Differentiation between metabolic incorporation and covalent binding in the labeling of macromolecules in the rat nasal mucosa and bone marrow by inhaled [14C]- and [3H]formaldehyde

An updated mode of action and human relevance framework evaluation for Formaldehyde-Related nasal tumors

Formaldehyde is a reactive aldehyde naturally present in all plant and animal tissues and a critical component of the one-carbon metabolism pathway. It is also a high production volume chemical used in the manufacture of numerous products. Formaldehyde is also one of the most well-studied chemicals with respect to environmental fate, biology, and toxicology-including carcinogenic potential, and mode of action (MOA). In 2006, a published MOA for formaldehyde-induced nasal tumors in rats concluded that nasal tumors were most likely driven by cytotoxicity and regenerative cell proliferation, with possible contributions from direct genotoxicity. In the past 15 years, new research has better informed the MOA with the publication of in vivo genotoxicity assays, toxicogenomic analyses, and development of ultra-sensitive methods to measure endogenous and exogenous formaldehyde-induced DNA adducts. Herein, we review and update the MOA for nasal tumors, with particular emphasis on the numerous studies published since 2006. These new studies further underscore the involvement of cytotoxicity and regenerative cell proliferation, and further inform the genotoxic potential of inhaled formaldehyde. The data lend additional support for the use of mechanistic data for the derivation of toxicity criteria and/or scientifically supported approaches for low-dose extrapolation for the risk assessment of formaldehyde.

Formaldehyde exposure and leukemia risk: a comprehensive review and network-based toxicogenomic approach

Formaldehyde is a widely used but highly reactive and toxic chemical. The International Agency for Research on Cancer classifies formaldehyde as a Group 1 carcinogen, based on nasopharyngeal cancer and leukemia studies. However, the correlation between formaldehyde exposure and leukemia incidence is a controversial issue. To understand the association between formaldehyde exposure and leukemia, we explored biological networks based on formaldehyde-related genes retrieved from public and commercial databases. Through the literature-based network approach, we summarized qualitative associations between formaldehyde exposure and leukemia. Our results indicate that oxidative stress-mediated genetic changes induced by formaldehyde could disturb the hematopoietic system, possibly leading to leukemia. Furthermore, we suggested major genes that are thought to be affected by formaldehyde exposure and associated with leukemia development. Our suggestions can be used to complement experimental data for understanding and identifying the leukemogenic mechanism of formaldehyde.

Using mechanistic information to support evidence integration and synthesis: a case study with inhaled formaldehyde and leukemia

Formaldehyde is one of the most comprehensively studied chemicals, with over 30 years of research focused on understanding the development of cancer following inhalation. The causal conclusions regarding the potential for leukemia are largely based on the epidemiological literature, with little consideration of cancer bioassays, dosimetry studies, and mechanistic research, which challenge the biological plausibility of the disease. Recent reanalyzes of the epidemiological literature have also raised significant questions related to the purported associations between formaldehyde and leukemia. Because of this, considerable scientific debate and uncertainty remain on whether there is a causal association between formaldehyde inhalation exposure and leukemia. Further complexity in evaluating this association is related to the endogenous production of formaldehyde. Multiple modes of action (MOA) have been postulated for the development of leukemia following formaldehyde inhalation that includes unsupported hypotheses of direct or indirect toxicity to the target cell population. Herein, the available evidence relevant to evaluating the postulated MOAs for leukemia following formaldehyde inhalation exposure is organized in the IPCS MOA Framework. The integration of all the available evidence clearly highlights the limited amount of data that support any of the postulated MOAs and demonstrates a significant amount of research supporting the null hypothesis that there is no causal association between formaldehyde inhalation exposure and leukemia. These analyses result in a lack of confidence in any of the postulated MOAs, increasing confidence in the conclusion that there is a lack of biological plausibility for a causal association between formaldehyde inhalation exposure and leukemia.

Beneficial effects of polyherbal formulation (Bronco-T) on formaldehyde-induced lung toxicity in male Wistar rats

Polyherbal compound (Bronco-T) has been extensively used as a traditional medicine for various therapies. However, very few report studies on anti-inflammatory and lung regeneration properties are evidenced. In the present study, we evaluated the beneficial actions and anti-inflammatory properties of polyherbal medicine, Bronco-T, exhibited by treating the lungs of rats exposed to formaldehyde to evaluate the beneficial properties. For this study, we divided into five groups': i.e. Group-I served as a control and the other four groups such as II, III, IV, and V are experimental. All animals maintained by regular feed and water ad libitum during the study. Formaldehyde vapors exposure at a single period of time (1 hour) daily (40%formaldehyde at room temperature) for 21 days period exposed all groups. The Bronco-T extracts about 50 mg/kg BW administered to experimental groups and group IV rats treated with 500μ grams/Kg BW salbutamol. To understand the impact of formaldehyde exposure on the beneficial effects of Bronco-T, we evaluated hematological parameters, bronchoalveolar lavage (BAL), histamine levels, and histological alterations of lung architecture. Formaldehyde-induced adverse effects in lung and increased histamine levels in BAL compared to Bronco-T-treated rats act as a preventive immunological role in blood toxicity and recovery of lung architecture in Bronco-T-treated rats. This study showed the evaluation of antihistamine levels through HPLC analysis. Bronco-T has antioxidant and anti-histamine properties as the widest therapeutic window, and we continue to evaluate the pharmacological evaluations needed in our further studies.

Considerations for refining the risk assessment process for formaldehyde: Results from an interdisciplinary workshop

Anticipating the need to evaluate and integrate scientific evidence to inform new risk assessments or to update existing risk assessments, the Formaldehyde Panel of the American Chemistry Council (ACC), in collaboration with the University of North Carolina, convened a workshop: "Understanding Potential Human Health Cancer Risk - From Data Integration to Risk Evaluation" in October 2017. Twenty-four (24) invited-experts participated with expertise in epidemiology, toxicology, science integration and risk evaluation. Including members of the organizing committee, there were 29 participants. The meeting included eleven presentations encompassing an introduction and three sessions: (1) "integrating the formaldehyde science on nasal/nasopharyngeal carcinogenicity and potential for causality"; (2) "integrating the formaldehyde science on lymphohematopoietic cancer and potential for causality; and, (3) "formaldehyde research-data suitable for risk assessment". Here we describe key points from the presentations on epidemiology, toxicology and mechanistic studies that should inform decisions about the potential carcinogenicity of formaldehyde in humans and the discussions about approaches for structuring an integrated, comprehensive risk assessment for formaldehyde. We also note challenges expected when attempting to reconcile divergent results observed from research conducted within and across different scientific disciplines - especially toxicology and epidemiology - and in integrating diverse, multi-disciplinary mechanistic evidence.

Six years after the NRC review of EPA's Draft IRIS Toxicological Review of Formaldehyde: Regulatory implications of new science in evaluating formaldehyde leukemogenicity

Shortly after the International Agency for Research on Cancer (IARC) determined that formaldehyde causes leukemia, the United States Environmental Protection Agency (EPA) released its Draft IRIS Toxicological Review of Formaldehyde ("Draft IRIS Assessment"), also concluding that formaldehyde causes leukemia. Peer review of the Draft IRIS Assessment by a National Academy of Science committee noted that "causal determinations are not supported by the narrative provided in the draft" (NRC 2011). They offered recommendations for improving the Draft IRIS assessment and identified several important research gaps. Over the six years since the NRC peer review, significant new science has been published. We identify and summarize key recommendations made by NRC and map them to this new science, including extended analysis of epidemiological studies, updates of earlier occupational cohort studies, toxicological experiments using a sensitive mouse strain, mechanistic studies examining the role of exogenous versus endogenous formaldehyde in bone marrow, and several critical reviews. With few exceptions, new findings are consistently negative, and integration of all available evidence challenges the earlier conclusions that formaldehyde causes leukemia. Given formaldehyde's commercial importance, environmental ubiquity and endogenous production, accurate hazard classification and risk evaluation of whether exposure to formaldehyde from occupational, residential and consumer products causes leukemia are critical.

