Benzene hematotoxicity and leukemogenesis

Intrinsic sexual dimorphism in the placenta determines the differential response to benzene exposure

Maternal immune activation (MIA) by environmental challenges is linked to severe developmental complications, such as neurocognitive disorders, autism, and even fetal/maternal death. Benzene is a major toxic compound in air pollution that affects the mother as well as the fetus and has been associated with reproductive complications. Our objective was to elucidate whether benzene exposure during gestation triggers MIA and its impact on fetal development. We report that benzene exposure during pregnancy leads MIA associated with increased fetal resorptions, fetal growth, and abnormal placenta development. Furthermore, we demonstrate the existence of a sexual dimorphic response to benzene exposure in male and female placentas. The sexual dimorphic response is a consequence of inherent differences between male and female placenta. These data provide crucial information on the origins or sexual dimorphism and how exposure to environmental factors can have a differential impact on the development of male and female offspring.

Kefir milk alleviates benzene-induced immunotoxicity and hematotoxicity in rats

The adverse health effects of benzene occupational and circumstance pollution exposure are an increasing concern. It leads to damage to various human tissues including bone marrow and ovarian tissues and many vital physiological processes. Previous studies showed that kefir is a rich probiotic, having protective effect, thanks to its antioxidant, anti-inflammatory, and immunomodulatory capacity. The purpose of this study was to evaluate the potential efficacy of kefir to remediate benzene toxicity in rat. Thirty-two female rats were randomly allocated and administered orally with benzene and/or kefir during a period of 21 consecutive days. At the end of the experiment, hematological and bone marrow cell changes were estimated. The animals exposed to benzene exhibited anemia and a significant decrease in the levels of white blood cell. Moreover, benzene led to the activation of gene expression of the pro-inflammatory cytokines interleukin-1β (IL-1β) and interleukin-6 (IL-6), a myelotoxicity in bone marrow cells. Our data showed that kefir treatment alleviated benzene-associated weight loss and increased the number of whole blood cells in peripheral blood and nucleated cells in the bone marrow. Furthermore, these physiological results were observed with animals showing high concentrations of lactic acid bacteria (LAB) determined from fecal samples, which are considered an indicator of kefir-associated microorganisms. Our study suggests that kefir is a potential nutritional supplement target to attenuate hematotoxicity induced by benzene.

Use of toxicant sensitivity distributions (TSD) for development of exposure guidelines for risk to human health from benzene

This technique for setting guideline values differs from that currently used by regulatory agencies throughout the world. Data for benzene were evaluated from epidemiological studies on human populations (29 studies). Exposure durations were evaluated in terms of Long Term Exposure (LTE) and Lifetime Exposure. All data was reported as Lowest Observed Adverse Effect Levels (LOAEL) and converted into exposure doses using Average Daily Dose (ADD) and Lifetime Average Daily Dose (LADD). These values were plotted as a Toxicant Sensitivity Distribution (TSD) which was the cumulative probability of LOAEL-ADD and LOAEL-LADD. From the TSD plots, linear regression equations gave correlation coefficients (R²) ranging from 0.69 to 0.97 indicating normal distributions. Guideline Values (GVs) for LTE (8hr/day) and Lifetime (24hr/70yrs) exposure to benzene were calculated using data from human epidemiological studies as 5% level of cumulative probability (CP) of LOAEL-ADD and LOAEL-LADD from the cumulative probability distributions (CPD). The derived guideline values from the human epidemiological studies were 92 μg/kg/day for LTE and 3.4 μg/kg/day for lifetime exposure. GV for LTE is appropriate for occupational exposure and GV derived for lifetime exposure appropriate for the general population. The guideline value for occupational exposure limit was below all the guideline values developed by regulatory agencies. But the general population guideline is within the range of values formulated by European Union, ATSDR, EPAQS, USEPA and OEHHA for air quality for the general population. This is an alternative method which eliminates the application of safety factors and other sources of errors in deriving guideline values for benzene.

Personal exposure to benzene of selected population groups and impact of commuting modes in Ho Chi Minh, Vietnam

Personal exposure to benzene of selected population groups, and impacts of traffic on commuters in Ho Chi Minh City were investigated. The study was carried out in June, July and November 2010. The preliminary data showed that on average, personal exposure to benzene for non-occupational people in Ho Chi Minh is ~18 μg/m(3) and most of the exposure is due to commuting. Benzene exposure during travelling by bus, taxi and motorcycle is, respectively, 22-30, 22-39 and 185-240 μg/m(3). Motorcycle-taxi drivers, petrol filling employees and street vendors suffer high daily exposures at 116, 52, 32 μg/m(3), respectively. Further measurements are needed for a better risk assessment and finding effective measures to reduce exposure.

