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Description of key information

A metabolism study is available for DIBC and DIBK assessing their metabolic pathways and identifying common metabolites. In addition several studies are available on the analogues MIBK and MIBC to further support this endpoint.

Key value for chemical safety assessment

Bioaccumulation potential:
no bioaccumulation potential
Absorption rate - oral (%):
100
Absorption rate - dermal (%):
100
Absorption rate - inhalation (%):
100

Additional information

Data on DIBC/DIBK

Absorption:

Oral:

There is no specific study addressing the oral, dermal or inhalation bioavailability of DIBC or DIBK. Given the presence of systemic toxicity following oral administration it is reasonable to conclude that DIBC and DIBK are bioavailable via the oral route. In addition, the in vivo metabolism study in rats using DIBC and DIBK showed that both substances were absorbed, and reached a Cmax (after a single oral gavage dose) at about 3 hours. From the study it is not possible to determine a ‘% absorption’, however DIBC and DIBK have molecular weights lower than 400, partition coefficients between 1 and 6 and are not ionized, therefore it is expected that oral bioavailability of these substances will be high, around 100%. The study indicates that there may be a difference in bioavailability of DIBC and DIBK due to the quantitative differences in metabolites observed, with DIBC being less available than DIBK. However for the purposes of this assessment both will be assumed to be 100% absorbed.

Inhalation:

The vapour pressure of these substances is low making it unlikely that there would be significant exposure to vapours and their water solubility is less than 1g/L, as such it is likely that their bioavailability via inhalation (e.g. aerosol exposure) would be less than 100%. However for the purposes of the assessment and in the absence of any conclusive data, 100% bioavailability will be assumed.

Dermal:

Via the dermal route, DIBC and DIBK are expected to be absorbed to some extent, again due to their low molecular weight, and their log Kow of between 1 and 6. DERMWIN v2 QSAR was used to predict dermal bioavailability. It predicted almost 100% bioavailability within 1 hour of exposure time. However when calibrating the tool for a structural analogue, 2 -ethyl-1 -hexanol, (a branched, secondary alcohol) it was found to significantly over predict dermal penetration.

Substance

Formula

MW

Log Kow

CAS

Family

2-ethyl-1-hexanol

C8H17OH

130

2.73

104-76-7  

secondary alcohol

2,6-Dimethyl-4-heptanol 

C9H19OH

144

3.08

108-82-7

secondary alcohol

 

Comparing the properties of 2EH and DIBC, they have similar molecular weights and lipophilicity, as such it is estimated that they will have similar dermal penetration potential. The measured dermal penetration of 2EH is approximately 5% (Food and Chemical Toxicology, Volume 48, Supplement 4, July 2010). Based on this, the dermal bioavailability is considered to be less than 10%. However, as indicated in the DNEL assessment, a conservative assessment of bioavilability of 100% was taken for all routes. As such it is considered that the DNELS include an additional layer of conservatism.

Metabolism:

A metabolism study on DIBC and DIBK is available.The objective of this study was to determine the pharmacokinetics of DIBK, DIBK isomer (4,6-dimethylheptan-2-one), DIBC, DIBC isomer (4,6-dimethylheptan-2-ol), and their potential major metabolites in plasma collected from NTac:SD rats following a single oral administration. A determination of comparable pharmacokinetics parameters (such as AUC) for both ketone and alcohol and their potential major metabolites would indicate bioequivalence of ketone and alcohol in themale NTac:SD rat.

In essence, the study assessed whether DIBC and DIBK had the same metabolic profiles – i.e. are the metabolised by the same enzymes and do they produce the same metabolites. The Hypothesis was that in the same way MIBC and MIBK ‘inter convert’ (see below), DIBC and DIBK would also ‘inter convert’ or produce common metabolic profiles.

