Developmental Toxicity of Inhaled Methyl Ethyl Ketone in Swiss Mice

Developmental Toxicity of Inhaled Methyl Ethyl Ketone in SwissMice. Schwetz, B. A., Mast, T. J., Weigel, R. J., Dill, J. A.,and Morrissey, R. E. (1991). Fundam. Appl. Toxicol. 16, 742–748.Methyl ethyl ketone (MEK) is a widely used industrial solventto which there is considerable human exposure. To assess thepotential for MEK to cause developmental toxicity in rodents,groups of Swiss (CD-1) mice were exposed to 0, 400, 1000, or3000 ppm MEK vapors 7 hr/day on Days 6–15 of gestation.Groups consisted of about 30 bred females each. Exposure ofpregnant mice to these concentrations of MEK did not resultin overt maternal toxicity although there was a slight, treatment-relatedincrease in relative liver weight which was statistically significantin the 3000 ppm group. Mild developmental toxicity was observedin the 3000 ppm group in the form of a reduction in mean fetalbody weight. This reduction was statistically significant forthe males only, although the relative decrease from the controlvalues was the same for both sexes. There was no increase inthe incidence of resorptions or the number of litters with resorptionsamong mice exposed to MEK. There was no significant increasein the incidence of any single malformation, but several malformationswhich were not observed in the concurrent control group or thecontrols of contemporary studies were present at a low incidence—cleftpalate, fused ribs, missing vertebrae, and syndactyly. Therewas also a significant trend for increased incidence of misalignedsternebrae, a developmental variation. In summary, pregnantSwiss (CD-1) mice were relatively insensitive to the toxic effectsof MEK at the inhaled concentrations used in this study. However,the offspring of the mice exhibited significant signs of developmentaltoxicity at the 3000 ppm exposure level. Neither maternal nordevelopmental toxicity was observed at 1000 ppm MEK or below.     

A 14-Week Vapor Inhalation Toxicity Study of Methyl Isobutyl Ketone

In a 2-week probe study male and female Fischer- 344 rats andB6C3F1 mice were exposed 6 hr/day to 2000, 500, 100, or 0 ppmmethyl isobutyl ketone (MIBK). At 2000 ppm there was a slightincrease in male rat liver weight (absolute and relative). Theonly changes observed histologically were increases in regenerativetubular epithelia and hyalin droplets in kidneys of male ratsexposed to 2000 or 500 ppm. Exposure levels for a subchronicstudy were 0, 50, 250, or 1000 ppm methyl isobutyl ketone vapors6 hr/day, 5 days per week, for 14 weeks. The 14 weeks of exposurehad no adverse effect on the clinical health or growth of ratsor mice. Male rats and male mice exposed to 1000 ppm MIBK hada slight but statistically significant increase in liver weightand the liver weight/body weight ratio. Liver weight was alsoincreased slightly in male mice exposed to 250 ppm. No grossor microscopic hepatic lesions related to MIBK exposure wereobserved. Furthermore, the only microscopic change observedwas an increase in the incidence and extent of hyahin dropletswithin proximal tubular cells of the kidneys of male rats exposedto 250 and 1000 ppm of MIBK. The relevance of the male rat kidneytubular effect to humans is not known. In conclusion, otherthan the male rat kidney effect, exposure of male and femalerats and mice to MIBK at levels up to 1000 ppm for 14 weekswas without significant toxicological effect.     

A 90-Day Vapor Inhalation Toxicity Study of Methyl Ethyl Ketone

A 90-Day Vapor Inhalation Toxicity Study of Methyl Ethyl Ketone.Cavender, F.L., Casey, H.W., Salem, H., Swenberg, J.A. and Gralla,E.J. (1983). Fundam. Appl. Toxicol. 3: 264–270. Male andfemale Fischer 344 rats were exposed to 0, 1250, 2500, or 5000ppm methyl ethyl ketone (MEK) vapors 6 hours per day, 5 daysper week for 90 days. The 90-day exposures had no adverse effecton the clinical health or growth of male or female rats exceptfor a depression of mean body weight in the 5000 ppm exposuregroup. The 5000 ppm animals had a slight but significant increasein liver weight, liver weight/body weight ratio, and liver weight/brainweight ratio at necropsy. Serum glutamic-pyruvic transaminase(SGPT) activity in the 2500 ppm female rats was elevated whilethe 5000 ppm female rats exhibited significantly decreased SGPTactivity. In addition, alkaline phosphatase, potassium and glucosevalues for the 5000 ppm female rats were increased. Specialneuro-pathological and routine pathological studies did notreveal any lesions that could be attributed to MEK exposure.     