The use of biomonitoring data in exposure and human health risk assessment: benzene case study

Abstract A framework of "Common Criteria" (i.e. a series of questions) has been developed to inform the use and evaluation of biomonitoring data in the context of human exposure and risk assessment. The data-rich chemical benzene was selected for use in a case study to assess whether refinement of the Common Criteria framework was necessary, and to gain additional perspective on approaches for integrating biomonitoring data into a risk-based context. The available data for benzene satisfied most of the Common Criteria and allowed for a risk-based evaluation of the benzene biomonitoring data. In general, biomarker (blood benzene, urinary benzene and urinary S-phenylmercapturic acid) central tendency (i.e. mean, median and geometric mean) concentrations for non-smokers are at or below the predicted blood or urine concentrations that would correspond to exposure at the US Environmental Protection Agency reference concentration (30 µg/m(3)), but greater than blood or urine concentrations relating to the air concentration at the 1 × 10(-5) excess cancer risk (2.9 µg/m(3)). Smokers clearly have higher levels of benzene exposure, and biomarker levels of benzene for non-smokers are generally consistent with ambient air monitoring results. While some biomarkers of benzene are specific indicators of exposure, the interpretation of benzene biomonitoring levels in a health-risk context are complicated by issues associated with short half-lives and gaps in knowledge regarding the relationship between the biomarkers and subsequent toxic effects.

3-(3,4-Dihydroxyphenyl)adenine, a urinary DNA adduct formed in mice exposed to high concentrations of benzene

Metabolism of benzene, an important environmental and industrial carcinogen, produces three electrophilic intermediates, namely, benzene oxide and 1,2- and 1,4-benzoquinone, capable of reacting with the DNA. Numerous DNA adducts formed by these metabolites in vitro have been reported in the literature, but only one of them was hitherto identified in vivo. In a search for urinary DNA adducts, specific LC-ESI-MS methods have been developed for the determination in urine of six nucleobase adducts, namely, 7-phenylguanine, 3-phenyladenine, 3-hydroxy-3,N(4) -benzethenocytosine, N(2) -(4-hydroxyphenyl)guanine, 7-(3,4-dihydroxyphenyl)guanine and 3-(3,4-dihydroxyphenyl)-adenine (DHPA), with detection limits of 200, 10, 260, 50, 400 and 200 pg ml(-1) , respectively. Mice were exposed to benzene vapors at concentrations of 900 and 1800 mg m(-3) , 6 h per day for 15 consecutive days. The only adduct detected in their urine was DHPA. It was found in eight out of 30 urine samples from the high-exposure group at concentrations of 352 ± 146 pg ml(-1) (mean ± SD; n = 8), whereas urines from the low-exposure group were negative. Assuming the DHPA concentration in the negative samples to be half of the detection limit, conversion of benzene to DHPA was estimated to 2.2 × 10(-6) % of the absorbed dose. Thus, despite the known high mutagenic and carcinogenic potential of benzene, only traces of a single DNA adduct in urine were detected. In conclusion, DHPA is an easily depurinating adduct, thus allowing indication of only high recent exposure to benzene, but not long-term damage to DNA in tissues.

The IARC october 2009 evaluation of benzene carcinogenicity was incomplete and needs to be reconsidered

I have been familiar with the toxicological and epidemiological literature on benzene since I was a member of the NIOSH Benzene Task Force in 1975. I also am familiar with the procedures of IARC Monographs meetings from past participation, and as observer I applied this experience to the Monograph 100 F review. In October of 2009, a Working Group (WG) of the International Agency for Research on Cancer (IARC) met in Lyon, France to evaluate the available evidence for site-specific cancer to humans for 33 chemical agents and related occupations previously categorized by IARC as human carcinogens. Generally, review and discussion of the epidemiological cancer literature related to benzene was limited due to the enormous amount of material needing to be covered since the last full monograph meeting on benzene in 1981, and because 32 other chemicals and occupations were also being evaluated. Moreover, among the 33 chemicals and occupations reviewed, there was some inconsistency in the use of studies for evaluating various cancers. In some situations, consideration could have been given to the inclusion of relevant unpublished, but readily available study results. Discussion and synthesis of the animal cancer studies and mechanistic data related to specific cancers also were limited. IARC's conclusion that there is sufficient evidence for benzene to cause acute non-lymphocytic leukemia only was based on an incomplete review. IARC should schedule another monographs meeting dedicated to a complete and full review and discussion of all potential cancers related to exposure to benzene and to benzene-containing mixtures.

The elderly as a sensitive population in environmental exposures: making the case

