The effect of chlorine substitution on the disposition of polychlorinated biphenyls following dermal administration.

The fate of selected polychlorobiphenyls (PCBs) was investigated following single dermal administration (0.4 mg/kg) to determine the effects of chlorine content and position on the disposition of PCBs following dermal absorption. Single dermal doses of (14)C-labeled mono-, di-, tetra- and hexachlorobiphenyls were administered to 1 cm(2) areas on the backs of F-344 male rats. Distribution of radioactivity in selected tissues and excreta was determined by serial sacrifice at time points up to 2 weeks. Unabsorbed radioactivity was removed from the dose site at either sacrifice or 48 h post-dose. The time course of radioactivity in the tissues showed a dependence on rate and extent of absorption. The most rapidly absorbed PCBs reached peak tissue concentrations at early times and were cleared from the tissues rapidly. The higher chlorinated PCBs were slowly absorbed and tended to accumulate in the adipose and skin after removal of unabsorbed dose. Excretion of absorbed radioactivity varied with chlorine content ranging from 27% to ca. 100% at 2 weeks post-dose. Excretion profiles following dermal doses tended to differ from profiles following equivalent IV doses, as did the metabolite profiles in excreta. Skin slice incubation experiments suggested that first pass metabolism in the dermal dose site was responsible for metabolism and disposition differences between routes of administration. The data further suggest that the rate of absorption, and therefore the disposition of PCBs following dermal administration may be mediated, either in part or fully, by transdermal metabolism.

A physiologically based pharmacokinetic model of p,p'-dichlorodiphenylsulfone.

A physiologically based pharmacokinetic model of the absorption, distribution, metabolism, and elimination of p,p'-dichlorodiphenylsulfone (DDS) in male and female rats and mice is presented. Data used in constructing the model come from single-dose intravenous administration of DDS to male Fischer 344 rats (10 mg/kg, with data taken up to 504 h after administration), from single-dose gavage administration to male rats (10, 100, or 1000 mg/kg, with data up to 72 h after administration), and from chronic feed studies in male and female rats and male and female B6C3F(1) mice (studies of duration from 2 weeks up to 18 months, with feed concentrations of DDS up to 300 ppm). The model uses diffusion-limited kinetic for the distribution of the parent compound. Because fewer data are available for the metabolites of DDS (at least five of which are known to exist in the data), the model groups the metabolites into one metabolic pathway and uses simpler flow-limited kinetics for the metabolites. The data show that the kinetics of DDS are nonlinear. Possible sources of nonlinearity considered in the model were nonlinear (Michaelis-Menten) metabolism, nonlinear absorption of DDS from the gut, and induction by DDS of its own metabolism. A model using Michaelis-Menten metabolism was not found to give a significantly better fit than one using first-order linear metabolism, but omitting either of the other nonlinear effects was found to give a significantly poorer fit to the data. Because the data from mice are limited compared to those from rats, there is more confidence in the model's description of DDS kinetics in rats than in its description of kinetics in mice.

Comparative xenobiotic metabolism between Tg.AC and p53+/- genetically altered mice and their respective wild types.

The use of transgenic animals, such as v-Ha-ras activated (TG:AC) and p53+/- mice, offers great promise for a rapid and more sensitive assay for chemical carcinogenicity. Some carcinogens are metabolically activated; therefore, it is critical that the altered genome of either of these model systems does not compromise their capability and capacity for metabolism of xenobiotics. The present work tests the generally held assumption that xenobiotic metabolism in the TG:AC and p53+/- mouse is not inherently different from that of the respective wild type, the FVB/N and C57BL/6 mouse, by comparing each genotype's ability to metabolize benzene, ethoxyquin, or methacrylonitrile. Use of these representative substrates offers the opportunity to examine arene oxide formation, aromatic ring opening, hydroxylation, epoxidation, O-deethylation, and a number of conjugation reactions. Mice were treated by gavage with (14)C-labeled parent compound, excreta were collected, and elimination routes and rates, as well as (14)C-derived metabolite profiles in urine, were compared between relevant treatment groups. Results of this study indicated that metabolism of the 3 parent compounds was not appreciably altered between either FVB/N and TG:AC mice or C57BL/6 and p53+/- mice. Further, expression of CYP1A2, CYP2E1, CYP3A, and GST-alpha in liver of naive genetically altered mice was similar to that of corresponding wild-type mice. Thus, these results suggest that the inherent ability of TG:AC and p53+/- mice to metabolize xenobiotics is not compromised by their altered genomes and would not be a factor in data interpretation of toxicity studies using either transgenic mouse line.

