Teratogenicity of acrylonitrile given to rats by gavage or by inhalation

Acrylonitrile induction of rodent neoplasia: Potential mechanism of action and relevance to humans

Acrylonitrile, an industrial chemical, is a multisite carcinogen in rats and mice, producing tumors in four tissues with barrier function, that is, brain, forestomach, Zymbal’s gland, and Harderian gland. To assess mechanism(s) of action (MoA) for induction of neoplasia and to evaluate whether the findings in rodents are indicative of human hazard, data on the potential key effects produced by acrylonitrile in the four rodent target tissues of carcinogenicity were evaluated. A notable finding was depletion of glutathione in various organs, including two target tissues, the brain, and forestomach, suggesting that this effect could be a critical initiating event. An additional combination of oxidative DNA damage and cytotoxic effects of acrylonitrile and its metabolites, cyanide, and 2-cyanoethylene oxide, could initiate pro-inflammatory signaling and sustained cell and tissue injury, leading to compensatory cell proliferation and neoplastic development. The in vivo DNA-binding and genotoxicity of acrylonitrile has been studied in several target tissues with no compelling positive results. Thus, while some mutagenic effects were reported in acrylonitrile-exposed rodents, data to determine whether this mutagenicity stems from direct DNA reactivity of acrylonitrile are insufficient. Accordingly, the induction of tumors in rodents is consistent primarily with a non-genotoxic MoA, although a contribution from weak mutagenicity cannot be ruled out. Mechanistic data to support conclusions regarding human hazard from acrylonitrile exposure is weak. Comparison of metabolism of acrylonitrile between rodents and humans provide little support for human hazard. Three of the tissues affected in bioassays (forestomach, Zymbal’s gland, and Harderian gland) are present only in rodents, while the brain is anatomically different between rodents and humans, diminishing relevance of tumor induction in these tissues to human hazard. Extensive epidemiological data has not revealed causation of human cancer by acrylonitrile.

Neoplasms in Rats Ingesting Acrylonitrile for Two Years

Male Sprague-Dawley rats ingested 0, 20 ppm, or 500 ppm of acrylonitrile in drinking water for 2 years. Rats receiving 500 ppm of acrylonitrile exhibited early mortality and retarded weight gain. Tumors of Zymbal's gland were associated in dose-response fashion with acrylonitrile exposure. Age-associated incidence of pituitary adenomas containing immunoreactive prolactin was decreased in acrylonitrile-treated rats. A decrease in pituitary tumor incidence also was observed in rats treated with low doses of acrylonitrile, suggesting that reduction in this tumor frequency was not because of early death. No increases were found in tumors of other organ systems, but a trend toward development of forestomach papillomas was noted in rats receiving the highest concentration of acrylonitrile.

Weight-of-the-evidence review of acrylonitrile reproductive and developmental toxicity studies

Risk assessment of acrylonitrile (AN) toxicity to humans has focused on potential carcinogenicity and acute toxicity. Epidemiological studies from China reported reproductive and developmental effects in AN workers, including infertility, birth defects, and spontaneous abortions. A weight-of-the-evidence (WoE) evaluation of the AN database assessed study strength, characterized toxicity, and identified no-observed-adverse-effect levels (NOAELs). The epidemiological studies do not demonstrate causality and are not sufficiently robust to be used for risk assessment. Rodent developmental studies showed fetotoxicity and malformations at maternally toxic levels; there was no unique developmental susceptibility. NOAELs for oral and inhalation exposures were 10 mg/kg/day and 12 ppm (6 h/day), respectively. Drinking-water and inhalation reproductive toxicity studies showed no clear effects on reproductive performance or fertility. Maternally toxic concentrations caused decreased pup growth. The drinking-water reproductive NOAEL was 100 ppm (moderate confidence due to study limitations). The inhalation exposure reproductive and neonatal toxicity high confidence NOAEL was 45 ppm (first generation 90 ppm) (6 h/day). The inhalation reproductive toxicity study provides the most robust data for risk assessment. Based on the WoE evaluation, AN is not expected to be a developmental or reproductive toxicant in the absence of significant maternal toxicity.

