Plagiochin E, a botanic-derived phenolic compound, reverses fungal resistance to fluconazole relating to the efflux pump

Comprehensive insights into herbal P-glycoprotein inhibitors and nanoformulations for improving anti-retroviral therapy efficacy

The worldwide HIV cases were 39.0 million (33.1-45.7 million) in 2022. Due to genetic variations, HIV-1 is more easily transmitted than HIV-2 and favours CD4 + T cells and macrophages, producing AIDS. Conventional HIV drug therapy has many drawbacks, including adherence issues leading to resistance, side effects that lower life quality, drug interactions, high costs limiting global access, inability to eliminate viral reservoirs, chronicity requiring lifelong treatment, emerging toxicities, and a focus on managing infections. Conventional dosage forms have bioavailability issues due to intestinal P-glycoprotein (P-gp) efflux, which can reduce anti-retroviral drug efficacy and lead to resistance. Use of phyto-constituents with P-gp regulating actions has great benefits for semi-synthetic modification to create formulations with greater bioavailability and reduced toxicity, which improves drug effectiveness. Lipid-based nanocarriers, solid lipid nanoparticles, nanostructured lipid carriers, polymer-based nanocarriers, and inorganic nanoparticles may inhibit P-gp efflux. Employing potent P-gp inhibitors within nanocarriers as a Trojan horse approach can enhance the intracellular accumulation of anti-retroviral drugs (ARDs), which are substrates for efflux transporters. This technique increases oral bioavailability and offers lower-dose options, boosting HIV patient compliance and lowering costs. Molecular docking of the inhibitor with P-gp may anticipate optimum binding and function, allowing drug efflux to be minimised.

Recent Advances in the Phytochemistry of Bryophytes: Distribution, Structures and Biological Activity of Bibenzyl and Bisbibenzyl Compounds

Research on bryophyte phytochemistry has revealed the presence of different phytochemicals like fatty acids, terpenoids, small phenolic molecules, etc. Small phenolic molecules, i.e., bibenzyls (of two aromatic rings) and bisbibenzyls (four aromatic rings), are unique signature molecules of liverworts. The first bisbibenzyls marchantin A and riccardin A were discovered in two consecutive years, i.e., 1982 and 1983, respectively, by Asakawa and coworkers. Since then, about 70 bisbibenzyls have been reported. These molecules are characterized and identified using different spectroscopic techniques and surveyed for different bioactivity and structure-activity relations. Biochemistry is determined by the season, geography, and environment. In this review, quantitative and qualitative information on bibenzyls and bisbibenzyl compounds and their distribution in different liverworts across, geographies along withtraditional to advanced extraction methods, and characterization techniques are summarized. Also, a comprehensive account of characteristic spectra of different bisbibenzyl compounds, their subtypes, and their basic skeleton patterns are compared. A comprehensive table is provided here for the first time presenting the quantity of bibenzyls, bisbenzyls, and their derivatives found in bryophytes, mentioning the spectroscopic data and mass profiles of the compounds. The significance of these compounds in different bioactivities like antibiotic, antioxidative, antitumor, antivenomous, anti-influenza, insect antifeedant, cytotoxic, and anticancerous activities are surveyed and critically enumerated.

Chitin Biosynthesis in Aspergillus Species

The fungal cell wall (FCW) is a dynamic structure responsible for the maintenance of cellular homeostasis, and is essential for modulating the interaction of the fungus with its environment. It is composed of proteins, lipids, pigments and polysaccharides, including chitin. Chitin synthesis is catalyzed by chitin synthases (CS), and up to eight CS-encoding genes can be found in Aspergillus species. This review discusses in detail the chitin synthesis and regulation in Aspergillus species, and how manipulation of chitin synthesis pathways can modulate fungal growth, enzyme production, virulence and susceptibility to antifungal agents. More specifically, the metabolic steps involved in chitin biosynthesis are described with an emphasis on how the initiation of chitin biosynthesis remains unknown. A description of the classification, localization and transport of CS was also made. Chitin biosynthesis is shown to underlie a complex regulatory network, with extensive cross-talks existing between the different signaling pathways. Furthermore, pathways and recently identified regulators of chitin biosynthesis during the caspofungin paradoxical effect (CPE) are described. The effect of a chitin on the mammalian immune system is also discussed. Lastly, interference with chitin biosynthesis may also be beneficial for biotechnological applications. Even after more than 30 years of research, chitin biosynthesis remains a topic of current interest in mycology.

Inferring Therapeutic Targets in Candida albicans and Possible Inhibition through Natural Products: A Binding and Physiological Based Pharmacokinetics Snapshot

Despite being responsible for invasive infections, fungal pathogens have been underrepresented in computer aided therapeutic target mining and drug design. Excess of Candida albicans causes candidiasis, causative of thrush and vaginal infection due to off-balance. In this study, we attempted to mine drug targets (n = 46) using a subtractive proteomic approach in this pathogenic yeast and screen natural products with inhibition potential against fructose-bisphosphate aldolase (FBA) of the C. albicans. The top compound selected on the basis of best docking score from traditional Indian medicine/Ayurvedic library was (4-Hydroxybenzyl)thiocarbamic acid, from the ZINC FBA inhibitor library was ZINC13507461 (IUPAC name: [(2R)-2-hydroxy-3-phosphonooxypropyl] (9E,12E)-octadeca-9,12-dienoate), and from traditional Tibetan medicine/Sowa rigpa was Chelerythrine (IUPAC name: 1,2-Dimethoxy-12-methyl-9H-[1,3]benzodioxolo[5,6-c]phenanthridin-12-ium), compared to the control (2E)-1-(4-nitrophenyl)-2-[(4-nitrophenyl)methylidene]hydrazine. No Ames toxicity was predicted for prioritized compounds while control depicted this toxicity. (4-Hydroxybenzyl)thiocarbamic acid showed hepatotoxicity, while Chelerythrine depicted hERG inhibition, which can lead to QT syndrome, so we recommend ZINC13507461 for further testing in lab. Pharmacological based pharmacokinetic modeling revealed that it has low bioavailability and hence, absorption in healthy state. In cirrhosis and renal impairment, absorption and plasma accumulation increased so we recommend further investigation into this occurrence and recommend high dosage in further tests to increase bioavailability.

