Chromosomal aberrations and micronuclei in lymphocytes in relation to alkylation of hemoglobin in workers exposed to ethylene oxide and propylene oxide

Combinations of genotoxic tests for the evaluation of group 1 IARC carcinogens

Many of the known human carcinogens are potent genotoxins that are efficiently detected as carcinogens in human populations but certain types of compounds such as immunosuppressants, sex hormones, etc. act via non-genotoxic mechanism. The absence of genotoxicity and the diversity of modes of action of non-genotoxic carcinogens make predicting their carcinogenic potential extremely challenging. There is evidence that combinations of different short-term tests provide a better and efficient prediction of human genotoxic and non-genotoxic carcinogens. The purpose of this study is to summarize the in vivo and in vitro comet assay (CMT) results of group 1 carcinogens selected from the International Agency for Research on Cancer and to discuss the utility of the comet assay along with other genotoxic assays such as Ames, in vivo micronucleus (MN), and in vivo chromosomal aberration (CA) test. Of the 62 agents for which valid genotoxic data were available, 38 of 61 (62.3%) were Ames test positive, 42 of 60 (70%) were in vivo MN test positive and 36 of 45 (80%) were positive for the in vivo CA test. Higher sensitivity was seen in in vivo CMT (90%) and in vitro CMT (86.9%) assay. Combination of two tests has greater sensitivity than individual tests: in vivo MN + in vivo CA (88.6%); in vivo MN + in vivo CMT (92.5%); and in vivo MN + in vitro CMT (95.6%). Combinations of in vivo or in vitro CMT with other tests provided better sensitivity. In vivo CMT in combination with in vivo CA provided the highest sensitivity (96.7%).

Propylene oxide: genotoxicity profile of a rodent nasal carcinogen

Propylene oxide (PO) is a DNA-reactive genotoxic agent; that is, it reacts with DNA to produce lesions in the genetic material. PO also induces tumors in rodents, although only at high concentrations and at portals of entry. This review of PO's genotoxicity profile is organized according to endpoints measured, that is, nonmutational or mutational endpoints, and as to whether the results were from in vitro or in vivo studies. In addition to results of experimental studies, PO's genotoxicity for humans is assessed by reviewing results of published biomarker studies. The weight of evidence indicates that although it is genotoxic, PO's potency as a DNA-reactive mutagen is weak. Other aspects of PO's overall tissue toxicities are also reviewed, with attention to glutathione (GSH) depletion and its consequences, that is, cell proliferation, death, and necrosis. These toxic tissue responses occur in the same anatomical regions in rodents as do the PO-induced tumors. Furthermore, some of these tissue toxicities can produce effects that may either augment PO's DNA-reactive mutagenicity or be genotoxic in themselves, not dependent on PO's DNA reactivity. Although its DNA reactivity may be a necessary component of PO's overall genotoxicity and rodent carcinogenicity, it is likely not sufficient, and the associated tissue toxicities, which are rate-limiting, also seem to be required. This complex mode of action has implications for estimations of PO's cancer potential in humans, especially at low exposure concentrations.

Genotoxic effects of ethylene oxide, propylene oxide and epichlorohydrin in humans: update review (1990-2001)

Ethylene oxide (EtO), propylene oxide (PO) and epichlorohydrin (ECH) are important industrial chemicals widely used as intermediates for various synthetic products. EtO and PO are also environmental pollutants. In this review we summarize data published during the period 1990-2001 concerning both the genotoxic and carcinogenic effects of these epoxides in humans. The use of DNA and hemoglobin adducts as biomarkers of exposure and the role of polymorphism, as well as confounding factors, are discussed. We have also included recent in vitro data comprising genotoxic effects induced by EtO, PO and ECH in mammalian cells. The uncertainties regarding cancer risk estimation still persist, in spite of the large database collected.