Reassessment of MTBE cancer potency considering modes of action for MTBE and its metabolites

A 1999 California state agency cancer potency (CP) evaluation of methyl tert-butyl ether (MTBE) assumed linear risk extrapolations from tumor data were plausible because of limited evidence that MTBE or its metabolites could damage DNA, and based such extrapolations on data from rat gavage and rat and mouse inhalation studies indicating elevated tumor rates in male rat kidney, male rat Leydig interstitial cells, and female rat leukemia/lymphomas. More recent data bearing on MTBE cancer potency include a rodent cancer bioassay of MTBE in drinking water; several new studies of MTBE genotoxicity; several similar evaluations of MTBE metabolites, formaldehyde, and tert-butyl alcohol or TBA; and updated evaluations of carcinogenic mode(s) of action (MOAs) of MTBE and MTBE metabolite's. The lymphoma/leukemia data used in the California assessment were recently declared unreliable by the U.S. Environmental Protection Agency (EPA). Updated characterizations of MTBE CP, and its uncertainty, are currently needed to address a variety of decision goals concerning historical and current MTBE contamination. To this end, an extensive review of data sets bearing on MTBE and metabolite genotoxicity, cytotoxicity, and tumorigenicity was applied to reassess MTBE CP and related uncertainty in view of MOA considerations. Adopting the traditional approach that cytotoxicity-driven cancer MOAs are inoperative at very low, non-cytotoxic dose levels, it was determined that MTBE most likely does not increase cancer risk unless chronic exposures induce target-tissue toxicity, including in sensitive individuals. However, the corresponding expected (or plausible upper bound) CP for MTBE conditional on a hypothetical linear (e.g., genotoxic) MOA was estimated to be ∼2 × 10(-5) (or 0.003) per mg MTBE per kg body weight per day for adults exposed chronically over a lifetime. Based on this conservative estimate of CP, if MTBE is carcinogenic to humans, it is among the weakest 10% of chemical carcinogens evaluated by EPA.

Formaldehyde induces toxicity in mouse bone marrow and hematopoietic stem/progenitor cells and enhances benzene-induced adverse effects

FA in air for 2 weeks, mimicking occupational exposure, then measured complete blood counts, nucleated BM cell count, and myeloid progenitor colony formation. We also investigated potential mechanisms of FA toxicity, including reactive oxygen species (ROS) generation, apoptosis, and hematopoietic growth factor and receptor levels. FA exposure significantly reduced nucleated BM cells and BM-derived colony-forming unit-granulocyte-macrophage (CFU-GM) and burst-forming unit-erythroid (BFU-E); down-regulated GM-CSFRα and EPOR expression; increased ROS in nucleated BM, spleen and CFU-GM cells; and increased apoptosis in nucleated spleen and CFU-GM cells. FA and BZ each similarly altered BM mature cells and stem/progenitor counts, BM and CFU-GM ROS, and apoptosis in spleen and CFU-GM but had differential effects on other end points. Co-exposure was more potent for several end points. Thus, FA is toxic to the mouse hematopoietic system, including BM stem/progenitor cells, and it enhances BZ-induced toxic effects. Our findings suggest that FA may induce BM toxicity by affecting myeloid progenitor growth and survival through oxidative damage and reduced expression levels of GM-CSFRα and EPOR.

Formaldehyde Stress Responses in Bacterial Pathogens

Formaldehyde is the simplest of all aldehydes and is highly cytotoxic. Its use and associated dangers from environmental exposure have been well documented. Detoxification systems for formaldehyde are found throughout the biological world and they are especially important in methylotrophic bacteria, which generate this compound as part of their metabolism of methanol. Formaldehyde metabolizing systems can be divided into those dependent upon pterin cofactors, sugar phosphates and those dependent upon glutathione. The more prevalent thiol-dependent formaldehyde detoxification system is found in many bacterial pathogens, almost all of which do not metabolize methane or methanol. This review describes the endogenous and exogenous sources of formaldehyde, its toxic effects and mechanisms of detoxification. The methods of formaldehyde sensing are also described with a focus on the formaldehyde responsive transcription factors HxlR, FrmR, and NmlR. Finally, the physiological relevance of detoxification systems for formaldehyde in bacterial pathogens is discussed.

Formaldehyde Exposure and Mortality Risks From Acute Myeloid Leukemia and Other Lymphohematopoietic Malignancies in the US National Cancer Institute Cohort Study of Workers in Formaldehyde Industries

OBJECTIVES

To evaluate associations between cumulative and peak formaldehyde exposure and mortality from acute myeloid leukemia (AML) and other lymphohematopoietic malignancies.

METHODS

Cox proportional hazards analyses.

RESULTS

Acute myeloid leukemia was unrelated to cumulative exposure. Hodgkin lymphoma relative risk estimates in the highest exposure categories of cumulative and peak exposures were, respectively, 3.76 (Ptrend = 0.05) and 5.13 (Ptrend = 0.003). There were suggestive associations with peak exposure observed for chronic myeloid leukemia, albeit based on very small numbers. No other lymphohematopoietic malignancy was associated with either chronic or peak exposure.

CONCLUSIONS

Insofar as there is no prior epidemiologic evidence supporting associations between formaldehyde and either Hodgkin leukemia or chronic myeloid leukemia, any causal interpretations of the observed risk patterns are at most tentative. Findings from this re-analysis do not support the hypothesis that formaldehyde is a cause of AML.

Formation, Accumulation, and Hydrolysis of Endogenous and Exogenous Formaldehyde-Induced DNA Damage

Formaldehyde is not only a widely used chemical with well-known carcinogenicity but is also a normal metabolite of living cells. It thus poses unique challenges for understanding risks associated with exposure. N(2-)hydroxymethyl-dG (N(2)-HOMe-dG) is the main formaldehyde-induced DNA mono-adduct, which together with DNA-protein crosslinks (DPCs) and toxicity-induced cell proliferation, play important roles in a mutagenic mode of action for cancer. In this study, N(2)-HOMe-dG was shown to be an excellent biomarker for direct adduction of formaldehyde to DNA and the hydrolysis of DPCs. The use of inhaled [(13)CD2]-formaldehyde exposures of rats and primates coupled with ultrasensitive nano ultra performance liquid chromatography-tandem mass spectrometry permitted accurate determinations of endogenous and exogenous formaldehyde DNA damage. The results show that inhaled formaldehyde only reached rat and monkey noses, but not tissues distant to the site of initial contact. The amounts of exogenous adducts were remarkably lower than those of endogenous adducts in exposed nasal epithelium. Moreover, exogenous adducts accumulated in rat nasal epithelium over the 28-days exposure to reach steady-state concentrations, followed by elimination with a half-life (t1/2) of 7.1 days. Additionally, we examined artifact formation during DNA preparation to ensure the accuracy of nonlabeled N(2)-HOMe-dG measurements. These novel findings provide critical new data for understanding major issues identified by the National Research Council Review of the 2010 Environmental Protection Agency's Draft Integrated Risk Information System Formaldehyde Risk Assessment. They support a data-driven need for reflection on whether risks have been overestimated for inhaled formaldehyde, whereas underappreciating endogenous formaldehyde as the primary source of exposure that results in bone marrow toxicity and leukemia in susceptible humans and rodents deficient in DNA repair.

The endogenous exposome

The concept of the Exposome is a compilation of diseases and one's lifetime exposure to chemicals, whether the exposure comes from environmental, dietary, or occupational exposures; or endogenous chemicals that are formed from normal metabolism, inflammation, oxidative stress, lipid peroxidation, infections, and other natural metabolic processes such as alteration of the gut microbiome. In this review, we have focused on the endogenous exposome, the DNA damage that arises from the production of endogenous electrophilic molecules in our cells. It provides quantitative data on endogenous DNA damage and its relationship to mutagenesis, with emphasis on when exogenous chemical exposures that produce identical DNA adducts to those arising from normal metabolism cause significant increases in total identical DNA adducts. We have utilized stable isotope labeled chemical exposures of animals and cells, so that accurate relationships between endogenous and exogenous exposures can be determined. Advances in mass spectrometry have vastly increased both the sensitivity and accuracy of such studies. Furthermore, we have clear evidence of which sources of exposure drive low dose biology that results in mutations and disease. These data provide much needed information to impact quantitative risk assessments, in the hope of moving towards the use of science, rather than default assumptions.