Pharmacological and Metabolic Interactions between Ethanol and Methyl n-Butyl Ketone, Methyl Isobutyl Ketone, Methyl Ethyl Ketone, or Acetone in Mice

Methyl n-butyl ketone (MnBK), methyl isobutyl ketone (MIBK),methyl ethyl ketone (MEK), and acetone are widely used industrialsolvents to which certain groups of workers are exposed. Pharmacologicaland metabolic interactions between these solvents and ethanolwere explored in male CD-1 mice. The effects of these solventson the duration of ethanol-induced loss of righting reflex andon ethanol elimination in mice were studied. The solvents weredissolved in corn oil and injected intraperitoneally 30 mm beforeethanol 4 g/kg ip. The four solvents prolonged significantlythe duration of ethanol-induced loss of righting reflex whengiven in the following doses (m mol/kg): MnBK, 3.75 and 5; MIBK,5; MEK, 5 and 10, acetone, 20 and 40. This prolongation wasdose related and increased as the dose of the solvent was increased.The concentrations of ethanol in blood or brain on return ofthe righting reflex were similar in solvent-treated and controlanimals, with the exception of the group of mice treated with40 mmol/kg acetone in which the ethanol concentrations weresignificantly lower than in control animals. The mean eliminationrate of ethanol was markedly reduced in mice treated with MnBK5 mmol/kg, MEK 15 mmol/ kg, and acetone 40 mmol/kg. All foursolvents reduced the activity of mouse liver alcohol dehy-drogenasein vitro. It is concluded that enhancement of the ethanol-inducedloss of righting reflex by these solvents in mice is well correlatedto reduced elimination rate of ethanol.     

Effects of short duration exposures to acetone and methyl ethyl ketone

Workers are commonly exposed to mixtures or combinations of chemical agents, and these mixtures often consist of solvents. One group of solvents that has been extensively studied for its neurotoxic properties has been the ketones. However, previous research has focused on neuropathies produced by extended exposures and not on the simple pharmacokinetics or the reversible central nervous system (CNS) effects from short-duration exposures. In this research, 137 volunteers were recruited and tested for neurobehavioral performance changes and biochemical indicators during and after a short-duration (4-h) exposure to either acetone at 250 ppm, methyl ethyl ketone (MEK) at 200 ppm, acetone at 125 ppm with MEK at 100 ppm, or a chemical-placebo. Ethanol (95%, 0.84 ml/kg) was used as a positive control. Testing took place in an environmental chamber with four test stations. The computer-controlled test regimen took 10 h, and several measures were collected: (1) biochemical measurements of venous blood and alveolar breath; (2) psychomotor tests of choice reaction time, visual vigilance, dual task (auditory tone discrimination and tracking), and memory scanning; (3) one sensorimotor (postural sway) test; and (4) one psychological (Profile of Mood States [POMS]) test. Blood and breath concentrations during and after exposure did not demonstrate any interaction between the two solvents, nor were statistically significant sex differences present during uptake or elimination. The 250-ppm acetone exposure produced small but statistically significant differences from controls in two measures of the auditory tone discrimination task, and on the anger-hostility scale (males only) of the POMS test. The other chemical exposure conditions, MEK at 200 ppm and combination MEK with acetone, produced no consistent statistically significant results, which suggests there was no potentiation of the acetone effects with the co-exposure to MEK or vice versa under these test conditions. Ethanol at 0.07-0.08% blood alcohol concentration (BAC) caused significant decrements on both the auditory tone and tracking tests in the dual task.     

