The carcinogenicity of environmental tobacco smoke

Male strain A/J mice were exposed for 6 h a day, 5 days a week to environmental tobacco smoke (ETS) generated from Kentucky 1R4F reference cigarettes. Chamber concentrations were 87 mg/m3 of total suspended particulate matter (TSP), 246 p.p.m. of CO and 16 mg/m3 of nicotine. After 5 months, 33% of the ETS exposed and 11% of the control animals had one or several lung tumors; the difference was statistically not significant. A second group of animals exposed for 5 months to ETS was allowed to recover for another 4 months in filtered air. When they were killed, 85% of the ETS animals had lung tumors (average number per lung: 1.4 +/- 0.2), whereas in the control group 38% had lung tumors (average number of lung tumors in all animals 0.5 +/- 0.2). The differences in tumor incidence and multiplicity were statistically significant. More than 80% of all tumors were adenomas, the rest adenocarcinomas. When animals were pretreated with a carcinogen, lung tumor multiplicity was lower in the ETS exposed animals after 5 months compared with controls injected with a carcinogen and kept in air. However, after an additional 4 month recovery period in air, lung tumor multiplicities were the same in ETS plus carcinogen exposed mice as in carcinogen-treated air-exposed controls. Histopathologic and morphometric analysis of the lung tissue failed to reveal any differences between ETS exposed and control animals. However, immediately after ETS exposure, immunohistochemistry revealed increased staining for CYP1A1 in airway epithelia and lung parenchyma; following recovery in air, the staining disappeared again. Analysis of cell kinetics showed an initial burst of increased DNA synthesis in the epithelial cells of the airways and a smaller early positive response in the parenchyma. Feeding of butylated hydroxytoluene during ETS exposure did not modulate lung tumor development. It was concluded that ETS is a pulmonary carcinogen in strain A/J mice.

A recommended occupational exposure limit for formaldehyde based on irritation

In recent years, several regulatory agencies and professional societies have recommended an occupational exposure limit (OEL) for formaldehyde. This article presents the findings of a panel of experts, the Industrial Health Foundation panel, who were charged to identify an OEL that would prevent irritation. To accomplish this task, they critiqued approximately 150 scientific articles. Unlike many other chemicals, a large amount of data is available upon which to base a concentration-response relationship for human irritation. A mathematical model developed by Kane et al. (1979) for predicting safe levels of exposure to irritants based on animal data was also evaluated. The panel concluded that for most persons, eye irritation clearly due to formaldehyde does not occur until at least 1.0 ppm. Information from controlled studies involving volunteers indicated that moderate to severe eye, nose, and throat irritation does not occur for most persons until airborne concentrations exceed 2.0-3.0 ppm. The data indicated that below 1.0 ppm, if irritation occurs in some persons, the effects rapidly subside due to "accommodation." Based on the weight of evidence from published studies, the panel found that persons exposed to 0.3 ppm for 4-6 h in chamber studies generally reported eye irritation at a rate no different than that observed when persons were exposed to clean air. It was noted that at a concentration of 0.5 ppm (8-h TWA) eye irritation was not observed in the majority of workers (about 80%). Consequently, the panel recommended an OEL of 0.3 ppm as an 8-h time-weighted average (TWA) with a ceiling value (CV) of 1.0 ppm (a concentration not to be exceeded) to avoid irritation. The panel believes that the ACGIH TLV of 0.3 ppm as a ceiling value was unnecessarily restrictive and that this value may have been based on the TLV Committee's interpretation of the significance of studies involving self-reported responses at concentrations less than 0.5 ppm. The panel concluded that any occupational or environmental guideline for formaldehyde should be based primarily on controlled studies in humans, since nearly all other studies are compromised by the presence of other contaminants. The panel also concluded that if concentrations of formaldehyde are kept below 0.1 ppm in the indoor environment (where exposures might occur 24 h/d) this should prevent irritation in virtually all persons. The panel could not identify a group of persons who were hypersensitive, nor was there evidence that anyone could be sensitized (develop an allergy) following inhalation exposure to formaldehyde. The panel concluded that there was sufficient evidence to show that persons with asthma respond no differently than healthy individuals following exposure to concentrations up to 3.0 ppm. Although cancer risk was not a topic that received exhaustive evaluation, the panel agreed with other scientific groups who have concluded that the cancer risk of formaldehyde is negligible at airborne concentrations that do not produce chronic irritation.

