Dithiocarbamates as potential confounders in butadiene epidemiology

Development of a unit risk factor for 1,3-butadiene based on an updated carcinogenic toxicity assessment

The Texas Commission on Environmental Quality (TCEQ) has developed an inhalation unit risk factor (URF) for 1,3-butadiene based on leukemia mortality in an updated epidemiological study on styrene-butadiene rubber production workers conducted by researchers at the University of Alabama at Birmingham. Exposure estimates were updated and an exposure estimate validation study as well as dose-response modeling were conducted by these researchers. This information was not available to the U.S. Environmental Protection Agency when it prepared its health assessment of 1,3-butadiene in 2002. An extensive analysis conducted by TCEQ discusses dose-response modeling, estimating risk for the general population from occupational workers, estimating risk for potentially sensitive subpopulations, effect of occupational exposure estimation error, and use of mortality rates to predict incidence. The URF is 5.0 x 10(-7) per microg/m(3) or 1.1 x 10(-6) per ppb and is based on a Cox regression dose-response model using restricted continuous data with age as a covariate, and a linear low-dose extrapolation default approach using the 95% lower confidence limit as the point of departure. Age-dependent adjustment factors were applied to account for possible increased susceptibility for early life exposure. The air concentration at 1 in 100,000 excess leukemia mortality, the no-significant-risk level, is 20 microg/m(3) (9.1 ppb), which is slightly lower than the TCEQ chronic reference value of 33 microg/m(3) (15 ppb) protective of ovarian atrophy. These values will be used to evaluate ambient air monitoring data so the general public is protected against adverse health effects from chronic exposure to 1,3-butadiene.

Dithiocarbamates and viral IL-10 collaborate in the immortalization and evasion of immune response in EBV-infected human B lymphocytes

Epstein-Barr virus (EBV) is implicated in the development of a number of human malignancies including several subtypes of non-Hodgkin lymphoma (NHL) [G. Pallesen, S.J. Hamilton-Dutoit, X. Zhou, The association of Epstein-Barr virus (EBV) with T cell lymphoproliferations and Hodgkin's disease: two new developments in the EBV Field, Adv. Cancer Res. 62 (1993) 179-239]. Lymphoproliferative disease and NHL occurring in severely immunosuppressed individuals almost always involve EBV and have been extensively studied and modeled in vitro. EBV has also been causally associated with some cases of NHL occurring in otherwise immunocompetent individuals. However, a direct role for EBV in the pathogenesis of neoplasms developing in the presence of an otherwise competent immune system has not been established. We investigated potential interactions between dithiocarbamates (DTC), an important class of thiono-sulfur compounds, and EBV leading to immortalization of human B lymphocytes and evasion of cell-mediated immune response in culture. Primary lymphocyte cultures employing wild-type and recombinant EBV mutants were used to assess the respective roles of DTC and viral genes in lymphocyte transformation and survival. Pretreatment of EBV-infected human B lymphocytes with DTC directly enhanced transformation in the absence of T cells (5 nM) and independently increased survival of transformed cells in the presence of competent autologous T cells (10 nM). Both DTC-induced transformation and immortalization of EBV-infected B lymphocytes were dependent on the expression of viral IL-10. These results provide a biological basis for studying collaborations between chemical and virus that alter lymphocyte biology, and provide a rationale for further molecular epidemiology studies to better understand the potential influence of these interactions on the development of NHL and perhaps other viral-associated malignancies.

