Phytochemical Investigation of Five Pentas Species for Leishmanicidal Principles

Abstract

Leishmaniasis is a disease complex instigated by a protozoan parasite of the genus Leishmania. Contemporary chemotherapies for leishmaniasis employ pentamidine (1), amphotericin B (2) and pentavalent antimonials (3). The efficacy of these drugs has deteriorated due to drug resistance. The drugs also pose unbearable side effects owing to their toxicity. Some metabolites from the genus Pentas (family Rubiaceae) have been reported to show antiprotozoal activity against Plasmodium species, but no studies on antileishmanial activity have been done. The study was focused on investigating five Pentas species for antileishmanial principles. The CH2Cl2/CH3OH (1:1) extracts of the roots and/or stems of Pentas bussei, P. longiflora, P. micrantha, P. parvifolia and P. zanzibarica were subjected to a combination of chromatographic separations resulting in the isolation of 14 compounds. The pure compounds were characterized by utilizing 1H NMR, 13C NMR, 1H-1H COSY, HMBC, HSQC and MS. The crude extract from the roots of Pentas parvifolia yielded busseihydroquinone B (51). The stem bark of Pentas parvifolia yielded β-stigmasterol (50) and β-amyrin (95). The aerial parts of P. parvifolia yielded vanillic acid (96), p-hydroxybenzoic acid (97) and protocatechuic acid (98). The aerial part of P. bussei yielded β-stigmasterol (50), a homoprenylated naphthoquinone (55), busseihydroquinone A (7), busseihydroquinone B (51), busseihydroquinone C (52) and methyl-8-hydroxy-1,4,6,7-tetramethoxy-2-naphthoate (47), which is a new compound. The aerial parts of P. micrantha yielded 2-methoxy-3-methyl-anthracene- 9,10-dione (72). The stem bark of P. zanzibarica yielded rubiadin-1-methyl ether (65) and rubiadin (64). The roots of Pentas longiflora yielded pentalongin (74). Pentalongin (74) showed antileishmanial activity (IC50 = 11 μM) against the antimony sensitive strain of Leishmania donavani (MHOM/IN/83/AG83). It also generated a substantial amount of nitric oxide in the cell culture (IC50 = 1.08 μM) relative to the positive control, miltefosine (4), (IC50 = 1.11 μM). Busseihydroquinone A (7) was oxidized with silver (I) oxide to yield 1-hydroxy-4,6-dimethoxy- 7,8-dioxo-7,8-dihydro-naphthalene-2-carboxylic acid methyl ester (99). Through computational modelling, the inhibitory potential of phytochemicals from the genus Pentas for Leishmania infantum trypanothione reductase was studied using UCSF Chimera 1.15. Among the studied compounds, schimperiquinone A (92) exhibited the highest affinity for the binding site of the receptor; with a binding energy of -10.9 kcal/mol. Anthraquinones generally showed superior inhibitory potency for Leishmania infantum trypanothione reductase than naphthoquinones. Overall, the phytochemicals from the genus Pentas showed sustained hydrogen bonds with Thr335, Lys60 and Cys52; these amino acid residues assist FAD to achieve a proper orientation towards the catalytic site of the enzyme. Therefore, the quinones from the genus Pentas have the potential to guide the development of antileishmanial drug agents. Given the distinctive binding mode of some of the anthraquinones and naphthoquinones observed here, the compounds should be subjected to in vitro and in vivo studies.