.4% and 77.1% respectively for group B fingernails. Most adverse events occurred infrequently; major changes in liver function tests were not noted. In conclusion, intermittent itraconazole therapy is highly effective and safe in patients with onychomycosis.

Altered lipid composition and enzyme activities of plasma membranes from Trypanosoma (Schizotrypanum) cruzi epimastigotes grown in the presence of sterol biosynthesis inhibitors

The accepted mechanism for the antiproliferative effects of sterol biosynthesis inhibitors (SBI) against the protozoan parasite Trypanosoma (Schizotrypanum) cruzi, the causative agent of Chagas' disease, is the depletion of specific parasite sterols that are essential growth factors and cannot be replaced by cholesterol, the main sterol present in the vertebrate host. However, the precise metabolic roles of these specific parasite sterols are unknown. We approached this problem by subjecting T. cruzi epimastigotes to two types of SBI, inhibitors of sterol C-14 demethylase and delta 24(25) methyl transferase, and investigating the modification of lipid composition and enzyme activities in the plasma membranes of the parasite. We found in purified plasma membrane from SBI-treated cells that, together with the expected changes in the sterol composition, there was also an inversion of the phosphatidylcholine (PC) to phosphatidylethanolamine (PE) ratio and a large increase in the content of saturated fatty acids esterified to phospholipids. The modification of the phospholipid headgroup composition correlated with a 70% reduction in the specific activity of the membrane-bound PC-PE-N-methyl transferase SBI-treated cells; it was shown that this inhibition was not due to a direct effect of the drug on the enzyme. Finally, the specific activity of the Mg(2+)-dependent, vanadate-sensitive ATPase present in the membranes was also inhibited by ca. 50% in SBI-treated cells. The results suggest that one of the primary effects of the depletion of endogenous sterols induced by SBI in T. cruzi is a modification of the cellular phospholipid composition as a consequence of a reduced activity of PE-PC-N-methyl transferase and probably of the acyl delta 9 and delta 6 desaturases; this, in turn, could affect the activity of other enzymatic and transport proteins.

Naturally azole-resistant Leishmania braziliensis promastigotes are rendered susceptible in the presence of terbinafine: comparative study with azole-susceptible Leishmania mexicana promastigotes

Leishmania braziliensis (isolate 2903) was naturally resistant to ketoconazole or the bis-triazole D0870, inhibitors of sterol C-14 demethylase, which produced only moderate effects on the proliferation of promastigotes at 10 microM. In contrast, Leishmania mexicana (isolate NR) was extremely susceptible to the azoles, as complete growth arrest and cell lysis were induced by incubation of the parasites with 0.05 microM concentrations of the drugs for 72 h. The opposite response was observed with terbinafine, an inhibitor of squalene epoxidase: L. braziliensis 2903 was three times more susceptible to the drug than L. mexicana NR (MICs of 5 and 15 microM, respectively). However, when the L. braziliensis stock was grown in the presence of 1 microM terbinafine, which by itself produced only marginal (< 10%) effects on growth, it became highly susceptible to the azoles, with an MIC of 0.03 microM. Analysis of cellular free sterols by high-resolution capillary gas chromatography coupled to mass spectrometry showed that 14-methyl sterols can support normal growth of L. braziliensis 2903 but not of L. mexicana NR. On the other hand, the higher susceptibility of the L. braziliensis isolate to terbinafine was correlated with a massive accumulation of squalene in the presence of the allylamine while no significant effects on L. mexicana sterol composition were observed at drug concentrations up to 1 microM. Thus, the > 300-fold increase in the susceptibility of L. braziliensis promastigotes to azoles in the presence of terbinafine was attributed to the combined effect of squalene and the methylated sterol precursors on the physical properties of the cell's membranes, leading to the loss of cell viability. Combination therapy with azoles and terbinafine in the treatment of human L. braziliensis infections deserves further study.

