Effects of etofibrate, clofibrate and nicotinic acid on lipid metabolism in normolipemic rats

Effect of etofibrate on bile production in the normolipidemic rat

1. The effect of etofibrate, the ethandiol-1,2 diester of nicotinic and clofibric acids on bile production was studied in male rats that received a daily dose of 300 mg of etofibrate/kg body weight by stomach tube for 10 days and were compared with control rats receiving the medium. 2. The bile duct was cannulated, animals were intravenously given 1 microCi (4-14C)-cholesterol/100 b.w. and bile was collected at different intervals for a total of 4 hr. 3. Etofibrate treatment decreased plasma cholesterol and triglyceride concentrations and increased the bile flow. The cummulative amount of both bile volume and total bile radioactivity secreted increased linearly in all the animals; the respective slopes being higher in etofibrate treated rats than in controls. 4. The main labelled component found in the bile was always bile acids rather than cholesterol and the proportion of each of these compounds was similar in both groups. Neither was any difference between the groups found in the concentration of bile acids, cholesterol and phospholipids nor in the cholesterol/(bile+phospholipid) ratio. 5. Besides other factors, the present results indicate that an increase in bile flow and biliary cholesterol excretion in its free form and after its conversion into bile acids should contribute to the hypocholesterolemic effect of etofibrate.

Etofibrate increases binding of low and high density lipoprotein to human platelets of patients with type II hyperlipoproteinemia

Previous work suggested an influence of etofibrate, a diester of nicotinic acid and clofibric acid, on lipoprotein receptors. Besides its beneficial effects on plasma lipoprotein levels of decrease in total cholesterol, LDL-cholesterol and triglycerides and increase in HDL-cholesterol, etofibrate was shown to inhibit platelet function. In order to further evaluate platelet-lipoprotein interactions, the effects of etofibrate on plasma lipids and lipoproteins on the specific binding of normal [111In]LDL and [111In]HDL onto platelets as well as its effect on platelet function were evaluated in 8 patients affected by Type II hyperlipoproteinemia (HLP). In all patients binding was saturable and indicated high affinity binding sites capable of binding 927 +/- 233 ng protein of [111In]LDL/10(9) platelets (Kd 12 +/- 3 micrograms protein/ml) and 1496 +/- 435 ng protein of [111In]HDL/10(9) platelets (Kd 14 +/- 3 micrograms protein/ml). The capacity of native LDL (HDL) to displace bound [111In]LDL ([111In]HDL) by half (IC50) amounted to 22 +/- 9 micrograms protein/ml (26 +/- 8 micrograms protein/ml). Following a 6-week treatment period with etofibrate (500 mg twice daily), decrease in plasma total cholesterol, LDL-cholesterol and apolipoprotein (apo) B and increase in HDL-cholesterol and apo AI was correlated to a significant (P < 0.01) increase in LDL- as well as HDL-receptor binding. The platelet binding capacity increased to 1085 +/- 212 ng protein/10(9) platelets (Kd 8 +/- 3 micrograms protein/ml) for [111In]LDL and to 1867 +/- 266 ng protein/10(9) platelets for [111In]HDL (Kd 11 +/- 3 micrograms protein/ml). Platelet function studies demonstrated significantly (P < 0.01) reduced platelet aggregation in response to ADP and thromboxane formation after 6 weeks of etofibrate therapy. These findings in patients with HPL Type II indicate in vivo upregulation of specific [111In]LDL as well as [111In]HDL binding sites on human platelets associated with reduced platelet activation following etofibrate therapy.