Metabolism and disposition of luminol in the rat.

1. The metabolism and disposition of Luminol (LMN, 3-aminophthalhydrazide), a widely used forensic and laboratory reagent that chemiluminesses upon oxidation, was determined as part of its overall toxicological characterization. 2. Radiolabelled LMN was well absorbed, metabolized and excreted following p.o. administration of a range of doses. About 90% of the total dose was recovered within 24 h of administration in urine in the form of two metabolites identified as LMN N8-glucuronide and LMN N8-sulphamic acid. 3-Aminophthalic acid, the oxidative product of LMN in the light-emitting reaction, was apparently not formed in vivo. 3. Metabolism and disposition of an i.v. administered dose was similar to that following gavage. Little or no LMN-derived radioactivity was present in tissue within 12 h post-dosing. Excretion of radioactivity in bile following i.v. injection was minimal (approximately 8% of the total dose in 6 h) and consisted of the same urinary-excreted glucuronide and sulphate conjugates. 4. LMN was not absorbed dermally in rat, potentially a major route of exposure to human. If the fate of LMN is similar between species, this compound should have little potential for either dermal absorption, bioaccumulation in tissues following other routes of exposure or chronic toxicity in humans.

Catechol metabolites of polychlorinated biphenyls inhibit the catechol-O-methyltransferase-mediated metabolism of catechol estrogens.

The catechol metabolites of estradiol, 2- and 4-hydroxyestradiol (2-OHE(2) and 4-OHE(2), respectively) are potent signaling molecules and are hypothesized to be central to estrogen-linked carcinogenesis. Methylation by catechol-O-methyltransferase (COMT) is the principal means of catechol estrogen (CE) deactivation in the liver and other tissues. The present studies were conducted to determine the effects of PCBs and catechol metabolites of PCBs on the COMT-mediated catabolism of 4-OHE(2) and 2-OHE(2) in vitro and in vivo. Liver homogenates of female Sprague-Dawley rats treated with Aroclor 1254 for 21 days (5 mg/kg/day) showed a 30 and 40% reduction of COMT activity toward 2-OHE(2) and 4-OHE(2), respectively. Incubation of [(3)H]-beta-estradiol with these same liver homogenates, followed by HPLC analysis, demonstrated an elevation of CEs and a nearly complete reduction in levels of methylated catechol estrogens. In classical enzyme kinetics studies, COMT was demonstrated to have a high affinity for catechol PCBs, with K(m)'s approximately equivalent to those of CEs. Catechol PCBs were also potent inhibitors of CE O-methylation. These data suggest that PCBs may significantly alter the metabolism of catechol estrogens in vivo and that this effect may be mediated by catechol metabolites of PCBs. It is further speculated that methyltransferase inhibition by PCB catechols may contribute to PCB-mediated endocrine effects and liver carcinogenesis.

Phenolphthalein metabolite inhibits catechol-O-methyltransferase-mediated metabolism of catechol estrogens: a possible mechanism for carcinogenicity.

Phenolphthalein (PT), used in over-the-counter laxatives, has recently been identified as a multisite carcinogen in rodents, but the molecular species responsible for the carcinogenicity is not known. A catechol metabolite of PT, hydroxyphenolphthalein (PT-CAT), was recently identified and may be the molecular species responsible for at least part of the toxicity/carcinogenicity of PT. We hypothesize that PT-CAT inhibits the enzyme catechol-O-methyltransferase (COMT) and therefore potentiates genotoxicity by either PT-CAT itself or the endogenous catechol estrogens (CEs) in susceptible tissues. The present studies were conducted to determine the effects of PT treatment and PT-CAT itself on the COMT-mediated metabolism of 4- and 2-hydroxyestradiol both in vitro and in vivo. Female mice were treated with PT (50 mg/kg/d) for 21 days and then euthanized. PT-CAT concentration in urine reached plateau levels by 7 days of exposure. An O-methylated metabolite of PT-CAT was detected in feces. In vitro experiments demonstrated that PT treatment resulted in an increase in free CEs, which are normally cleared by COMT and a concurrent decrease in the capacity of hepatic catechol clearance by COMT. In vitro, PT-CAT was a substrate of COMT, with kinetic properties within the range measured with endogenous substrates. PT-CAT was an extremely potent mixed-type inhibitor of the O-methylation of the catechol estrogens, with 90-300 nM IC50s. The above data, when taken together, suggest that chronic administration of PT may enhance metabolic redox cycling of both PT-CAT and the catechol estrogens and this, in turn, may contribute to PT-induced tumorigenesis.