Following a single oral gavage dose (700 mg/kg bw/day) of either DIBC or DIBK (each containing both isomers) to rats, analysis of the plasma demonstrated the presence of the following biomarkers:

DIBK

·        2,6-dimethylheptan-4-one (DIBK major isomer)

·        4,6-dimethylheptan-2-one (DIBK minor isomer)

·        DIBK alcohol (2-hydroxy-2,6-dimethylheptan-4-one)

·        DIBC alcohol (2,6-dimethylheptan-1,4-diol)

DIBC

·        2,6-dimethylheptan-4-ol (DIBC major isomer)

·        4,6-dimethylheptan-2-one (DIBK minor isomer)

·        DIBK alcohol (2-hydroxy-2,6-dimethylheptan-4-one)

·        DIBC alcohol (2,6-dimethylheptan-1,4-diol)

Although there appear to be some quantitative kinetic differences in the metabolism of the two substances, qualitatively, it is clear that exposure to DIBK and DIBC lead to exposure to common metabolites via common metabolic processes. The main difference in metabolic profile between DIBK and DIBC was the amount of DIBC alcohol produced. Following administration of DIBK, the amount of DIBC alcohol produced was substantially higher (20 times) compared to the amount of DIBC alcohol produced following administration of DIBC. The levels of all other metabolites were comparable. It appears likely that there is some enterohepatic circulation and this could account for the apparent quantitative differences in the levels of metabolites measured for DIBK versus DIBC. It is also possible that DIBC was less bioavailable than DIBK via the oral route, however confirmatory data are not available for this hypothesis.

It appears that there are two major pathways for the metabolism of DIBC (major isomer). The first is oxidation to DIBK and then addition of a second alcohol group to form the DIBK alcohol. The second pathway is conversion to DIBC alcohol. The minor isomer of DIBC appears to be oxidised to form the minor DIBK isomer. There is no data on the subsequent fate of this metabolite. There is also no data on the potential conjugates of these metabolites.

DIBK (major isomer) is also metabolised via the same two pathways, with one leading to formation of the DIBK alcohol and the second involving the reduction of the ketone to an alcohol group and then addition of a second alcohol to form the DIBC alcohol. The minor isomer of DIBC was not detected, indicating that the ketone group in the minor isomer of DIBK does not appear to be reduced to an alcohol.

It should be noted that the structure of the DIBC alcohol and the DIBK alcohol differ in the position of the additional alcohol group. The exact structure of the DIBC alcohol is not confirmed, but tentatively identified based on LC/MS analysis.

Based on these data it appears that both DIBK and DIBC are metabolised by the same enzyme systems and produce the same metabolites. The metabolites are structurally similar to those produced by MIBC and MIBK (alcohol <-> ketone; addition of second alcohol group) and the enzymatic processes are the same.

Excretion

In the metabolism study on DIBK and DIBC, the parent compounds and metabolites had disappeared from the plasma within 40-50 hours, and the Cmax of the parent compound was approximately 3 hours. As such, the parent and metabolites appear to be removed effectively with half lives of between 5 and 10 hours.

Due to the structural similarity of the parent compounds and metabolites of DIBC and DIBK to MIBC and MIBK it is predicted that excretion would occur primarily via the urine. However some conjugation and excretion via the feaces cannot be ruled out.

Supporting data on MIBK/MIBC

The low molecular weight (100.16 g/mol), log Pow (1.9), and water solubility along with the physical state (liquid) of methyl i-butyl ketone (MIBK) favour its absorptionviavarious routes of exposure (oral, dermal, and inhalation). Consistent with this prediction, pharmacokinetic analysis of MIBK demonstrated that the compound was rapidly absorbed into the systemic circulation of rats following oral administration with maximum plasma concentrations occurring at 0.25 hours post-dosing. Signs of toxicity and systemic effects observed in experimental animals following acute oral and acute and repeated inhalation exposure to MIBK are also indicative of systemic absorptionviathese routes of exposure. Following absorption, MIBK was slowly eliminated from the plasma and remained detectable up to 9 hours post-dosing in rats, but not beyond 12 hours. Distribution of MIBK to tissues was demonstrated by the systemic effects in liver and kidneys in mice and rats after repeated inhalation exposure. In developmental toxicity studies, fetal effects were only observed at doses that caused maternal toxicity. Toxicological evidence of central nervous system effects suggests that MIBK may cross the blood-brain barrier in mice and rats. Metabolic data indicate that MIBK is rapidly and extensively metabolized to its major metabolite 4-hydroxymethyl-4-methyl-2-pentanone [i.e., diacetone alcohol (DAA)] and to a minor extent (0.05%) to methyl i-butyl carbinol (MIBC). Metabolism to DAA occursviaoxidation by the mixed function oxidase and metabolism to MIBC occursviareduction by alcohol dehydrogenase. Hydroxylation products of MIBK, such as MIBC and DAA, are expected either to be conjugated with sulfate or glucuronic acid and excreted in urine or to enter intermediary metabolism to be converted to carbon dioxide. Based on the available metabolic data and taking into consideration its low molecular weight and log Pow value, MIBK is not expected to bioaccumulate. 

