Induction of cytochromes P450.

The induction of cytochromes P450 (CYPs) has been appreciated for some time but an understanding of the mechanisms involved has been poorly understood until recently. The discovery of the role of nuclear receptors such as the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR) has provided a major trigger for research in this area. This work has provided an explanation for species differences in hepatic induction. The production of a PXR crystal structure in the presence and absence of known high affinity ligands has offered the possibility of predicting structures which may bind to the receptor and hence act as inducing agents in man. An improvement in the technology of hepatocyte culture, access to good quality human hepatocytes and the miniaturisation of cultured preparations has meant that the potential of this technique to predict induction in man has been realised. Molecular biological techniques have also proved essential in both the science and the quantitation of CYP induction. The use of transient transfection cell based systems coupled with reporter gene assays have meant that dose response curves can be generated for many chemicals. Assays have been developed to measure the increase of the corresponding CYP mRNAs in primary hepatocytes and some cell lines with a high degree of sensitivity and specificity (allowing the quantitation of closely related CYPs). Although CYP induction is not usually considered as a major drawback in drug development, the aim should be to eliminate or reduce the inducing effects of a new drug to a minimum. Thus, it is essential to increase our understanding of the complex mechanisms that regulate induction and to pay attention to both the dose and the physicochemical and structural properties of CYP inducing agents.