A state-of-the-art review and prospective therapeutic applications of prenyl flavonoids as chemosensitizers against antifungal multidrug resistance in Candida albicans

Multidrug resistance (MDR) in the opportunistic pathogen Candida albicans is defined as non-susceptibility to at least one agent in two or more drug classes. This phenomenon has been increasingly reported since the rise in the incidence of fungal infections in immunocompromised patients at the end of the last century. After the discovery of efflux pump overexpression as a principal mechanism causing MDR in Candida strains, drug discovery targeting fungal efflux transporters has had a growing impact. Chemosensitization aims to enhance azole intracellular concentrations through combination therapy with transporter inhibitors. Consequently, the use of drug efflux inhibitors combined with the antifungal agent will sensitize the pathogen. As a result, the use of lower drug concentrations will reduce possible adverse effects on the host. Through an extensive revision of the literature, this review aims to provide an exhaustive and critical analysis of the studies carried out in the past two decades, regarding the chemosensitization strategy to cope with multidrug resistance in C. albicans. This work provides a deep analysis of the research about the inhibition of drug-efflux membrane transporters by prenylated flavonoids and the interactions of these phytocompounds with azole antifungals as an approach to chemosensitize multidrug-resistant C. albicans strains. We highlight the importance of prenylflavonoids and their particular chemical and pharmacological characteristics that make them excellent candidates with therapeutic potential as chemosensitizers. Finally, we propose the need for further research of prenyl flavonoids as inhibitors of drug-efflux mediated fungal resistance.

Anti-Inflammatory Effect of Liverwort (Marchantia polymorpha L.) and Racomitrium Moss (Racomitrium canescens (Hedw.) Brid.) Growing in Korea

Bryophytes contain a variety of bioactive metabolites, but studies about the anti-inflammatory effect of bryophytes are meager. Therefore, the present study aimed to compare the anti-inflammatory effect of methanol extract of Marchantia polymorpha L. (liverwort) and Racomitrium canescens (Racomitrium moss) in lipopolysaccharide (LPS)-induced HaCaT cells. To evaluate the anti-inflammatory effect of liverwort and Racomitrium moss, the levels of nitric oxide (NO) production and the mRNA expression of inducible nitric oxide synthase (iNOS), cyclooxygenase-2 (COX-2) and tumor necrosis factor-α (TNF-α), and interleukin (IL)-6 and IL-1β in LPS-induced HaCaT cells were measured. The methanol extract of liverwort and Racomitrium moss significantly decreased LPS-induced NO production in HaCaT cells. When compared with Racomitrium moss extract, pre-treatment with methanol extract of liverwort markedly inhibited the expression of iNOS, COX-2, IL-6, and IL-1β at the concentration of 100 µg/mL with the exception of TNF-α. Further, liverwort extract markedly attenuated the production of TNF-α, IL-6, and IL-1β in the culture medium. In addition, ethyl acetate and butanol fractions obtained from the methanol extract of liverwort showed remarkable inhibitory activity against the production of NO in LPS-stimulated HaCaT cells. The LC-MS data revealed the presence of bisbibenzyl types of bioactive components in the methanol extract of liverwort. These data demonstrate that liverwort extract exhibits effective inhibitory activity against the production of inflammatory mediators in LPS-induced HaCaT cells and may be useful for the treatment of inflammation-mediated diseases.

Recent progress on anti-Candida natural products

Candida is an intractable life-threatening pathogen. Candida infection is extremely difficult to eradicate, and thus is the major cause of morbidity and mortality in immunocompromised individuals. Morevover, the rapid spread of drug-resistant fungi has led to significant decreases in the therapeutic effects of clinical drugs. New anti-Candida agents are urgently needed to solve the complicated medical problem. Natural products with intricate structures have attracted great attention of researchers who make every endeavor to discover leading compounds for antifungal agents. Their novel mechanisms and diverse modes of action expand the variety of fungistatic agents and reduce the emergence of drug resistance. In recent decades, considerable effort has been devoted to finding unique antifungal agents from nature and revealing their unusual mechanisms, which results in important progress on the development of new antifungals, such as the novel cell wall inhibitors YW3548 and SCY-078 which are being tested in clinical trials. This review will present a brief summary on the landscape of anti-Candida natural products within the last decade. We will also discuss in-depth the research progress on diverse natural fungistatic agents along with their novel mechanisms.

Bryo-Activities: A Review on How Bryophytes Are Contributing to the Arsenal of Natural Bioactive Compounds against Fungi

Usually regarded as less evolved than their more recently diverged vascular sisters, which currently dominate vegetation landscape, bryophytes seem having nothing to envy to the defensive arsenal of other plants, since they had acquired a suite of chemical traits that allowed them to adapt and persist on land. In fact, these closest modern relatives of the ancestors to the earliest terrestrial plants proved to be marvelous chemists, as they traditionally were a popular remedy among tribal people all over the world, that exploit their pharmacological properties to cure the most different diseases. The phytochemistry of bryophytes exhibits a stunning assortment of biologically active compounds such as lipids, proteins, steroids, organic acids, alcohols, aliphatic and aromatic compounds, polyphenols, terpenoids, acetogenins and phenylquinones, thus it is not surprising that substances obtained from various species belonging to such ancestral plants are widely employed as antitumor, antipyretic, insecticidal and antimicrobial. This review explores in particular the antifungal potential of the three Bryophyta divisions-mosses (Musci), hornworts (Anthocerotae) and liverworts (Hepaticae)-to be used as a sources of interesting bioactive constituents for both pharmaceutical and agricultural areas, providing an updated overview of the latest relevant insights.

