Cytotoxic xanthonoids from the stem bark of garcinia hanburyi collected in Viet Nam

Vietnam Journal of Science and Technology 58 (2) (2020) 133-142 doi:10.15625/2525-2518/58/2/14367 CYTOTOXIC XANTHONOIDS FROM THE STEM BARK OF GARCINIA HANBURYI COLLECTED IN VIETNAM Nguyen Thi Kim An 1, 2, * , Ngo Dai Quang 3 , Pham Quoc Long 4 , Tran Thi Thu Thuy 4, * 1 Hanoi University of Industry, 289 Cau Dien Street, North Tu Liem district, Ha Noi, Viet Nam 2 Graduate University of Science and Technology - VAST, 18 Hoang Quoc Viet, Cau Giay, Ha Noi, Viet Nam 3 Vietnam National Chemical Group - 1A Trang Tien, Hoan Kiem, Ha Noi, Viet Nam 4 Institute of Natural Products Chemistry, VAST, 18 Hoang Quoc Viet, Cau Giay, Ha Noi, Viet Nam * Email: thuytran.inpc@gmail.com, kimansp@gmail.com Received: 26 August 2019; Accepted for publication: 9 January 2020 Abstract. Five prenylated caged xanthones: desoxymorellin (1), morellic acid (2), gambogic acid (3), isogambogic acid (4) and 10α-butoxy gambogic acid (5), were isolated from the stem bark of Garcinia hanburyi collected in Vietnam. Their structures were elucidated by detailed 1D, 2D NMR, MS spectroscopic analyses and compared with reported data. Two major constituents, gambogic acid and isogambogic acid, displayed strong cytotoxicity against LLC and SK-LU-1cell lines with IC50 values in the range from 0.35±0.05 µM to 2.26±0.19 µM. This is the first study on the cytotoxicity of gambogic acid and isogambogic acid against LLC and SK-LU-1 lung cancer cell lines. Keywords: prenylated caged xanthones, Garcinia hanburyi, gambogic acid, cytotoxicity, LLC and SK-LU-1. Classification numbers: 1.1.1, 1.1.6, 1.4.7. 1. INTRODUCTION Garcinia hanburyi Hook. f. (G. hanburyi), an evergreen tree belonging to the Guttiferae family, distributes in the rainforest of Vietnam, Thailand, Cambodia and the southern part of China. The resin of G. hanburyi, which is also called gamboge, has been used as pigment and in folk medicine as a potent purgative and for infected wounds [1]. Previous phytochemical studies revealed that prenylated caged xanthones which contain a unique 4-oxatricyclo[4.3.1.0 3,7 ]dec-8- en-2-one scaffold were found from the resin, fruits and other parts of the plant [2-10]. This special group of xanthones is considered as main bioactive group with potent biological activities such as anti-tumor [3-11], anti-HIV [1], anti-bacterial [2], anti- inflammatory [12] and neurotrophic activities [12]. Nguyen Thi Kim An, Ngo Dai Quang, Pham Quoc Long, Tran Thi Thu Thuy 134 The phytochemical of resin of G. hanburyi collected in Phu Quoc, Vietnam was investigated with the isolation of five caged xanthones, assigned as gambogic acid, isogambogic acid, isomorellic acid, isomorellin and isomoreollin B [13]. In this continuing investigation on the sterm bark of this species, we report herein the isolation, structural elucidation and cytotoxicity evaluation against lung cancer cell lines of five prenylated caged xanthones (1-5), namely desoxymorellin (1), morellic acid (2), gambogic acid (3), isogambogic acid (4) and 10α-butoxy gambogic acid (5) from the dichloromethane (DCM) extract of the stem bark of G. hanburyi collected in Phu Quoc island. 