Subchronic Inhalation Toxicity of Methyl Isoamyl Ketone in Rats

Subchronic Inhalation Toxicity of Methyl Isoamyl Ketone in Rats.KATZ, G. V., RENNER, E. R., JR., AND TERHAAR, C. J. (1986).Fundam. Appl. Toxicol. 6, 498–505. Rats were exposed byinhalation, 6 hr/day, 5 days/week, to target vapor concentrationsof 2000, 1000, or 0 ppm of methyl isoamyl ketone (MIAK) for12 exposures spanning 16 days, and 2000, 1000, 200, or 0 ppmfor 69 exposures spanning 96 days. Body weights, hematology,and serum clinical chemistry determinations were comparableto controls in both inhalation studies. Clinical signs of toxicitywere lethargy and decreased aural response (2000 ppm, 2-weekstudy; 2000 and 1000 ppm, 90-day study) and nasal and eye irritation(2000 and 1000 ppm, 90-day study). In addition, the excretionof gel-like casts in seminal fluid was seen in males exposedto 2000 and 1000 ppm in both studies. increases in absoluteand relative liver and kidney weights were observed in bothsexes following exposure to 2000 and 1000 ppm in the 2-weekand 90-day studies. Liver weight increases were exposure dependentand in the 90-day study reflected hepatocyte hypertrophy observedon microscopic examination. Microscopic kidney changes werehyalin degeneration or hyalin droplet formation in males inthe 2-week (2000 and 1000 ppm) and 90-day (2000 ppm) studies;and minor to moderate regeneration of tubular epithelium (2000and 1000 ppm) in both studies. Minor tubular epithelium regenerationwas seen in females exposed to 2000 ppm for 90 days. The toxicityof MIAK following inhalation exposure was not as extensive orsevere as that resulting from a prior study in which male ratswere dosed orally with 2000 mg/kg/day (a dose comparable to2000 ppm) for 13 weeks. The 90-day inhalation exposure no-observed-effectlevel for toxicity was 200 ppm MIAK.     

Effects of acetone, methyl ethyl ketone and methyl isobutyl ketone on a match-to-sample task in the baboon.

Acetone, methyl ethyl ketone (MEK) and methyl isobutyl ketone (MIBK) were evaluated for effects on a delayed match-to-sample discrimination task in the juvenile baboon. The animals were exposed to 1/2 the threshold limit value (TLV) of each gas for 24 hr per day during a 7-day period. They were also exposed to a combination of MEK and MIBK at the same exposure concentrations. Each exposure condition affected accuracy of performance minimally but resulted in increased and decreased extra responses during the delay intervals. Response times were slowed under acetone, MEK or MIBK. In contrast to the effects of the individual gases, exposure to a combination of the same doses of MEK and MIBK produced a consistent increase in extra responses during delay and a concomitant decrease in response times. Changes in tissue uptake and metabolism are suggested as possible mechanisms to explain this observation.     

Simulation of repeated dose kinetics of methyl isobutyl ketone in humans from experimental single-dose inhalation exposure

Methyl isobutyl ketone (MIBK) is a solvent used in numerous products and processes and may be present in the air of the workplace as a vapor. The American Conference of Governmental Industrial Hygienists (ACGIH) threshold limit value-time-weighted average (TLV-TWA) and TLV-short term exposure limit (TLV-STEL) for MIBK are 50 and 75 ppm, respectively. These workplace air concentration limits were set to protect workers from irritation, neurasthenic symptoms and possible adverse effects to their livers and kidneys. A recent revision of the ACGIH limit value has been proposed, to reduce the current TLV-TWA to 30 ppm. This article predicts the kinetics and accumulation of MIBK in humans exposed repeatedly in various exposure scenarios (8, 12, and 24h/day for 7 days) to the current ACGIH TLV-TWA of 50 ppm. The kinetic parameters of the model were derived from published human time-course blood MIBK data from a single 2h inhalation exposure to 48.9 ppm MIBK. The model correctly simulated single exposure experimental data with a rapid rise in blood concentration to 1.06 microg/ml within 1h and approached >or=99% steady-state blood level in 4h of exposure. MIBK was predicted to be rapidly eliminated from blood after terminating the exposure, reaching 0.53 microg/ml and 0.13 microg/ml within 0.5 and 2h post-exposure, respectively. Within 4h after the termination of exposure, blood concentration would be expected to <1% of the steady-state concentration. On the basis of these results, it is concluded that accumulation of MIBK in workers due to repeated inhalation exposure is not likely to occur at the current TLV-TWA concentration of 50 ppm.     

Effects of Methyl Ethyl Ketone Pretreatment on Hepatic Mixed-Function Oxidase Activity and on in Vivo Metabolism of N-Hexane

1. Male Fischer-344 rats were given methyl ethyl ketone (MEK; 1.87 ml/kg), a potentiator of the neurotoxicity of n-hexane, by gavage for 4 days prior to a single inhalation exposure to n-hexane (1000 ppm).

