Design, synthesis, and biological evaluation of low-toxic lappaconitine derivatives as potential analgesics

The C₁₈-diterpenoid alkaloid lappaconitine (LA) is a non-addictive analgesic used in China. The toxicity (LD₅₀ = 11.7 mg/kg) limits its application. Two series of LA derivatives, including amides and sulfonamides (1-93), were designed and synthesized by modification on their C4 acetamidobenzoate side chains in this work. In vivo analgesic activity and toxicity of all derivatives were evaluated, and the structure-activity relationship was summarized. Six lead compounds (35, 36, 39, 49, 70, and 89) exhibited approximate analgesic activity to LA but with significantly reduced toxicity. The therapeutic index of these compounds is 14-30 times that of LA. In vivo metabolism study of the lead compounds 39, 49, 70, and 89 were conducted by UPLC-MS^E, indicating the reason for the low toxicity of the potential derivatives might be they are difficult to metabolize to toxic metabolite N-deacetyllappaconitine compared to LA. The effects of lead compounds on sodium channels and hERG channels were also studied by ion channel reader (ICR) which further revealed their analgesic and toxicity-attenuating mechanisms. Sodium channel assay revealed that the analgesic mechanism of these lead compounds was inhibiting the Na_v 1.7 channels. Taken together, compound 39 was provided as a new analgesic lead compound with significantly low toxicity and comparable activity to LA.