The toxicology of environmental tobacco smoke

It has by now become obvious that environmental tobacco smoke (ETS) may pose a health risk to nonsmokers. Epidemiological data suggest that exposure to ETS may increase the risk of developing lung cancer, cardiovascular disease, intrauterine growth retardation, predisposition to chronic lung disease, and sudden infant death syndrome. The human populations most at risk from ETS exposure appear to be neonates, young children, and possibly the fetus while in utero. Experimental studies with cigarette sidestream smoke (SS) have successfully duplicated several of these disease conditions in laboratory animals, particularly the effects of SS on fetal growth, lung maturation, and altered airway reactivity. The availability of animal models may open the way to fruitful experimental studies on mechanisms that help us to better understand disease.

Correlation between cumulative labeling indices measured in the terminal bronchioles and in the centriacinar region in the lungs of rats exposed to oxidant air pollutants

Male Sprague-Dawley rats were exposed on 7 consecutive days for 8 h a night to 0.6 ppm ozone, 10.8 ppm NO2 or a mixture of the two gases. Cumulative labeling indices were measured in the epithelium of the terminal bronchioles and the adjacent centriacinar region. An excellent correlation in labeling indices between the two sites was found. It is concluded that the labeling index in the terminal bronchioles represents a sensitive measurement for ozone-induced lung injury.

Histochemical characterization of non-neuroendocrine tumors and neuroendocrine cell hyperplasia induced in hamster lung by 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone with or without hyperoxia

Lung tumors induced by 4-(methylnitrosamine)-1-(3-pyridyl)-1-butanone (NNK) with or without hyperoxia have frequent K-ras mutations but only rare p53 mutations, suggesting that this may be a model for non-small cell lung cancers. The goals of the present study were (1) to characterize the histopathology of lung tumors induced in hamsters by NNK with or without O2 and (2) as a corollary, to quantitate the pulmonary neuroendocrine cell hyperplasia in the different treatment groups early and late in the treatment period. Lung tumors induced by NNK with or without O2 were 71% adenomas, 22% adenocarcinomas, approximately 4% bronchoalveolar carcinomas, and approximately 4% squamous/adenosquamous carcinomas. One-half of all tumors were positive for the Clara cell antigen CC10 and 21% of NNK-induced tumors were mucin positive, compared with 2% of NNK/O2-induced tumors (P = 0.003). Immunostaining for PGP9.5 was positive in 5% of tumors induced by NNK alone, but in none of NNK/O2-induced tumors (P = 0.024). Abundant proliferating cell nuclear antigen occurred in 55% of NNK-induced tumors, compared with 19% of NNK/O2-induced tumors (P = 0.009). These data indicate that NNK with or without O2 induces non-neuroendocrine lung tumors. Hyperoxia appears to inhibit cell proliferation and suppress mucinous and partial neuroendocrine differentiation in some of these tumors.