Derivation of noncancer reference values for acrylonitrile

Dose-response assessments were conducted for the noncancer effects of acrylonitrile (AN) for the purposes of deriving subchronic and chronic oral reference dose (RfD) and inhalation reference concentration (RfC) values. Based upon an evaluation of available toxicity data, the irritation and neurological effects of AN were determined to be appropriate bases for deriving reference values. A PBPK model, which describes the toxicokinetics of AN and its metabolite 2-cyanoethylene oxide (CEO) in both rats and humans, was used to assess the dose-response data in terms of an internal dose measure for the oral RfD values, but could not be used in deriving the inhalation RfC values. Benchmark dose (BMD) methods were used to derive all reference values. Where sufficient information was available, data-derived uncertainty factors were applied to the points of departure determined by BMD methods. From this assessment, subchronic and chronic oral RfD values of 0.5 and 0.05 mg/kg/day, respectively, were derived. Similarly, subchronic and chronic inhalation RfC values of 0.1 and 0.06 mg/m(3), respectively, were derived. Confidence in the reference values derived for AN was considered to be medium to high, based upon a consideration of the confidence in the key studies, the toxicity database, dosimetry, and dose-response modeling.

Two-generation reproductive toxicity study of inhaled acrylonitrile vapors in Crl:CD(SD) rats

To assess the effects of acrylonitrile (AN) exposure on reproduction, Sprague-Dawley rats (25/sex/group) were exposed to vapor atmospheres of AN via whole-body inhalation at concentrations of 0, 5, 15, 45 (two offspring generations) and 90 ppm (one offspring generation), 6 h daily, 1 litter/generation, through F2 weanlings on postnatal day 28. After approximately 3 weeks of direct exposure following weaning, exposure of the F1 animals at 90 ppm was terminated due to excessive systemic toxicity in the males. There were no exposure-related mortalities in adult animals, no functional effects on reproduction or effects on reproductive organs, and no evidence of cumulative toxicity or of enhanced toxicity in pregnant and lactating dams or in developing animals. Adult systemic toxicity was limited to body weight and/or food consumption deficits in both sexes and generations (greater in males) at 45 and 90 ppm and increased liver weights in the 90 ppm F0 males and females and 45 ppm F1 males. Neonatal toxicity was expressed by F1 offspring weight decrements at 90 ppm. Clinical signs of local irritation during and immediately following exposure were observed at 90 ppm. Microscopic lesions of the rostral nasal epithelium, representing local site-of-contact irritation, were observed in some animals at 5 to 45 ppm. The no-observed-adverse-effect level (NOAEL) for reproductive toxicity over two generations and neonatal toxicity of AN administered to rats via whole-body inhalation was 45 ppm. The NOAEL for reproduction was 90 ppm for the first generation. The NOAEL for parental systemic toxicity was 15 ppm.

EVALUATION OF THE POTENTIAL DEVELOPMENTAL TOXICITY OF 3-AMINOPENTANENITRILE (3-APN) IN THE RAT

The potential developmental toxicity of 3-aminopentanenitrile (3-APN) was assessed in rats. Groups of 25 time-mated female Crl:CD(SD)IGS BR rats were orally gavaged at daily dose levels of 0, 5, 30, 100 or 300 mg/kg over days 6-20 of gestation (days 6-20G); the day of copulation plug detection was designated day 0G. The dams were euthanized on day 21G and their abdominal and thoracic viscera were examined grossly. The fetuses were weighed, sexed, and examined for external, visceral, and skeletal alterations. Evidence of maternal and developmental toxicity was seen at 100 and 300 mg/kg. Regarding maternal toxicity, there were compound-related, statistically significant reductions in maternal body weight and food consumption at 100 and 300 mg/kg. The incidence of alopecia was significantly increased at these levels as well. Regarding developmental toxicity, mean fetal weight was slightly but significantly reduced at 100 and 300 mg/kg. In addition, at 300 mg/kg, there were significant increases in several skeletal variations (wavy ribs and skull, rib, and vertebral ossification delays) consistent with developmental delay. There was no evidence of either maternal or developmental toxicity at 5 or 30 mg/kg. Thus, the maternal and developmental no-observed-effect level (NOEL) was 30 mg/kg. Because developmental toxicity was observed only after administration of doses that also produced signs of maternal toxicity, 3-APN is not considered to be a selective developmental toxicant in the rat.

