Chiral Separation of Tacrine-Based Metabolites: A Step Towards a Safe, Highly Active Drug

Abstract Tacrine is one of the key structural motifs frequently explored in the search for new therapeutic candidates targeting Alzheimer’s disease (AD). Its structural modification primarily focuses on preserving activity while simultaneously mitigating its hepatotoxicity. The present study evaluates cholinesterase inhibitory activity and hepatotoxicity profiles of chiral tacrine-based hydroxy metabolites as potential drug candidates. Comprehensive chiral screening revealed that normal phase and supercritical fluid chromatography (SFC) are effective in the chiral discrimination of tacrine-based hydroxy metabolites. Molecular modeling highlighted the importance of hydrogen bonding and π–hydrogen bonding in selectand–selector interactions. Based on the chiral screening, the enantiomers of the most active racemate, 9-amino-6-chloro-1,2,3,4-tetrahydroacridin-1-ol (1-OH-6-Cl-THA), were semipreparatively isolated. Their absolute configurations were determined using single-crystal X-ray diffraction (scXRD) and electronic circular dichroism (ECD). The bioactivity assays identified the R -enantiomer as the eutomer, which possesses superior acetylcholinesterase inhibitory potency compared to the racemate and tacrine. These findings underline the need for tailored chiral methods in drug discovery and support the further investigation of R -(−)-1-OH-6-Cl-THA in animal models.