Species comparison in P450 induction: effects of dexamethasone, omeprazole, and rifampin on P450 isoforms 1A and 3A in primary cultured hepatocytes from man, Sprague–Dawley rat, minipig, and beagle dog

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Abstract

Induction of P450 isoforms 1A (CYP1A) and 3A (CYP3A) by model inducers dexamethasone, omeprazole and rifampin was evaluated in primary cultured hepatocytes from man and laboratory animals. Inducer-specific species-differences were observed. Results with human hepatocytes from six human donors consistently show that both rifampin and dexamethasone were inducers of CYP3A activity (measured as testosterone 6β-hydroxylase activity), with rifampin being more potent. Conversely, in rat hepatocytes, dexamethasone was a potent CYP3A inducer while rifampin was not an inducer. Rifampin but not dexamethasone induced CYP3A in minipig and beagle dog hepatocytes. Omeprazole was a potent inducer of CYP1A activity (measured as ethoxyresorufin-O-deethylase activity) in human, beagle dog and minipig hepatocytes, and not an inducer in rat hepatocytes. The species-differences observed suggest that human hepatocytes represent the most appropriate preclinical experimental system for the evaluation of P450 induction in human.

Introduction

Cytochrome P450 induction is a well-established mechanism of pharmacokinetic drug–drug interactions. Induction of cytochrome P450 by one drug can lead to enhanced metabolism of a co-administered drug that is a substrate of the induced P450 isoform. The enhanced metabolism may lead to lowering of the plasma concentration of the affected drug to that below the intended efficacious level. Information on P450 induction potential can be estimated before clinical trials via the evaluation of the P450 isoform activities in livers in animals, for instance, animals from toxicological studies, which have been treated for a prolonged period (days) with the drugs or drug candidates in question. However, it is now known that species-differences exist in the effects of some P450 inducers. Rifampin, one of the most potent CYP3A inducer found in man, has been found to cause clinically significant interactions with oral contraceptives [1], and cyclosporin [2], [3], leading to undesired pregnancies and organ rejection, respectively. However, this drug is an extremely weak CYP3A inducer in the most commonly used laboratory animal species for toxicological studies — Sprague–Dawley (SD) rats. Results with the SD rat have failed to reproduce the significant CYP3A inducing effects of rifampin in man [4].

It is now generally believed that primary cultured hepatocytes represent a relevant experimental model for the evaluation of P450 induction potential of natural and synthetic chemicals [5]. The use of primary hepatocytes provides the advantages of savings in materials and cost as compared to in vivo experimentation with laboratory animals. Furthermore, human induction potential of a drug or drug candidate can be investigated in primary human hepatocytes. This is especially important when experimentation in human in vivo is impractical, for instance, during drug discovery and development phases where data on enzyme induction potential are used as one of the selection criteria for lead drug candidates.

In this study, we investigated the induction potential of three human pharmaceuticals which are known to be P450 inducers: dexamethasone, omeprazole, and rifampin, in hepatocytes from four animal species: man, SD rat, beagle dog, and minipig. The major goals are to investigate the following: (1) Do primary hepatocytes reproduce the in vivo species-differences? (2) Which of the three laboratory animal species is most like man in response to these inducers?

Section snippets

Chemicals

Salicylamide, 7-ethoxyresorufin, 7-hydroxyresorufin, testosterone, dexamethasone, omeprazole, and rifampin were purchased from Sigma Chemical Co. (St. Louis, MO), 11β-testosterone and 6β-testosterone from Ultrafine Inc. (Manchester, UK).

Human and animal livers

Human livers were obtained from approved human tissue distribution agencies. The livers were procured for liver transplantation but were found not suitable for transplantation. Livers from beagle dogs and minipigs were obtained from Covance (Reston, VA).

Dexamethasone versus rifampin in CYP3A induction

The induction potential of the potent human CYP3A inducer, rifampin, was compared to that for dexamethasone in 6 independent experiments using human hepatocytes from 6 donors (Fig. 1). Rifampin was consistently found to be a more potent inducer than dexamethasone. Rifampin induced CYP3A4 activity to the maximum levels ranged from approx. 400% in Donor 4 (Fig. 1D) to over 3000% in Donor 1 (Fig. 1A). In all six experiments, the effects of dexamethasone were lower than that for rifampin, with the

Discussion

P450 induction has been reported for human hepatocytes [8], [9], [10], [11], [12], [13], [14], [15], rat hepatocytes [16], [17], canine hepatocytes [18], [19] and porcine hepatocytes [20], [21]. This study represents the first reported study comparing human hepatocytes to hepatocytes from these commonly used laboratory animals, in the effects of the three known human P450 inducing drugs: rifampin, omeprazole, and dexamethasone. The experiments with hepatocytes from the different animal species

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