Current industrial practices in assessing CYP450 enzyme induction: preclinical and clinical

Abstract

Induction of drug metabolizing enzymes, such as the cytochromes P450 (CYP) is known to cause drug-drug interactions due to increased elimination of co-administered drugs. This increased elimination may lead to significant reduction or complete loss of efficacy of the co-administered drug. Due to the significance of such drug interactions, many pharmaceutical companies employ screening and characterization models which predict CYP enzyme induction to avoid or attenuate the potential for drug interactions with new drug candidates. The most common mechanism of CYP induction is transcriptional gene activation. Activation is mediated by nuclear receptors, such as AhR, CAR, and PXR that function as transcription factors. Early high throughput screening models utilize these nuclear hormone receptors in ligand binding or cell-based transactivation/reporter assays. In addition, immortalized hepatocyte cell lines can be used to assess enzyme induction of specific drug metabolizing enzymes. Cultured primary human hepatocytes, the best established in vitro model for predicting enzyme induction and most accepted by regulatory agencies, is the predominant assay used to evaluate induction of a wide variety of drug metabolizing enzymes. These in vitro models are able to appropriately predict enzyme induction in patients when compared to clinical drug-drug interactions. Finally, transgenic animal models and the cynomolgus monkey have also been shown to recapitulate human enzyme induction and may be appropriate in vivo animal models for predicting human drug interactions.

Fig. 1

Illustration of domains of nuclear receptors (a) and structure of a gene construct with promoter region and target gene or transfection vector for a transactivation assay with promoter region and reporter gene (b)

Fig. 2

Example of how efficacious drug concentration can affect the predictability of induction potential from a plot of PXR transactivation vs. drug concentration. A, B, and C represent multiple efficacious concentrations for a hypothetical drug

Fig. 3

A representation of in vitro and in vivo induction models in order of decreasing throughput and increasing reliability/predictability (open squares: current models; open circles: future models). Assays such as the transactivation and immortalized hepatocyte assays are extremely high throughput, however they lack the complexity of human hepatocytes which represent the multitude of transcription factors/receptors and their interactions and dynamic drug exposure which occurs in patients