Metabolic disposition of antihyperlipidemic agents in man and laboratory animals

A novel prodrug strategy for extremely hydrophobic agents: conjugation to symmetrically branched glycerol trimer improves pharmacological and pharmacokinetic properties of fenofibrate

Management of a lipophilic-hydrophilic balance is a key element in drug design to achieve desirable pharmacokinetic characters. Therefore we have created unique modular molecules, symmetrically branched oligoglycerols (BGL), as an alternative way to endow hydrophobic molecules with sufficient hydrophilicity. We have successfully demonstrated amelioration of the water solubility and thermal stability of several hydrophobic agents by covalent conjugation to BGL so far. However, it has not been clarified whether the molecular modification by BGL also improves the pharmacological and/or pharmacokinetic properties indeed. Recently, we synthesized a novel BGL-prodrug derivative of fenofibrate, which is an antihyperlipidemic agent and one of the most hydrophobic medicinal compounds currently used clinically, by conjugating fenofibric acid to symmetrically branched glycerol trimer (BGL003), the simplest BGL. We have previously demonstrated that the hydrophilicity and water solubility of fenofibrate are improved more than 2000 times just by conjugation to the BGL003. To verify our hypothesis that the prodrug strategy with BGL should improve pharmacological efficacy and pharmacokinetic properties of extremely hydrophobic agents such as fenofibrate by the rise in hydrophilicity, we evaluated the BGL003-prodrug derivative of fenofibrate (FF-BGL) using rodent models. Here we demonstrate that the lipid-lowering effects of fenofibrate are much potentiated by chemical conjugation to BGL003 without exhibiting significant toxicity. Plasma concentration of fenofibric acid, an active metabolite of fenofibrate, after single oral administration of FF-BGL was more than 3 times higher than that of fenofibrate, in accordance. In fasting rats, plasma concentration of fenofibric acid after fenofibrate administration was curtailed into less than half of that in ad libitum-fed rats, while FF-BGL showed about the same plasma level even in the starving rats. This is the first report showing that BGL-prodrug improves pharmacological and pharmacokinetic properties as well as hydrophilicity of highly hydrophobic compounds. Furthermore, prodrug strategy using BGL suggests the possibility of diminishing the food-drug interaction effects, which should be advantageous for promoting drug compliance. BGL will be a suitable prodrug strategy to ameliorate physical, pharmacological, and pharmacokinetic characteristics of extremely hydrophobic compounds.

Effects of clofibric and beclobric acid in rat and monkey hepatocyte primary culture: influence on peroxisomal and mitochondrial beta-oxidation and the activity of catalase, glutathione S-transferase and glutathione peroxidase

The effect of hypolipidaemic compounds on peroxisomal fatty acid beta-oxidation and on peroxisome morphology in the liver differs widely between rodent and primate species. We studied the relative importance of peroxisomal and mitochondrial beta-oxidation of palmitate in primary cultures of hepatocytes isolated from rat and monkey liver in the absence or presence of clofibric acid or beclobric acid. It was demonstrated that it is possible to differentiate between peroxisomal and mitochondrial beta-oxidation activities in intact cells. Overall beta-oxidation of palmitate was ca. 30% higher in rat hepatocytes than in monkey liver cells. In both monkey and rat cell cultures the mitochondrial component was over 90% of the total palmitate beta-oxidation. In rat hepatocyte culture clofibric acid and beclobric acid caused a 5- to 8-fold stimulation of peroxisomal beta-oxidation, while in monkey cells this activity was not significantly increased. However, in cells derived from both species mitochondrial palmitate beta-oxidation was increased (rat 2.5-fold; monkey 1.5-fold). These results indicate that the species differences in the increase in peroxisomal fatty acid oxidation are not a result of an inability to metabolize fatty acids in rat liver cell mitochondria. A comparison of the activity of enzymes involved in the detoxification of hydrogen peroxide showed that catalase and glutathione-S-transferase activity is 2.9-fold higher in monkey hepatocytes than in rat liver cells, while glutathione peroxidase activity was 1.6-fold higher in rat cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduction of rat liver endoplasmic reticulum Ca(2+)-ATPase activity and mobilization of hepatic intracellular calcium by ciprofibrate, a peroxisome proliferator

