Elsevier

Fitoterapia

Volume 120, July 2017, Pages 108-116
Fitoterapia

Diterpenoids with diverse scaffolds from Vitex trifolia as potential topoisomerase I inhibitor

https://doi.org/10.1016/j.fitote.2017.06.006Get rights and content

Abstract

Eleven new compounds, including six labdane (16), three halimane (79), and two clerodane (1011) diterpenoids and 16 known analogues (1227), were isolated from the leaves of Vitex trifolia. The structures of 111 were established by extensive 1D- and 2D-NMR and HRMS spectroscopic data. The absolute configurations of compounds 3, 7, and 10 were assigned using X-ray diffraction. Compounds 127 were evaluated for DNA topoisomerases I (Top1) inhibitory activity and cytotoxicity against HCT 116 cells. Compounds 8 and 11 exhibited equipotent Top1 inhibitory activity to the positive control, camptothecin (CPT), at 100 μM. Compounds 8, 9, 16, and 27 showed moderate cytotoxicity at low micromolar concentrations.

Introduction

Vitex genus (Verbenaceae) comprises of almost 250 species worldwide, of which 14 species are distributed in mainland China [1]. Various types of diterpenoids, including labdane [2], [3], [4], [5], [6], [7], halimane [7], [8], [9], [10], norlabdane [11], [12], clerodane [12], and abietane diterpenoids [12], [13], [14], were isolated from the Vitex genus. Vitex trifolia L., a small deciduous shrub, is widely distributed in Fujian, Guangdong, Guangxi, and Yunnan provinces in mainland China [15]. The fruits of V. trifolia are traditionally used for the treatment of cold, wind-heat, nervous headache, and rheumatism [15]. Its leaves have been reported to exhibit cytotoxic activity [16]. V. trifolia is rich in numerous diterpenoids possessing cytotoxic [17], trypanosidal [11], and antitubercular [10] activities.

DNA topoisomerases I (Top1) is an essential human nuclear enzyme that relaxes DNA supercoiling in various cellular metabolic processes. However, the inhibition of Top1 activity blocks DNA transcription and replication, leading to cell death [18]. Camptothecin, a natural alkaloid from the plants of Camptotheca acuminate [19], has been identified to solely target cellular Top1. Three camptothecin derivatives, topotecan, irinotecan, and belotecan, were approved for clinical cancer treatment, which means that Top1 is a validated target of anticancer drugs [20].

In our continuing search for bioactive constituents from traditional Chinese medicine, the leaves of V. trifolia were investigated. This led to the purification of six new labdane (16), three new halimane (79), two new clerodane (1011) diterpenoids, and 16 known analogues (1227) (Fig. 1). The structures of all new compounds were established by the interpretation of their physical data and X-ray diffraction analyses, and the known compounds were identified by the comparison of their experimental data with the reported values. These isolates were then assayed for Top1 inhibitory activity and cytotoxicity against HCT 116 cell lines.

Section snippets

General experimental procedures

Optical rotations were obtained with a Perkin-Elmer 341 automatic polarimeter. UV spectra were recorded using a Shimadzu UV-2450 spectrophotometer. ECD spectra were acquired on an Applied Photophysics Chirascan spectrometer. IR spectra were determined on a Bruker Tensor 37 infrared spectrophotometer with KBr pellets. The 1H (400 MHz), 13C (100 MHz), and 2D NMR spectra were obtained on a Bruker AM-400 NMR spectrometer with TMS as an internal reference. HRESIMS were acquired on a Shimadzu

Results and discussion

Compound 1 was isolated as a white amorphous powder. Its molecular formula was established as C24H40O7 based on HRESIMS ion at m/z 463.2653 [M + Na]+ (calcd for C24H40O7Na, 463.2666), and corresponds to five degrees of unsaturation. The IR spectra showcased absorptions for hydroxy group (3367 cm 1) and carbonyl group (1736 cm 1). The 1H NMR spectrum exhibited four methyl singlets (δH 0.92, 0.96, 1.20, 2.02), one methyl doublet (δH 1.05), one methyl triplet (δH 1.19), and four oxygenated methine

Conclusions

Twenty-seven diterpenoids, including eleven new one with diverse frameworks, were isolated from the leaves of V. trifolia. Four compounds (8, 9, 16, and 27) showed moderate cytotoxicity at micromolar range. Compounds 8 and 11 showed equipotent Top1 inhibition to CPT. Compound 8 was most cytotoxic against human colorectal carcinoma cells HCT 116 with Top1 inhibition of +++. The results indicated that Top1 possibly is one cellular target of diterpenoid 8 from V. trifolia.

Conflict of interest

The authors declare no competing financial interests.

Acknowledgments

This study was supported in part by the Guangdong Province Frontier and Key Technology Innovation Program (2015B010109004), the National Natural Science Foundation of China (Nos. 81471138, 81573310, 81371793), and the Medical Scientific Research Foundation of Guangdong Province (No. A2014212).

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