Increased cholesterol epoxide hydrolase activity in clofibrate-fed animals

Cholesterol epoxide hydrolase (mCE) is a microsomal enzyme that hydrolyzes cholesterol-5,6-epoxides (CE) to cholestanetriol (CT). In the present study, hepatic mCE activity was measured in mice pretreated with several different xenobiotics known to induce a variety of hepatic drug-metabolizing enzymes. Only the phenoxyacetate hypolipidemics (clofibrate and ciprofibrate, included in the diet for 14 days) were found to increase hepatic mCE activity (1.8-fold increase). Clofibrate administration also increased rabbit hepatic (2.9-fold increase) and rat and rabbit renal mCE activity (1.5- and 2.3-fold increase respectively). On a subcellular level, mCE activities in rabbit nuclear and light mitochondrial fractions were increased (3.3- and 1.8-fold increase respectively), whereas activities in the cytosolic and heavy mitochondrial fractions were unchanged. In rabbits, clofibrate administration enhanced the hepatic microsomal hydrolysis of CE without increasing the hydrolysis of arene epoxides (benzopyrene-4,5-oxide) or fatty acid epoxides (methyl cis-9,10-epoxystearate). Increase of tissue mCE activity may significantly enhance tissue CT levels. In this light, it is worth noting similarities in the mechanisms of hypocholesterolemic action caused by clofibrate or CT administration.     

The pharmacokinetics of a thiazole benzenesulfonamide beta 3-adrenergic receptor agonist and its analogs in rats, dogs, and monkeys: improving oral bioavailability.

The pharmacokinetics and oral bioavailability of (R)-N-[4-[2-[[2-hydroxy-2-(pyridin-3-yl)ethyl]amino]ethyl]phenyl]-4-[4-[4-(trifluoromethylphenyl]thiazol-2-yl]benzenesulfonamide (1), a 3-pyridyl thiazole benzenesulfonamide beta3-adrenergic receptor agonist, were investigated in rats, dogs, and monkeys. Systemic clearance was higher in rats (approximately 30 ml/min/kg) than in dogs and monkeys (both approximately 10 ml/min/kg), and oral bioavailability was 17, 27, and 4%, respectively. Since systemic clearance was 25 to 40% of hepatic blood flow in these species, hepatic extraction was expected to be low, and it was likely that oral bioavailability was limited either by absorption or a large first-pass effect in the gut. The absorption and excretion of 3H-labeled 1 were investigated in rats, and only 28% of the administered radioactivity was orally absorbed. Subsequently, the hepatic extraction of 1 was evaluated in rats (30%) and monkeys (47%). The low oral bioavailability in rats could be explained completely by poor oral absorption and hepatic first-pass metabolism; in monkeys, oral absorption was either less than in rats or first-pass extraction in the gut was greater. In an attempt to increase oral exposure, the pharmacokinetics and oral bioavailability of two potential prodrugs of 1, an N-ethyl [(R)-N-[4-[2-[ethyl[2-hydroxy-2-(3-pyridinyl)ethyl]amino]ethyl]phenyl]-4-[4-[4-(trifluoromethyl)phenyl]thiazol-2-yl]benzenesulfonamide; 2] and a morpholine derivative [(R)-N-[4-[2-[2-(3-pyridinyl)morpholin-4-yl]ethyl]phenyl]-4-[4-[4-(trifluoromethyl)- phenyl]thiazol-2-yl]benzenesulfonamide; 3], were evaluated in monkeys. Conversion to 1 was low (<3%) with both derivatives, and neither entity was an effective prodrug, but the oral bioavailability of 3 (56%) compared with 1 (4%) was significantly improved. The hypothesis that the increased oral bioavailability of 3 was due to a reduction in hydrogen bonding sites in the molecule led to the design of (R)-N-[4-[2-[[2-hydroxy-2-(pyridin-2-yl)ethyl]amino]ethyl]phenyl]-4-[4-(4-trifluoromethylphenyl)thiazol-2-yl]benzenesulfonamide (4), a 2-pyridyl beta3-adrenergic receptor agonist with improved oral bioavailability in rats and monkeys.     

