Cumulative effects of repeated doses of compounds transformed into reactive metabolites

Thresholds in Acute and Long Term Animal Studies

The possibilities to define thresholds in general pharmacology and toxicology depend upon the proper knowledge and determination of the most sensitive parameters and relevant targets leading to clinical effects or toxicity. We are generally confronted with the statistical summation of very numerous individual responses and effects at the enzymological, sub-cellular, cellular, organ, or systemic level. In consequence, biological actions of chemicals reveal typical dose-response characteristics described by simple mathematics, and more or less sharp thresholds. Thresholds and targets can change after single and repeated (cumulative) doses by means of altered kinetics and metabolism, by adaptation and modified sensitivity of targets and/or receptors. Most sensitive targets and parameters do not always determine the real thresholds for clinical toxicity. Threshold and specificity of carcinogens can also be modified by similar conditions mentioned. Nutritional factors play an important role in experimental car-dnogenesis, and also for the cancer incidence in man. Frequently, secondary mechanisms such as overloading, induction of cellular growth, mechanical and chemical irritation and damage, and other conditions have simulated real carcinogenic properties of chemical molecules. The important question on the existence and validity of thresholds in carc/nogenes/s must be connected with a solid evaluation of the mode of action of the individual compounds. Otherwise, mathematical calculations of safety limits, definitions of acceptable risks and no-effect levels lack any biological foundation. Finally, the individual perception and social acceptance of risks reveal certain thresholds.

Vinyl Chloride—A Classical Industrial Toxicant of New Interest

The carcinogenicity of vinyl chloride in humans was recognized in 1974 based on observations of hepatic angiosarcomas in highly exposed workers. A multiplicity of endpoints has been demonstrated. The primary target organ, the liver, displays differential susceptibilities of hepatocytes and sinusoidal cells, which are modified by factors of age and dose. There is consistency in organotropism between experimental animals and humans. Vinyl chloride is a pluripotent carcinogen, predominantly directed toward hepatic endothelial (sinusoidal) cells, and second toward the parenchymal cells of the liver. The similarity of results between experimental animals and humans is a solid basis of an amalgamation of experimental and epidemiological risk estimates. Vinyl chloride requires metabolic activation for carcinogenicity and mutagenicity, and toxicokinetics are a key to interpret the dose response. Practically the entire initial metabolism of vinyl chloride is oxidative. At higher exposure concentrations this is nonlinear, and metabolic saturation of metabolism in rats is reached at about 250 ppm. This is consistent with the plateau of hepatic angiosarcoma incidence in rat bioassays. Physiologically based pharmacokinetic/toxicokinetic (PBPK) models have been developed and successfully applied within the frame of human cancer risk assessments. The major DNA adduct induced by vinyl chloride (approximately 98% of total adducts in rats), 7-(2-oxoethyl)guanine, is almost devoid of promutagenic activity. The clearly promutagenic "etheno" adducts N2,3-ethenoguanine and 3,N4-ethenocytosine each represent approximately 1% of the vinyl chloride DNA adducts in rats, and 1,N6-ethenoadenine is found at even lower concentrations. Etheno adducts appear to have a long persistence and are repaired by glycosylases. Vinyl chloride represents a human carcinogen for which a series of mechanistic events connects exposure with the carcinogenic outcome. These include (1) metabolic activation (to form chloroethylene oxide), (2) DNA binding of the reactive metabolite (to exocyclic etheno adducts), (3) promutagenicity of these adducts, and (4) effects of such mutations on protooncogenes/tumor suppressor genes at the gene and gene product levels. In rat hepatocytes, a further event is a biomarker response. Cancer prestages (enzyme-altered foci), as quantitative biomarkers, provide a tool to study dose response even within low dose ranges where a carcinogenic risk cannot be seen in cancer bioassays directly. Such biomarker responses support a linear nonthreshold extrapolation for low-dose assessment of carcinogenic risks due to vinyl chloride. Published risk estimates based on different sets of data (animal experiments, epidemiological studies) appear basically consistent, and on this basis an angiosarcoma risk of approximately 3 x 10(-4) has been deduced by extrapolation, for exposure to 1 ppm vinyl chloride over an entire human working lifetime. An important point that should be considered in regulatory standard settings is the presence of a physiological background of those etheno DNA adducts, which are also produced by vinyl chloride. Likely reasons for this background are oxidative stress and lipid peroxidation. In essence, fundamentals of the hepatocarcinogenicity of vinyl chloride appear now well established, providing a solid scientific basis for regulatory activities.