1,3-Butadiene and leukemia among synthetic rubber industry workers: exposure-response relationships

Previous research updated the mortality experience of North American synthetic rubber industry workers during the period 1944-1998, determined if leukemia and other cancers were associated with several employment factors and carried out Poisson regression analysis to examine exposure-response associations between estimated exposure to 1,3-butadiene (BD) or other chemicals and cancer. The present study used Cox regression procedures to examine further the exposure-response relationship between several unlagged and lagged, continuous, time-dependent BD exposure indices (BD parts per million (ppm)-years, the total number of exposures to BD concentrations >100 ppm ("peaks") and average intensity of BD) and leukemia, lymphoid neoplasms and myeloid neoplasms. All three BD exposure indices were associated positively with leukemia. Using continuous, untransformed BD ppm-years the regression coefficient (beta) from an analysis that controlled only for age was 2.9 x 10(-4) (p<0.01); the regression coefficient adjusted for all covariates (age, year of birth, race, plant, years since hire and dimethyldithiocarbamate) was similar in magnitude (beta=3.0 x 10(-4), p=0.04). Lagging exposure had minimal impact on the results for leukemia for any of the three BD exposure indices. In models that controlled only for age, lymphoid neoplasms were associated with BD ppm-years and myeloid neoplasms, with BD peaks, but neither trend was statistically significant after adjusting for multiple covariates. The present results support the presence of a causal relationship between high cumulative exposure and high intensity of exposure to BD and leukemia.

Chemical Exposures in the Synthetic Rubber Industry and Lymphohematopoietic Cancer Mortality

OBJECTIVE

This study evaluated the association between exposure to several chemicals and mortality from lymphohematopoietic cancer (LHC) among 16,579 synthetic rubber industry workers who were followed up from 1943 to 1998.

METHODS

Poisson regression analyses examined LHC rates in relation to butadiene, styrene, and DMDTC exposure. Models provided maximum likelihood estimates of the relative rate for the contrast between categories of one agent, adjusting for other agents and for additional potential confounders.

RESULTS

Cumulative exposure to 1,3-butadiene was associated positively with all leukemia (relative rates of 1.0, 1.4, 1.2, 2.9, and 3.7, respectively, for exposures of 0, >0 to <33.7, 33.7 to <184.7, 184.7 to <425.0, and 425.0+ ppm-years), chronic myelogenous leukemia and to a lesser extent with chronic lymphocytic leukemia. Adjusting for styrene and DMDTC attenuated these associations. After controlling for butadiene, neither styrene nor DMDTC displayed a consistent exposure-response trend with all leukemia, chronic myelogenous leukemia, or chronic lymphocytic leukemia.

CONCLUSIONS

This study found a positive association between butadiene and leukemia that was not explained by exposure to other agents examined.

Childhood cancers and atmospheric carcinogens

STUDY OBJECTIVES

To retest previous findings that childhood cancers are probably initiated by prenatal exposures to combustion process gases and to volatile organic compounds (VOCs); and to identify specific chemical hazards.

DESIGN

Birth and death addresses of fatal child cancers in Great Britain between 1966 and 1980, were linked with high local atmospheric emissions of different chemical species. Among migrant children, distances from each address to the nearest emissions "hotspot" were compared. Excesses of outward over inward migrations show an increased prenatal or early infancy risk.

SETTING AND SUBJECTS

Maps of emissions of many different substances were published on the internet by the National Atmospheric Emissions Inventory and "hotspots" for 2001 were translated to map coordinates. Child cancer addresses were extracted from an earlier inquiry into the carcinogenic effects of obstetric radiographs; and their postcodes translated to map references.

MAIN RESULTS

Significant birth proximity relative risks were found within 1.0 km of hotspots for carbon monoxide, PM10 particles, VOCs, nitrogen oxides, benzene, dioxins, 1,3-butadiene, and benz(a)pyrene. Calculated attributable risks showed that most child cancers and leukaemias are probably initiated by such exposures.

CONCLUSIONS

Reported associations of cancer birth places with sites of industrial combustion, VOCs uses, and associated engine exhausts, are confirmed. Newly identified specific hazards include the known carcinogens 1,3-butadiene, dioxins, and benz(a)pyrene. The mother probably inhales these or related materials and passes them to the fetus across the placenta.

