In vitro activities of antifungal agents alone and in combination against fluconazole-susceptible and -resistant strains of Candida dubliniensis

Antifungal resistance, combinations and pipeline: oh my!

Invasive fungal infections are a strong contributor to healthcare costs, morbidity and mortality, especially amongst hospitalized patients. Historically, Candida was responsible for approximately 15% of all nosocomial bloodstream infections. In the past 10 years, the epidemiology of Candida species has altered, with increasing prevalence of resistant species. With rising fungal resistance, especially in Candida spp., the demand for novel antifungal therapies has exponentially increased over the last decade. Newer antifungal agents have become an attractive option for patients needing long-term therapy for infections or those requiring antifungal prophylaxis. Despite advances in coverage of non-Candida pathogens with newer agents, clinical scenarios involving multidrug-resistant fungal pathogens continue to arise in practice. Combination antifungal therapy can lead to a host of side-effects, some of which can be drug limiting. Additional antifungal therapies with enhanced fungal spectrum of activity and decreased rates of adverse effects are warranted. Fosmanogepix, ibrexafungerp, olorofim and rezafungin may help fill some of these gaps in the antifungal armamentarium. This article is part of the Challenges and strategies in the management of invasive fungal infections Special Issue: https://www.drugsincontext.com/special_issues/challenges-and-strategies-in-the-management-of-invasive-fungal-infections.

Styrylpyridinium Derivatives as New Potent Antifungal Drugs and Fluorescence Probes

The incidence of Candida glabrata infections increases every year due to its higher resistance to commonly used antifungal drugs. We characterized the antifungal mechanism of action of eight new styrylpyridinium derivatives, with various N-alkyl chains (-C₆H₁₃, -C₈H₁₇, -C₁₀H₂₁, -C₁₂H₂₅) and different substituents, on C. glabrata strains differing in their drug resistance due to the presence or absence of two major drug-efflux pumps. We found that the tested styrylpyridinium compounds affected the growth of C. glabrata cells in a compound- and strain-dependent manner, and apparently they were substrates of CgCdr1 and CgCdr2 pumps. Further, we determined the impact of the tested compounds on plasma membrane integrity. The ability to cause damage to a plasma membrane depended on the compound, its concentration and the presence of efflux pumps, and corresponded well with the results of growth and survival tests. We also tested possible synergism with three types of known antifungal drugs. Though we did not observe any synergism with azole drugs, styrylpyridinium compounds 5 and 6 together with FK506 demonstrated excellent antifungal properties, whereas compounds 2, 3, 5, and 6 exhibited a significant synergistic effect in combination with terbinafine. Based on our results, derivatives 2 and 6 turned out to be the most promising antifungal drugs. Moreover, compound 6 was not only able to effectively permeabilize the yeast plasma membrane, but also exhibited significant synergism with FK506 and terbinafine. Finally, we also characterized the spectroscopic properties of the tested styrylpyridinium compounds. We measured their absorption and fluorescence spectra, determined their localization in yeast cells and found that their fluorescence characteristics differ from the properties of current commercial vacuolar styrylpyridinium markers and allow multi-color staining. Compounds 1, 3, 7, and 8 were able to accumulate in plasma and vacuolar membranes, and compounds 2, 5, and 6 stained the whole interior of dead cells. In summary, of the eight tested compounds, compound 6 is the most promising antifungal drug, compound 8, due to its minimal toxicity, is the best candidate for a new vacuolar-membrane probe or new benchmark substrate of C. glabrata Cdr pumps, and derivative 5 for a new vital dye.

Successful Treatment of Refractory Candidal Granuloma by Itraconazole and Terbinafine in Combination with Hyperthermia and Cryotherapy

Candidal granuloma is a rare and refractory disease in clinical practice, usually reported in immunocompromised patients. We report a 57-year-old man who presented with candidal granuloma caused by Candida tropicalis. The diagnosis was confirmed according to histopathology and molecular identification. Prolonged duration of initial antifungal therapy did not obtain satisfactory improvement. Finally, the refractory disease was successfully treated by itraconazole and terbinafine in combination with hyperthermia and cryotherapy. The "blackish-red dot" dermoscopic sign of the verrucous granuloma gradually resolved during the treatment.