Bibenzyls and bisbybenzyls of bryophytic origin as promising source of novel therapeutics: pharmacology, synthesis and structure-activity

BACKGROUND

The amphibian, non-vascular, gametophyte-dominant, bio-indicator class, bryophytes; with their wide ranges of habitat have attained importance due to their promising medicinal attributions and therapeutic role; mostly aided by presence of aromatic bibenzyl and bisbybenzyl class of compounds. Bibenzyls are steroidal ethane derivatives, resembling the structural moiety of bioactive dihydro-stilbenoids or iso-quinoline alkaloids. These stress triggered secondary metabolites are the by-products of the flavonoid biosynthetic pathway. Different classes of bryophytes (Bryophyta, Marchantiophyta and Anthocerotophyta) possess different subtypes of bibenzyls and dimeric bisbibenzyls. Among the liverwort, hornwort and mosses, former one is mostly enriched with bibenzyl type constituents as per the extensive study conducted for phytochemical deposit. Considering macrocyclic and acyclic group of bibenzyls and bisbybenzyls, generally marchantin type compounds are reported vividly for significant biological activity that includes neuro-nephro-cardio-protection besides anti-allergic, anti-microbial, anti-apoptotic and cytotoxic activities studied on in-vitro and in-vivo models or on cell lines.

RESULT

The critical analysis of reported chemical and pharmaceutical attributions of bibenzyls and bis-bibenzyls yielded detailed report on this compound class along with their application, mode of action, natural source, techniques of synthesis, extraction procedure, isolation and characterization. Further, the structure activity relationship studies and bioactivity of bibenzyls derived from non-bryophytic origin were also summarized.

CONCLUSION

This review encompasses prospective biological application of botanical reservoir of this primarily ignored, primeval land plant group where recent technical advances has paved the way for qualitative and quantitative isolation and estimation of novel compounds as well as marker components to study their impact on environment, as bio-control agents and as key leads in future drug designing. Graphical abstract.

Sodium New Houttuyfonate Inhibits Candida albicans Biofilm Formation by Inhibiting the Ras1-cAMP-Efg1 Pathway Revealed by RNA-seq

Here, we aim to investigate the antifungal effect and mechanism of action of sodium new houttuyfonate (SNH) against Candida albicans. Microdilution analysis results showed that SNH possesses potent inhibitory activity against C. albicans SC5314, with a MIC₈₀ of 256 μg/mL. Furthermore, we found that SNH can effectively inhibit the initial adhesion of C. albicans. Inverted microscopy, crystal violet staining, scanning electron microscopy and confocal laser scanning microscopy results showed that morphological changes during the transition from yeast to hypha and the biofilm formation of C. albicans are repressed by SNH treatment. We also found that SNH can effectively inhibit the biofilm formation of clinical C. albicans strains (Z103, Z3044, Z1402, and Z1407) and SNH in combination with fluconazole, berberine chloride, caspofungin and itraconazole antifungal agents can synergistically inhibit the biofilm formation of C. albicans. Eukaryotic transcriptome sequencing and qRT-PCR results showed that SNH treatment resulted in significantly down-regulated expression in several biofilm formation related genes in the Ras1-cAMP-Efg1 pathway (ALS1, ALA1, ALS3, EAP1, RAS1, EFG1, HWP1, and TEC1) and significantly up-regulated expression in yeast form-associated genes (YWP1 and RHD1). We also found that SNH can effectively reduce the production of key messenger cAMP in the Ras1-cAMP-Efg1 pathway. Furthermore, using Galleria mellonella as an in vivo model we found that SNH can effectively treat C. albicans infection in vivo. Our presented results suggest that SNH exhibits potential antibiofilm effects related to inhibiting the Ras1-cAMP-Efg1 pathway in the biofilm formation of C. albicans.

Silva L, Gonçales RA, Neves BJ, Soares CM, Pereira M. Setting New Routes for Antifungal Drug Discovery Against Pathogenic Fungi

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Fungal diseases are life-threatening to human health and responsible for millions of deaths around the world. Fungal pathogens lead to a high number of morbidity and mortality. Current antifungal treatment comprises drugs, such as azoles, echinocandins, and polyenes and the cure is not guaranteed. In addition, such drugs are related to severe side effects and the treatment lasts for an extended period. Thus, setting new routes for the discovery of effective and safe antifungal drugs should be a priority within the health care system. The discovery of alternative and efficient antifungal drugs showing fewer side effects is time-consuming and remains a challenge. Natural products can be a source of antifungals and used in combinatorial therapy. The most important natural products are antifungal peptides, antifungal lectins, antifungal plants, and fungi secondary metabolites. Several proteins, enzymes, and metabolic pathways could be targets for the discovery of efficient inhibitor compounds and recently, heat shock proteins, calcineurin, salinomycin, the trehalose biosynthetic pathway, and the glyoxylate cycle have been investigated in several fungal species. HSP protein inhibitors and echinocandins have been shown to have a fungicidal effect against azole-resistant fungi strains. Transcriptomic and proteomic approaches have advanced antifungal drug discovery and pointed to new important specific-pathogen targets. Certain enzymes, such as those from the glyoxylate cycle, have been a target of antifungal compounds in several fungi species. Natural and synthetic compounds inhibited the activity of such enzymes and reduced the ability of fungal cells to transit from mycelium to yeast, proving to be promisor antifungal agents. Finally, computational biology has developed effective approaches, setting new routes for early antifungal drug discovery since normal approaches take several years from discovery to clinical use. Thus, the development of new antifungal strategies might reduce the therapeutic time and increase the quality of life of patients.

Overexpression of efflux pump transporter genes and mutations in ERG11 pave the way to fluconazole resistance in Candida tropicalis: A study from a North India region

INTRODUCTION

Fluconazole resistance in Candida tropicalis health care-associated infections is increasing. We investigated the role of efflux pump and mutations in ERG11p conferring fluconazole resistance in C. tropicalis.

MATERIALS AND METHODS

A total of 17 C. tropicalis clinical isolates, including fluconazole-resistant and fluconazole-susceptible/fluconazole-susceptible dose-dependent isolates, were collected from a tertiary care centre in a North India region between 2015 and 2018. Antifungal susceptibility, reversal of fluconazole resistance by tacrolimus, ERG11 amplification and its sequencing and a quantitative polymerase chain reaction (PCR) assay for expression analysis of ERG11, MDR1 and CDR1 genes were performed.

RESULTS AND DISCUSSION

Synergism between fluconazole and tacrolimus was observed in all resistant C. tropicalis isolates. Overexpression of all the three genes, MDR1, ERG11 and CDR1, was observed in resistant isolates (P = 0.05). Among resistant isolates, mutations leading to amino acid substitution were seen in two, Ct10 (glysine464serine) and Ct16 (tyrosine132phenylalanine; serine154phenylalanine).