Historical estimation of exposure to 1,3-butadiene, styrene, and dimethyldithiocarbamate among synthetic rubber workers

Quantitative estimates of exposure to 1,3-butadiene (BD), styrene (STY), and dimethyldithiocarbamate (DMDTC) were developed for a follow-up study of workers at six North American synthetic rubber plants. Procedures entailed identifying tasks and jobs involving exposure, identifying factors influencing historical changes in exposure potential, and using mathematical models to calculate job- and time-period-specific exposures. Exposure metrics included 8-hour time-weighted average (TWA) intensity, the annual number of peak exposures (BD: >100 ppm, STY: >50 ppm) and TWA intensity below and above the peak threshold. The 5th and 95th percentiles of the approximate probability distribution of each exposure estimate served as its 90% uncertainty interval. Job- and year-specific estimates were linked with subjects' work histories to obtain cumulative exposure indices. Exposure estimates varied among tasks, jobs, plants, and time periods. BD TWAs were approximately 10 ppm during the 1940s-1960s and declined during the 1970s and 1980s. STY TWAs were always <2 ppm. DMDTC exposure began in the 1950s, was high through the 1960s, and later declined. BD peak exposure accounted for a large proportion of cumulative BD exposure, whereas almost none of the STY exposure was experienced at levels >50 ppm. Exposure indices were correlated. Exposures were higher than previously estimated. Multiple correlations among DMDTC, BD, and STY exposure estimates make it difficult to estimate agent-specific effects. Limitations of the methodology include the potential inaccuracy of the estimates, the lack of adequate industrial hygiene data to validate the estimates, the additional inaccuracy of linkage with poorly specified job groups, and the potential for differential exposure misclassification because the jobs and work areas where excess leukemia mortality occurred were well-known at the time of this study. Nevertheless, the new exposure estimates were highly correlated with the old, yielding equivalent exposure ranking of workers and were comparable to limited industrial hygiene data published by NIOSH.

Comparative carcinogenicity of 1,3-butadiene, isoprene, and chloroprene in rats and mice

1,3-Butadiene, isoprene (2-methyl-1,3-butadiene), and chloroprene (2-chloro-1,3-butadiene) are high-production-volume chemicals used mainly in the manufacture of synthetic rubber. Inhalation studies have demonstrated multiple organ tumorigenic effects with each of these chemicals in mice and rats. Sites of tumor induction by these epoxide-forming chemicals were compared to each other and to ethylene oxide, a chemical classified by the National Toxicology Program (NTP) and by the International Agency for Research on Cancer (IARC) as carcinogenic to humans. For this group of chemicals, there are substantial species differences in sites of neoplasia; neoplasia of the mammary gland is the only common tumorigenic effect in rats and mice. Within each species, there are several common sites of tumor induction; these include the hematopoietic system, circulatory system, lung, liver, forestomach, Harderian gland, and mammary gland in mice, and the mammary gland and possibly the brain, thyroid, testis, and kidney in rats. For studies in which individual animal data were available, mortality-adjusted tumor rates were calculated, and estimates were made of the shape of the exposure-response curves and ED10 values (i.e. exposure concentrations associated with an excess risk of 10% at each tumor site). Most tumorigenic effects reported here were consistent with linear or supralinear models. For chloroprene and butadiene, the most potent response was for the induction of lung neoplasms in female mice, with ED10 values of 0.3 ppm. Based on animal cancer data, isoprene and chloroprene are listed in the NTP's Report on Carcinogens (RoC) as reasonably anticipated to be a human carcinogen. Butadiene is listed in the RoC as known to be a human carcinogen 'based on sufficient evidence of carcinogenicity from studies in humans, including epidemiological and mechanistic information', with support from experimental studies in laboratory animals. Epidemiology data for isoprene and chloroprene are not considered adequate to evaluate the potential carcinogenicity of these agents in humans.