Inhaled formaldehyde induces DNA-protein crosslinks and oxidative stress in bone marrow and other distant organs of exposed mice

Formaldehyde (FA), a major industrial chemical and ubiquitous environmental pollutant, has been classified as a leukemogen. The causal relationship remains unclear, however, due to limited evidence that FA induces toxicity in bone marrow, the site of leukemia induction, and in other distal organs. Although induction of DNA-protein crosslinks (DPC), a hallmark of FA toxicity, was not previously detected in the bone marrow of FA-exposed rats and monkeys in studies published in the 1980s, our recent studies showed increased DPC in the bone marrow, liver, kidney, and testes of exposed Kunming mice. To confirm these preliminary results, in the current study we exposed BALB/c mice to 0, 0.5, 1.0, and 3.0 mg m(-3) FA (8 hr per day, for 7 consecutive days) by nose-only inhalation and measured DPC levels in bone marrow and other organs of exposed mice. As oxidative stress is a potential mechanism of FA toxicity, we also measured glutathione (GSH), reactive oxygen species (ROS), and malondialdehyde (MDA), in the bone marrow, peripheral blood mononuclear cells, lung, liver, spleen, and testes of exposed mice. Significant dose-dependent increases in DPC, decreases in GSH, and increases in ROS and MDA were observed in all organs examined (except for DPC in lung). Bone marrow was among the organs with the strongest effects for DPC, GSH, and ROS. In conclusion, exposure of mice to FA by inhalation induced genotoxicity and oxidative stress in bone marrow and other organs. These findings strengthen the biological plausibility of FA-induced leukemogenesis and systemic toxicity.

Formaldehyde exposure and leukemia: critical review and reevaluation of the results from a study that is the focus for evidence of biological plausibility

A recent study (Zhang et al., 2010) has provided results attributed to aneuploidy in circulating stem cells that has been characterized as providing potential support for proposed mechanisms for formaldehyde to impact bone marrow. A critical review of the study, as well as a reanalysis of the underlying data, was performed and the results of this reanalysis suggested factors other than formaldehyde exposure may have contributed to the effects reported. In addition, although the authors stated in their paper that "all scorable metaphase spreads on each slide were analyzed, and a minimum of 150 cells per subject was scored," this protocol was not followed. In fact, the protocol to evaluate the presence of monosomy 7 or trisomy 8 was followed for three or less samples in exposed workers and six or less samples in non-exposed workers. In addition, the assays used (CFU-GM) do not actually measure the proposed events in primitive cells involved in the development of acute myeloid leukemia. Evaluation of these data indicates that the aneuploidy measured could not have arisen in vivo, but rather arose during in vitro culture. The results of our critical review and reanalysis of the data, in combination with recent toxicological and mechanistic studies, do not support a mechanism for a causal association between formaldehyde exposure and myeloid or lymphoid malignancies.

Formaldehyde carcinogenicity research: 30 years and counting for mode of action, epidemiology, and cancer risk assessment

Formaldehyde is a widely used high production chemical that is also released as a byproduct of combustion, off-gassing of various building products, and as a fixative for pathologists and embalmers. What is not often realized is that formaldehyde is also produced as a normal physiologic chemical in all living cells. In 1980, chronic inhalation of high concentrations of formaldehyde was shown to be carcinogenic, inducing a high incidence of nasal squamous cell carcinomas in rats. Some epidemiologic studies have also found increased numbers of nasopharyngeal carcinoma and leukemia in humans exposed to formaldehyde that resulted in formaldehyde being considered a Known Human Carcinogen. This article reviews the data for rodent and human carcinogenicity, early Mode of Action studies, more recent molecular studies of both endogenous and exogenous DNA adducts, and epigenetic studies. It goes on to demonstrate the power of these research studies to provide critical data to improve our ability to develop science-based cancer risk assessments, instead of default approaches. The complexity of constant physiologic exposure to a known carcinogen requires that new ways of thinking be incorporated into determinations of cancer risk assessment for formaldehyde, other endogenous carcinogens, and the role of background endogenous DNA damage and mutagenesis.

Is exposure to formaldehyde in air causally associated with leukemia?--A hypothesis-based weight-of-evidence analysis

Recent scientific debate has focused on the potential for inhaled formaldehyde to cause lymphohematopoietic cancers, particularly leukemias, in humans. The concern stems from certain epidemiology studies reporting an association, although particulars of endpoints and dosimetry are inconsistent across studies and several other studies show no such effects. Animal studies generally report neither hematotoxicity nor leukemia associated with formaldehyde inhalation, and hematotoxicity studies in humans are inconsistent. Formaldehyde's reactivity has been thought to preclude systemic exposure following inhalation, and its apparent inability to reach and affect the target tissues attacked by known leukemogens has, heretofore, led to skepticism regarding its potential to cause human lymphohematopoietic cancers. Recently, however, potential modes of action for formaldehyde leukemogenesis have been hypothesized, and it has been suggested that formaldehyde be identified as a known human leukemogen. In this article, we apply our hypothesis-based weight-of-evidence (HBWoE) approach to evaluate the large body of evidence regarding formaldehyde and leukemogenesis, attending to how human, animal, and mode-of-action results inform one another. We trace the logic of inference within and across all studies, and articulate how one could account for the suite of available observations under the various proposed hypotheses. Upon comparison of alternative proposals regarding what causal processes may have led to the array of observations as we see them, we conclude that the case for a causal association is weak and strains biological plausibility. Instead, apparent association between formaldehyde inhalation and leukemia in some human studies is better interpreted as due to chance or confounding.

Guidance for the classification of carcinogens under the Globally Harmonised System of Classification and Labelling of Chemicals (GHS)

The United Nations Conference on Environment and Development (UNCED) has developed criteria for a globally harmonised system of classification and labelling of chemicals (GHS). With regard to carcinogenicity, GHS distinguishes between Category 1 ('known or presumed human carcinogens') and Category 2 ('suspected human carcinogens'). Category 1 carcinogens are divided into Category 1A ('known to have carcinogenic potential for humans'), based largely on human evidence, and 1B ('presumed to have carcinogenic potential for humans'), based largely on experimental animal data. Concerns have been raised that the criteria for applying these carcinogenicity classifications are not sufficiently well defined and potentially allow different conclusions to be drawn. The current document describes an attempt to reduce the potential for diverse conclusions resulting from the GHS classification system through the application of a series of questions during the evaluation of data from experiments with rodents; epidemiological data, which could lead to Category 1A, have not been considered. Answers to each question can lead either to a classification decision or to the next question, but this process should only be implemented in an environment of informed scientific opinion. The scheme is illustrated with five case studies. These questions are: (1) Has a relevant form of the substance been tested? (2) Is the study design relevant to human exposure? (3) Is there a substance-related response? (4) Is the target tissue exposure relevant to humans? (5) Can a mode of action be established? (6) Is the mode of action relevant to humans? (7) What is the potency?

