Synthesis and Biological Evaluation of RBG Derivatives as Nrf2 Activators for the Treatment of Parkinson’s Disease

Nuclear factor erythroid 2-related factor 2 (Nrf2) is a master regulator of the cellular antioxidant response and a promising therapeutic target for Parkinson’s disease (PD). Resibufogenin (RBG), a bioactive bufadienolide from toad venom, has been identified as a potential Nrf2 agonist; however, its application is limited by cytotoxicity and poor drug-like properties. Herein, we report the rational design, synthesis, and biological evaluation of a series of RBG derivatives modified at the C3, C14–C15, and C17 positions. Systematic structure–activity relationship (SAR) studies identified 2-5c, featuring a C3 2-chloroacryloyl group and a C17 pyrimidine substitution, as a potential Nrf2 activator (EC50 = 4.18 μM), exhibiting approximately 7-fold greater activity than RBG. Importantly, 2-5c demonstrated neuroprotective effects in MPP+-induced BV2 microglial cells and effectively ameliorated motor deficits in an MPTP-induced PD mouse model. These findings suggest that 2-5c represents a promising candidate for further investigation in the development of novel Nrf2-based therapies for PD.