Comparative toxicity of dithiocarbamates and butadiene metabolites in human lymphoid and bone marrow cells

Apparent differences in the pattern of leukemia risk have been observed between workers employed in 1,3-butadiene (BD) monomer production and those working in styrene-butadiene rubber production (SBR). There are a number of possible explanations for these discrepancies, including differences in disease classification and diagnosis as well as possible quantitative and qualitative differences in occupational exposure between these two industries. This led us to evaluate the possibility that the pattern of disease observed in SBR might be influenced by the presence of an important class of biologically reactive chemicals, dithiocarbamates (DTC), that were present in SBR but not BD monomer production. Therefore, we compared the immunotoxic and hematotoxic activities of DTC and BD metabolites in human immune and hematopoietic cells. Relative to the mouse, human CD34+ bone marrow cells are relatively resistant to the direct effects of BD metabolites, with only the bis-oxide producing any evidence of suppression of clonogenic response at concentrations between 1 and 10 microM. Similarly, treatment of human CD4+ lymphocytes with known (2,3-epoxybutene) and putative BD metabolites (D,L-butane-bis-oxide, (2S,3R)-3-epoxybutane-1,2-diol) does not result in appreciable T-cell toxicity at concentrations likely to be encountered in vivo. In contrast, treatment of human cells with DTC at concentrations as low as 100 nM results in significant suppression of hematopoietic clonogenic response and T-lymphocyte function. Additional studies in our laboratory and others suggest a role for copper in DTC toxicity in both human lymphocytes and bone marrow cells, although the pattern of altered transcriptional regulation observed is markedly different in these two cell populations. These results are consistent with the pattern of DTC toxicity previously observed in clinical and molecular studies.

Future research needs for the health assessment of 1,3-butadiene

In recent years, several organizations have evaluated the health effects of 1,3-butadiene. Dissimilar conclusions about the carcinogenicity of 1,3-butadiene have been reached, due to differences in interpretation of the same data. Although 1,3-butadiene has been extensively studied, various uncertainties and data gaps remain which will require further clarification to reduce the uncertainties in future health assessments. This paper discusses future research needs respective of cancer and non-cancer effects that would be useful for future assessments as to how 1,3-butadiene affects health.

A review of the epidemiology of 1,3-butadiene and chloroprene

Butadiene epidemiologic research has focused primarily on one cohort of workers in the North American styrene-butadiene rubber (SBR) industry and on the largest cohort of workers in the United States butadiene monomer industry. The most recent studies of these populations are characterized by carefully enumerated study populations, extremely long and high quality mortality follow-up, accurate job categorizations, detailed exposure assessments, and comprehensive statistical analyses. Leukemia was clearly associated with increasing estimated butadiene exposure in the SBR study, but not in the monomer industry study. This has lead to hypotheses about exposure differences between these two industries and the presence of co-factors or confounders in the SBR industry. Research presented at this symposium should shed some light on these hypotheses. The chloroprene epidemiologic literature, on the other hand, is in an early stage of development. The existing studies are limited by poor exposure characterization, lack of control of potential confounding factors, incompleteness in cohort enumeration, short follow-up periods, and small numbers of cancer cases. The state of the science for chloroprene would be advanced by arranging more comprehensive studies than those that have been conducted to date.

The influence of co-exposure to dimethyldithiocarbamate on butadiene metabolism

Treatment of rats and mice with a single oral dose of dimethyldithiocarbamate (DMDTC; 250 mg/kg) had a marked effect on hepatic CYP2E1 and aldehyde dehydrogenase activities, measured in vitro, for up to 24 h after dosing. The same treatment did not affect CYP2A6, glutathione S-transferase, epoxide hydrolase, alcohol dehydrogenase activities or hepatic glutathione levels. As a consequence of the loss of CYP2E1 activity, butadiene metabolism in liver fractions from DMDTC treated rats and mice was markedly reduced, as was the metabolism of the mono-epoxide to the di-epoxide in mouse liver. The conversion of the mono-epoxide to the diol by epoxide hydrolases was not affected by DMDTC treatment. Urinary excretion of radioactivity, following dosing with DMDTC and exposure to 200 ppm C-14 butadiene for 6 h, was markedly reduced in rats, but increased in mice. The profiles of urinary metabolites were qualitatively similar from mice exposed to butadiene to those exposed after dosing with DMDTC. In the rat, pre-dosing with DMDTC resulted in the formation of three additional urinary metabolites following exposure to butadiene. Overall, DMDTC appears to impact qualitatively and quantitatively on the metabolism of butadiene. The nature and full significance of these changes has yet to be characterised.

