Therapeutic effects of water-soluble echinocandin compounds on Pneumocystis pneumonia in mice

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.

Dectin-1 is required for host defense against Pneumocystis carinii but not against Candida albicans

Dectin-1 is a C-type lectin involved in the recognition of beta-glucans found in the cell walls of fungi. We generated dectin-1-deficient mice to determine the importance of dectin-1 in the defense against pathogenic fungi. In vitro, beta-glucan-induced cytokine production from wild-type dendritic cells and macrophages was abolished in cells homozygous for dectin-1 deficiency ('dectin-1-knockout' cells). In vivo, dectin-1-knockout mice were more susceptible than wild-type mice to pneumocystis infection, even though their cytokine production was normal. However, pneumocystis-infected dectin-1-knockout macrophages did show defective production of reactive oxygen species. In contrast to those results, wild-type and dectin-1-knockout mice were equally susceptible to candida infection. Thus, dectin-1 is required for immune responses to some fungal infections, as protective immunity to pneumocystis, but not to candida, required dectin-1 for the production of antifungal reactive oxygen species.

From natural products to clinically useful antifungals

In our search for natural products with a broad spectrum of antifungal activity as lead compounds for novel treatments for mycoses, we have isolated echinocandin-type lipopeptide FR901379 and lipopeptidolactone FR901469, as novel water-soluble antifungal agents that inhibit the synthesis of 1,3-beta-glucan, a key component of the fungal cell wall. Since the cell wall is a feature unique to fungi and is not present in nonfungal eukaryotic cells, inhibitors of the synthesis of fungal cell wall components such as 1,3-beta-glucan have potential for selective toxicity to fungi and not to the host. In this short review, we describe efforts directed at synthetic modification of FR901469 and FR901379 with the ultimate goal of identifying new entities with suitable profiles as development candidate compounds. The main thrust of our work to date has been replacement of the highly flexible lipophilic side chains of the natural products with a view to reducing the hemolytic potential associated with these compounds, and to enhance chemical stability and/or in vivo antifungal efficacy. As a result of these efforts, we recently discovered a novel analog, FK463 (micafungin). Micafungin is currently in phase III clinical trials worldwide as a parenteral agent for various mycoses, and a new drug application (NDA) was recently filed in Japan.

FR131535, a novel water-soluble echinocandin-like lipopeptide: synthesis and biological properties

The synthesis and biological properties of a novel water-soluble echinocandin-like lipopeptide, FR131535, are described. This compound displayed potent in vitro and in vivo antifungal activities. The hemolytic activity of FR901379 was reduced by replacing the acyl side chain. This compound showed good water-solubility, comparable to the natural product FR901379.

Prophylactic effect of FK463, a novel antifungal lipopeptide, against Pneumocystis carinii infection in mice

The prophylactic effect of FK463, a new water-soluble echinocandin-like lipopeptide with inhibitory activity against 1, 3-beta-D-glucan synthase, against Pneumocystis carinii infection was investigated with the severe combined immunodeficient (SCID) mouse model. Treatment with FK463, pentamidine, and saline only was performed for 6 weeks from the day after the SCID mice were inoculated intranasally with infected lung homogenates. FK463 at 0.2 or 1.0 mg/kg of body weight, pentamidine at 4 mg/kg, or saline was subcutaneously administered daily into the backs of the SCID mice. The effects of the drugs were evaluated by detection of P. carinii cysts in mouse lung homogenates by toluidine blue O staining, lung histology, and PCR amplification of a P. carinii-specific DNA fragment from the lungs. P. carinii cysts were detected in the lungs of all mice administered saline. In contrast, no cysts were detected in mice administered both doses of FK463 and pentamidine. A specific DNA fragment was amplified from all mice administered saline and at least half or more of the mice administered FK463 and pentamidine. These results indicate that FK463 acts on cyst wall formation but not on trophozoite proliferation and is extremely effective in preventing P. carinii-associated pneumonia. These results suggest that FK463 is potentially useful as a prophylactic agent against P. carinii infection.

Pneumocystis carinii infection in non-AIDS patients

Infection with the opportunist fungus Pneumocystis carinii remains a significant cause of morbidity and mortality in non-HIV-infected individuals immunosuppressed by a wide range of malignancies, transplantation and inflammatory conditions. Glucocorticoid use appears to be an independent risk factor for the development of Pneumocystis carinii pneumonia. Transmission from infected to susceptible patients may occur, albeit infrequently. A diagnosis of Pneumocystis carinii pneumonia may be achieved in the majority of cases by DNA detection using polymerase chain reaction on oropharyngeal mouth washes.