Use of haemoglobin adducts in exposure monitoring and risk assessment

Many industrial bulk chemicals are oxiranes or alkenes that are easily metabolised to oxiranes in mammalian systems. Many oxiranes may react with DNA and are therefore mutagenic in vitro. Some oxiranes have been shown to be carcinogenic in rodents in vivo as well. Despite the very limited evidence of the carcinogenicity of oxiranes in humans, they should be considered potential human carcinogens. As a consequence, exposure to these compounds should be minimised and controlled. Twenty-five years ago, Ehrenberg and co-workers suggested that exposure to oxiranes might be determined through the measurement of the adducts they form with haemoglobin (Hb). Ten years later, a modification of the Edman degradation was developed at Stockholm University that allowed determination of adducts with the N-terminal valine of Hb by GC-MS. In our laboratory, this methodology was modified and adapted for analysis on an industrial scale. Since 1987, exposure of operators in our facilities to ethylene oxide (EO) has been routinely monitored by determination of N-(2-hydroxyethyl)valine in Hb. Biological monitoring programmes for propylene oxide (PO) and 1,3-butadiene (BD) were developed later. In this review, the methodology and its results are discussed as a tool in human risk assessment of industrial chemicals. Two major advantages of Hb adduct determinations in risk assessment are (1) the qualitative information on the structure of reactive intermediates that may be obtained through the mass spectrometry, which may provide insight in the molecular toxicology of compounds such as BD, and (2) the possibility of reliable determination of exposure over periods of several months with limited number of samples for compounds such as ethylene oxide (EO), propylene oxide (PO) and BD which form stable adducts with Hb. Since good correlations between the airborne concentrations of these chemicals with their respective adducts have been established, Hb adducts can also be used to quantitate airborne exposure which is of paramount importance as exposure assessment is usually one of the weaker parameters in risk assessment.

Induction and persistence of micronuclei, sister-chromatid exchanges and chromosomal aberrations in splenocytes and bone-marrow cells of rats exposed to ethylene oxide

Studies on the induction and persistence of ethylene oxide (EO) induced chromosomal alterations in rat bone-marrow cells and splenocytes following in vivo exposure were carried out. Rats were exposed to ethylene oxide either chronically by inhalation (50-200ppm, 4 weeks, 5 days/week, 6h/day) or acutely by intraperitoneal injection (i.p.) at dose levels of 50-100ppm.Spontaneous- and induced-frequencies of micronuclei (MN), sister-chromatid exchanges (SCEs) and chromosomal aberrations were determined in rat bone-marrow cells, and in splenocytes following in vitro mitogen stimulation. Unstable chromosomal aberrations were studied in whole genome using standard Giemsa staining technique and fluorescence in situ hybridisation using probe for chromosome #2 was employed to detect chromosome translocations. Following chronic exposure, the cytogenetic analyses were carried out at days 5 and 21 in rat splenocytes, to study the induction and persistence of sister-chromatid exchanges. Following chronic exposure, ethylene oxide was effective in inducing SCEs, and markedly cells with high frequency SCEs were observed and they in-part persisted until day 21 post-exposure. However, no significant effect was observed in rat splenocytes for induction of MN and chromosomal aberrations. Following acute exposure, both SCEs and MN were increased significantly in rat bone-marrow cells as well as splenocytes.In conclusion, this study indicates that ethylene oxide at the concentrations employed by intraperitoneal injection or inhalation in adult rats is mutagenic and can induce both SCEs and MN.

Tissue distribution of DNA adducts in male Fischer rats exposed to 500 ppm of propylene oxide: quantitative analysis of 7-(2-hydroxypropyl)guanine by 32P-postlabelling

7-(2-Hydroxypropyl)guanine (7-HPG) constitutes the major adduct from alkylation of DNA by the genotoxic carcinogen, propylene oxide. The levels of 7-HPG in DNA of various organs provides a relevant measure of tissue dose. 7-Alkylguanines can induce mutation through abasic sites formed from spontaneous depurination of the adduct. In the current study the formation of 7-HPG was investigated in male Fisher 344 rats exposed to 500 ppm of propylene oxide by inhalation for 6 h/day, 5 days/week, for up to 20 days. 7-HPG was analyzed using the 32P-postlabelling assay with anion-exchange cartridges for adduct enrichment. In animals sacrificed directly following 20 days of exposure, the adduct level was highest in the respiratory nasal epithelium (98.1 adducts per 10(6) nucleotides), followed by olfactory nasal epithelium (58.5), lung (16.3), lymphocytes (9.92), spleen (9.26), liver (4.64), and testis (2.95). The nasal cavity is the major target for tumor induction in the rat following inhalation. This finding is consistent with the major difference in adduct levels observed in nasal epithelium compared to other tissues. In rats sacrificed 3 days after cessation of exposure, the levels of 7-HPG in the aforementioned tissues had, on the average, decreased by about one-quarter of their initial concentrations. This degree of loss closely corresponds to the spontaneous rate of depurination for this adduct (t 1/2 = 120 h), and suggests a low efficiency of repair for 7-HPG in the rat. The postlabelling assay used had a detection limit of one to two adducts per 10(8) nucleotides, i.e. it is likely that this adduct could be analyzed in nasal tissues of rats exposed to less than 1 ppm of propylene oxide.