Distribution of DNA adducts caused by inhaled formaldehyde is consistent with induction of nasal carcinoma but not leukemia

Inhaled formaldehyde is classified as a known human and animal carcinogen, causing nasopharyngeal cancer. Additionally, limited epidemiological evidence for leukemia in humans is available; however, this is inconsistent across studies. Both genotoxicity and cytotoxicity are key events in formaldehyde nasal carcinogenicity in rats, but mechanistic data for leukemia are not well established. Formation of DNA adducts is a key event in initiating carcinogenesis. Formaldehyde can induce DNA monoadducts, DNA-DNA cross-links, and DNA protein cross-links. In this study, highly sensitive liquid chromatography-tandem mass spectrometry-selected reaction monitoringmethods were developed and [(13)CD(2)]-formaldehyde exposures utilized, allowing differentiation of DNA adducts and DNA-DNA cross-links originating from endogenous and inhalation-derived formaldehyde exposure. The results show that exogenous formaldehyde induced N(2)-hydroxymethyl-dG monoadducts and dG-dG cross-links in DNA from rat respiratory nasal mucosa but did not form [(13)CD(2)]-adducts in sites remote to the portal of entry, even when five times more DNA was analyzed. Furthermore, no N(6)-HO(13)CD(2)-dA adducts were detected in nasal DNA. In contrast, high amounts of endogenous formaldehyde dG and dA monoadducts were present in all tissues examined. The number of exogenous N(2)-HO(13)CD(2)-dG in 1- and 5-day nasal DNA samples from rats exposed to 10-ppm [(13)CD(2)]-formaldehyde was 1.28 +/- 0.49 and 2.43 +/- 0.78 adducts/10(7) dG, respectively, while 2.63 +/- 0.73 and 2.84 +/- 1.13 N(2)-HOCH(2)-dG adducts/10(7) dG and 3.95 +/- 0.26 and 3.61 +/- 0.95 N(6)-HOCH(2)-dA endogenous adducts/10(7) dA were present. This study provides strong evidence supporting a genotoxic and cytotoxic mode of action for the carcinogenesis of inhaled formaldehyde in respiratory nasal epithelium but does not support the biological plausibility that inhaled formaldehyde also causes leukemia.

Inhalation of formaldehyde does not induce systemic genotoxic effects in rats

Male Fischer-344 rats were exposed to formaldehyde (FA) by inhalation for 4 weeks (6 h/day, 5 days/week). Groups of six rats each were exposed to the target concentrations of 0, 0.5, 1, 2, 6, 10 and 15 ppm. Potential systemic genotoxic effects were investigated as part of a comprehensive study on local and systemic toxic and genotoxic effects. For this purpose, peripheral blood samples were obtained by puncturing the retro-orbital venous plexus at the end of the exposure period. Blood sampling was carried out in a randomized sequence and samples were coded by sequence number to ensure blind evaluation. Blood samples were used for the comet assay, the sister chromatid exchange test (SCE test) and the micronucleus test (MNT). DNA migration in the comet assay was measured both directly and after irradiation of the blood samples with 2 Gy gamma-radiation. The latter modification of the comet assay was included to increase its sensitivity for the detection of DNA-protein cross-links (DPX). The following positive control groups were included: one group (six animals) was treated with 50mg/kg methyl methanesulfonate (MMS) once by gavage 4h before blood sampling. Another group (six animals) was treated twice orally with 10mg/kg cyclophosphamide (CP) with an interval of 24 h. The last application of CP was 24h before blood sampling. For the comet assay, four slides were analysed from each blood sample, two without and two with irradiation. From each slide, 50 randomly selected cells were measured by image analysis, and tail intensity (% tail DNA) and tail moment were evaluated. For the SCE test, blood was cultured for 56 h in the presence of BrdU (10 microg/ml for the last 35 h) and SCE were counted in 30 second-division metaphases per sample. The MNT with peripheral blood was performed according to the instructions for the micronucleus analysis kit MICROFLOW (Litron Laboratories). Approximately 20,000 cells per sample were analysed by flow cytometry and the percentage of reticulocytes with micronuclei (MN) was determined. The positive control substances induced a significant effect in the genotoxicity tests and thus demonstrated the sensitivity of the test systems. FA did not induce any significant effect in any of the genotoxicity tests performed. It can be concluded that inhalation of FA in a 28-day study with FA concentrations up to 15 ppm does not lead to systemic genotoxic effects in the blood of rats.

Is propylene oxide induced cell proliferation in rat nasal respiratory epithelium mediated by a severe depletion of water-soluble non-protein thiol?

Propylene oxide (PO) concentrations >or=300 ppm induced cell proliferation and tumors in rat nasal respiratory epithelium (NRE). Cell proliferation was suggested to result from depletion of glutathione (GSH) in NRE. In order to substantiate this hypothesis, cell proliferation - measured by bromodeoxyuridine incorporation into DNA of the epithelium lining middle septum, dorsal medial meatus, and medial and lateral surfaces of the nasoturbinate in transverse nasal sections taken immediately posterior to the upper incisor teeth - and water-soluble non-protein thiol (NPSH) in NRE were determined after exposing male Fischer 344 rats to 50 ppm, 100 ppm, 200 ppm, or 300 ppm PO (6 h/day, 3 days). Both parameters were also investigated after treating rats for 3 days with diethylmaleate (DEM; 2 x 250 mg/kg/day or 500 + 150 mg/kg/day) or buthionine sulfoximine (BSO; 500 mg/kg/day). Exposure to 50 ppm PO and treatment with 2 x2 50 mg/kg/day DEM resulted in NPSH levels approximating 50% and 80% of the level in untreated controls, respectively. Cell proliferation did not increase. After exposures to >or= 100 ppm PO or treatment with BSO or 500 + 150 mg/kg/day DEM, NPSH was depleted to <or=1/3 of the control level and cell proliferation increased 2.0-3.7-fold the control value. In conclusion, profound perturbation of the GSH status may represent a crucial step in PO induced rat nasal tumorigenicity.

Your prodrug releases formaldehyde: should you be concerned? No!

The title of this commentary contains a frequently asked question whenever someone presents or proposes a prodrug strategy that releases formaldehyde as a result of bioconversion of a prodrug to parent drug. Formaldehyde, a highly water-soluble one-carbon molecule, is endogenous to cells, tissues, and body fluids. Although formaldehyde is generated and incorporated into essential metabolic processes by the human body, exposure to large amounts of formaldehyde vapor can irritate the nasal mucosa and may potentially be carcinogenic. It also gives a positive Ames test. Metabolism of both endogenous and exogenous formaldehyde involves rapid oxidation to formic acid catalyzed by glutathione dependent and independent dehydrogenases in the liver and erythrocytes. Balancing this rapid detoxification pathway is endogenous formation from normal metabolic processes and exogenous formaldehyde input, resulting in approximately 0.1 mM systemic levels. The possibility that formaldehyde released upon bioconversion of prodrugs might induce toxicity has been repeatedly stated, but no convincing evidence for this perceived toxicity has been documented in experimental studies. Therefore, as pharmaceutical chemists and not as toxicologists, we present our perspective on the apparent concern with release of formaldehyde as a by-product of in vivo bioconversion of selective prodrugs, and suggest that in comparison to the total amount of daily endogenous formaldehyde production from metabolism, and exogenous exposure from food and the environment, the amount generated by prodrugs is minute and is unlikely to cause any systemic toxicity in humans. Such an argument does not preclude formaldehyde-based toxicity assessment of a prodrug. Instead, it reduces the risk that in vivo liberation of formaldehyde will cause undue toxicity.

Formaldehyde and glutaraldehyde and nasal cytotoxicity: case study within the context of the 2006 IPCS Human Framework for the Analysis of a cancer mode of action for humans

Formaldehyde and glutaraldehyde cause toxicity to the nasal epithelium of rats and mice upon inhalation. In addition, formaldehyde above certain concentrations induces dose-related increases in nasal tumors in rats and mice, but glutaraldehyde does not. Using the 2006 IPCS human framework for the analysis of cancer mode of action (MOA), an MOA for formaldehyde was formulated and its relevance was tested against the properties of the noncarcinogenic glutaraldehyde. These compounds produce similar patterns of response in histopathology and in genotoxicity tests (although formaldehyde has been much more extensively tested studied). The MOA is based on the induction of sustained cytotoxicity and reparative cell proliferation induced by formaldehyde at concentrations that also induce nasal tumors upon long-term exposure. Data on dose dependency and temporal relationships of key events are consistent with this MOA. While a genotoxic MOA can never be ruled out for a compound that is clearly genotoxic, at least in vitro, the nongenotoxic properties fundamental to the proposed MOA can explain the neoplastic response in the nose and may be more informative than genotoxicity in risk assessment. It is not yet fully explained why glutaraldehyde remains noncarcinogenic upon inhalation, but its greater inherent toxicity may be a key factor. The dual aldehyde functions in glutaraldehyde are likely to produce damage resulting in fewer kinetic possibilities (particularly for proteins involved in differentiation control) and lower potential for repair (nucleic acids) than would be the case for formaldehyde. While there have been few studies of possible glutaraldehyde-associated cancer, the evidence that formaldehyde is a human carcinogen is strong for nasopharyngeal cancers, although less so for sinonasal cancers. This apparent discrepancy could be due in part to the classification of human nasal tumors with tumors of the sinuses, which would receive much less exposure to inhaled formaldehyde. Evaluation of the human relevance of the proposed MOA of formaldehyde in rodents is restricted by human data limitations, although the key events are plausible. It is clear that the human relevance of the formaldehyde MOA in rodents cannot be excluded on either kinetic or dynamic grounds.