Medicinal herbs in the United States: research needs.

Virtually all cultures have, throughout history, used a variety of plants or materials derived from plants for the prevention and treatment of disease. Evidence of the beneficial therapeutic effects of these medicinal herbs is seen in their continued use. Additionally, the development of modern chemistry permitted the isolation of chemicals from medicinal herbs that have served as drugs or starting materials for the synthesis of many important drugs used today. Many more modern drugs have been synthesized as a result of knowledge gained from studies of mechanisms of actions of chemicals first isolated from medicinal herbs. Thus, medicinal herbs have played a major role in the development of modern medicine and continue to be widely used in their original form. Whereas it is generally agreed that most medicinal herbs are safe under the conditions used, some are toxic and should be avoided even though they are readily available, and others have significant adverse side effects when misused. Also, little has been done to investigate potential adverse effects that may be associated with extended or high-dose use of medicinal herbs. Thus, concern has been expressed that the lack of quality control used in the preparation of medicinal herbs, plus their unregulated sale and uninformed use, pose potential adverse health effects for consumers. There is also concern regarding potential herb/herb or herb/drug interactions and possible untoward health effects of medicinal herbs in sensitive subpopulations such as the young and the elderly and certain genetically predisposed individuals. In this paper, we discuss these concerns at some length and make recommendations for additional research and education discussed in the recent International Workshop to Evaluate Research Needs on the Use and Safety of Medicinal Herbs.

In vitro estrogenicity of the catechol metabolites of selected polychlorinated biphenyls.

A considerable body of work has demonstrated that phenolic polychlorinated biphenyl (PCB) metabolites, structural analogues to estradiol, bind to the soluble estrogen receptor (ER) and that hydroxy PCB-ER complexes will translocate into the nucleus and bind to ER response elements in cultured cells. Although catechol estrogens exhibit weak estrogenic activity, the catechol PCB metabolites which are structurally similar to these ER agonists have gone untested for potential estrogenicity. In the present work we have assessed the estrogenicity of this second group of PCB metabolites, the catechols. The test compounds used in the present study were chosen to elucidate the effects of chlorine and catechol position on in vitro estrogenicity. Cultured HeLa cells, transfected with the estrogen reporter gene ERET81CAT and mouse ER cDNA, were incubated with PCB catechols. The cells were harvested at 28 h posttransfection and assayed for chloramphenicol acetyl transferase (CAT) activity. The responses elicited by the PCB catechols tested fell within the range of effect measured for the catechol estrogens and phenolic PCBs, and were within the range previously reported for other "environmental estrogens" such as nonylphenol and o,p'-DDT. Maximal measured responses were achieved at concentrations approximately two to three orders of magnitude higher than that of 17-beta-estradiol, indicating that PCB catechols have estrogenic activity in vitro. The extent of chlorination and the position of the catechol (3,4 vs 2,3 substitution) were important in determining estrogenicity in the compounds tested. The 2,3-catechol showed no detectable activity in this system, while activity of the 3, 4-catechols increased with the degree of chlorination. The observed estrogenicity of PCB catechols suggests that further oxidative metabolism of estrogenic PCB phenolic metabolites would not necessarily result in lowering the total estrogenic burden of a PCB-exposed organism. The present results imply that if estrogenic activity is assigned to an individual phenol, the potential contribution of its catechol metabolites to the total estrogenic burden should also be taken into consideration.

Dose-dependent metabolism of benzene in hamsters, rats, and mice.

