Design, synthesis and biological evaluation of esculetin derivatives as anti-tumour agents

Esculetin, a naturally catecholic coumarin, possess multiple pharmacological activities including antitumour, anti-inflammatory and anti-oxidant. However, the extensive phase II metabolism and rapid elimination from the human body significantly hinder esculetin and its derivatives as drug leads/candidates. To improve both the metabolic stability and the anti-tumour activity of esculetin via rational modification, a series of C-4 and C-8 substituted derivatives were designed and synthesized by perchloric acid catalysed von Pechmann reaction and Mannich reaction, respectively. The in vitro metabolic half-life in human liver S9 and anti-tumour activities in A549 and B16 cell lines of the newly synthesized compounds were assayed. Of these compounds, 8-(pyrrolidin-1-ylmethyl)-4-trifluoromethyl esculetin 15 was the most potent candidate compound, with almost a 20-fold increase in antiproliferative activity and a 3-fold prolonged half-life in human liver S9 compared with the parent compound 1. In addition, the potential structure-activity relationship and structure-metabolic stability relationship were discussed. Notably, the introduction of a nitrogen containing group as a hydrogen bond acceptor at the C-8 position of esculetin can improve both the metabolic stability and anti-tumour activity. All of these findings are very helpful for the structural modification of esculetin and other bioactive phenolic compounds to improve their phase II metabolic stability and bioactivity synchronously.