Studies with etofibrate in the rat. Part II: A comparison of the effects of prolonged and acute administration on plasma lipids, liver enzymes and adipose tissue lipolysis

To contribute to the understanding of the hypolipidemic action of etofibrate, which is the 1,2-ethandiol ester of clofibric acid and nicotinic acid, 300 mg of this drug/kg body weight or of the medium were administered daily by a stomach tube to normolipidemic rats. Some animals were decapitated at the 10th day of daily treatment (prolonged treatment), whereas others were studied at different times after one single administration (acute treatment). In animals on prolonged treatment etofibrate decreased plasma levels of cholesterol, triacylglycerols, free fatty acids (FFA) and glycerol, as well as the total and unesterified cholesterol concentrations, in liver microsomes. In these rats, etofibrate increased the activity of liver cytosolic glycerol-3-P dehydrogenase, whereas it decreased the activity of both microsomal HMG-CoA reductase and cholesterol 7 alpha-hydroxylase and did not affect acyl-CoA: cholesterol acyltransferase (ACAT). At 3, 5 and 7 h after acute treatment, etofibrate decreased plasma levels of triacylglycerols, glycerol and FFA, and this effect disappeared at 24 h, whereas plasma cholesterol did not change 3 h after etofibrate but decreased at 5 and 7 h and remained low after 24 h, and a similar change was found in the liver microsomes free cholesterol concentration. However, with the exception of a significant reduction in cytosolic glycerol-3-P dehydrogenase at 7 h and in ACAT at 5 h, acute etofibrate treatment did not affect the activity of the liver enzymes studied. At low concentrations (10(-5) M) in the incubation medium, etofibrate decreased the release of both FFA and glycerol by epididymal fat pad pieces incubated in vitro. These findings together with those previously reported by us in rats using a similar etofibrate treatment protocol [6] indicate that etofibrate decreases the availability of lipolytic products in the liver by acting on their release from adipose tissue and on their intrinsic hepatic metabolism. Consequently, this drug would decrease liver VLDL triacylglycerol synthesis and secretion, which together with facilitating the clearance of circulating triacylglycerols causes its hypotriglyceridemic effect. The hypocholesterolemic effect of etofibrate after acute treatment may be a secondary consequence of the reduced liver VLDL production caused by decreased adipose tissue lipolysis, but after prolonged treatment, this effect also seems to be influenced by the inhibition of HMG-CoA reductase activity which would reduce cholesterol synthesis.

The effect of etofibrate on cholesterol and bile acid metabolism in the hamster

Hamsters were given etofibrate at a dose of 300 mg/kg body wt, by gavage for 5 days, while being fed a chow diet. After treatment, serum cholesterol levels were 27% lower compared to those of the control animals. A similar trend was observed for triglyceride levels. Hepatic lipid levels were unchanged by the treatment. HMG-CoA reductase and acylCoA cholesterol acyltransferase were decreased while cholesterol 7 alpha-hydroxylase was not significantly modified by etofibrate. A choleretic effect and an increase of cholesterol excretion into hepatic bile was observed in treated animals. Nevertheless, composition and cholesterol saturation index of gallbladder bile were similar in control and treated animals. With respect to controls, hepatic bile of treated hamsters contained a lesser amount of cholic and deoxycholic acid and a greater amount of ursodeoxycholic acid.

Studies with etofibrate in the rat. Part I: Effects on glycerol, free fatty acid and triacylglycerol metabolism