The disposition of oral doses of [14C]benzene was investigated using a range of doses that included lower levels (0.02 and 0.1 mg/kg) than have been studied previously in rat, mouse, and in hamster, a species which has not been previously examined for its capacity to metabolize benzene. Saturation of metabolism of benzene was apparent as the dose increased, and a considerable percentage of the highest doses (100 mg/kg) was exhaled unchanged. Most of the remainder of the radioactivity was excreted as metabolites in urine, and significant metabolite-specific changes occurred as a function of dose and species. Phenyl sulfate was the predominant metabolite in rat urine at all dose levels (64-73% of urinary radioactivity), followed by prephenlmercapturic acid (10-11%). Phenyl sulfate (24-32%) and hydroquinone glucuronide (27-29%) were the predominant metabolites formed by mice. Mice produced considerably more muconic acid (15%), which is derived from the toxic metabolite muconaldehyde, than did rats (7%) at a dose of 0.1 mg/kg. Unlike both rats and mice, hydroquinone glucuronide (24-29%) and muconic acid (19-31%) were the primary urinary metabolites formed by hamsters. Two metabolites not previously detected in the urine of rats or mice after single doses, 1,2,4-trihydroxybenzene and catechol sulfate, were found in hamster urine. These data indicate the hamsters metabolize benzene to more highly oxidized, toxic products than do rats or mice.

Disposition of 2-methylimidazole in rats.

2-Methylimidazole (2-MI), widely used as a chemical intermediate, is also present in cigarette smoke and may form in food and forage as a result of ammoniation of simple sugars. 2-MI has been shown to be neurotoxic in several animal species and to alter serum levels of T3, T4, and thyroid-stimulating hormone (TSH) in the rat, apparently leading to hyperplasia of thyroid follicular cells. In order to better characterize 2-MI-induced toxicity, the disposition of [2-(14)C]-2-MI has been investigated following p.o. administration of either 5, 50, or 150 mg/kg to male F344 rats. Excretion data indicated that absorption of 2-MI was both rapid and proportional to dose in the range studied. Approximately 90% of the total dose was eliminated in urine within 24 h. Most of the remaining 14C was excreted in feces and as expired 14CO2. Excretion data were similar following i.v. administration of 5 mg/kg. Little or no enterohepatic circulation of compound occurred, since biliary excretion of 2-MI-derived 14C was negligible. Approximately 70% of the 14C excreted in urine, following all dosing, consisted of parent compound. High-performance liquid chromatography (HPLC) chromatograms for all treatment groups were similar, indicating that metabolism of 2-MI in rats was not affected by dose or route of administration.

The effect of chlorine substitution on the dermal absorption of polychlorinated biphenyls.

The fate of selected mono-, di-, tetra-, and hexachlorobiphenyls was investigated following single dermal administration (0.4 mg/kg) to determine the effects of chlorine substitution on the dermal absorption and disposition of polychlorinated biphenyls (PCBs). Single dermal doses of 14C-labeled mono-, di-, tetra-, and hexachlorobiphenyls were administered to 1-cm2 areas on the backs of F-344 male rats. Unabsorbed radioactivity was removed from the dose site either at euthanasia or 48 h postdose. Distribution of radioactivity in the dose site and selected tissues was determined by serial sacrifice at time points up to 2 weeks. Dermal penetration varied inversely with degree of chlorination and at 48 h ranged from ca. 100% for monochlorobiphenyl to ca. 30% for the hexachlorobiphenyl. Penetration rate constants correlated well with log kow. PCBs were retained in the epidermis for up to 2 weeks postdose. The data from these studies suggest that systemic absorption of PCBs involves a combination of sequential processes including penetration across the stratum corneum, possibly metabolism in the epidermis and/or dermis, adsorption to proteins, and finally absorption into the systemic circulation. The skin favors the rapid absorption of less chlorinated PCBs, but the relatively rapid metabolism and elimination of these compounds would result in lower body burdens. More highly chlorinated PCBs penetrate less rapidly but remain in the site of exposure and slowly enter the systemic circulation. The dermal absorption of a commercial PCB mixture was modeled, and the results suggest that the net result of the differences in absorbance rates would be a greater body burden of higher chlorinated PCBs relative to those that have a lower chlorine content.