Comparative developmental toxicities of aliphatic nitriles: in vivo and in vitro observations

The effects on embryonic development of a series of eight saturated (acetonitrile, propionitrile, and n-butyronitrile) and unsaturated (acrylonitrile, methacrylonitrile, allylnitrile, cis-2-pentenenitrile, and 2-chloroacrylonitrile) nitriles were compared in vitro using the whole embryo culture system. Day 10 rat embryos were cultured for 46 h in rat serum in the presence of either of these chemicals. All the tested chemicals produced concentration-dependent decreases in growth and differentiation and increases in the incidences of morphologically abnormal embryos. A wide range of embryotoxic potency was observed, with 2-chloroacrylonitrile and acetonitrile at the extremes (lowest effect levels of 50 microM and 40 mM, respectively). No common pattern could be drawn for all the eight nitriles tested in vitro, although there were some similarities between the malformations elicited by propionitrile and n-butyronitrile or between those elicited by the five unsaturated nitriles. Presence of a rat hepatic microsomal fraction and NADPH in the culture medium enhanced the embryotoxic effects of the five unsaturated nitriles tested but had no effects on saturated nitriles embryotoxicity. In addition to these in vitro experiments, pregnant rats were given a single oral dose of each compound on Day 10 of gestation and the embryos were evaluated on Day 12 of gestation, i.e., at a time of development corresponding to the developmental stage at the end of the whole embryo culture. All the nitriles investigated produced the characteristic defects developed by embryos exposed to sodium cyanide in utero or in culture. Our results provide further evidence that maternal production of cyanide may contribute to the developmental toxicity of saturated and unsaturated nitriles and suggest that distinct metabolites derived from microsomal metabolism of unsaturated nitriles may also play a role.

Sodium thiosulfate protects against acrylonitrile-induced elevation of glial fibrillary acidic protein levels by replenishing glutathione

Acrylonitrile (ACN) like many organic solvents produce neurotoxicity by elevating brain glial fibrillary acidic protein (GFAP), a putative biomarker of astrogliosis. In this study we tested the hypothesis that sodium thiosulfate (STS) protective action against ACN-induced astrogliosis is glutathione (GSH) mediated. Male Sprague-Dawley rats were administered for 2 weeks intraperitoneal doses (50 mg/kg body weight) of the ACN, with or without STS as outline in the Methods section. Specific brain regions were tested for GFAP, GSH and glutathione-S-transferase (GST) enzyme activity. In the brain regions tested STS significantly (P</=0.05) maintained GFAP levels at the basal saline control levels, when compared to ACN-treated groups. STS also significantly (P</=0.05) increased GSH levels in these brain regions with a corresponding increased in GST enzyme activity. Although the data indicated that STS antidotal action against ACN-induced neurotoxicity is likely to involve GSH and GST activity, other complex series of mechanisms may be involved.

Mutagenicity, carcinogenicity, and teratogenicity of acrylonitrile

Acrylonitrile (AN) is an important intermediary for the synthesis of a variety of organic products, such as artificial fibres, household articles and resins. Although acute effects are the primary concern for an exposure to AN, potential genotoxic, carcinogenic and teratogenic risks of AN have to be taken seriously in view of the large number of workers employed in such industries and the world-wide population using products containing and possibly liberating AN. An understanding of the effect of acrylonitrile must be based on a characterization of its metabolism as well as of the resulting products and their genotoxic properties. Tests for mutagenicity in bacteria have in general been positive, those in plants and on unscheduled DNA synthesis doubtful, and those on chromosome aberrations in vivo negative. Wherever positive results had been obtained, metabolic activation of AN appeared to be a prerequisite. The extent to which such mutagenic effects are significant in man depends, however, also on the conditions of exposure. It appears from the limited data that the ultimate mutagenic factor(s), such as 2-cyanoethylene oxide, may have little opportunity to act under conditions where people are exposed because it is formed only in small amounts and is rapidly degraded. The carcinogenic action of AN has been evaluated by various agencies and ranged from 'reasonably be anticipated to be a human carcinogen' to 'cannot be excluded', the most recent evaluation being 'possibly carcinogenic to humans'. Animal data that confirm the carcinogenic potential of AN have certain limitations with respect to the choice of species, type of tumors and length of follow up. Epidemiological studies which sometimes, but not always, yielded positive results, encounter the usual difficulties of confounding factors in chemical industries. Exposure of workers to AN should continue to be carefully monitored, but AN would not have to be considered a cancer risk to the population provided limitations on releases from consumer products and guidelines on AN in water and air are enforced. AN is teratogenic in laboratory animals (rat, hamster) at high doses when foetal/embryonic (and maternal) toxicity already is manifest. Pregnant workers should not be exposed to AN. In view of the small concentrations generally encountered outside plants, women not professionally exposed would appear not to be at risk of teratogenic effects due to AN. Future research should concentrate on the elucidation of the different degradation pathways in man and on epidemiological studies in workers including pregnant women, assessing also, if possible, individual exposure by bio-monitoring.