The US population is aging. CDC has estimated that 20% of all Americans will be 65 or older by the year 2030. As a part of the aging process, the body gradually deteriorates and physiologic and metabolic limitations arise. Changes that occur in organ anatomy and function present challenges for dealing with environmental stressors of all kinds, ranging from temperature regulation to drug metabolism and excretion. The elderly are not just older adults, but rather are individuals with unique challenges and different medical needs than younger adults. The ability of the body to respond to physiological challenge presented by environmental chemicals is dependent upon the health of the organ systems that eliminate those substances from the body. Any compromise in the function of those organ systems may result in a decrease in the body's ability to protect itself from the adverse effects of xenobiotics. To investigate this issue, we performed an organ system-by-organ system review of the effects of human aging and the implications for such aging on susceptibility to drugs and xenobiotics. Birnbaum (1991) reported almost 20 years ago that it was clear that the pharmacokinetic behavior of environmental chemicals is, in many cases, altered during aging. Yet, to date, there is a paucity of data regarding recorded effects of environmental chemicals on elderly individuals. As a result, we have to rely on what is known about the effects of aging and the existing data regarding the metabolism, excretion, and adverse effects of prescription medications in that population to determine whether the elderly might be at greater risk when exposed to environmental substances. With increasing life expectancy, more and more people will confront the problems associated with advancing years. Moreover, although proper diet and exercise may lessen the immediate severity of some aspects of aging, the process will continue to gradually degrade the ability to cope with a variety of injuries and diseases. Thus, the adverse effects of long-term, low-level exposure to environmental substances will have a longer time to be manifested in a physiologically weakened elderly population. When such exposures are coupled with concurrent exposure to prescription medications, the effects could be devastating. Public health officials must be knowledgeable about the sensitivity of the growing elderly population, and ensure that the use of health guidance values (HGVs) for environmental contaminants and other substances give consideration to this physiologically compromised segment of the population.

Benzene-induced hematopoietic neoplasms including myeloid leukemia in Trp53-deficient C57BL/6 and C3H/He mice

This research focused on three major questions regarding benzene-induced hematopoietic neoplasms (HPNs). First, why are HPNs induced equivocally and at only threshold level with low-dose benzene exposure despite the significant genotoxicity of benzene even at low doses both in experiments and in epidemiology? Second, why is there no linear increase in incidence at high-dose exposure despite a lower acute toxicity (LD(50) > 1000 mg/kg body weight; WHO, 2003, Benzene in drinking-water. Background document for development of WHO Guidelines for Drinking-Water Quality)? Third, why are particular acute myeloid leukemias (AMLs) not commonly observed in mice, although AMLs are frequently observed in human cases of occupational exposure to benzene? In this study, we hypothesized that the threshold-like equivocal induction of HPNs at low-dose benzene exposure is based on DNA repair potential in wild-type mice and that the limited increase in HPNs at a high-dose exposure is due to excessive apoptosis in wild-type mice. To determine whether Trp53 deficiency satisfies the above hypotheses by eliminating or reducing DNA repair and by allowing cells to escape apoptosis, we evaluated the incidence of benzene-induced HPNs in Trp53-deficient C57BL/6 mice with specific regard to AMLs. We also used C3H/He mice, AML prone, with Trp53 deficiency to explore whether a higher incidence of AMLs on benzene exposure might explain the above human-murine differences. As a result, heterozygous Trp53-deficient mice of both strains showed a nonthreshold response of the incidence of HPNs at the lower dose, whereas both strains showed an increasing HPN incidence up to 100% with increasing benzene exposure dose, including AMLs, that developed 38% of heterozygous Trp53-deficient C3H/He mice compared to only 9% of wild-type mice exposed to the high dose. The detection of AMLs in heterozygous Trp53-deficient mice, even in the C57BL/6 strain, implies that benzene may be a potent inducer of AMLs also in mice with some strain differences.

ATSDR evaluation of health effects of benzene and relevance to public health

As part of its mandate, the Agency for Toxic Substances and Disease Registry (ATSDR) prepares toxicological profiles on hazardous chemicals found at Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) National Priorities List (NPL) sites that have the greatest public health impact. These profiles comprehensively summarize toxicological and environmental information. This article constitutes the release of portions of the Toxicological Profile for Benzene. The primary purpose of this article is to provide public health officials, physicians, toxicologists, and other interested individuals and groups with an overall perspective on the toxicology of benzene. It contains descriptions and evaluations of toxicological studies and epidemiological investigations and provides conclusions, where possible, on the relevance of toxicity and toxicokinetic data to public health.

Cluster of Hodgkin’s lymphoma in residents near a non-operational petroleum refinery

This report examines the prevalence rate of Hodgkin’s disease in an American mid-west town located directly south of a non-operational oil refinery. The refinery has a history of benzene-containing gasoline leaks dating back to the early 1900s. Exposure data were assessed through the Toxic Release Inventory (TRI) data as published by the Environmental Protection Agency (EPA) and supplemented by exposure simulations using variations of residential exposure times and odour levels and the benzene content of the gasoline. Prevalence rates depended on the size of the population in question. The population size varied greatly between sources, with the more conservative and consistent estimates being reported by the local government and United States Census Bureau and the highest population figure being reported by the Agency for Toxic Substances Disease Registry. The prevalence of Hodgkin’s disease for the residents within 1 mile from the refinery was found to be elevated for every population figure, ranging from 72.11 cases per 100,000 using the ATSDR’s population to 182.34 per 100,000, whereas the prevalence for Hodgkin’s disease in all the United States is only 22 cases of Hodgkin’s disease per 100,000 people. The prevalence value reported in this report should be given greater weight than what would have been calculated using data from the ATSDR. Because of its significantly increased value compared with the rest of the United States, it provides evidence of benzene’s role as a causative agent in the etiology of Hodgkin’s disease.

