Antibodies as a Probe in Cytochrome P450 Research

Abstract

ABSTRACT:Cytochrome P450 (P450) is the superfamily of enzymes responsible for biotransformation of endobiotics and xenobiotics. However, their large isoform multiplicity, inducibility, diverse structure, widespread distribution, polymorphic expression, and broad overlapping substrate specificity make it difficult to measure the precise role of each individual P450 to the metabolism of drugs (or carcinogens) and hamper the understanding of the relationship between the genetic/environmental factors that regulate P450 phenotype and the responses of the individual P450s to drugs. The antibodies against P450s have been useful tools for the quantitative determination of expression level and contribution of the epitope-specific P450 to the metabolism of a drug or carcinogen substrate in tissues containing multiple P450 isoforms and for implications in pharmacogenetics and human risk assessment. In particular, the inhibitory antibodies are uniquely suited for reaction phenotyping that helps to predict human pharmacokinetics for clinical drugdrug interaction potential in drug discovery and development.Many enzyme systems exist in the liver and in extrahepatic tissues, such as intestine and kidney, that can contribute to the clearance of drugs from the systemic circulation. Among the various phase I and phase II drug-metabolizing enzymes, the cytochrome P450 (P450) enzymes play a key role in xenobiotic and endobiotic metabolic processing. It has been estimated that approximately two thirds of marketed drugs are metabolized by this enzyme system (Rendic and Di Carlo, 1997;Guengerich, 2003;Williams et al., 2004). In fact, members of three P450 subfamilies (CYP1, CYP2, and CYP3) are largely responsible for the metabolic clearance of drugs and xenobiotics (Rendic and Di Carlo, 1997;Guengerich, 2003;Williams et al., 2004). Multiplicity and overlapping substrate specificity of P450 have greatly complicated efforts at understanding the precise role of individual P450 isoforms in the metabolism of drugs and drug candidates. Therefore, over recent years, a variety of reagents and tools have been developed for in vitro and in vivo studies, and it is now possible to determine which specific P450 isoform(s) is (are) involved in the metabolism of a given compound (Rodrigues, 1999;Rodrigues and Rushmore, 2002;Lu et al., 2003;Williams et al., 2003). Given the importance of fraction of drug metabolized (f m ) in assessing the potential of in vivo drug-drug interaction (DDI), P450 reaction-phenotyping studies are conducted at multiple stages during drug discovery and development. Irrespective of the strategy, information related to P450-mediated metabolism is included in regulatory documents and the absorption, distribution, metabolism, and elimination (ADME) and DDI sections of the product label, where precautions concerning the coadministration of other drugs are listed along with recommendations for dose adjustment (Obach et al., 2006;Huang et al., 2007). Antibodies that are specific to individual P450s can be used to determine P450 pr...

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