Comparative studies of the hepatic effects of di- and mono-n-octyl phthalates, di-(2-ethylhexyl) phthalate and clofibrate in the rat

The oral administration of di-n-octyl phthalate (DNOP), mono-n-octyl phthalate (MNOP), di-(2-ethylhexyl) phthalate (DEHP) and clofibrate to young male Sprague-Dawley rats for 14 days resulted in liver enlargement. Morphological examination of liver sections from DEHP and clofibrate treated rats, but not from either DNOP or MNOP treated animals, revealed increased numbers of peroxisomes (microbodies). Both DEHP and clofibrate treatment markedly stimulated the activities of certain peroxisomal marker enzymes whereas DNOP and MNOP produced only marginal effects. Similarly both DEHP and clofibrate, but not DNOP or MNOP, increased microsomal cytochrome P-450 content and markedly stimulated microsomal lauric acid hydroxylation activity. The results thus demonstrate that whilst the branched chain phthalate ester DEHP induced peroxisomal proliferation, the straight chain analogue DNOP and its metabolite MNOP were essentially inactive. In addition, DEHP treatment appeared to induce similar form(s) of cytochrome P-450 in rat liver to those previously described after clofibrate administration.     

氯苯丁酯引起大鼠肝肿大与肝细胞生长因子产生的可能关系

目的研究氯苯丁酯引起的大鼠肝脏肿大与血浆中ＨＧＦ水平变化的关系。方法氯苯丁酯（５００，４００，３００，１００ｍｇ·ｋｇ－１·ｄ－１）加入饲料中喂养大鼠，在不同时间处死大鼠，取出肝脏称重和制备血清，以原代培养大鼠肝细胞ＤＮＡ合成检测ＨＧＦ的生物学活性。结果氯苯丁酯处理后的大鼠血清能显著增加原代培养大鼠肝细胞３Ｈ－ＴｄＲ的参入，ｃｐｍ值随着血清被稀释而逐渐降低，但仍高于对照组。大鼠血清中ＨＧＦ的释放在给药后４ｄ达到最高水平，为正常对照组的６倍以上，给药２ｗｋ后仍维持在正常水平的２倍以上。而氯苯丁酯引起肝脏肿大发生在给药２ｗｋ后，并维持在正常水平的１．８倍以上。结论氯苯丁酯引起的肝脏肿大可能部分与ＨＧＦ促肝细胞的增殖作用有关     

Change of the sex-dependent response to clofibrate in F344 rat liver during postnatal development

Although it has been reported that male rats are more responsive than females to peroxisome proliferation induced by clofibrate, these sex differences have been confirmed in young adult rats. Using 4-, 8-, and 12-week-old F344 rats, postnatal change of the sex-dependent response to clofibrate was investigated. These animals were administered 200 mg/kg body wt./day clofibrate by gavage for 7 days. In 4-week-old rats clofibrate-dependent changes (hepatomegaly, induction of hepatic microsomal and peroxisomal enzymes, proliferation of smooth endoplasmic reticulum and peroxisomes of hepatocytes) were slight in both sexes. In 8- and 12-week-old rats clofibrate-induced changes of males were moderate, whereas those of females were slight. These results suggest that the responsiveness of immature rat to clofibrate is weak and in males the susceptibility is gradually strong during postnatal development.     

Toxicological studies on a benzofurane derivative. II. Demonstration of peroxisome proliferation in rat liver

The uricosuric drug benzbromarone (3,5-dibromo-4-hydroxyphenyl)-1-(2-ethyl-3-benzofuranyl)methanone, a benzofurane derivative, was studied for its effects on parameters related to hepatic peroxisome proliferation. Groups of male F-344 rats were fed either basal diet, the peroxisome proliferator clofibrate at 5000 ppm as a comparison compound, or benzbromarone at two doses, 1000 and 2000 ppm. Benzbromarone and clofibrate produced hepatomegaly and increases in the activities of catalase, acyl CoA oxidase, malate dehydrogenase, and glycerol-3-phosphate dehydrogenase. Benzbromarone and clofibrate also both induced similar histologic and ultrastructural changes in hepatocytes, including induction of peroxisomes. Therefore, benzbromarone acted as a peroxisome-proliferating agent in rats under these conditions. Benzbromarone differs from other peroxisome proliferators in its chemical structure, uricosuric action, and the morphology of liver peroxisomes that were induced by exposure.     

