James A. Popp

A Critical Review of the Literature on Hydrogen Sulfide Toxicity

The information available on the biological activity of hydrogen sulfide has been examined for present status of critical results pertaining to the toxicity of hydrogen sulfide. This review of the literature is intended as an evaluative report rather than an annotated bibliography of all the source material examined on hydrogen sulfide. The information was selected as it might relate to potential toxic effects of hydrogen sulfide to man and summarized, noting information gaps that may require further investigation. Several recommendations are listed for possible consideration for either toxicological research or additional short- and long-term tests. Two bibliographies have been provided to assist in locating references considered in this report: (1) literature examined but not cited and (2) reference citations. The majority of the references in the first bibliography were considered peripheral information and less appropriate for inclusion in this report.

Covalent Binding of Inhaled Formaldehyde to DNA in the Respiratory Tract of Rhesus Monkeys: Pharmacokinetics, Rat-to-Monkey Interspecies Scaling, and Extrapolation to Man

DNA-protein cross-links were formed in the respiratory tract of rhesus monkeys exposed to [14C]formaldehyde (0.7, 2, or 6 ppm; 6 hr). Concentrations of cross-links (pmol/mg DNA) were highest in the mucosa of the middle turbinates; lower concentrations were produced in the anterior lateral wall/septum and nasopharynx. Very low concentrations were found in the larynx/trachea/carina and in the proximal portions of the major bronchi of some monkeys exposed to 6 ppm but not to 0.7 ppm. No cross-links were detected in the maxillary sinuses or lung parenchyma. The pharmacokinetics of cross-link formation in the nose were interpreted using a model in which the rate of formation is proportional to the tissue concentration of formaldehyde. The model includes both saturable and nonsaturable elimination pathways and describes regional differences in DNA binding as having an anatomical rather than a biochemical basis. Using this model, the concentration of cross-links formed in corresponding tissues of different species can be predicted by scaling the pharmacokinetic parameter that depends on minute volume (V) and quantity of nasal mucosal DNA (MDNA). The concentration-response curve for the average rate of cross-link formation in the turbinates, lateral wall, and septum of rhesus monkeys was predicted from that of F-344 rats exposed under similar conditions. There was significant overlap between predicted and fitted curves, implying that V and MDNA are major determinants of the rate of cross-link formation in the nasal mucosa of different species. Concentrations of cross-links that may be produced in the nasal mucosa of adult men were predicted based on experimental data in rats and monkeys. The results suggest that formaldehyde would generate lower concentrations of cross-links in the nasal mucosa of humans than of monkeys, and much lower concentrations in humans than in rats. The rate of formation of DNA-protein cross-links can be regarded as a surrogate for the delivered concentration of formaldehyde. Use of this surrogate should decrease the uncertainty of human cancer risk estimates derived by interspecies extrapolation by providing a more realistic measure of the delivered concentration at critical target sites.

Dinitrotoluene: Acute Toxicity, Oncogenicity, Genotoxicity, and Metabolism

Dinitrotoluene (DNT) is a major commodity chemical; over six hundred million pounds of DNT were used in the chemical industry in 1980. Interest in the toxicology of this important chemical was greatly increased when separate oncogenicity assays yielded the conflicting results that DNT was either not hepatocarcinogenic or produced a 100% incidence of hepatocellular carcinomas in male rats in one year. Research revealed pronounced differences in the activity of the DNT isomers and provided the reason for the dissimilar results of the various carcinogenicity studies. Cell culture genetic toxicology assays failed to predict the potent carcinogenic activity of any isomer of DNT. Only when the complex pattern of metabolic activation of DNT began to unfold and genotoxic activity was assessed in the appropriate target organ in the intact treated animal was the potent genotoxic activity of DNT revealed, and the reasons for the negative in vitro results understood. The DNTs have been extensively tested for reproductive effects in animals and humans, and the metabolism and disposition of each of the six possible isomers have been studied. This work has provided valuable information in establishing the risk of these compounds to humans.