Ethylene oxide: evaluation of genotoxicity data and an exploratory assessment of genetic risk

A risk estimate of the heritable effects of ethylene oxide exposure, using the parallelogram approach, as suggested by Frits Sobels, is described. The approach is based on available data on the ethylene oxide-induced responses for the same genetic endpoint in somatic cells of both laboratory animals and humans, and for germ cell mutations in the same laboratory animal. Human germ cell effects are estimated. The available data sets for this approach were evaluated. We consider this as complementary to the genetic risk assessment carried out by U.S. EPA scientists, in which the risk from heritable (reciprocal) translocations induced by ethylene oxide was estimated. In the present study we restricted our assessment to dominant mutations. The sensitivity factor relating mouse to man was based on ethylene oxide-induced HPRT mutant frequencies in lymphocytes in vivo. From this comparison, it could be concluded that occupational exposure for 1 year to 1 ppm ethylene oxide would lead to a risk of a dominantly inherited disease in the offspring of 4 x 10(-4) above the background level. The uncertainty interval of this figure is quite large (0.6-28) x 10(-4). The values are compatible with the existing estimates of the corresponding risk from exposure to low LET radiation when the genotoxic potency ratio of ethylene oxide and radiation is considered. This risk estimation approach has allowed us to identify additional data that are required for a more complete risk estimation of the heritable effects of ethylene oxide, or indeed any mutagenic chemical.

The parallelogram approach in studies of genotoxic effects

Over the past two decades mutagenicity tests have been used for the identification of potential human mutagens and have had an ancillary role, as supportive evidence in the assessment of human carcinogens. The demonstration of human germinal mutagens has been beyond the main scope of short-term testing strategies. However, just as mutagenicity tests have been useful in detecting potential carcinogens so should carcinogenicity tests assist the identification of presumptive germ cell mutagens. Cancer is an easily observable phenotype of mutation for genotoxic carcinogens and multi-site carcinogens or gonadal carcinogens logically could be germ cell mutagens. Thus carcinogenicity and mutagenicity data for a given genotoxic chemical should be considered together in the identification of putative germinal mutagens. Clearly, most classified human carcinogens are genotoxic thus helping to build the case for human germ cell mutagenicity. This paper describes the issues involved in such thinking and suggests an enhanced parallelogram approach incorporating the cancer endpoint. The enhanced parallelogram is explored using 1,3-butadiene and ethylene oxide as examples. The obvious lack of data for extrapolations using the parallelogram method suggests the need for targeted studies specifically designed for use in this approach.

Chromosomal aberrations in mouse lymphocytes exposed in vivo and in vitro to aliphatic epoxides

Mouse lymphocytes in vivo or in vitro were exposed for 24 h to 4 aliphatic epoxides, glycidyl 1-naphthyl ether, glycidyl 4-nitrophenyl ether, 1-naphthyl-propylene oxide and trichloropropylene oxide (TCPO), and tested for the induction of chromosomal aberrations (CA). These epoxides were among the most genotoxic aliphatic epoxides in our previous studies. With the exception of TCPO, the test epoxides caused significant increases in CA in vivo compared to a negative control. There were concentration related increases in CA for all 4 epoxides in vitro and TCPO produced the greatest cellular toxicity and genotoxic effects towards cultured lymphocytes. The difference in the order of genotoxicity for the two test systems can be explained on the basis of a much shorter half-life for TCPO than for the other epoxides.

Degree of alkylation of macromolecules in vivo from variable exposure

Measurements of adducts formed with blood proteins, particularly haemoglobin, are increasingly being used to monitor human exposures to genotoxic chemicals. Information about the relationships between levels of genotoxic chemicals in the environment, e.g., concentration in the air, and levels of protein adducts in the blood is particularly important in setting safety standards and assessing risks. This paper describes the relationships between level of exposure to alkylating agents and level of haemoglobin adducts, considering the zero-order kinetics of the disappearance of these adducts. For comparison the corresponding relationship for adducts to macromolecules subjected to turnover, with first-order kinetics of disappearance, is described. For chemically stable and unstable adducts different exposure situations are considered: acute, chronic, intermittent and varying exposure levels. It is shown how an optimum solution of the problem of establishing the relationship between long-term exposure at varying levels (e.g., in work environments) and adduct level can be reached. Through mathematical derivations, which are given, expressions applicable to various exposure patterns are obtained and presented.

