Comparison of lifibrol to other lipid-regulating agents in experimental animals

Mode of action framework analysis for receptor-mediated toxicity: The peroxisome proliferator-activated receptor alpha (PPARα) as a case study

Several therapeutic agents and industrial chemicals induce liver tumors in rodents through the activation of the peroxisome proliferator-activated receptor alpha (PPARα). The cellular and molecular events by which PPARα activators induce rodent hepatocarcinogenesis has been extensively studied and elucidated. This review summarizes the weight of evidence relevant to the hypothesized mode of action (MOA) for PPARα activator-induced rodent hepatocarcinogenesis and identifies gaps in our knowledge of this MOA. Chemical-specific and mechanistic data support concordance of temporal and dose-response relationships for the key events associated with many PPARα activators including a phthalate ester plasticizer di(2-ethylhexyl) phthalate (DEHP) and the drug gemfibrozil. While biologically plausible in humans, the hypothesized key events in the rodent MOA, for PPARα activators, are unlikely to induce liver tumors in humans because of toxicodynamic and biological differences in responses. This conclusion is based on minimal or no effects observed on growth pathways, hepatocellular proliferation and liver tumors in humans and/or species (including hamsters, guinea pigs and cynomolgous monkeys) that are more appropriate human surrogates than mice and rats at overlapping dose levels. Overall, the panel concluded that significant quantitative differences in PPARα activator-induced effects related to liver cancer formation exist between rodents and humans. On the basis of these quantitative differences, most of the workgroup felt that the rodent MOA is "not relevant to humans" with the remaining members concluding that the MOA is "unlikely to be relevant to humans". The two groups differed in their level of confidence based on perceived limitations of the quantitative and mechanistic knowledge of the species differences, which for some panel members strongly supports but cannot preclude the absence of effects under unlikely exposure scenarios.

Lifibrol as a model compound for a novel lipid-lowering mechanism of action

Lifibrol is a potent lipid-lowering drug with an unknown mechanism of action. We investigated its effects on lipoprotein and sterol metabolism in normocholesterolemic male participants. Seven participants were treated for 4 weeks with 600 mg/d lifibrol and 9 with 40 mg/d pravastatin in a double-blind randomized parallel-group trial. Kinetic studies were performed at baseline and under acute and chronic treatment. Turnover of apolipoprotein B-100 was investigated with endogenous stable-isotope labeling, and kinetic parameters were derived by multicompartmental modeling. Lathosterol and cholesterol metabolism were investigated using mass isotopomer distribution analysis (MIDA) after [1-(13)C]acetate labeling. Carbon metabolism was investigated by calculating the total isotope incorporation into newly formed sterols and measuring the disposal of acetate by (13)CO(2) breath analysis. Total- and low-density lipoprotein (LDL) cholesterol decreased by 18% and 27% under lifibrol and by 17% and 28% under pravastatin, respectively, whereas very-low-density lipoprotein (VLDL) cholesterol, triglycerides, and high-density lipoprotein (HDL) cholesterol did not change. Very-low-density lipoprotein apoB fractional synthesis and production increased under lifibrol but remained unchanged under pravastatin. Low-density lipoprotein apoB fractional synthesis and production increased under pravastatin but remained unchanged under lifibrol. Mass isotopomer distribution analysis indicated that both drugs decrease endogenous sterol synthesis after acute administration, but pravastatin had more powerful effects. Carbon-13 appearance in breath was higher during pravastatin than during lifibrol treatment. Mass isotopomer distribution analysis and carbon metabolism analysis indicated compartmentalization at the site of sterol synthesis, thus suggesting differential effects of the 2 drugs. Although having comparable lipid-lowering properties, lifibrol seems to have a mechanism of action distinct from that of statins. Lifibrol could serve as a model compound for the development of new lipid-lowering agents.

Prediction of PPAR-alpha ligand-mediated physiological changes using gene expression profiles

Peroxisome proliferator-activated receptor (PPAR)-alpha controls the transcription of a variety of genes involved in lipid metabolism and is the target receptor for the hypolipidemic drug class of fibrates. In the present study, the molecular and physiological effects of seven different PPAR-activating drugs have been examined in a rodent model of dyslipidemia. The drugs examined were selected to display varying potencies and efficacies toward PPAR-alpha. To help elucidate the link between the gene regulation elicited by PPAR-alpha ligands and the concomitant physiological changes, we have used cDNA microarray analysis to identify smaller gene sets that are predictive of the function of these ligands. A number of genes showed strong correlations to the relative PPAR-alpha efficacy of the drugs. Furthermore, using multivariate analysis, a strong relationship between the drug-induced triglyceride lowering and the transcriptional profiles of the different drugs could be found.

