Hypolipidemics Carcinogenicity and Extrapolation of Experimental Results for Human Safety Assessments

Dyslipoproteinemias represent a group of disorders closely related to alterations of cholesterol and triglycerides. The alterations of these lipids are considered important risk factors in coronary heart disease and indicate the need for clinically effective and safe drugs.

Hypolipidemic agent therapy, however, does not appear without risk since the administration of these agents is by necessity, on a long-term basis. In the conduct of animal safety studies with some hypolipidemics, hyperplastic nodules or tumors developed in the liver of rodents. Data from the literature seem to indicate that the tumor response in rodents varies with the type of hypolipidemic drug administered. This paper summarizes the studies with the new lipid-regulating agent gemfibrozil. Aside from conventional long-term studies in rodents, the ultrastructural aspects of the liver were analyzed in several species and genotoxicity assays and short-term tests for hepatocarcinogenicity were conducted. Thus, it was possible to obtain an overview of these biological phenomena in order to allow for safety extrapolations. The biological behavior of these liver nodules showed that gemfibrozil and clofibrate-induced hepatocytes had not undergone malignant transformation. Further, the phenomenon of peroxisome proliferation, a characteristic event that follows hypolipidemic administration in rodents, was not confirmed in primate or human liver. Peroxisome proliferation has been linked to the process of hepatocarcinogenesis in rodents, although genotoxicity assays were negative and initiation/promotion tests failed to elicit tumors or nodules in a system where hepatocarcinogens manifest their activity.

Thus, hypolipidemics such as gemfibrozil or Clofibrate may possess low tumorigenic potential with low risk due to the lack of correlation between these tests. Nevertheless, these agents are indicated for specific lipoprotein phenotype alteration with the resulting clinical benefits.

Hypolipidemics Carcinogenicity and Extrapolation of Experimental Results for Human Safety Assessments

Dyslipoproteinemias represent a group of disorders closely related to alterations of cholesterol and triglycerides. The alterations of these lipids are considered important risk factors in coronary heart disease and indicate the need for clinically effective and safe drugs.

Hypolipidemic agent therapy, however, does not appear without risk since the administration of these agents is by necessity, on a long-term basis. In the conduct of animal safety studies with some hypolipidemics, hyperplastic nodules or tumors developed in the liver of rodents. Data from the literature seem to indicate that the tumor response in rodents varies with the type of hypolipidemic drug administered. This paper summarizes the studies with the new lipid-regulating agent gemfibrozil. Aside from conventional long-term studies in rodents, the ultrastructural aspects of the liver were analyzed in several species and genotoxicity assays and short-term tests for hepatocarcinogenicity were conducted. Thus, it was possible to obtain an overview of these biological phenomena in order to allow for safety extrapolations. The biological behavior of these liver nodules showed that gemfibrozil and clofibrate-induced hepatocytes had not undergone malignant transformation. Further, the phenomenon of peroxisome proliferation, a characteristic event that follows hypolipidemic administration in rodents, was not confirmed in primate or human liver. Peroxisome proliferation has been linked to the process of hepatocarcinogenesis in rodents, although genotoxicity assays were negative and initiation/promotion tests failed to elicit tumors or nodules in a system where hepatocarcinogens manifest their activity.

Thus, hypolipidemics such as gemfibrozil or clofibrate may possess low tumorigenic potential with low risk due to the lack of correlation between these tests. Nevertheless, these agents are indicated for specific lipoprotein phenotype alteration with the resulting clinical benefits.

Three-dimensional and histochemical studies of peroxisomes in cultured hepatocytes by quick-freezing and deep-etching method

Primary cultured mouse hepatocytes were treated with clofibric acid to induce peroxisome proliferation. They were briefly fixed with paraformaldehyde and centrifuged to prepare pellets. The hepatocytes were split open to remove cytoplasmic soluble proteins and fixed with glutaraldehyde. They were routinely incubated for catalase enzyme cytochemistry and fixed in osmium tetroxide. The specimens were quickly frozen, deeply etched and rotary shadowed by platinum and carbon. Peroxisomes were identified as osmium reaction products on replica membranes. In treated hepatocytes, the number of peroxisomes was considerably increased and smooth membranous structures resembling sER (referred to as 'peroxisome-forming sheets') showed hypertrophy. The rER associated with intermediate filaments was significantly decreased and 'peroxisome-forming sheets' appeared, being accompanied with budding and fragmentation of peroxisomes.