Ciprofibrate, a peroxisome proliferating agent, induces cell proliferation in rodent liver during the early periods of exposure. Since Ca2+ plays an important role in mitogenesis, we have investigated the effects of ciprofibrate on hepatic endoplasmic reticulum (ER) Ca(2+)-ATPase, which in part regulates Ca2+ homeostasis. A single oral dose of 200 mg/kg ciprofibrate to male F344 rats produced a transient decrease in liver microsomal Ca(2+)-ATPase activity to 48% of control levels at 24 hr post-exposure. Activity had returned to control levels by 48 and 72 hr after exposure. The decrease in Ca(2+)-ATPase activity was not a function of non-specific enzymatic inhibition, since activity of another microsomal enzyme, glucose-6-phosphatase, was not altered in ciprofibrate-exposed rats. Using an ATP-driven 45Ca2+ accumulation assay, rats exposed to 25, 100 and 200 mg/kg ciprofibrate exhibited a dose-dependent inhibition of liver microsomal Ca2+ accumulation at 24 hr post-exposure. Analysis of Western immunoblots using a polyclonal antibody to the liver ER Ca(2+)-ATPase revealed a marginal increase in Ca(2+)-ATPase protein content in microsomes prepared from ciprofibrate-exposed rats compared to controls 24 hr post-exposure. These data indicate that the reduction of Ca(2+)-ATPase activity is not attributable to diminished Ca(2+)-ATPase protein content in vivo and, therefore, is due to a functional inhibition of the enzyme. Ciprofibrate also produced a concentration-dependent inhibition of rat liver ER Ca(2+)-ATPase activity in vitro (IC50 approximately 170 microM). In freshly isolated rat hepatocytes, ciprofibrate elevated the free intracellular calcium concentration ([Ca2+]i) in the presence and absence of extracellular calcium. Collectively, these results suggest that ciprofibrate mobilizes hepatic [Ca2+]i via inhibition of the ER Ca(2+)-ATPase. These events may lead to an environment of elevated [Ca2+]i during the early stages of ciprofibrate exposure and may serve to augment Ca(2+)-dependent processes, thus playing a pivotal role in the acute mitogenic response.

Influence of single and concurrent clofibrate and phenobarbital administration on cytochrome P450-dependent mixed function oxidase activities and peroxisome proliferation in male rat liver

The influence of both single and concurrent administration of phenobarbital and clofibrate on hepatomegaly, cytochrome P450-dependent mixed function oxidase activities, and peroxisome proliferation in male rat liver have been studied. Both xenobiotics separately increase the liver: body weight ratio and their combined administration results in greater hepatomegaly than either compound alone. Both compounds induce NADPH-cytochrome c(P450) reductase activity and laurate omega- and omega-1-hydroxylase activities, but only phenobarbital induces pentoxyresorufin-O-dealkylase. None of the drug treatments induced microsomal cytochrome b5. Phenobarbital did not cause peroxisome proliferation and inhibited the corresponding clofibrate-dependent proliferation. Taken collectively, our studies have demonstrated that concomitant treatment with phenobarbital and clofibrate are largely permissive with respect to the hepatic mixed function oxidase system but have opposing effects on the phenomenon of peroxisome proliferation in the same tissue.