Guinea pigs as models to study the hypocholesterolemic effects of drugs

Guinea pigs are useful models to investigate the mechanisms of the hypocholesterolemic effects of drugs. Like humans, guinea pigs are one of the few species that carry the majority of cholesterol in LDL. This animal model has also been shown to develop atherosclerosis when challenged with hypercholesterolemic diets. In addition, plasma lipid profiles in males, females and ovariectomized guinea pigs, a model for menopause, follow similar patterns to those observed in humans. In this report, drugs aimed at lowering plasma cholesterol and triglycerides in hyperlipidemic individuals are reviewed. Studies analyzing the hypolipidemic effect of HMG-CoA reductase inhibitors, acyl CoA cholesterol acyltransferase inhibitors, fibrates, bile acid resins, apical sodium bile acid transporter inhibitors, and others show that guinea pigs and humans have comparable responses to drug therapy. In addition, results from the limited clinical reports addressing specific effects of drugs on LDL catabolism or VLDL synthesis are in agreement with observations in guinea pigs. From the review of these studies, it is apparent that the guinea pig is a useful animal model to further explore the mechanisms of action of lipid lowering drugs including effects on specific receptors and regulatory enzymes involved in cholesterol metabolism and on early atherosclerosis development.

ABBREVIATIONS

ACAT, acyl-CoA:cholesterol acyltransferase; ASBT, apical sodium co-dependent bile acid transporter; ApoB, apolipoprotein B; CHD, coronary heart disease; CYP7, cholesterol 7alpha-hydroxylase; HDL, high density lipoprotein; HMG-CoA, 3-hydroxy-3-methylglutaryl coenzyme A; FCR, free catabolic rate; LDL, low density lipoprotein; PPAR, peroxisome proliferators-activated receptor; TC, total cholesterol; TG, triglycerides; VLDL, very low density lipoprotein.

Increase in hepatic expression of SREBP-2 by gemfibrozil administration to rats

It is well known that gemfibrozil increases the biliary output of cholesterol and phospholipids, but we have little knowledge about the impact these changes have on liver cholesterol and phospholipid biosynthetic pathways. In the present study, no changes were detected in liver lipids and CTP:phosphocholine cytidylyltransferase after gemfibrozil administration to rats. On the contrary, 3-hydroxy-3-methylglutaryl-CoA reductase mRNA (9.9-fold) and Rd activity (16.7-fold) and phosphatidate phosphohydrolase activity (1.7-fold) increased, while plasma apo B-cholesterol (40%) and triglyceride (43%) levels decreased. As a part of a compensatory homeostatic response, we report for the first time that gemfibrozil administration to rats increased the hepatic sterol regulatory element binding protein-2 (SREBP-2) mRNA (2.9-fold) and mature protein (2.2-fold) levels. An early increase in the transcriptional activity of SREBP-2 elicited by gemfibrozil administration might be responsible for the observed changes in HMG-CoA reductase, phosphatidate phosphohydrolase, and SREBP-2 expression.

Guinea pigs as models for cholesterol and lipoprotein metabolism

Guinea pigs carry the majority of their plasma cholesterol in LDL, making them a unique animal model with which to study hepatic cholesterol and lipoprotein metabolism. In this review, the benefits and advantages of using this particular model are discussed. How dietary factors such as soluble fiber, cholesterol and fatty acids that vary in saturation and chain length affect hepatic cholesterol homeostasis and influence the synthesis, intravascular processing and catabolism of lipoproteins is reviewed. In addition, alterations in hepatic cholesterol metabolism and plasma lipoproteins as affected by treatment with cholestyramine or 3-hydroxyl-3-methylglutaryl coenzyme A reductase inhibitors, exercise, marginal intake of vitamin C, ovariectomy (a model for menopause) and similarities to the human situation are addressed. A review of guinea pigs as models for early atherosclerosis development is also presented.