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.

Risk assessment of formaldehyde for the general population in Japan

Formaldehyde is used in the production of resins, molding compounds, photographic film, bactericide, and tissue preservative. The purpose of this study was to provide an up-to-date critical review of the information to the toxicological profile of formaldehyde, and to assess the risk of formaldehyde for the general population in Japan. Inhaled formaldehyde is an effective sensory irritant at a dosage of 0.5 ppm in mice. Following inhalation in laboratory animals, more than 6 ppm formaldehyde causes degenerative non-neoplastic effects in mice and monkeys and nasal tumors in rats. It is considered that formaldehyde induces genotoxic effects directly in vitro and secondarily in vivo. Sensory irritation of the eyes and respiratory tract in response to inhalation exposure to formaldehyde has been reported at 0.08 ppm and above in human study. Formaldehyde is carcinogenic at the site of contact as a consequence of epithelial cell regenerative proliferation resulting from cytotoxicity and mutation, based on studies in both animals and humans. Levels of formaldehyde in atmosphere detected in rural, suburban, and urban areas in Japan were 2.5-3.2 ppb from 1998 to 2003. The majority of the population is exposed to atmosphere concentrations of formaldehyde less than those associated with sensory irritation. The reference concentration of formaldehyde in atmosphere for the Japanese general population is recommended to be 0.01 ppm.

The implausibility of leukemia induction by formaldehyde: a critical review of the biological evidence on distant-site toxicity

Formaldehyde is a naturally occurring biological compound that is present in tissues, cells, and bodily fluids. It is also a potent nasal irritant, a cytotoxicant at high doses, and a nasal carcinogen in rats exposed to high airborne concentrations. The normal endogenous concentration of formaldehyde in the blood is approximately 0.1 mM in rats, monkeys, and humans, and it is 2- to 4-fold higher in the liver and nasal mucosa of the rat. Inhaled formaldehyde enters the one-carbon pool, and the carbon atom is rapidly incorporated into macromolecules throughout the body. Oxidation to formate catalyzed by glutathione-dependent and -independent dehydrogenases in nasal tissues is a major route of detoxication and generally precedes incorporation. The possibility that inhaled formaldehyde might induce various forms of distant-site toxicity has been proposed, but no convincing evidence for such toxicity has been obtained in experimental studies. This review summarizes the biological evidence that pertains to the issue of leukemia induction by formaldehyde, which includes: (1) the failure of inhaled formaldehyde to increase the formaldehyde concentration in the blood of rats, monkeys, or humans exposed to concentrations of 14.4, 6, or 1.9 ppm, respectively; (2) the lack of detectable protein adducts or DNA-protein cross-links (DPX) in the bone marrow of normal rats exposed to [3H]- and [14C]formaldehyde at concentrations as high as 15 ppm; (3) the lack of detectable protein adducts or DPX in the bone marrow of glutathione-depleted (metabolically inhibited) rats exposed to [3H]- and [14C]formaldehyde at concentrations as high as 10 ppm; (4) the lack of detectable DPX in the bone marrow of Rhesus monkeys exposed to [14C]formaldehyde at concentrations as high as 6 ppm; (5) the failure of formaldehyde to induce leukemia in any of seven long-term inhalation bioassays in rats, mice, or hamsters; and (6) the failure of formaldehyde to induce chromosomal aberrations in the bone marrow of rats exposed to airborne concentrations as high as 15 ppm or of mice injected intraperitoneally with formaldehyde at doses as high as 25 mg/kg. Biological evidence that might be regarded as supporting the possibility of leukemia induction by formaldehyde includes: (1) the detection of cytogenetic abnormalities in circulating lymphocytes in seven studies of human subjects exposed to ambient concentrations in the workplace (but not in seven other studies of human subjects or in rats exposed to 15 ppm); (2) the induction of leukemia in rats in a single questionable drinking water study with formaldehyde concentrations as high as 1.5 g/L (but not in three other drinking water studies with concentrations as high as 1.9 or 5 g/L); (3) the detection of chromosomal aberrations in the bone marrow of rats exposed to very low concentrations of formaldehyde (0.4 or 1.2 ppm) (but not in another study at concentrations as high as 15 ppm); and (4) an apparent increase in the fraction of protein-associated DNA (assumed to be due to DPX) in circulating lymphocytes of humans exposed to ambient concentrations in the workplace (1-3 ppm). This evidence is regarded as inconsequential for several reasons, including lack of reproducibility, inadequate reporting of experimental methods, inconsistency with other data, or insufficient analytical sensitivity, and therefore, it provides little justification for or against the possibility that inhaled formaldehyde may be a leukemogen. In contrast to these inconclusive findings, the abundance of negative evidence mentioned above is undisputed and strongly suggests that there is no delivery of inhaled formaldehyde to distant sites. Combined with the fact that formaldehyde naturally occurs throughout the body, and that multiple inhalation bioassays have not induced leukemia in animals, the negative findings provide convincing evidence that formaldehyde is not leukemogenic.

A mathematical model for the absorption and metabolism of formaldehyde vapour by humans

Epidemiological studies of occupational exposure to formaldehyde gas (HCHO) have suggested possible links between concentration and duration of exposure, and elevated risks of leukaemia and other cancers at sites distant from the site of contact. Formaldehyde is a highly water soluble gas which, when inhaled, reacts rapidly at the site of contact and is quickly metabolised by enzymes in the respiratory tissue. Inhaled formaldehyde is almost entirely absorbed in the respiratory tract and, for formaldehyde induced toxicity to occur at distant sites, HCHO must enter the blood and be transported to systemic tissues via the circulatory system. A mathematical model describing the absorption and removal of inhaled formaldehyde in the nasal tissue is therefore formulated to predict the proportion of formaldehyde entering into the blood. Accounting for the spatial distribution of the formaldehyde concentration and the metabolic activity within the mucosa, the concentration of formaldehyde in the mucus, the epithelium and the blood has been determined and was found to attain a steady-state profile within a few seconds of exposure. The increase of the formaldehyde concentration in the blood was predicted to be insignificant compared with the existing pre-exposure levels in the body, indicating that formaldehyde is rapidly removed in the nasal tissue. The results of the model thus suggest that it is highly unlikely that following inhalation by the nose, formaldehyde itself will cause toxicity at sites other than the initial site of contact in the respiratory tract.

Mortality from solid cancers among workers in formaldehyde industries

In industrial workers, formaldehyde exposure has been associated with cancer of the nasal cavities, nasopharynx, prostate, lung, and pancreas; however, these associations are inconsistent and remain controversial. Animals exposed to formaldehyde show excesses of nasal cancer. In an extended follow-up of a large cohort of formaldehyde-exposed workers, the authors evaluated mortality from solid cancers (1,921 deaths) among 25,619 workers (865,708 person-years) employed in 10 US formaldehyde-producing or -using facilities through 1994. Exposure assessment included quantitative estimates of formaldehyde exposure. Standardized mortality ratios and relative risks were calculated. Compared with that for the US population, mortality from solid cancers was significantly lower than expected among subjects exposed and nonexposed to formaldehyde (standardized mortality ratios = 0.91 and 0.78, respectively). Relative risks for nasopharyngeal cancer (nine deaths) increased with average exposure intensity, cumulative exposure, highest peak exposure, and duration of exposure to formaldehyde (p-trend = 0.066, 0.025, <0.001, and 0.147, respectively). Formaldehyde exposure did not appear to be associated with lung (744 deaths), pancreas (93 deaths), or brain (62 deaths) cancer. Although relative risks for prostate cancer (145 deaths) were elevated for some measures of formaldehyde exposure, the trend was inconsistent. In this cohort of formaldehyde-industry workers, some evidence was found of an exposure-response relation with mortality from nasopharyngeal cancer (based on small numbers) but not for cancers of the pancreas, brain, lung, or prostate.

