Carltonine B derivatives: Synthesis and structure-activity relationship insights for selective butyrylcholinesterase...

Butyrylcholinesterase (BChE) is recognized as a promising therapeutic target for the late stages of Alzheimer's disease (AD) due to its role in the hydrolysis of acetylcholine (ACh), while acetylcholinesterase (AChE) activity declines during disease progression. Here, we have reported an efficient chemistry procedure for the naturally occurring Amaryllidaceae alkaloid carltonine B, along with the design and synthesis of 36 novel carltonine-based analogues to determine structure-activity relationship (SAR). Most of the synthesized compounds exhibited potent and selective human BChE (hBChE) inhibition, with IC₅₀ values ranging from low micromolar to nanomolar concentrations. The drug-like properties of the molecules were assessed by in silico tools, using the blood-brain barrier (BBB) score algorithm, and subsequently validated by in vitro permeability assessment via parallel artificial membrane permeability assay (PAMPA). The derivatives exhibited potent hBChE inhibition in the low micromolar to submicromolar range, while their cytotoxicity against human neuroblastoma (SH-SY5Y) cells was observed only at higher micromolar concentrations, indicating a favorable safety profile. The synthesized alkaloid carltonine B (37) and its N-ethyl derivative (38) emerged as the most potent and selective hBChE inhibitors, with IC₅₀ values of 0.014 ± 0.002 μM and 0.013 ± 0.001 μM, respectively. Enzyme kinetic studies were conducted to elucidate the inhibition mechanism toward hBChE enzyme. Compound 37 demonstrated competitive inhibition with K_i value of 0.055 μM. In contrast, compound 38 showed a noncompetitive inhibition profile, with a K_i value of 0.067 μM. Molecular modeling suggested that the superior potency of compounds 37 and 38 arises from their more optimal engagement of the BChE active-site gorge compared to compound 33. For the additional safety assessment, CYP inhibition assay revealed that compounds 37 and 38 may pose a risk of CYP3A4-mediated drug-drug interactions during chronic administration.