2. MATERIALS AND METHODS 2.1. General NMR spectra were recorded on a Bruker Avance 500 spectrometer at 500 and 125 MHz for 1 H and 13 C, respectively, at Institute of Chemistry - Vietnam Academy of Science and Technology (VAST). Chemical shifts are shown in δ (ppm) with tetramethylsilane (TMS) as an internal reference. ESI-MS spectra were performed on an Agilent 1100 LC/MS at Institute of Natural Products Chemistry – VAST. Melting points were measured on Buchi B545 apparatus (no correction). Column chromatographic separations were conducted by using silica gel 60 (Merck, 5-40 μm), silica gel 60 (Merck, 40-63 μm), silica gel 100 (Merck, 63-200 μm), and C18- reversed-phase silica gel (RP-18, Merck, 15-25 μm). Visualization of TLC plates was detected under UV light (at 254 and 365 nm) and staining with vanilin-H2SO4 10% solution. Ellipticine and dimethyl sulfoxide (DMSO) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Fetal bovine serum (FBS), DMEM medium, trichloroacetic acid (TCA) and acetic acid were obtained from GIBCO-Invitrogen (Carlsbad, CA, USA). Lung cancer cell lines were provided by Professor J. M. Pezzuto, Long-Island University, US and Professor Jeanette Maier, University of Milan, Italy. 2.2. Plant materials The stem bark of G. hanburyi was collected in Phu Quoc island - Kien Giang province, in December 2015. The plant materials were identified by Dr. Nguyen Quoc Binh, Vietnam National Museum of Nature. The voucher specimen (GH2015129) is deposited at Institute of Natural Products Chemistry - Vietnam Academy of Science and Technology. 2.3. Extraction and isolation The stem bark of G. hanburyi (2.5 kg) was cut into small pieces and then were dried in the oven at 45 o C in three days to achieve 2.1 kg dried stem bark. The dried stem bark was grounded into powder and extracted with methanol (MeOH) (3 L x 3) at room temperature using conventional ultrasound-assisted technique. The solvent was evaporated under reduced pressure to give a dark brown residue (325.0 g). The residue was further extracted with DCM (500 mL × 3) to yield DCM extract (71.9 g). The left residue was then extracted with ethyl acetate (EtOAc) (500 mL × 3) to afford EtOAc extract (122.4 g). The crude DCM extract was subjected to column chromatography (CC) over silica gel, eluted successively with a gradient of n-hexane-EtOAc (v/v, 100:0 to 3:1), DCM-EtOAc (v/v, 15:1 to 3:1) and DCM-MeOH (v/v, 9:1 to 1:2) to afford ten fractions (Frs. GHT1-GHT10). Cytotoxic xanthonoids from the stem bark of Garcinia hanburyi collected in Viet Nam 135 Fraction GHT1 (3.4 g) was separated by CC using eluent of 2% acetone in n-hexane to give five subfractions (GHT1.1-GHT1.5). Further chromatography of subfraction GHT1.4 (0.5 g) over silica gel using n-hexane-acetone (v/v, 50:1) as the mobile phase to yield compound 1 (30 mg). Fraction GHT7 (7.5 g) was