Reduced accumulation of drug in Candida krusei accounts for itraconazole resistance

Due to intrinsic resistance Candida krusei is emerging as a systemic pathogen in AIDS patients undergoing fluconazole therapy, but acquired resistance to itraconazole has not been studied biochemically. We report here studies on the basis for azole resistance and sterol composition in C. krusei. An itraconazole-resistant isolate showed reduced susceptibility to azole drugs in in vitro growth inhibition studies. Accumulation of 14 alpha-methyl-3,6-diol under azole treatment was associated with growth arrest. In vitro ergosterol biosynthesis and type II binding studies suggested no alteration in the affinity to azole drugs of the target enzyme, the cytochrome P-450 sterol 14 alpha-demethylase, in the resistant isolate. Resistance was associated with a decreased intracellular content of drug in the resistant isolate.

Comparison of D0870, a new triazole antifungal agent, to fluconazole for inhibition of Candida albicans cytochrome P-450 by using in vitro assays

D0870 was 12 to 15 times more active than fluconazole in experiments to determine the MIC for growth arrest for two isolates of Candida albicans. A biochemical comparison of in vitro sterol biosynthesis in cell extracts showed only a twofold superiority of D0870 over fluconazole. A large differentiation (10-fold) in 50% saturating concentrations obtained by examining the binding of the azoles to microsomal P-450 was observed in a type II binding spectrophotometric assay, possibly reflecting the differential affinity for more than one P-450 enzyme. Additional mechanisms besides affinity for the target enzyme sterol 14 alpha-demethylase, such as differential intracellular accumulation of drug, may contribute to the differences in antifungal activity.

Comparison of ketoconazole- and fluconazole-induced hepatotoxicity in a primary culture system of rat hepatocytes

Ketoconazole (KT) and fluconazole (FLU) are azole antifungal agents with a broad spectrum of activity against both superficial and systemic mycoses. KT is also an anticancer agent in the treatment of advanced prostate cancer. In many clinical and retrospective studies, KT has been reported to cause liver damage, i.e. chemical hepatitis. Histologic analysis of KT induced hepatotoxicity shows massive centrilobular necrosis in which the hepatotoxicity was not thought to be mediated through an immunoallergic mechanism. According to the medical literature, the pattern of hepatic injury appears to be primarily of the hepatocellular type. Because of the documented reports of KT and FLU hepatotoxicity, a cytotoxicity comparison of KT and FLU was implemented. The objective of this comparison was to evaluate the cytotoxicity of these azoles such that future mechanistic investigations of hepatotoxicity could be performed. The relative hepatotoxicity of KT and FLU was evaluated using primary cultures of postnatal rat hepatocytes. Cytotoxicity was evaluated by measuring the leakage of the cytosolic enzyme, lactate dehydrogenase (LDH), into the medium; by assessing mitochondrial reduction of 3-(4,5-dimethythiazol-2yl)-2,5-diphenyl tetrazolium bromide (MTT); by assessing lysosomal uptake of neutral red (NR); and by gross morphology (phase contrast microscopy). The cultures were exposed to various concentrations of KT (56-188 microM) for 0.5-4 h and to various concentrations of FLU (50 microM to 1.0 mM) for 0.5-6 h. There was a significant increase (P < 0.05) in LDH leakage and a large decrease in MTT reduction and lysosomal uptake of NR at 4 h for KT. One millimolar FLU had minimal effects on the LDH leakage and MTT reduction. These results demonstrate that KT is a more potent cytotoxicant than FLU; and its toxicity was expressed in a dose- and time-dependent manner.

P450 inhibitors of use in medical treatment: focus on mechanisms of action

A number of cytochrome P450s are targets for compounds that are clinically used or under clinical evaluation for treatment of patients with mycotic infections, such as dermatophytosis, superficial and systemic candidiasis, cryptococcosis and aspergillosis, with skin diseases, such as psoriasis or ichthyosis, and other retinoid-sensitive malignancies, e.g., neuro-ectodermal glioma. Some of the P450 inhibitors are candidates for the treatment of hirsutism or prostate cancer, others are potent inhibitors of the P450 isomerase involved in the synthesis of thromboxane A2, a potent platelet aggregation inducer and vasoconstrictor.