2. Samples of blood, liver, testis and sciatic nerve were obtained and analysed for n-hexane, MEK and their metabolites by g.l.c.-mass spectrometry.

3. Pretreatment with MEK increased the concentrations of 2,5-hexanedione (2,5-HD; the proximal neurotoxin) in blood, sciatic nerve and testis relative to concentrations in the tissues in sham-treated controls.

4. Concentrations of 2,5-dimethylfuran, a metabolite of 2,5-HD, were increased in all four tissues tested.

5. After 1-7 days treatment with MEK, the activity of 7-ethoxycoumarin O-deethylase was increased (up to 500%), but benzphetamine N-demethylase activity was virtually unaffected.

6. Hence, the potentiating effects of MEK on the neurotoxicity of n-hexane appear to arise, at least in part, from the activating effects of MEK on selected microsomal enzymes responsible for n-hexane activation.     

Safety assessment of MIBK (methyl isobutyl ketone)

MIBK (Methyl Isobutyl Ketone) is an aliphatic ketone that functions as both a denaturant and solvent in cosmetic products. Current use in cosmetic products is very limited, but MIBK is reported to be used in one nail correction pen (volume = 3 ml) at a concentration of 21%. The maximum percutaneous absorption rate in guinea pigs is 1.1 micromol/min/cm2 at 10 to 45 min. Metabolites include 4-hydroxy-4-methyl-2-pentanone (oxidation product) and 4-methyl-2-pentanol (4-MPOL) (reduction product). Values for the serum half-life and total clearance time of MIBK in animals were 66 min and 6 h, respectively. In clinical tests, most of the absorbed MIBK had been eliminated from the body 90 min post exposure. MIBK was not toxic via the oral or dermal route of exposure in acute, short-term, or subchronic animal studies, except that nephrotoxicity was observed in rats dosed with 1 g/kg in a short-term study. MIBK was an ocular and skin irritant in animal tests. Ocular irritation was noted in 12 volunteers exposed to 200 ppm MIBK for 15 min in a clinical test. A depression of the vestibulo-oculomotor reflex was seen with intravenous infusion of MIBK (in an emulsion) at 30 microM/kg/min in female rats. The no-observed-effect level in rats exposed orally to MIBK was 50 mg/kg. Both gross and microscopic evidence of lung damage were reported in acute inhalation toxicity studies in animals. Short-term and subchronic inhalation exposures (as low as 100 ppm) produced effects in the kidney and liver that were species and sex dependent. Dermal doses of 300 or 600 mg/kg for 4 months in rats produced reduced mitotic activity in hair follicles, increased thickness of horny and granular cell layers of the epidermis, a decrease in the number of reactive centers in follicles (spleen), an increase in the number of iron-containing pigments in the area of the red pulp (spleen), and a reduction in the lipid content of the cortical layer of the adrenal glands. Neuropathological changes in the most distal portions of the tibial and ulnar nerves were observed in young adult rats which inhaled 1500 ppm MIBK for up to 5 months. No adverse effects were seen in any other neurological end point by any route of exposure in other studies using rats or other animal species. Clinical tests demonstrated a threshold for MIBK-induced irritation of the lungs at 0.03 to 0.1 mg/L after 1 min of respiration. MIBK was not mutagenic in the Ames test or in a mitotic gene-conversion assay in bacteria. Mammalian mutagenicity test results were also negative in the following assays: mouse lymphoma, unscheduled DNA synthesis, micronucleus, cell transformation, and chromosome damage. MIBK did not induce any treatment-related increases in embryotoxicity or fetal malformations in pregnant Fischer 344 rats or CD-1 mice that inhaled MIBK at concentrations of 300, 1000, or 3000 ppm. There was evidence of treatment-related maternal toxicity only at the highest concentration tested. MIBK applied to the tail of rats daily at doses of 300 or 600 mg/kg for 4 months produced changes in the testes, including a reduction in the number of spermatocytes, spermatids, and spermatozoa. An ongoing carcinogenicity study of MIBK being conducted by the National Toxicology Program will be considered when the results are available. On the basis of the information that is currently available, MIBK is considered safe as used in nail polish removers and as an alcohol denaturant in cosmetic products.     