fractionated by CC with a gradient of n-hexane-acetone (v/v, 50:1 to 0:100) to give eight subfractions, GHT7.1-GHT7.8. Subfraction GHT7.3 (1.85 g) was separated by chromatography over RP-18 silica gel eluting with MeOH-H2O (v/v, 6:1) to give nine subfractions (Frs. GHT7.3.1-GHT7.3.9). Subfraction GHT7.3.6 (0.25 g) was further purified by CC (n-hexane-acetone, v/v, 50:1) to obtain compound 2 (10 mg). Fraction GHT4 (11.9 g) was chromatographed over silica gel, eluting with a gradient of hexane-EtOAc-CH3COOH (v/v/v, 50:1:0.01 to 10:1:0.01) to produce four subfractions (GHT4.1-GHT4.4). Subfraction GHT4.1 (1.8 g) was separated by CC using eluent of 10 % acetone in n-hexane follow by purifying on RP-18 silica gel (MeOH-H2O, v/v, 5:1), to give compound 3 (820 mg). Fraction GHT8 (9.5 g) was loaded to a silica gel CC with a gradient of n-hexane-acetone (v/v, 20:1 to 0:100) to yield five subfractions (Frs. GHT8.1-GHT8.5). Compound 4 (470 mg), was derived from subfraction GHT8.4 (3.84 g) by repeated purification on C18-reversed-phase silica gel chromatography with eluent of MeOH-H2O (v/v, 5:1). Fraction GHT6 (7.4 g) was fractionated by employing CC over silica gel eluting with a gradient of n-hexane-EtOAc (v/v, 60:1 to 0:100) to yield five subfractions (Frs. GHT6.1-GHT6.5). Subfraction GHT6.3 (0.9 g) was further isolated by CC over RP-18 silica gel using MeOH-H2O (v/v, 6:1) as the mobile phase. Compound 5 (0.03 g) was achieved from subfractions GHT6.3.4 by repeating separation on RP-18 silica gel eluting with MeOH-H2O (v/v, 5:1). Desoxymorellin (1): yellow oil; 1 H and 13 C NMR (CDCl3), see Tables 1 and 2. ESI-MS m/z 531.25 [M+H] + (calcd for C33H39O6, 531.27). Morellic acid (2): bright yellow solid, m.p. 90-92 o C; 1 H and 13 C NMR (CDCl3), see Tables 1 and 2. ESI-MS m/z 561.22 [M+H] + (calcd for C33H37O8, 561.25). Gambogic acid (3): orange solid, m.p. 86-89 o C; 1 H and 13 C NMR data (CDCl3), see Tables 1 and 2. ESI-MS m/z 629.35 [M+H] + (calcd for C38H45O8, 629.31). Isogambogic acid (4): orange solid, m.p. 88-90 o C; 1 H and 13 C NMR (CDCl3), see Tables 1 and 2. ESI-MS m/z 629.32 [M+H] + (calcd for C38H45O8, 629.31). 10α-butoxy gambogic acid (5): light yellow oil; 1H and 13C NMR (CDCl3), see Tables 1 and 2. ESI-MS m/z 701.34 [M+H] + (calcd for C42H53O9, 701.37). 2.4. Cytotoxicity assay Cytotoxicity against human lung carcinoma (SK-LU-1) and Lewis lung carcinoma (LLC) cell lines was evaluated by using MTT method according to the described protocol [14] at Institute of Biotechnology – VAST. SK-LU-1 and LLC cell lines were cultured in a DMEM media mixed with 10 % heat- inactivated fetal bovine serum (FBS), 1 % antibiotic PSF (100 units/mL penicilline, 100 µg/mL streptomycin and fungizone), 2 mM L-glutamine, 1.5 g/L sodium bicarbonate, 4.5 g/L glucose, 10 mM HEPES and 1.0 mM sodium pyruvate using incubator with 5 % CO2 at 37 o C. Cultured cells were added in 96 well plates at the density of 4 x 10 4 cells/well and grown for 48 h. Compounds 1-5 with various concentrations in DMSO (10 L) and control (0.5 % of dimethyl sulfoxide) were added to cells. After incubating cells for 48 h, media was removed and the cell monolayer attached to the wall was fixed with cold 20 % (wt/vol) TCA for 1 h at 4 o C and stained with 1X SRB staining solution at room temperature (22-25 o C) for 30 min. The Nguyen Thi Kim An, Ngo Dai Quang, Pham Quoc Long, Tran Thi Thu Thuy 136 unbound dye was removed by washing repeatedly with 1 % (v/v) acetic acid. Finally, the protein-bound dye was dissolved in 10 mM Tris base solution for OD determination at 515-540 nm on an ELISA Plate Reader (Bio-Rad). Cell viability was assessed through MTT [3-(4,5- dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay as described previously [14]. Ellipticine, with concentrations of 10 μg/mL, 2 g/mL, 0.4 μg/mL, 0.08 μg/mL, was used as a positive control. 3. RESULTS AND DISCUSSION 3.1. Isolation of the caged xanthones 1-5 Compounds 1-5 were isolated from the DCM extract of the stem bark of G. hanburyi using repeated column chromatography on silica gel and C18-reversed-phase silica gel eluting with appropriate solvent mixtures. The structures of the isolated compounds are shown in Figure 1. Figure 1. Structures of compounds 1-5. Compound 1 was obtained as yellow oil. The 1 H NMR spectrum of 1 revealed the presence of one chelated ortho-hydroxy group [ H 12.90 (1H, s, OH-6)], eight tertiary methyl groups [ H 1.09 (3H, s), 1.31 (3H,s), 1.39 (3H, s), 1.46 (6H, s), 1.69 (3H, s), 1.73 (3H, s), 1.79 (3H, s)], two prenyl olefinic protons [ H 4.46 (1H, t, J = 7.0 Hz, H-27), 5.24 (1H, dd, J = 6.5, 7.5 Hz, H-32)], a cis-double bond [ H 5.54 (1H, d, J = 10.0 Hz, H-3), 6.66 (1H, d, J = 10.0 Hz, H-4)] and one proton of , -unsaturated ketone group [ H 7.46 (1H, d, J = 7.0 Hz, H-10)] (Table 1). The 13 C NMR and HSQC spectra contained signals due to eight methyl, three methylene, seven methine, and fifteen quaternary carbons including two carbonyl carbons at C 203.5 and 179.6, which Cytotoxic xanthonoids from the stem bark of Garcinia hanburyi collected in Viet Nam 137 were characterized as signals of C-12 and C-8 of the caged xanthonoid skeleton (Table 2). The NMR spectra of 1 also contained characteristic signals of a bicyclo[2,2,2]octane ring at [ H 3.51 (dd, J = 6.5, 5.0 Hz), C 47.0, CH-11] , [ H 2.35 and 1.34 (dd), C 25.5, CH2-21] and [ H 2.51 (d, J = 9.5), C 49.2, CH-22] (Tables 1, 2). These spectral data suggested that 1 contained a caged xanthonoid moeity [15-19] with two prenyl side chains. The positions of the substituents at C-2, C-17 and C-13 were determined by HMBC experiment with correlations between: H-19, H-20 ( H 1.46) /C-3 ( C 126.1); H-31( H 3.35)/C-16 ( C 157.5) and C-18 ( C 160.6); H-26 ( H 2.59)/C- 12 ( C 203.5), C-13 ( C 84.7) and C-14 ( C 90.5). In addition, the HMBC correlations of H-10/C- 11, C-21; H-21/C=O (C-12); H-22, H-24 ( H 1.73), H-25 ( H 1.31)/C-23 ( C 83.2) confirmed the presence of a bicyclo[2,2,2]octane ring. On the basis of the evidences and upon comparison the spectral data of compound 1 with those of previously reported [20], compound 1 was assigned as desoxymorellin. Table 1. 