Follow-up study on the carcinogenicity of vinyl chloride and vinylidene chloride in rats and mice: tumor incidence and mortality subsequent to exposure

A novel application of the Margin of Exposure approach: segregation of tobacco smoke toxicants

This paper presents a rationale for utilising a Margin of Exposure (MOE) approach to the segregation of tobacco smoke toxicants for risk assessment and management purposes. Future regulatory frameworks and product modifications aimed at tobacco harm reduction could utilise data that segregate toxicants using associations with specific diseases caused by cigarette smoking together with an indication of their relative contribution to that disease. Compounds with MOEs >10,000 accompanied by appropriate narrative are considered "low priority for risk management actions". This paper applies the MOE model to representative examples of tobacco smoke toxicants associated with respiratory tract carcinogenesis and other respiratory diseases. A multiplicity of published dose response data on individual toxicants has been used to determine the range of possible MOE values, thus demonstrating the consistency of the relationships. Acetaldehyde, acrolein, acrylonitrile, cadmium, ethylene oxide, formaldehyde and isoprene all segregate with MOEs <10,000 and should be considered as high priority for exposure reduction research whereas benzo(a)pyrene and vinyl chloride segregate with an MOE >10,000 and therefore may be considered as a low priority. 1,3-Butadiene, m-/p-cresols, NNK and NNN are assumed to segregate with high priority although additional data would be required to complete a full MOE assessment.

Vinyl chloride and the liver

Vinyl chloride monomer is a known cause of angiosarcoma of the liver. It also has other toxic effects on the liver, and it has recently been suggested that exposure to vinyl chloride also causes hepatocellular carcinoma. However, the data on which this conclusion is based is incomplete. There is inadequate ascertainment of unequivocal diagnoses. In the largest studies lack of data meant that confounding diseases such as viral hepatitis or alcoholic liver disease could not be assessed. At best, the increase in risk is minimal, based on more than 22,000 exposed workers and more than 640,000 person years of observation. However, based on the available data the hypothesis that vinyl chloride causes or contributes to the development of hepatocellular carcinoma remains unproven.

Vinyl chloride: a case study of data suppression and misrepresentation

When the U.S. Environmental Protection Agency (EPA) finalized its 2000 update of the toxicological effects of vinyl chloride (VC), it was concerned with two issues: the classification of VC as a carcinogen and the numerical estimate of its potency. In this commentary we describe how the U.S. EPA review of VC toxicology, which was drafted with substantial input from the chemical industry, weakened safeguards on both points. First, the assessment down-plays risks from all cancer sites other than the liver. Second, the estimate of cancer potency was reduced 10-fold from values previously used for environmental decision making, a finding that reduces the cost and extent of pollution reduction and cleanup measures. We suggest that this assessment reflects discredited scientific practices and recommend that the U.S. EPA reverse its trend toward ever-increasing collaborations with the regulated industries when generating scientific reviews and risk assessments.

Evaluation of the carcinogenicity of 1,1-dichloroethylene (vinylidene chloride)

The U.S. Environmental Protection Agency has classified 1,1-dichloroethylene (vinylidene chloride; VDC) as a "C" carcinogen and has developed an inhalation unit risk value and an oral cancer slope factor for this chemical. The development and use of these cancer potency estimates for risk assessment purposes are questionable. The inhalation unit risk value is based on increased kidney adenocarcinomas in Swiss mice from one study. This type of cancer was not increased in female mice or in rats or hamsters in the same study nor in male mice of a similar strain in another study with higher VDC exposures. The VDC oral cancer slope factor is based on a non-statistically significant increase in adrenal pheochromocytomas in male rats following oral exposure in a standard National Toxicology Program chronic bioassay. Both human and animal literature relevant to VDC carcinogenicity was reviewed according to the USEPA draft Guidelines for Carcinogen Risk Assessment with the objective of determining the weight-of-evidence for VDC carcinogenicity. We conclude that information currently available for VDC is most appropriately characterized in a weight-of-evidence narrative by the descriptor "inadequate for an assessment of human carcinogenic potential." For chemicals with this descriptor, dose-response assessment is not indicated. Under this guidance, quantitative estimates of cancer risks associated with VDC exposure are inappropriate until additional, more definitive evidence for human carcinogenicity becomes available.

