Design, Synthesis, and Mechanistic Evaluation of l-Theanine Derivatives Targeting Cathepsin D for Anti-Hepatic Fibrosis

Hepatic fibrosis represents a major global public health challenge, yet effective therapeutic interventions remain limited. In this study, we synthesized 40 derivatives through systematic structural modification of l-theanine and identified compound 9a was a potent antifibrotic agent. In vitro experiments revealed that compound 9a dose-dependently inhibited TGFβ1-induced activation of hepatic stellate cells (LX-2 and mHSC). Moreover, in both rat bile duct ligation (BDL) and mouse methionine-choline-deficient high-fat diet (CDAHFD) induced liver fibrosis models, compound 9a significantly attenuated hepatic injury, fibrosis, and inflammation, demonstrating robust hepatoprotective effects. Mechanistic investigations showed that compound 9a directly interacts with Cathepsin D and promotes its degradation, thereby suppressing the expression of fibrogenic and inflammatory genes. Pharmacokinetic studies demonstrated that compound 9a undergoes metabolic conversion to yield pharmacologically active metabolites 10 and 11c. Collectively, these results highlight compound 9a as a promising l-theanine-based candidate for the treatment of hepatic fibrosis.