Effect of clofibrate administration on the esterification and deesterification of steroid hormones by liver and extrahepatic tissues in rats

Treatment of rats with clofibrate markedly stimulated the liver microsomal esterification of estradiol, testosterone, pregnenolone, dehydroepiandrosterone, and corticosterone by acyl-CoA:steroid acyltransferase. This enzyme catalyzes the esterification of estradiol with long-chain fatty acids in both liver and extrahepatic tissues. In untreated control rats, brain had the highest acyltransferase activity per milligram of microsomal protein for estradiol esterification (3- to 4-fold higher than in the liver). Although, treatment of rats with clofibrate stimulated the esterification of estradiol by 9- to 14-fold in the liver, estradiol esterification in kidney, lung, brain, uterus, fat, and mammary glands was not increased, indicating that liver may be uniquely sensitive to induction of acyl-CoA:estradiol acyltransferase by clofibrate. In additional studies, esterase activity for hydrolysis of the oleoyl ester of estradiol was determined in control and clofibrate-treated rats. Clofibrate administration increased esterase activity by an average of 107% in fat and 70% in liver. The results indicate that treatment of rats with clofibrate stimulates the hepatic formation of highly lipophilic fatty acid esters that can be hydrolyzed in the liver and in extrahepatic tissues to the parent steroid hormone by a clofibrate-inducible esterase.     

Translactational induction of CYP4A expression in 10.5-day neonatal rats by the hypolipidemic drug clofibrate

Lactating mothers of 7.5-day neonatal rats were injected intraperitoneally with 500 mg kg⁻¹ clofibrate for 3 consecutive days at 24-hour intervals; 24 hours after the final injection, the maternal cytochrome P450 4A (CYP4A) mRNA levels had risen 14- and 2.5-fold above the constitutive levels of expression seen in the liver and kidney, respectively. Lactational transfer of clofibrate to the suckling 10.5-day litter was demonstrated by the 15- and 5-fold elevation observed in the neonatal hepatic and renal CYP4A mRNAs, respectively, following suckling from drug-induced mothers. A significant decrease in the relative liver weights of these neonatal pups was seen following clofibrate exposure via maternal milk, in total contrast to the normally observed increase in liver/body weight ratios of rats treated with clofibrate. Western blot analysis using a polyclonal goat anti-rat CYP4A1 antibody also demonstrated a rise in the CYP4A protein levels in both the mothers and their litters following maternal clofibrate treatment.     

Pharmacological studies with beclobrate, a new hypolipidemic agent

A new diphenylmethane derivative with potent hypolipidemic activity, ethyl-(+/-)-2-[[alpha-(p-chlorophenyl)-p-tolyl]-oxy]-2-methylbutyrate (Sgd 24774, beclobrate) has been investigated in animals. From a comparison of the ED25 values of beclobrate and clofibrate, the new drug is 11 times more potent with respect to its hypocholesterolemic activity and 36 times more hypotriglyceridemic in normally fed rats, and lowers fructose-induced hypertriglyceridemia in rats 20 times as effectively as does clofibrate. On a similar basis of comparison, the hepatomegalic effect of beclobrate in rats is 22 times that of clofibrate. High doses of beclobrate did not reveal any other peripheral or central effects in a wide range of pharmacological tests, indicating a high specificity of the action of the drug on blood lipids. On the basis of the results of interaction studies performed with beclobrate in animals, administration of the substance in man should be largely free from risk.     

Hypolipidemia and peroxisome proliferation induced by phenoxyacetic acid herbicides in rats

Male Wistar rats were treated daily by gavage with two phenoxy herbicides, 2,4-dichlorophenoxyacetic acid (2,4-D)(100-200 mg/kg body wt) and 4-chloro-2-methylphenoxyacetic acid (MCPA) (100-200 mg/kg body wt), and with the chemically different glyphosate N-phosphonomethyl glycine (300 mg/kg body wt) 5 days per week for 2 weeks. A hypolipidemic drug, clofibrate [ethyl-2-(4-chlorophenoxy)-2-methylpropionate], which is structurally related to phenoxy acids, was used as a positive control (200 mg/kg body wt). 2,4-D and MCPA had several effects similar to those of clofibrate: all three compounds induced proliferation of hepatic peroxisomes, decreased serum lipid levels, and increased hepatic carnitine acetyltransferase and catalase activities. 2,4-D and MCPA, but not clofibrate, decreased lipoprotein lipase activity in the adipose tissue to about a third of the control value but did not change the lipoprotein lipase activity in the heart muscle. The data suggest that these compounds cause hypolipidemia not by enhancing the storage of peripheral lipids in adipose tissue but by preferentially increasing lipid utilization in the liver. Glyphosate caused no peroxisome proliferation or hypolipidemia, suggesting that these effects are associated with the structural similarity between phenoxy acid herbicides and clofibrate.     

