Secondary metabolites from the fruit peels of Durio zibethinus L. and their cytotoxic activity

Abstract Three new glucosides, ziberthinosides A–C (1–3), and six known compounds, cleomiscosins A–D (4–7), sporogen AO-2 (8) and sapinofuranone A (9), were isolated from a methanol extract of Durio zibethinus fruit peels. Their chemical structures were elucidated by analyses of their 1 D/2D NMR and HR-ESI-MS data as well as by comparison with the literature values. The absolute configuration of sporogen AO-2 (8) was confirmed by ECD calculation. Of the isolates, compounds 8 and 9 were cytotoxic toward all MCF7, HepG2 and SK-LU-1 cell lines, with IC50 values ranging from 10.7 to 34.9 μM. Our results contribute to better understanding of the chemical constituents of the high valued durian fruit and their pharmacological effect, such as cytotoxicity. Graphical Abstract


Introduction
Durian (Durio zibethinus L.), belonging to Bombacaceae family, is a popular and commercially valuable fruit crop that is widely cultivated in the South and Southeast areas of Asia.The durian fruit is rich in nutrients; its peel (covers 65-70% weight) has been used in the traditional medicine to treat stomach upset, indigestion, tuberculosis and fever (Chi 2012).Previous phytochemical investigations on the durian fruits have led in the isolation and identification of secondary metabolites including flavonoids, fatty acid, sulphur-containing compounds and polysaccharides (Feng et al. 2016;Liang et al. 2022).The methanol extract has been shown to have numerous pharmacological activities, such as antioxidant, anti-inflammatory, anti-cancer, antimicrobial and anticoagulant activities, as well as immunomodulatory effect (Charoenphun and Klangbud 2022;Zamakshshari et al. 2022).In our continuing research on bioactive constituents from Vietnamese medicinal plants (Hanh et al. 2021a;Hanh et al. 2021b;Truong et al. 2021), we report herein the isolation and structural identification of three new (1-3) and six known compounds (6-9) from the durian fruit peels.Additionally, cytotoxicity of all the compounds against three human cancer cell lines, including HepG2, MCF7 and SK-LU-1, was also evaluated by the SRB assay.
The complete planar structure of 1 was identified by the further analysis of the 1 H-1 H COSY and HMBC spectra.The configuration of 2-methylbutyric acid groups was suggested as S based on the biosynthetic consideration with respect to the similar durianols A-C isolated from the same species (Feng et al. 2016).Thus, the structure of 1 was established as a new compound, named ziberthinoside A.
In conclusion, the phytochemical investigation of the MeOH extract derived from D. zibethinus fruit peels led to the isolation of three new and five known compounds.Of those, 8 and 9 were shown to be cytotoxic toward all MCF7, HepG2 and SK-LU-1 cell lines, with IC 50 values ranging from 10.7 to 34.9 lM.It is noted that this is the first report on the isolation of cleomiscosins C (6) and D ( 7), sporogen AO-2 (8) and sapinofuranone A (9) from the genus Durio and the ECD calculation and cytotoxicity of compound 8.Our results contribute to better understanding of the chemical constituents of the high valued durian fruit and their cytotoxicity.

General experimental procedures
Specific optical rotation values were identified using the Jasco P2000 polarimeter.NMR spectra of the isolated compounds were conducted on a Bruker AVANCE III 500 and 600 MHz spectrometer.HR-ESI-MS analyses were measured on an Agilent 1290 UPLC/6530 Accurate-Mass QTOF LC/MS system.Column chromatography was carried out using silica gel (40-63 mm) or ODS (150 mm) as adsorbents.Thin layer chromatography (TLC) was carried out on pre-coated plates.Open column chromatography was performed using silica gel or reversed phase C18 as a stationary phase.Preparative high performance liquid chromatography (prep.HPLC) was conducted on an Agilent 1200 system (Agilent Technologies, USA) equipped with a G1361A preparative pump, a G1315D diode array detector, a G2260A preparative autosampler and a COSMOSIL 5C18-MS-II Packed column (250 Â 10 mm, 5 lm).Thin layer chromatography was carried out with pre-coated plates (silica gel 60 F 254 or RP-18 F 254S ), and the spots were visualized by UV detection at 254 nm by spraying a 10% aqueous H 2 SO 4 solution, followed by heating.

Plant material
The peels of durian fruits were collected in October 2021 from the local fruit markets of Hanoi, Vietnam and were identified by one of the authors, Dang Viet Cuong.A voucher specimen (No: IMBC-SR.10.2020) was deposited at the Institute of Marine Biochemistry, VAST, Vietnam.

Cytotoxicity assay
The cytotoxicity of the isolated compounds against HepG2, MCF7 and SK-LU-1 human cancer cell lines was examined using the SRB assay (see the Supplementary material) (Monks et al. 1991;Quang et al. 2022).
Compound 1 was isolated as white amorphous powder.The HR-ESI-MS spectrum of 1 exhibited a quasi-molecular ion [M þ NH 4 ] þ at m/z 366.2135 (calcd for C 16 H 32 NO 8 Compound 3 was afforded as white amorphous powder.Quasi-molecular ion peaks[M þ NH 4 ] þ at m/z 410.2415 (calcd for C 18 H 36 NO 9 þ , 410.2385) and [M þ Na] þ at m/z 415.1946 (calcd for C 18 H 32 NaO 9 þ , 415.1939) were observed in the HR-ESI-MS, suggesting the molecular formula of C 18 H 32 O 9 .