Antiproliferative polycyclic polyprenylated acylphloroglucinols from Garcinia paucinervis

Abstract Bioassay-guided isolation of the stems of Garcinia paucinervis led to one new adamantane-type polycyclic polyprenylated acylphloroglucinols (PPAPs), (-)-garpauvinin A (1), and four known analogues (2–5). The structure and absolute configuration of 1 was established via spectroscopic techniques and ECD method. All the isolates displayed moderate antiproliferative activity against HL-60, PC-3 and Caco-2 human cancer cell lines with IC50 values ranging from 0.81 to 19.92 μM, and exhibited low toxicity on WPMY-1 normal human cells, showing selectivity between normal and malignant prostate cells. The biosynthetic pathways of the isolated PPAPs were proposed. Graphical Abstract


Introduction
The polycyclic polyprenylated acylphloroglucinols (PPAPs) and caged xanthones with stunningly complex molecular architectures are an extremely rich source of secondary metabolites with a wide array of fascinating biological profiles, especially antitumor activity, from the genus Garcinia of Guttiferae (Clusiaceae) family (Ciochina and Grossman 2006;Richard et al. 2012;Kim and Choi 2014;Niu et al. 2017).In recent years, the genus has gained considerable attention from pharmaceutical industries by reason of their immense remedial qualities (Hemshekhar et al. 2011).For example, gambogic acid was recently advanced into a phase II clinical trial for the treatment of lung cancer in China (Tian et al. 2016).Garcinia is a large genus of shrubs or polygamous trees and occurs exclusively in tropical Asia, Africa and Polynesia.The phytochemistry of roots, stems, leaves, fruits and seeds of the Garcinia species with a variety of simple and complex bioactive substances have been investigated (Kumar et al. 2013).In previous study, our group have identified a series of novel antitumor agents from Garcinia species (Li et al. 2016;Jia et al. 2017Jia et al. , 2018Jia et al. , 2019)).G. paucinervis, a karst-endemic evergreen tree growing at altitudes of 300-800 metres above sea level, is only distributed in southwest China and northern Vietnam (Hu et al. 2017).This species not only has medicinal value, including treating burns and scalds, clearing heat, detumescence, and detoxification, but also has economic benefits, such as military industry, construction, shipbuilding, and quality furniture.However, these aforementioned characteristics have in turn led to excessive deforestation, inevitably (Hu et al. 2017;Tan et al. 2020).In light of the restriction of specific growing environment, a sharp decline in the number of G. paucinervis caused it to become an endangered species of national secondary protected plants in China (Jia et al. 2019).In continuous searching for novel bioactive constituents from the Garcinia species, the stems of G. paucinervis were investigated by bioassay-guided isolation.
The dichloromethane (CH 2 Cl 2 ) extract displayed antiproliferative effects against human cancer cell lines of myeloid leukaemia HL-60, prostate cancer PC-3 and colon carcinoma Caco-2 with the IC 50 values of 3.10, 21.82 and 16.50 μg/ml, respectively.One new and four known PPAPs (Figure 1) were isolated from the stems of G. paucinervis.The isolation and structural elucidation of new compound as well as antiproliferative activities of all compounds are described herein.
The biosynthetic pathways to the PPAPs 1-5 were proposed (Figure 2).The same precursor MPAPs (Tian et al. 2016;Yang et al. 2018) (Nguyen et al. 2012), respectively.The prenyl group at C-7 of c cyclizes with C-6 to get the known compound garciniagifolone A, followed by cyclisation of the side chain at C-5 to produce 1. e could undergo cyclizing to gain 2.

Plant material
The stems of Garcinia paucinervis were collected in Xishuangbanna of Yunnan province, China, in March 2012, and identified by Mr. Yu Chen at Xishuangbanna Tropical Botanic Garden of the Chinese Academy of Sciences.A voucher specimen (JSLJ-1203) has been deposited at the Department of Natural Products Chemistry, Shenyang Pharmaceutical University, Shenyang, China.

Computational methods
The time-dependent density functional theory (TD-DFT) ECD calculations for optimised conformers were subjected at the B3LYP/6-31 + G** level with a CPCM model in MeOH solvent, and the calculated ECD curves of 1 were generated by Boltzmann weighting of the selected low-energy conformers using SpecDis 1.51 (Frisch et al. 2010;Bruhn et al. 2013) with σ = 0.3 eV at −15 nm shift.

Antiproliferative assay
All compounds were screened for their antiproliferative activities carried out according to the described procedure (Cheng et al. 2018).Compounds 1-5 were dissolved in dimethyl sulfoxide (DMSO).5-Fluorouracil (5-FU) was regarded as a positive control.As indicated, data were represented as means ± SD based on three replicates.
a Positive control.b sI: selectivity index.
μM than the positive controls of 5-FU with IC 50 values of 30.59 μM and 38.77 μM, respectively.Meanwhile, all compounds showed low antiproliferative activities again WPMY-1 with IC 50 values from 20.67 to 65.64 μM.The results may provide useful clues to develop PPAPs that can be used as precursors for structural modifications, making them potential therapeutic agents for cancer.