Pharmacokinetics and metabolism of vinyl fluoride in vivo and in vitro

Vinyl fluoride (VF) is an inhalation carcinogen at concentrations of 25 ppm or greater in rats and mice. The main neoplastic lesion induced in rodents was hepatic hemangiosarcomas, and mice were more sensitive than rats. In a first set of experiments, groups of three rats or five mice were exposed to VF in a closed-chamber gas uptake system at starting concentrations ranging from 50 to 250 ppm. Chamber concentrations of VF were measured every 10-12 min by gas chromatography. Partition coefficients were determined by the vial equilibration technique and used as parameters for a physiologically based pharmacokinetic (PBPK) model. Mice showed a higher whole-body metabolic capacity compared to rats (Vmax = 0.3 vs 0.1 mg/hr-kg). Both species had an estimated Km of < or = 0.02 mg/liter. The specificity for the oxidation of VF in vivo was determined by selective inhibition or induction of CYP 2E1. Inhibition with 4-methylpyrazole completely impaired VF uptake in rats and mice, whereas induction with ethanol (rats only) increased the metabolic capacity by two- to threefold. The pharmacokinetics of VF were also investigated in vitro. Microsomes from rat and mouse liver were incubated in a sealed vial with VF and an NADPH-regenerating system. Headspace concentrations (10-300 ppm) were monitored over time by gas chromatography. Consistent with the in vivo data, VF was metabolized faster by mouse microsomes than by rat microsomes (Vmax = 3.5 and 1.1 nmol/hr-mg protein, respectively). The rates of metabolism by human liver microsomes were generally in the same range as those found with rat liver microsomes (Vmax = 0.5-1.3 nmol/hr-mg protein), but one sample was similar to mice (Vmax = 3.3 nmol/ hr-mg protein). Metabolic rates in human microsomes were found to correlate with the amount of CYP 2E1 as determined by Western blotting and by chlorzoxazone 6-hydroxylation. It is concluded that the greater metabolic capacity of mice for VF both in vivo and in vitro may contribute to their greater susceptibility to tumor formation. CYP 2E1 is clearly the main isozyme involved in the oxidation of VF in all species tested. VF pharmacokinetics and metabolism in humans may depend upon the interindividual variability in the expression level of CYP 2E1. The excellent correspondence between in vivo and in vitro kinetics in rodents improves. substantially the degree of confidence for human in vivo predictions from in vitro data.

Genetic toxicology of vinyl chloride--a review

Vinyl chloride (VC) is a colorless gas with a mild, sweet odor. It is extensively used in the production of vinyl chloride polymer, copolymer resin, packaging materials, wire and cable coatings as well as in industrial and laboratory intermediates. It is toxic and also carcinogenic in experimental animals. The wide human exposure to this compound in different industries throughout the world causes great concern for human health. In the present review an attempt has been made to evaluate and update the genotoxic effects of vinyl chloride based on the available literature.

Effect of exposure route on potency of carcinogens

To compare the effectiveness of different exposure routes for the induction of cancer in experimental animals, the estimated dose associated with a 25% additional risk of cancer (RRD(25)) was calculated using a group of carcinogenic agents for which both inhalation and oral ingestion cancer bioassays were available. Comparisons were made of 14 agents in rats and 9 in mice. Seven of the nine compared in mice were also compared in rats. Among rats, 8 of 14 agents were more effective via the oral route, while 7 of 9 were more effective via the oral route in mice. The variation in RRD(25) values with route, however, was less than 10-fold for all the agents tested in mice and for 11 of 14 tested in rats. An overall difference in potency with route could not be detected statistically. In rats, differences in potency greater than 10-fold were found for asbestos, vinyl chloride, and hydrazine. In the case of asbestos, the agent was in the form of relatively insoluble particulate matter. The greater potency via inhalation is likely due to longer residence time of the particles in the deep lung than in the gut, allowing for a greater degree of particle dissolution with an accompanying increase in bioavailability. Vinyl chloride was generally tested by inhalation at doses high enough to saturate activation pathways, resulting in underestimates of low-dose potency. Many of the smaller potency differences with route, as well as those for hydrazine, were considered likely to be the result of variability in the design and/or quality of studies. It was concluded that, if the design and conduct of the experiments were adequate, if agents in the form of relatively insoluble particulate matter are eliminated, and if corrections are made to account for incomplete activation, then large errors during route extrapolation are unlikely to occur.