Biochemical effects and zonal heterogeneity of peroxisome proliferation induced by perfluorocarboxylic acids in rat liver

Rats were treated for 5 to 14 days with perfluoroacetate, perfluorobutyrate and perfluorooctanoate. Alterations in hepatic morphology with special reference to the peroxisomal compartment were investigated by light and electron microscopy following cytochemical staining of catalase activity with the alkaline 3,3'-diaminobenzidine medium. All three compounds induced hepatomegaly and peroxisome proliferation. Perfluorobutyrate and perfluorooctanoate were found to be more active than perfluoroacetate. Perfluorooctanoate-induced peroxisome proliferation was more prevalent in centrilobular than in periportal hepatocytes. Peroxisomes in centrilobular liver cells frequently were of round shape, exhibited diameters of up to 1.5 microns and were predominantly located within smooth endoplasmic reticulum-glycogen areas. In periportal cells, however, clusters of polymorphous peroxisomes ranging from 250 to 1,100 nm in diameter were observed at the periphery of smooth endoplasmic reticulum-glycogen regions. Peroxisome proliferation was accompanied by a change of peroxisomal and mitochondrial enzyme activities, in particular an increase in peroxisomal palmitoyl-CoA oxidation. Significant alterations in the concentration of peroxisomal matrix and membrane polypeptides were also noted. Within the first 2 days, perfluorooctanoate treatment exerted a strong hypolipidemic activity and both compounds perfluorooctanoate and perfluorobutyrate raised the level of hepatic free acid-soluble CoA nearly 10-fold as compared with control livers. The results suggest perfluorinated carboxylic acids to be model substances suitable to correlate biochemical and morphological parameters with the zonal heterogeneity of the peroxisomal compartment in rat liver. Due to the manifold hepatic effects, contact of humans with perfluorinated carboxylic acids or their metabolic precursors may represent a severe health risk.

Serial section analysis of mouse hepatic peroxisomes

The ultrastructure and organization of mouse hepatic peroxisomes were investigated using serial thin sections and the alkaline diaminobenzidine technique for visualization of the peroxidatic activity of catalase. Mouse periportal hepatocytes exhibit three classes of peroxisomes which display morphological and cytochemical heterogeneity: 1) large, circular to ovoid organelles containing a crystalline nucleoid, 2) small, circular to elongate, anucleoid particles, and 3) tail-like extensions which are devoid of both catalase activity (only traces of reaction deposits) and a crystalline core. Serial section analysis reveals that these profiles correspond to three diverse interconnecting peroxisomal segments which constitute a highly complex organelle. In particular, the large nucleoid-containing peroxisomal segment exhibits an intimate relationship to the endoplasmic reticulum. However, direct membrane continuities between the two compartments are never observed. With respect to the complex structure of the organelle the following conclusions can be drawn concerning biochemical studies on liver peroxisomes: 1) During homogenization and subcellular fractionation procedures, fragmentation of peroxisomes into particles of different size classes should be expected. 2) These peroxisomal fragments are inhomogeneous with respect to their matrix contents and possess at least one rupture site on their membrane surface. 3) Soluble matrix and, to a lesser degree, membrane components of peroxisomes contribute to the soluble fraction. 4) Crude microsomal fractions are regularly contaminated by peroxisomal membrane fragments.

Quantitative microscopy comparison of peroxisome proliferation by the lipid-regulating agent gemfibrozil in several species

Peroxisome proliferation, a well-documented subcellular reaction which follows the administration of hypolipidemic agents, has been well studied in rodents. However, quantitative studies of this phenomenon in other species of laboratory animals are not readily available even though these species are commonly used as predictors of tolerance or safety in humans. The quantitative stereologic studies reported here compared the effects of the new hypolipidemic agent gemfibrozil on hepatic peroxisomes of monkeys, dogs, hamsters and rats of both sexes under several treatment schedules. Gemfibrozil was administered to rats at 300 mg per kg per day for 1 year in the diet; to hamsters at 400 mg per kg per day for 2 weeks by diet admixture; to dogs at 300 mg per kg per day in gelatin capsules for 1 year; and to monkeys at 300 mg per kg day for 3 months by gavage. These dose levels were selected on the basis of tolerance from preliminary studies in each species. At the end of each experimental interval, liver samples were processed for quantitative microscopy. Peroxisomes from male rats were enlarged and the number of peroxisomes per cell were increased 7-fold over controls, resulting in a 20-fold increased peroxisome volume per cell. Statistically, significant increases also occurred in female rats and the difference between treated and controls was 3-fold for both number and volume of peroxisomes per cytoplasmic unit volume. In hamsters, peroxisomes were proliferated and were of significantly smaller size to the extent that the volume of cytoplasm occupied by peroxisomes was not significantly changed. In dogs, the number of peroxisomes per cell was increased and the volume fraction was significantly increased in females only. The number of peroxisomes in young monkeys did not change after treatment, and the peroxisome volume was decreased in males and increased in females. Aged monkeys had increased number of peroxisomes per hepatocyte with increased volume fraction. These results indicate significant differences in the magnitude and direction of peroxisome changes, reflecting species-dependent organelle response to hypolipidemic agents. The order of susceptibility of peroxisome proliferation in laboratory animals is dog less than monkey less than hamster less than rat.