Potentiation of allergic bronchoconstriction by repeated exposure to formaldehyde in guinea-pigs in vivo

BACKGROUND

Indoor formaldehyde (FA) might worsen allergies and be an underlying factor for the increasing incidence and severity of asthma; the exact mechanism, however, remains unclear.

OBJECTIVE

The present study examined the effects of repeated exposure to FA on methacholine- and antigen-induced bronchoconstriction in guinea-pigs in vivo.

METHODS

First, non-sensitized guinea-pigs were transnasally treated with 0.1 or 1.0% FA or saline three times a week for 6 weeks, and increasing concentrations of methacholine (50, 100, and 200 microg/mL) were inhaled at 5-min intervals. Second, guinea-pigs pre-treated with transnasal administration of FA or saline using the same protocol were passively sensitized with anti-ovalbumin (OA) serum 7 days before antigen challenge. Third, guinea-pigs were actively sensitized with OA and pre-treated with transnasal administration of FA or saline using the same protocol. The lateral pressure of the tracheal tube (Pao) was measured under anesthesia and artificial ventilation.

RESULTS

The antigen-induced increase in Pao in actively sensitized guinea-pigs was significantly potentiated by FA exposure in a dose-dependent manner. The dose-response curve of the methacholine-induced increase in Pao in non-sensitized guinea-pigs or of the antigen-induced increase in Pao in passively sensitized guinea-pigs was not altered by FA exposure. Transnasal administration of FA significantly increased the serum anti-OA homocytotropic antibody titre (IgG) as measured by the passive cutaneous anaphylaxis reaction in actively sensitized guinea-pigs.

CONCLUSION

The results suggest that repeated exposure to FA worsens allergic bronchoconstriction through enhancing antigen sensitization.

Genotoxicity--threshold or not? Introduction of cases of industrial chemicals

Many industrially and environmentally important industrial carcinogens display effects that lead them to be viewed and regulated as 'genotoxic compounds'. Some of these chemicals cause experimental tumours only at high or toxic doses. The current view is that non-threshold principles should be applied for risk assessments and to define permissible exposure values. The toxicological impact of underlying mechanisms is frequently not well investigated and understood. The classification of carcinogens is now in a state of discussion. In Germany, the 'MAK-Commission' has issued new recommendations to distinguish between 5 groups of proven and suspected carcinogens. This proposal includes a category of 'substances with carcinogenic potential for which genotoxicity plays no or at most a minor role'. Another category comprises 'substances with carcinogenic and genotoxic potential, the potency of which is considered so low that, provided that the MAK-value is observed, no significant contribution to human cancer risk is to be expected'. There is also a number of apparently genotoxic carcinogens where the existence of 'practical thresholds' is at least debated. One outstanding example is vinyl acetate, which must be viewed against the background of discussions on other industrial high-volume chemicals like formaldehyde, acrylonitrile, acrylamide and trichloroethylene. Main arguments in favour or against thresholds of carcinogenicity of these individual compounds are summarised. Current instruments of regulation should be adjusted to allow adequate consideration of carcinogenic effects of chemicals that are practically relevant at high doses only. Also, research into this field is encouraged.

Neurotoxic effects of acute and subacute formaldehyde exposures in mice

In this study, the effects of acute and subacute formaldehyde (FA) exposures on spontaneous locomotor activity (SLMA), wet dog shake (WDS) behavior and pentylenetetrazole (PTZ) induced seizures were evaluated in Balb/C mice. SLMA was concentration dependently reduced after acute FA exposures at 1.8, 3.2, 4.5, 6.4, 9.7, and 14.8 ppm. The incidence of WDS behavior was increased only after acute FA exposures at 1.8, 3.2 and 6.4-ppm. PTZ-injections caused more intensive seizures in mice acutely exposed to FA only at 1.8 ppm. Meanwhile, the incidence of PTZ induced seizures was significantly lower after acute FA exposure at 14.8 ppm. SLMA was also reduced after subacute FA exposure at 2.0 ppm for 3 weeks. The inhibitory effects were significant after 1-week exposure at this concentration, but a tolerance developed at the end of the second week. As the concentration increased to 3.2 ppm, SLMA has found to be reduced after 2-week exposure. There was no change either on the incidence of WDS or on the parameters of PTZ-induced seizures, due to the subacute exposures of FA at the respective concentrations. In conclusion, based upon these data, acute and subacute exposures of FA produce a significant behavioral depression on mice. The data also suggest that acute FA exposures at low concentrations (such as 1.8 ppm) may increase the excitability of central nervous system (CNS).

A review of adverse pregnancy outcomes and formaldehyde exposure in human and animal studies

We examine the potential for reproductive and developmental effects from formaldehyde exposure. Formaldehyde is unlikely to reach the reproductive system in humans in concentrations sufficient to cause damage since it is rapidly metabolized and detoxified upon contact with the respiratory tract. While there are effects seen in in vitro studies or after injection, there is little evidence of reproductive or developmental toxicity in animal studies under exposure levels and routes relevant to humans. Most of the epidemiology studies examined spontaneous abortion and showed some evidence of increased risk (meta-relative risk=1.4, 95% CI 0.9-2.1). We found evidence of reporting biases and publication biases among the epidemiology studies and when these biases were taken into account, we found no evidence of increased risk of spontaneous abortion among workers exposed to formaldehyde (meta-relative risk=0.7, 95% CI 0.5-1.0). The small number of studies on birth defects, low birth weight, and infertility among formaldehyde workers; the limitations in the design of these studies; and the inconsistent findings across these studies make it difficult to draw conclusions from the epidemiology data alone. However, information from experimental studies and studies of metabolism indicate reproductive impacts are unlikely at formaldehyde exposures levels observed in the epidemiology studies.

Mode of action and tissue dosimetry in current and future risk assessments

Two fundamental concepts have emerged to organize contemporary approaches to chemical risk assessment - mode of action and tissue dosimetry. Mode of action specifies the nature of the interactions between the chemical and the body that lead to toxic responses and should, under optimal circumstances, also specify the form of the tissue dose that leads to these effects. This paper highlights recent development of biologically based dose response (BBDR) models for specific toxic endpoints that use knowledge on mode of action to specify measures of dose. These dose measures then are used to support low dose and interspecies extrapolations. We first focus on a series of dose response models developed for several compounds that produce nasal toxicity. These examples demonstrate a range of model structures from simple dosimetry models (methylmethacrylate) to linkage of dosimetry with specific biological processes involved in carcinogenesis (formaldehyde). Two BBDR models with dioxin illustrate the organization of biological and dosimetry information into specific testable hypotheses that could distinguish these different models and lead to a more uniform approach to risk assessment for this compound. A final section discusses the impact of molecular biology and the genomic revolution in relation to development of BBDR models for specific toxic endpoints.

Kinetics of propylene oxide metabolism in microsomes and cytosol of different organs from mouse, rat, and humans

Kinetics of the metabolic inactivation of 1,2-epoxypropane (propylene oxide; PO) catalyzed by glutathione S-transferase (GST) and by epoxide hydrolase (EH) were investigated at 37 degrees C in cytosol and microsomes of liver and lung of B6C3F1 mice, F344 rats, and humans and of respiratory and olfactory nasal mucosa of F344 rats. In all of these tissues, GST and EH activities were detected. GST activity for PO was found in cytosolic fractions exclusively. EH activity for PO could be determined only in microsomes, with the exception of human livers where some cytosolic activity also occurred, representing 1-3% of the corresponding GST activity. For GST, the ratio of the maximum metabolic rate (V(max)) to the apparent Michaelis constant (K(m)) could be quantified for all tissues. In liver and lung, these ratios ranged from 12 (human liver) to 106 microl/min/mg protein (mouse lung). Corresponding values for EH ranged from 4.4 (mouse liver) to 46 (human lung). The lowest V(max) value for EH was found in mouse lung (7.1 nmol/min/mg protein); the highest was found in human liver (80 nmol/min/mg protein). K(m) values for EH-mediated PO hydrolysis in liver and lung ranged from 0.83 (human lung) to 3.7 mmol/L (mouse liver). With respect to liver and lung, the highest V(max)/K(m) ratios were obtained for GST in mouse and for EH in human tissues. GST activities were higher in lung than in liver of mouse and human and were alike in both rat tissues. Species-specific EH activities in lung were similar to those in liver. In rat nasal mucosa, GST and EH activities were much higher than in rat liver.