Metabolism and disposition of dimethyl hydrogen phosphite in rats and mice.

A study of dimethyl hydrogen phosphite (DMHP) by the National Toxicology Program (NTP) indicated that chronic administration by oral gavage resulted in an increased incidence of neoplastic lesions in the lungs and forestomachs of Fischer 344 rats but not in B6C3F1 mice. The current study was designed to evaluate the metabolic basis, if any, of this species selectivity by studying the metabolism and disposition of [14C]DMHP in the respective strains of rats and mice. Results of this study indicate that DMHP administered at a range of dose of 10-200 mg/kg was readily and near completely absorbed from the gastrointestinal tracts of rats and mice. DMHP-derived radioactivity was eliminated primarily as CO2 in the expired air, 44-57%, and urine, 28-49%, and very little was collected in feces, 1-2%, or as volatile organics, 2-3%. DMHP-derived radioactivity was widely distributed in tissues of rats and mice, with the highest concentrations observed in the liver, kidneys, spleen, lungs, and forestomach, and the lowest in brain, skeletal muscle, and adipose tissue. The disappearance of radioactivity from mouse tissues was approximately twice as rapid as from rat tissues. In vitro, DMHP was metabolized to formaldehyde by the microsomal fractions of liver, lungs, kidneys, forestomach, and glandular stomach. In vivo, DMHP was metabolized to the product of demethylation, monomethyl hydrogen phosphite (MMHP), which was excreted in urine. Results of this study indicate that the NTP carcinogenicity study with DMHP was carried out within the dose range in which the absorption, metabolism, and disposition of DMHP are linear in both species. Apparent species-dependent differences in the metabolism and disposition of DMHP are limited to the more rapid metabolism and elimination by the mouse. Therefore, the species-dependent variations in the carcinogenicity of DMHP are most likely attributable to factors other than metabolism and disposition.

Diethanolamine absorption, metabolism and disposition in rat and mouse following oral, intravenous and dermal administration.

1. The disposition of [14C]diethanolamine (DEA) (1) was determined in rat after oral, i.v. and dermal administration, and in mouse after dermal administration. 2. Oral administration of DEA to rat was by gavage of 7 mg/kg doses once and after daily repeat dosing for up to 8 weeks. Oral doses were well absorbed but excreted very slowly. DEA accumulated to high concentrations in certain tissues, particularly liver and kidney. The steady-state of bioaccumulation was approached only after several weeks of repeat oral dosing, and the half-life of elimination was approximately 1 week. 3. DEA was slowly absorbed through the skin of rat (3-16% in 48 h) after application of 2-28 mg/kg doses. Dermal doses ranging from 8 to 80 mg/kg were more readily absorbed through mouse skin (25-60%) in 48 h of exposure, with the percent of the applied dose absorbed increasing with dose. 4. Single doses (oral or i.v.) of DEA were excreted slowly in urine (c. 22-25% in 48 h) predominantly as the parent compound. There was minimal conversion to CO2 or volatile metabolites in breath. The profile of metabolites appearing in urine changed after several weeks of repeat oral administration, with significant amounts of N-methylDEA and more cationic metabolites appearing along with unchanged DEA.

Using structural information to create physiologically based pharmacokinetic models for all polychlorinated biphenyls.

Physiologically based pharmacokinetic (PBPK) models are useful in describing the distribution, metabolism, and fate of xenobiotics across multiple species. The eventual goal of the present research is to create PBPK models for all 209 polychlorinated biphenyls (PCBs). Key parameters in any PBPK model are the tissue-to-blood partition coefficients. Tissue:blood partition coefficients relate the compound's concentration in a target tissue to its concentration in blood under equilibrium conditions. Data on the adipose:plasma partition coefficients of 24 PCBs were used in a regression analysis to find an expression for the adipose:plasma partition coefficient as a function of molecular structure. Using stepwise regression, it was found that three simple structural descriptors were sufficient to predict adipose:plasma partition coefficients for all 209 PCB congeners. Data on the distribution of PCBs among blood components were used to derive the adipose:blood partition coefficient from the adipose:plasma partition coefficient. The lipid contents of liver, muscle, and skin were used to derive the tissue:blood partition coefficient for those tissues from the adipose:blood partition coefficient. These results allow for the calculation of tissue:blood partition coefficients for liver, skin, muscles, and fat for all 209 PCB congeners.