Structural design of microbicidal cationic oligomers and their synergistic interaction with azoles against Candida albicans

Membrane-disrupting synthetic antimicrobial polymers have been well developed as antimicrobial peptide (AMP) mimics to mitigate antimicrobial resistance (AMR). However, synthetic polymers possess inherent drawbacks, being a mixture of different chain lengths, which restricts their clinical applications. In fact, synthetic oligomers with defined chain length and molecular structure could be better representatives of AMPs. Herein, a series of novel imidazolium-ammonium oligomers developed in this work exhibit excellent broad spectrum antimicrobial activity, specifically the salient structure dependent high efficiency against C. albicans. Moreover, synergistic effect emerged when the combined azoles and synthetic oligomers were applied against C. albicans. The detail structural coupling between azoles and oligomers was scrutinized through molecular dynamics simulations to unravel the interaction details with the atomistic resolution. The labile interaction between oligomer and azoles facilitated the transfer of drug into fungal cells, which can be a synergistic solution to prevent the development of resistance on C. albicans.

Refractory sporotrichosis due to Sporothrix brasiliensis in humans appears to be unrelated to in vivo resistance

Sporotrichosis is a subacute to chronic infection caused by members of the Sporothrix schenckii complex. Itraconazole is the first choice antifungal drug for treating this infection, with terbinafine and potassium iodide as alternatives and amphotericin B used in cases of severe infections. Correlation of antifungal susceptibility data with the clinical outcome of the patients is scarce. The aim of this study was to correlate clinical and mycological data in patients with refractory sporotrichosis. In this work, antifungal susceptibilities, determined according to the reference M38-A2 CLSI protocol, of 25 Sporothrix strains, isolated from seven human cases of sporotrichosis with adversities in the treatment, are presented. Tested drugs included itraconazole, ketoconazole, posaconazole, voriconazole, terbinafine, and amphotericin B. Fungi were identified using the T3B PCR fingerprinting. This method identified all strains as Sporothrix brasiliensis and also demonstrated a high degree of similarity between the strains. In general, voriconazole was ineffective against all strains, and elevated minimal inhibitory concentrations (MICs) were observed for amphotericin B. High itraconazole and terbinafine MICs were not observed in S. brasiliensis isolates from patients of this study. Moreover, a significant increase in itraconazole and terbinafine MIC values from strains isolated from the same patient in different periods was not observed. The results suggest that the antifungal susceptibility to terbinafine and itraconazole determined by the reference method does not play an important role in therapeutic failure of sporotrichosis and that acquisition of resistance during prolonged antifungal treatment is not likely to occur in S. brasiliensis.

Antifungal Therapy: New Advances in the Understanding and Treatment of Mycosis

The high rates of morbidity and mortality caused by fungal infections are associated with the current limited antifungal arsenal and the high toxicity of the compounds. Additionally, identifying novel drug targets is challenging because there are many similarities between fungal and human cells. The most common antifungal targets include fungal RNA synthesis and cell wall and membrane components, though new antifungal targets are being investigated. Nonetheless, fungi have developed resistance mechanisms, such as overexpression of efflux pump proteins and biofilm formation, emphasizing the importance of understanding these mechanisms. To address these problems, different approaches to preventing and treating fungal diseases are described in this review, with a focus on the resistance mechanisms of fungi, with the goal of developing efficient strategies to overcoming and preventing resistance as well as new advances in antifungal therapy. Due to the limited antifungal arsenal, researchers have sought to improve treatment via different approaches, and the synergistic effect obtained by the combination of antifungals contributes to reducing toxicity and could be an alternative for treatment. Another important issue is the development of new formulations for antifungal agents, and interest in nanoparticles as new types of carriers of antifungal drugs has increased. In addition, modifications to the chemical structures of traditional antifungals have improved their activity and pharmacokinetic parameters. Moreover, a different approach to preventing and treating fungal diseases is immunotherapy, which involves different mechanisms, such as vaccines, activation of the immune response and inducing the production of host antimicrobial molecules. Finally, the use of a mini-host has been encouraging for in vivo testing because these animal models demonstrate a good correlation with the mammalian model; they also increase the speediness of as well as facilitate the preliminary testing of new antifungal agents. In general, many years are required from discovery of a new antifungal to clinical use. However, the development of new antifungal strategies will reduce the therapeutic time and/or increase the quality of life of patients.

