Confronting antifungal resistance, tolerance, and persistence: Advances in drug target discovery and delivery systems

Synthesis and antifungal evaluation of novel triazole derivatives bearing a pyrazole-methoxyl moiety

Life-threatening invasive fungal infections pose a serious threat to human health. A series of novel triazole derivatives bearing a pyrazole-methoxyl moiety were designed and synthesized in an effort to obtain antifungals with potent, broad-spectrum activity that are less susceptible to resistance. Most of these compounds exhibited moderate to excellent in vitro antifungal activities against Candida albicans SC5314 and 10,231, Cryptococcus neoformans 32,609, Candida glabrata 537 and Candida parapsilosis 22,019 with minimum inhibitory concentration (MIC) values of ≤0.125 μg/mL to 0.5 μg/mL. Use of recombinant Saccharomyces cerevisiae strains showed compounds 7 and 10 overcame the overexpression and resistant-related mutations in ERG11 of S. cerevisae and several pathogenic Candida spp. Despite being substrates of the C. albicans and Candida auris Cdr1 drug efflux pumps, compounds 7 and 10 showed moderate potency against five fluconazole (FCZ)-resistant fungi with MIC values from 2.0 μg/mL to 16.0 μg/mL. Growth kinetics confirmed compounds 7 and 10 had much stronger fungistatic activity than FCZ. For C. albicans, compounds 7 and 10 inhibited the yeast-to-hyphae transition, biofilm formation and destroyed mature biofilm more effectively than FCZ. Preliminary mechanism of action studies showed compounds 7 and 10 blocked the ergosterol biosynthesis pathway at Erg11, ultimately leading to cell membrane disruption. Further investigation of these novel triazole derivatives is also warranted by their predicted ADMET properties and low cytotoxicity.

Discovery of Novel α,β-Unsaturated Amide Derivatives as Candidate Antifungals to Overcome Fungal Resistance

In our previous study, coumarin-containing CYP51 inhibitor A32 demonstrated potent antiresistance activity. However, compound A32 demonstrated unsatisfied metabolic stability, necessitating modifications to overcome these limitations. In this study, α,β-unsaturated amides were used to replace the unstable coumarin ring, which increased metabolic stability by four times while maintaining antifungal activity, including activity against resistant strains. Subsequently, the sterol composition analysis and morphological observation experiments indicated that the target of these novel compounds is lanosterol 14α-demethylase (CYP51). Meanwhile, biofilm growth was inhibited and resistance genes (ERG11, CDR1, CDR2, and MDR1) expression was downregulated to find out how the antiresistance works. Importantly, compound C07 demonstrated the capacity to stimulate reactive oxygen species, thus displaying potent fungicidal activity. Moreover, C07 exhibited encouraging effectiveness in vivo following intraperitoneal administration. Additionally, the most potent compound C07 showed satisfactory pharmacokinetic properties and low toxicity. These α,β-unsaturated amide derivatives, particularly C07, are potential candidates for treating azole-resistant candidiasis.

Exploring emerging drug responses in Cryptococcus

Cryptococcosis imposes a considerable burden on public health, and emerging drug responses to anticryptococcal drugs remain to be addressed. In this forum article we discuss the emerging drug responses of Cryptococcus, focusing on the critical nature of understanding such responses in order to improve the effectiveness of anticryptococcal therapeutics.

Design of Proteolytic-Resistant Antifungal Peptides by Utilizing Minimum d-Amino Acid Ratios

Antifungal peptides are an appealing alternative to standard antifungal medicines due to their unique mechanism of action and low-level resistance. However, their susceptibility to protease degradation keeps hindering their future development. Herein, a library was established to design peptides with protease resistance and high antifungal activity. The peptides were incorporated with minimal D-amino acids to further improve the protease stability. The most active peptide, IR3, demonstrated good antifungal activity and low toxicity, and its molecular integrity was maintained after protease hydrolysis for 8 h at 2 mg/mL. Furthermore, IR3 could permeate the fungal cell wall, disrupt the cell membrane, produce reactive oxygen species, and induce apoptosis in fungal cells. In vivo experiments confirmed that IR3 could effectively treat fungal keratitis. Collectively, these findings suggest that IR3 is a promising antifungal agent and may be beneficial in the design and development of protease-resistant antifungal peptides.