Investigations on the relationship between DNA ethenobase adduct levels in several organs of vinyl chloride-exposed rats and cancer susceptibility

The levels of 1,N6-ethenodeoxyadenosine (epsilon dAdo) and 3,N4-ethenodeoxycytidine (epsilon dCyd) were measured in DNA of several target organs of vinyl chloride (VC)-exposed rats. Seven-day-old (group I) and 13-week-old (group II) BD VI rats were exposed during 2 weeks to 500 ppm VC in air (7 hr per day and 7 days per week). epsilon dAdo and epsilon dCyd were measured by a combination of prepurification of DNA hydrolysates by HPLC and competitive radioimmunoassay using specific murine monoclonal antibodies. Both ethenodeoxynucleosides were detected in liver, lungs and brain (levels ranging from 0.6 x 10(-7) to 1.3 x 10(-7) for epsilon dAdo/2'-deoxyadenosine and from 1.95 x 10(-7) to 4.92 x 10(-7) for epsilon dCyd/2'-deoxycytidine) but not in kidneys of group I rats. In group II rats, only liver DNA was analysed and the levels of each adduct were six times lower than in young (group II) rats. These findings are in good agreement with the organotropism and the age-related sensitivity of VC-induced carcinogenesis in rodents.

Long-term assays for carcinogenicity

During the past two decades, the rodent bioassay for detection of chemical carcinogens has reached a high standard of performance with both an increased number of animals and dose levels and a more detailed assessment of findings. However, the basic principles of testing and evaluation of results have remained essentially unchanged. Problems such as the length of the testing, use of maximum tolerated dose (MTD), selection of strains, variability of spontaneous tumors, discordant results between mouse and rat, and the classification of chemical carcinogens according to their mechanism of action have all remained unsolved. By contrast, the results of short-term tests and of other biological analyses do not always show a direct correlation with those of the long-term bioassays; this can be interpreted as an indication of different mechanisms of carcinogenicity. Currently available medium-term tests may detect carcinogenic activity of chemicals at particular organs in a period of time (weeks to months) relatively shorter than that of the 2-year carcinogenesis bioassay, and they may also provide additional information on mechanisms of carcinogenicity.

Incorporation of biological information in cancer risk assessment: example--vinyl chloride

Vinyl chloride (VC) is used as an example to demonstrate how biological information can be incorporated into quantitative risk assessment. The information included is the pharmacokinetics of VC in animals and humans and the data-generated hypothesis that VC primarily affects the initiation stage of the multistage carcinogenesis. The emphasis in this paper is on the improvement of risk assessment methodology rather than the risk assessment of VC per se. Sufficient data are available to construct physiologically-based pharmacokinetic models for both animals and humans. These models are used to calculate the metabolized dose corresponding to exposure scenarios in animals and in humans. On the basis of the data on liver angiosarcomas and carcinomas in rats, the cancer risk per unit of metabolized dose is comparable, irrespective of routes (oral or inhalation) of exposure. The tumor response from an intermittent/partial lifetime exposure is shown to be consistent with that from a lifetime exposure when VC is assumed to affect the first (initiation) stage of the multistage carcinogenic process. Furthermore, the risk estimates calculated on the basis of animal data are shown to be consistent with the human experience.