Neurotoxicity of Methyl Chloride in Continuously versus Intermittently Exposed Female C57BL/6 Mice

Neurotoxicity of Methyl Chloride in Continuously versus IntermittentlyExposed Female C57BL/6 Mice. LANDRY, T. D., QUAST, J. F., GUSHOW,T. S., and MATTSSON, J. L. (1985). Fundam. Appl. Toxicol. 5,87–98. This study evaluated the relationship between methylchloride (MeCl) exposure duration and neurotoxicity. FemaleC57BL/6 mice were exposed to MeCl for 11 days, either continuously(22 hr/day) to 15, 50, 100, 150, or 200 ppm, or intermittently(5.5 hr/day) to 150, 400, 800, 1600, or 2400 ppm. This strainand sex of mouse was chosen because it is sensitive to MeClneurotoxicity and was a good candidate to allow the evaluationof morphological effects and the quantitation of functionaleffects. A simple quantitative relationship between neurotoxicityand continuous vs intermittent exposure was not observed. Althoughthe no-observable-effect levels for continuous and intermittentMeCl exposures were very nearly proportionate to exposure concentrationmultiplied by duration, the dose—response curve was muchsteeper for continuously exposed mice. Cerebellar granular celllayer degeneration was observed in mice exposed continuouslyto 100 ppm MeCl and in mice exposed intermittently to 400 ppm.This histopathologic effect was observed at lower concentrationsthan a decrement in rotating rod running performance. No effectswere observed in mice exposed to 50 ppm continuously or to 150ppm intermittently. Continuous exposure to MeCl produced thecerebellar lesion with less effect on other tissues than didintermittent exposure. In mice exposed to 2400 ppm intermittently,there were renal and hematopoietic effects in addition to relativelyslight cerebellar granular cell layer degeneration. These 2400-ppmexposed mice developed hemoglobinuria, apparently as a resultof intravascular hemolysis. Although the effect of exposureduration on MeCl toxicity was complex, this study indicatedthat careful judgment is necessary when extrapolating intermittentexposure data to a continuous exposure situation.     

Toxicity and carcinogenicity of methyl isobutyl ketone in F344N rats and B6C3F1 mice following 2-year inhalation exposure

Methyl isobutyl ketone (MIBK) is primarily used as a denaturant for rubbing alcohol, as a solvent and in the manufacture of methyl amyl alcohol. Inhalation of vapors is the most likely route of exposure in the work place. In order to evaluate the potential of MIBK to induce toxic and carcinogenic effects following chronic exposure, groups of 50 male and 50 female F344/N rats and B6C3F1 mice were exposed to MIBK at concentrations of 0, 450, 900, or 1800ppm by inhalation, 6h/day, 5 days per week for 2 years. Survival was decreased in male rats at 1800ppm. Body weight gains were decreased in male rats at 900 and 1800ppm and in female mice at 1800ppm. The primary targets of MIBK toxicity and carcinogenicity were the kidney in rats and the liver in mice. In male rats, there was increased mineralization of the renal papilla at all exposure concentrations. The incidence of chronic progressive nephropathy (CPN) was increased at 1800ppm and the severity was increased in all exposed groups. There were also increases in renal tubule hyperplasia at all exposure concentrations, and in adenoma and adenoma or carcinoma (combined) at 1800ppm; these lesions are thought to represent a continuum in the progression of proliferative lesions in renal tubule epithelium. These increases may have resulted from the increased severity of CPN, either through alpha2micro-globulin-dependent or -independent mechanisms. An increase in mononuclear cell leukemia at 1800ppm was an uncertain finding. Adrenal medulla hyperplasia was increased at 1800ppm, and there was a positive trend for increases in benign or malignant pheochromocytomas (combined). In female rats, there were increases in the incidence of CPN in all exposure concentrations and in the severity at 1800ppm, indicating that CPN was increased by mechanisms in addition to those related to alpha2micro-globulin. There were renal mesenchymal tumors, which have not been observed in historical control animals, in two female rats at 1800ppm. The relationship of these tumors to exposure to MIBK was uncertain. Hepatocellular adenomas, and adenoma or carcinoma (combined) were increased in male and female mice exposed to 1800ppm. There were also treatment-related increases in multiple adenomas in both sexes.     