The effect of hypolipidemic agents on the hepatic microsomal drug-metabolizing enzyme system of the rat. Induction of cytochrome(s) P-450 with specificity toward terminal hydroxylation of lauric acid

The effect of chronic dietary administration of the hypolipidemic agents, clofibrate, methylclofenapate, fenofibrate, and tibric acid, on the hepatic drug-metabolizing enzyme system of the albino rat has been studied. Each compound caused dose-dependent increase in liver size and cytochrome P-450 (methylclofenapate = fenofibrate = tibric acid greater than clofibrate). NADPH-cytochrome c reductase activity was increased only after clofibrate and methylclofenapate treatment. There was no overall increase in the metabolism of a number of commonly used model substrates in parallel with the cytochrome P-450 induction. Aminopyrine and ethoxyresorufin dealkylation, biphenyl 4-hydroxylation, testosterone 16 alpha-hydroxylation, and o-aminophenol and chloramphenicol glucuronidation showed no change or inhibition, whereas ethoxycoumarin and phenacetin dealkylation and testosterone 6 beta-hydroxylation were increased (only up to 2-fold). Using clofibrate as a representative of this class of pharmacological agent, the enzymatic changes were essentially reversed within 6 days after removal of drug from the diet. Clofibrate administration also increased liver size and, to a lesser extent, hepatic cytochrome P-450 content in the albino (CD-1) mouse but had no effect in the marmoset monkey. In the rat, clofibrate administration specifically increased the hepatic microsomal omega-hydroxylation of lauric acid approximately 28-fold, which contrasted with the specific increase in (omega - 1)-hydroxylation caused by phenobarbital administration. The specific increase in microsomal cytochrome P-450-mediated omega-oxidation of a medium length, straight chain, saturated fatty acid is similar to the documented increase in peroxisomal and mitochondrial fatty acid beta-oxidation caused by administration of hypolipidemic agents.     

The metabolic stability of acyl-CoA: cholesterol O-acyltransferase (ACAT) inhibitors,N-(4-benzyloxy-3, 5-dimethoxycinnamoyl)-N'-(2, 4-dimethylphenyl)piperazine (YIC-708-424) and its derivatives in rat liver and intestinal epithelium

The metabolic stability of the acyl-CoA: cholesterol O-acyltransferase (ACAT) inhibitor N-(4-benzyloxy-3, 5-dimethoxycinnamoyl)-N'-(2, 4-dimethylphenyl)piperazine (YIC-708-424) and its n-alkoxy derivatives containing an alkyl chain of 3 or 7 to 10 carbons, which exhibited different hypocholesterolemic activities, was investigated in vivo and in vitro in rats. After the oral administration of YIC-708-424 to rats at a dose of 5 mg/kg/d for 7 d, the parent compound was not detected in the blood. On the other hand, when the n-alkoxy derivatives were administered to rats, an increase in the alkyl chain length produced a progressive increase in the blood concentration of the parent compound. Both in the blood of rats administered YIC-708-424 and in the reaction mixture after the incubation of YIC-708-424 with rat hepatic 9000 x g supernatants, an inactive major metabolite, N-(4-benzyloxy-3, 5-dimethoxycinnamoyl)-N'-(4-carboxyl-2-methylphenyl)piperazine, was observed. The ratio of the maximum velocity to the apparent Michaelis-Menten constant (V(max)/K(m)) for the degradation of the n-propyloxy derivative in rat hepatic and intestinal microsomes was almost equivalent to that of YIC-708-424. On the other hand, an increase in the alkyl chain length of n-alkoxy derivatives produced a progressive decrease in V(max)/K(m) for the degradation of these compounds. Additionally, the in vivo hypocholesterolemic activities of YIC-708-424 and its n-alkoxy derivatives were positively correlated with the blood concentration of the parent compound and were negatively correlated with their V(max)/K(m). These results suggest that the metabolic stability of ACAT inhibitors in the liver and intestinal epithelium, which are the major target organs of these compounds, has a strong influence on their pharmacological activities in vivo.     