The importance of pharmacokinetic and receptor studies in drug safety evaluation

The importance of pharmacokinetic and receptor studies in the preclinical and clinical safety evaluation of candidate drugs is reviewed with reference to a number of recently developed drugs. Different aspects of the relationships between pathways of metabolism, pharmacokinetics, receptor interactions, and drug toxicity are illustrated. The failure of animal toxicity studies to predict drug toxicity in humans, due to species differences in metabolism and pharmacokinetics, is illustrated by reference to the anti-inflammatory antiviral terpenoid carbenoxolone, the antiasthmatic candidate drug FPL 52757, and the cardiotonic drug amrinone. The false prediction of adverse effects in man from toxicity manifested in experimental animals, due to species differences in pharmacokinetics or receptor activities, is exemplified with reference to the antiepileptic valproic acid, the hypolipidemic drug ciprofibrate, the antipeptic ulcer drug, omeprazole, and the progestogen lynestrenol. Finally, the importance of adequate, repeat-dose, clinical pharmacokinetic studies in patients as distinct from healthy volunteers to evaluate any effect of the disease state, in the elderly and the young to examine the effects of age, and in sufficiently large populations to detect genetic anomalies and idiosyncrasies is illustrated by reference to the anti-rheumatoid drug benoxaprofen, the antiangina drug perhexiline, and the diuretic tienilic acid.

The comparative pharmacokinetics and gastric toxicity of bezafibrate and ciprofibrate in the rat

1. The comparative gastric toxicology and pharmacokinetics of two phenoxyisobutyrate derivatives have been evaluated in the Fischer rat. 2. After oral administration of single daily doses for 7 days, the plasma elimination half-life for bezafibrate was rapid (t1/2 of 4-5 h) in comparison to ciprofibrate (t1/2 of 76 h). 3. The area under the plasma drug concentration versus time curve (AUC) 0-24 (micrograms.h/ml +/- SD) for bezafibrate (dose 125 mg/kg per day) was 1553 +/- 334, which was less than half the value of 3748 +/- 358 achieved by ciprofibrate (10 mg/kg per day) after 7 days. 4. Oral administration of ciprofibrate at 10 mg/kg every 48 h produced similar sustained plasma concentrations to those achieved by bezafibrate 125 mg/kg dosed every 12 h. The AUC 0-48 values (micrograms.h/ml +/- SD) achieved were 5124 +/- 450 for bezafibrate compared to 4207 +/- 240 for ciprofibrate. 5. In chronic oral multidose studies with ciprofibrate and bezafibrate, similar gastric toxicity (neuroendocrine cell hyperplasia) occurred in the rat when dose regimens were adjusted to compensate for the pharmacokinetic differences between these two drugs.

Assessment of gastric antisecretory effects of phenoxyisobutyrate derivatives in the rat and the mouse

After the observation of a low incidence of gastric carcinoid tumours in rats, but not in mice, given ciprofibrate for 2 years, ciprofibrate and related compounds were investigated for gastric antisecretory activity. A significant inhibition of gastric secretion, similar to that induced by comparable doses of cimetidine, was observed in the fischer rat 1.5 h after a single oral (200 or 500 mg kg-1) or intraduodenal (100 or 300 mg kg-1) administration of ciprofibrate, bezafibrate, and clofibric acid. Ciprofibrate had prolonged antisecretory activity when compared with bezafibrate or ranitidine. Prolonged inhibition of gastric secretion is proposed as the primary cause of gastric carcinoids in the rat, since in a comparative evaluation, antisecretory activity was observed in the rat but not in the mouse.

Species variation in gastric toxicity following chronic administration of ciprofibrate to rat, mouse, and marmoset

Comparative oral toxicity studies with ciprofibrate have been undertaken in the mouse, rat, and marmoset for up to 26 weeks. Chronic administration of ciprofibrate (20 mg/kg/day) produced a prolonged, modest, but statistically significant hypergastrinemia in the rat. Morphological changes in the rat stomach included increased eosinophilia and hypertrophy of oxyntic cells after 2 or more weeks treatment and hyperplasia of the neuroendocrine (NE) cells after 8 weeks treatment. In contrast, only a transient hypergastrinemia was induced, but not sustained in the mouse at the same dose level over an 8-week time period. No morphological changes were detected in the stomach of this species. In the marmoset treatment, up to 80 mg/kg/day for 26 weeks failed to induce hypergastrinemia and no significant alterations in gastric NE cells were detected.