Effects of itraconazole on cytochrome P-450-dependent sterol 14 alpha-demethylation and reduction of 3-ketosteroids in Cryptococcus neoformans

As in other pathogenic fungi, the major sterol synthesized by Cryptococcus neoformans var. neoformans is ergosterol. This yeast also shares with most pathogenic fungi a susceptibility of its cytochrome P-450-dependent ergosterol synthesis to nanomolar concentrations of itraconazole. Fifty percent inhibition of ergosterol synthesis was reached after 16 h of growth in the presence of 6.0 +/- 4.7 nM itraconazole, and complete inhibition was reached at approximately 100 nM itraconazole. This inhibition coincided with the accumulation of mainly eburicol and the 3-ketosteroid obtusifolione. The radioactivity incorporated from [14C]acetate in both compounds represents 64.2% +/- 12.9% of the radioactivity incorporated into the sterols plus squalene extracted from cells incubated in the presence of 10 nM itraconazole. The accumulation of obtusifolione as well as eburicol indicates that itraconazole inhibits not only the 14 alpha-demethylase but also (directly or indirectly) the NADPH-dependent 3-ketosteroid reductase, i.e., the enzyme catalyzing the last step in the demethylation at C-4. This latter inhibition obviates the synthesis of 4,4-demethylated 14 alpha-methylsterols that may function at least partly as surrogates of ergosterol. Eburicol and obtusifolione are unable to support cell growth, and the 3-ketosteroid has been shown to disturb membranes. The complete inhibition of ergosterol synthesis and the accumulation of the 4,4,14-trimethylsterol and of the 3-ketosteroid together with the absence of sterols, such as 14 alpha-methylfecosterol and lanosterol, which can partly fulfill some functions of ergosterol, are at the origin of the high activity of itraconazole against C. neoformans. Fifty percent inhibition of growth achieved after 16 h of incubation in the presence of 3.2 +/- 2.6 nM itraconazole.

Experimental chemotherapy with combinations of ergosterol biosynthesis inhibitors in murine models of Chagas' disease

We report the effects of ketoconazole and the bistriazole ICI 195,739 acting alone or in combination with the allylamine terbinafine (Lamisil) on murine models of Chagas' disease. Mice infected with 10(5) Trypanosoma (Schizotrypanum) cruzi blood trypomastigotes and treated orally with 30 mg of ketoconazole per kg of body weight per day for 7 days, starting at 24 h postinoculation, had 100% survival after 35 days, while controls (untreated) or animals that received 15 mg of ketoconazole or 100 mg of terbinafine per kg/day by the same route had 0% survival after the same period of time. However, all mice receiving the combination of 15 mg of ketoconazole plus 100 mg of terbinafine per kg/day survived for 35 days after infection; it was shown that the survival of the animals treated with this combination was statistically greater than that obtained with either drug acting alone and was indistinguishable from that observed with the high doses of ketoconazole, indicating a synergistic action of the drugs in vivo. However, most animals that survived after the 7-day treatments were not cured, as indicated by a delayed but persistent parasitemia. When the treatment was extended to 14 days, 100% survival was obtained 10 weeks after inoculation for mice treated with 30 mg of ketoconazole per kg/day and the combination of 15 mg of ketoconazole per kg/day plus 100 mg of terbinafine per kg/day, while two-thirds of the mice treated with 15 mg of ketoconazole per kg/day alone were alive after the 14-day treatment; controls or animals that received 100 mg of terbinafine per kg/day did not survive after 25 days. Parasitemia in all surviving mice was negative after 55 days but parasitological cure, as assessed by subinoculation of organs in naive animals, was predominant only in animals that received the combined drug treatment. We also investigated the bistriazole ICI 195,739 and found, as reported previously, that just 1 mg of the compound per kg/day administered orally for 5 days was enough to protect most mice from death 30 days after inoculation, but no parasitological cures were observed. However, in the protocol used in the present study, the protective activity of ICI 195,739 at suboptimal doses (0.5 mg/kg/day) could be enhanced when it was used in combination with terbinafine at doses of the allylamine that by themselves induced no significant protection. Survival of the mice was inversely correlated with the levels of parasitemia in all cases. Extension of the treatment period with the triazole to 15 days at 1 mg/kg/day afforded definitive protection against death, with parasitological cure being achieved in 50% of mice at 10 weeks postinoculation, but no enhancement of its activity at suboptimal doses was observed when it was used in combination with terbinafine during this extended observation period. Taken together, these results supports the proposition that ketoconazole used in combination with terbinafine could be useful in the treatment of humans with Chagas' disease because it can promote parasitological cure without the need to resort to the use of high levels of the azole, which is known to interfere with hepatic function and steroid synthesis in the host. They also support the conclusions of previous in vitro studies which suggested that the triazole ICI 195,739 blocks the proliferation of T. cruzi by a mechanism which differs from those of classical ergosterol biosynthesis inhibitors.