Health effects classification and its role in the derivation of minimal risk levels: developmental effects

Agency for Toxic Substances and Disease Registry (ATSDR) utilizes chemical-specific minimal risk levels (MRLs) to assist in evaluating the public health risk associated with exposure to hazardous substances. The MRLs are derived based on the health effects data compiled from current literature searches and presented in ATSDR's toxicological profiles. Health effects are categorized according to their degree of severity (e.g., serious, less serious, minimal, and not adverse). This evaluation is important, because each respective category can be assigned a different amount of uncertainty, thus affecting the final value of the calculated MRL. From the total of 272 MRLs derived as of December 1997, 21 were based on developmental effects. ATSDR's ranking of developmental health effects as described in the Guidance for Developing Toxicological Profiles and specific examples of how the categorized health effects were used in MRL derivations are provided in this paper.

Correlation of structural class with no-observed-effect levels: a proposal for establishing a threshold of concern

The relationship between chemical structure and toxicity was explored through the compilation of a large reference database consisting of over 600 chemical substances tested for a variety of endpoints resulting in over 2900 no-observed-effect levels (NOELs). Each substance in the database was classified into one of three structural classes using a decision tree approach. The resulting cumulative distributions of NOELs for each of the structural classes differed significantly from one another, supporting the contention that chemical structure defines toxicity. The database was used to derive a threshold of acceptable human exposure for each of the structural classes that could be applied in the absence of specific toxicity data on a substance within one of the three structural classes. The human exposure thresholds provide guidance on the degree of testing and evaluation required for substances that lack toxicity data.

Modulation of acrylonitrile-induced embryotoxicity in vitro by glutathione depletion

The effects of glutathione (GSH) depletion on the embryotoxicity of acrylonitrile were assessed in vitro using the rat whole-embryo culture system. Day 10 rat embryos were cultured in rat serum medium for 6 h in the presence of 250 microM L-buthionine-S,R-sulfoximine (BSO), a specific inhibitor of GSH synthesis, to deplete GSH in both embryo and visceral yolk sac. Following pretreatment, conceptuses were cultured for an additional 21 h in the presence of 152, 228, or 304 microM acrylonitrile. At the end of the culture period, conceptuses were assessed for survival, growth and development, malformations, and the protein and glutathione content of embryos and yolk sacs were assayed. Acrylonitrile alone produced concentration-related and statistically significant decreases in yolk sac diameter, crown-rump length, head length and number of somite pairs, as well as in embryonic and yolk sac proteins. The chemical also caused dysmorphogenesis of the brain and of the caudal extremity, and a concentration-related and statistically significant increase in GSH content in the yolk sac. Pretreatment with BSO significantly enhanced the embryotoxic effects of acrylonitrile. The conceptuses displayed further decreases in functional yolk sac circulation, yolk sac diameter, crown-rump and head length, when compared to either acrylonitrile or BSO alone. The incidence of caudal malformations and the severity of brain malformations produced by acrylonitrile were also increased. Marked decreases in embryonic and yolk sac GSH contents were observed after exposure to BSO alone or in combination with acrylonitrile.(ABSTRACT TRUNCATED AT 250 WORDS)