CONCLUSION

Overexpression in efflux pump transporter genes, together with mutations in ERG11, lead to fluconazole resistance among C. tropicalis. To the best of our knowledge, this is the first study on the C. tropicalis fluconazole resistance mechanism from the North India region.

Targeting the fungal cell wall: current therapies and implications for development of alternative antifungal agents

Fungal infections are a worldwide problem associated with high morbidity and mortality. There are relatively few antifungal agents, and resistance has emerged within these pathogens for the newest antifungal drugs. As the fungal cell wall is critical for growth and development, it is one of the most important targets for drug development. In this review, the currently available cell wall inhibitors and suitable drug candidates for the treatment of fungal infections are explored. Future studies of the fungal cell wall and compounds that have detrimental effects on this important outer structural layer could aid in antifungal drug discovery and lead to the development of alternative cell wall inhibitors to fill gaps in clinical therapies for difficult-to-treat fungal infections.

Promising Antifungal Targets Against Candida albicans Based on Ion Homeostasis

In recent decades, invasive fungal infections have been increasing significantly, contributing to high incidences and mortality in immunosuppressed patients. Candida albicans (C. albicans) is the most prevalent opportunistic fungal pathogen in humans that can cause severe and often fatal bloodstream infections. Current antifungal agents have several limitations, including that only a small number of classes of antifungals are available, certain of which have severe toxicity and high cost. Moreover, the emergence of drug resistance is a new limitation to successful patient outcomes. Therefore, the development of antifungals with novel targets is an essential strategy for the efficient management of C. albicans infections. It is widely recognized that ion homeostasis is crucial for all living cells. Many studies have identified that ion-signaling and transduction networks are central to fungal survival by regulating gene expression, morphological transition, host invasion, stress response, and drug resistance. Dysregulation of ion homeostasis rapidly mediates cell death, forming the mechanistic basis of a growing number of compounds that elicit antifungal activity. Most of the potent antifungals have been widely used in the clinic, and certain of them have low toxicity, meaning that they may be expected to be used as antifungal drugs in the future. Hence, we briefly summarize the homeostasis regulation of several important ions, potential antifungal targets based on these ion-signaling networks, and antifungal compounds based on the disruption of ion homeostasis. This summary will help in designing effective drugs and identifying new targets for combating fungal diseases.

Fungicidal action of geraniol against Candida albicans is potentiated by abrogated CaCdr1p drug efflux and fluconazole synergism

Among the several mechanisms of multidrug resistance (MDR), overexpression of drug efflux pumps CaCdr1p and CaMdr1p belonging to ATP binding cassette (ABC) and major facilitator superfamily (MFS) respectively remain the predominant mechanisms of candidal infections. Therefore inhibiting or modulating the function of these transporters continues to draw attention as effective strategy to combat MDR. We have previously reported the antifungal potential of Geraniol (Ger), a natural monoterpenoid from Palmarosa oil, against Candida albicans. Herein, we explored the fungicidal nature of Ger. The Rhodamine 6G (R6G) and Nile red accumulation confirms the specific effect on CaCdr1p. Mechanistic insights with Candida cells overexpressing CaCdr1p and CaMdr1p revealed that Ger specifically modulates CaCdr1p activity. Kinetic studies further unraveled the competitive inhibition of Ger for R6G efflux as evident from increased apparent Km without affecting Vmax value. The effect of Ger on CaCdr1p was substantiated by molecular docking analyses, which depicted in-silico binding affinity of Ger with CaCdr1p and explored that Ger binds to the active site of CaCdr1p with higher binding energy. Although RT-PCR and western blot revealed no change in expressions of CDR1 and CaCdr1p, confocal microscopy images however depicted CaCdr1p mislocalization in presence of Ger. Interestingly, Ger was synergistic (FICI<0.5) with fluconazole (FLC) which is a well known antifungal drug. Furthermore, Ger sensitizes the FLC sensitive and resistant clinical matched pair of isolates Gu4/Gu5 and led to abrogated R6G efflux and depleted ergosterol. Furthermore, Rhodamine B labeling demonstrates altered mitochondrial potential with Ger which suggest possible linkage of dysfunctional mitochondria with CaCdr1p activity. We also estimated phenotypic virulence marker extracellular phospholipase activity which was considerably diminished along with inhibited cell adherence and biofilm biomass. Lastly, antifungal efficacy of Ger was demonstrated by enhanced survival of Caenorhabditis elegans model and negligible hemolytic activity (20%). Together, modulation of efflux pump activity by Ger and FLC synergism represent a promising approach for combinatorial treatment of candidiasis.

Chemical Constituents of Bryophytes: Structures and Biological Activity

Comparatively little attention has been paid to the bryophytes for use in the human diet or medicine in spite of the presence of 23 000 species globally. Several hundred new compounds have been isolated from the liverworts (Marchantiophyta), and more than 40 new carbon skeletons of terpenoids and aromatic compounds were found. Most of the liverworts studied elaborate characteristic odiferous, pungent, and bitter-tasting compounds, of which many show antimicrobial, antifungal, antiviral, allergic contact dermatitis, cytotoxic, insecticidal, anti-HIV, plant growth regulatory, neurotrophic, NO production and superoxide anion radical release inhibitory, muscle relaxing, antiobesity, piscicidal, and nematocidal activities. The biological effects ascribed to the liverworts are mainly due to lipophilic sesqui- and diterpenoids, phenolic compounds, and polyketides, which are the principal constituents of their oil bodies. Some mosses and liverworts produce significant levels of vitamin B₂ and tocopherols, as well as prostaglandin-like highly unsaturated fatty acids. The most characteristic chemical phenomenon of the liverworts is that most of the sesqui- and diterpenoids are enantiomers of those found in higher plants. In this review, the chemical constituents and potential medicinal uses of bryophytes are discussed.

Synergistic in vitro activity of sodium houttuyfonate with fluconazole against clinical Candida albicans strains under planktonic growing conditions

CONTEXT

Fluconazole resistance is an intractable problem of treating Candida albicans, calling for more antifungal agents to enhance the activity of fluconazole.