Sensory irritation by toluene diisocyanate in single and repeated exposures

The effects of single and repeated exposures to toluene diisocyanate (TDI) at concentrations ranging from 0.007 to 2 ppm were investigated using an animal model to detect the level of sensory irritation caused by this chemical. Time-response relationships showed the slow development of the response with exposure duration and a very slow recovery following 3 hr of exposure. Concentration-response relationships also showed that the level of response was not only dependent upon exposure concentration, but also on duration of exposure and that TDI is the most potent sensory irritant tested so far. Repeated exposures to TDI at or above 0.023 ppm resulted in cumulative effects. On the basis of a prior prediction proposed from the results obtained with this animal model, it is suggested that the Threshold Limit Value (TLV) for industrial exposure be reduced to 0.006 ppm as a time weighted average (TWA) and no exposure above 0.02 ppm be permitted, even for short exposures.     

Developmental Toxicity Evaluation of Inhaled Methyl Isobutyl Ketone in Fischer 344 Rats and CD-1 Mice

Developmental Toxicity Evaluation of Inhaled Methyl IsobutylKetone in Fischer 344 Rats and CD-1 Mice. Tyl, R. W., FRANCE,K. A., FISHER, L. C., PRITTS, I. M., TYLER, T. R., PHILLIPS,R. D., and MORAN, E. J. (1987). Fundam. Appl. Toxicol. 8, 310–327.Pregnant Fischer 344 rats and CD-1 mice were exposed to methylisobutyl ketone vapor (CAS No. 108-10-1) by inhalation on GestationalDays 6 through 15 at concentrations of 0, 300, 1000, or 3000ppm (mean analytical values of 0, 305, 1012, and 2997 ppm, respectively).The animals were sacrificed on Gestational Day 21 (rats) or18 (mice), and live fetuses were examined for external, visceral,and skeletal alterations. In rats, exposure to 3000 ppm resultedin maternal toxicity expressed as clinical signs, decreasedbody weight and body weight gain, increased relative kidneyweight, and decreased food consumption, and in fetotoxicityexpressed as reduced fetal body weight per litter and reductionsin skeletal ossification. In mice, exposure to 3000 ppm resultedin maternal toxicity expressed as exposure-related increasesin deaths (12.0%, 3/25 dams), clinical signs, and increasedabsolute and relative liver weight, and in fetotoxicity expressedas increased incidence of dead fetuses, reduced fetal body weightper litter, and reductions in skeletal ossification. No treatment-relatedincreases in embryotoxicity or fetal malformations were seenin either species at any exposure concentration tested. Therewas no evidence of treatment-related maternal, embryo, or fetaltoxicity (including malformations) at 1000 or 300 ppm in eitherSpecies.     

Disposition of Butadiene Epoxides in Sprague-Dawley Rats Following Exposures to 8000 ppm 1,3-Butadiene: Comparisons with Tissue Epoxide Concentrations Following Low-Level Exposures

1,3-Butadiene (BD), a compound used extensively in the rubberindustry, is weakly carcinogenic in Sprague-Dawley rats afterchronic exposures to concentrations of 1000 and 8000 ppm. Conversely,in B6C3F1 mice, tumors occur after chronic exposures to concentrationsas low as 6.25 ppm. Previously, we have shown that tissue concentrationsof the mutagenic BD metabolites, butadiene monoepoxide (BDO)and butadiene diepoxide (BDO₂), are present in greater concentrationsin mice than in rats following acute exposures to low levels(100 ppm or less). This disparity was particularly significantfor the diepoxide. We hypothesized that if these epoxides areinvolved in the carcinogenic response of BD, then they willalso be present in rat tissues at relatively high concentrationsfollowing exposures to 8000 ppm BD. In the present study, concentrationsof the BD epoxides, BDO and BDO₂, were determined in blood offemale Sprague-Dawley rats following a single 6-h exposure and10 repeated exposures to a target concentration of 8000 ppmBD. Concentrations of these epoxides were also determined ina number of other tissues, including the primary rat targetorgan—mammary gland—following 10 repeated exposures.Blood concentrations of BDO were 4030 pmol/g ± 191 followinga 6-h exposure and were 18% lower following the 10-day exposure.Blood concentrations of BDO₂, following the 8000 ppm exposures,were very similar to those previously observed after exposuresto 62.5 ppm BD (11 ± 1 and 17 ± 1 pmol/g followingexposures of 6h and 6h/day for 10 days, respectively.) Concentrationsof BDO ranged from 740 ± 110 (femur) to 8990 ±1150 (fat) pmol/g tissue. Concentrations of BDO₂ were similaramong eight tissues analyzed, ranging from 5 ± 1 (femur)to 17 ± 3 (heart) pmol/g tissue. Tissue concentrationsof butadiene mono-epoxide were increased by 17- to 50-fold intissues from rats exposed by inhalation to 8000 ppm BD as comparedto tissues from rats exposed to 62.5 ppm BD. Based on earlierstudies at our institute the internal dose of BD increases approximately14-fold in the 8000 ppm-exposed rats compared to rats exposedto 62.5 ppm BD. Concentrations of butadiene diepoxide in rattissues following an exposure to 8000 ppm BD were similar tothose observed in rat tissues following exposures to 62.5 ppmBD. This study shows that pathways responsible for the accumulationof BDO₂ in rats are saturated following low-level BD exposures.This suggests that the primary determinant of BD tumorigenicityin rats is not butadiene diepoxide. The high levels of BDO observedin rat mammary tissue suggest that this metabolite may be amore important determinant of BD carcinogenesis in the rat.     