Six-month exposure of strain A/J mice to cigarette sidestream smoke: cell kinetics and lung tumor data

Male strain A/J mice were exposed to sidestream smoke (SS) generated from burning Kentucky 1R4F reference cigarettes. Chamber concentrations were 4 mg/m3 of total suspended respirable particulate matter (TSP). Animals were exposed 6 hr a day, 5 days a week. One-week cumulative labeling indices were significantly increased in the large intrapulmonary airways during the 1st week and in the respiratory epithelium of the nasal and maxillar turbinates during the first 3 weeks of exposure and then returned to control values. Subsequently, signs of increased cell proliferation were again found in the nasal and maxillar turbinates during the 9th and 16th exposure weeks. The experiment was terminated after 6 months. The number of animals bearing lung tumors was the same in smoke-exposed as in filtered air-exposed animals as was the average number of tumors per lung. Analysis of the DNA of individual tumors obtained from exposed and control mice for K-ras mutations suggested that exon 2 might be a specific target for SS. It was concluded that (1) duration of exposure was too short or (2) concentration of TSP was too low to reveal a possible carcinogenic potential of SS in strain A/J mice or that (3) SS is not carcinogenic in strain A mice.

Mutations of the Ki-ras protooncogene in 3-methylcholanthrene and urethan-induced and butylated hydroxytoluene-promoted lung tumors of strain A/J and SWR mice

Mutations of the Ki-ras protooncogene in 190 lung tumors initiated in male A/J and SWR mice by 3-methylcholanthrene(MCA) or urethan and promoted by butylated hydroxytoluene (BHT), were evaluated by utilizing polymerase chain reaction (PCR) and sequencing analysis. The most common mutation pattern was a GC to CG transversion at the first base of codon 12/13. The predominant mutation pattern at codon 61 was an AT to TA transversion and the next frequent one an AT to GC transition. Mutations of Ki-ras codon 12/13 were found in 44% (A/J) and 13% (SWR) of MCA-induced and in 94% (A/J) and 43% (SWR) of MCA plus BHT-induced lung tumors. Mutations of the Ki-ras codon 61 were found in 31% (A/J) and 13% (SWR) of urethan-induced and 69% (A/J) and 44% (SWR) of urethan plus BHT-induced lung tumors. These data suggest that in strain A/J mice the 2 carcinogens produce Ki-ras mutations and that BHT promotes the activations of Ki-ras protooncogenes in lung tumors.

Environmental tobacco smoke: experimental facts and societal issues

Involuntary exposure to environmental tobacco smoke (ETS) in public or in working places is considered to be a serious risk to human health. This symposium addressed several issues of toxicological interest that are associated with exposure to ETS. Epidemiologic evidence obtained in human studies suggests that "passive smoking" increases the risk of developing lung cancer in nonsmokers and favors the development of respiratory tract infections in children. Comparatively few data are available from animal studies that provide experimental support of the observations. Exposure of pregnant or neonate rats to cigarette sidestream smoke (SS) affects developmental patterns of drug metabolizing enzymes that may persist up to 90 days. In young roosters, SS accelerates the development of arteriosclerotic plaques. On the other hand, exposure of adult rats for up to 90 days induces only transient signs of damage in the nasal passages, but not in the deep lung, and this only at extremely high concentrations of ETS. So far, experimental toxicology has provided comparatively few data on the correlation between exposure to ETS and adverse health effects. yet, such data are needed, particularly since many conclusions drawn from the epidemiological studies remain open to criticism and questions.

Cumulative labeling indices in epithelial cell populations of the respiratory tract after exposure to ozone at low concentrations

Male Sprague-Dawley and F344 rats were exposed to concentrations of 0.12 ppm of ozone for 12 hr a night or for 24 hr a day, to 0.24 ppm for 24 hr a day, and to 0.36 ppm for 8 hr a night. Cumulative labeling indices were measured during the first and the third week of exposure in the terminal bronchioles, the large intrapulmonary airways, the trachea, and the anterior nasal passages. The most sensitive indicator for exposure to ozone after a 1-week exposure was an increased labeling index in the epithelium lining the terminal bronchioles. Sprague-Dawley and F-344 rats were equally sensitive to ozone. In the large intrapulmonary airways and in the maxillar turbinates, an increased cumulative labeling index was observed only at the highest dose (concentration x time). During the third week of exposure, no increases in labeling index were found in the large intrapulmonary airways and terminal bronchioles, whereas in the nasal passages the highest ozone concentration continued to elicit a proliferative response. Animals were also exposed for 1 week to ozone and then allowed to recover in air for another week. Renewed exposure to ozone produced a smaller, but still significant, increase in the labeling index compared to that in animals exposed to ozone for the first time. It was concluded that the determination of the cumulative labeling index in the terminal bronchioles may serve as a method to explore dose and time effects of such concentrations of ozone as may be encountered in heavily polluted urban environments.

