Studies of the microsomal drug metabolism system in warfarin-resistant and -susceptible rats

Differential expression of cytochrome P450 genes between bromadiolone-resistant and anticoagulant-susceptible Norway rats: a possible role for pharmacokinetics in bromadiolone resistance

BACKGROUND

Anticoagulant resistance in Norway rats, Rattus norvegicus (Berk.), has been suggested to be conferred by mutations in the VKORC1 gene, encoding the target protein of anticoagulant rodenticides. Other factors, e.g. pharmacokinetics, may also contribute to resistance, however. To examine the involvement of pharmacokinetics in bromadiolone resistance in male and female rats, liver expression profiles of seven cytochrome P450 genes from a Danish bromadiolone-resistant rat strain (with an Y139C-VKORC1 mutation) were compared with profiles from an anticoagulant-susceptible strain.

RESULTS

In the presence of bromadiolone, the Cyp2e1, Cyp2c13, Cyp3a2 and Cyp3a3 genes were significantly overexpressed, while Cyp2c12 expression was suppressed in resistant female rats compared with susceptible females. Relative to susceptible males, resistant males showed significant overexpression of the Cyp2a1, Cyp2e1, Cyp3a2 and Cyp3a3 genes. On exposure to bromadiolone, females had higher Cyp2e1 expression than males, which possibly explains why female rats are generally more tolerant to anticoagulants than male rats.

CONCLUSION

Results suggest that bromadiolone resistance in a Danish strain of Norway rats involves enhanced anticoagulant metabolism catalysed by cytochrome P450-2e1, -3a2 and -3a3. This pharmacokinetically based bromadiolone resistance is to some extent sex differentiated, as female resistance furthermore seems to involve overexpression of cytochrome P450-2c13 and suppression of P450-2c12, whereas male resistance appears to involve P450-2a1 overexpression.

Suppression of CYP3A2 mRNA expression in the warfarin-resistant roof rat, Rattus rattus: possible involvement of cytochrome P450 in the warfarin resistance mechanism

1. The continual use of warfarin as a rodenticide has caused the development of populations of warfarin-resistant roof rat. To study the biochemical mechanism of warfarin resistance, the mRNA expression levels of the major P450 forms in the warfarin-resistant and -susceptible roof rat liver following exposure to warfarin were quantified by competitive RT-PCR. 2. The constitutive levels of CYP2C11 and CYP3A2 mRNAs in the warfarin-resistant and -susceptible roof rat were extremely low compared with those in the STD rat. In response to warfarin administration, the CYP3A2 mRNA level in the warfarin-susceptible rat increased to about 3-fold of that before the treatment, whereas in the warfarin-resistant roof rat, CYP3A2 mRNA remained at a low level. 3. The present results suggest the possibility that reduced synthesis of CYP3A2 mRNA is involved in the warfarin-resistant mechanism in the roof rat.

Pharmacokinetics and pharmacodynamics in toxicology

1. Pharmacokinetics aids interpretation of the dose-response relationship in individual toxicology studies. 2. When used to compare across studies, even in a single species other factors, including variation in pharmacodynamic response, must be taken into account. Variation in pharmacodynamic response becomes more profound when one compares across species. 3. Examples do occur where plasma concentration-response relationships are constant across species, particularly when corrected for unbound drug. These examples should not be taken as support, however, of a general universal principle. 4. Owing to multiple factors such as species differences in receptors, enzymes and ion channels, dose or plasma concentration-response relationships can vary enormously across species. In the light of this, the results of toxicology studies should be viewed as qualitative rather than quantitative. Once sufficient clinical experience is gained the human database is the overriding measure of drug safety.

Hepatic microsomal warfarin metabolism in warfarin-resistant and susceptible mouse strains: influence of pretreatment with cytochrome P-450 inducers

In the present paper, the heterogeneity of hepatic cytochrome P-450 isoenzymes in the mouse has been probed, using warfarin as the substrate. Both sex and strain differences in the in vitro microsomal metabolism of warfarin have been investigated in male and female warfarin-resistant HC and warfarin-susceptible LAC-grey mouse strains. Animals were either untreated or treated with the cytochrome P-450 inducers phenobarbitone, beta-napthoflavone or clofibrate. In both sexes and strains of mice, metabolism of warfarin was stereoselective in favour of the R(+) enantiomer. However, regioselectively was different in both strains and sexes of untreated animals. After pretreatment with phenobarbitone, increases in the rate of formation of 4' and 7-hydroxy R(+) and S(-) warfarin metabolites in HC mice were observed, compared with untreated animals. In LAC-grey mice increases in 4'-, 6-, 7- and 8-hydroxy R(+) and S(-) warfarin metabolites were noted, compared with untreated animals. This data indicated that different amounts or forms of cytochrome P-450s were responsible for warfarin metabolism after phenobarbitone treatment in the two strains. Pretreatment of animals with beta-napthoflavone resulted in significant decreases in the rat of R(+) warfarin metabolism in both strains and sexes of mice indicating that the beta-naphthoflavone-inducible cytochrome P-450 isoenzymes were less active in the metabolism of warfarin, as compared to the uninduced isoenzymes. In addition, the cytochrome P-450 isoenzyme composition in the two mouse strains was different after clofibrate pretreatment, as reflected in reduced levels of some warfarin metabolites and a reduced total metabolism of warfarin, consistent with the narrow substrate specificity of clofibrate-induced cytochrome P450IVA1 for fatty acid hydroxylation. Accordingly, it is clear that both the basal and xenobiotic inducible hepatic cytochrome P-450 isoenzymes in warfarin-resistant and susceptible mice are different and therefore have implications for the in vivo disposition of warfarin.