Upper respiratory tract toxicity of inhaled methylvinyl ketone in F344 rats and B6C3F1 mice.

The National Toxicology Program is conducting a chemical class study to investigate the structure-activity relationships for the toxicity of alpha,beta-unsaturated ketones. Methylvinyl ketone (MVK) was selected for study because it is a representative straight-chain aliphatic alpha,beta-unsaturated ketone and because of its extensive use and widespread exposure. Short-term inhalation studies of MVK were conducted to provide toxicity data for comparison with other alpha,beta-unsaturated ketones and for use in designing chronic toxicity and carcinogenicity studies. In 2-week studies, rats and mice were exposed to 0, 0.25, 0.5, 1, 2, 4, or 8 ppm MVK 6 h/day, 5 days/week for 12 exposures. Morbidity and early deaths occurred in all male and female rats after 1 exposure and in 2 male mice after 10 exposures to 8 ppm. Rats exhibited nasal cavity toxicity and lung necrosis at 4 ppm. No toxicity was observed in animals exposed to less than 2 ppm. Based on these results a 13-week study was conducted at 0, 0.5, 1, and 2 ppm MVK. As observed in the 2-week study, the nasal cavity was the main target organ and rats were more sensitive than mice. Respiratory and olfactory epithelial necrosis were prominent by day 21 in the rat. At study termination these lesions were still evident but not as severe as noted earlier. Additionally, changes such as olfactory epithelial regeneration and metaplasia (respiratory) as well as respiratory epithelial hyperplasia and metaplasia (squamous) were clearly evident. Nasal lesions in mice were limited to a subtle squamous metaplasia of transitional and/or respiratory epithelium covering predominantly the tips of naso- and maxilloturbinates in Levels I and II. A transient, leukopenia was observed in rats exposed to 2 ppm, however, this effect was not present after 13 weeks of exposure. In mice, leukocyte counts were significantly decreased at all exposure concentrations after 13 weeks of exposure. Absolute testicular and epididymal weights and sperm counts were decreased at the high dose only. MVK can be characterized as a reactive, direct-acting gaseous irritant. MVK exposure causes the same nasal cavity lesions as the cyclic alpha,beta-unsaturated ketone, 2-cyclohexen-1-one, although at lower exposure concentrations.     

Predicting age-appropriate pharmacokinetics of six volatile organic compounds in the rat utilizing physiologically based pharmacokinetic modeling.

The capability of physiologically based pharmacokinetic models to incorporate age-appropriate physiological and chemical-specific parameters was utilized to predict changes in internal dosimetry for six volatile organic compounds (VOCs) across different ages of rats. Typical 6-h animal inhalation exposures to 50 and 500 ppm perchloroethylene, trichloroethylene, benzene, chloroform, methylene chloride, or methyl ethyl ketone (MEK) were simulated for postnatal day 10 (PND10), 2-month-old (adult), and 2-year-old (aged) rats. With the exception of MEK, predicted venous blood concentrations of VOCs in the aged rat were equal or up to 1.5-fold higher when compared to the adult rat at both exposure levels, whereas levels were predicted to be up to 3.8-fold higher in the case of PND10 at 50 ppm. Predicted blood levels of MEK were similar in the adult and aged rat, but were more than 5-fold and 30-fold greater for PND10 rats at 500 and 50 ppm, respectively, reflecting high water solubility along with lower metabolic capability and faster ventilation rate per unit body weight (BW) of PND10 animals. Steady-state blood levels of VOCs, simulated by modeling constant exposure, were predicted to be achieved in the order PND10 > adult > aged, largely due to increasing fat volume. The dose metric, total amount metabolized per unit liver volume was generally much lower in PND10 than in adult rats. The blood:air partition coefficient, fat volume, and fat blood flow were identified as critical determinants for the predicted differences in venous blood concentrations between the adult and aged. The lower metabolic capability, largely due to a smaller liver size, and faster ventilation rate per unit BW of PND10 animals contribute the most to the differences between PND10 and adult rats. This study highlights the pharmacokinetic differences and the relevant parameters that may contribute to differential susceptibility to the toxic effects of VOCs across life stages of the rat.     

