Tropidia Curculioides: Secondary metabolites and derivatives with antimycobacterial and leishmanicidal activity

Abstract

Background: Arunachal Pradesh the north-eastern state of India is a natural habitat of 550 species of orchids including 37 species of medicinal importance. Although these plants are regularly used by ethnic population, very few scientific investigations have been conducted to establish their pharmacological potential. In our previous study, we evaluated antimycobacterial and leishmanicidal activity of extracts prepared from three relatively unexplored medicinal orchids. In the present study, we conducted screening of the compounds isolated from Tropidia curculioides (Tc) for the above-mentioned activities. Furthermore, we prepared synthetic analogs of the most active compound. Objective: Evaluation of antimycobacterial and leishmanicidal activity of the isolated compounds and preparation of synthetic analogs of the most active compound followed by evaluation of biological activities. Materials and Methods: Antimycobacterial activity was evaluated by colorimetric redox indicator assay, and the leishmanicidal activity was assessed by 3-(4,5-dimethylthiazol-2-Yl)-2,5-diphenyltetrazolium bromide assay. The root, stem, and leaves of Tc were extracted with methanol:water (9:1) followed by fractionation with diethyl ether (Et2O) and n-butanol solvent. All these fractions were evaluated for biological activity to identify the most active fraction. Further, chromatography of the most active fraction (Tc root Et2O fraction) afforded three compounds, namely, 4-hydroxybenzaldehyde (1), 4,4'-dihydroxydiphenylmethane (2), and 3,5-dihydroxy-4-methoxybenzoic acid (3). Standard synthetic procedures were followed to prepare the analogs of the most active compound. Results: The screening result identified compound 2 with maximum antimycobacterial activity (minimum inhibitory concentration [MIC] – 125 μg/ml) and leishmanicidal activity (IC50 100 μg/ml). Three synthetic analogs were prepared targeting the methane linkage of compound 2. The N-benzylmethylamine derivative (6) showed the highest activity with MIC 15.62 μg/ml against Mycobacterium and IC50 31.25 μg/ml against Leishmania spp. Conclusion: The promising result of N-benzylmethylamine analog could be explored to obtain new antimycobacterial and leishmanicidal agent. Abbreviations used: MIC: Minimum inhibitory concentration; NMR: Nuclear magnetic resonance; CRI: Colorimetric redox indicator; OADC: Oleic acid, dextrose, and catalase; MDR: Multidrug Resistant; MTT: 3-(4,5-dimethylthiazol-2-Yl)-2, 5-diphenyltetrazolium bromide; Diethyl ether: Et2O.