Miconazole induces aneuploidy-mediated tolerance in Candida albicans that is dependent on Hsp90 and calcineurin

Antifungal resistance and antifungal tolerance are two distinct terms that describe different cellular responses to drugs. Antifungal resistance describes the ability of a fungus to grow above the minimal inhibitory concentration (MIC) of a drug. Antifungal tolerance describes the ability of drug susceptible strains to grow slowly at inhibitory drug concentrations. Recent studies indicate antifungal resistance and tolerance have distinct evolutionary trajectories. Superficial candidiasis bothers millions of people yearly. Miconazole has been used for topical treatment of yeast infections for over 40 years. Yet, fungal resistance to miconazole remains relatively low. Here we found different clinical isolates of Candida albicans had different profile of tolerance to miconazole, and the tolerance was modulated by physiological factors including temperature and medium composition. Exposure of non-tolerant strains with different genetic backgrounds to miconazole mainly induced development of tolerance, not resistance, and the tolerance was mainly due to whole chromosomal or segmental amplification of chromosome R. The efflux gene CDR1 was required for maintenance of tolerance in wild type strains but not required for gain of aneuploidy-mediated tolerance. Heat shock protein Hsp90 and calcineurin were essential for maintenance as well as gain of tolerance. Our study indicates development of aneuploidy-mediated tolerance, not resistance, is the predominant mechanism of rapid adaptation to miconazole in C. albicans, and the clinical relevance of tolerance deserves further investigations.

Pitavastatin Calcium Confers Fungicidal Properties to Fluconazole by Inhibiting Ubiquinone Biosynthesis and Generating Reactive Oxygen Species

Fluconazole (FLC) is extensively employed for the prophylaxis and treatment of invasive fungal infections (IFIs). However, the fungistatic nature of FLC renders pathogenic fungi capable of developing tolerance towards it. Consequently, converting FLC into a fungicidal agent using adjuvants assumes significance to circumvent FLC resistance and the perpetuation of fungal infections. This drug repurposing study has successfully identified pitavastatin calcium (PIT) as a promising adjuvant for enhancing the fungicidal activity of FLC from a comprehensive library of 2372 FDA-approved drugs. PIT could render FLC fungicidal even at concentrations as low as 1 μM. The median lethal dose (LD50) of PIT was determined to be 103.6 mg/kg. We have discovered that PIT achieves its synergistic effect by inhibiting the activity of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase, thereby impeding ubiquinone biosynthesis, inducing reactive oxygen species (ROS) generation, triggering apoptosis, and disrupting Golgi function. We employed a Candida albicans strain that demonstrated a notable tolerance to FLC to infect mice and found that PIT effectively augmented the antifungal efficacy of FLC against IFIs. This study is an illustrative example of how FDA-approved drugs can effectively eliminate fungal tolerance to FLC.

Regulation of Charge Transfer Pathway in Ag-ZnIn2S4 Nanowires for Visible Photodynamic Therapy on Candida Albicans Infections

Photodynamic therapy (PDT) is receiving extensive attention as an antimicrobial strategy that does not cause drug resistance by reactive oxygen species (ROS). Herein, hierarchical Ag-ZnIn2S4 (Ag-ZIS) core-shell nanowires are synthesized by in situ Metal-Organic Framework derived method for efficient PDT of Candida albicans (C. albicans). The core-shell structure enables spatial synergy strategy to regulate the charge transfer pathway under visible light excitation, in which the Ag nanowires are like the highway for the photogenerated electrons. The enhanced charge carrier separation efficiency greatly increased the chances for the generation of ROS. As expected, the optimized Ag-ZIS nanowires exhibit excellent performance for inactivation of C. albicans under visible light irradiation (λ ≥ 420 nm, 15 min), and the effective sterilization concentration is as high as 107CFU mL-1. Moreover, in vivo infection experiments suggested that the PDT effect of Ag-ZIS nanowires on the mouse wound healing is better than that of the clinical Ketoconazole drug. The PDT antifungal mechanism of Ag-ZIS nanowires is also investigated, and superoxide anion is found to be the predominant active species to causes C. albicans damage. This work provides a new perspective for designing novel interface structures to regulate charge transfer to achieve efficient PDT antifungal therapy.