OBJECTIVE

This work investigates the anti-C. albicans activities of sodium houttuyfonate (SH) and/or fluconazole and the associated mechanism.

MATERIALS AND METHODS

The minimum inhibitory concentrations (MICs) of SH and fluconazole both ranging from 0.5 to 1024 μg/mL were determined by broth microdilution method in 19 C. albicans isolates, and their fractional inhibitory concentration index (FICI) was evaluated by checkerboard assay. After MIC_SH and/or MIC_fluconazole treatments, the expressions of IFD6, PHR1, ZAP1, ADH5, BGL2, XOG1 and FKS1 were analyzed by quantitative reverse transcription polymerase chain reaction (qRT-PCR) in C. albicans 1601.

RESULTS AND CONCLUSION

The MICs of SH alone ranged from 32 to 256 μg/mL and decreased 2-16-fold in combination. SH showed strong synergism with fluconazole with FICI <0.13-0.5. In C. albicans 1601, we observed that (i) the expression of the seven genes increased notably in a range between 3.71- and 12.63-fold (p < 0.05) when SH was used alone, (ii) the combined use of SH and fluconazole slightly inhibited the expression of IFD6 and PHR1 by 1.23- and 1.35-fold (p > 0.05), but promoted evidently the expression of ZAP1, ADH5, XOG1 and FKS1 by 1.98-, 3.56-, 4.10- and 2.86-fold (p < 0.05). The results suggested SH to be a potential synergist to enhance the antifungal activity of fluconazole in C. albicans resistant isolates, and the underlying mechanism may be associated with β-1,3-glucan synthesis and transportation.

Natural Products as Alternative Choices for P-Glycoprotein (P-gp) Inhibition

Multidrug resistance (MDR) is regarded as one of the bottlenecks of successful clinical treatment for numerous chemotherapeutic agents. Multiple key regulators are alleged to be responsible for MDR and making the treatment regimens ineffective. In this review, we discuss MDR in relation to P-glycoprotein (P-gp) and its down-regulation by natural bioactive molecules. P-gp, a unique ATP-dependent membrane transport protein, is one of those key regulators which are present in the lining of the colon, endothelial cells of the blood brain barrier (BBB), bile duct, adrenal gland, kidney tubules, small intestine, pancreatic ducts and in many other tissues like heart, lungs, spleen, skeletal muscles, etc. Due to its diverse tissue distribution, P-gp is a novel protective barrier to stop the intake of xenobiotics into the human body. Over-expression of P-gp leads to decreased intracellular accretion of many chemotherapeutic agents thus assisting in the development of MDR. Eventually, the effectiveness of these drugs is decreased. P-gp inhibitors act by altering intracellular ATP levels which are the source of energy and/or by affecting membrane contours to increase permeability. However, the use of synthetic inhibitors is known to cause serious toxicities. For this reason, the search for more potent and less toxic P-gp inhibitors of natural origin is underway. The present review aims to recapitulate the research findings on bioactive constituents of natural origin with P-gp inhibition characteristics. Natural bioactive constituents with P-gp modulating effects offer great potential for semi-synthetic modification to produce new scaffolds which could serve as valuable investigative tools to recognize the function of complex ABC transporters apart from evading the systemic toxicities shown by synthetic counterparts. Despite the many published scientific findings encompassing P-gp inhibitors, however, this article stand alones because it provides a vivid picture to the readers pertaining to Pgp inhibitors obtained from natural sources coupled with their mode of action and structures. It provides first-hand information to the scientists working in the field of drug discovery to further synthesise and discover new P-gp inhibitors with less toxicity and more efficacies.

Antifungal effects of phytocompounds on Candida species alone and in combination with fluconazole

Invasive fungal infections caused by Candida spp. remain the most predominant nosocomial fungal infections. Owing to the increased use of antifungal agents, resistance of Candida spp. to antimycotics has emerged frequently, especially to fluconazole (FLC). To cope with this issue, new efforts have been dedicated to discovering novel antimycotics or new agents that can enhance the susceptibility of Candida spp. to existing antimycotics. The secondary metabolites of plants represent a large library of compounds that are important sources for new drugs or compounds suitable for further modification. Research on the anti-Candida activities of phytocompounds has been carried out in recent years and the results showed that a series of phytocompounds have anti-Candida properties, such as phenylpropanoids, flavonoids, terpenoids and alkaloids. Among these phytocompounds, some displayed potent antifungal activity, with minimum inhibitory concentrations (MICs) of ≤8 µg/mL, and several compounds were even more effective against drug-resistant Candida spp. than FLC or itraconazole (e.g. honokiol, magnolol and shikonin). Interestingly, quite a few phytocompounds not only displayed anti-Candida activity alone but also synergised with FLC against Candida spp., even leading to a reversal of FLC resistance. This review focuses on summarising the anti-Candida activities of phytocompounds as well as the interactions of phytocompounds with FLC. In addition, we briefly overview the synergistic mechanisms and present the structure of the antimycotic phytocompounds. Hopefully, this analysis will provide insight into antifungal agent discovery and new approaches against antifungal drug resistance.

Efflux as a mechanism of antimicrobial drug resistance in clinical relevant microorganisms: the role of efflux inhibitors

INTRODUCTION

Microbial resistance against antibiotics is a serious threat to the effective treatment of infectious diseases. Several mechanisms exist through which microorganisms can develop resistance against antimicrobial drugs, of which the overexpression of genes to produce efflux pumps is a major concern. Several efflux transporters have been identified in microorganisms, which infer resistance against specific antibiotics and even multidrug resistance. Areas covered: This paper focuses on microbial resistance against antibiotics by means of the mechanism of efflux and gives a critical overview of studies conducted to overcome this problem by combining efflux pump inhibitors with antibiotics. Information was obtained from a literature search done with MEDLINE, Pubmed, Scopus, ScienceDirect, OneSearch and EBSCO host. Expert opinion: Efflux as a mechanism of multidrug resistance has presented a platform for improved efficacy against resistant microorganisms by co-administration of efflux pump inhibitors with antimicrobial agents. Although proof of concept has been shown for this approach with in vitro experiments, further research is needed to develop more potent inhibitors with low toxicity which is clinically effective.