Genotoxicity of intermittent co-exposure to benzene and toluene in male CD-1 mice

Benzene is an important industrial chemical. At certain levels, benzene has been found to produce aplastic anemia, pancytopenia, myeloblastic anemia and genotoxic effects in humans. Metabolism by cytochrome P450 monooxygenases and myeloperoxidase to hydroquinone, phenol, and other metabolites contributes to benzene toxicity. Other xenobiotic substrates for cytochrome P450 can alter benzene metabolism. At high concentrations, toluene has been shown to inhibit benzene metabolism and benzene-induced toxicities. The present study investigated the genotoxicity of exposure to benzene and toluene at lower and intermittent co-exposures. Mice were exposed via whole-body inhalation for 6h/day for 8 days (over a 15-day time period) to air, 50 ppm benzene, 100 ppm toluene, 50 ppm benzene and 50 ppm toluene, or 50 ppm benzene and 100 ppm toluene. Mice exposed to 50 ppm benzene exhibited an increased frequency (2.4-fold) of micronucleated polychromatic erythrocytes (PCE) and increased levels of urinary metabolites (t,t-muconic acid, hydroquinone, and s-phenylmercapturic acid) vs. air-exposed controls. Benzene co-exposure with 100 ppm toluene resulted in similar urinary metabolite levels but a 3.7-fold increase in frequency of micronucleated PCE. Benzene co-exposure with 50 ppm toluene resulted in a similar elevation of micronuclei frequency as with 100 ppm toluene which did not differ significantly from 50 ppm benzene exposure alone. Both co-exposures - 50 ppm benzene with 50 or 100 ppm toluene - resulted in significantly elevated CYP2E1 activities that did not occur following benzene or toluene exposure alone. Whole blood glutathione (GSH) levels were similarly decreased following exposure to 50 ppm benzene and/or 100 ppm toluene, while co-exposure to 50 ppm benzene and 100 ppm toluene significantly decreased GSSG levels and increased the GSH/GSSG ratio. The higher frequency of micronucleated PCE following benzene and toluene co-exposure when compared with mice exposed to benzene or toluene alone suggests that, at the doses used in this study, toluene can enhance benzene-induced clastogenic or aneugenic bone marrow injury. These findings exemplify the importance of studying the effects of binary chemical interactions in animals exposed to lower exposure concentrations of benzene and toluene on benzene metabolism and clastogenicity. The relevance of these data on interactions for humans exposed at low benzene concentrations can be best assessed only when the mechanism of interaction is understood at a quantitative level and incorporated within a biologically based modeling framework.

Benzene's toxicity: a consolidated short review of human and animal studies

A large population of humans is exposed to benzene from various occupational and environmental sources. Benzene is an established human and animal carcinogen. Exposure to benzene has been associated with leukaemia in humans and several types of malignancies in animals. The exact mechanism of benzene-induced toxicity is poorly understood. It is believed that benzene exerts its adverse effects by metabolic activation to toxic metabolites. Certain benzene metabolites are genotoxic and mutagenic. This consolidated short-review is composed of human and animal studies to summarize the adverse effects of benzene with special reference to molecular mechanisms involved in benzene-induced toxicity.

Immunotoxic effects of chemicals: A matrix for occupational and environmental epidemiological studies

BACKGROUND

Many biological and chemical agents have the capacity to alter the way the immune system functions in human and animals. This study evaluates the immunotoxicity of 20 substances used widely in work environments.

METHODS

A systematic literature search on the immunotoxicity of 20 chemicals was performed. The first step was to review literature on immunotoxicity testing and testing schemes adopted for establishing immunotoxicity in humans. The second step consisted of providing a documentation on immunotoxicity of substances that are widely used in work environment, by building tables for each chemical of interest (benzene, trichloroethylene, PAHs, crystalline silica, diesel exhausts, welding fumes, asbestos, styrene, formaldehyde, toluene, vinyl chloride monomer, tetrachloroethylene, chlorophenols, 1,3-butadiene, mineral oils, P-dichlorobenzene, dichloromethane, xylene, 1,1,1-trichloroethane, ethylene oxide). The third step was the classification of substances; an index (strong, intermediate, weak, nil) was assigned on the basis of the evidence of toxicity and type of immunotoxic effects (immunosuppression, autoimmunity, hypersensitivity) on the basis of the immune responses. Finally substances were assigned a score of immunotoxic power.

RESULTS

Tables have been produced that include information for the 20 substances of interest, based on 227 animal studies and 94 human studies. Each substance was assigned an index of immunotoxic evidence, a score of immunotoxic power and type of immunotoxic effect.

CONCLUSIONS

This matrix can represent a tool to identify chemicals with similar properties concerning the toxicity for the immune system, and to interpret epidemiological studies on immune-related diseases.

