Objective To conduct structural modification of rhein by using hydroxydecanoic acid and decanol as modifiers, increase lipid solubility of rhein, reduce toxic effects, and improve anti-inflammatory and antioxidant effects for potential clinical application.

Method Firstly, based on the structure-activity relationship of rhein, the carboxyl group in rhein was selected for esterification with ten-carbon hydroxydecanoic acid and decanol to synthesize four rhein derivatives including rhubarb-5-hydroxydecanol (R-5HD), rhubarb-hydroxydecanol (R-DA), rhubarb-10-hydroxydecanoic acid (R-10HA) and rhubarb-10-hydroxy-2-denoic acid (R-10HDA). The chemical structural characteristic of rhein derivatives were investigated by ¹H nuclear magnetic resonance (¹H NMR) and Fourier transform infrared spectroscopy (FTIR). Secondly, the CCK-8 kit was used to determine the effects of rhein derivatives on survival rates of mouse mononuclear macrophage cells RAW 264.7. The most suitable concentration ranges of rhein and its derivatives were determined according to the survival rates of cells. Finally, to further verify the anti-inflammatory activity of rhein derivatives, a lipopolysaccharide (LPS)-induced RAW 264.7 cell model was constructed, and the reactive oxygen species (ROS) level, nitric oxide (NO) release amount, as well as tumour necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and interleukin-6 (IL-6) expressions were determined. The antioxidant activity of rhein derivatives were analyzed by the ROS fluorescent probe 2,7-dichlorofluorescin diacetate (DCFH-DA).

Result Four rhein derivatives were prepared successfully and proved by ¹H NMR and FTIR. The in vitro anti-inflammatory model validation showed that the cytotoxicity of rhein after modification was significantly reduced; Four rhein derivatives significantly increased the cell viability of RAW 264.7 compared with rhein. The rhein derivatives had better anti-inflammatory and antioxidant activity in the LPS-treated RAW 264.7 cell model. All four rhein derivatives significantly reduced NO release, as well as the expressions of TNF-α, IL-6 and IL-β. All four rhein derivatives significantly inhibited ROS production compared to rhein.

Conclusion This study focuses on the structural modification of rhein by hydroxydecanoic acid and decanol. In vitro experiment confirmes that R-DA has stronger anti-inflammatory and antioxidant activities, which provides a theoretical basis for the subsequent research and development of rhein derivatives.