1 H (500 MHz) NMR data of compounds 1-5 in CDCl3: H (ppm) J (Hz). Position 1 2 3 4 5 3 5.54 (d, 10.0) 5.48 (d, 10.0) 5.38 (d, 10.0) 5.44 (dd, 10.5, 7.0) 5.44 (d, 10.0) 4 6.66 (d, 10.0) 6.58 (d, 10.0) 6.60 (d, 10.0) 6.67 (dd, 10.5, 2.5) 6.66 (dd, 10.0, 1.5) 6-OH 12.90 (s) 12.80 (br s) 12.77 (s) 12.75 (br s) 11.95 (s) 9 - - - - 3.18 (m) 10 7.46 (d,7.0) 7.55 (d, 6.5) 7.55 (d, 7.0) 7.55 (dd, 7.0, 2.5) 4.42 (dd, 4.5, 1.5) 11 3.51 (dd, 6.5, 5.0) 3.48 (m) 3.47 (m) 3.50 (dt, 7.0, 2.5) 2.81 (m) 19 1.46 (s) 1.40 (s) 1.38 (s) 1.38 (s) 1.36 (s) 20 1.46 (s) 1.44 (s) 1.76 (overlapped), 1.59 (m) 1.78 (overlapped), 1.65 (overlapped) 1.77 (m), 1.63 (m) 21 2.35 (dd, 13.5, 4.5); 1.34 (overlapped) 1.38 (overlapped); 2.32 (dd, 13.5, 4.5) 2.31 (dd, 13.5, 5.0); 1.34-1.36 (m) 2.33 (dd, 13.5, 4.5); 1.34-1.36 (m) 1.94 (m), 1.38 (m) 22 2.51 (d, 9.5) 2.53 (d, 9.5) 2.51 (d, 9.0) 2.52 (d, 9.5) 2.50 (d, 8.5) 24 1.73 (s) 1.29 (s) 1.69 (s) 1.71 (s) 1.35 (s) 25 1.31 (s) 1.65 (s) 1.29 (s) 1.29 (s) 1.15 (s) 26 2.59 (d, 7.5) 2.92 (t, 7.5, 7.0) 2.95 (dd, 15.5, 8.0) 2.63 (dd, 6.0, 3.0), 2.58 (dd, 6.0) 3.20 (m), 3.10 (m) Nguyen Thi Kim An, Ngo Dai Quang, Pham Quoc Long, Tran Thi Thu Thuy 138 27 4.46 (t, 7.0) 6.02 (t, 7.0, 6.5) 6.09 (t, 7.5) 6.49 (t, 7.0) 6.61 (dt, 7.0, 1.0) 29 1.09 (s) - - 1.36 (s) - 30 1.39 (s) 1.74 (s) 1.74 (s) - 1.96 (s) 31 3.35 (m) 3.16 (dd, 14.5, 5.0), 3.32 (dd, 9.0, 8.0) 3.29 (dd, 15.0, 8.0), 3.14 (dd, 15.0, 5.0) 3.29 (dd, 16.0, 6.5) 3.28 (m), 3.19 (m) 32 5.24 (dd, 7.5, 6.5) 5.05 (t, 6.5, 6.0) 5.04 (overlapped) 5.13 (dt, 7.0, 1.5) 5.03 (dt, 6.0, 1.5) 34 1.79 (s) 1.74 (s) 1.72 (s) 1.73 (s) 1.73 (s) 35 1.69 (s) 1.65 (s) 1.62 (s) 1.65 (s) 1.62 (s) 36 - - 2.01 (m) 2.03 (m) 2.08 (m) 37 - - 5.04 (overlapped) 5.07 (dt, 8.5, 2.5) 5.09 (dt, 7.0, 1.5) 39 - - 1.64 (s) 1.66 (s) 1.56 (s) 40 - - 1.55 (s) 1.55 (s) 1.66 (s) Table 2. 13 C (125 MHz) NMR data of compounds 1-5 in CDCl3: C (ppm). Position 1 2 3 4 5 2 78.4 78.7 81.3 81.3 81.0 3 126.1 126.1 124.5 124.8 125.1 4 115.6 115.4 115.9 115.9 115.9 5 103.0 103.2 102.8 102.9 102.8 6 157.8 157.6 157.6 157.6 156.4 7 100.6 100.6 100.5 100.5 101.8 8 179.6 179.0 178.9 178.8 193.8 9 133.8 133.8 133.4 133.4 48.6 10 133.8 135.4 135.3 135.3 72.3 11 47.0 46.9 46.8 46.9 44.2 12 203.5 203.5 203.3 202.9 208.4 13 84.7 84.1 83.8 83.7 86.4 14 90.5 90.9 90.9 90.7 88.4 16 157.5 157.3 157.4 157.4 155.7 17 108.3 108.1 107.6 107.9 108.9 Cytotoxic xanthonoids from the stem bark of Garcinia hanburyi collected in Viet Nam 139 18 160.6 161.2 161.5 161.4 161.2 19 28.3 28.3 27.7 27.4 27.2 20 28.3 28.3 42.0 41.9 41.9 21 25.5 25.2 25.2 25.4 20.0 22 49.2 49.1 49.0 49.1 43.6 23 83.2 83.8 84.1 83.7 82.4 24 30.1 28.9 29.9 30.0 29.7 25 29.1 29.9 28.8 29.1 27.2 26 28.8 29.4 29.3 29.0 28.0 27 117.9 136.8 137.8 136.8 139.0 28 134.9 126.1 127.8 128.9 127.6 29 16.7 170.1 170.2 11.4 171.6 30 25.6 20.8 20.7 171.1 20.6 31 21.7 21.7 21.6 21.6 21.5 32 122.2 122.2 122.3 122.2 122.6 33 131.6 131.6 131.5 131.9 131.3 34 18.2 18.2 