Ozone, NO, and NO2: oxidant air pollutants and more

This article reviews the acute and chronic toxicity of the three oxidant air pollutants ozone, nitric oxide (NO), and nitrogen dioxide (NO2). The toxicity of binary mixtures of NO2 with other inhaled agents is also discussed. Newer studies are emphasized, especially those published in the last 5 years or still in press. Very recent data from our laboratory that suggest a new cellular mechanism of importance in lung injury in animals exposed to mixtures of ozone and NO2 that may have general relevance with regard to the effects of oxidant air pollutants on the lung are also presented.

Nicotine- or epinephrine-induced uteroplacental vasoconstriction and fetal growth in the rat

We examined the relationship between nicotine-induced vasoconstriction in pregnant rat dams and fetal growth during the third trimester of pregnancy. Pregnant rats were continuously treated between days 13 and 19 of gestation with either nicotine (9.6, 4.8 or 2.4 mg/kg/day), epinephrine (0.72 microgram/kg/day), or saline via continuous infusion from a subcutaneously implanted osmotic minipump. Placental weights in rats treated with high dose nicotine and dams' body weights were severely reduced. However, fetal weights were not affected. Blood flows in uterus and placenta were quantified by measurement of tissue content of 85Sr-labelled microspheres injected via a carotid artery catheter. Both nicotine and epinephrine caused a significant reduction (> 40%) in uterine and placental blood flow. We conclude that vasoconstriction alone as a result of nicotine or epinephrine administration during the last trimester of gestation does not necessarily reduce nutrient supply to the fetus and does not affect fetal growth in rats.

Effects of exposure to nicotine and to sidestream smoke on pregnancy outcome in rats

Nicotine-delivering transdermal patches were applied to the back of timed-pregnant Sprague-Dawley rats. Pregnancy failure was 100% in animals exposed to 3.5 mg of nicotine per day during the entire pregnancy and 50% in animals exposed to the same amount during the first trimester. Application of 1.75 mg of nicotine per day resulted in a 50% pregnancy failure when exposure occurred during the entire pregnancy. In animals exposed for the first half of pregnancy to cigarette sidestream smoke, under conditions where plasma nicotine levels reached about 25% of those observed following exposure to 1.75 mg of nicotine per day, the average litter size was reduced by about 25%. It is concluded that continuous exposure to nicotine early during pregnancy may adversely affect pregnancy outcome in rats.

Short-term effects of sidestream smoke on respiratory epithelium in mice: cell kinetics

Male strain A/J and C57BL/6 mice were exposed on five consecutive days, for 6 hr a day, to sidestream smoke generated from Kentucky 1R4F reference cigarettes. Chamber concentrations were 1 mg/m3 of total suspended particulate matter and 528 to 549 micrograms/m3 of nicotine. Cumulative labeling indices in the airways and in the pulmonary parenchyma were measured following 1, 3, or 5 days exposure to unfiltered or filtered sidestream smoke. A significantly increased labeling index was found in A/J mice in the epithelium lining large intrapulmonary airways and terminal bronchioles after 3 and 5 days exposure to unfiltered smoke, whereas following exposure to filtered smoke labeling indices remained normal. The alveolar labeling index was not increased following smoke exposure. In C57BL/6 mice, sidestream smoke did not produce signs of increased cell proliferation in the respiratory tract. It is concluded that the response to sidestream smoke inhalation in mice may depend upon the strain of mice examined.

