A novel rat hepatic clofibrate-inducible cytochrome P450 that is not a lauric acid hydroxylase

Effect of clofibrate on the chiral inversion of ibuprofen in healthy volunteers

OBJECTIVES

To determine the influence of the hypolipidemic drug clofibrate on the stereoselective metabolism of ibuprofen in humans.

METHODS

Healthy male subjects (n = 12) ingested a dose of 400 mg pseudoracemic ibuprofen (200 mg R-ibuprofen, 160 mg S-ibuprofen, and 40 mg 13C-S-ibuprofen) on two occasions after either pretreatment with clofibrate (2 gm/day over 1 week) or no pretreatment in a randomized order.

RESULTS

When subjects were pretreated with clofibrate, clearances of R-ibuprofen and 13C-S-ibuprofen increased significantly from 55.0 and 66.4 ml/min to 186.2 and 106.7 ml/min (p < 0.01), respectively. This increase was similarly reflected in the clearance by inversion of R-ibuprofen (control, 36.0 ml/min; treated, 118.8 ml/min; p < 0.01), as well as in the clearance by noninversion (control, 19.0 ml/min; treated, 67.4 ml/min; p < 0.01). Unbound clearance values significantly increased for R-ibuprofen (control, 19.5 L/min; treated, 38.7 L/min) but not for 13C-S-ibuprofen (11.8 versus 10.6 L/min, respectively). The fractional inversion of ibuprofen calculated from the urinary metabolite data was increased after clofibrate pretreatment (clofibrate group, 66.4%; control, 53.5%; p < 0.01). However, this was not evident when fractional inversion was calculated from the plasma concentration-time data for the unmetabolized drug.

CONCLUSIONS

Clofibrate altered the stereoselective disposition of ibuprofen in healthy volunteers by increased formation of R-ibuprofenoyl-coenzyme A rather than by an effect on oxidative metabolism of ibuprofen. This interaction has potential therapeutic implications.

Extraperoxisomal targets of peroxisome proliferators: mitochondrial, microsomal, and cytosolic effects. Implications for health and disease

Peroxisome proliferators are a structurally diverse group of compounds that include the fibrate hypolipidemic drugs, the phthalate ester industrial plasticizers, the phenoxy acid herbicides, and the anti-wetting corrosion inhibitors perfluorinated straight-chain monocarboxylic fatty acids. Administration of these chemicals to rodents results in a number of effects, the most prominent being hepatomegaly and induction of peroxisomal enzyme activities. Several of these compounds have also been associated with the production of liver tumors in rodents and are classified as nongenotoxic hepatocarcinogens. Experimental evidence suggests that humans are not susceptible to these effects following exposure to peroxisome-proliferating compounds. This has led to the proposal that an "actual threat to humans" from exposure to one of these compounds seems "rather unlikely". Indeed, recent reports suggest that peroxisome proliferators may prove valuable as antitumor agents in humans. However, this assessment is preliminary given that peroxisome proliferators also produce a myriad of extraperoxisomal effects in livers and other tissues of experimental animals. Such effects include both stimulation and inhibition of mitochondrial and microsomal metabolism and alteration of the activities of various cytosolic enzymes. These responses may be directly or indirectly related to the effects on peroxisomes or may be totally independent of these events. Whether the extraperoxisomal effects of these compounds occur in humans is not known and their potential impact on human health remains to be investigated.

Experimental models for evaluating enzyme induction potential of new drug candidates in animals and humans and a strategy for their use

Experimental models that have application for evaluating enzyme induction potential have been described in order of increasing complexity. The main focus was on models that have had wide application thus far. However, many new models are currently being developed that may have future applications in evaluating enzyme induction potential. A strategy to evaluate the enzyme induction potential of drug candidates was outlined. This scheme uses a combination of new and established techniques to evaluate data in a stepwise manner that is appropriate to the drug's current stage of development.

Selective inhibition of mammalian lanosterol 14 alpha-demethylase by RS-21607 in vitro and in vivo

