Antiprotozoal and Antibacterial Activity of Ravenelin, a Xanthone Isolated from the Endophytic Fungus Exserohilum rostratum.

Jeferson Rodrigo Souza Pina,Anna Caroline Campos Aguiar,Luciano Almeida Watanabe,Kátia Da Silva Calabrese,Andrey Moacir Do Rosario Marinho,Josiwander Miranda Carvalho,Guilherme Eduardo De Souza,Fernando Almeida-Souza,Heriberto Rodrigues Bitencourt,Rafael Victorio Carvalho Guido,João Victor Silva-Silva,Juan Matheus Pereira Fernandes,Patrícia Santana Barbosa Marinho

doi:10.3390/molecules26113339

Abstract

The natural compound ravenelin was isolated from the biomass extracts of Exserohilum rostratum fungus, and its antimicrobial, antiplasmodial, and trypanocidal activities were evaluated. Ravenelin was isolated by column chromatography and HPLC and identified by NMR and MS. The susceptibility of Gram-positive and Gram-negative bacteria strains to ravenelin was determined by microbroth dilution assay. Cytotoxicity was evaluated in hepatocarcinoma cells (HepG2) and BALB/c peritoneal macrophages by using MTT. SYBR Green I-based assay was used in the asexual stages of Plasmodium falciparum. Trypanocidal activity was tested against the epimastigote and intracellular amastigote forms of Trypanosoma cruzi. Ravenelin was active against Gram-positive bacteria strains, with emphasis on Bacillus subtilis (MIC value of 7.5 µM). Ravenelin’s antiparasitic activities were assessed against both the epimastigote (IC50 value of 5 ± 1 µM) and the intracellular amastigote forms of T. cruzi (IC50 value of 9 ± 2 µM), as well as against P. falciparum (IC50 value of 3.4 ± 0.4 µM). Ravenelin showed low cytotoxic effects on both HepG2 (CC50 > 50 µM) and peritoneal macrophage (CC50 = 185 ± 1 µM) cells with attractive selectivity for the parasites (SI values > 15). These findings indicate that ravenelin is a natural compound with both antibacterial and antiparasitic activities, and considerable selectivity indexes. Therefore, ravenelin is an attractive candidate for hit-to-lead development.

Highlights

Secondary metabolites produced by microorganisms, including fungi and bacteria, have shown useful applications in different areas of human life development [1]
Isocoumarins analogs have been obtained from cultures of E. rostratum isolated as endophytic fungi of Stemona sp., and had their biological activity assessed against a resistant strain of P. falciparum (K1, multidrug-resistant strain)
The compound ravenelin 1 was isolated from the fraction A3 of the ethyl acetate extract from E. rostratum by a preparative high-performance liquid chromatography with a photodiode array detector (HPLC-PAD) (Figure 1)

Summary

Introduction

Secondary metabolites produced by microorganisms, including fungi and bacteria, have shown useful applications in different areas of human life development [1]. The natural products of endophytic fungi show a variety of both biological activities and chemical classes, including alkaloids, steroids, terpenoids, flavonoids, glycosides, xanthones, isocoumarins, quinones, phenylpropanoids, lignans, aliphatic metabolites, and lactones, among others [5]. The increasing number of resistant pathogens (bacteria and fungi) to the antimicrobials available [6] and the loss of effectiveness to antiprotozoal treatment [7] have motivated the investigation of bioactive compounds from natural sources. In this sense, secondary metabolites isolated from endophytic fungi may play a role against microbial and parasite resistance [8]. The most potent compound of the series showed inhibitory activity in the submicromolar range (IC50 of 0.68 μM) [9]

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