Mevinolin (lovastatin) potentiates the antiproliferative effects of ketoconazole and terbinafine against Trypanosoma (Schizotrypanum) cruzi: in vitro and in vivo studies

We have studied the antiproliferative effects of mevinolin (lovastatin), an inhibitor of 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase, on the protozoan parasite Trypanosoma (Schizotrypanum) cruzi and its ability to potentiate the action of specific ergosterol biosynthesis inhibitors, such as ketoconazole and terbinafine, both in vitro and in vivo. Against the epimastigote form in vitro, mevinolin produced a dose-dependent reduction of the growth rate up to 25 microM, but at 50 and 75 microM, complete growth arrest and cell lysis took place after 144 and 96 h, respectively. A systematic study of the effects of mevinolin combined with ketoconazole and terbinafine, which act at different points in the ergosterol biosynthesis pathway, on the proliferation of epimastigotes indicated a synergic action, as shown by concave isobolograms and fractional inhibitory concentration indexes ranging from 0.17 to 0.54. Analysis of the sterol composition and de novo sterol synthesis in control and treated cells by thin-layer and gas-liquid chromatographies showed that the antiproliferative effects of the drug alone and in combination were correlated with the depletion of the endogenous ergosterol pool and particularly with a critical (exogenous) cholesterol/endogenous 4-desmethyl sterol ratio in the cells. When we studied the effects of mevinolin on the amastigote form proliferating inside Vero cells in vitro, only very modest effects on the parasites were observed up to 0.75 microM; above this concentration, significant deleterious effects on the host cells were found. However, when the same concentration of the drug was combined with ketoconazole, it was able to reduce by a factor of 10 the concentration of the azole required to eradicate the parasite (from 10 to 1 nM), again indicating a synergic action. On the other hand, a combination of mevinolin and terbinafine had only additive effects on amastigotes, but a ternary combination of mevinolin, ketoconazole, and terbinafine was again clearly synergistic. In vivo studies with a murine model of Chagas' disease showed that mevinolin can also potentiate the therapeutic effects of ketoconazole in this system; combined treatment with the two drugs at doses that alone offered only limited protection against the parasite was able to essentially eliminate circulating parasites and produce complete protection against death. These results confirm the synergic action against the proliferative stages of T. cruzi both in vitro and in vivo and in vivo of combined ergosterol biosynthesis inhibitors that act at different points in the pathway and suggest that mevinolin combined with azoles, such as ketoconazole, can be used in the treatment of human Chagas' disease.

Posttreatment itraconazole levels in the nail. New implications for treatment in onychomycosis

BACKGROUND

A problem in the treatment of onychomycosis is the lengthy duration of therapy. The pharmacokinetics of itraconazole suggest a potential for briefer treatment.

OBJECTIVE

This study was designed to investigate itraconazole nail kinetics in 39 patients with onychomycosis in relation to their therapeutic outcome.

METHODS

All patients received itraconazole for 3 months at a dose of 100 or 200 mg daily. Itraconazole levels of distal nail clippings were determined during a 6-month posttherapy period.

RESULTS

Therapeutic itraconazole concentrations were found in the nail plates of fingernails and toenails for up to 6 months after treatment. Cure of the toenails was observed in 79% of the patients treated with the 200 mg dosage and in 26% of those treated with 100 mg at 6 months after therapy.