Acute and chronic drug-induced hepatitis

Adverse drug reactions may mimic almost any kind of liver disease. Acute hepatitis is often due to the formation of reactive metabolites in the liver. Despite several protective mechanisms (epoxide hydrolases, conjugation with glutathione), this formation may lead to predictable toxic hepatitis after hugh overdoses (e.g. paracetamol), or to idiosyncratic toxic hepatitis after therapeutic doses (e.g. isoniazid). Both genetic factors (e.g. constitutive levels of cytochrome P-450 isoenzymes, or defects in protective mechanisms) and acquired factors (e.g. malnutrition, or chronic intake of alcohol or other microsomal enzyme inducers) may explain the unique susceptibility of some patients. Formation of chemically reactive metabolites may also lead to allergic hepatitis, probably through immunization against plasma membrane protein epitopes modified by the covalent binding of the reactive metabolites. This may be the mechanism for acute hepatitis produced by many drugs (e.g. amineptine, erythromycin derivatives, halothane, imipramine, isaxonine, alpha-methyldopa, tienilic acid, etc.). Genetic defects in several protective mechanisms (e.g. epoxide hydrolase, acetylation) may explain the unique susceptibility of some patients, possibly by increasing exposure to allergenic, metabolite-altered plasma membrane protein epitopes. Like toxic idiosyncratic hepatitis, allergic hepatitis occurs in a few patients only. Unlike toxic hepatitis, allergic hepatitis is frequently associated with fever, rash or other hypersensitivity manifestations; it may be hepatocellular, mixed or cholestatic; it promptly recurs after inadvertent drug rechallenge. Lysosomal phospholipidosis occurs frequently with three antianginal drugs (diethylaminoethoxyhexestrol, amiodarone and perhexiline). These cationic, amphiphilic drugs may form phospholipid-drug complexes within lysosomes. Such complexes resist phospholipases and accumulate within enlarged lysosomes, forming myeloid figures. This phospholipidosis has little clinical importance. In a few patients, however, it is associated with alcoholic-like liver lesions leading to overt liver disease and, at times, cirrhosis. Subjects with a deficiency in a particular isoenzyme of cytochrome P-450 poorly metabolize perhexiline and are at higher risk of developing liver lesions. Prolonged, drug-induced liver-cell necrosis may also lead to subacute hepatitis, chronic hepatitis or even cirrhosis. This usually occurs when the drug administration is continued, either because the liver disease remains undetected or because its drug aetiology is overlooked. Several autoantibodies may be present.(ABSTRACT TRUNCATED AT 400 WORDS)

Metabolic activation of the tricyclic antidepressant amineptine--II. Protective role of glutathione against in vitro and in vivo covalent binding

Incubation of [11-14C]amineptine (1 mM) with an NADPH-generating system and hamster liver microsomes resulted in the in vitro covalent binding of an amineptine metabolite to microsomal proteins; this binding was decreased by 41-71% in the presence of cysteine, lysine, glycine or glutathione (0.5 mM). An inverse relationship was found between the concentration of glutathione in the incubation mixture (0.25-4 mM) and the extent of covalent binding in vitro, which became undetectable at concentrations of glutathione of 2 mM and higher. Administration of [11-14C]amineptine (300 mg/kg-1 i.p.) to hamsters pretreated with phorone (500 mg/kg i.p.) resulted in the in vivo covalent binding of an amineptine metabolite to hepatic proteins. This binding was increased by phenobarbital-pretreatment and decreased by piperonyl butoxide-pretreatment. After various doses of phorone (150-500 mg/kg), an inverse relationship was found between hepatic glutathione content and in vivo covalent binding. Administration of amineptine alone (300 mg/kg i.p.) depleted hepatic glutathione by 16% only; in these animals, in vivo covalent binding was undetectable from background. Amineptine (300 mg/kg i.p.) did not produce hepatic necrosis, even in hamsters pretreated with phorone and/or phenobarbital. We conclude that physiologic concentrations of glutathione essentially prevent the in vivo covalent binding of an amineptine metabolite to hepatic proteins, and that this binding does not produce liver cell necrosis in hamsters.

