Antibacterial agents inhibit fungi
With the wide application of antibiotics, immunosuppressants and other treatment methods, the infection rate and fatality rate of invasive fungi, such as Candida albicans, Cryptococcus neoformans and Aspergillus fumigatus, are constantly increasing, posing a serious threat to human health. Fungi are eukaryotes with similar biological characteristics to mammalian cells, so conventional antifungal therapy has limited effects and large side effects. Designing drugs for different targets of fungi and mammalian cells can more effectively inhibit fungal growth and reduce the toxic side effects on the body. Cell wall is a unique and essential structure of fungi. In recent years, with the in-depth study of the components of fungal cell wall, a series of drugs aimed at inhibiting the synthesis of key components of cell wall have been developed and applied.
Fungal cell wall structural characteristics
Fungal cell wall is a complex polysaccharide structure, which plays an important role in maintaining fungal morphology and resisting changes in external physical and chemical environment, such as osmotic pressure and pH fluctuations. The integrity and stability of the cell wall is essential for the survival of the fungus, and any damage can lead to cell membrane rupture and even cell lysis. The composition and structure of the cell wall, which varies with different stages of the fungal life cycle, is not only a key interface for the interaction between fungi and host cells, but also directly related to the pathogenicity and virulence of invasive fungi.
The fungal cell wall is mainly composed of β-glucan, chitin and mannoprotein, and the proportion of each component varies according to the fungal species. Beta-glucans are linked by a glucoside bond between glucose and consist mainly of beta-1, 3-glucan and a small amount of beta-1, 6-glucan. Chitin, also known as chitin, is a polysaccharide structure formed by the polymerization of N-acetylglucosamine monomer by the β-1, 4-glucoside bond, and has a large tensile strength. These polysaccharide components are synthesized by plasma membrane-associated synthetases and transported to the cell wall, while mannoglycoproteins are synthesized in the endoplasmic reticulum and transported to the cell surface after Golgi modification.
Fungal cell wall components. (ZHU, X., 2022)
The cell wall is usually divided into two inner and outer layers, each with a different composition. The inner layer is mainly a three-dimensional network structure formed by β-1, 3-glucan through hydrogen bonding, and its end is connected with β-1, 6-glucan and chitin, which plays a supporting role. The outer layer is rich in mannoglycoproteins linked to β-1, 6-glucan, forming a peptidoglycan network that enhances the mechanical strength of the cell wall. The inner and outer layers of the cell wall are stabilized by hydrogen bonding and side chain interactions.
The cell walls of certain fungi, such as Cryptococcus neoformans, also contain a unique alpha-1,3-1, 4-glucan structure, which is formed by alternating connections of alpha-1,3 and alpha-1,4 bonds, and is involved in building a hydrophobic rigid scaffold inside the cell wall and forming a hydrated soft matrix with β-glucan on the outer layer. The role of alpha-1, 3-glucan in the outer cell wall is to promote fungal adhesion to host cells and immune escape, and is essential for the pathogenicity and virulence of invasive fungi.
Antifungal agents targeting cell walls
At different morphogenetic stages of the fungal life cycle, the cell wall must be constantly rebuilt, including the breaking of old cross chains, the insertion of new synthetic chains of polysaccharides and proteins into the cell wall, and the formation of new cross chains. Targeted synthetases that interfere with key components of the cell wall can lead to the reduction of cell wall components, barrier destruction, and then induce abnormal cytoplasmic structure, cell membrane rupture, and cell death. Common antifungal agents targeting cell walls and their mechanisms of action are described below.
Beta-1, 3-glucan synthase inhibitors
β-1, 3-glucan is mainly located in the inner layer of fungal cell wall and plays an important role in the stability of fungal structure. β-1, 3-glucan is produced by β-1, 3-glucan synthetase catalyzed the transport of uridine diphosphate glucose, which leads to the decrease of β-1, 3-glucan content in the cell wall by targeting the key group of the synthetase, resulting in cell rupture and death.
Classification of β-1, 3-glucan synthase inhibitors The most studied non-competitive inhibitors of β-1, 3-glucan synthase are echinocandin and papulacandin derivatives. Echinocandin is a non-ribosomal cyclic hexapeptide, and the location and conformation of its fatty acid side chain are important for its antifungal activity, which may be related to the intercalation of the side chain in the phospholipid bilayer of the fungal plasma membrane. Echinocandin binds to the catalytic subunit of β-1, 3-glucan synthase in a non-competitive manner, reducing the integrity of fungal cell walls and leading to fungal death. Natural echinocandin is produced by fungi, but due to its low solubility and high toxicity, it cannot be directly used in the clinic. The echinocandins currently approved for clinical treatment are caspofungin, anidulafungin and micafungin.
The main novel glucan synthase inhibitors currently under development include Cidara Therapeutics' echinocandin CD101 and Scynexis' non-echinocandin SCY-078. CD101 obtained by structural modification of the choline part at the core of echinocandins improves its solubility and stability in plasma, aqueous solution and buffer solution, which greatly solves the problem of short half-life of traditional echinocandins. CD101 has similar antifungal activity to other echinocandins in vitro and can effectively inhibit candida.