AmBisome® Formulations for Pediatrics: Stability, Cytotoxicity, and Cost-Effectiveness Studies

Liposomal amphotericin B (Ambisome®) is the gold standard for the treatment and prevention of fungal infections both in the adult and pediatric populations. The lyophilized dosage form has to be reconstituted and diluted by hospital staff, but its management can be challenging due to the spontaneous tendency of amphotericin B to form aggregates with different biological activity. In this study, the colloidal stability of the liposomes and the chemical stability of amphotericin B were investigated over time at storage conditions. Three liposomal formulations of amphotericin B at 4.0 mg/mL, 2.0 mg/mL, and 0.2 mg/mL were prepared and assayed for changes regarding the dimensional distribution, zeta potential, drug aggregation state, and onset of by-products. Our analyses highlighted that the most diluted formulation, kept at room temperature, showed the greatest changes in the aggregation state of the drug and accordingly the highest cytotoxicity. These findings are clinically relevant since the lower dosages are addressed to the more vulnerable patients. Therefore, the centralization of the dilution of AmBisome® at the pharmacy is of fundamental importance for assuring patient safety, and at the same time for reducing medication waste, as we demonstrated using the cost-saving analysis of drug expense per therapy carried out at the G. Gaslini children hospital.

Brain glucose induces tolerance of Cryptococcus neoformans to amphotericin B during meningitis

Antibiotic tolerance is the ability of a susceptible population to survive high doses of cidal drugs and has been shown to compromise therapeutic outcomes in bacterial infections. In comparison, whether fungicide tolerance can be induced by host-derived factors during fungal diseases remains largely unknown. Here, through a systematic evaluation of metabolite-drug-fungal interactions in the leading fungal meningitis pathogen, Cryptococcus neoformans, we found that brain glucose induces fungal tolerance to amphotericin B (AmB) in mouse brain tissue and patient cerebrospinal fluid via the fungal glucose repression activator Mig1. Mig1-mediated tolerance limits treatment efficacy for cryptococcal meningitis in mice via inhibiting the synthesis of ergosterol, the target of AmB, and promoting the production of inositolphosphorylceramide, which competes with AmB for ergosterol. Furthermore, AmB combined with an inhibitor of fungal-specific inositolphosphorylceramide synthase, aureobasidin A, shows better efficacy against cryptococcal meningitis in mice than do clinically recommended therapies.

Contribution of N-Linked Mannosylation Pathway to Candida parapsilosis and Candida tropicalis Biofilm Formation

Background

Mycoses are a growing threat to human health, and systemic candidiasis caused by Candida parapsilosis and Candida tropicalis is frequent in immunocompromised patients. Biofilm formation is a virulence factor found in these organisms, as sessile cells adhere to surfaces, the stratification and production of extracellular matrix provides protection and resistance to antifungal drugs. Previous evidence indicated that the N-linked mannosylation pathway is relevant to C. albicans biofilms, but its contribution to other species remains unknown.

Methods

C. parapsilosis and C. tropicalis och1∆ mutants, which have a disrupted N-linked mannosylation pathway, were used to form biofilms. In addition, wild-type and mutant cells were also treated to remove N-linked mannans or block this pathway. Biofilms were analyzed by quantifying the included fungal biomass, and extracellular matrix components. Moreover, gene expression and secreted hydrolytic enzymes were also quantified in these biofilms.

Results

The och1∆ mutants showed a reduced ability to form biofilms in both fungal species when compared to the wild-type and control strains. This observation was confirmed by trimming N-linked mannans from walls or blocking the pathway with tunicamycin B. According to this observation, mutant, and treated cells showed an altered composition of the extracellular matrix and increased susceptibility to antifungal drugs when compared to control or untreated cells. The gene expression of secreted virulence factors, such as aspartyl proteinases and phospholipases, was normal in all the tested cells but the secreted activity was reduced, suggesting a defect in the secretory pathway, which was later confirmed by treating cells with brefeldin A.

Conclusion

Proper N-linked mannosylation is required for biofilm formation in both C. parapsilosis and C. tropicalis. Disruption of this posttranslational modification affected the secretory pathway, offering a link between glycosylation and biofilm formation.