ABSORPTION

Gastrointestinal Absorption

The absorption of MIBK was studied in male Sprague-Dawley rats orally administered a single dose of 5 mmol/kg body weight of MIBK in corn oil, equivalent to 501 mg/kg body weight, by gavage (Guillaumat, 2004; Gingell et al., 2003). MIBK was rapidly absorbed into the systemic circulation following oral exposure, with a mean maximum plasma concentration (Cmax) of 0.644 mmol/L occurring at 0.25 hours [(time to maximum plasma concentration (tmax)] post-administration. MIBK was detected at very low levels (0.006 mmol/L) at 9 hours post-administration.

Plasma MIBK concentrations were 5.3, 8.4, and 16.1 µg/mL in rats at 1 hour after the last of 3 daily gavage exposures to 1.5, 3.0, and 6.0 mmol/kg MIBK (150, 300, or 601 mg/kg-day), indicating rapid and exposure level-related oral absorption into the bloodstream (Duguay and Plaa, 1993, 1995). The relative uptake of MIBK from the gastrointestinal tract has not been quantified in humans or in animals. Effects observed in laboratory animals following oral exposure to MIBK provide qualitative evidence that it is absorbed from the gastrointestinal tract in toxicologically relevant quantities.

 

Respiratory Tract Absorption

MIBK vapor is absorbed relatively well in humans. Hjelm et al.(1990) measured the pulmonary retention of inhaled MIBK in eight men exposed to MIBK at 10, 100, or 200 mg/m3 (2.4, 24, or 49 ppm) for 2 h by comparing the exhaled concentration with the inhaled concentration. Regardless of the exposure concentration, about 60% of the inhaled MIBK was retained by the body. The respiratory retention rate was fairly constant during the 2-h exposure. The blood concentration of MIBK rose quite rapidly, and no plateau was reached during the 2-h exposure.

Another study, however, shows that MIBK blood concentration can reach a plateau in 2 h. Dick et al. (1992) conducted a neurobehavioral study in which MIBK concentrations in the blood and the breath were measured in human volunteers exposed to MIBK at 88 ppm for 4 h. The mean concentrations in 13 men and 12 women combined are presented in the following Table. MIBK reached plateau concentrations in the blood and breath as early as 2 h into the exposure.

 

TABLE: Blood and Breath Concentrations in Volunteers Exposed to MIBK at 88 ppm (Dick et al. 1992)

 

2-h Exposure

4-h Exposure

90-min Post-Exp

20-h Post-Exp

MIBK in blood (µg/mL)

 

10.6

10.5

0.2

ND

MIBK in breath (ppm)

10.6

0.6

0.1

ND

ND: below detection limit

 

MIBK absorption is not as well studied in rodents as in humans. Duguay and Plaa (1993, 1995) measured the plasma concentration of MIBK given by inhalation to ratsin a study on MIBK's potentiation of the cholestatic effects of taurocholate or manganese on bilirubin in rats. MIBK reached about 5, 8, or 14µg/mL in the plasma 1 h after a 4-h exposure of rats at 200, 400, or 600 ppm.