Effect of lifibrol on the metabolism of low density lipoproteins and cholesterol

Lifibrol is a powerful cholesterol-lowering drug of unknown mechanism of action. This investigation was carried out to determine whether the major action of lifibrol is to enhance clearance of low density lipoproteins (LDL) through the LDL-receptor pathway, and if so, whether the drug exerts its action by altering the excretion of bile acids (acidic steroids), the absorption of cholesterol, or the synthesis of cholesterol. In a first study, in two patients with complete absence of LDL receptors, lifibrol therapy had essentially no effect on plasma LDL concentrations; in two others who had a marked reduction in LDL-receptor activity, response to the drug was attenuated. These findings suggest that lifibrol requires an intact LDL-receptor pathway to exert its action. In a second study, in patients with primary moderate hypercholesterolemia, isotope kinetic studies showed that lifibrol enhanced the fractional catabolic rate of LDL-apolipoprotein B (apo B), but had no effect on transport rates of LDL; these observations likewise support the probability that lifibrol acts mainly to increase the activity of the LDL-receptor pathway. However, in a third study in hypercholesterolemic patients, lifibrol therapy failed to increase acidic steroid excretion, inhibit cholesterol absorption, or reduce net cholesterol balance. Furthermore, lifibrol treatment did not significantly reduce urinary excretion of mevalonic acid. In contrast, in a parallel study, simvastatin therapy, which is known to inhibit cholesterol synthesis, gave the expected decrease in net cholesterol balance and reduction in urinary excretion of mevalonic acid. Thus, lifibrol, like statins, appears to increase the activity of LDL receptors; but in contrast to findings with statins, it was not possible to detect a significant decreased synthesis of cholesterol, either from balance studies or from urinary excretion of mevalonic acid. This finding raises the possibility that lifibrol activates the LDL-receptor pathway through a different mechanisms which remains to be determined.

Lack of predictability of classical animal models for hypolipidemic activity: a good time for mice?

Hypolipidemic drugs that are efficacious in man are not always active in classical animal models of dyslipidemia. Inhibitors of HMG-CoA reductase (statins) do not lower plasma cholesterol in rats, but yet this species was alone in providing activity for fibrate-type drugs. Nicotinic acid possesses many desirable features with regard to clinical use, but most of these actions are lacking in rats and monkeys. The metabolism of low density lipoproteins in hamsters is widely thought to be similar to that in humans, yet neither statins or fibrates lower plasma lipids in these species. With the advent of mouse models expressing specific human genes (or disruption of genes) it is now possible to re-examine the effect of established drugs and to characterize new hypolipidemic compounds with respect to site and mechanism of action. Drug responses observed in humans are now being seen in such mouse models (e.g. HDL elevation with fenofibrate in mice with the human apo A-I gene). Moreover, mice are now being screened for compounds that lower plasma (human) Lp(a), or lower plasma cholesterol in the absence of LDL receptors. It is proposed that these new genetic mouse models may afford a more focused examination of drug action and provide, for new compounds, better prediction of the human response.

Opposite effects of bezafibrate and gemfibrozil in both normal and hypertriglyceridemic rats

Chow and sucrose-fed rats were used as animal models to study the dose-responses of bezafibrate and gemfibrozil in normolipidemic and hypertriglyceridemic states, respectively. Although both drugs lowered plasma triglycerides (TG) to about the same extent in chow-fed rats, gemfibrozil lowered liver TG as well as plasma total and LDL-cholesterol (LDL-C), but elevated HDL-cholesterol (HDL-C) and plasma apo E concentrations. Bezafibrate produced opposite effects, namely, decreased HDL-C, apo E and liver TG, and tended to increase LDL-C. TG lowering for both drugs in chow-fed rats was not due to changes in TG secretion (production) in normal rats but was associated with enhanced LPL activity. In hypertriglyceridemic rats both drugs modestly reduced TG secretion rates about 40% at a dose producing maximal TG lowering, but again, gemfibrozil elevated and bezafibrate lowered HDL-C and apo E. Unlike gemfibrozil, bezafibrate induced the appearance of LDL-C in hypertriglyceridemic rats which was not detected in control animals, and also tended to increase rather than decrease plasma apo B levels. Finally, changes in liver TG concentration (mg/g) in hypertriglyceridemic rats were opposite for these drugs, resulting in significant drug-related differences in liver TG content (mg/organ). From these data we postulate that, although similar with regard to TG lowering activity and mechanisms thereof, gemfibrozil and bezafibrate produce fundamentally different effects on LDL, HDL and apolipoprotein metabolism (apo B and apo E) in rats which may relate to potential differential effects on reverse cholesterol transport and atherogenesis.