Decreased fetal weights in rats exposed to sidestream cigarette smoke

Pregnant Sprague-Dawley rats were exposed to sidestream cigarette smoke (SS) for 6 hr a day, at a concentration of 1 mg/m3 of respirable total suspended particulate material (TSP) on Days 3, 6-10, and 13-17 of pregnancy. Controls were kept in an identical chamber without smoke exposure. The animals were killed on Day 20 of gestation. No differences were found in maternal body weight gain or average daily food consumption between the smoke-exposed and control groups. The numbers of fetuses and of implantation sites per litter were comparable among the groups. None of the pups showed any gross malformations and no difference was found between controls and SS-exposed pups when examined for reduced skeletal ossifications. However, there was a small but significant reduction in mean pup weight. We conclude that intermittent exposure of rats to sidestream cigarette smoke at concentrations severalfold greater than those encountered in smokey public indoor environments causes intrauterine growth retardation.

Airway epithelial labeling index as an indicator of ozone induced lung injury

Rats were implanted subcutaneously with a bromodeoxyuridine-filled minipump and then were exposed to ozone delivered at a low dose rate (0.4 ppm during 12 h per night) or at a high dose rate (0.8 ppm during 6 h per night). Three and 7 days after pump implantation the cumulative labeling indices were measured in the alveolar zone and in the airways. Greater alveolar labeling indices were observed 7 days after implantation of the minipumps than after 3 days in all groups, but no ozone-related changes were found in the alveoli of rats in either experimental group at either time. After 3 or 7 days, the labeling index in the large intrapulmonary airways and in the terminal bronchioli of the rats exposed to the higher dose rate (0.8 ppm) was increased. In rats exposed to the lower dose rate (0.4 ppm) the labeling index was significantly elevated in the terminal bronchioli after 3 days and in both the terminal bronchioli and large intrapulmonary airways after 7 days. In the terminal bronchioli the extent of cell proliferation appeared to be defined by dose rate rather than by cumulative exposure. It is concluded that measurement of the airway labeling index is a sensitive indicator of the response of the rat lung to acute exposure to ozone.

Failure of ozone and nitrogen dioxide to enhance lung tumor development in hamsters

We tested the hypothesis that the two common oxidant air pollutants, ozone and nitrogen dioxide, modulate the development of respiratory tract tumors in Syrian golden hamsters. The animals received subcutaneous injections of the carcinogen diethylnitrosamine (20 mg/kg) twice a week while being exposed continuously to an atmosphere of 0.8 parts per million (ppm)* of ozone or 15 ppm of nitrogen dioxide. Animals were killed 16 weeks or 24 to 32 weeks after the beginning of the treatment. Ozone delayed the appearance of tracheal tumors and reduced the incidence of tumors in the lung periphery. A suspected neuroendocrine differentiation of those lung tumors could not be established by immunocytochemistry due to overfixation of tissues. On the other hand, ozone seemed to mitigate development of hepatotoxic lesions mediated by diethylnitrosamine. In animals treated with diethylnitrosamine and exposed to nitrogen dioxide, fewer tracheal tumors and no lung tumors were found. Only a few lung tumors were produced in animals treated with diethylnitrosamine and kept in an atmosphere of 65% oxygen. The previously observed neuroendocrine nature of tumors induced by simultaneous exposure to diethylnitrosamine and hyperoxia could not be established because the long fixation of tissues precluded immunocytochemical stains. Animals treated with diethylnitrosamine and kept in filtered air while being housed in wire-mesh cages developed fewer lung tumors than animals given the same treatment and kept on conventional bedding in shoebox cages. Although all inhalants tested are known to produce substantial cell proliferation in the respiratory tract, it was not possible to document whether this would enhance lung tumor development. The role of the two common air pollutants, ozone and nitrogen dioxide, as possible additional risks in the pathogenesis of lung cancer in animals continues to remain uncertain.