Inhalation two-generation reproductive toxicity study of methyl isobutyl ketone in rats

To evaluate whether methyl isobutyl ketone (MIBK) affects reproductive performance, a two-generation reproduction study was conducted. MIBK was administered to 30 Sprague-Dawley rats/sex/group via whole-body inhalation at concentrations of 0, 500, 1000, or 2000 ppm, 6 h daily, for 70 days prior to mating. F(0) and F(1) females were exposed from mating through gestation day 20 and from postnatal day 5; F(2) litters were maintained through postnatal day 21. No treatment-related mortality of adult animals occurred. There was a dose-related increase in adult animals with no or a decreased response to a sound stimulus at 1000 and 2000 ppm; however, no adverse clinical signs occurred 1 h after exposure, suggesting this was a transient sedative effect. Clinical signs of central nervous system (CNS) depression in the pups were observed and one F(1) pup died after initial exposure to 2000 ppm on postnatal day 22; subsequently exposure was delayed until postnatal day 28. Decreased body weight gain and slight decreased food consumption were observed during the first 2 weeks of exposure in both generations at 2000 ppm. There were no adverse effects on male and female reproductive function or landmarks of sexual maturation. Increased F(0) and F(1) liver weights with associated centrilobular hypertrophy occurred in rats at 2000 ppm, indicative of an adaptive response. Increased male kidney weights at all exposure concentrations, associated with hyaline droplets, were indicative of male rat-specific nephropathy. Other than acute sedative effects, the no-observed-adverse-effect level (NOAEL) for parental systemic effects (excluding male rat kidney) was 1000 ppm, based on transient decreased body weight and food consumption; for reproductive effects, 2000 ppm, the highest concentration tested; and for neonatal toxicity, 1000 ppm (based on acute CNS depressive effects).     

Metabolic interaction and disposition of methyl ethyl ketone and m-xylene in rats at single and repeated inhalation exposures

1. Rats were exposed to m-xylene (300 ppm) and methyl ethyl ketone (MEK, 600 ppm) vapour, separately and in combination. 2. Repeated exposures to m-xylene enhanced liver drug-metabolizing capacity, whereas MEK showed no effects. After mixed exposure the cytochrome P-450-dependent monooxygenase activities were additively or synergistically induced. 3. In the presence of MEK the overall metabolism of xylene was strongly inhibited both after single and repeated exposures, an effect accompanied by elevation of xylene concentration in blood (18-29%) and fat (25-32%). 4. The 24-h excretion of the urine metabolites of m-xylene was decreased by 22-24% in mixed exposures: the excretion of methylhippuric acid was decreased (29%), but that of 2,4-dimethylphenol increased (9-35%). 5. After repeated inhalation exposures the excretion of xylene metabolites in urine was consistently higher, whereas the concentrations of xylene in fat (but not the concentration of MEK) were lower than after a single treatment, conceivably due to accelerated metabolic clearance of xylene. 6. Thioether excretion in urine was enhanced in xylene-treated rats (7-13-fold), but was not influenced by the induced changes in the metabolism of xylene. Xylene inhalation caused liver GSH to decrease slightly (10%), as did inhalation of MEK, but the latter did not enhance the excretion of thioethers. 7. MEK is a potent inhibitor of the side-chain oxidation of m-xylene producing methylhippuric acid, but not of its ring oxidation to 2,4-dimethylphenol, and exhibits a synergistic inducing effect on liver enzymes responsible for the oxidation of m-xylene. The increased ring oxidation of m-xylene was not associated with increased production of reactive metabolites indicated by GSH-depletion or thioether formation.