Formaldehyde as a potential human leukemogen: an assessment of biological plausibility

The International Agency for Research on Cancer (IARC, 2004) recently reevaluated the epidemiological data on formaldehyde and concluded that there was "strong but not sufficient evidence for a causal association between leukaemia and occupational exposure to formaldehyde." This conclusion was tempered since a mechanism for leukemia induction could not be identified. Chemically induced leukemia is a well-studied phenomenon with benzene and a number of cancer chemotherapeutic drugs recognized as capable of causing this effect. Abundant in vitro and in vivo data in animals and humans demonstrate that exposure to sufficient doses of these recognized leukemogens can initiate a cascade of events leading to hematopoietic toxicity and the subsequent development of leukemia. This review addresses the biological plausibility that formaldehyde might be capable of causing any type of leukemia by providing a broad overview of the scientific data that must be considered in order to support or refute a conclusion that a particular substance might be leukemogenic. Data on benzene and selected chemotherapeutic cancer drugs are used as examples and are briefly summarized to demonstrate the similar biological events thought to result in leukemogenesis. These data are compared and contrasted with the available data on formaldehyde in order to judge whether they fulfill the criteria of biological plausibility that formaldehyde would be capable of inducing leukemia as suggested by the epidemiological data. Based on the epidemiological data, it is reasonable to expect that if formaldehyde was capable of inducing leukemia, in vivo and in vitro data would offer supporting evidence for biological plausibility. In particular, there is (1) no evidence to suggest that formaldehyde reaches any target organ beyond the site of administration including the bone marrow, (2) no indication that formaldehyde is toxic to the bone marrow/hematopoietic system in in vivo or in vitro studies, and (3) no credible evidence that formaldehyde induces leukemia in experimental animals. As discussed in this review, based on the key biological events that occur in the process of chemically induced leukemia, there is inadequate biological evidence currently available to corroborate existing weak epidemiological associations. This provides an insufficient database to conclude that there is a causal relationship for formaldehyde and leukemia risk.

Genotoxicity of benzene and its metabolites

The potential role of genotoxicity in human leukemias associated with benzene (BZ) exposures was investigated by a systematic review of over 1400 genotoxicity test results for BZ and its metabolites. Studies of rodents exposed to radiolabeled BZ found a low level of radiolabel in isolated DNA with no preferential binding in target tissues of neoplasia. Adducts were not identified by 32P-postlabeling (equivalent to a covalent binding index <0.002) under the dosage conditions producing neoplasia in the rodent bioassays, and this method would have detected adducts at 1/10,000th the levels reported in the DNA-binding studies. Adducts were detected by 32P-postlabeling in vitro and following high acute BZ doses in vivo, but levels were about 100-fold less than those found by DNA binding. These findings suggest that DNA-adduct formation may not be a significant mechanism for BZ-induced neoplasia in rodents. The evaluation of other genotoxicity test results revealed that BZ and its metabolites did not produce reverse mutations in Salmonella typhimurium but were clastogenic and aneugenic, producing micronuclei, chromosomal aberrations, sister chromatid exchanges and DNA strand breaks. Rodent and human data were compared, and BZ genotoxicity results in both were similar for the available tests. Also, the biotransformation of BZ was qualitatively similar in rodents, humans and non-human primates, further indicating that rodent and human genotoxicity data were compatible. The genotoxicity test results for BZ and its metabolites were the most similar to those of topoisomerase II inhibitors and provided less support for proposed mechanisms involving DNA reactivity, mitotic spindle poisoning or oxidative DNA damage as genotoxic mechanisms; all of which have been demonstrated experimentally for BZ or its metabolites. Studies of the chromosomal translocations found in BZ-exposed persons and secondary human leukemias produced by topoisomerase II inhibitors provide some additional support for this mechanism being potentially operative in BZ-induced leukemia.

Exacerbation of benzene pneumotoxicity in connexin 32 knockout mice: enhanced proliferation of CYP2E1-immunoreactive alveolar epithelial cells

The pulmonary pathogenesis triggered by benzene exposure was studied. Since the role of the connexin 32 (Cx32) gap junction protein in mouse pulmonary pathogenesis has been suggested, in the present study, we explored a possible role of Cx32 in benzene-induced pulmonary pathogenesis using the wild-type (WT) and Cx32 knockout (KO) mice. The mice were exposed to 300 ppm benzene by inhalation for 6 h per day, 5 days per week for a total of 26 weeks, and then sacrificed to evaluate the pneumotoxicity or allowed to live out their life span to evaluate the reversibility of the lesions and tumor incidence. Our results clearly revealed exacerbated pneumotoxicity in the benzene-exposed Cx32 KO mice, characterized by diffuse granulomatous interstitial pneumonia, markedly increased mucin secretion of bronchial/bronchiolar and alveolar epithelial cells, and hyperplastic alveolar epithelial cells positive for CYP2E1. But the results did not indicate any enhancement of pulmonary tumorigenesis in the Cx32 KO mice though the number of animals was small.