Leukotrienes and α-naphthylisothiocyanate-induced liver injury

α-Naphthylisothiocyanate (ANIT) administration to rats results in periportal hepatic inflammation and injury. Glutathione (GSH) appears to be necessary for the liver injury to occur. The leukotrienes (LTs) are metabolites of arachidonic acid and potent mediators of inflammation that have been implicated in certain liver injury models. Inasmuch as GSH is a cofactor for the synthesis of cysteinyl-LTs and since inflammation is a prominent component of ANIT injury, we hypothesized that LTs are involved in producing the hepatic insult that results from ANIT administration. To test this hypothesis, rats were treated with one of several inhibitors of LT biosynthesis, A63162, Zileuton or MK-886. Each of these agents prevented the formation of LTB₄ in Ca⁺⁺ ionophore-stimulated whole blood from rats treated with the inhibitors. A63162 attenuated the hepatic parenchymal injury caused by ANIT and resulted in a modest decrease in ANIT-induced cholestasis. In contrast, neither Zileuton nor MK-886 attenuated liver injury. AT-125 (Acivicin) inhibits γ-glutamyl transferase (GGT), the enzyme that catalyzes the formation of LTD₄ from LTC₄. AT-125 pretreatment did not prevent ANIT-induced hepatic parenchymal insult. It did, however, ameliorate the cholestasis caused by ANIT. In conclusion, the partial protection afforded by A63162 and AT-125 likely results from, effects unrelated to the formation of LTs, since Zileuton and MK-886 inhibited LT synthesis without affording protection. The lack of protection by Zileuton and MK-886 in the face of LT synthesis inhibition suggests that LTs are not necessary for the expression of injury after ANIT administration.     

Low density lipoprotein-lowering and high density lipoprotein-elevating effects of nicardipine in rats

Oral administration of a calcium antagonist, nicardipine hydrochloride (simply designated as nicardipine), in doses of 10-100 mg/kg tended to decrease total serum cholesterol and to increase high density lipoprotein (HDL) cholesterol in the normal rat. These effects of nicardipine were much greater than those of clofibrate, a standard cholesterol-lowering drug. Neither nicardipine nor clofibrate caused significant alteration in serum triglyceride and phospholipid. In hypercholesterolemic rats, nicardipine increased significantly HDL cholesterol with a reduction of total serum cholesterol, whereas clofibrate did not change HDL cholesterol. Separation of serum lipoproteins either by ultracentrifugation in various densities of KBr-NaCl solution or by polyacrylamide gel electrophoresis demonstrated that nicardipine increased preferentially HDL2 (density: 1.063-1.125), with a reduction of the low density lipoprotein (LDL) (density: 1.006-1.063) level in hypercholesterolemic rats. Serum triglyceride and liver phospholipid were increased slightly by nicardipine with no clear dose-dependency. Clofibrate also increased serum triglyceride. In normal rats, neither nicardipine nor nicotinic acid inhibited sterol biosynthesis from [1-14C]acetate in the liver, whereas clofibrate inhibited sterol production. Oral administration of [4-14C]cholesterol to hypercholesterolemic rats indicated that nicardipine had no inhibitory effect on the intestinal absorption of cholesterol.     

Comparative study on the in vivo and in vitro antilipolytic effects of etofibrate, nicotinic acid and clofibrate in the rat

The release of both glycerol and free fatty acids (FFA) into a medium by epididymal fat pad pieces from fed rats incubated in Krebs Ringer bicarbonate-albumin buffer supplemented or not with epinephrine decreased more in the presence of etofibrate than in the presence of equimolecular doses of nicotinic acid or clofibrate. The first drug was the only one to stimulate the rate of fatty acid re-esterification when incubations were done under basal conditions. By 3 h after their acute oral administration all three drugs decreased plasma FFA levels, although the effect from etofibrate was largest, the drugs enhanced or decreased plasma glycerol levels depending on both the dose and the time after treatment. Plasma triglycerides also decreased at 3 h after oral drug administration, and this effect was similar with etofibrate and nicotinic acid but less with clofibrate. With the exception of a decrease at 7 h after the highest dose (1.2 mmol/kg) of either etofibrate or nicotinic acid (but not clofibrate), plasma cholesterol levels remained stable at 7 h after the respective treatments. Thus, the hypocholesterolemic effect of these drugs seems secondary to their hypotriglyceridemic effect, which would be a consequence of their respective antilipolytic actions, and follows an efficiency sequence of etofibrate, nicotinic acid and clofibrate.     