Cinnamaldehyde and its derivatives, a novel class of antifungal agents

The last few decades have seen an alarming rise in fungal infections, which currently represent a global health threat. Despite extensive research towards the development of new antifungal agents, only a limited number of antifungal drugs are available in the market. The routinely used polyene agents and many azole antifungals are associated with some common side effects such as severe hepatotoxicity and nephrotoxicity. Also, antifungal resistance continues to grow and evolve and complicate patient management, despite the introduction of new antifungal agents. This suitation requires continuous attention. Cinnamaldehyde has been reported to inhibit bacteria, yeasts, and filamentous molds via the inhibition of ATPases, cell wall biosynthesis, and alteration of membrane structure and integrity. In this regard, several novel cinnamaldehyde derivatives were synthesized with the claim of potential antifungal activities. The present article describes antifungal properties of cinnamaldehyde and its derivatives against diverse classes of pathogenic fungi. This review will provide an overview of what is currently known about the primary mode of action of cinnamaldehyde. Synergistic approaches for boosting the effectiveness of cinnamaldehyde and its derivatives have been highlighted. Also, a keen analysis of the pharmacologically active systems derived from cinnamaldehyde has been discussed. Finally, efforts were made to outline the future perspectives of cinnamaldehyde-based antifungal agents. The purpose of this review is to provide an overview of current knowledge about the antifungal properties and antifungal mode of action of cinnamaldehyde and its derivatives and to identify research avenues that can facilitate implementation of cinnamaldehyde as a natural antifungal.

Discovery of 4-acetyl-3-(4-fluorophenyl)-1-(p-tolyl)-5-methylpyrrole as a dual inhibitor of human P-glycoprotein and Staphylococcus aureus Nor A efflux pump

Polysubstituted pyrrole natural products, lamellarins, are known to overcome multi-drug resistance in cancer via the inhibition of p-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) efflux pumps. Herein, a series of simplified polysubstituted pyrroles, prepared via a one-pot domino protocol, were screened for P-gp inhibition in P-gp overexpressing human adenocarcinoma LS-180 cells using a rhodamine 123 efflux assay. Several compounds showed the significant inhibition of P-gp at 50 μM, as indicated by increase in the intracellular accumulation of Rh123 in LS-180 cells. Furthermore, pyrrole 5i decreased the efflux of digoxin, a FDA approved P-gp substrate in MDCK-MDR1 cells with an IC50 of 11.2 μM. In in vivo studies, following the oral administration of a P-gp substrate drug, rifampicin, along with compound , the Cmax and AUC0-∞ of rifampicin was enhanced by 31% and 46%, respectively. All the compounds were then screened for their ability to potentiate ciprofloxacin activity via the inhibition of Staphylococcus aureus Nor A efflux pump. Pyrrole showed the significant inhibition of S. aureus Nor A efflux pump with 8- and 4-fold reductions in the MIC of ciprofloxacin at 50 and 6.25 μM, respectively. The molecular docking studies of compound with the human P-gp and S. aureus Nor A efflux pump identified its plausible binding site and key interactions. Thus, the results presented herein strongly indicate the potential of this scaffold for its use as multi-drug resistance reversal agent or bioavailability enhancer.

Synergistic antifungal effect of glabridin and fluconazole

The incidence of invasive fungal infections is increasing in recent years. The present study mainly investigated glabridin (Gla) alone and especially in combination with fluconazole (FLC) against Cryptococcus neoformans and Candida species (Candida albicans, Candida tropicalis, Candida krusei, Candida parapsilosis and Candida Glabratas) by different methods. The minimal inhibitory concentration (MIC) and the minimal fungicidal concentration (MFC) indicated that Gla possessed a broad-spectrum antifungal activity at relatively high concentrations. After combining with FLC, Gla exerted a potent synergistic effect against drug-resistant C. albicans and C. tropicalis at lower concentrations when interpreted by fractional inhibitory concentration index (FICI). Disk diffusion test and time-killing test confirming the synergistic fungicidal effect. Cell growth tests suggested that the synergistic effect of the two drugs depended more on the concentration of Gla. The cell envelop damage including a significant decrease of cell size and membrane permeability increasing were found after Gla treatment. Together, our results suggested that Gla possessed a synergistic effect with FLC and the cell envelope damage maybe contributed to the synergistic effect, which providing new information for developing novel antifungal agents.

Osthol and curcumin as inhibitors of human Pgp and multidrug efflux pumps of Staphylococcus aureus: reversing the resistance against frontline antibacterial drugs

Curcumin and osthol are identified as NorA pump inhibitors.

ML212: A small-molecule probe for investigating fluconazole resistance mechanisms in Candida albicans

The National Institutes of Health Molecular Libraries and Probe Production Centers Network (NIH-MLPCN) screened >300,000 compounds to evaluate their ability to restore fluconazole susceptibility in resistant Candida albicans isolates. Additional counter screens were incorporated to remove substances inherently toxic to either mammalian or fungal cells. A substituted indazole possessing the desired bioactivity profile was selected for further development, and initial investigation of structure–activity relationships led to the discovery of ML212.

Phytochemical and biological studies of bryophytes

The bryophytes contain the Marchantiophyta (liverworts), Bryophyta (mosses) and Anthocerotophyta (hornworts). Of these, the Marchantiophyta have a cellular oil body which produce a number of mono-, sesqui- and di-terpenoids, aromatic compounds like bibenzyl, bis-bibenzyls and acetogenins. Most sesqui- and di-terpenoids obtained from liverworts are enantiomers of those found in higher plants. Many of these compounds display a characteristic odor, and can have interesting biological activities. These include: allergenic contact dermatitis, antimicrobial, antifungal and antiviral, cytotoxic, insecticidal, insect antifeedant, superoxide anion radical release, 5-lipoxygenase, calmodulin, hyaluronidase, cyclooxygenase, DNA polymerase β, and α-glucosidase and NO production inhibitory, antioxidant, piscicidal, neurotrophic and muscle relaxing activities among others. Each liverwort biosynthesizes unique components, which are valuable for their chemotaxonomic classification. Typical chemical structures and biological activity of the selected liverwort constituents as well as the hemi- and total synthesis of some biologically active compounds are summarized.

