The synthesis and activity evaluation of N‐acylated analogs of echinocandin B with improved solubility and lower toxicity

Echinocandins Localized to the Target-Harboring Cell Surface Are Not Degraded but Those Entering the Vacuole Are

Echinocandin antifungal drugs have a broad spectrum of activities and excellent safety profiles. These agents noncompetitively inhibit the formation of the major polysaccharide component of the fungal cell wall, a reaction catalyzed by the membrane-bound β-glucan synthase (GS) protein complex. We have developed fluorescent probes of three echinocandin drugs: caspofungin (CSF), anidulafungin (ANF), and rezafungin (RZF). Fluorescent echinocandins had the same spectrum of activities as the parent echinocandins, supporting the fact that conjugation of the dye did not alter their mode of action. Of the three echinocandins, ANF has the most potent in vitro activity. Investigation of the subcellular distribution of the fluorescent echinocandins in live Candida yeast cells revealed that despite their high structural similarity, each of the drug probes had a unique subcellular distribution pattern. Fluorescent CSF, which is the least potent of the three echinocandins, accumulated in Candida vacuoles; fluorescent ANF localized in the extracellular environment and on the yeast cell surface where the target GS resides; and fluorescent RZF was partitioned between the surface and the vacuole over time. Recovery of fluorescent CSF from Candida cells revealed substantial degradation over time; functional vacuoles were necessary for this degradation. Under the same conditions, fluorescent ANF was not degraded. This study supports the "target-oriented drug subcellular localization" principle. In the case of echinocandins, localization to the cell surface can contribute to improved potency and accumulation in vacuoles induces degradation leading to drug deactivation.

Echinocandins - structure, mechanism of action and use in antifungal therapy

With increasing number of immunocompromised patients as well as drug resistance in fungi, the risk of fatal fungal infections in humans increases as well. The action of echinocandins is based on the inhibition of β-(1,3)-d-glucan synthesis that builds the fungal cell wall. Caspofungin, micafungin, anidulafungin and rezafungin are semi-synthetic cyclic lipopeptides. Their specific chemical structure possess a potential to obtain novel derivatives with better pharmacological properties resulting in more effective treatment, especially in infections caused by Candida and Aspergillus species. In this review we summarise information about echinocandins with closer look on their chemical structure, mechanism of action, drug resistance and usage in clinical practice. We also introduce actual trends in modification of this antifungals as well as new methods of their administration, and additional use in viral and bacterial infections.

Natural products targeting the synthesis of β(1,3)-D-glucan and chitin of the fungal cell wall. Existing drugs and recent findings

BACKGROUND

During the last three decades systemic fungal infections associated to immunosuppressive therapies have become a serious healthcare problem. Clinical development of new antifungals is an urgent requirement. Since fungal but not mammalian cells are encased in a carbohydrate-containing cell wall, which is required for the growth and viability of fungi, the inhibition of cell wall synthesizing machinery, such as β(1,3)-D-glucan synthases (GS) and chitin synthases (CS) that catalyze the synthesis of β(1-3)-D-glucan and chitin, respectively, represent an ideal mode of action of antifungal agents. Although the echinocandins anidulafungin, caspofungin and micafungin are clinically well-established GS inhibitors for the treatment of invasive fungal infections, much effort must still be made to identify inhibitors of other enzymes and processes involved in the synthesis of the fungal cell wall.

PURPOSE

Since natural products (NPs) have been the source of several antifungals in clinical use and also have provided important scaffolds for the development of semisynthetic analogues, this review was devoted to investigate the advances made to date in the discovery of NPs from plants that showed capacity of inhibiting cell wall synthesis targets. The chemical characterization, specific target, discovery process, along with the stage of development are provided here.

METHODS

An extensive systematic search for NPs against the cell wall was performed considering all the articles published until the end of 2020 through the following scientific databases: NCBI PubMed, Scopus and Google Scholar and using the combination of the terms "natural antifungals" and "plant extracts" with "fungal cell wall".

RESULTS

The first part of this review introduces the state of the art of the structure and biosynthesis of the fungal cell wall and considers exclusively those naturally produced GS antifungals that have given rise to both existing semisynthetic approved drugs and those derivatives currently in clinical trials. According to their chemical structure, natural GS inhibitors can be classified as 1) cyclic lipopeptides, 2) glycolipids and 3) acidic terpenoids. We also included nikkomycins and polyoxins, NPs that inhibit the CS, which have traditionally been considered good candidates for antifungal drug development but have finally been discarded after enduring unsuccessful clinical trials. Finally, the review focuses in the most recent findings about the growing field of plant-derived molecules and extracts that exhibit activity against the fungal cell wall. Thus, this search yielded sixteen articles, nine of which deal with pure compounds and seven with plant extracts or fractions with proven activity against the fungal cell wall. Regarding the mechanism of action, seven (44%) produced GS inhibition while five (31%) inhibited CS. Some of them (56%) interfered with other components of the cell wall. Most of the analyzed articles refer to tests carried out in vitro and therefore are in early stages of development.

CONCLUSION

This report delivers an overview about both existing natural antifungals targeting GS and CS activities and their mechanisms of action. It also presents recent discoveries on natural products that may be used as starting points for the development of potential selective and non-toxic antifungal drugs.