SCY-078 has a triterpene structure different from echinocandin and is a semi-synthetic derivative of the natural product enfumafungin. Clinical studies have shown that SCY-078 has strong inhibitory activity against strains of Candida albicans and Candida glabrum. At present, two phase II clinical trials of SCY-078 have been completed. The results of the first study indicate that the clinical cure rate of patients with acute moderate to severe vulvovaginal candida infection who take SCY-078 orally during treatment is higher than that of the oral fluconazole group. Results from the second study showed a safe and well-tolerated oral daily dose of SCY-078 of 750 mg, with a therapeutic effect similar to standard treatment. No adverse events occurred during treatment in either study.
The antifungal activity of papulacandin was discovered in 1977. It was first isolated and purified by Ciba-Geigy, and its main active ingredient is papulacandin B. Similar to the mechanism of antifungal action of echinocandin, papulacandin also inhibits the production of β-1, 3-glucan by targeting β-1, 3-glucan synthase.
Resistance to beta-1, 3-glucan synthetase inhibitors clinical resistance to echinocandin usually occurs with repeated administration and occasionally in patients with brief use. One of the most common mechanisms by which fungi acquire resistance is through mutations in genes that alter the conformation of drug targets, thereby reducing drug binding and efficacy. In response to the inhibitory effect of echinocandin derivatives on synthase, fungi can induce drug resistance by altering the configuration of Fks subunits in β-1, 3-glucan synthase. Another mechanism of fungal resistance stems from the short half-life of existing echinocandins, which makes it impossible to maintain effective concentrations for long periods of time, thus giving the surviving fungi the opportunity to mutate.
Chitin synthase inhibitors
Chitin is connected to β-1,3-glucan through hydrogen bonds and has high toughness and plays an important role in maintaining the stability of fungal cell wall structures. By targeting UDP-N-acetylglucosamine synthase, chitin synthase inhibitors can effectively inhibit chitin synthesis, resulting in changes in the osmotic pressure of the fungal cytoplasm, and then rupture and death. Nicormycin Z is a natural nucleoside peptide antifungal antibiotic that competitively inhibits chitin synthase. Subsequent in vivo and in vitro experiments showed that nicormycin Z has a certain therapeutic effect on chronic abscesses caused by Blastomaria dermatidis, but has limited therapeutic effect on invasive infections caused by Cryptococcus neoformans. In addition to its limited therapeutic spectrum of fungi, the clinical use of nicormycin Z is also limited by its extremely short half-life.
Glycosylphosphatidylinositol (GPI) anchored protein inhibitors
GPI is an important molecule that anchors cell surface proteins in the plasma membrane of eukaryotic cells. In fungi, GPI acts as a bridge between the cell membrane and the cell wall by assisting in the cross-linking of β-1,3-glucans and glycoproteins. A large number of chemical modifications are required from the synthesis of GPI to the final formation of biologically functional GPI anchored proteins. The key rate-limiting enzymes are Gwt1 and Mcd4 enzymes. By inhibiting these two rate-limiting enzymes, the synthesis of GPI-anchored proteins can be effectively reduced, which in turn leads to cell wall structural disorder and inhibits the virulence of fungi.
The new drug APX001A can specifically inhibit the inositol acylation of GPI by Gwt1 enzyme, thereby blocking the synthesis of GPI-anchored protein. Drugs with similar mechanisms of action to APX001A include G884, G365 and Gepinacin. Mcd4 is an ethanolamine phosphotransferase, which can also disrupt the structure of fungal cell walls by inhibiting its activity. Currently, new drugs that inhibit GPI-anchored proteins by inhibiting Mcd4 enzyme include M743 and M720.
Antifungal agents at BOC Sciences
Other antibiotics inhibiting fungi
Amphotericin B and nystatin are polyene antibiotics, which belong to the macrolide class antibiotics. They mainly target ergosterol in fungal cell membranes and damage cell membrane permeability, resulting in leakage of important substances (such as potassium ions) within the cells., ultimately causing the death of fungal cells. Azole antibiotics (such as fluconazole and itraconazole) interfere with the biosynthesis of ergosterol by inhibiting the activity of 14-alpha-demethylase, a key enzyme involved in the synthesis of ergosterol, thereby destroying the permeability of fungal cell membranes.
Secondary metabolites produced by Pseudomonas are the main source of antibacterial metabolites and pesticides. Chen et al. isolated 923Pseudomonas mosselii strain from the rice rhizosphere. From the strain product, they purified and identified the natural antibiotic pyrazole triazine, which can specifically inhibit the plant bacterial pathogen Xanthonas and the fungal pathogen Magnaporthe oryzae.
Reference
- ZHU, X., et al. A review of the study on antifungal drugs acting on cell wall. Mycosystema. 2022, 41(6): 871-877.