Nucleophilic selectivity as a determinant of carcinogenic potency (TD50) in rodents: a comparison of mono- and bi-functional alkylating agents and vinyl chloride metabolites

Using published data, the carcinogenic potency (TD50) in rodents of a series of monofunctional alkylating agents, bifunctional antitumor drugs and the vinyl chloride (VC) metabolites chloroethylene oxide (CEO) and chloroacetaldehyde (CAA) was compared to their nucleophilic selectivity (Swain and Scott's constant s or initial ratio of 7-/O6-alkylguanine in DNA). A positive correlation between the log of TD50 estimates and the s values for a series of 14, mostly monofunctional, alkylating agents was observed. This linear relationship also included 2 bifunctional chloroethylnitrosoureas, although their carcinogenic potency was compared to their initial 7-/O6-alkylguanine ratio rather than their s values (n = 16, r = 0.91, p less than 0.005). In addition, the carcinogenic potency of 2 alkyl sulfates, which is not yet known accurately, may correlate with their nucleophilic selectivity through the same relationship. By contrast, 2 methyl halides and 5 bifunctional antitumor drugs (nitrogen mustards and azyridinyl derivatives) did not follow this linear relationship: at similar nucleophilic selectivity, they were more potent carcinogens than the above 18 alkylating agents; this may hold true for CEO and CAA too, although further carcinogenicity experiments are needed to calculate their precise TD50 values. The possible molecular mechanisms involved in tumor induction by these agents are discussed on the basis of these findings. Comparison of the estimated TD50 for CEO, CAA and VC in rodents confirms that CEO is the ultimate carcinogenic metabolite of VC and suggests that only a very small proportion of metabolically generated CEO is available for DNA alkylation in vivo.

Effects of vinyl chloride on liver function of exposed workers, evaluated by measurements of plasma clearance of the 99mTc-N-2,4-dimethylacetanilido-iminodiacetate complex

In 77 males exposed occupationally to vinyl chloride (VC), the plasma clearance (Cl) of 99mTc-N(2,4-dimethylacetanilido)iminodiacetate ('HEPIDA' complex) was determined. The results were juxtaposed with a scaled assessment of liver parenchyma performance based upon clinical examination and a series of biochemical tests. Detection of the diagnosable damage of liver parenchyma by means of the reduced clearance was sensitive (90%) at the reasonable specificity of 74%. Probability of exclusion of liver damage in patients with the clearance above 240 ml min-1 1.73 m-2 amounted to 92%. There was a significant correlation between degree of exposure to VC and the frequency of low clearance values. It appears that the periodic determination of the 99mTc-HEPIDA clearance in workers exposed to VC allows the assessment of incipient liver damage and signals the need for prophylactic measures.

A cohort study on vinyl chloride manufacturers in Italy: study design and preliminary results

A cohort mortality study of 5000 vinyl chloride manufacturers is ongoing in 9 Italian plants. They represent the entire workforce of those ever employed in the production of the monomer and its polymerization. The objectives of the study are to investigate the mortality of the exposed population and to clear up the carcinogenic spectrum of vinyl chloride. This article gives the results for 3 out of 9 plants, Rosignano, Ferrara and Ravenna, which represent about 25% of the total cohort. The expected deaths have been calculated using the mortality rates of the Italian population. For the deceased persons information from the death certificates were used in the analysis of mortality; additional clinical and pathological data were collected (best pathological evidence, b.p.e.). In Ferrara a statistically significant excess for all malignant tumors and lung cancer was detected. In Rosignano and Ravenna the number of observed deaths were small and therefore no comments can be made on cancer mortality. The cohort study is ongoing in the 6 remaining cohorts and the future analysis will consider duration and level of exposure and latency.