18.2 18.1 18.1 35 25.7 25.7 25.6 25.5 25.6 36 - - 22.7 22.7 22.9 37 - - 123.8 123.8 123.8 38 - - 131.8 131.8 131.9 39 - - 25.6 25.7 25.6 40 - - 17.6 17.6 17.6 O O O O O O HOOC H H H H O 5 HMBCCOSY O O O O O O H H H H H H 1 Figure 2. COSY correlations, key HMBC correlations and/or NOESY correlations of compounds 1 and 5. Nguyen Thi Kim An, Ngo Dai Quang, Pham Quoc Long, Tran Thi Thu Thuy 140 Compound 2 was isolated as a bright yellow solid. The NMR spectra of compound 2 exhibited 33 carbon signals, equal to the number of carbons in 1 (Table 2). However, the 1 H- NMR spectrum of 2 demonstrated the disappearance of a methyl group and the shift to the upperfield of the olefinic proton H-27 (at H 6.02 in 2 compared to 4.46 in 1) (Table 1). Moreover, the 13 C-NMR spectrum exhibited the appearance of one more carbonyl carbon at C 170.1 besides the two carbonyl carbons C-8, -12 (at C 179.0 and 203.5, respectively). Thus, the NMR data of 2 suggested the replacement of the methyl group in compound 1 by a carboxyl group (C-29). By comparing the NMR data of 2 with reported data of morellic acid [21], we concluded that compound 2 was morellic acid. Compounds 3 and 4 were separated as yellow solids. Comparison of NMR data of 3 or 4 and 2 (morellic acid) revealed the only difference was that one of the methyl groups bonded to C-2 in 2 was replaced by a prenyl group in 3 and 4 (Tables 1-2). The structures of 3 and 4 were elucidated as gambogic acid and isogambogic acid from their NMR data and by comparison with reported data [3, 21-22]. These compounds were previously isolated from the resin of G. hanburyi growing in Phu Quoc island [13]. Compound 5 was obtained as light yellow oil. The NMR spectra of compound 5 were quite similar to those of compounds 3 and 4, except for the disappearance of H-10 signal at H 7.55 ppm (1H, s) and the appearance of one n-butoxy group and two methine protons at H 3.18 (1H, m, H-9) and 4.42 (1H, dd, J = 4.5 & 1.5Hz, H-10). The COSY and HSQC spectra of compound 5 illustrated the correlations of six 1 H - 1 H and 1 H - 13 C spin systems represented as bold lines (Figure 2), in which the n-butoxy group was appeared at [ H 3.51 (1H, m), 3.38 (1H, m), C 68.1, CH-1'], [ H 1.47 (2H, m), C 31.6, CH-2'], [ H 1.27 (2H, m,), C 19.2, CH-3'] and [ H 0.85 (3H, t, J = 7.0, 7.5), C 13.8, CH-4']. Moreover, the HMBC correlations of H-9/C-10 ( C 72.3), C-11 ( C 44.2); H-21 ( H 1.94, 1.38)/C-10; H-22 ( H 2.50)/C-10, C-14 ( C 88.4) and H-10/C-1' indicated that the double bond between C-9 and C-10 was oxidized. Furthermore, the signal of the metyl group in the 4-methylbut-3-en-4-oic group was assigned at C 20.6 ppm (C-30) suggested the Z form of the double bond C-27 and C-28. Based on the above evidences and upon comparison of the reported values [23], compound 5 was identified as 10α-butoxy gambogic acid. 3.2. In vitro cytotoxicity Table 3. Cytotoxicity of compounds 1-5 against LLC and SK-LU-1 cells. Compound IC50 (μM) LLC SK-LU-1 1 N/A N/A 2 N/A N/A 3 0.37 ± 0.08 0.35 ± 0.05 4 2.26 ± 0.19 