Assessing DNA damage and health risk using biomarkers

Carcinogenesis is a multi-stage and prolonged process. At the present time, our knowledge of biological activities along the process is incomplete, therefore, a variety of experimental data are used to assess health risk from exposure to environmental chemicals. However, experimental approaches may not be adequate unless human data are available to support the assessment. In this brief review, benzene (CAS No. 71-43-2), a well-established human leukemogen, will be used as an example to illustrate the challenge in assessing toxicological mechanisms and cancer risk. Benzene has been shown to form DNA-adducts in experimental animals but the adducts have proved elusive of detection in human. Several toxic metabolites of benzene have been identified but the metabolite(s) responsible for the carcinogenic activities is unknown. Furthermore, the significant differences between rodents and human in response to benzene exposure are not understood. Therefore, the bone marrow specificity for the induction of leukemia in human by benzene remains to be elucidated. These complications illustrate the complexity of the assessment process and identify serious information gaps. These information gaps can be viewed as research opportunities to provide more precise data for assessment of toxicological effects and health risk.

Benzene and lymphohematopoietic malignancies in humans

BACKGROUND

Quantitative evaluations of benzene-associated risk for cancer have relied primarily on findings from a cohort study of highly exposed U.S. rubber workers. An epidemiologic investigation in China (NCI/CAPM study) extended quantitative evaluations of cancer risk to a broader range of benzene exposures, particularly at lower levels.

METHODS

We review the evidence implicating benzene in the etiology of hematopoietic disorders, clarify methodologic aspects of the NCI/CAPM study, and examine the study in the context of the broader literature on health effects associated with occupational benzene exposure.

RESULTS

Quantitative relationships for cancer risk from China and the U.S. show a relatively smooth increase in risk for acute myeloid leukemia and related conditions over a broad dose range of benzene exposure (below 200 ppm-years mostly from the China study and above 200 ppm-years mostly from the U.S. study).

CONCLUSIONS

Risks of acute myeloid leukemia and other malignant and nonmalignant hematopoietic disorders associated with benzene exposure in China are consistent with other information about benzene exposure, hematotoxicity, and cancer risk, extending evidence for hematopoietic cancer risks to levels substantially lower than had previously been established. Published 2001 Wiley-Liss, Inc.

A model for the induction of aplastic anemia by subcutaneous administration of benzene in mice

Long-term exposure to benzene vapors is associated with hematological diseases such as leukemia, lymphoma and aplastic anemia. CD(1) male mice were randomly assigned to six groups: 1B(10), 1B(15), 1B(20), 2B(10), 2B(15), and 2B(20.) 1B mice were administered 2 ml/kg (1940 mg/kg) subcutaneous injection (in the dorsal region) of benzene 5 days a week, and 2B mice were exposed 3 days a week (Monday, Wednesday and Friday) until a total of 10, 15 and 20 doses were completed. About 48 h after treatment completion, leukocyte, erythrocyte, and bone marrow cells were counted, and spleen histopathology was analyzed. 1B(15) and 1B(20) mice showed lethargy and irritability, 80% body and 42% spleen weight loss (P<0.001), while body and spleen weight loss were less severe in 2B mice (12 and 48%, respectively). After exposure to 20 benzene doses, 1B(20) and 2B(20) mice showed decreased hemoglobin concentrations, and erythrocyte, leukocyte and bone marrow cell counts (37, 34, 80 and 50%, respectively in group 1B(20); P<0.001; and 12, 48, 62 and 62%, respectively in group 2B(20)). Thrombocytopenia occurred only in group 2B. Both benzene-treatment schemes caused aplastic anemia, however, the disease was masked by spleen toxicity in group 1B. Scheme 2 allowed mice survival and caused less non-hematological effects. We establish here a reproducible and inexpensive experimental model to induce aplastic anemia in mice by subcutaneous injection of 2 ml/kg benzene, using two short-term treatment schemes.

Persistence of aneuploid immature/primitive hemopoietic sub-populations in mice 8 months after benzene exposure in vivo

Benzene (bz) is a common environmental contaminant associated with increased risk of myeloid leukemia. Chronic bz exposure in vivo increases the frequency of aneuploid circulating lymphocytes in humans. However, there is no information about persistence of bz-associated aneuploidy in immature/primitive cells, at risk of leukemic transformation, after bz exposure in vivo. We explored the relationship between the induction and persistence of aneuploidy in primitive hemopoietic cells from mice that received oral doses of bz in vivo. Short- and long-term persistence of aneuploidy were evaluated in immature/primitive sub-populations (Lin(-)c-kit(+)Sca-1(+)), as well as lymphoid and myeloid cells, 6 days and 2-8 months after exposure. Mice receiving bz in a corn oil carrier, or corn oil alone, both have increased aneuploidy frequencies (1-5%, compared to <1% in untreated controls) in all sub-populations, 6 days after exposure. However, unlike bz-induced aneuploidy, corn oil-induced aneusomies are transient, with frequencies returning to background levels in lymphoid and myeloid cells, 9 weeks after exposure. The frequency (5-9%) of aneuploid lymphocytes and myeloid cells is higher at 9 weeks than at 6 days, suggesting that bz disrupts chromosomal segregation in differentiated cells and/or progenitors. About 8 months after bz exposure, the Lin(-)c-kit(+)Sca-1(+) sub-population contains up to 14% aneuploid cells with numerical chromosomal aberrations affecting chromosomes 2 or 11. These data demonstrate that bz induces DNA copy number changes in immature/primitive cells, and that these changes persist for long periods. Although, initial exposures are not leukemogenic, subsequent exposures of cells to genotoxins or oxidative radicals that induce additional genetic hits may increase the risk of transformation. The contribution of bz-induced aneuploidy in immature/primitive cells to leukemogenesis remains to be determined.