Disposition of xenobiotics: species differences

Species differences are a major obstacle in predicting toxicity of xenobiotics from one species to another. Species differences in toxicity of drugs and other chemicals may be due to differences in pharmacokinetics or pharmacodynamics. This paper illustrates the point that species differences in pharmacokinetics of xenobiotics may be the result of differences in any of the processes contributing to the disposition of a xenobiotic. This is an important point because biotransformation as a cause of species differences for the disposition of xenobiotics has been overemphasized in the past, whereas only scant attention has been paid to the other 3 major contributing processes to disposition, viz. absorption, distribution, and excretion. This brief overview presents a balanced examination of all 4 major processes (absorption, distribution, biotransformation and excretion) as they affect the pharmacokinetics of xenobiotics in various species.

Induction of peroxisomal fatty acyl-CoA oxidase and microsomal laurate hydroxylase activities by beclobric acid and two metabolites in primary cultures of rat hepatocytes

Beclobrate [2-(4-[(4-chlorophenyl)methyl]phenoxy)-2-methylbutyric acid ethyl ester], a structural analog of clofibrate, is used clinically as a lipid-lowering agent. Although, like clofibrate, beclobrate produces a profound hepatomegalic response in rodents, no studies of this drug on hepatic peroxisome proliferation have appeared. We have examined, relative to clofibric acid (CPIB), the concentration-dependent effects of beclobric acid (Beclo), the activity moiety of beclobrate, and two oxidized metabolites [a carbinol (M2) and a benzophenone (M3)] on peroxisomal fatty acyl-CoA oxidase (FACO) and microsomal laurate hydroxylase (LH) activities in primary cultures of rat hepatocytes. All compounds induced FACO and LH activities in a concentration-dependent manner after a 72 hr incubation with the cultured cells. Beclo was 4.8- and 6.5-fold more potent than CPIB as an inducer of FACO and LH respectively. M2 and M3 were more potent than Beclo as inducers of FACO and LH. Additionally, all compounds produced significant elevations relative to untreated control cultures in cellular lactate dehydrogenase activity (1.6- to 2.2-fold). We conclude that (1) Beclo is more potent than CPIB as an inducer of peroxisome proliferation-associated enzyme activities; (2) two metabolites of Beclo are more potent than the parent molecule as inducers of these activities and (3) these metabolites may contribute to the lipid-lowering and/or hepatomegalic effects of beclobrate in rats.

Effect of clofibric acid on the molecular species composition of diacyl glycerophosphocholine of rat liver microsomes

The effect of administering p-chlorophenoxyisobutyric (clofibric) acid to rats on the molecular species composition of diacyl-glycerophosphocholine (GPC) of rat liver microsomes was studied. Microsomal choline glycerophospholipids were converted to 1,2-diradyl-3-acetylglycerol and were separated into molecular species by reverse-phase high performance liquid chromatography. Diacyl-GPC consisted of 17 different molecular species. The predominant species were arachidonoyl derivatives, such as 18:0-20:4 (22.2% of the total) and 16:0-20:4 (22.0%). Administration of clofibric acid to rats caused a marked increase in 16:0-18:1 species of diacyl-GPC from 8% to 30%, making these the predominant species of diacyl-GPC in clofibric acid-fed rats. Also, a significant decrease (50% of controls) in 18:0-18:2 and 18:0-20:4 species was observed, whereas the decrease in molecular species containing 16:0 at the 1-position such as 16:0-18:2 and 16:0-20:4 was small (approximately 85% of control). The results show that clofibric acid caused marked changes in the molecular species composition of diacyl-GPC. The participation of 1-acyl-GPC acyltransferase and stearoyl-CoA desaturase in the regulation of the molecular species composition of diacyl-GPC is discussed.