CONCLUSION

The data suggest that the drug reaches the nail via incorporation into the matrix and by diffusion from the nail bed and is eliminated with regrowth of the nail after discontinuation of treatment.

Mode of action of anti-Candida drugs: focus on terconazole and other ergosterol biosynthesis inhibitors

A large proportion of the presently available antifungal agents are claimed to derive their activity from interaction with the biosynthesis of ergosterol, the key sterol in most pathogenic fungi. An important target for the allylamines, naftifine and terbinafine, is the squalene epoxidase. Interaction with the epoxidation step results in a decreased availability of ergosterol and an accumulation of squalene. Although the squalene epoxidase is clearly the primary target for this class of antifungals, it still remains an open question whether the fungistatic or fungicidal effects originate from a decrease in ergosterol or squalene accumulation. Indeed, preliminary evidence suggests that squalene does not change the physicochemical properties of membranes. Much more is known about the primary and secondary effects of the azole antifungals, such as miconazole, ketoconazole, terconazole, and itraconazole. Most of the imidazole and triazole derivatives are highly potent and selective inhibitors of the cytochrome P-450-dependent 14 alpha-demethylation of lanosterol (P-45014DM). Their potency and selectivity are determined by the nitrogen heterocycle and to a much greater extent by the hydrophobic N-1 substituent. The triazole antifungals, terconazole and itraconazole, combine a high affinity for Candida P-45014DM with an exceptionally low effect on mammalian cytochrome P-450.

Antiproliferative effects and mechanism of action of ICI 195,739, a novel bis-triazole derivative, on epimastigotes and amastigotes of Trypanosoma (Schizotrypanum) cruzi

The in vitro antiproliferative effects of ICI 195,739, a recently developed bis-triazole derivative (T. Boyle, D. J. Gilman, M. B. Gravestock, and J. M. Wardleworth, Ann. N.Y. Acad. Sci. 544:86-100, 1988; J. F. Ryley, S. McGregor, and R. G. Wilson, Ann. N.Y. Acad. Sci. 544:310-328, 1988), on epimastigotes and amastigotes of Trypanosoma (Schizotrypanum) cruzi and some aspects of its mechanism of action are described. Despite previous claims that triazole compounds act on susceptible organisms by essentially the same mechanism demonstrated for the imidazole compounds, i.e., by interfering with the synthesis of ergosterol at the level of the cytochrome P-450-dependent C-14 demethylation of lanosterol, our results indicate that ICI 195,739 acts on T. cruzi epimastigotes by a dual mechanism which involves blockade of ergosterol byosynthesis and a second, still-unidentified target whose alteration leads to immediate growth arrest. Although ICI 195,739 blocks ergosterol biosynthesis at the level of C-14 lanosterol demethylation, as shown by gas-liquid and thin-layer chromatography, growth arrest in ICI 195,739-treated cells is not related to the depletion of the endogenous ergosterol pool, contrary to what was previously found for ketoconazole, the reference compound among antifungal and antiprotozoal azole derivatives. Consistent with this observation is the fact that the concentration of ICI 195,739 required to inhibit de novo synthesis of ergosterol in epimastigotes by 50% is 60 nM, which is essentially identical to that previously found for ketoconazole under identical conditions, while the minimum concentration required to produce complete growth inhibition is 0.1 microM, which is 300 times lower than that of ketoconazole. With respect to the intracellular amastigote form proliferating inside vertebrate (Vero) cells, 10 nM is sufficient to eradicate the parasite completely in 96 h, with no effects on the host cells; this concentration is identical to that previously found for ketoconazole. Growth inhibition and morphological alterations induced by ketoconazole can be reserved by exogenously added ergosterol but not by cholesterol; for ICI 195, 739, neither sterol is capable of reserving the drug effects. Contrary to what was observed for ketoconazole, the in vitro antiproliferative effects of ICI 195, 739 on both forms of the parasite are not potentiated by the simultaneous presence of terbinafine, an allylamine which blocks ergosterol production by the parasite at a different level of the sterol biosynthetic pathway. These results, together with those of an accompanying study of the ultrastructural alterations induced by the drug, strongly support the notion that ICI 195, 739 acts on T. cruzi by a novel combination of biochemical and cellular effects, which could explain its extraordinary potency in vivo against the parasite.