Chlorinated hydrocarbon-induced peroxisomal enzyme activity in relation to species and organ carcinogenicity

Trichloroethylene (TCE), perchloroethylene (PER), and pentachloroethane (PENT) are widely used industrial chemicals that cause an increased incidence of hepatocellular carcinoma in mice and a very low incidence of renal tubular adenocarcinoma in rats. A recent study (C. R. Elcombe, M. S. Rose, and I.S. Pratt (1985), Toxicol. Appl. Pharmacol. 79, 365-376) suggested that the species difference in the hepatocarcinogenicity of TCE seen between rats and mice was due to a species difference in peroxisomal proliferation and cell proliferation. The purpose of the present investigation was to understand better the association of peroxisome proliferation in the species-specific hepatocarcinogenicity, and nephrocarcinogenicity of TCE, PER, and PENT. TCE (1000 mg/kg body wt), PER (1000 mg/kg body wt), PENT (150 mg/kg body/wt), the metabolite trichloroacetic acid (TCA; 500 mg/kg body wt) or the potent peroxisome proliferating agent Wy-14,643 (WY; 50 mg/kg body wt) was administered by gavage to male F-344 rats and B6C3F1 mice for 10 days. Cyanide-insensitive palmitoyl CoA oxidation activity (PCO) was used to measure the peroxisome proliferation response. Of the chlorinated hydrocarbons, TCE and PER elevated PCO activity in mouse liver whereas only TCE elevated rat liver and kidney PCO. All agents increased PCO activity in the kidneys of mice. None of the chlorinated hydrocarbons induced a PCO response stronger than WY. These results support an association between peroxisome proliferation and hepatic tumors in mice following TCE and PER, but not PENT, administration and suggest that chlorinated hydrocarbon-induced peroxisome proliferation does not correlate with species-specific renal carcinogenicity.

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

Abstract 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.

Formaldehyde concentrations in the blood of rhesus monkeys after inhalation exposure

The effect of subchronic exposure to formaldehyde (HCHO; 6 ppm; 6 hr/day, 5 days/wk for 4 wk) on the HCHO concentration in the blood of three rhesus monkeys was investigated. Immediately after the final exposure, the monkeys were sedated, and blood samples were withdrawn 7 min after the end of exposure. The HCHO concentration in the blood, determined by gas chromatography-mass spectrometry was 1.84 +/- 0.15 micrograms/g blood and did not differ significantly after a further 45 hr without exposure to HCHO (2.04 +/- 0.40 micrograms/g blood). The average concentration of HCHO in the blood of exposed monkeys was also not significantly different from that of three unexposed controls (2.42 +/- 0.09 micrograms/g blood). However, individual monkeys differed significantly from one another with respect to their blood concentrations of HCHO. These results indicate that subchronic inhalation exposure of non-human primates to HCHO has no significant effect on the HCHO concentration in the blood, and that the average concentration of HCHO in the blood of monkeys is similar to that in the blood of humans.

Perspectives on the mechanism of action of the splenic toxicity of aniline and structurally-related compounds.

Aniline and several structurally-related aromatic amines produce spleen tumours in rats given high doses of compound in 2-year bioassay studies. Evaluation of the pathogenesis of the splenic lesions and characterization of the disposition of radiolabelled aniline in animals suggests that the spleen tumours may be a secondary response resulting from chemically-mediated erythrocyte toxicity. It is proposed that compound-derived toxicity to erythrocytes results in scavenging of damaged red blood cells by the spleen, initiating a series of events which may contribute to the development of spleen tumours. These events potentially include (i) specific accumulation of the parent compound or toxic metabolite(s) carried to the spleen by erythrocytes; (ii) deposition of erythrocytic debris, particularly iron, which may catalyse tissue-damaging free-radical reactions; and (iii) induction of splenic hyperplasia resulting from erythrocyte overload. Linkage of the splenic tumorigenicity of these aromatic amines to an initial toxic event in the erythrocyte suggests that the carcinogenicity of such compounds may be determined by a definable threshold dose, i.e. the events leading to the carcinogenicity are not initiated until the capacity of the red blood cell to cope with the toxic insult is exceeded.

Potential role of α-2μ-globulin, protein droplet accumulation, and cell replication in the renal carcinogenicity of rats exposed to trichloroethylene, perchloroethylene, and pentachloroethane