Comparative absorption of lead from contaminated soil and lead salts by weanling Fischer 344 rats.

A 44-day dosed feed study was performed to compare the bioavailability of lead from contaminated soil versus two lead salts and the effect of soil on gastrointestinal absorption of ingested lead. Male Fischer rats (approximately 4 weeks of age) received lead, 17, 42, or 127 ppm, in the form of lead acetate, lead sulfide, lead-contaminated soil, or combinations thereof in the diet for 7, 15, or 44 days. Control soil was added to the diets of some animals to determine how it might alter lead bioavailability. Blood Delta-aminolevulinic acid dehydratase (Delta-ALAD) and blood, bone, kidney, and liver lead were determined in groups of animals at each time-point. Blood Delta-ALAD was inhibited in a dose-dependent manner and to the greatest degree in the lead acetate and lead acetate/control soil groups, followed by the lead sulfide and lead-contaminated soil groups. Bone and tissue lead levels increased in a dose-dependent manner and were greatest in animals receiving lead acetate and significantly less in animals receiving lead sulfide and lead-contaminated soil. Blood lead levels were generally greatest by 7 days and stabilized at lower levels thereafter. Bone lead concentration-time patterns did not demonstrate the biphasic change seen with tissues and continued to increase in most treatment groups through the course of the study. The presence of soil in the diet clearly attenuated the absorption of lead acetate, but had little effect on the absorption of lead sulfide. Results of these studies confirm previous observations that lead absorption is highly dependent on the form of lead ingested and the matrix in which it is ingested. More important, these studies demonstrate that lead in soil may be significantly less available than estimated by current default assumptions and that the presence of soil may decrease the availability of lead from lead salts on which the default assumptions are based. Results presented here also demonstrate that the weanling rat may represent an appropriate model that could be used to obtain relatively rapid and economical estimates of the availability of lead in complex matrices such as soil.

Comparative metabolism and disposition of ethoxyquin in rat and mouse. I. Disposition.

1. The biological fate of the antioxidant [3-14C]ethoxyquin (EQ) was investigated in the male F344 rat and the B6C3F1 mouse following either p.o. or i.v. administration. 2. The disposition of single doses up to 25 mg/kg was similar in the rat and mouse. About 90% of a total dose was excreted in urine and faeces within 24 h post-dosing. In contrast, no more than 60% of a higher dose of 250 mg/kg was excreted within 24 h following p.o. administration. 3. Metabolism of EQ was rapid in both the rat and mouse following either p.o. or i.v. administration. Little or no parent compound was detected in cumulative 24-h excreta. 5. EQ-derived radioactivity bioaccumulated in some tissues following repeated exposure to rat of either 25 or 250 mg/kg by gavage. However, the fold-increases in concentrations of EQ-derived radioactivity in tissues following repeated administration of the higher dose were generally less than those observed following repeated administration of the lower dose. Repeated high dose administration may overcome delayed gastric emptying (observed following single dose administration of 250 mg/kg) and/or lead to auto-induction of EQ metabolism.

Comparative metabolism and disposition of ethoxyquin in rat and mouse. II. Metabolism.

1. The major pathways of ethoxyquin (EQ) metabolism in both the rat and mouse are O-deethylation and conjugation to endogenous substrates. 2. The two major EQ-derived metabolites excreted in rat urine were in the form of sulphate conjugates, 1,2-dihydro-6-hydroxy-2,2,4-trimethylquinoline sulphate, and 1,2,3,4-tetrahydro-3,6-dihydroxy-4-methylene-2,2-dimethylquinoline sulphate. The latter apparently arises from an intramolecular rearrangement of the 3,4-epoxide of ethoxyquin. 3. Mouse urine contained one major glucuronide, 1,2-dihydro-6-hydroxy-2,2,4-trimethylquinoline glucuronide as well as one major sulphate conjugate, 1,2-dihydro-6-hydroxy-2,2,4-trimethylquinoline sulphate. 4. EQ-derived radioactivity was excreted in rat bile, mainly as GSH conjugates, with little unchanged EQ present. Two of the biliary metabolites are glutathione conjugates of ethoxyquin 3,4-epoxide; the third appears to be a conjugate of either ethoxyquin 7,8-epoxide or 2,2,4-trimethylquinol-6-one.