Benzene--a review of the literature from a health effects perspective

A literature review of the impact on human health of exposure to benzene was conducted. Special emphasis in this report is given to the health effects reported in excess of national norms by participants in the Benzene Subregistry of the National Exposure Registry--people having documented exposure to benzene through the use of benzene-contaminated water for domestic purposes. The health effects reported in excess (p < or = .01) by some or all of the sex and age groups studied were diabetes, kidney disease, respiratory allergies, skin rashes, and urinary tract disorders; anemia was also increased for females, but not significantly so.

Bone marrow morphology in patients with neutropenia due to chronic exposure to organic solvents (benzene): early lesions

The authors present the histological and cytological features of bone marrow (BM) in 152 employees from the steel plant of Cubatão (S. Paulo-Brazil) who presented with neutropenia, due to chronic exposure to benzene and its homologues. All patients were male. Mean age was 35 years. At the time of this study, all patients were removed from risk areas because of the hematological abnormality. BM morphology was characterized by a hypocellular hemopoiesis (82%). Decrease of the granulocytic precursors (86%) was the most outstanding feature. Erythroid and megakaryocytic series were diminished in 40% and 57% of the cases, respectively. Eosinophilia in BM was observed in 71% of the cases. Cell atypias and stromal changes (necrosis, increase in reticulin fibres) were frequent.

Co-exposure to gasoline vapor decreases benzene metabolism in Fischer-344 rats

The metabolic interactions of benzene and gasoline vapor were investigated in male Fischer-344 rats. A closed chamber gas-uptake exposure system was used to obtain inhalation uptake curves for benzene alone and benzene in the presence of gasoline vapor. Exposure to benzene as a component of gasoline vapor resulted in a decrease of benzene metabolism. A physiologically based pharmacokinetic model of benzene metabolism was used to quantitatively determine the extent of the inhibitory effect of gasoline vapor on benzene metabolism. This observed inhibitory effect cannot be accounted for by the presence of toluene in gasoline vapor.

Hematotoxic effects of benzene analyzed by mathematical modeling

The hematopoietic cell response to benzene intoxication in mice (during and after long-term inhalation) was analyzed by a mathematical model of murine hematopoiesis. Two complementary methods, Time-Curve and Steady-State Analysis, were developed to identify target cells for benzene toxicity and to quantify the extent of damage in different stages of development of these target cells. We found that (i) erythropoietic cells were the most sensitive; (ii) granulopoietic cells were about half as sensitive as erythropoietic and (iii) hematopoietic stem cells exhibited a sensitivity that ranged between that of erythropoietic and granulopoietic cells. A dose-response relationship between benzene levels and damage in target cells (valid from 1 to more than 900 ppm) was derived that was linear for doses up to 300 ppm and plateaued thereafter. This relationship indicated that benzene-induced hematotoxicity is subject to a saturable process. Recovery of hematopoiesis following chronic benzene intoxication was simulated for different doses and preceding exposure periods. The impaired recovery following exposure periods greater than 8 weeks could be explained by a severe reduction in the maximum self-maintenance of stem cells. This study indicates that the present mathematical model represents a useful approach to investigate alternate hypotheses for the action of hematotoxic agents.

Increased erythrocyte volume in car repair painters and car mechanics

The biological effect of occupational long term low level exposure to organic solvents was studied in 17 car repair painters and 28 car mechanics, and compared with a control group consisting of 46 healthy men not exposed to organic solvents. The erythrocyte count (RBC) was significantly decreased for both the painters and the mechanics compared with the control group, and a significant increase in the mean erythrocyte volume (MCV) was seen in painters. The mode in size distribution of the erythrocytes (MAXRBC) was shifted towards larger cell volumes for both painters and mechanics. When data from painters and mechanics were combined, the exposed group showed a significantly reduced erythrocyte count (RBC), an increased mean erythrocyte volume (MVC), and an increased mean platelet volume (MPV) compared with the controls. Exposure measurements in combination with analysis of haematological parameters may be a tool for early detection of cellular changes in the blood caused by exposure to solvents, before the appearance of clinical symptoms.

Parental occupations of children with leukaemia in west Cumbria, north Humberside, and Gateshead

OBJECTIVE

To determine whether parental occupations and chemical and other specific exposures are risk factors for childhood leukaemia.

DESIGN

Case-control study. Information on parents was obtained by home interview.

SETTING

Three areas in north England: Copeland and South Lakeland (west Cumbria); Kingston upon Hull, Beverley, East Yorkshire, and Holderness (north Humberside), and Gateshead.

SUBJECTS

109 children aged 0-14 born and diagnosed as having leukaemia or non-Hodgkin's lymphoma in study areas during 1974-88. Two controls matched for sex and date and district of birth were obtained for each child.

MAIN OUTCOME MEASURES

Occupations of parents and specific exposure of parents before the children's conception, during gestation, and after birth. Other adults living with the children were included in the postnatal analysis.

