Comparison of the Bioactivation Potential of the Antidepressant and Hepatotoxin Nefazodone with Aripiprazole, a Structural Analog and Marketed Drug

Abstract

ABSTRACT:In vitro metabolism/bioactivation of structurally related central nervous system agents nefazodone (hepatotoxin) and aripiprazole (nonhepatotoxin) were undertaken in human liver microsomes in an attempt to understand the differences in toxicological profile. NADPH-supplemented microsomal incubations of nefazodone and glutathione generated conjugates derived from addition of thiol to quinonoid intermediates. Inclusion of cyanide afforded cyano conjugates to iminium ions derived from ␣-carbon oxidation of the piperazine ring in nefazodone and downstream metabolites. Although the arylpiperazine motif in aripiprazole did not succumb to bioactivation, the dihydroquinolinone group was bioactivated via an intermediate monohydroxy metabolite to a reactive species, which was trapped by glutathione. Studies with synthetic dehydroaripiprazole metabolite revealed an analogous glutathione conjugate with molecular weight 2 Da lower. Based on the proposed structure of the glutathione conjugate(s), a bioactivation sequence involving aromatic ortho-or para-hydroxylation on the quinolinone followed by oxidation to a quinone-imine was proposed. P4503A4 inactivation studies in microsomes indicated that, unlike nefazodone, aripiprazole was not a time-and concentration-dependent inactivator of the enzyme. Overall, these studies reinforce the notion that not all drugs that are bioactivated in vitro elicit a toxicological response in vivo. A likely explanation for the markedly improved safety profile of aripiprazole (versus nefazodone) despite the accompanying bioactivation liability is the vastly improved pharmacokinetics (enhanced oral bioavailability, longer elimination half-life) due to reduced P4503A4-mediated metabolism/bioactivation, which result in a lower daily dose (5-20 mg/day) compared with nefazodone (200-400 mg/day). This attribute probably reduces the total body burden to reactive metabolite exposure and may not exceed a threshold needed for toxicity.

Simultaneous Screening of Glutathione and Cyanide Adducts Using Precursor Ion and Neutral Loss Scans-Dependent Product Ion Spectral Acquisition and Data Mining Tools

“…Nefazodone has been demonstrated to undergo bioactivation at the 3-chlorophenylpiperazine motif to form reactive quinone-imine species, which can be trapped by GSH to form adducts [29,35,36]. In addition, Alfa-carbon oxidation of the piperazine ring in nefazodone results in iminium ions, which are hard electrophiles and can be trapped by cyanide to form CN adducts [27,36] (Scheme 1).…”

“…In addition, Alfa-carbon oxidation of the piperazine ring in nefazodone results in iminium ions, which are hard electrophiles and can be trapped by cyanide to form CN adducts [27,36] (Scheme 1). In this study, nefazodone was chosen as a model compound to examine the effectiveness of the current approach (Figure 1).…”

“…GM1 and GM2 eluted at 6.3 and 6.5 min, respectively, and showed the identical [M+H] + ion ( Figure 3a) and EPI spectrum (Figure 3b). The [M+H] + ion at m/z 807 corresponds to the addition of GSH to a dihydroxylated nefazodone metabolite [29,36]. The ER scan spectrum around m/z 807 exhibited an isotope envelop that is consistent with the presence of both GSH and 13 …”

Contribution of Artifacts toN-Methylated Piperazine Cyanide Adduct Formation In Vitro fromN-Alkyl Piperazine Analogs

“…In CN-trapping assays performed on secondary alicyclic amines, such as the nefazodone N-dealkylation product, significant CN adduct was found with an addition of 14 Da in the molecule (Argoti et al, 2005;Bauman et al, 2008). In our CN-trapping screening on Ndealkylated alicyclic compounds, significant CN adducts with an additional 14 Da were also observed and confirmed as the addition of a methylene group (unpublished data).…”

A Pharmacokinetic Modeling Approach to Predict the Contribution of Active Metabolites to Human Efficacious Dose

“…An understanding of metabolism and other clearance mechanisms in vitro and in preclinical species and how they translate to the clinic is a pivotal component of these predictions. Drug metabolites can not only cause or contribute to drug-drug interactions (Rambeck et al, 1990;Ho et al, 2003;Isoherranen et al, 2009;Yu and Tweedie, 2013;Yu et al, 2015) and other safety concerns (Bauman et al, 2008), but can also potentially contribute to efficacy (Drayer, 1976;Fura, 2006;Obach 2013). In such cases, predictions of human efficacious dose should include the contribution of active metabolites to efficacy.…”