Disintegration of lung peroxisomes during differentiation of type II cells to type I cells in butylated hydroxytoluene-administered mice

Ultrastructural and cytochemical changes in peroxisomes of Type II alveolar cells were investigated in butylated hydroxytoluene (BHT)-administered mouse lungs. Male mice were given a single injection of BHT (400 mg/kg, ip) and sacrificed 1 to 7 days later. By means of tracheal infusion, lungs were fixed with a 2% glutaraldehyde or half-strength Karnovsky solution. Type I epithelium was selectively destroyed by BHT and was replaced by cuboidal Type II cells. Type II cells proliferated and some became squamous, extended their cytoplasm, and might differentiate into Type I cells (Hirai, Witschi, and Côté (1977) Exp. Mol. Pathol. 27, 295-308). Peroxisomes, Type II cell constituents, were clustered around and continuous with endoplasmic reticulum. The shape of the peroxisomes became indistinct after Type I cell injury by BHT. Also the density of the matrix was reduced in proportion to the reduction in the peroxidatic activity of catalase. These changes were accompanied by a decrease in the number of peroxisomes. New pinocytotic vesicles, one of the Type I cell characteristics, were generated at the apical and basal cell surfaces. Therefore, these cells had characteristics intermediate between Type I and Type II cells. These findings may indicate further evidence of the origin of Type I cells from Type II cells.

Reaction patterns of cell organelles in vitamin B6 deficiency. Ultrastructural-morphometric analysis of the liver parenchymal cell

Under the influence of seven weeks of vitamin B6-free nutrition, the hepatocyte single volume is diminished in the face of an unchanged nuclear single volume, posing the question of whether there is proportional atrophy of all cell organelles or single organelle atrophy with counter-regulatory hyperplasia in other cellular compartments. The nucleoli hypertrophy, as the result of segregation and protein synthesis inhibition. The rough endoplasmic reticulum experiences hypoplasia, but the ultrastructural-morphometric analysis does not determine which metabolic steps are attacked. By contrast one observes hyperplasia in the smooth endoplasmic reticulum, which may be regarded as a counter-regulatory mechanism to the choking of protein synthesis. As with the rough endoplasmic reticulum, the mitochondria also show hypoplasia. This is due to the dependency of the mitochondria upon the endoplasmic protein synthesis. The disturbed functional output of the endoplasmic reticulum is seen in the reduced number of orthoperoxisomes as well. The Golgi apparatus experiences hypoplasia as a result of B6 hypovitaminosis, and this must be considered in the context of fatty change in the liver.

Hepatic effect of two hypolipidemic drugs in rats

Some hepatic effects of the hypolipidemic agents 3,9-di-3-pyridyl-2,4,8,10-tetraoxaspiro-5,5-undecane (compound A) and 2-(4-dibenzofuranyloxy)-2-methylpropionic acid (compound B) were investigated in male rats. The animals were treated orally with these drugs and a reference compound-clofibrate for 10 weeks, the daily doses being 250, 300 and 300 mg/kg body weight respectively. All three drugs caused hepatomegaly with a normal microscopic appearance in liver cells. In rats given compound A, part of some liver cells could be occupied by numerous membranes of smooth endoplasmic reticulum. The hepatocytes of the rats treated with compound B or clofibrate showed a marked increase in microbody profiles and an elevated hepatic catalase activity in comparison to the control animals. Neither the microbodies nor the catalase activity were affected by compound A. Hypolipidemic effects were demonstrated with all three compounds, the most potent activity being shown by compound B. Treatment with this agent resulted in significantly higher catalse activity than with clofibrate.