Acrylonitrile interaction with testicular DNA in rats

In the present study we report the in vivo interaction of acrylonitrile (VCN) with testicular tissue in rats. Covalent binding of radioactivity to testicular tissue DNA was examined for a period of 72 hr after a single oral dose (46.5 mg/kg) of [2,3-14C] VCN. Maximal covalent binding was observed at 0.5 hr (8.9 mumol VCN equivalent/mol nucleotide). Binding decreased gradually thereafter but was still detected (2.5 mumol VCN equivalent/mol nucleotide) at 72 hr following VCN administration. Further, we examined the effects of VCN on DNA synthesis and repair in the testes of rats following a single oral dose (46.5 mg/kg) of VCN to clarify the impact of the covalent binding observed on the testicular genetic material. A significant decrease in DNA synthesis (80% of control) was observed at 0.5 hr after treatment. At 24 hr following acrylonitrile administration, testicular DNA synthesis was severely inhibited (38% of control). Testicular DNA repair was increased 1.5-fold at 0.5 hr and more than 3.3-fold at 24 hr following treatment with VCN. These results suggest that VCN can act as a multipotent genotoxic agent by alkylating DNA in testicular tissue and may affect the male reproductive function by interfering with testicular DNA synthesis and repair processes.

Effect of glutathione depletion on the irreversible association of acrylonitrile with tissue macromolecules after oral administration to rats

Binding of acrylonitrile and its reactive metabolites to tissue macromolecules, especially nucleic acids, may be responsible for its carcinogenicity in rats. Both acrylonitrile and its primary metabolite, 2-cyanoethylene oxide, also react with glutathione. To better understand the role of glutathione in the manifestation of acrylonitrile toxicity, the irreversible binding to tissue macromolecules was assessed in control and glutathione-depleted F-344 rats treated with a 4 mg/kg dose (po) of [2,3-14C]acrylonitrile. Glutathione was depleted in rat tissues by the administration of a combined intraperitoneal phorone/buthionine sulfoximine treatment (300 mg/kg and 2 mmol/kg, respectively) given 30 min prior to acrylonitrile administration. The amount of total radioactivity recovered from brain, stomach (target organs), liver, kidney, lung, and blood (nontarget organs) was similar between control and glutathione-depleted rats. However, stomach, lung, blood, and liver showed an increase in total radioactivity content after glutathione depletion by phorone/buthionine sulfoximine treatment. Glutathione depletion also caused an increase in acrylonitrile-derived non-dialysable radioactivity (MW greater than 3500 Da) in liver, lung, kidney, stomach, blood, and brain macromolecules between 6 and 24 hr after the dose. There was no organ-specific accumulation of radiolabel in RNA in control rats. However, an increase in the radiolabel associated with nucleic acids in the target organs but not in the nontarget organs was measured in glutathione-depleted rats. Urinary excretion of thiocyanate, a metabolite derived from the epoxide pathway, was also increased by 300% in glutathione-depleted rats. These results suggest that glutathione might play a role in the extent of 2-cyanoethylene oxide formation and in the distribution of the radiolabel among tissues.

Effect of glutathione depletion on the uptake of acrylonitrile vapors and on its irreversible association with tissue macromolecules

Cellular GSH may influence the metabolism of the rodent brain and forestomach carcinogen acrylonitrile (ACN) and its subsequent binding to tissue macromolecules. To investigate the role of GSH in ACN metabolism and binding to macromolecules, we studied the effect of GSH depletion on the irreversible association of radiolabel with tissue macromolecules in male F-344 rats given a 4 mg/kg dose of [2,3-14C]ACN by inhalation. A combined phorone/buthionine sulfoximine treatment (300 mg/kg and 2 mmol/kg, respectively) was given 30 minutes prior to ACN exposure to deplete GSH. The uptake of ACN vapor by control rats was biphasic and characterized by a rapid phase lasting about 60 min and by a slower phase from 60 min to the end of exposure. The rate of uptake for both phases was linearly related to the initial concentration of ACN in the chamber. GSH depletion caused an increase in the rate of ACN uptake in both phases. It also caused a decrease in total radioactivity recovered in brain, stomach, liver, kidney, and blood and a concomitant decrease in the ACN-derived nondialyzable radioactivity in these organs. In control rats, accumulation of radiolabel was greatest in brain RNA, but no radioactivity was detected in DNA of any organ examined. In GSH-depleted rats, the radiolabel concentration was higher in brain RNA than in the liver or stomach RNA, but was also 50% lower than that observed in brain RNA of control rats. Urinary excretion of thiocyanate (SCN-), a metabolite derived from the epoxide pathway of ACN metabolism, was doubled in GSH-depleted rats. These results suggest that GSH might be involved in the distribution of ACN-derived reactive species and, therefore, might play a role in the binding of ACN-derived species to tissue macromolecules and nucleic acids.