Metabolic activation of the tricyclic antidepressant amineptine--I. Cytochrome P-450-mediated in vitro covalent binding

Incubation of [14C]amineptine (1 mM) with hamster liver microsomes resulted in the irreversible binding of an amineptine metabolite to microsomal proteins. Covalent binding measured in the presence of various concentrations of amineptine (0.0625-1 mM) followed Michaelis-Menten kinetics. Pretreatment with phenobarbital increased not only the Vmax, but also the Km, for this binding. Covalent binding required NADPH and molecular oxygen and was decreased when the incubation was made in the presence of inhibitors of cytochrome P-450 such as piperonyl butoxide (4 mM), SKF 525-A (4 mM) or carbon monoxide (80:20 CO-O2 atmosphere). In contrast, binding was increased when microsomes from untreated hamsters were incubated in the presence of 0.5 mM 1,1,1-trichloropropene 2,3-oxide, an inhibitor of epoxide hydrolase. Metabolic activation also occurred in kidney microsomes. In vitro covalent binding to kidney microsomal proteins required NADPH and was decreased by piperonyl butoxide (4 mM) but was not increased by pretreatment with phenobarbital. We conclude that amineptine is activated by hamster liver and kidney microsomes into a chemically reactive metabolite that covalently binds to microsomal proteins.

Selenite-induced protection of bromobenzene hepatotoxicity in male rats

Acute treatment with sodium selenite effectively reduces bromobenzene hepatotoxicity in male, Sprague-Dawley rats. Hepatocellular damage was ameliorated as shown by marked decreases in plasma alanine and aspartate aminotransferase (ALT and AST) activities. A single dose of selenite (12.5 or 30 mumol Se/kg, ip) was administered to rats at 4, 24, 48, or 72 hr before injection of bromobenzene (7.5 mmol/kg, ip). Plasma ALT and AST activities and hepatic glutathione (GSH) content were measured 24 hr after bromobenzene treatment. As the length of time of selenite pretreatment increased, the extent of reduction of bromobenzene-induced elevation in plasma enzyme activities by selenite was enhanced, and generally, in a dose-related manner with optimal protection occurring in rats pretreated 72 hr prior with selenite. However, depletion of liver GSH by bromobenzene was not affected by selenite treatment. Hepatic GSH levels and GSH detoxication enzyme activities were measured at various intervals in rats treated with selenite alone. Selenite increased hepatic GSH content 20 to 25% at both 24 and 48 hr after injection, with a return to GSH control levels at 72 hr. Selenite treatment produced slight decreases in GSH peroxidase activity but did not alter GSH S-transferase activity. These studies suggest that the reduction of bromobenzene hepatotoxicity by selenite does not involve alterations in the activity of hepatic GSH detoxication enzymes; however, the data suggest that factors in addition to selenite-induced changes in hepatic glutathione levels are also involved.

Inhalation pharmacokinetics based on gas uptake studies. I. Improvement of kinetic models

An improve pharmacokinetic model is described for inhalation of volatile xenobiotics from a closed gas phase system. This model is based on steady-state kinetics and covers metabolic elimination processes of either first-order, zero-order, or Michaelis-Menten characteristics. It is emphasized that the distribution of a volatile compound between gas phase and organism under steady-state conditions may be much different from a static equilibrium obtained in absence of metabolism, as it is observed after application of a metabolic inhibitor. A re-analysis or previous experimental data on dose-dependent pharmacokinetics of different haloethylenes reveals that, in general, the metabolic elimination processes of the rapidly equilibrating mono-haloethylenes (and vinylidene fluoride) can be resolved with excellent accuracy into sections of first-order and zero-order kinetics. Other compounds show a more smooth transition from first-order elimination (at lower atmospheric concentrations) into conditions of saturation (dichloroethylenes, trichloroethylene). The analyses are consistent with a recent concept of Andersen (1980) that metabolic elimination of inhaled xenobiotics is limited by either the capacity of metabolic enzymes or factors of transport to the metabolic sites.

Morphologic alterations in patients with alpha-methyldopa-induced liver damage after short- and long-term exposure

Alpha-methyldopa-induced histologic alterations were investigated in 21 patients with hepatic injury after short- and long-term exposure. Seven patients developed liver injury within 6 months and 24 after several years (mean, 5 years) of exposure. Histologic findings and clinical and biochemical data differed significantly in the two groups. Morphologic analysis of the short-term-treated group revealed marked parenchymatous degeneration, focal, confluent and massive necrosis, and inflammation. Fatty accumulation and increased fibrous trabeculae were characteristic for the patients treated for long term. All patients in the short-term-exposed group had acute and severe hepatitis. Four of them had icterus. Two patients died of hepatic coma. Patients in the long-term-treated group had for several months initially mild but increasing discomfort, dyspepsia, nausea, and colics. Liver function tests in these groups revealed differences in serum albumin, bilirubin, and transferase levels. No changes were observed in alkaline phosphatase and Thrombotest. Fat accumulation and fibrous trabeculae suggest that the alterations precede the clinical symptoms and biochemical signs of hepatitis. The findings show that alpha-methyldopa may induce hepatocellular injury after short- and long-term exposure.