Pharmacodynamics of formaldehyde: applications of a model for the arrest of DNA replication by DNA-protein cross-links

A variety of evidence suggests that formaldehyde (HCHO)-induced DNA-protein cross-links (DPX) are genotoxic as a result of their ability to arrest DNA replication. Although DPX can be removed and the DNA can be repaired, failure to remove the blockage prior to cell division or excision followed by incomplete repair could cause cell death or a mutation. To characterize the concentration and time dependence of this mechanism, a biologically based model for DNA replication in the presence of DPX was developed based on the assumptions that (1) DPX are formed randomly in the DNA and (2) a replication fork can advance up to but not past a DPX. Using a combination of Poisson and binomial statistics, a quantitative relationship between the amount of newly synthesized DNA and the concentration of DPX was derived, which predicts that the rate of DNA replication should decrease nonlinearly with increasing concentrations of DPX. Because the latter is a nonlinear function of the airborne concentration of HCHO, an inverse sigmoidal relationship is predicted between the rate of DNA replication and the concentration of inhaled formaldehyde. The model was parameterized using data derived from a study of the incorporation of [methyl-(14)C]thymidine monophosphate into the DNA of the nasal respiratory mucosa of Fischer-344 rats exposed to (3)HCHO and H(14)CHO (6 ppm, 6 h). The model was then applied to measurements of DNA replication in the nasal mucosa of experimental animals exposed to wide ranges of H(14)CHO (rats: 0.7, 2, 6, or 15 ppm, 3 h; rhesus monkeys: 0.7, 2, or 6 ppm, 6 h). The results indicate that, at airborne concentrations above 6 ppm in rats, there is a marked decrease (ca. 62% at 15 ppm) in the amount of newly synthesized DNA due to DPX formation during a single 6-h exposure to HCHO. The arrest of DNA replication at high HCHO concentrations could result in cytolethality or genotoxicity, both of which are critical factors in the induction of rat nasal cancer by HCHO. However, at concentrations below 2 ppm in monkeys or 1 ppm in rats, the decrease in the rate of DNA replication is predicted to be <1% after a 6-h exposure. This small decrease is probably undetectable using currently available techniques. The parameterized model suggests that the arrest of DNA replication by DPX is mainly a high-dose phenomenon and that at ambient exposure concentrations it is unlikely to be a major risk factor.

Autoradiographic imaging of formaldehyde adducts in mice: possible relevance for vascular damage in diabetes

The activity of semicarbazide-sensitive amine oxidase (SSAO) has been reported to be elevated in blood from diabetic patients. It has been suggested that the enzyme is involved in the development of complications such as retinopathies, nephropathies and neuropathies, which are associated with advanced diabetes, possibly by the formation of toxic metabolites. Under the influence of SSAO, methylamine is deaminated to formaldehyde which is known to react with various macromolecules. It has therefore been proposed that specific inhibition of SSAO could be of therapeutic value for treatment of diabetic patients. The present results provide evidence that treatment with an SSAO inhibitor potently reduces the levels of irreversible adducts. In this study, 14C-methylamine was given intraperitoneally to NMRI mice, and the tissue distribution of irreversibly bound methylamine metabolites was estimated by an autoradiographic method. Such radioactive residues occurred in high concentrations in the intestinal wall, brown adipose tissue, spleen and bone marrow. By inhibiting SSAO irreversibly with hydralazine before giving 14C-methylamine to the mice, it was possible to determine the resynthesis rate of SSAO in different tissues. A complete recovery of SSAO activity was seen in the intestinal wall after 6 days, whereas only about 60% was recovered in adipose tissue after 14 days. This suggests that factors controlling the synthesis of SSAO differ in these tissues, or that these tissues express different forms of enzymes.

DNA damage in the nasal passageway: a literature review

The purpose of this review is to provide a compilation of work examining DNA damage in the nasal cavity. There are numerous methods to identify and quantify damage to DNA and the diversity of methods and toxicologic endpoints is illustrated by the range of studies presented here. There are a large number of independent studies measuring endpoints in the upper respiratory tract; however, with regard to toxicant induced DNA damage in the nasal passageway, the effects of two compounds, 4-(N-Methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and formaldehyde (HCHO), appear to have been extensively characterized. The body of work on NNK and formaldehyde have provided insights into molecular mechanisms of DNA damage and repair and induced cell replication and its relationship to nasal cancer. With new technologies and molecular techniques, the sensitivity to enable evaluations of the minute quantities of nasal tissue available in test species and human biopsy impact the study of the nasal-toxicant interactions. As methods used to characterize DNA damage increase in sensitivity, the importance of both exogenous and endogenous sources of DNA damage, steady-state levels of cellular damage, repair, and resulting mutations, low-dose exposure assessments and inter-species extrapolation will become increasingly complex. Additional studies of DNA damage in the nasal passage will undoubtedly challenge future estimations of risk and impact what are perceived to be acceptable levels of exposure to known and predicted carcinogens. The aim of this paper is to provide to the interested scientist literature relevant to the effects of agents on nasal DNA, so that areas of insufficient information can be identified and used to further develop and expand the knowledge base for nasal DNA toxicant interactions.

Propylene oxide: mutagenesis, carcinogenesis and molecular dose

The results from mutagenic and carcinogenic studies of propylene oxide (PO) and the current efforts to develop molecular dosimetry methods for PO-DNA adducts are reviewed. PO has been shown to be active in several bacterial and mammalian mutagenicity tests and induces site of contact tumors in rodents after long-term administration. Quantitation of N7-(2-hydroxypropyl)guanine (7-HPG) in nasal and hepatic tissues of male F344 rats exposed to 500 ppm PO (6 h/day; 5 days/week for 4 weeks) by inhalation was performed to evaluate the potential of high concentrations of PO to produce adducts in the DNA of rodent tissues and to obtain information necessary for the design of molecular dosimetry studies. The persistence of 7-HPG in nasal and hepatic tissues was studied in rats killed three days after cessation of a 4-week exposure period. DNA samples from exposed and untreated animals were analyzed for 7-HPG by two different methods. The first method consisted of separation of the adduct from DNA by neutral thermal hydrolysis, followed by electrophoretic derivatization of the adduct and gas chromatography-high resolution mass spectrometry (GC-HRMS) analysis. The second method utilized 32P-postlabeling to quantitate the amount of this adduct in rat tissues. Adducts present in tissues from rats killed immediately after cessation of exposure were 835.4 +/- 80.1 (respiratory), 396.8 +/- 53.1 (olfactory) and 34.6 +/- 3.0 (liver) pmol adduct/mumol guanine using GC-HRMS. Lower values, 592.7 +/- 53.3, 296.5 +/- 32.6 and 23.2 +/- 0.6 pmol adduct/mumol guanine were found in respiratory, olfactory and hepatic tissues of rats killed after three days of recovery. Analysis of the tissues by 32P-postlabeling yielded the following values: 445.7 +/- 8.0 (respiratory), 301.6 +/- 49.2 (olfactory) and 20.6 +/- 1.8 (liver) pmol adduct/mumol guanine in DNA of rats killed immediately after exposure cessation and 327.1 +/- 21.7 (respiratory), 185.3 +/- 29.2 (olfactory) and 15.7 +/- 0.9 (liver) pmol adduct/mumol guanine after recovery. Current methods of quantitation did not provide evidence for the endogenous formation of this adduct in control animals. These studies demonstrated that the target tissue for carcinogenesis has much greater alkylation of DNA than liver, a tissue that did not exhibit a carcinogenic response.