Overcoming fluconazole resistance in Candida albicans clinical isolates with tetracyclic indoles

Continuing efforts to discover novel means of combating fluconazole resistance in Candida albicans have identified an indole derivative that sensitizes strains demonstrating resistance to fluconazole. This tetracycle (3, ML229) does not appear to act through established Hsp90 or calcineurin pathways to chemosensitize C. albicans, as determined in Saccharomyces cerevisiae models, and may be a useful probe to uncover alternative resistance pathways.

Secondary Metabolites from Plants Inhibiting ABC Transporters and Reversing Resistance of Cancer Cells and Microbes to Cytotoxic and Antimicrobial Agents

Fungal, bacterial, and cancer cells can develop resistance against antifungal, antibacterial, or anticancer agents. Mechanisms of resistance are complex and often multifactorial. Mechanisms include: (1) Activation of ATP-binding cassette (ABC) transporters, such as P-gp, which pump out lipophilic compounds that have entered a cell, (2) Activation of cytochrome p450 oxidases which can oxidize lipophilic agents to make them more hydrophilic and accessible for conjugation reaction with glucuronic acid, sulfate, or amino acids, and (3) Activation of glutathione transferase, which can conjugate xenobiotics. This review summarizes the evidence that secondary metabolites (SM) of plants, such as alkaloids, phenolics, and terpenoids can interfere with ABC transporters in cancer cells, parasites, bacteria, and fungi. Among the active natural products several lipophilic terpenoids [monoterpenes, diterpenes, triterpenes (including saponins), steroids (including cardiac glycosides), and tetraterpenes] but also some alkaloids (isoquinoline, protoberberine, quinoline, indole, monoterpene indole, and steroidal alkaloids) function probably as competitive inhibitors of P-gp, multiple resistance-associated protein 1, and Breast cancer resistance protein in cancer cells, or efflux pumps in bacteria (NorA) and fungi. More polar phenolics (phenolic acids, flavonoids, catechins, chalcones, xanthones, stilbenes, anthocyanins, tannins, anthraquinones, and naphthoquinones) directly inhibit proteins forming several hydrogen and ionic bonds and thus disturbing the 3D structure of the transporters. The natural products may be interesting in medicine or agriculture as they can enhance the activity of active chemotherapeutics or pesticides or even reverse multidrug resistance, at least partially, of adapted and resistant cells. If these SM are applied in combination with a cytotoxic or antimicrobial agent, they may reverse resistance in a synergistic fashion.

Bisbibenzyls, a new type of antifungal agent, inhibit morphogenesis switch and biofilm formation through upregulation of DPP3 in Candida albicans

The yeast-to-hypha transition plays a crucial role in the pathogenesis of C. albicans. Farnesol, a quorum sensing molecule (QSM) secreted by the fungal itself, could prevent the formation of hyphae and subsequently lead to the defect of biofilm formation. The DPP3, encoding phosphatase, is a key gene in regulating farnesol synthesis. In this study, we screened 24 bisbibenzyls and 2 bibenzyls that were isolated from bryophytes or chemically synthesized by using CLSI method for antifungal effect. Seven bisbibenzyls were found to have antifungal effects with IC₈₀ less than 32 µg/ml, and among them, plagiochin F, isoriccardin C and BS-34 were found to inhibit the hyphae and biofilm formation of C. albicans in a dose-dependent manner. To uncover the underlying relationship between morphogenesis switch and QSM formation, we measured the farnesol production by HPLC-MS and quantified Dpp3 expression by detecting the fluorescent intensity of green fluorescent protein tagged strain using Confocal Laser Scanning microscopy and Multifunction Microplate Reader. The DPP3 transcripts were determined by real-time PCR. The data indicated that the bisbibenzyls exerted antifungal effects through stimulating the synthesis of farnesol via upregulation of Dpp3, suggesting a potential antifungal application of bisbibenzyls. In addition, our assay provides a novel, visual and convenient method to measure active compounds against morphogenesis switch.

Piperazinyl quinolines as chemosensitizers to increase fluconazole susceptibility of Candida albicans clinical isolates

The effectiveness of the potent antifungal drug fluconazole is being compromised by the rise of drug-resistant fungal pathogens. While inhibition of Hsp90 or calcineurin can reverse drug resistance in Candida, such inhibitors also impair the homologous human host protein and fungal-selective chemosensitizers remain rare. The MLPCN library was screened to identify compounds that selectively reverse fluconazole resistance in a Candida albicans clinical isolate, while having no antifungal activity when administered as a single agent. A piperazinyl quinoline was identified as a new small-molecule probe (ML189) satisfying these criteria.

Curcumin acts synergistically with fluconazole to sensitize a clinical isolate of Candida albicans showing a MDR phenotype

The primary objective of this work was to evaluate the capability of curcumin, a natural compound found in the Curcuma longa plant, to sensitize a clinical isolate of Candida albicans, which was found to have a high resistance to fluconazole. In addition, we assessed whether the resistance of this isolate was the result of the existence of efflux pumps, which could confer a multiple drug resistance phenotype. To evaluate azole resistance, we used the Clinical Laboratory Standard Institute (CLSI) MIC assays procedures with minor modifications. For evaluation of synergistic interaction of curcumin and fluconazole, checkerboard experiments were employed. Nile red and Rhodamine 6G accumulation assays were used to evaluate efflux pump activity. Curcumin was found to have a great capability to inhibit fluconazole resistance of the isolate of C. albicans. It was capable of restoring its sensitivity to this azole when used at 11 μM. Analysis with different azoles and the two indicated dyes showed that an efflux pump could be acting and contributing to the resistance of this isolate to fluconazole. The results suggest that a major facilitator superfamily (MFS) transporter might be involved in this process.