The antioxidant action of ketoconazole and related azoles: comparison with tamoxifen and cholesterol

The azole antifungal drug ketoconazole was found to inhibit Fe(III)-ascorbate dependent lipid peroxidation using either rat liver microsomes or ox-brain phospholipid liposomes as the substrate. It also inhibited microsomal peroxidation induced by the Fe(III)-ADP/NADPH system. The related azoles, miconazole and clotrimazole, were much weaker inhibitors than ketoconazole. Ketoconazole was approximately equipotent with the triphenylethylene anticancer drug tamoxifen in the microsomal system and was almost as effective as 4-hydroxytamoxifen in the liposomal system. Ketoconazole introduced into phospholipid liposomes during their preparation inhibited Fe(III)-ascorbate induced lipid peroxidation to a greater extent than similarly introduced cholesterol, ergosterol or tamoxifen. Miconazole and clotrimazole were again poor inhibitors of lipid peroxidation in this system. These antioxidant effects of ketoconazole may be due to membrane stabilization in the systems used. The implications of our findings for the clinical applications of these drugs are discussed.

Ultrastructural alterations induced by two ergosterol biosynthesis inhibitors, ketoconazole and terbinafine, on epimastigotes and amastigotes of Trypanosoma (Schizotrypanum) cruzi

We report the ultrastructural alterations induced during the proliferative stages of Trypanosoma (Schizotrypanum) cruzi, the causative agent of Chagas' disease, by two ergosterol biosynthesis inhibitors, ketoconazole and terbinafine, which had previously been shown to be potent growth inhibitors whose effects are potentiated when used in combination (J. A. Urbina, K. Lazardi, T. Aguirre, M. M. Piras, and R. Piras, Antimicrob. Agents Chemother. 32:1237-1242, 1988). Epimastigotes treated with a low concentration of ketoconazole (1 microM), which blocks ergosterol biosynthesis at the level of C-14 demethylation of lanosterol and induces cell lysis coincident with total ergosterol depletion, showed gross alterations of the kinetoplast-mitochondrion complex, which swelled and lost the organization of its inner membrane and the electron-dense bodies of its matrix. Thus, coincident with the beginning of cell lysis, the kinetoplast-mitochondrion complex occupied greater than 80% of the cell volume, while other subcellular structures such as the nucleus and subpellicular microtubules were not affected. Terbinafine, which blocks ergosterol synthesis in these cells at the level of squalene synthetase and thus leads to almost immediate arrest of growth at concentrations greater than 1 microM, produced proliferation of glycosomelike bodies, binucleated cells (arrest at cytokinesis), and eventually massive vacuolization. When the drugs were combined, the predominant effect was mitochondrial swelling, which was more drastic and took place earlier than that observed in cells treated with ketoconazole alone. In amastigotes proliferating in Vero cells, ketoconazole at the concentration required to eradicate the parasites (10 nM) produced mitochondrial swelling, the appearance of autophagic vacuoles containing partially degraded subcellular material, and finally a general breakdown of the subcellular structures. Terbinafine at 3 microM induced more limited ultrastructural damage to the amastigotes consistent with increased vacuolization of the cells and the appearance of occasional autophagic vacuoles. When the drugs were used in combination, just 1 nM was required for the total eradication of parasites, the ultrastructural effects were more extensive, and cell disintegration occurred earlier than when any of the drugs was used alone at a much higher concentration. No effect of the drugs on the ultrastructure of the host cells were observed at any of the concentrations tested.