In rats, plasma MIBK concentrations were 5.0, 8.1, and 14.3 µg/mL immediately following the last of 3 daily 4-hour inhalation exposures to 200, 400, or 600 ppm MIBK (819, 1639, and 2458 mg/m3), indicating rapid and exposure level-related respiratory absorption into the bloodstream (Duguay and Plaa, 1995). In the rat, inhalation exposures to atmospheric concentrations of 200, 400, or 600 ppm MIBK for 4 hours resulted in absorption of the same amount of MIBK as from the oral administration of 1.5, 3.0, or 6.0 mmol/kg, respectively.

 

Dermal absorption

The percutaneous uptake rate in guinea pigs exposed epicutaneously to MIBK peaked at 10 to 45 minutes after the onset of a 150-minute exposure; the maximum uptake rate ranged from 0.11 to 2.0 µmol/min/cm and averaged 1.1 µmol/min/cm (Hjelm et al., 1991).

The penetration rate predicted from the solubility and the octanol-water partition coefficient (log P = 1.38) is 0.95 mg/cm2/hr ( Fiserova-Bergerova et al., 1990).

 

 

DISTRIBUTION

MIBK is likely to be widely distributed in the body because it is absorbed readily into the bloodstream after inhalation exposure (Hjelm et al., 1990). MIBK partitions approximately equally between red blood cells and plasma in rat and human blood; in plasma MIBK is associated primarily with proteins rather than being dissolved in plasma water (Lam et al., 1990). High lipid solubility indicates that MIBK may partition rapidly to lipid-rich tissues, such as nervous tissue.

Concentrations of MIBK and its principal metabolite, 4-hydroxy-4-methyl-2-pentanone, in rat plasma, liver, and lung tissue were positively related to exposure level shortly after the last of 3 daily oral or inhalation exposures (Duguay and Plaa, 1995) or after a single oral administration (Guillaumat, 2004; Gingell et al., 2003).

MIBK accumulated rapidly in brain tissue of mice that received a single intraperitoneal dose of 5 mmol MIBK/kg, peaking at 30 minutes post-exposure, but it was completely eliminated from the brain by 90 minutes post-exposure (Granvil et al., 1994). The brain concentration of 4-hydroxy-4-methyl-2-pentanone continued to increase throughout the 90-minute post-exposure period.

The distribution of MIBK in blood was studied by Lam et al. (1990). In rats exposed to MIBK at 512 ppm for 2 h, MIBK reached a concentration of 25.3µg/mL in blood with 51.2% distributed to red blood cells (RBCs) and the balance in plasma immediately after the exposure. Apparently, MIBK distributed similarly in human blood because Lam et al. (1990) found that 49.4% of MIBK added to human blood in vitro at 0.8 mg/mL resided with RBCs. For human RBCs, 68% of MIBK was associated with hemoglobin. In human plasma, 80% of MIBK was associated with plasma proteins. Therefore, the majority of MIBK in human blood was associated with proteins.

 

METABOLISM

Based on studies in rodents, MIBK is metabolized by either oxidation at the omega-1 carbon to form a hydroxylated ketone or reduction of the carbonyl group to form an alcohol.

Following a single dose of 5 mmol/kg bw of MIBK in corn oil, equivalent to 501 mg/kg body weight, by gavage to male Sprague-Dawley rats (Guillaumat, 2004; Gingell et al., 2003), the main metabolite, 4-hydroxymethyl-4-methyl-2-pentanone [i.e., diacetone alcohol (DAA)], was detected in plasma shortly after oral exposure to MIBK. Plasma levels of HMP slowly increased to a Cmaxof 2.03 mmol/L 9 hours after dosing and remained detectable at 12 hours post-dosing; negligible levels of MIBC were also detectable in plasma after oral dosing of MIBK. Neither MIBK nor MIBC were detectable in 12-hour samples. No metabolite other than HMP and MIBC was detected in the blood. The plasma levels of methyl isobutyl carbinol (MIBC) were very low (<0.012 mmol/L) all over the study. The major material in the blood was HMP, with a Cmax of 2.03 mmol/L at 9 hours and remained detectable at 12 hours post-dosing. Neither MIBK nor MIBC were detectable in 12-hour samples. No compounds other than HMP and MIBK were detected in the blood. The 12-hour area under the plasma concentration time curve (AUC0-12 h) for MIBK, MIBC and HMP were 0.089, 3.558 and 17, 436mmol·hour/L, respectively. HMP and MIBK represented 79% and 20% of the total AUC, respectively. Based on the results of this study, MIBK is rapidly absorbed into the blood in rats following oral exposure and is rapidly and extensively metabolized to HMP (the major metabolite in blood after 3 hours).