Effect of clofibrate on DNA synthesis in rat liver and kidney

A single dose of clofibrate (400 mg/kg), given to rats, increased the incorporation of (^3H)thymidine into liver DNA, in a period of 20–30 h after administration. However, (^3H)thymidine incorporation into hepatic DNA of rats treated repeatedly was identical to that of control animals. After the administration of a single dose of clofibrate a small increase in (^3H)thymidine incorporation also occurred in kidney DNA; repeated doses, however, resulted in a marked suppression of labeling.Dedicated to Professor Dr. med. Herbert Remmer on the occasion of his 65th birthday     

2,3-Diarylcyclopentenones as orally active, highly selective cyclooxygenase-2 inhibitors

Cyclopentenones containing a 4-(methylsulfonyl)phenyl group in the 3-position and a phenyl ring in the 2-position are selective inhibitors of cyclooxygenase-2 (COX-2). The selectivity for COX-2 over COX-1 is dramatically improved by substituting the 2-phenyl group with halogens in the meta position or by replacing the phenyl ring with a 2- or 3-pyridyl ring. Thus the 3,5-difluorophenyl derivative 7 (L-776,967) and the 3-pyridyl derivative 13 (L-784,506) are particularly interesting as potential antiinflammatory agents with reduced side-effect profiles. Both exhibit good oral bioavailability and are potent in standard models of pain, fever, and inflammation yet have a much reduced effect on the GI integrity of rats compared to standard nonsteroidal antiflammatory drugs.     

Diethyl (4baalpha,4calpha,9aalpha,9balpha)-3,6-dichlorocyclobuta[1,2-b:3,4-b']bisbenzofuran-9a,9b(4bH,4cH)-dicarboxylate: the cis,syn photodimer of ethyl 5-chlorobenzofuran-2-carboxylate, an analogue related to the antilipidemic drug clofibrate.

The antilipidemic properties of diethyl (4balpha,4calpha,9aalpha,4balpha)-3,6-dichlorocyclobutal[1,2-b:3,4-b']bisbenzofuran-9a,9b(4bH,4cH)-dicarboxylate, herein termed dimer 8, were studied in sucrose-fed and in Triton-induced hyperlipidemic rats. Whereas clofibrate (0.4 mmol/kg) exhibited both anticholesterolemic and antitriglyceridemic activity, dimer 8 showed only antitriglyceridemic properties at the lower dose (0.2 mmol/kg) in sucrose-fed rats. Dimer 8 only lowered serum triglycerides levels in Triton WR-1339 hyperlipidemic rats, whereas clofibrate lowered both cholesterol and triglyceride levels. In the chronic sucrose-fed model, dimer 8 and clofibrate lowered hepatic HMG-CoA reducatase activity and produced significant elevations in several parameters of hepatic drug metabolism. No positive relationship between serum cholesterol lowering and reduction of hepatic HMG-CoA reductase activity was observed by these agents in sucrose-fed rats.     

Phenobarbital, beta-naphthoflavone, clofibrate, and pregnenolone-16alpha-carbonitrile do not affect hepatic thyroid hormone UDP-glucuronosyl transferase activity, and thyroid gland function in mice

The effects of representative liver enzyme inducers such as clofibrate (CLO), phenobarbital (PB), pregnenolone-16alpha-carbonitrile (PCN), and beta-naphthoflavone (NF) on hepatic microsomal thyroxin (T4)- UDP-glucuronosyl transferase (UGT) and triiodothyronine (T3)- UGT activities and thyroid function were evaluated in OF-1 male mice after a 14-day po administration. CLO, PB, and PCN induced histological liver hypertrophy, increases in liver weights, in microsomal protein and cytochrome P450 contents as well as increases in specific UGT activities. Despite this, no significant changes in T4-UGT and T3-UGT activities occurred after treatment by any of these compounds. Furthermore, no significant changes in serum T4 and T3 levels were observed and thyroid histology was not affected. NF treatment induced microvacuolation of hepatocytes but did not affect any of the other tested parameters. The results show that, in contrast to the widely described effects in rats, liver enzyme inducers do not affect hepatic thyroid hormone metabolism and thyroid function in mice, suggesting that this species should be less sensitive to thyroid tumor promotion by hepatic microsomal enzyme inducers than rats.