Etofibrate is the 1,2-ethandiol diester of clofibric acid and nicotinic acid that decreases circulating levels of triacylglycerols and cholesterol. To understand the mechanism by which the drug affects plasma triacylglycerols, normolipemic rats were treated daily with 300 mg of etofibrate/kg body weight or with the medium by a stomach tube. They were decapitated on the 10th day, and showed lower levels of plasma beta-hydroxybutyrate, glycerol, free fatty acids (FFA), total triacylglycerols and cholesterol and VLDL triacylglycerols and cholesterol, whereas glucose and RIA-determined insulin levels were unmodified. Epididymal fat pad pieces from etofibrate-treated rats incubated in vitro released more glycerol but the same amount of FFA to the medium, and had greater uptake of [U-14C]glycerol for [14C]acylglycerol formation. In the presence of heparin, they also showed an enhanced release of lipoprotein lipase activity to the medium. The disappearance from plasma of intravenously administered [1-14C]palmitate was faster in the etofibrate-treated rats, and although they showed a decrease in 14C-esterified fatty acids of neutral lipids in both liver and plasma VLDL, there was an increase in liver 14C-labelled water-soluble components. After intravenous [U-14C]glycerol administration, there was a decrease in plasma VLDL [14C]acylglycerol and [14C]glucose and in liver [14C]acylglycerol, but an increase in plasma [14C]lactate. In the liver, etofibrate treatment heightened the cytosolic glycerol-3-phosphate dehydrogenase activity and the total carnitine concentration, whereas it reduced triacylglycerol and cholesterol concentrations. It is proposed that etofibrate enhances the reesterification of fatty acids and glycerol in adipose tissue, which, together with its augmented lipoprotein lipase activity, may facilitate the clearance of circulating triacylglycerols. These effects may act concomitantly with the decreased synthesis of triacylglycerols, secondary to the increased utilization of their precursors, acyl-CoA and glycerol-3-phosphate, in other pathways, causing the reduction of plasma VLDL triacylglycerols produced by etofibrate treatment.

Further assessment of glunicate hypolipidaemic activity in the rat

The hypolipidaemic activity of glunicate administered orally to rats has been further evaluated. In the fasted normolipidaemic rat, glunicate, in single doses of 10-300 mg kg-1 (as nicotinic acid equivalents), exerted a dose-dependent reduction of plasma triglycerides and cholesterol. At the lower doses, the HDL: total cholesterol ratio was enhanced, but at higher doses it was depressed. Similar results were obtained in the fasted obese Zucker rat, but the acute effects of glunicate seemed less marked in this strain. When the same doses of glunicate were given for 7-8 consecutive days no alteration of the plasma lipid pattern was observed, the reason being that the hypocholesterolaemic action of single doses of glunicate is reduced or abolished in fed animals; this was not due to pharmacokinetic factors depending on feeding. The same lack of hypocholesterolaemic action in fed animals occurred with nicotinic acid.

Effect of clofibrate on peroxisomal and mitochondrial beta-oxidation in chicken liver

The effect of a 0.25% clofibrate diet for 2 weeks on peroxisomal and mitochondrial beta-oxidation in chicken liver was studied. The activities of antimycin antimycin A-insensitive palmitoyl-CoA oxidation (peroxisomal beta-oxidation) and carnitine acetyltransferase increased about two-fold. The activities of palmitoyl-CoA-dependent O2 consumption (mitochondrial beta-oxidation) and carnitine palmitoyltransferase were also slightly activated by the administration of clofibrate, but not significant. Thus, clofibrate may be a typical drug which activates the peroxisomal beta-oxidation more than the mitochondrial one in various species. The effect of clofibrate on peroxisomal carnitine acetyltransferase was the same as that on the mitochondrial one in chicken liver. Serum lipids were not lowered, but hepatomegaly was observed in the present experiment with chicken.

Hypolipidemic effects of clofibrate and selected chroman analogs in fasted rats: I. Chow-fed animals

The hypolipidemic properties of ethyl 6-chlorochroman-2-carboxylate (II), ethyl 6-phenylchroman-2-carboxylate (III) and ethyl 6-cyclohexylchroman-2-carboxylate (IV) were compared to clofibrate (I) in fasted normolipidemic rats. The chroman analog II, like its parent compound, clofibrate, reduced serum and alpha-lipoprotein cholesterol concentrations. Although analog III had no effect on serum cholesterol, it caused a slight elevation of alpha-lipoprotein cholesterol concentration. Serum free cholesterol was increased and LCAT activity was reduced in clofibrate-treated rats. The hypolipidemic agents had no consistent effect on liver lipid concentrations and liver microsomal HMG-CoA reductase activity. In addition, we have shown that drug efficacies varied directly with seasonal variations in serum lipid concentrations.