Antihyperlipidemic properties of beta-pyridylcarbinol. A review of preclinical studies

The available preclinical literature on the antihyperlipidemic properties of beta-pyridylcarbinol is reviewed. Similarities between the pharmacological profiles for beta-pyridylcarbinol and nicotinic acid, and evidence for the metabolic conversion of beta-pyridylcarbinol to nicotinic acid are discussed. Several reviews discussing the antihyperlipidemic effects of beta-pyridylcarbinol (beta-PC, nicotinyl alcohol, Roniacol) and nicotinic acid (NA) have appeared during the last 15 years (1-6). However, continuing clinical interest in the ability of nicotinic acid analogs to reduce plasma lipids indicated that an update and critical evaluation of the preclinical literature on this subject would be of value in order to permit a more complete assessment of the relevance of several animal models to effects in human subjects. The literature reviewed included (a) preclinical studies of beta-PC where it was the sole compound examined; (b) comparative studies of beta-PC and NA; and (c) studies relating to the metabolism of beta-PC. The literature chosen included experiments involving fasted animals, satiated animals, and effects of Triton-induced hyperlipidemia. Data on other pharmacological properties of beta-PC and/or NA that might contribute to antihyperlipidemic efficacy (e.g., fibrinolysis, inhibition of platelet aggregation, erythrocyte membrane changes) were also included where available.

Effect of the antilipolytic compound acipimox on peroxisome marker enzymes, lipid pattern and biotransformation related functions in rat liver

The antilipolytic drug acipimox (5-methylpyrazine-2-carboxylic acid 4-oxide) was given to male rats for 1 week at 500, 1000 and 2000 mg/kg/day and for 2, 6 and 7 months at 20, 100 and 500 mg/kg/day. The peroxisome proliferative effect was evaluated determining the activity of catalase and carnitine acetyltransferase, the rate of cyanide-insensitive palmitoyl CoA oxidation and the electrophoretic profile of liver polypeptides. Hepatic lipid content and distribution were evaluated after 2 and 6 months' treatment. The effect on liver detoxificating function was evaluated by assaying glutathione, cytochrome P-450, glutathione-S-transferase and glutathione-reductase activities after 7 months' treatment. Sub-acute and chronic treatment with a wide range of acipimox doses did not cause hepatomegaly, liver peroxisome proliferation or liver steatosis and did not change some important biochemical variables related to detoxification and biotransformation mechanisms. Acipimox given to rats does not have the negative side-effects of other compounds and seems a safe blood lipid lowering drug.

Effect of AY-25,712 on fatty acid metabolism in rats

The effect of AY-25,712 [2-methyl-2-phenyl-3(2H)-furanone-5-carboxylic acid] on various aspects of free fatty acid (FFA) and triglyceride metabolism was studied in male rats. Serum triglycerides were lowered by a single oral dose of AY-25,712 or nicotinic acid, but not of clofibrate. Unlike with clofibrate, when AY-25,712 or nicotinic acid was given in the diet, serum triglycerides were not affected. In vitro, both AY-25,712 and nicotinic acid suppressed the theophylline-induced FFA release by epididymal fat pads, but had no effect on lipolysis induced by norepinephrine. Both AY-25,712 and nicotinic acid enhanced the activity of adipose tissue lipoprotein lipase. The initial decrease in plasma FFA and triglycerides, and in liver triglycerides after a single oral dose of nicotinic acid was followed by a rebound to levels which, at later time intervals, wee significantly higher than in controls. AY-25,712 was more potent than nicotinic acid in lowering plasma FFA and triglycerides as well as liver triglycerides, but produced no such rebound effect. The data show that, except for the absence of this rebound effect, the mode of action of AY-25,712 in rats resembles that of nicotinic acid and differs from that of clofibrate.