Teratogenic effects of trichloroacetonitrile in the Long-Evans rat

Trichloroacetonitrile (TCAN) is among a number of contaminants found in drinking water produced by reactions of chlorine with background organic material. Long-Evans rats were intubated with TCAN (0, 1, 7.5, 15, 35, 55 mg/kg) in a tricaprylin vehicle on gestation days 6-18. The highest dose tested (55 mg/kg) was lethal in 21% of the dams and produced 100% resorptions in two-thirds of the survivors. Only one maternal death was seen at the next-lower dose; however, fetal weight and viability were decreased in a dose-related manner. The percentage of embryolethality was 13.9% at the lowest dose and 78.4% at the high dose, with resorption of entire litters seen at 7.5 mg/kg and above. At all doses, cardiovascular (interventricular septal defect, levocardia, common carotid, and right-sided aortic arch and ductus arteriosus) and urogenital (hypoplastic, missing, misplaced and fused kidneys, and hypoplastic uterine horns) malformations were seen in the offspring. Frequency of these malformations was dose related, ranging from 8% to 35% at the 1.0- and 35-mg/kg doses, respectively. The incidence of total soft tissue malformations was statistically significant at 15 and 35 mg/kg. There were no significant treatment-related changes in the incidence of skeletal malformations. The no-effect dose was established by statistical analysis to be 1.0 mg/kg/day.

Testicular effects of acrylonitrile in mice

Daily oral administration of acrylonitrile (10 mg/kg body weight) to mice for a period of 60 days caused a significant decrease in the activity of testicular sorbitol dehydrogenase and acid phosphatase, and an increase in that of lactate dehydrogenase and beta-glucuronidase. Histopathological studies revealed degeneration of the seminiferous tubules. A decrease in the sperm counts of the epididymal spermatozoa was also observed in the animals of the acrylonitrile-exposed group. These observations suggest that acrylonitrile may affect the male reproductive function by causing testicular injury.

Primary brain tumours in Fischer 344 rats chronically exposed to acrylonitrile in their drinking-water

Acrylonitrile (ACN) has been tested for carcinogenicity by various routes in a number of rat strains. At relatively high levels of administration (e.g. 500 ppm in the drinking-water) there were statistically significant increases in microscopically detectable primary brain tumours, which were difficult to classify. In a further study of ACN-induced brain tumours, ACN was administered to groups of 50 male and 50 female F-344 rats from 6 wk of age at levels of 0, 100 and 500 ppm in the drinking-water. A fourth group of 300 rats (147 males, 153 females), was also given 500 ppm ACN. Neurological signs were observed in 0, 4, 16 and 29, respectively, of the rats in these four groups within 12-18 months. Among the treated animals, females died slightly earlier than males. Few controls of either sex had died by month 18, but, apart from those killed for tumour donation, a high proportion of the rats in the 500-ppm groups had died by that time. Of the 49 brain tumours found in rats exposed to 500 ppm ACN, 11 were only detectable microscopically, 28 were 1-5 mm in diameter and 10 were greater than 5 mm. Despite this variation in size, all the tumours were similar in cellular and architectural features. They were densely cellular, with occasional areas of focal necrosis, and were infiltrative at the margins. They were negative for glial fibrillary acidic protein (GFAP). Ultrastructurally, the tumour cells showed intermingling cytoplasmic processes but no glial filaments and no neurosecretory granules or specialized cell contacts. Samples of tumour tissue were successfully grown in culture, but transplantation of samples from these cultures (observed for up to 12 wk) was unsuccessful. However, a direct intracerebral transplantation from a large tumour was successful.

