Mechanisms of action and resistance of the major antifungal agents.
Abstract
Candida species, members of the normal body flora, are opportunistic mycosis agents that can cause infections associated with high morbidity and mortality rates in the presence of underlying predisposing factors. In recent studies, it has been reported that the incidence of invasive Candida infections caused by Candida species, such as non-albicans Candidaparapsilosis, Candida tropicalis, Candida glabrata, and Candida auris, in which antifungal drug resistance is more common, has increased, in addition to Candidaalbicans, the most frequently detected Candida species. In this context, the objective of this review article is to discuss the molecular mechanisms and biofilm-related factors responsible for the antifungal drug resistance developed in Candida species.
Keywords
- Candida spp.
- antifungal drug resistance
- azoles
- echinocandins
- amphotericin B
- biofilm
1. Introduction
The epidemiology of invasive
The results of the SENTRY study where antifungal resistance was investigated in
The progressive increase of
In this context, molecular mechanisms and biofilm-related factors responsible for resistance to antifungal drugs in
2. Antifungal drugs
The emergence of acquired drug resistance in common
Azoles bind to 14-α-demethylase, which is one of the critical enzymes (Erg11p) during ergosterol biosynthesis, leading to the disruption of fungal ergosterol synthesis and accumulation of toxic sterols. Echinocandins act by blocking the catalytic subunit of the glucan synthase enzyme encoded by the
Fluconazole, which is included in the azole group, is often the drug of choice for the treatment of most
Given the limited number of antifungals used today, several clinical studies are underway for the development of antifungals. Some of these studies feature promising drugs, such as fosmanogepix (a novel Gwt1 enzyme inhibitor), ibrexafungerp (a first-in-class triterpenoid), olorofime (a novel dihydroorotate dehydrogenase enzyme inhibitor), opelconazole (a novel triazole optimized for inhalation), and rezafungin (an echinocandin designed to be dosed once weekly) are currently in the final phase [3].
3. Antifungal drug resistance
Antifungal drug resistance refers to stable genetic changes that increase the probability of failure in a treatment applied against a fungal pathogen included in a particular class of antifungal drugs [2]. In addition to several clinical factors pertaining to the host, the mechanisms of action of antifungals, the acquired resistance related to the mutations observed in
Generally speaking, resistance mechanisms cannot be transferred between
4. Detection of antifungal drug resistance
4.1 Phenotypic methods
In vitro antifungal susceptibility testing (AFST) is a tool commonly used to detect antifungal drug resistance or the possibility of failure of antifungal therapy. AFST measures the ability of a particular organism to grow in vitro in the presence of a particular drug. This measured growth indicates the minimal inhibitory concentration (MIC), that is, the lowest drug concentration that completely stops or significantly reduces fungal growth. Antifungal drug resistance is quantitatively determined phenotypically by determining the MIC value [14, 16]. To this end, broth microdilution (BMD) based reference methods that have been standardized for AFST by the Clinical and Laboratory Standards Institute (CLSI) and the European Committee for Antimicrobial Susceptibility Testing (EUCAST), are used [14].
Standardization of the tests enabled the determination of clinical breakpoints (CBP) and epidemiological cut-off values (ECV) for azoles, echinocandin, and AMB against
Today, in addition to the reference methods, commercial tests (
4.2 Molecular methods
Phenotypic AFST has some major limitations. Therefore, other methods have been developed for the molecular detection of resistance-related genetic mutations independent of culture for the isolation of
Molecular detection of resistance relies on DNA technologies used in the detection of relevant mutations in genes associated with drug resistance, including methods, such as Sanger sequencing, pyrosequencing, real-time PCR, Luminex technology, and next-generation sequencing (NGS). Among these methods, NGS has the ability to detect novel mutations that play a role in the phenotypic resistance of clinical isolates. However, these methods have limited use in the direct detection of resistance. Although there are specific tests for the direct detection of
5. Antifungal resistance mechanisms
5.1 Resistance mechanisms to azole group of antifungal drugs
Ergosterol, which makes up most of the sterols in the fungal cell membrane, is formed by the conversion of lanosterol to ergosterol by the enzyme lanosterol 14-alpha-demethylase, which is encoded by the
Among the molecular resistance mechanisms developed against the azole group of antifungal drugs are alteration or overproduction of lanosterol 14-alpha-demethylase, which is involved in the synthesis of ergosterol and is the target of the antifungal drug, and mechanisms that ensure the excretion of the antifungal drug out of the cell [3].