Aspects of anticoagulant action: a review of the pharmacology, metabolism and toxicology of warfarin and congeners

Warfarin is the most widely used anticoagulant in the treatment of thromboembolism in man. It has also been used extensively as a rodenticidal agent. Insofar as its clinical use is concerned, it is now clear that many of the drug interactions observed in patients are mediated via metabolic or pharmacokinetic factors. An understanding of the disposition of warfarin is therefore essential if one is to predict the likely response in patients undergoing anticoagulant therapy with this compound. Warfarin-resistance has been reported in both man and rodents. Understanding resistance in both man and rodents is important for effective anticoagulant therapy, and in control of resistant strains of rodents. Warfarin resistance in rat strains does not appear to have a metabolic or pharmacokinetic basis; in this species, resistance is thought to be due to differences in permeability to, or affinity for a receptor. Apart from its clinical and rodenticidal uses, warfarin is an excellent substrate for probing the heterogeneity of cytochrome P.450, since its metabolic oxidation is mediated by this mixed function oxidase. This review draws together much of the current published literature on the pharmacology, metabolism and toxicology of warfarin and related congeners.

Inheritance of Scottish-type resistance to warfarin in the Norway rat

The inheritance of resistance to the rodenticide, warfarin, in the Norway rat, Rattus norvegicus, derived from a wild rat population in Scotland was studied in the backcross, intercross and testcross. The resistance was found to be due to a major gene with about the same map position in Linkage Group I as the warfarin-resistance gene, Rw2, which occurs in the wild rat population in Wales. In heterozygotes, the Scottish resistance gene, unlike the Welsh gene, is incompletely penetrant in expression, though the penetrance was found to increase markedly in response to selection. Differences between the Scottish and Welsh types of resistance suggest that the two resistance genes are allelic.

Relationship between induction of aryl hydrocarbon hydroxylase and de novo synthesis of cytochrome P-448 (P1-450) in mice

Phenobarbital, 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT), benzpyrene, 3-methylcholanthrene (3-MC) or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were administered i.p. for 1 or 3 days to genetically "responsive" (C57BL/6J) and genetically "non-responsive" (DBA/2J) mice. 3-MC or benzpyrene stimulated aryl hydrocarbon hydroxylase (AHH) activity in C57BL/6J (B6) mice but not in DBA/2J (D2) mice. TCDD induced AHH activity in both B6 and D2 mice. Time-course studies showed that in the first 12 h after a single injection of 3-MC to B6 mice there was no shift in the reduced cytochrome P-450-CO complex absorption spectra from 450 to 448 nm, although AHH activity increased 4-5 times over (above) that of the control group. The relationship between induction of AHH activity by polycyclic hydrocarbons in B6 mice and the concomitant synthesis of cytochrome P-448 is discussed.

A survey of the response of different strains of mice to substances metabolised by microsomal oxidation; hexobarbitone, zoxazolamine and warfarin

Sixteen strains of mice were compared with respect to their hexobarbitone sleeping time and their zoxazolamine paralysis time. The strains were A2G, CBA, CE, C3H, C57BL, C57L, DBA, F/st, ICFW, NMRI, NZB, Schneider, Simpson, SM, TO and 129/rr. All the strains except 129 Rr were also tested for survival on a diet containing 0.05% racemic Warfarin. There was highly significant interstrain correlation between hexobarbitone sleeping time and zoxazolamine paralysis time (r = 0.72) and between hexobarbitone sleeping time Warfarin survival (r = 0.68). There was a significant correlation between zoxazolamine paralysis time and Warfarin survival (r = 0.56). The correlations can be explained if: (1) there is a genetically determined interstrain variable which is some common component of the microsomal mixed-function oxidase systems involved in the hydroxylation of the three substances; (2) the anticoagulant action of Warfarin is caused more by a hydroxylated metabolite of Warfarin than by Warfarin itself. Phenobarbitone pretreatment shortened hexobarbitone sleeping times and zoxazolamine paralysis times, but its effect was greater in those strains with longer initial hexobarbitone sleeping times and zoxazolamine paralysis times. Piperonyl butoxide pretreatment lengthened hexobarbitone sleeping times, but had no effect on zoxazolamine paralysis times. Warfarin survival was unaltered by pretreatment with either phenobarbital or piperonyl butoxide.