p,p'-Dichlorodiphenyl sulfone metabolism and disposition in rats.

p, p'-Dichlorodiphenyl sulfone (DDS) is a lipophilic monomer used extensively in the synthesis of high temperature plastics. Studies of the fate of uniformly labeled [14C]DDS in the rat have established that it is readily absorbed from the gastrointestinal tract, distributed to all tissues examined, and concentrated in adipose tissue. After intravenous administration of 10 mg/kg and determination of the time course of DDS distribution, increasing accumulation of DDS in adipose was observed up to 24 hr, followed by slow elimination with a half-life of approximately 12 days. DDS equivalents in tissues were primarily (> 90%) parent compound, whereas excreted DDS equivalents were primarily (> 80%) present as metabolites. On repeat oral dosing at 10 mg/kg, levels of DDS in tissues seemed to reach steady state after approximately 2 weeks, at which time the concentrations in adipose reached 265 micrograms/g tissue. Hepatic cytochrome P450 (CYP) content, ethoxyresorufin O-deethylase activities, and levels of metabolites arising from phase I metabolism were doubled after repeat oral administration of DDS, but benzphetamine N-demethylase activity was unchanged. Thus, it seems that DDS induces CYP1A forms, but not CYP2B isozymes. DDS-derived radioactivity was excreted primarily in feces and to a lesser extent in urine as a phenolic metabolite and its glucuronide. The aglycone of this glucuronide was isolated and characterized by NMR and MS as 3-hydroxy-4,4'-dichlorodiphenyl sulfone.

Cell proliferation as a determining factor for the carcinogenicity of chemicals: studies with mutagenic carcinogens and mutagenic noncarcinogens.

Recent work in our laboratory has examined mechanisms whereby chemicals produce mutagenicity in short-term in vitro assays yet fail to produce carcinogenesis in 2-year rodent bioassays. These studies have used mutagenic structural analogs of carcinogenic and noncarcinogenic chemicals for comparison. Our previous studies have determined that differences in the metabolism and disposition of these chemicals were not responsible for their observed carcinogenic differences, but that carcinogenicity correlated with the ability of the respective isomer to induce cell proliferation in the target organ. Mutagenic noncarcinogens such as 2,6-diaminotoluene (DAT), 1-nitropropane (NP), dimethoate, dioxathion, and dichlorvos failed to induce an increase in cell turnover in the target organs. An increase in cell proliferation was observed following exposure to the mutagenic carcinogen analogs 2,4-DAT (liver), 2-NP (liver), and tris(2,3-dibromopropyl)phosphate (kidney). Our recent studies have used transgenic (Big Blue) mice to detect in vivo mutagenesis induced by DAT isomers. Results of these studies demonstrate that administration of the carcinogenic isomer, 2,4-DAT, resulted in an increase in in vivo mutation frequency, whereas administration of the noncarcinogenic isomer, 2,6-DAT, failed to do so. These results indicate that cell proliferation may be requisite for expression of chemical-induced mutagenicity in vivo and thereby accommodate expression of carcinogenicity.

Rotenone, an anticarcinogen, inhibits cellular proliferation but not peroxisome proliferation in mouse liver.

In previous National Toxicology Program (NTP) studies, rotenone reduced the background incidence of hepatocellular carcinoma in male B6C3F1 mice. In the present studies, rotenone reduced the basal hepatic labeling index of male B6C3F1 mice in a dose-dependent fashion and inhibited hepatocellular proliferation, but not peroxisome proliferation, induced by the peroxisome proliferator Wy-14,643. These results indicate that reduction of hepatic tumors by rotenone may have been due to decreased liver cell replication, that peroxisome proliferation can be induced in the absence of hepatocellular proliferation and suggest rotenone as a potential tool in studies of relationships of cell proliferation, peroxisomal proliferation and hepatocarcinogenesis.