Chemical carcinogens. A review and analysis of the literature of selected chemicals and the establishment of the Gene-Tox Carcinogen Data Base. A report of the U.S. Environmental Protection Agency Gene-Tox Program

The literature on 506 selected chemicals has been evaluated for evidence that these chemicals induce tumors in experimental animals and this assessment comprises the Gene-Tox Carcinogen Data Base. Three major sources of information were used to create this evaluated data base: all 185 chemicals determined by the International Agency for Research on Cancer to have Sufficient evidence of carcinogenic activity in experimental animals, 28 selected chemicals bioassayed for carcinogenic activity by the National Toxicology Program/National Cancer Institute and found to induce tumors in mice and rats, and 293 selected chemicals which had been evaluated in genetic toxicology and related bioassays as determined from previous Gene-Tox reports. The literature data on the 239 chemicals were analyzed by the Gene-Tox Carcinogenesis Panel in an organized, rational and consistent manner. Criteria were established to assess individual studies employing single chemicals and 4 categories of response were developed: Positive, Negative, Inconclusive (Equivocal) and Inconclusive. After evaluating each of the individual studies on the 293 chemicals, the Panel placed each of the 506 chemicals in an overall classification category based on the strength of the evidence indicating the presence or absence of carcinogenic effects. An 8-category decision scheme was established using a modified version of the International Agency for Research on Cancer approach. This scheme included two categories of Positive (Sufficient and Limited), two categories of Negative (Sufficient and Limited), a category of Equivocal (the evidence of carcinogenicity from well-conducted and well-reported lifetime studies had uncertain significance and was neither clearly positive nor negative), and three categories of Inadequate (the evidence of carcinogenicity was insufficient to make a decision, however, the data suggested a positive or negative indication). Of the 506 chemicals in the Gene-Tox Carcinogen Data Base, 252 were evaluated as Sufficient Positive, 99 as Limited Positive, 40 as Sufficient Negative, 21 as Limited Negative, 1 as Equivocal, 13 as Inadequate with the data suggesting a positive indication, 32 as Inadequate with the data suggesting a negative indication, and 48 Inadequate with the data not suggesting any indication of activity. This data base was analyzed and examined according to chemical class, using a 29 chemical class scheme.(ABSTRACT TRUNCATED AT 400 WORDS)

Oncogenic response of strain A/J mice to inhaled chemicals

Strain A/J mice were exposed by inhalation for 6 h/d, 5 d/wk, for 6 mo to carbon disulfide, 1,2-dibromoethane, ethylene oxide, naphthalene, nitrogen dioxide, or vinyl chloride. Significant increases in pulmonary adenoma formation were observed following exposure to 300 ppm carbon disulfide; 20 and 50 ppm 1,2-dibromoethane; 70 and 200 ppm ethylene oxide; 10 ppm nitrogen dioxide; and 50, 200, and 500 ppm vinyl chloride compared to control animals. Repeated studies with 1,2-dibromoethane, ethylene oxide, and vinyl chloride gave similarly significant results. Exposure of mice to 30 ppm naphthalene did not elicit a significant adenoma response. Histopathological examination of lungs from animals in these studies revealed multiple alveolar adenomas. Results from earlier studies with these chemicals, using strain A mice and Swiss mice, and bioassay information with rats and mice were compared with these data. These results provide further information for the validation of this in vivo model as a tool for predicting oncogenic potential following chemical exposure.

Non-reversibility of vinyl chloride carcinogenesis in rodents

A review of the data obtained from various studies on carcinogenicity of vinyl chloride (VC) in rodents, particularly on the effect of dose, age, duration of exposure and potential reversibility of lesions, revealed that vinyl chloride-induced carcinogenicity in rodents was dose and time related; no recovery occurred in mice even after only 1 month of VC exposures or in rats after 6-month exposures. In addition, younger animals (2 months old) were more susceptible to VC-induced carcinogenicity than animals held for 6 or 12 months prior to exposure. Initial 6 or 12 month exposures were adequate to detect the carcinogenic potential of VC. The above information was used as a basis for discussion on design of carcinogenicity studies. Possibility of determining the carcinogenic potential of a compound in a shorter period than the traditional 2 year studies in rodents was discussed in consideration with appropriate doses, species, age and exposure duration. Although this approach may be applicable to a strong carcinogen, it was not considered practicable in case of weak or unknown carcinogens.