Cytotoxic Carbazole Alkaloids from the Stems of Murraya koenigii

Murraya koenigii is a tropical to subtropical small tree belonging to the Rutaceae family. The plant is native to India and is distributed in most of Southern Asia. The leaves of this plant are highly aromatic, and young leaves are used as a vegetable herb in the Southern part of Thailand. Various parts of the plant have been reported as a folk medicine. The leaves, stems, and roots are used externally in skin eruptions and bites of venomous animals, while the barks and roots are used as a stimulant [1]. Previous phytochemical investigations led to the isolation of a number of carbazole alkaloids, some of which have been indicated for their antioxidant, antitumor, antimicrobial, antiinflammatory, antitrypanocidal, and mosquitocidal activities [2–5]. In the course of our ongoing search for bioactive compounds from Rutaceae medicinal plants, we found that the EtOAc extract of the stems of M. koenigii exhibited cytotoxicity against three human cancer cell lines, including oral cavity cancer (KB), breast cancer (MCF7), and small cell lung cancer (NCI-H187). In this paper, we describe the isolation and cytotoxic activity of the crude extract and pure carbazole alkaloids from the stems of M. koenigii. The EtOAc extract of M. koenigii stems showed cytotoxic activity against KB, MCF7, and NCI-H187 cancer cell lines, with IC50 values of 35.54, 17.56, and 14.86 g/mL, respectively. The activity-guided investigation of this extract resulted in the identification of nine earlier isolated carbazole alkaloids from M. koenigii, including murrayanine (1) [6], koenoline (2) [7], mahanimbinol (3) [8], girinimbine (4) [9], mahanimbine (5) [9], furostifoline (6) [10], murrayazoline (7) [9], murrayaquinone A (8) [9], and murrayazolidine (9) [11]. Compound 6 was isolated for the first time from M. koenigii. All structures were characterized by spectroscopic methods and comparison of their physical and spectral data with reported values. Murrayanine (1). Yellow solid, mp 158–161 C. 1H NMR (400 MHz, CDCl3, , ppm, J/Hz): 10.04 (1H, s, CHO), 8.69 (1H, br.s, NH), 8.18 (1H, br.s, H-2), 8.10 (1H, d, J = 8.0, H-5), 7.51 (1H, m, H-8), 7.49 (1H, m, H-7), 7.45 (1H, br.s, H-2), 7.31 (1H, m, H-6), 4.06 (3H, s, OCH3). 13C NMR (100 MHz, CDCl3, , ppm): 191.8 (CHO), 146.0 (C-1), 139.4 (C-8a), 134.0 (C-8b), 130.1 (C-3), 126.6 (C-7), 123.6 (C-4a and C-4b), 120.7 (C-5 and C-6), 120.4 (C-4), 111.5 (C-8), 103.5 (C-2), 55.8 (OCH3). Koenoline (2). Brown viscous oil. 1H NMR (400 MHz, CDCl3, , ppm, J/Hz): 8.28 (1H, br.s, NH), 8.03 (1H, d, J = 8.0, H-5), 7.65 (1H, br.s, H-4), 7.46 (1H, br.d, J = 8.0, H-8), 7.41 (1H, m, H-7), 7.22 (1H, m, H-6), 6.95 (1H, br.s, H-2), 4.80 (2H, s, H-1 ), 4.01 (3H, s, OCH3), 2.17 (1H, br.s, OH). 13C NMR (100 MHz, CDCl3, , ppm): 145.7 (C-1), 139.2 (C-8a), 132.7 (C-3), 129.1 (C-8b), 125.8 (C-7), 125.7 (C-4a), 124.0 (C-4b), 120.5 (C-5), 119.5 (C-6), 111.7 (C-4), 111.0 (C-8), 105.6 (C-2), 66.5 (C-1 ), 55.5 (OCH3). Mahanimbinol (3). Brown viscous oil. 1H NMR (400 MHz, CDCl3, , ppm, J/Hz): 7.93 (1H, br.d, J = 7.6, H-5), 7.85 (1H, br.s, NH), 7.68 (1H, s, H-4), 7.36 (1H, br.d, J = 8.0, H-8), 7.30 (1H, br.t, J = 7.6, 7.2, H-6), 7.17 (1H, br.t, J = 7.2, H-7), 5.37 (1H, t, J = 6.8, H-2 ), 5.21 (1H, br.s, OH), 5.07 (1H, m, H-6 ), 3.62 (2H, d, J = 6.8, H-1 ), 2.40 (3H, s, CH3-3), 2.12 (4H, m, H-4 and -5 ), 1.90 (3H, s, CH3-9 ), 1.66 (3H, s, CH3-8 ), 1.59 (3H, s, CH3-10 ). 13C NMR (100 MHz, CDCl3, , ppm): 151.2 (C-2), 139.4 (C-8a), 139.0 (C-3 ), 138.4 (C-8b), 132.1 (C-7 ), 124.2 (C-6), 123.6 (C-4b and C-6 ), 121.3 (C-2 ), 119.3 (C-4, C-5 and C-7), 117.0 (C-3), 116.6 (C-4a), 110.3 (C-8), 107.8 (C-1), 39.6 (C-4 ), 26.4 (C-5 ), 25.6 (CH3-8 ), 24.7 (C-1 ), 17.7 (CH3-10 ), 16.5 (CH3-3), 16.4 (CH3-9 ).