Alveolar and airway cell kinetics in the lungs of rats exposed to nitrogen dioxide, ozone, and a combination of the two gases

Rats carrying minipumps filled with 5-bromo-2'deoxyuridine were exposed to ozone, NO2, or a mixture of the two gases using four different protocols: (A) ozone 0.2 ppm, NO2 3.6 ppm, or their mixture for 24 hr a day; (B) ozone 0.2 ppm, NO2 7.2 ppm, or their mixture for 12 hr per night; (C) ozone 0.6 ppm, NO2 10.8 ppm, or their mixture for 8 hr per night; and (D) ozone 0.8 ppm, NO2 14.4 ppm, or their mixture for 6 hr per night. After three consecutive daily exposures, the animals were returned to filtered air and killed 7 days after implantation of the minipump. Alveolar labeling indices were comparable to control values except in the group of animals exposed for 6 hr nightly to a combination of 0.8 ppm of ozone and 14.4 ppm of NO2. Labeling indices in the peripheral airways were the most sensitive exposure index since they were significantly increased over control values in all animals exposed to ozone, NO2, or a mixture of the two gases, regardless of concentration or exposure duration. Labeling indices increased with elevated dose rate, i.e., concentration of the gases in the inspired air. The response to the combined gases was greater than the calculated sum of the responses to the two individual gases for the three higher dose rates in the large airways and for the highest dose rate in the peripheral airways. The results led to the following conclusions: (1) By the criterion of analysis of cell kinetics in rat large and peripheral airways, neither ozone, NO2, nor their mixture follows Haber's law (c x t = k) over the concentration ranges studied; and (2) at the higher dose rates studied, there is a more than additive (synergistic) airway response to the combination of ozone and NO2.

A new model of progressive pulmonary fibrosis in rats

Sprague-Dawley rats were exposed for 6 h daily to 0.8 ppm of ozone and 14.4 ppm of nitrogen dioxide. Approximately 7 to 10 wk after the initiation of exposure, animals began to demonstrate respiratory insufficiency and severe weight loss. About half of the rats died between Days 55 and 78 of exposure; no overt ill effects were observed in animals exposed to filtered air, to ozone alone, or to nitrogen dioxide. Biochemical findings in animals exposed to ozone and nitrogen dioxide included increased lung content of DNA, protein, collagen, and elastin, which was about 300% higher than the control values. The collagen-specific crosslink hydroxy-pyridinium, a biomarker for mature collagen in the lung, was decreased by about 40%. These results are consistent with extensive breakdown and remodeling of the lung parenchyma and its associated vasculature. Histopathologic evaluation showed severe fibrosis, alveolar collapse, honeycombing, macrophage and mast cell accumulation, vascular smooth muscle hypertrophy, and other indications of severe progressive interstitial pulmonary fibrosis and end-stage lung disease. This unique animal model of progressive pulmonary fibrosis resembles the final stages of human idiopathic pulmonary fibrosis and should facilitate studying underlying mechanisms and potential therapy of progressive pulmonary fibrosis.

K-ras and p53 point mutations in 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone-induced hamster lung tumors

Lung tumors were induced in Syrian golden hamsters by s.c. injection of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). After 40 weeks lung tumor tissue was isolated. Administration of the NNK and exposure of the animals to an atmosphere of 65% oxygen resulted in a statistically significant reduction in tumor size but did not alter the histological tumor type or tumor incidence when compared with carcinogen treated animals maintained under ambient air. Histologically, lung tumors had the morphologic features of adenomas and adenocarcinomas with approximately 15% being squamous cell carcinomas. Lung tumors were examined for mutations in the Ki-ras oncogene and the p53 tumor suppressor gene by direct sequencing. The Ki-ras mutation frequency in RNA isolated from pooled tumors and in DNA isolated from individual tumors were found to be identical. Activated Ki-ras alleles were detected in 77-94% of tumors. All mutations observed (from a total of 65) except one were GC-AT. The Ki-ras mutations resulted in amino acid substitutions at either codons 12 or 13. No mutations were detected at the 61st codon. Examination of the same tumors for p53 mutations showed only one point mutation. We conclude that the NNK treatment in Syrian golden hamsters results in a distinctive mutation pattern in the Ki-ras gene whereas p53 gene mutations may not play a major role at this stage in hamster lung tumorigenesis.

