Systemic fungal infections pose a serious clinical problem. Treatment options are limited, and antifungal drug resistance is increasing. In addition, a substantial proportion of patients do not respond to therapy despite being infected with fungi that are susceptible to the drug. The discordance between overall treatment outcome and low levels of clinical resistance may be attributable to antifungal drug tolerance. In this Review, we define and distinguish resistance and tolerance and discuss the current understanding of the molecular, genetic and physiological mechanisms that contribute to those phenomena. Distinguishing tolerance from resistance might provide important insights into the reasons for treatment failure in some settings. In this Review, Berman and Krysan define and distinguish resistance and tolerance, and discuss the current understanding of the molecular, genetic and physiological mechanisms that contribute to those phenomena. Distinguishing tolerance from resistance might provide important insights into the reasons for treatment failure in some settings.

Drug resistance and tolerance in fungi.

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.

/pdf/antifungal-therapy-new-advances-in-the-understanding-and-2nk9zo7suk.pdf

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

Like other microorganisms, free-living Candida albicans is mainly present in a three-dimensional multicellular structure, which is called a biofilm, rather than in a planktonic form. Candida albicans biofilms can be isolated from both abiotic and biotic surfaces at various locations within the host. As the number of abiotic implants, mainly bloodstream and urinary catheters, has been increasing, the number of biofilm-associated bloodstream or urogenital tract infections is also strongly increasing resulting in a raise in mortality. Cells within a biofilm structure show a reduced susceptibility to specific commonly used antifungals and, in addition, it has recently been shown that such cells are less sensitive to killing by components of our immune system. In this review, we summarize the most important insights in the mechanisms underlying biofilm-associated antifungal drug resistance and immune evasion strategies, focusing on the most recent advances in this area of research.

/pdf/recent-insights-into-candida-albicans-biofilm-resistance-4b4omjxumt.pdf

Recent insights into Candida albicans biofilm resistance mechanisms

Increase in invasive fungal infections over the past few years especially in immunocompromised patients prompted the search for new antifungal agents with improved efficacy Current antifungal armoury includes very few effective drugs like Amphotericin B; new generation azoles, including voriconazole and posaconazole; echinocandins like caspofungin and micafungin to name a few Azole class of antifungals which target the fungal cell membrane are the first choice of treatment for many years because of their effectiveness As the fungal cell membrane is predominantly made up of sterols, glycerophospholipids and sphingolipids, the role of lipids in pathogenesis and target identification for improved therapeutics were largely pursued by researchers during the last few years Present review focuses on cell membrane as an antifungal target with emphasis on membrane biogenesis, structure and function of cell membrane, cell membrane inhibitors, screening assays, recent advances and future prospects

https://sfamjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/jam.13301

Fungal cell membrane-promising drug target for antifungal therapy.

CYP51 (Erg11) belongs to the cytochrome P450 monooxygenase (CYP) superfamily and mediates a crucial step of the synthesis of ergosterol, which is a fungal-specific sterol. It is also the target of azole drugs in clinical practice. In recent years, researches on fungal CYP51 have stepped into a new stage attributing to the discovery of crystal structures of the homologs in Candida albicans, Cryptococcus neoformans and Aspergillus fumigatus. This review summarizes the functions, structures of fungal CYP51 proteins, and the inhibitors targeting these homologs. In particular, several drug-resistant mechanisms associated with the fungal CYP51s are introduced. The sequences and crystal structures of CYP51 proteins in different fungal species are also compared. These will provide new insights for the advancement of research on antifungal agents.

/pdf/the-fungal-cyp51s-their-functions-structures-related-drug-3848bpx4sn.pdf

The Fungal CYP51s: Their Functions, Structures, Related Drug Resistance, and Inhibitors

Candida albicans is a clinically important fungus and is capable of forming biofilms, which contributes to the emergence of fluconazole resistance. Here, sessile minimum inhibitory concentrations (SMICs) of fluconazole combined with doxycycline against biofilms of C. albicans were determined, and the results of SMICs were compared with minimum inhibitory concentrations (MICs) of planktonic cells. SMICs and MICs were determined by microdilution checkerboard method, and the interactions between two drugs were interpreted by two models of fractional inhibitory concentration index and the percentage of growth difference (ΔE). For the biofilms formed over 4, 8, and 12 h, synergism was displayed by the combination of doxycycline(1-64 mg L(-1)) and fluconazole, and the fluconazole SMIC reduced from 64-512 mg L(-1) to 1-16 mg L(-1) against all the tested isolates. Calcium homeostasis is an important factor in growth of C. albicans. In this study, the impact of calcium channel blocker on the drug combination was observed by plate streaking and determined by liquid methods quantitatively. Obvious enhancement of antifungal effect appeared by combination of three drugs. These results show us that fluconazole combined with doxycycline could be effective against C. albicans biofilm, and the combined antifungal mechanism is associated with calcium.