The discovery of selective lanosterol 14 alpha-demethylase inhibitors may lead to novel hypolipidemic drugs. RS-21607, (2S,4S)-cis-2[1H-imidazol-1-yl)methyl]-2-[2-(4-chlorophenyl)ethyl]-4- [[(4-aminophenyl)thio]methyl]-1,3-dioxolane, was characterized as a tight-binding, competitive inhibitor of lanosterol 14 alpha-demethylase purified from rat liver. The apparent Ki was determined to be 840 pM and found to be similar in hepatic microsomes from human, rat, and hamster. RS-21607, which contains two chiral centers, was a more effective lanosterol 14 alpha-demethylase inhibitor than its three stereoisomers. In vitro, RS-21607 had a greater affinity for lanosterol 14 alpha-demethylase than the other cytochromes P450 evaluated: CYP7, CYP27, CYP11A1, CYP19, CYP17, CYP11B1, CYP21, CYP3A4, CYP4A, CYP2D6, CYP1A2, CYP2C9, and 27-hydroxycholesterol 7 alpha-hydroxylase. The other stereoisomers were not as selective as RS-21607. Doses of 3-30 mg/kg RS-21607 given orally to hamsters caused a dose-dependent decrease in cholesterol biosynthesis with a corresponding accumulation of 24,25-dihydrolanosterol. RS-21607 inhibited the enzyme and cholesterol biosynthesis in hamster liver by 50% at 18 h following a 30 mg/kg oral dose. This was interpreted to indicate that RS-21607 is able to distribute to the site of action in hamsters and inhibit the target enzyme. In the same dose range, the plasma concentrations of testosterone, corticosterone, and progesterone, the endpoints for the cytochromes P450 involved in steroid biosynthesis, were relatively unaffected. These data show RS-21607 to be an effective and selective inhibitor of lanosterol 14 alpha-demethylase, both in vivo and in vitro. RS-21607 interacted with the purified enzyme to produce a type II binding spectrum, consistent with an interaction between the imidazole moiety and the heme. The electrostatic contribution of the imidazole binding was investigated using the desimidazole analog of RS-21607. The apparent Ki for the desimidazole compound (65 microM) was similar to the apparent Km for the substrate DHL (79 microM). Together, these data confirm that the ligand attached to the imidazole in RS-21607 is a good non-sterol substitute for DHL, i.e., binding to the enzyme with similar affinity, and that the coordination of the imidazole to the heme provides a major electrostatic contribution for the inhibition of lanosterol 14 alpha-demethylase by RS-21607. RS-21607 was also observed to increase the accumulation of 3 beta-hydroxy-24,25-dihydrolanost-8-en-32-al, the second intermediate in the multistep oxidation, but not the first intermediate. 24,25-dihydrolanost-8-ene-3 beta,32-diol.(ABSTRACT TRUNCATED AT 400 WORDS)

Species differences in 5 alpha-androstane-3 beta,17 beta-diol hydroxylation by rat, monkey, and human prostate microsomes

The 6 alpha-, 7 alpha-, and 7 beta-hydroxylation of 5 alpha-androstane-3 beta,17 beta-diol by rat prostate microsomes appears to be catalyzed by a single, high-affinity cytochrome P450 enzyme. In the present study we have examined the hydroxylation of 5 alpha-androstane-3 beta,17 beta-diol by prostate microsomes from cynomolgus monkeys and from normal subjects and patients with benign prostatic hyperplasia. Our results suggest that although rat, monkey, and human prostate microsomes catalyze the 6 alpha-, 7 alpha-, and 7 beta-hydroxylation of 5 alpha-androstane-3 beta,17 beta-diol, these pathways of oxidation in monkeys and humans are not catalyzed by a single cytochrome P450 enzyme. The ratio of the three metabolites was not uniform among prostate microsomal samples from individual humans or monkeys. The 6 alpha-hydroxylation of 5 alpha-androstane-3 beta,17 beta-diol varied independently of both the 7 alpha- and 7 beta-hydroxylation, which varied in unison. The 6 alpha-, 7 alpha-, and 7 beta-hydroxylation of 5 alpha-androstane-3 beta,17 beta-diol by monkey prostate microsomes appeared to be differentially affected by in vivo treatment of monkeys with beta-naphthoflavone or dexamethasone. Treatment of a monkey with dexamethasone appeared to cause a 2.5-fold increase in both the 7 alpha- and the 7 beta-hydroxylation of 5 alpha-androstane-3 beta,17 beta-diol without increasing the 6 alpha-hydroxylation. The 7 alpha- and 7 beta-hydroxylation of 5 alpha-androstane-3 beta,17 beta-diol by human and monkey prostate microsomes, but not the 6 alpha-hydroxylation, was inhibited by antibody against rat liver NADPH-cytochrome P450 reductase. Similarly, the 7 alpha- and 7 beta-hydroxylation of 5 alpha-androstane-3 beta,17 beta-diol by human prostate microsomes, but not the 6 alpha-hydroxylation, was markedly inhibited (greater than 85%) by equimolar concentrations of the imidazole-containing antimycotic drugs ketoconazole, clotrimazole, and miconazole. These results suggest that the 7 alpha- and 7 beta-hydroxylation of 5 alpha-androstane-3 beta,17 beta-diol by monkey and human prostate microsomes is catalyzed by a cytochrome P450 enzyme, whereas the 6 alpha-hydroxylation is catalyzed by a different enzyme which may or may not be a cytochrome P450 monooxygenase. The hydroxylation of 5 alpha-androstane-3 beta,17 beta-diol by prostate microsomes from normal human subjects was quantitatively and qualitatively similar to its hydroxylation by prostate microsomes from patients with benign prostatic hyperplasia.(ABSTRACT TRUNCATED AT 400 WORDS)