Total Syntheses of the Amaryllidaceae Alkaloids Carltonines A–C and the Neuroprotective and Immunomodulatory Evaluation of Carltonine B

The inhibition of butyrylcholinesterase (BChE) represents an emerging approach for the treatment of Alzheimer’s disease (AD). In 2020, three previously undescribed Amaryllidaceae alkaloids, carltonines A–C (1–3), were isolated, and their inhibitory activity on BChE was reported. Herein, we describe the first, nonstereoselective total synthesis of carltonine A–C (1–3) and confirmed their inhibitory activity using Ellman’s assay, with nanomolar BChE inhibition by carltonine B (2). Further kinetic studies revealed that carltonine B (2) acts as a reversible, competitive inhibitor of BChE. In addition, racemic carltonine B (2) was evaluated in a phenotypic screening using murine hippocampal nerve cells, where it demonstrated protective effects against glutamate-induced oxytosis and ferroptosis at concentrations ranging from 5 to 10 μM and against iodoacetic acid-induced ATP depletion. Finally, in studies using lipopolysaccharide-activated microglial N9 cells, carltonine B (2) exhibited immunomodulatory effects by downregulating pro-inflammatory markers such as NOS2 and IL-1β without affecting anti-inflammatory signaling pathways.