Dihydroptychantol A, a macrocyclic bisbibenzyl derivative, induces autophagy and following apoptosis associated with p53 pathway in human osteosarcoma U2OS cells

Dihydroptychantol A (DHA), a novel macrocyclic bisbibenzyl compound extracted from liverwort Asterella angusta, has antifungal and multi-drug resistance reversal properties. Here, the chemically synthesized DHA was employed to test its anti-cancer activities in human osteosarcoma U2OS cells. Our results demonstrated that DHA induced autophagy followed by apoptotic cell death accompanied with G₂/M-phase cell cycle arrest in U2OS cells. DHA-induced autophagy was morphologically characterized by the formation of double membrane-bound autophagic vacuoles recognizable at the ultrastructural level. DHA also increased the levels of LC3-II, a marker of autophagy. Surprisingly, DHA-mediated apoptotic cell death was potentiated by the autophagy inhibitor 3-methyladenine, suggesting that autophagy may play a protective role that impedes the eventual cell death. Furthermore, p53 was shown to be involved in DHA-mediated autophagy and apoptosis. In this connection, DHA increased nuclear expression of p53, induced p53 phosphorylation, and upregulated p53 target gene p21(Waf1/Cip1). In contrast, cytoplasmic p53 was reduced by DHA, which contributed to the stimulation of autophagy. In relation to the cell cycle, DHA decreased the expression of cyclin B₁, a cyclin required for progression through the G₂/M phase. Taken together, DHA induces G₂/M-phase cell cycle arrest and apoptosis in U2OS cells. DHA-induced apoptosis was preceded by the induction of protective autophagy. DHA-mediated autophagy and apoptosis are associated with the cytoplasmic and nuclear functions of p53.

Synergistic mechanisms of retigeric acid B and azoles against Candida albicans

AIMS

To clarify the underlying synergistic antifungal mechanisms of retigeric acid B (RAB) in combination with azoles against Candida albicans.

METHODS AND RESULTS

Increased accumulation of rhodamine 123 in C. albicans was measured by both spectrophotometric method and flow cytometry. The inhibitory properties to the drug efflux of C. albicans were determined spectrophotometrically. The decreased cellular ergosterol synthesis was measured using its unique spectrophotometric absorbance profile, and the downregulation expression levels of CDR1 and ERG11 were detected by real-time reverse transcription polymerase chain reaction. Transmission electron microscopy investigation found the wrinkled cell membrane and the impaired cell wall.

CONCLUSIONS

RAB synergizes the antifungal effect of azoles against C. albicans by inhibiting efflux pump activity, targeting the ergosterol biosynthesis pathway and increasing the fluidity for the resulted ergosterol depletion.

SIGNIFICANCE AND IMPACT OF THE STUDY

Investigating the mechanism of the synergy between RAB and azoles against C. albicans will help us to uncover the antifungal roles of this lichen-derived triterpene acid and find its possible clinical applications in overcoming fungal resistance.

The inhibitory effect of a macrocyclic bisbibenzyl riccardin D on the biofilms of Candida albicans

Biofilm formation plays a key role in the life cycles and subsistence of many microorganisms. The human fungal pathogen Candida albicans has a high propensity to develop biofilms and resulted resistant to traditional antifungal agents. Biofilms are composed of a mixture of cell types, including yeast, pseudohyphal and hyphal cells, and hyphae are a prominent feature of biofilms. Riccardin D is a macrocyclic bisbibenzyl isolated from the liverwort Dumortiera hirsute in our laboratory. In the present investigation, the XTT [2,3-bis(2-methoxy-4-nitro-5-sulfo-phenyl)-2H-tetrazolium-5-carboxanilide] reduction assay and live/dead cell staining were employed for evaluating the effects of riccardin D on C. albicans biofilms. The results demonstrated that riccardin D can interfere with the biofilm formation. To investigate whether this effect was due to the inhibition of hyphae formation, morphological observation and real-time reverse transcriptase polymerase chain reaction (RT-PCR) were employed for evaluating the effects of riccardin D on the hyphae formation and the expression of hyphae specific genes. The results showed that the hyphae formation was strongly inhibited and the mRNA expression levels of hyphae specific genes were downregulated after riccardin D treatment. We concluded that riccardin D interfered with the biofilm formation of C. albicans through downregulating the expression of hyphae specific genes and inhibiting the formation of hyphae.

Ring strain and total syntheses of modified macrocycles of the isoplagiochin type

Macrocycles of the bisbibenzyl-type are natural products that are found exclusively in bryophytes (liverworts). The molecular framework of the subtype "isoplagiochin" is of substantial structural interest because of the chirality of the entire molecule, which arises from two biaryl axes in combination with two helical two-carbon units in a cyclic arrangement. From a structural as well as a synthetic point of view we report on the total synthesis of compounds which possess more rigid two-carbon biaryl bridges like stilbene (E or Z) or even tolane moieties which were introduced starting with a Sonogashira protocol. The McMurry method proved to be a powerful tool for the cyclization to these considerably ring-strained macrocycles.

Secondary metabolites in bryophytes: an ecological aspect

Bryophytes frequently grow in an unfavorable environment as the earliest land plants, and inevitably biosynthesize secondary metabolites against biotic or abiotic stress. They not only defend against the plant competition, microbial attack, and insect or animal predation, but also function in UV protection, drought tolerance, and freezing survival. This review covers the ecological aspect of secondary metabolites in bryophytes and is taxonomically presented according to the ecological significances.

The effect of plagiochin E alone and in combination with fluconazole on the ergosterol biosynthesis of Candida albicans

Plagiochin E (PLE), a macrocyclic bis(bibenzyl) isolated from the liverwort Marchantia polymorpha, has been reported to have antifungal activity and resistance reversal effects on Candida albicans. In order to understand the underlying mechanisms, we studied the effects of PLE alone and in combination with fluconazole (FLC) on the ergosterol biosynthetic pathway against both FLC-sensitive and FLC-resistant strains by analyzing the sterol content and the ergosterol pathway gene (ERG) expression. Relative quantitative analysis of different ergosterol precursors was carried out by employing the hyphenated technique of gas chromatography-high resolution mass spectrometry (GC-HR-MS). We observed that for FLC-resistant strain PLE itself can cause the accumulation of lanosterol and the decrease of 14alpha-methylfecosterol. When it combined with FLC, a significant decrease was observed in ergosterol formation and corresponding accumulation of 14alpha-methylated sterols was also found. Real-time reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed that the transcription level of ERG11 was decreased in FLC-resistant strain when exposed to PLE alone or PLE plus FLC. These results suggest that PLE potentiates FLC antifungal activity by interfering with the FLC-targeted ergosterol biosynthesis pathway.