A brief review of formaldehyde carcinogenesis in relation to rat nasal pathology and human health risk assessment

The 1980 report that inhaled formaldehyde induced nasal squamous cell carcinomas in rats had a significant societal impact and resulted in extensive research in the fields of rodent nasal pathology and human cancer risk assessment. This article presents an overview of the evolution of these events. It is concluded that the nasal passages of humans and rats are fundamentally identical biological target organs. Nevertheless, in the case of human health risk assessment, minor differences between these species may be critically important. Special attention should be paid to interspecies differences in nasal dosimetry and local metabolism; thus, chemical toxicity data derived from rats require careful interpretation when used for human risk assessments. In the case of formaldehyde, it is recommended that low-concentration (< or = 2 ppm airborne exposure) extrapolation, where no tissue damage is observed, be uncoupled from the responses at high concentrations (> or = 6 ppm), where epithelial degeneration, regenerative cell replication, and inflammation appear to be essential driving forces in formaldehyde carcinogenesis. The presence of treatment-related nasal lesions in rats following exposure to chemicals should always be treated as an indication of a potential human health risk, whether exposure is by the inhalation, oral, or dermal route.

DNA adducts: biological markers of exposure and potential applications to risk assessment

DNA adducts have been investigated extensively during the past decade. This research has been advanced, in part, by the development of ultrasensitive analytical methods, such as 32P-postlabeling and mass spectrometry, that enable detection of DNA adducts at concentrations as low as one adduct per 10(9) to 10(10) normal nucleotides. Studies of mutations in activated oncogenes such as ras, inactivated tumor suppressor genes such as p53, and surrogate genes such as hprt provide linkage between DNA adducts and carcinogenesis. The measurement of DNA adducts, or molecular dosimetry, has important applications for cancer risk assessment. Cancer risk assessment currently involves estimating the probable effects of carcinogens in humans based on results of animal bioassays. Estimates of risk are then derived from mathematical models that fit data of tumor incidence at the high animal exposures and extrapolate to probable human exposures that may be orders of magnitude lower. Molecular dosimetry could extend the observable range of mechanistic data several orders of magnitude lower than can be achieved in carcinogenesis bioassays. This measurement also compensates automatically for individual and species differences in toxicokinetic factors, as well as any nonlinearities that affect the quantitative relationships between exposure and molecular dose. As a result, molecular dosimetry can provide a basis for conducting high- to low-dose, route-to-route, and interspecies extrapolations. The incorporation of such data into risk assessment promises to reduce uncertainties and produce more accurate estimates of risk compared to current methods.

Formaldehyde mechanistic data and risk assessment: endogenous protection from DNA adduct formation

Exposures of rodents to airborne formaldehyde (FA) produce dose-related toxicity, enhanced cell proliferation and squamous cell carcinomas in the nasal passages. The mechanism of FA-induced tumor formation involves DNA-protein crosslink formation and enhanced cell proliferation secondarily to cytotoxicity. The mucociliary apparatus and glutathione protect against low-dose FA-induced effects. Consequently, the mechanistic information is consistent with a very sublinear dose-response curve for tumor formation. The sublinear dose-response of nasal DNA-protein crosslinks levels in rodents and monkeys has been used in the risk assessment of FA.

Subacute immunotoxicity study of formaldehyde in male rats

Immune functions were examined in male rats following 28 day oral administration of formaldehyde by gastric tube at dose levels of 0, 20, 40, and 80 mg/kg. Routine parameters examined included hematology, clinical chemistry, and body, thymus, kidney, and liver weights. In addition, cellularity of spleen and lymph nodes, histology of spleen, thymus, lymph nodes, liver kidney, small and large intestines, and histochemistry of spleen and lymph nodes were evaluated. Immune parameters evaluated included serum hemagglutinin antibody response; antibody plaque forming cell response to sheep erythrocytes (lymphocyte-dependent antigen); microbicidal activity of Candida albicans; and phagocytic activity by adhesion of microspheric hydrophilic synthetic particles to leukocyte cell membrane. Body weights were slightly decreased at high dose level (80 mg/kg). The difference was significant when compared to the controls. The lymph node weights were significantly increased in rats receiving formaldehyde. The cellularity of lymphoid organs was not influenced after 28 day exposure to formaldehyde.

Cytogenetic analysis of pulmonary lavage and bone marrow cells of rats after repeated formaldehyde inhalation

Cytogenetic analyses were conducted on bone marrow and pulmonary lavage cells from rats that received repeated inhalation exposures to formaldehyde. Male Sprague-Dawley rats were exposed to 0, 0.5, 3, or 15 ppm formaldehyde for 6 h per day, 5 days per week, for 1 and 8 weeks. There was no significant increase in chromosomal abnormalities in the bone marrow cells of formaldehyde-exposed rats relative to controls. There was a statistically significant increase in chromosomal aberrations in the pulmonary lavage cells from rats that inhaled 15 ppm. There were 7.6 and 9.2% of the scored pulmonary lavage cells that had aberrations following 1 and 8 weeks, respectively, of 15 ppm formaldehyde exposure (with control levels of 3.5 and 4.8%, respectively). The predominant damage seen was chromatid breaks. These findings indicate that marginal but statistically significant genotoxic effects could be detected locally in lung alveolar macrophages, but not distally in bone marrow, following repeated formaldehyde exposures only at a high concentration that is carcinogenic to rats. The biological significance of this effect is uncertain since formaldehyde is not considered to be a lung carcinogen in rats.

Formaldehyde in drinking water: comparative hazard evaluation and an approach to regulation

Formaldehyde, a widely used industrial chemical to which humans are ubiquitously exposed, presented cause for concern when it was demonstrated to be carcinogenic in laboratory animals. Risk assessment protocols subsequently applied to formaldehyde are of questionable validity in light of the results of recent mechanistic investigations of biological responses to formaldehyde. Further, the hazard of ingested formaldehyde is not addressed in current assessment protocols. This paper addresses the potential human health risks accompanying low-level exposure to formaldehyde as a contaminant in drinking water. In this exposure scenario, noncarcinogenic risk from inhalation of formaldehyde from drinking water is evaluated through knowledge of the metabolism and biological effects of formaldehyde exposure. Noncarcinogenic risk from ingestion of formaldehyde in drinking water is evaluated from the perspective gained by comparison with dietary sources of formaldehyde. Carcinogenic risk to humans is evaluated in light of recent investigations into the mechanisms underlying biological responses to formaldehyde exposure. Finally, based on a comparison of ingestion of formaldehyde in drinking water with ingestion of naturally occurring formaldehyde in foods, a comparative hazard approach to formaldehyde regulation is offered as a supplement to the rigid evaluation protocols currently used.

Inhibition and recovery of mammalian respiratory ciliary function after formaldehyde exposure

Formaldehyde (HCHO) has been reported to impair mucociliary clearance. The present investigation using rabbit and porcine tracheal explants in vitro examined (1) the impairment of ciliary activity, an essential component of mucociliary clearance; (2) the reversibility of ciliary dysfunction after HCHO exposure; and (3) the mechanism by which ciliary activity is reduced by HCHO. HCHO treatment of rabbit tracheal rings significantly decreased zones of active ciliated epithelium in direct proportion to concentration and exposure duration. There was also a significant concentration-dependent reduction of ciliary beat frequency. Removal of HCHO permitted recovery of zones of ciliary activity to normal beat frequencies; greater inhibitory concentrations of HCHO required greater time for return of function. Treatment of porcine tracheae with increasing concentrations of HCHO for time periods inhibitory to rabbit ciliary activity correspondingly reduced the yield of cilia extractable from treated epithelium. Furthermore, the specific activity of ATPase of extracted ciliary axonemes was diminished with increasing HCHO concentration, indicating loss of function. A recovery period following identical exposures of the porcine tracheae to the lower HCHO concentrations resulted in normal yields of functionally intact ciliary axonemes. Similarly, a recovery period after the highest HCHO concentration produced more functional axonemes than obtained from exposed tracheae without a recovery period, although less than normal yields. Therefore, ciliary dysfunction elicited by a defined range of HCHO concentrations is reversible. The yield and functional integrity of ciliary axonemes from epithelium exposed to HCHO with a recovery period are significantly greater than those without such a recovery period, suggesting an alteration and subsequent repair of epithelial surface components following HCHO exposure.