Leukemia and exposure to 1,3-butadiene, styrene and dimethyldithiocarbamate among workers in the synthetic rubber industry

This study evaluated relations between exposure to 1,3-butadiene (BD), styrene (STY) and dimethyldithiocarbamate (DMDTC) and mortality from leukemia among synthetic rubber industry workers. Subjects were 13130 men employed for at least 1 year during 1943-1991 at any of six plants that manufactured synthetic rubber. Death certificates and medical records identified workers with leukemia. Cumulative exposure estimates were based on plant- and time period-specific process and task characteristics, linked to subjects' work histories. Poisson regression estimated relative rates (RRs) for workers exposed to each agent compared to unexposed workers. Leukemia (N=59) was positively associated with BD ppm-years (RRs of 1.0, 1.2, 2.0 and 3.8, for exposures of 0, >0-<86.3, 86.3-<362.2 and 362.2+ ppm-years; only the RR for the highest exposure category was statistically significant), STY ppm-years (RRs of 1.0, 1.2, 2.3 and 3.2, for exposures of 0, >0-<20.6, 20.6-<60.4 and 60.4+ ppm-years; only the RR for the highest exposure category was statistically significant) and DMDTC mg-years/cm (RRs of 1.0, 2.3, 4.9 and 2.9, for 0, >0-<566.6, 566.6-<1395.1 and 1395.1+ mg-years/cm; the RR for each non-zero exposure category was statistically significant) after adjusting for age and years since hire. After further adjusting each agent-specific set of RRs for the other two agents, a positive but imprecise relation remained for BD and DMDTC but not for STY. The association with BD was stronger for ppm-years due to exposure intensities >100 ppm than for ppm-years due to lower concentrations. BD and DMDTC, but not STY, were positively associated with leukemia in multivariable analyses. The independent effect of each agent was difficult to evaluate because of correlations with other agents and imprecision.

Application of process chemistry and SAR modelling to the evaluation of health findings of lower olefins

Epidemiology studies show increased leukemia mortality among styrene butadiene rubber (SBR) workers but not among butadiene monomer production employees. A detailed review of the SBR manufacturing process indicates that sodium dimethyldithiocarbamate (DMDTC) introduced into the SBR manufacturing process for a period in the 1950s coincides with increased leukemia mortality. Using the Computer-Optimized Molecular Parametric Analysis of Chemical Toxicity (COMPACT), we assessed the enzyme (cytochrome P450) substrate specificity of an olefin series including 1,3-butadiene (BD) and also modeled its interaction with DMDTC. These analyses showed correlation of a structural/electronic parameter--the COMPACT radius--with the presence or absence of cytogenetic activity and also found that DMDTC would inhibit the oxidative metabolism of BD at least at high concentrations. Both DMDTC and its diethyl analog (DEDTC) bind with CYP 2E1 and CYP 2A6. Both of these isoforms are important in the initial oxidative metabolism of butadiene and other olefins. In co-exposure studies in mice of DMDTC with BD or with epoxybutene (EB), we found that there was a reduced increase in genotoxic activity based on micronuclei induction compared with BD or EB exposure alone. Treatment with DMDTC significantly increased the protein carbonyl contents of hepatic microsomes compared with that of controls, a finding that may be related to DMDTC's activity as a prooxidant. Co-exposure with DMDTC and EB increased hepatic microsomal carbonyls to levels significantly greater than those of DMDTC-treated mice, while EB administration in the absence of DMDTC did not change protein carbonyls relative to those of controls. The increase in hepatic microsomal protein carbonyls suggests that DMDTC may modulate EB metabolism towards the formation of reactive intermediates that react with proteins. The present molecular modeling and mechanistic studies suggest that co-exposure of BD and DMDTC is a plausible biological hypothesis regarding increased leukemia risk among SBR workers.