CYTOTOXICITY OF TACRINE AND VELNACRINE METABOLITES IN CULTURED RAT, DOG AND HUMAN HEPATOCYTES

Clinical trials with tacrine (THA) and its principal (1-OH) metabolite (velnacrine) for the treatment of Alzheimer's disease have been hampe red by adverse hepatic events that were undetected in preclinical stud ies. As part of integrated in vivo/in vitro efforts to characterize th e role of metabolites in these events, cultured cells were evaluated f or their suitability for further mechanistic studies. The relative cyt otoxic potentials of THA, three monohydroxy metabolites of THA (includ ing velnacrine, a racemate), the two velnacrine enantiomers, and sever al known and suspected dihydroxy velnacrine metabolites were determine d. Cytotoxicity was evaluated in 24-hour cultures by morphology and by the Neutral Red Uptake Assay. All test articles were evaluated in pri mary rat hepatocytes and in a human hepatoma cell line (HepG2). THA an d velnacrine were also tested in a rat hepatoma cell line (H4) and in primary dog hepatocytes. The metabolic competency of each cell type wa s determined. Sensitivity to THA and velnacrine was greatest in H4 cel ls, followed by primary rat and HepG2 cells; dog cells were least sens itive. In HepG2 cells, THA was clearly more cytotoxic (LC50:54 mug/ml) than its monohydroxy metabolites (LC50 values: 84 to 190 mug/ml); dih ydroxy velnacrine metabolites were the least cytotoxic (LC50 values:25 1 to 434 mug/ml); the relative order was comparable in primary rat hep atocytes. Roles for reactive metabolites and/or altered metabolic capa bilities of Alzheimer's patients are suggested.