5.1.1 Point mutations in the ERG11 gene
Mutations leading to amino acid changes in the hotspot (HS) region of the ERG11 gene can cause azole resistance by causing changes in the structure of the target protein and a decrease in the binding affinity of the drug [20].
Most of the amino acid changes that occur in the
Mutation of Y132F either alone or in combination with R398I has been reported in fluconazole-resistant
In another resistance mechanism, the defect in the
On the other hand, chemical diversity in a core unit structure within the azole family facilitates the development of cross-resistance. For example, some mutations in
5.1.2 Overexpression of the ERG11 gene
Often the level of overexpression is minimal or can be observed with other resistance mutations. Studies have shown that overexpression often involves Upc2p, a zinc cluster transcription factor induced upon depletion of ergosterol. Mutations in Upc2p result in gain-of-function (GOF) for this regulator, resulting in constitutive transcriptional activity and increased Erg11p production [3]. In addition to managing the regulation of many other genes (not only
However, overexpression of
5.1.3 Overexpression of membrane transportes
The efflux pumps are the proteins responsible for the excretion of exogenous or endogenous substances out of the cell by transporting them across the cell membrane. Accordingly, the efflux pumps throw drugs out of the cell, reducing their intracellular concentrations and thus their effects on the cell. There are two types of efflux pumps associated with drug resistance: ATP binding cassette (ABC) transporters (
The ABC and MFS transporters in pathogenic yeasts are mainly overexpressed by GOF mutations in
5.2 Resistance mechanisms to echinocandin group of antifungal drugs
Echinocandins act by inhibiting the two catalytic subunits of the BDG synthase enzyme complex encoded by the
Studies have identified several mutations associated with echinocandin resistance in the HS1 and HS2 regions of
In recent studies, in addition to stating that the most appropriate way to determine the echinocandin resistance mechanisms is the sequence analysis of the HS region, the importance of whole-genome analysis of the
The fact that echinocandin-resistant isolates, especially
5.3 Resistance mechanisms to polyene group of antifungal drugs
Polyenes are a group of antifungal drugs that target ergosterol-containing membranes and bind to sterols in the cell membrane, forming channels, and thereby disrupting the integrity of the membrane [19].
AMB is fungicidal, and resistance to AMB is usually observed intrinsically. Acquired resistance in susceptible species is rare [18]. The mechanism deemed to be responsible for AMB resistance in
Some strains of the
5.4 Multi-drug resistance and related resistance mechanisms
Although intrinsic multi-drug resistance (MDR) is rare among
The resistance to azole usually develops over time depending on more than one mechanism, including the
The mechanisms of action of antifungal drugs used in the treatment of
Antifungal class | Antifungal Drug | Mechanism(s) of Action | Mechanism(s) of resistance |
---|---|---|---|
Azoles | Fluconazole Voriconazole Posaconazole Itraconazole İsavuconazole | İnhibition of the 14-α-demethylase, which is one of the critical enzymes (Erg11p) during ergosterol biosynthesis, leads to the disruption of fungal ergosterol synthesis and accumulation of toxic sterols. İnhibition of fungal cell membrane function and growth. | Overexpression of cell membrane eflux pumps, decreasing drug concentration. Alteration of the target enzyme, decreasing affinity to the binding site (point mutation in ERG11 gene). Upregulation of the target enzyme (overexpression of ERG11 gene). |
Echinocandins | Caspofungin Micafungin Anidulafungin | İnhibition of the catalytic subunit of the glucan synthase enzyme encoded by the İnhibition of the biosynthesis of β-1,3-D-glucan, the primary cell wall polymer. | Regulated expression of glucan biosynthesis genes. Point mutations in FKS1 and FKS2 genes. |
Polyenes | Amphotericin B | Polyenes, bind to ergosterol, leading to the formation of pores in the cell membrane, disrupting the osmotic balance and ultimately the death of the fungal cell Oxidative damage. | Replacement of cell membrane sterols. Mutations in the |
5.5 Biofilm and antifungal resistance
The most fundamental features of
Biofilm development progresses through four main phases over a 24- to 48-hour period: adherence, initiation, maturation, and dispersal [25, 26]. Accordingly, the adherence of the yeast cell to the surface (adherence phase) is followed by the cell proliferation phase (initiation phase), which is accompanied by hyphal growth. Subsequently, the maturation of the biofilm structure (maturation phase) begins with the assembly of hyphae and the aggregation of the extracellular matrix (ECM). Finally, yeast cells detached from the upper parts of the biofilm layer are dispersed to the environment in order to initiate the same process in other foci (dispersal phase) [23, 26].
