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

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

In Vitro and In Vivo Studies To Characterize the Clearance Mechanism and Potential Cytochrome P450 Interactions of Anidulafungin▿

Anidulafungin is a novel semisynthetic echinocandin with potent activity against Candida (including azole-resistant isolates) and Aspergillus spp. and is used for serious systemic fungal infections. The purpose of these studies was to characterize the clearance mechanism and potential for drug interactions of anidulafungin. Experiments included in vitro degradation of anidulafungin in buffer and human plasma, a bioassay for antifungal activity, in vitro human cytochrome P450 inhibition studies, in vitro incubation with rat and human hepatocytes, and mass balance studies in rats and humans. Clearance of anidulafungin appeared to be primarily due to slow chemical degradation, with no evidence of hepatic-mediated metabolism (phase 1 or 2). Under physiological conditions, further degradation of the primary degradant appears to take place. The primary degradation product does not retain antifungal activity. Anidulafungin was not an inhibitor of cytochrome P450 enzymes commonly involved in drug metabolism. Mass balance studies showed that anidulafungin was eliminated in the feces predominantly as degradation products, with only a small fraction (10%) eliminated as unchanged drug; fecal elimination likely occurred via biliary excretion. Only negligible renal involvement in the drug's elimination was observed. In conclusion, the primary biotransformation of anidulafungin is mediated by slow chemical degradation, with no evidence for hepatic enzymatic metabolism or renal elimination

Considerations from the IQ Induction Working Group in Response to Drug-Drug Interaction Guidances from Regulatory Agencies: Focus on CYP3A4 mRNA in vitro response thresholds, variability, and clinical relevance

Here the IQ induction working group presents an assessment of best practice for data interpretation of in vitro induction, specifically, response thresholds, variability, application of controls and translation to clinical risk assessment with focus on CYP3A4 mRNA. Single concentration control data and Emax/EC50 data for prototypical CYP3A4 inducers were compiled from many human hepatocyte donors, repeated over time, in different laboratories. Clinical CYP3A induction and in vitro data were gathered for 51 compounds, 16 of which were proprietary from IQ member companies. A large degree of variability was observed in both the clinical and in vitro induction responses gathered, yet analysis confirmed in vitro data are able to predict clinical induction risk. Following extensive examination of this large dataset, the following recommendations are proposed: • CYP induction should continue to be evaluated in three separate human donors in vitro. • In light of empirically divergent responses in rifampicin control and most test inducers, normalization of data to percent positive control appears to be of limited benefit. • Two-fold induction, with concentration dependence, is an acceptable threshold for positive identification of in vitro CYP3A4 mRNA induction. • To reduce the risk of false positives, in the absence of a concentration dependent response, induction ≥ 2-fold should be observed in more than one donor to classify a compound as an in vitro inducer. • If qualifying a compound as negative for CYP3A4 mRNA induction, the magnitude of maximal rifampicin response in that donor should be ≥10-fold. • Inclusion of a negative control adds no value beyond that of the vehicle control