R 76713 and enantiomers: selective, nonsteroidal inhibitors of the cytochrome P450-dependent oestrogen synthesis

The triazole derivative, R 76713 and its enantiomers R 83839(-) and R 83842(+) are effective inhibitors of the aromatization of androstenedione. For human placental microsomes, the (+) enantiomer (R 83824) is about 1.9- and 32-times more active than the racemate (IC50 2.6 nM) and the (-) enantiomer, respectively. R 83842 is about 30- and 1029-times more active than 4-hydroxyandrostene-3,17-dione and aminoglutethimide. This potency might originate from its high affinity for the microsomal cytochrome P450 (P450). Indeed, R 83842, compared to R 76713 and R 83839, forms a more stable P450-drug complex. Difference spectral measurements indicate that the triazole nitrogen N-4 coordinates to the haem iron. The reversed type 1 spectral changes suggest that R 76713 is able to displace the substrate from its binding place and the stable complex formed in particular with the (+) enantiomer suggests that its N-1-substituent occupies a lipophilic region of the apoprotein moiety. Kinetic analysis implies that there is a competitive part in the inhibition of the human placental aromatase by R 76713. The Ki values for R 76713, R 83842 and R 83839 are 1.3 nM, 0.7 nM and 18 nM, respectively. These results are indicative of stereospecificity for binding. Up to 10 microM, R 76713 and its enantiomers have no statistically significant effect on the regio- and stereoselective oxidations of testosterone in male rat liver microsomes. All three compounds have no effect on the P450-dependent cholesterol synthesis, cholesterol side-chain cleavage and 7 alpha-hydroxylation and 21-hydroxylase. At 10 microM, R 76713 has a slight effect on the bovine adrenal 11 beta-hydroxylase. This effect originates mainly from R 83839, the less potent aromatase inhibitor. On the other hand, the inhibition of the 17,20-lyase of rat testis observed at concentrations greater than or equal to 0.5 microM, originates rather from R 83842. However, 50% inhibition is only achieved at 1.8 microM R 83842, i.e. at a concentration about 1300-times higher than that needed to reach 50% inhibition of the human placental aromatase.

Saperconazole: a selective inhibitor of the cytochrome P-450-dependent ergosterol synthesis in Candida albicans, Aspergillus fumigatus and Trichophyton mentagrophytes

The N-1-substituted triazole antifungal, saperconazole, is a potent inhibitor of ergosterol synthesis in Candida albicans, Aspergillus fumigatus and Trichophyton mentagrophytes. Fifty % inhibition is already achieved at nanomolar concentrations. The saperconazole-induced inhibition of ergosterol synthesis coincides with an accumulation of 14-methylated sterols, such as 24-methylenedihydrolanosterol, lanosterol, obtusifoliol, 14 alpha-methylfecosterol, 14 alpha-methylergosta-8,24(28)-dien-3 beta-6 alpha-diol and 14 alpha-methylergosta-5,7,22,24(28)-tetraenol. This indicates that saperconazole interferes with the cytochrome P-450 (P-450)-dependent 14 alpha-demethylation of lanosterol and/or 24-methylenedihydrolanosterol. Saperconazole forms stable drug-P-450-complexes by binding via its free triazole nitrogen to the heme iron and via its N-1 substituent to the apoprotein moiety. The triazole derivative is a highly selective inhibitor of the 14 alpha-demethylase in fungal cells. It is a poor inhibitor of the 14 alpha-demethylation of lanosterol in rat and human liver cells. Saperconazole is, at concentrations as high as 10 microM, devoid of effects on the P-450-dependent cholesterol side-chain cleavage and 11 beta-hydroxylase, 17,20-lyase,21-hydroxylase and aromatase. Saperconazole does not interfere with the 2 alpha, 6 alpha-, 6 beta- and 7 alpha-hydroxylations of testosterone in microsomes from male rat liver. At high concentrations (greater than 5 microM) an inhibition of the 16 beta-hydroxylations is seen.