2.02 ± 0.27 5 N/A N/A Ellipticine 1.99 ± 0.16 1.79 ± 0.33 N/A: no activities Five isolated compounds were evaluated for their cytotoxicity against lung cancer cells (LLC, SK-LU-1), with ellipticine as the positive control. Deoxymorelline (1), morellic acid (2) Cytotoxic xanthonoids from the stem bark of Garcinia hanburyi collected in Viet Nam 141 and 10-butoxy-gambogic acid (5) showed no activities whereas gambogic acid (4) and isogambogic acid displayed potent toxicity against tested cancer cell lines. The IC50 values of gambogic acid (3) and isogambogic acid (4) are given in the Table 3. The test result also revealed that these two compounds displayed higher cytotoxic activity against SK-LU-1 cell line than against LLC cell line. Furthermore, gambogic acid (3) showed higher activity, with IC50 values of 0.37 and 0.35 μM against LLC and SK-LU-1 cell lines, respectively. 4. CONCLUSIONS Chemical constituents study of the sterm bark of Garcinia hanburyi Hook. f. (Guttiferae) collected from Phu Quoc island leads to the isolation and identification of five prenylated caged xanthones, desoxymorellin (1), morellic acid (2), gambogic acid (3), isogambogic acid (4) and 10α-butoxy gambogic acid (5). Among these xanthones, gambogic acid (3) and isogambogic acid (4) presented potent cytotoxic activities to LLC and SK-LU-1 cancer cells. Especially, gambogic acid (3) may provide us a clue for a lead structure of anticancer agents. Acknowledgement: We thank the Institute of Natural Products Chemistry (VAST) for funding and Assoc. Professor Do Thi Thao, Institute of Biotechnology (VAST) for the cytotoxicity determinations. REFERENCES 1. Ravichandran S., Chen B., Cai X., Fu R. - Anticancer and multidrug-resistance reversing potential of traditional medicinal plants and their bioactive compounds in leukemia cell lines, Chin. J. Nat. Med. 12 (2014) 881-894. 2. Yaowapa S., Vatcharin R., Souwalak P. - Antibacterial caged-tetraprenylated xanthones from the fruits of Garcinia hanburyi, Chem. Pharm. Bull. 53 (2005) 850-852. 3. Asano J., Chiba K., Tada M., Yoshi T., Souwalak P. - Cytotoxic xanthones from Garcinia hanburyi, Phytochem. Lett. 41 (1996) 815-820. 4. Deng X. Y., Pan S. L., Zhao S. Y., Wu M. Q., Sun Z. Q., Chen X. H., Shao Z. Y. - Cytotoxic alkoxylated xanthones from the resin of Garcinia hanburyi, Fitoterapia 83 (2012) 1548-1552. 5. Han Q. B., Wang Y. L., Yang L., Tso T. F., Qiao C. F., Song J. Z., Xu L. J., Chen S. L., Yang D. J, Xu H. X. - Cytotoxic polyprenylated xanthones from the resin of Garcinia hanburyi, Chem. Pharm. Bull. 54 (2006) 265-267. 6. Han Q. B., Yang L., Wang Y. L., Qiao C. F., Song J. Z., Sun H. D., Xu H. X. - A pair of novel cytotoxic polyprenylated xanthone epimers from gamboges, Chem. Biodivers. 3 (2006) 101-105. 7. Tao S. J., Guan S. H., Wang W., Lu Z. Q., Chen G. T., Sha N., Yue Q. X., Liu X., Guo D. A. - Cytotoxic polyprenylated xanthones from the resin of Garcinia hanburyi, J. Nat. Prod. 72 (2009) 117–124. 