Quantitative assessment of a human carcinogenic potency for propylene oxide

The potential for causing carcinogenic and mutagenic effects is the main concern when assessing the risks associated with low-level exposures of humans to the industrially important epoxide, propylene oxide (PO). The available basic information used in estimation of carcinogenic risk has been reviewed. It is concluded that the published data from gavage studies in rodents are less appropriate and that observed cancer incidences from long-term inhalation should preferably be utilized for quantitative risk assessment. Furthermore, PO and ethylene oxide (EO) are directly acting alkylating agents which exhibit several similarities. Although data are less comprehensive for PO than for EO, PO appears to yield a rather uniform alkylation pattern in various tissues. Also, similarly to EO, PO is probably detoxified at a rate which does not vary widely in various mammalian species, including man. For these reasons, the surface-based extrapolation model for estimation of the human equivalent dose may not be appropriate, and the previously derived carcinogenic potency factors should be revised downward. Alternative risk estimates are provided. From the most relevant available studies, we propose a carcinogenic potency factor of 0.001 (mg/kg/day)-1 for PO in humans by inhalation.

Genotoxic effects of ethylene oxide and propylene oxide: a comparative study

The two alkylating agents ethylene oxide (EO) and propylene oxide (PO) were compared for genotoxic effectiveness in various test systems. The study was undertaken partly to shed light on the difference between the compounds found after chronic exposure of monkeys (Lynch et al., 1984) where EO but not PO was able to induce SCE and chromosomal aberrations. In the present study EO was found to be 5-10 times more effective than PO with respect to gene conversion and reverse mutation in Saccharomyces cerevisiae D7 and sister-chromatid conversion in S. cerevisiae RS112. In contrast, the abilities of the two compounds to induce point mutation in S. typhimurium strains and SCE in human lymphocytes were approximately equal. One possible cause of EO being more effective than PO in certain respects, discussed on the basis of inference from earlier studies, is an expected difference in ability to cause strand breaks via alkylation of DNA-phosphate groups.

An epidemiological study of cancer risk among workers exposed to ethylene oxide using hemoglobin adducts to validate environmental exposure assessments

Cancer morbidity was investigated in a cohort of 2,170 ethylene oxide (EO)-exposed workers from 2 plants producing disposable medical equipment. The subjects had been employed for at least 1 year during the periods 1970-1985 and 1964-1985, respectively. The exposure to EO was assessed for each of six job categories in the plants with respect to each calendar year, on which basis values for individual cumulative exposure to EO (ppm-years) were calculated. The levels of hydroxyethyl adducts to N-terminal valine (HOEtVal) in hemoglobin fitted well with the values estimated for airborne exposure to EO. No increased cancer incidence was found [standardized morbidity ratio (SMR), 0.78; 95% CI, 0.49-1.21)]. No leukemia was observed, but one case of non-Hodgkin's lymphoma, one case of myeloma, and one case of polycythemia vera were diagnosed as compared with two expected hematopoietic and lymphatic tumors (SMR, 1.54; 95% CI, 0.32-4.5). No stomach cancer was detected as compared with the 0.5 case expected. There were no significant exposure-response associations between estimates of exposure to EO and cancer morbidity.

Monitoring exposure to simple epoxides and alkenes through gas chromatographic determination of hemoglobin adducts

A method for monitoring exposure to ethylene oxide (EO) and propylene oxide (PO) and their corresponding alkenes through the analysis of adducts to N-terminal valine in hemoglobin (Hb) using gas chromatography (GC) and electron-capture detection has been developed. The method is a further development of the so-called N-alkyl Edman method, which has thus far been carried out using gas chromatography-mass spectrometry (GC/MS). The correlation between GC and GC/MS determinations of adduct levels in human samples was found to be good. The newly developed GC method enables the determination of adducts to Hb from EO and PO down to levels of about 100 pmol/g globin. This adduct level corresponds to the expected increment from ethene in inhaled tobacco smoke in a smoker of about 10 cigarettes/day or from an average exposure to about 50 ppb EO or 1 ppm PO during working hours.

Alkylation of DNA and hemoglobin in the mouse following exposure to propene and propylene oxide

Male CBA mice were exposed to propene, unlabelled or 14C-labelled, by inhalation, or to 14C-labelled propylene oxide by intraperitoneal injection. 2-Hydroxypropyl adducts to guanine-N-7 in DNA of various organs and to N-terminal valine and histidine-N pi in hemoglobin were measured. The adduct levels observed show that propylene oxide is the major primary metabolic product of propene. A direct comparison of propylene oxide with the homologous compound ethylene oxide on the basis of adduct levels introduced (in DNA and in hemoglobin) at equimolar injected amounts, shows that propylene oxide is 6-10 times less effective than ethylene oxide.