CYP3A4-Mediated Bioactivation of the β-Amyloid Precursor Protein-Cleaving Enzyme 1 Inhibitor JNJ-54861911 Results in Redox-Neutral Addition of Glutathione via Catalysis by Glutathione S-Transferase α1, Identified as the Major Target Protein in Human Hepatocytes

The β-amyloid precursor protein-cleaving enzyme 1 (BACE1) inhibitor JNJ-54861911, a candidate for the treatment of Alzheimer's disease, was withdrawn from clinical trials due to drug-induced liver injury (DILI). This paper describes our investigation of the metabolism of JNJ-54861911 to understand the potential contribution to the observed DILI. In human hepatocytes, JNJ-54861911 is metabolized by CYP450 3A4 to a reactive intermediate (RI), which undergoes glutathione (GSH) addition at C6 of the 2-amino-4-methyl-1,3-thiazin-4-yl moiety via glutathione S-transferase α1 (GSTA1) catalysis. Despite the preponderant role of CYP3A4 as an enabler, the adduct has the same level of oxidation as that of JNJ-54861911. The exact mechanism of RI formation might involve a sulfoxide (with further reduction) or tautomeric forms of JNJ-54861911 bearing a reactive thiazinium cation activating both the C2 and C6 positions. The cell pellet from the human hepatocyte incubated with C-JNJ-54861911 was analyzed via gel electrophoresis, resulting in the identification of a major protein adduct on GSTA1, a cross-link resulting from the addition of GSH and lysine 120 to JNJ-54861911, most likely on position C6 and on the nitrile, respectively. Ultimately, this major adduct might only account for 15-25% of the total covalent binding (CVB). Other important contributors to CVB might exist, like the bioactivation of the major diaminothiazine metabolite (DIAT). The level of covalent binding (CVB) burden (fraction of the dose resulting in CVB) is clearly below 1 mg/day, with a low daily dose of 25 mg. Despite this limited magnitude of CVB, it could still contribute to the liver enzyme elevations observed in approximately 20% of the patients and to the few cases of severe immune-mediated DILI. The latter could occur through a haptenization phenomenon and/or by inducing stress in hepatocytes. Such stress may activate an innate immune response, which, in turn, promotes the adaptive immune response.