8. Wang L. L., Li Z. L., Xu P. Y., Liu X. Q., Pei Y. H., Jing Y. K., Hua H. M. - A new cytotoxic caged polyprenylated xanthone from the resin of Garcinia hanburyi, Chin. Chem. Lett. 19 (2008) 1221-1223. Nguyen Thi Kim An, Ngo Dai Quang, Pham Quoc Long, Tran Thi Thu Thuy 142 9. Ren Y. L., Yuan C. H., Chai H. B., Ding Y. Q., Li X. C., Daneel F., Kinghorn A. D. - Absolute configuration of (-)-gambogic acid, an antitumor agent, J. Nat. Prod. 74 (2011) 460-463. 10. Tao S. J., Guan S. H., Li X. G., Guo D. A. - A highly rearranged pentaprenylxanthonoid from the resin of Garcinia hanburyi, Helv. Chim. Acta 93 (2010) 1395-1400. 11. Wang X. J., Lu N., Yang Q., Dai Q. S., Tao L., Guo X. K., Guo Q. L., You Q. D. - Spectacular modification of Gambogic acid on microwave irradiation in methanol: isolation and structure identification of two products with potent anti-tumor activity, Bioorg. Med. Chem. Lett. 20 (2010) 2438-2442. 12. Anantachoke N., Tuchinda P., Kuhakarn C., Pohmakotr M., Reutrakul V. - Prenylated caged xanthones: chemistry and biology, Pharm. Biol. 50 (2012) 78-91. 13. Nguyen T. K. A., Ngo D. Q., Pham Q. L., Tran T. T. T. - Polyprenylated caged xanthones from the resin of Garcinia hanburyi growing in Vietnam, J. Chem. 57 (4e3,4) (2019) 306- 312. 14. Monks A., Scudiero D., Skehan P., Shoemaker R., Paull K., Vistica D., Hose C., Langley J., Cronise P., Vaigro-Wolff A., Gray-Goodrich M. - Feasibility of a high-flux anticancer drug screen using a diverse panel of cultured human tumor cell lines. J. Natl. Cancer Inst. 83 (11) (1991) 757-766. 15. Nguyen H. D., Trinh B. T. D., Nguyen L. H. D. - Guttiferones Q-S, cytotoxic polyisoprenylated benzophenones from the pericarp of Garcinia cochinchinensis, Phytochem. Lett. 4 (2011) 129-133. 16. Chien S. C., Chyu C. F, Chang I. S., Chiu H. L., Kuo Y. H. - A novel polyprenylated phloroglucinol, garcinialone, from the roots of Garcinia multiflora, Tetrahedron Lett. 49 (2008) 5276-5278. 17. Han Q. B., Yang N. Y., Tian H. L., Qiao C. F., Song J. Z., Chang D. C., Chen S. L., Luo K. Q., Xu H. X. - Xanthones with growth inhibition against HeLa cells from Garcinia xipshuanbannaensis, Phytochemistry 69 (2008) 2187-2192. 18. Nguyen L. H. D., Vo H. T., Pham H. D., Connolly J. D., Harrison L. J. - Xanthones from the bark of Garcinia merguensis, Phytochemistry 63 (2003) 467-470. 19. Nguyen L. H. D., Harrison L. J. - Xanthones and triterpenoids from the bark of Garcinia vilersiana, Phytochemistry 53 (2000) 111-114. 20. Bhat H. B., Nair P. M., Venkataraman K. The colouring matters of G. morrella, Part V. Isolation of deoxymorellin and dihydroisomorellin, Indian J. Chem. 2 (1964) 405-410. 21. Karanjgaonkar C. G.., Nair P. M., Venkataraman K. - Morellic, isomorellic and gambogic acids a, b , Tetrahedron Lett. 7 (1966) 687-691. 22. Lin L. J., Lin L. Z., Pezzuto J. M., Cordell G. A. - Isogambogic acid and isomorellinol from Garcinia hanburyi, Magn. Reson. Chem. 31 (1993) 340-347. 23. Chen Y., He S., Chu Tang C., Li J., Yang G. - Caged polyprenylated xanthones from the resin of Garcinia hanburyi, Fitoterapia 109 (2016) 106-112.