Modulation of N-nitrosodiethylamine-induced hamster lung tumors by ozone

Male Syrian Golden hamsters were treated with subcutaneous injections of N-nitrosodiethylamine (DEN), 20 mg/kg, twice a week for 24 weeks. Half the animals were kept in filtered air and the other half was exposed continuously to an atmosphere of 0.8 ppm of ozone. After 6 months, no more DEN injections were given and all animals were kept in air until termination of the experiment at 7 months. It was found that the animals kept in ozone developed half as many peripheral lung tumors as did the animals kept in air; however, the difference was not statistically significant. Tumors of the trachea, bronchi, nasal cavity and liver developed with the same incidence whether the animals were exposed to ozone or not. It was concluded that ozone, an agent known to produce cell proliferation in the respiratory tract, does not enhance the development of tumors in the peripheral lung or in the nasal cavity of hamsters.

Cationized Bowman-Birk protease inhibitor as a targeted cancer chemopreventive agent

The conjugate of the Bowman-Birk inhibitor (BBI) with poly(D-lysine) (PDL-ss-BBI) has been suggested as a lung-targeted anti-carcinogenic agent. The authors demonstrate that PDL-ss-BBI, given i.p., reduces the tumor number in the lungs of 3-methylcholanthrene treated mice (61-71% compared to control group) in a dose-dependent manner, but is toxic to the treated animals at a high dosage. In order to develop a better lung-targeted anti-carcinogenic agent, spermine-conjugated BBI (spermine-BBI) was synthesized by coupling BBI to spermine through amide bonds using a carbodiimide-mediated reaction. Results from in vitro transformation assays demonstrated that spermine-BBI was at least as effective as BBI in reducing the transformation yield in C3H10T1/2 cells. When injected intravenously into mice [125I]spermine-BBI accumulated to a greater extent in the lungs and the liver compared to BBI. The in vitro cytotoxicity of spermine-BBI in C3H10T1/2 cells was 30-fold less than that of PDL-ss-BBI. These results suggest that spermine-BBI is likely to be an improved cancer chemopreventive agent compared to BBI or PDL-ss-BBI.

Synergistic interaction of nitrogen dioxide and ozone on rat lungs: acute responses

Rats were exposed for 6 hr per day to either ozone alone (0.2-0.8 ppm), nitrogen dioxide (NO2) alone (3.6-14.4 ppm), or to combinations of these two oxidant air pollutants. Their response was quantified by changes in the total protein content of lung lavage supernatants or by changes in the content of specific cell types in the lung lavage pellets. A concentration-dependent synergistic response was observed when rats were exposed to the combination of ozone and NO2. Apparent threshold concentrations for the observation of synergistic interaction between ozone and NO2 were assay specific, with epithelial cell content of lung lavage fluid being the most sensitive parameter evaluated, showing positive interaction (greater than additive response) at the lowest concentrations tested. Concurrent exposure to ozone and NO2 was necessary to elicit greater than additive responses; no such interactions were seen upon sequential exposure to ozone or NO2 in either order of presentation. Based upon apparent disappearance rates of ozone in the chambers during exposure of rats to ozone and NO2, we modelled the predicted outcomes based upon the assumption that the two oxidant gases were reacting to form nitrogen pentoxide (N2O5) in the chambers. Agreement between predicted concentrations of ozone and NO2 and those actually observed was excellent. Based upon such modelling estimates and our acute toxicological data, we conclude that synergistic toxicologic interactions between ozone and NO2 are found only at concentrations very much higher than would be encountered in environmental or occupational settings. It remains to be determined whether there are any chronic toxicological responses to exposure to combinations of ozone and NO2 at concentrations below the thresholds for observing acute responses.