Trichloroethylene (TCE), perchloroethylene (PER), and pentachloroethane (PENT) are used extensively as industrial solvents. These agents cause an increased incidence of renal tumors in male, but not female, rats. Male and female F-344 rats were gavaged for 10 days with TCE (1000 mg/kg), PER (1000 mg/kg), and PENT (150 mg/kg) to determine if chlorinated hydrocarbon-induced changes in levels of renal alpha-2 mu-globulin (alpha 2 mu), protein droplet accumulation (PDA), and cell replication were male rat specific. The animal strain, dose, and route of administration were the same as previous chronic bioassays in order to better understand the relationship between alpha 2 mu, PDA, and cell replication to the sex-specific renal carcinogenicity. In male rats, increases in protein droplet and crystalloid accumulation in the cytoplasm of the P2 segment of the proximal tubule were evident after PER and more notably PENT administration. Cell replication rates in male rats increased specifically in the histologically damaged P2 segments after PER or PENT exposure. Protein droplets and cell replication did not differ from controls in TCE-treated male rats or in female rats treated with TCE, PER, or PENT. Immunohistochemical staining for alpha 2 mu revealed a marked correlation between the presence of alpha 2 mu and the protein droplets. Renal alpha 2 mu concentrations in male rats increased after PER or PENT but not TCE administration. The protein droplet nephropathy induced in male rats after PER and PENT treatment appears identical to that observed with other male-rat-specific renal carcinogens such as unleaded gasoline. The differences observed in male and female rats after chlorinated hydrocarbon exposure suggest that increases in cell replication may be directly linked to the male-rat-specific protein alpha 2 mu. Since compensatory cell division is postulated to affect all stages of the carcinogenic process, the increased incidence of renal tumors in male rats after PER or PENT treatment may be related to nephrotoxicity and resulting enhanced cell replication. Mechanisms involved in TCE-induced renal carcinogenicity appear to be different from PER- and PENT-induced renal carcinogenicity.

Carcinogenicity of inhaled benzene in CBA mice.

This study investigated benzene-induced neoplasia in CBA/Ca mice, with special emphasis on hematopoietic tissues. Ten-week-old male CBA/Ca mice were exposed to 300 ppm benzene via inhalation for 6 hr/day, 5 days/week, for 16 weeks and held 18 months after the last exposure. There were 125 benzene-exposed and 125 sham-exposed mice. Malignant lymphoma was a statistically significant cause of early mortality in the benzene-exposed mice. Fourteen benzene-exposed mice developed lymphoma (lymphoblastic, lymphocytic, or mixed) as compared to only 2 sham-exposed mice. Benzene-exposed mice also developed preputial gland squamous cell carcinomas (60% in benzene-exposed vs 0% in sham-exposed) and had an increased incidence of lung adenomas (36% vs 14%). Moderate to marked granulocytic hyperplasia was present in benzene-exposed animals, with a 36% incidence in the bone marrow and 6% in the spleen, as compared to the sham-exposed with 8 and 0%, respectively. Interpretation of the granulocytic response as a direct effect of benzene was complicated by the presence of inflammation in the mice. Although inhaled benzene was clearly carcinogenic in CBA mice, it did not induce granulocytic leukemia.

Dose-related effects of the hepatocarcinogen, Wy-14,643, on peroxisomes and cell replication☆

The dose and time dependency of peroxisome proliferation and hepatocyte replication was evaluated in the liver of rats fed the peroxisome proliferator and hepatocarcinogen, Wy-14,643. Male F344 rats were fed NIH07 diet blended with Wy-14,643 at 0, 5, 10, 50, 100, or 1000 ppm for 1, 3, 6, or 13 weeks. Hepatomegaly was induced by Wy-14,643 at all doses and at all time points. Peroxisome proliferation was present in rats fed 5 ppm Wy-14,643 as early as 1 week, as determined by the peroxisome-specific NAD+ reduction of palmitoyl CoA (PCO) and the peroxisome-associated activity of carnitine acetyltransferase (CAT) (5- and 11-fold over control, respectively). The elevations of PCO and CAT were dose-dependent from 5 to 50 ppm and then plateaued from 50 to 1000 ppm throughout the treatment period. Hepatocellular replication, evaluated by nuclear histoautoradiography ([3H]thymidine labeling, 6-day infusion), was increased in all Wy-14,643 dose groups after 1 week of treatment (5 ppm, 4-fold; 10 ppm, 5-fold; 50 ppm, 13-fold; 100 ppm, 12-fold; and 1000 ppm, 13-fold over controls). However, in 5 and 10 ppm groups this cell replication returned to control levels by 3 weeks. In contrast, 50, 100, and 1000 ppm groups had sustained increases in cell replication up to 13 weeks (13 weeks: 6-, 7-, and 9-fold over controls, respectively). We have demonstrated that Wy-14,643 can induce peroxisome proliferation at 5 ppm, a dose 200 times lower than the dose shown to be highly hepatocarcinogenic in rats (100% incidence by 60 weeks).(ABSTRACT TRUNCATED AT 250 WORDS)