Hepatic microsomal induction profile of carbamic acid [[2,6-bis(1- methylethyl)phenoxy] sulfonyl]-2,6-bis(1-methylethyl) phenyl ester, monosodium salt (PD138142-15), a novel lipid regulating agent

Induction of hepatic microsomal cytochrome P450 produced by carbamic acid [2,6-bis(1-methylethyl)phenoxy]sulfonyl]-2,6-bis(1-methylethyl) phenyl ester, monosodium salt (PD138142-15), a novel water-soluble inhibitor of acyl-CoA: cholesterol acyltransferase, was examined in male and female rats, dogs, and monkeys, and in male guinea pigs. Relative to control, PD138142-15 increased hepatic microsomal total spectral P450 in all species examined. Hepatic microsomal ethoxyresorufin-O-deethylase, pentoxyresorufin-O-dealkylase, and peroxisomal carnitine acetyltransferase activities and cyanide-insensitive Beta-oxidation were affected only marginally. Erythromycin-N-demethylase activity was increased (2- to 6-fold) in all three species in which it was examined (rat, dog and pig). Marked increases in immunoreactive P450 3A were noted in the rats and dogs, while slight increases were seen in monkeys. Pharmacokinetic studies of PD138142-15 in rats and dogs revealed pronounced decreases (80-90%) in plasma Cmax and AUC within 2 weeks of initiation of daily dosing. In spite of the marked decline in plasma drug levels, efficacy in dogs, as determined by serum cholesterol levels, was maintained for up to 6 weeks with continued dosing. Potential acid (gastric) breakdown products of PD 138142-15 were examined for their hepatic cytochrome P450 induction profiles in rats adn were found to differ both quantitatively and qualitatively from profiles produced by the parent compound. This suggested that induction observed in rats was due to parent PD138142-15 and not to any of the known potential acid breakdown products. The cumulative data establish that PD 138142-15 is an inducer of P450 3A in rats and dogs. The results also suggest that P450 3A is induced in monkeys and pigs as well, although the data are less definitive. Decreases in plasma drug levels imply that the compound may be an autoinducer in dogs and rats. The maintenance of efficacy in spite of decreased drugs levels in dogs suggests that the effects on serum cholesterol are due to a metabolite or that cholesterol lowering effects occur before the compound is metabolized by the liver.

New targets for lipid lowering and atherosclerosis prevention

The effectiveness of plasma lipid lowering in the clinic is well supported by a growing number of contributions, indicating the significant improvement in cardiovascular risk in primary and particularly in secondary prevention. While these studies have clearly indicated that the more potent agents for cholesterol reduction can provide a very effective help, other pathways of lipid metabolism have gained interest. These should be evaluated, in the hope of providing a more complete answer to the question of regulating lipid absorption, distribution, and tissue deposition. In addition to newer more potent systemic lipid-lowering drugs (in particular hydroxymethylglutaryl coenzyme A reductase inhibitors), nonsystemic agents, including cholesterol sequestrants, are receiving attention. Some of these are effective at low concentrations, thus providing a potentially powerful tool for plasma cholesterol regulation. Another area of development is that of acyl coenzyme A cholesterol acyltransferase inhibitors, i.e., drugs interfering with cholesterol esterification in tissues, particularly in the arterial wall; the major problem with these seems to be that of poor tolerability and of lack of definitive proof of plasma cholesterol reduction in humans. At present, drugs for the treatment of elevated lipoprotein(a) levels are not available, with few exceptions; in this case, a better understanding of the regulation of lipoprotein(a) metabolism and of the potential benefit of treatment seems necessary. Elevation of congenitally low high density lipoprotein cholesterol levels may also be an important target: microsomal enzyme inducers have been tested, but have not provided a clinically significant response; drugs with a mixed endocrine-hypolipidemic activity possibly may prove effective. Other targets, e.g., the correction of the lipoprotein pattern characterized by "small low density lipoprotein," and the development of drugs specifically acting on the cholesteryl ester transfer protein and lipoprotein lipase systems, are being explored. Finally, new areas of development are in recombinant apolipoproteins (apo's) and in gene therapy. One case, i.e., that of apo A-I/HDL, is entering the clinical field; the mutant apo A-IMilano might provide help because of a combined cholesterol removing/fibrinolytic activity. In the case of gene therapy, at present, data on low density lipoprotein receptor replacement are encouraging. Further options, such as gene transfer in the arterial wall to induce vascular protection/disobliteration of occlusions, are also being tested.