RESULTS

Few risk factors were identified for mothers, although preconceptional association with the food industry was significantly increased in case mothers (odds ratio 2.56; 95% confidence interval 1.32 to 5.00). Significant associations were found between childhood leukaemia and reported preconceptional exposure of fathers to wood dust (2.73, 1.44 to 5.16), radiation (3.23, 1.36 to 7.72), and benzene (5.81, 1.67 to 26.44); ionising radiation alone gave an odds ratio of 2.35 (0.92 to 6.22). Raised odds ratios were found for paternal exposure during gestation, but no independent postnatal effect was evident.

CONCLUSION

These results should be interpreted cautiously because of the small numbers, overlap with another study, and multiple exposure of some parents. It is important to distinguish periods of parental exposures; identified risk factors were almost exclusively restricted to the time before the child's birth.

In vivo metabolic interactions of benzene and toluene

The metabolic interactions of benzene and toluene co-exposure were investigated in male Fischer rats. A closed recirculated exposure system was used to obtain inhalation uptake curves for individual chemicals as well as for a mixture of the two compounds. Pharmacokinetic parameters for benzene and toluene individually were determined in previous experimental studies. These values were incorporated into a physiologically based pharmacokinetic model which simulated the inhalation uptake process for both chemicals simultaneously. An optimal fit to the uptake curves for simultaneous exposure was obtained by adjusting the metabolic interaction terms for each chemical. Mutual suppression of metabolism was apparent. Toluene more effectively inhibited benzene metabolism than the reverse. This simulation approach for analyzing gas uptake data provided a method to determine the metabolic interactions occurring upon inhalation exposure to two different chemicals. Such analyses will prove useful in improving predictive toxicokinetic models.

Pharmacokinetics of benzene

A physiologically based pharmacokinetic model was developed and used to describe the pharmacokinetics of benzene in three species: mice, rats, and humans. For each species, the body was divided into five anatomical compartments, consisting of liver, fat, bone marrow, and muscle, and organs such as brain, heart, kidney, and viscera, connected by the arterial and venous blood flow pathways. Metabolism of benzene followed Michaelis-Menten (nonlinear) kinetics in all species and occurred primarily in the liver compartment and, to a lesser extent, in the bone marrow. Comparison of model results with empirical data on inhalation, gavage, and intraperitoneal and subcutaneous injection in mice, rats, and humans, demonstrates the utility of a physiological pharmacokinetic model in describing the pharmacokinetics of benzene in three species across multiple routes of exposure.

Possible implications from results of animal studies in human risk estimations for benzene: nonlinear dose-response relationship due to saturation of metabolism

To date, all risk assessment studies on benzene have been based almost exclusively on epidemiological data. We have attempted a more integrated and quantitative evaluation of carcinogenic risk for humans, trying to utilize, in addition to the epidemiological data, all data available, specifically data on metabolism, genotoxicity, and carcinogenicity in small rodents. An integrated evaluation of the globality of the available data seems to suggest a progressive saturation of metabolic capacity both for man and rodents between 10 and 100 ppm. The most susceptible target cells seem to be different in humans (predominant induction of myelogenous leukemia) and small rodents (induction of a wide variety of tumors). Nevertheless, both epidemiological and experimental carcinogenicity data tend to indicate a flattening of the response for the highest dosages, again suggesting a general saturation of mechanisms of metabolic activation, extended to different target tissues. From a quantitative point of view, the data suggest a carcinogenic potency at 10 ppm two to three times higher than that computable by a linear extrapolation from data in the 100 ppm range. These observations are in accord with the recent proposal of the European Economic Community of reducing benzene time-weighted average occupational levels from 10 to 5 ppm.

Correlation between the induction of micronuclei in bone marrow by benzene exposure and the excretion of metabolites in urine of CD-1 mice

Male and female CD-1 mice received single oral doses of benzene (220, 440, and 880 mg/kg) and were pretreated with modifiers of the mixed-function oxidase enzyme activities. Urinary metabolites (MT) (0-24 and 24-48 hr) were quantified by high-performance liquid chromatography. The micronucleus test was performed at 30 h. The following pretreatments were used to correlate micronucleus formation and the excreted benzene MT: 3-Methylcholanthrene and beta-naphthoflavone led to a marked increase in micronuclei (MN) and MT, whereas phenobarbital caused a slight increase, and SKF-525A had no effect. MN and MT were decreased when benzene was administered by the ip route or toluene was given simultaneously. Females had a lower number of MN and excreted more unconjugated phenol than did males. Muconic acid, hydroquinone, and phenol glucuronide and MN correlated well. They were dependent on both the dose and route of administration of benzene, being most inducible by P-448 inducers, in males more than females. The administration of hydroquinone induced MN, but phenol or catechol (200, 250, and 150 mg/kg, po, respectively) did not, and none of these compounds yielded trans, trans-muconic acid, a benzene MT in urine. This study establishes that benzene myeloclastogenicity is a function of its metabolism and that quantification of urinary metabolites could provide reliable correlates of this effect in vivo.