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

SDZ 89-485, a new triazole antimycotic agent, potently inhibited ergosterol biosynthesis in cells of Candida. Trichophyton and Aspergillus. Biosynthesis was measured both by incorporation of radiolabelled acetate and also by methylation of the sterol side chain. Inhibition was accompanied by accumulation of radiolabel in 4,4-dimethylsterols and to a lesser extent in the 4-methylsterols, consistent with inhibition of lanosterol 14-demethylation. No other steps were affected. Ergosterol biosynthesis in the mycelial growth form of Candida albicans was about twice as sensitive to the drug as that in the yeast form. Inhibition by SDZ 89-485 was qualitatively and quantitatively similar to that shown by other systemically active azoles (ketoconazole, itraconazole, ICI 195,739) in Candida cells and cell-free preparations. Rat liver cell-free cholesterol biosynthesis was much less sensitive to inhibition by SDZ 89-485 than was the equivalent Candida system (selectivity ratio of about 300). In contrast to miconazole and econazole, SDZ 89-485 had no uncoupling effect on respiration in C. albicans cells. The structure of SDZ 89-485 contains an asymmetric centre, the drug being the (-)(R) enantiomer. The (+)(S) enantiomer 89-486, which is antifungally much less effective, was at least 10-fold less active than SDZ 89-485 as an ergosterol biosynthesis inhibitor in all test systems employed. The difference between the two enantiomers appears to reside in stereo-selectivity at the level of interaction with the demethylase enzyme.

Biochemical approaches to selective antifungal activity. Focus on azole antifungals

Azole antifungals (e.g. the imidazoles: miconazole, clotrimazole, bifonazole, imazalil, ketoconazole, and the triazoles: diniconazole, triadimenol, propiconazole, fluconazole and itraconazole) inhibit in fungal cells the 14 alpha-demethylation of lanosterol or 24-methylenedihydrolanosterol. The consequent inhibition of ergosterol synthesis originates from binding of the unsubstituted nitrogen (N-3 or N-4) of their imidazole or triazole moiety to the heme iron and from binding of their N-1 substituent to the apoprotein of a cytochrome P-450 (P-450(14)DM) of the endoplasmic reticulum. Great differences in both potency and selectivity are found between the different azole antifungals. For example, after 16h of growth of Candida albicans in medium supplemented with [14C]-acetate and increasing concentrations of itraconazole, 100% inhibition of ergosterol synthesis is achieved at 3 x 10(-8) M. Complete inhibition of this synthesis by fluconazole is obtained at 10(-5) M only. The agrochemical imidazole derivative, imazalil, shows high selectivity, it has almost 80 and 98 times more affinity for the Candida P-450(s) than for those of the piglet testes microsomes and bovine adrenal mitochondria, respectively. However, the topically active imidazole antifungal, bifonazole, has the highest affinity for P-450(s) of the testicular microsomes. The triazole antifungal itraconazole inhibits at 10(-5) M the P-450-dependent aromatase by 17.9, whereas 50% inhibition of this enzyme is obtained at about 7.5 x 10(-6)M of the bistriazole derivative fluconazole. The overall results show that both the affinity for the fungal P-450(14)DM and the selectivity are determined by the nitrogen heterocycle and the hydrophobic N-1 substituent of the azole antifungals. The latter has certainly a greater impact. The presence of a triazole and a long hypdrophobic nonligating portion form the basis for itraconazole's potency and selectivity.

Fungistatic activity of miconazole against Candida albicans in relation to pH and concentration of nonprotonated drug

At less than 10(-5) M, miconazole (MCZ) exerts a fungistatic effect on Candida albicans, presumably by interfering with ergosterol biosynthesis. The imidazole moiety of MCZ is subject to protonation (pKa approximately 6.5). Based on pKa and greater water solubility of protonated (MCZH+) versus nonprotonated (MCZo) drug, fungistatic action ought to be markedly affected by environmental pH, but apparently it is not. In this report growth phase, pH, and concentrations of MCZ and MCZo have been studied in relation to fungistasis. Yeasts were grown in a synthetic liquid medium and MCZ effects were monitored by viability determinations. Results showed that fungistatic activity is little affected by growth phase and is largely independent of drug concentration and pH. Antagonism of fungistasis by low pH was demonstrated only at less than 10(-7) M MCZ. Data supported earlier proposals that MCZo is required for biological activity and suggested that target sites are saturated at very low levels of drug.