Maternal toxicity: a possible etiological factor in embryo-fetal deaths and fetal malformations of rodent-rabbit species

Data from animal teratology studies were surveyed to determine whether embryo-fetal mortality and fetal malformations result from a primary action of the agent on the conceptus or if they are secondary to maternal toxicity--a consequence of administration with high dose levels of test chemicals. A fairly strong association between embryo-fetal mortality and maternal toxicity was revealed by analysis of data from hamsters, mice, rats, and rabbits in 234 studies of chemical and physical agents, of which 83 were conducted at both maternotoxic and nonmaternotoxic doses, 94 only at maternotoxic doses, and 49 at nonmaternotoxic doses. In the above studies, only nine chemicals (four each in hamsters and rabbits and one in rats) were reported to induce embryo-fetal deaths at apparently nonmaternotoxic doses. These findings tend to suggest a contributory role for maternal toxicity in the induction of embryo-fetal deaths. The previously reported hypothesis that certain fetal defects in mice may perhaps be caused by maternal toxicity was also found to be true in a review of data on hamsters, rats, and rabbits. Salient maternal toxicity-associated fetal malformations were exencephaly, encephalocele, micro- or anophalmia, and fused ribs in hamsters and defective (fused, missing, or extra) ribs, vertebrae, and sternebrae, ex-, an-, or microphthalmia, and cleft palate in rats and rabbits. These malformations occurred at low frequencies, generally with no readily apparent dose-response relationship. Presumptive evidence indicates that embryo-fetal deaths, and the above-mentioned fetal malformations in experimental animals, which in published literature are presently attributed to chemical induction for a large number of chemicals, may be a consequence of maternal toxicity per se.

Acrylonitrile: a suspected human carcinogen

The literature on carcinogenicity of acrylonitrile (an important intermediate in the chemical industry) is reviewed. The three main conclusions are: (1) Acrylonitrile has genotoxic effects in various tests in microorganisms and in mammal cells. (2) Chronic exposure to acrylonitrile causes tumours in rats. (3) Results of epidemiological studies indicate that acrylonitrile may be a human carcinogen. From this it is clear that acrylonitrile is very probably carcinogenic to humans. Therefore the authors plead for a reduction of acrylonitrile standards to the lowest practicable limit.

Acrylonitrile-induced gastrointestinal hemorrhage and the effects of metabolism modulation in rats

Acrylonitrile (VCN) is a heavily used monomer in plastic and fiber industries. Quantitatively, VCN-induced gastrointestinal hemorrhage is time and dose dependent and is not the result of a direct irritating action of VCN on gastric tissues. The effect of cytochrome P-450 enzymes inducers was studied. Pretreatment with phenobarbital decreased the VCN-induced GI blood loss (55%), while Aroclor 1254 drastically increased it (240%). VCN administration to rats treated with cobalt chloride or SKF 525A (cytochrome P-450 enzymes inhibitors) resulted in a significant protection against GI bleeding (10 and 40%, respectively). Treatment with diethylmaleate (a known depletor of reduced glutathione) prior to VCN administration, produced no significant change in the VCN-induced GI bleeding. Potassium cyanide (KCN) administration to rats failed to produce significant GI bleeding. These results indicate that metabolic activation of the VCN molecule, to a metabolite(s) other than cyanide by the cytochrome P-450 enzymes, is a prerequisite for VCN to induce gastric hemorrhage.

Distribution and accumulation of [2,3-14C]acrylonitrile in rat after single injection

The distribution and accumulation of acrylonitrile after the single i.p. injection of [2,3-14C]acrylonitrile were examined by whole-body autoradiography and by the determination of 14C radioactivities in several tissues and subcellular fractions after a whole-body perfusion. 14C Radioactivity was seen strongly in blood, particularly in red blood cells, and in several tissues including lung, liver, and kidney. Longer retention of radioactivity in brain and muscle was observed. At the subcellular level a relatively high specific radioactivity was seen in cytosol fractions of the brain, liver, and kidney.