5.5.1 Adhesion phase
During the adhesion phase, yeast cells adhere to a surface and form a basal layer that will anchor the biofilm to the surface. Adhesins specific to
5.5.2 Initiation phase
The adhesion phase is followed by the initiation phase, which is characterized by the onset of hyphae formation and leads to the formation of a hyphae network that will contribute to the overall strength of the biofilm. This phase is critical for the healthy development of the biofilm. At this phase, virulence factors specific to the cell type and transcriptional regulators play a role [23, 25].
5.5.3 Maturation phase
The next phase is maturation. Hyphal yeast cells produce exo-polymeric substances (EPS), which virtually act as adhesives. A mature
5.5.4 Dispersal phase
The final phase is characterized by the dispersal of the mature forms of yeast cells and/or biofilm fragments. In this way, biofilm formation occurs in different regions, and the infection becomes systemic [23, 25]. The ability of biofilm-associated yeast cells to disperse and thereby initiate new biofilm formation is of clinical significance in terms of giving rise to invasive diseases and candidemia [26]. Various components, such as transcriptional regulators, cell wall proteins, and chaperones play an important role in this phase [23, 25]. The steps of biofilm formation of C. albicans are shown in Figure 1 [28].
Despite their fundamental similarities, bacterial and fungal biofilms differ in structural and developmental aspects. Dispersal from the bacterial biofilms occurs predominantly at the end of the biofilm life cycle. On the other hand, dispersal from the fungal biofilms featuring
Studies investigating environmental signals regulating the dispersion from fungal biofilms are still in their infancy and have particular aspects, which differ from the studies that investigate the dispersion from bacterial biofilms. Dispersal from bacterial biofilms is triggered by factors, such as nutrition, carbon limitation, hypoxia, low nitric oxide (NO) levels, and a decrease in cellular bis-(3′-5′)-cyclic dimeric guanosine monophosphate (c-di-GMP) levels [30, 31].
In comparison, dispersal from the fungal biofilms featuring
Dispersion from fungal biofilms depends on the balance between yeast and the hyphae community. PES1 (Pescadillo ribosomal biogenesis factor-1), which controls the production of lateral yeast from hyphal filaments as key regulators of dispersion, and NRG1 (Neuregulin-1), a negative regulator of filamentation, have been reported to play a role in this balance [29]. In addition, it was stated in another study that the presence of histone deacetylase, which enables proper biofilm formation and multifactorial drug resistance development in
Dispersin B, which is among the matrix-degrading enzymes in bacteria, and DNase I (Deoxyribonuclease I), another enzyme, have attracted attention as an antibiofilm and pro-dispersal agent. In fungal biofilms, a complex hyphae structure and the presence of abundant EPS (extracellular polymeric substances) prevent fragmentation. Although none of the well-known dispersins has been identified in fungal biofilms, DNase has been found to cause degradation by acting on the biofilm matrix in the treatment of
The aim of the biofilm dispersion is to prevent biofilm-induced infections and to develop new treatment approaches [31]. The key step in the fight against microorganisms in the biofilm is the disintegration and dissolution of the biofilm structure or its conversion into a planktonic cell form with no antibiotic resistance properties [33].
Biofilm formation is a complex and multi-phase process controlled by a wide variety of transcriptional regulators (TR). TRs play a key role in the microbial response given to environmental stimuli and regulate the cellular development and routine biological functions of the cells. Studies have shown that a network of nine basic TRs (BCR1, EFG1, NDT80, ROB1, TEC1, BRG1, FLO8, GAL4, and RFX2) is required for biofilm formation both in vitro and in vivo [23, 25].
Cells aggregated within biofilm clusters induce the host’s immune response and the development of resistance to antifungals. Candidiasis, which features a versatile interaction with the host, often involves the formation of surface-associated biofilms. Compared to planktonic cells,
The effect of biofilm formation on the development of antifungal resistance is multifactorial. Factors, such as the increase in the density of cells and cell membrane sterols, the presence of a complicated extracellular matrix, and the expression of antifungal resistance genes may lead to the development of antifungal resistance [26].
Biofilms formed by
6. Conclusion
In conclusion, the incidence of invasive
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