Antifungal activities of tacrolimus in combination with antifungal agents against fluconazole-susceptible and fluconazole-resistant Trichosporon asahii isolates

The antifungal activity of tacrolimus in combination with antifungal agents against different fungal species has been previously reported. Here we report the in vitro interactions between tacrolimus and amphotericin B, fluconazole, itraconazole, and caspofungin against 30 clinical isolates of both fluconazole-susceptible and fluconazole-resistant Trichosporon asahii. For these analyses, we used the broth microdilution method based on the M27-A3 technique and checkerboard microdilution method. Tacrolimus showed no activity against T. asahii strains (minimal inhibitory concentrations, MICs > 64.0 μg mL⁻¹). However, a larger synergistic interaction was observed by the combinations tacrolimus + amphotericin B (96.67%) and tacrolimus + caspofungin (73.33%) against fluconazole-susceptible isolates. Combinations with azole antifungal agents resulted in low rates of synergism for this group (fluconazole + tacrolimus = 40% and itraconazole + tacrolimus = 10%). Antagonistic interactions were not observed. For the fluconazole-resistant T. asahii group, all tested combinations showed indifferent interactions. The synergism showed against fluconazole-susceptible T. asahii isolates suggests that the potential antifungal activity of tacrolimus deserves in vivo experimental investigation, notably, the combination of tacrolimus with amphotericin B or caspofungin.

Silver and gold nanostructures: antifungal property of different shapes of these nanostructures on Candida species

The shape of nanoparticles is an important determinant of their physical and chemical properties, possibly including the little-explored area of their use as antifungal agents. Therefore, we evaluated the in vitro antifungal activities of three different shapes of silver and gold nanostructures, including nanocubes, nanospheres, and nanowires, on Candida albicans, C. glabrata and C. tropicalis, using the microdilution and disk diffusion methods as per the guidelines of the Clinical and Laboratory Standards Institute. We found that silver and gold nanocubes had higher antifungal properties against the test species than nanospheres and nanowires. While some isolates were resistant to silver and gold nanospheres and nanowires, none of the isolates were resistant to silver and gold nanocubes. The occurrence of resistance is a new finding which should be further explored.

Possible mechanisms of the antifungal activity of fluconazole in combination with terbinafine against Candida albicans

CONTEXT

Candidiasis is a term describing infections by yeasts from the genus Candida, the majority Candida albicans. Treatment of such infections often requires antifungals such as the azoles, but increased use of these drugs has led to selection of yeasts with increased resistance to these drugs.

OBJECTIVE

Combination therapy would be one of the best strategies for the treatment of candidiasis due to increased resistance to azoles.

MATERIALS AND METHODS

The antifungal activities of fluconazole and terbinafine were evaluated in vitro alone and in combination using broth microdilution test and time kill study. Eventually the expression level of selected genes involved in ergosterol biosynthesis of Candida was evaluated using semi-quantitative RT-PCR.

RESULTS

The obtained results showed the significant MICs ranging from 0.25 to 8 µg/mL followed by FICs ranged from 0.37 to 1 in combination with fluconazole/terbinafine. Our findings have demonstrated that the combination of fluconazole and terbinafine could also significantly reduce the expression of ERG1, 3, and 11 in the cell membrane of Candida in all concentrations tested ranging from 1.73- to 6.99-fold.

DISCUSSION AND CONCLUSION

This study was undertaken with the ultimate goal of finding the probable targets of fluconazole/terbinafine in C. albicans by looking at its effects on cell membrane synthesis.

Chloroquine sensitizes biofilms of Candida albicans to antifungal azoles

Biofilms formed by Candida albicans, a human pathogen, are known to be resistant to different antifungal agents. Novel strategies to combat the biofilm associated Candida infections like multiple drug therapy are being explored. In this study, potential of chloroquine to be a partner drug in combination with four antifungal agents, namely fluconazole, voriconazole, amphotericin B, and caspofungin, was explored against biofilms of C. albicans. Activity of various concentrations of chloroquine in combination with a particular antifungal drug was analyzed in a checkerboard format. Growth of biofilm in presence of drugs was analyzed by XTT-assay, in terms of relative metabolic activity compared to that of drug free control. Results obtained by XTT-metabolic assay were confirmed by scanning electron microscopy. The interactions between chloroquine and four antifungal drugs were determined by calculating fractional inhibitory concentration indices. Azole resistance in biofilms was reverted significantly (p < 0.05) in presence of 250 μg/mL of chloroquine, which resulted in inhibition of biofilms at very low concentrations of antifungal drugs. No significant alteration in the sensitivity of biofilms to caspofungin and amphotericin B was evident in combination with chloroquine. This study for the first time indicates that chloroquine potentiates anti-biofilm activity of fluconazole and voriconazole.