Synergistic effect of doxycycline and fluconazole against Candida albicans biofilms and the impact of calcium channel blockers

Candida albicans biofilms are intrinsically resistant to antimicrobial agents. Previous work demonstrated that the antifungal activity of fluconazole against C. albicans biofilms is notably enhanced by doxycycline. In order to explore the synergistic mechanism of fluconazole and doxycycline, we investigated the changes of efflux pump gene expression, intracellular calcium concentration and cell cycle distribution after drug intervention in this study. The expression levels of CDR1, CDR2 and MDR1 were determined by real-time PCR, and the results showed that fluconazole alone could stimulate the high expression of CDR1, CDR2 and MDR1, and the combination of doxycycline and fluconazole downregulated the gene overexpression induced by fluconazole. Intracellular calcium concentration was determined using Fluo-3/AM by observing the fluorescence with flow cytometry. A calcium fluctuation, which started 4 h and peaked 8 h after the treatment with fluconazole, was observed. The combined drugs also initiated a calcium fluctuation after 4 h treatment and showed a peak at 16 h, and the peak was higher than that stimulated by fluconazole alone. The cell cycle was measured using flow cytometry. Fluconazole alone and the combined drugs both induced a reduction in the percentages of S-phase cells and an elevation in the percentages of cells in the G2/M phase. The results of this research showed that the synergism of fluconazole and doxycycline against C. albicans biofilms is associated with blockade of the efflux pump genes CDR1, CDR2 and MDR1, and stimulation of high intracellular calcium concentration. The findings of this study are of great significance in the search for new antifungal mechanisms.

Synergistic effect of fluconazole and doxycycline against Candida albicans biofilms resulting from calcium fluctuation and downregulation of fluconazole-inducible efflux pump gene overexpression.

Candida krusei is intrinsically resistant to fluconazole (FLC). This study aimed to investigate whether tacrolimus (FK506) could enhance the susceptibility of FLC against C. krusei. The tested strains included the following: five isolates with minimal inhibitory concentration (MIC) of 32 μg mL(-1), two with MIC of 256 μg mL(-1), and one with MIC of 512 μg mL(-1). MICs of FK506 and FLC alone and in combination were determined by checkerboard assay, with data analyzed by fractional inhibitory concentration index model. The time-kill curves were plotted to investigate the antifungal activity at 0, 6, 12, 24, and 48 h after drug exposure. The results revealed that FK506 reduced the resistance of all isolates obviously and degree of reduction in MICs varied with susceptibilities of strains. Addition of FK506 resulted in a fourfold and 16-fold downward shift in MICs of the isolates with MICs of 32 μg mL(-1) and of ≥ 256 μg mL(-1), respectively. The synergy was further confirmed by the time-kill assay. When they were in combination against CK4/CK9 with MIC of 32/256 μg mL(-1), there was a 2.25/2.03 log10 CFU mL(-1) decrease at 24 h compared with FLC alone, respectively. In conclusion, combination of FK506 and FLC may represent a promising approach of overcoming the intrinsic resistance of C. krusei to FLC.

A promising approach of overcoming the intrinsic resistance of Candida krusei to fluconazole (FLC) – combining tacrolimus with FLC

There is an increasing concern about Candida glabrata due to its high isolation frequency in candidiasis recently and notorious drug resistance to fluconazole. Drug combination is one effective approach to counteract drug resistance. This study aimed to test whether a combination of fluconazole and tacrolimus (FK506) had a synergistic effect on C. glabrata, and to seek the potential mechanisms underlying the synergistic effects. In vitro effects of fluconazole and FK506 against C. glabrata with different susceptibilities were investigated by a chequerboard method and a time–kill curve method. The mechanistic studies against the resistant C. glabrata were performed from two aspects: quantification of expression levels of fluconazole resistance genes (ERG11, CDR1, PDH1 and SNQ2) by real-time quantitative PCR and functional assays of drug efflux pumps. The addition of FK506 resulted in a decrease in the MIC of fluconazole from 32 to 8 µg ml−1 against the dose-dependent susceptible C. glabrata, and from 256 to 16 µg ml−1 against the resistant C. glabrata, respectively. The synergy was further confirmed by the time-kill assay. The expression levels of the ERG11 and SNQ2 genes were significantly downregulated after exposure to the drug combination, whereas that of the CDR1 gene was significantly upregulated, and no significant change in expression of PDH1 gene was observed. Flow cytometric assays showed that FK506 reduced the efflux of fluconazole. Tacrolimus enhanced the susceptibility of fluconazole against resistant C. glabrata by reducing the expression levels of the ERG11 and SNQ2 genes and inhibiting fluconazole efflux.

Resistance reversal induced by a combination of fluconazole and tacrolimus (FK506) in Candida glabrata.

The Effect of Fluconazole in Combination with Rifampin in vitro on Candida albicans Biofilms

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