Synthesis of rhein derivatives and evaluation of their anti-inflammatory activity

Objective Rhein possesses a variety of biological activities, including anticancer, antibacterial and anti-inflammatory effects. However, its poor solubility, toxic effects and low bioavailability limit its clinical application. In this study, hydroxydecanoic acid and decanol were chosen as modifiers for the structural modification of rhein. This study aims to increase the lipid solubility of rhein, reduce the toxic effects, and improve the 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 5-decanol rhubarbate (R-5HD), rhubarbic acid decanol (R-DA), 10-hydroxydecanoic acid rhubarbic acid (R-10HA), 10-hydroxydec-2-denoic acid (R-10HDA). The chemical structural characteristic of the rhein derivatives were investigated by ¹H nuclear magnetic resonance hydrogen spectroscopy (¹H NMR) and Fourier transform infrared spectroscopy (FTIR). Secondly, the CCK-8 kit was used to determine the effects of rhein derivatives on cell viability of mouse mononuclear macrophage leukemia cells RAW 264.7. The most suitable concentration range of rhein and its derivatives was determined according to the cell viability. 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), nitric oxide (NO), tumour necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and interleukin-6 (IL-6) expression were determined. The antioxidant activity of the rhein derivatives were analyzed by the ROS fluorescent probe ethyl dichlorofluorescin diacetate (DCFH-DA).

Result In this study, four rhein derivatives were prepared successfully proved by ¹H NMR and FTIR methods. Four rhein derivatives significantly increased the cell viability of RAW 264.7 compared with rhein. The LPS-induced RAW 264.7 cell model revealed that all four rhein derivatives significantly reduced NO release, and R-DA significantly reduced the expression of TNF-α and IL-6. R-5HD and R-DA significantly inhibited ROS production compared to rhein.

Conclusion Rhein was esterified with decahydroxy decanoic acid or decanol. The experimental results showed that rhein significantly reduced the cytotoxicity of rhein after modification, and showed better anti-inflammatory and antioxidant activity in the LPS-treated RAW 264.7 cell model. This study focused on the structural modification of rhein by decahydroxycapric acid or decanol. In vitro experiments confirmed that R-DA had strong anti-inflammatory and antioxidant activities, which provided theoretical basis for the subsequent research and development of rhein derivatives.