In vivo biotransformation and biliary excretion of 1-14C-acrylonitrile in rats

1-14C-Acrylonitrile (VCN) was give orally to rats, 27% of the given dose was excreted in bile in 6 h. When 1-14C-VCN was given to overnight fasted or cobaltous chloride treated rats, a significant increase in the biliary excretion occurred. Pretreatment of rats with phenobarbital produced no change, while diethyl maleate pretreatment significantly decreased the portion of the dose excreted in bile in 6 h. Four metabolites of 1-14C-VCN have been isolated from the collected bile, and characterized. The two major biliary metabolites were found to be glutathione (GSH) conjugates of VCN, indicating the importance of GSH in VCN biotransformation.

The disposition of [14C]acrylonitrile in rats

1. The disposition of [1,2-14C]acrylonitrile and acrylo[14C]nitrile has been studied following intraperitoneal and oral administration to rats. 2. Most of the 14C found in the tissues was associated with erythrocytes, liver and kidneys. Loss of 14C from liver and kidneys occurred fairly rapidly, but the 14C in the erythrocytes was still mostly retained 48 h after administration. Significant differences in the rates of 14C loss from tissues occurred with [1,2-14C]acrylonitrile and acrylo[14C]nitrile given orally. 3. The 14C from both labelled forms of acrylonitrile was excreted mostly in the urine (82-93% dose) with a smaller amount exhaled unchanged in the breath (3-7% dose) in 24 h.

Teratogenic effects of aliphatic nitriles

Intraperitoneal injection of acrylonitrile at 1.51-2.26 mmole/kg (80-120 mg/kg) or propionitrile at 0.54-1.51 mmole/kg (30-83 mg/kg) on the morning of Day 8 of gestation in the hamster induced exencephaly, encephalocoeles, and rib fusions and bifurcations in the offspring. These doses of the aliphatic nitriles also resulted in obvious toxicity to the dams. Multiple intraperitoneal injections of sodium thiosulfate at 4.03 mmole/kg (1 gm/kg) protected both dams and embryos against toxicity. When the larger doses of either acrylonitrile or propionitrile were given in the presence of sodium thiosulfate, teratogenic effects were observed in the absence of overt signs of maternal poisoning. A survey of the literature describes many studies which demonstrate that acrylonitrile and propionitrile are converted in vivo to toxicologically significant concentrations of cyanide and that sodium thiosulfate, an established cyanide antagonist, can provide protective actions against poisoning by either acrylonitrile or propionitrile. The observations suggest that the teratogenic effects of both acrylonitrile and propionitrile are related to the metabolic release of cyanide.

Occupational chemicals tested for teratogenicity

Literature was surveyed concerning the teratological testing of chemicals, to which large numbers of workers are occupationally exposed. They include metals, plastics monomers and additives, solvents, and other organic chemicals. The effective doses used in the studies were compared to the potential exposures in the occupational environment as regulated by hygienic standards. In light of the animal experiments, the TLVs for some organic chemicals, particularly for acrylonitrile, methacrylate esters, styrene, carbon disulfide, chloroform, methylene chloride, toluene and xylene, appeared too high to provide absolute safety for pregnant workers. The mechanisms of teratogenesis and the validity of the animal experiments were also considered.

Effects of acrylonitrile on rat liver cytochrome P-450, benzo(a)pyrene metabolism and serum hormone levels

Intraperitoneal injections of acrylonitrile (AN), 33 mg/kg, into male rats daily for 3 consecutive days resulted in a 20% decrease in the liver microsomal content of cytochrome P-450. The in vitro formation of 9-hydroxy and 9,10-dihydrodiol metabolites of benzo(a)pyrene in liver microsomes was inhibited. Serum corticosterone levels were markedly decreased (30% of th values for control animals) demonstrating a disturbed pituitary-adrenal axis. A central effect of AN is supported by the finding that prolactin concentrations fell by approximately 60% in exposed animals. AN also induced a doubling FSH levels, whereas LH concentrations were the same as in the control animals. These results suggest an impaired spermatogenesis in the exposed animals.

Distribution of [1-14C] acrylonitrile in rat and monkey

The distribution of [1-14C] acrylonitrile (ACN) in rat and monkey has been studied by whole-body autoradiography, after being administered orally and intravenously to rats and orally to monkeys. Uptake of radioactivity was seen in the blood, liver, kidney, lung, adrenal cortex and stomach mucosa.