Induction of drug metabolizing enzymes: A survey of in vitro methodologies and interpretations used in the pharmaceutical industry—Do they comply with FDA recommendations?
Introduction
Drug metabolizing enzyme induction can have significant implications to the pharmacokinetics and toxicity of drugs; therefore, it is important to establish the induction potential of new chemical entities (NCEs) before they move up through the development pipeline. Recently, the FDA has published a draft guidance document outlining guidelines by which researchers should design drug–drug interaction (DDI) assays [1]. In addition, there are three other main papers which describe in some detail the considerations for enzyme induction: (1) Tucker et al. [2], which was a summary report from a conference held in Basel in November 2000; (2) Bjornsson et al. [3], a comprehensive outline of experimental conditions and interpretation recommendations; (3) Huang [4] which was a preliminary concept paper to the 2006 guidelines. The aim of these summary papers is to help industry towards an accepted standardized method by which to carry out and interpret DDI assays. If pharmaceutical companies wish to submit induction data, it makes sense to follow guidelines set out by the FDA. However, different companies develop compounds from different chemical classes and for different therapeutic areas. Moreover, different sites of the same company may well have their own preferred assay, depending on the stage of drug development (screening, lead optimisation, etc.), for determining the induction potential of NCEs. With this in mind, a survey (carried out in November 2005) was constructed to determine (1) if the majority of pharmaceutical companies were strictly adhering to the 2004 FDA draft guidance [4] (since this was the latest version at that time), (2) how they interpreted the data and (3) what decisions were made according to the results. Data from the literature was collated to examine the predictive value of the two 2004 FDA criteria for assessing the induction potential, namely, fold induction and percentage of the positive control. In this assessment, induction of CYP3A4 was used as an example because it is the most cited of the cytochrome P450 (CYP) enzymes and, in most cases rifampin was used as the positive control. In contrast, reports on CYP1A2 induction compare test compound induction with a variety of positive controls, such as β-naphthoflavone (BNF), omeprazole and 3-methycholanthrine (3-MC), which made the comparison between the two criteria more difficult. The draft FDA guidelines have changed since the survey was carried out, therefore, the changes and their significance will also be discussed.
Section snippets
The survey
Thirty researchers from 27 companies were asked to complete a questionnaire containing 19 questions. Of the companies involved, 23 were European and 7 were US-based from a total of 9 countries. For three companies, two researchers were from the same company; in one case, the two researchers were from European and US sites of the same company. Responses were collected between November 2005 and February 2006. The questions that comprised the survey focused on three main areas:
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In vitro methods and
Choice of species and enzyme isoforms
All 30 participants determined induction potential in human hepatocytes but some also used hepatocytes from animal species such as rat (seven researchers), dog (three researchers), monkey (two researchers) and pig (one researcher). It is generally considered that human hepatocytes are the best model for predicting interactions in humans because there are known differences in the induction response between animal and human hepatocytes to the same inducers. The well-known example of this is
Acknowledgements
The authors would like to sincerely thank the participants of this survey, the identity of whom must remain undisclosed but you know who you are!
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