DiVincenzo et al.(1976) identified 4-methyl-2-pentanol and 4-hydroxy-4-methyl-2-pentanone as MIBK metabolites in blood of guinea pigs. DiVincenzo et al.(1976) identified 4-hydroxy-4-methyl-2-pentanone and 4-methyl-2-pentanol as the MIBK metabolites in the serum of guinea pigs administered MIBK at 450 mg/kg intraperitoneally.

Duguay and Plaa (1993) could not detect 4-methyl-2-pentanol in the plasma of rats 1 h after a 4-h inhalation exposure to MIBK at 200 ppm, but 4-hydroxy-4-methyl-2-pentanone was found at 5µg/mL. However, both 4-hydroxy-4-methyl-2-pentanone (at about 6-7µg/mL) and 4-methyl-2-pentanol (at about 4-5µg/mL) were detected in the plasma of rats 1 h after a 4-h exposure to MIBK at 400 or 600 ppm (Duguay and Plaa 1993).

Hjelm et al.(1990) evaluated for these metabolites in the urine of human volunteers and found them both to be at concentrations below the detection limit of 5 nmol/L within a 3-hour post-exposure period. Blood levels of potential MIBK metabolites were not quantified in the Hjelm et al. (1990) study. Hjelm et al. (1990) suggested the source of the apparent discrepancy for why MIBK metabolites were detected in the blood of guinea pig but not in the urine of humans could be the lower dose of MIBK used in the Hjelm et al. (1990) study, or perhaps the qualitative and/or quantitative differences in metabolism of MIBK between man and guinea pig. In addition, the urinary excretion of the metabolites may be delayed and therefore the 3-hour post-exposure period may have been too short to permit detection of the metabolites in the urine. Hjelm et al.(1990) suggested that, in humans, 4-methyl-2-pentanol and 4-hydroxy-4-methyl-2-pentanone may either undergo further metabolism to be eliminated as CO2via the lungs or intermediate metabolites may be stored in tissues.

Vézina et al.(1990) found that either single or repeated oral doses of MIBK induced significant increases in hepatocellular cytochrome P-450 content and the hepatic activities of aniline hydroxylase and 7-ethoxycoumarin O-deethylase in rats, suggesting that the liver is involved in the metabolism of MIBK. Similarly, Brondeau et al. (1989) reported increased hepatic cytochrome P-450 content and glutathione-S-transferase activity in rats (but not mice) exposed once by inhalation to MIBK. Hepatic total cytochrome P-450 concentrations were significantly increased in New Zealand male rabbits treated orally with 5 mmol MIBK/kg daily for 3 days (Kobusch et al., 1987). Furthermore, the hepatic mixed-function oxidase activities for aminopyrine N-demethylation, 7-ethoxycoumarin dealkylation, and aniline hydroxylation were increased significantly.

 

ELIMINATION AND EXCRETION

After a single dose of 5 mmol/kg bw of MIBK in corn oil, equivalent to 501 mg/kg body weight, by gavage to male Sprague-Dawley rats (Guillaumat, 2004; Gingell et al., 2003), the half live of elimination of MIBK and 4-Hydroxy-4-methyl-2-pentanone from the blood were 2.5 and 4.8 hours respectively. Only a trace amount of a second metabolite, 4-methyl-2-pentanol, was detected at any time.

The half-life of MIBK in the serum of guinea pigs administered a single 450 mg/kg intraperitoneal dose was estimated to be 66 minutes, based on single blood samples collected from different guinea pigs at intervals up to 16 hours post-dosing (DiVincenzo et al., 1976). 4-Hydroxy-4-methyl-2-pentanone was cleared from the blood within 16 hours, and only a trace amount of a second metabolite, 4-methyl-2-pentanol, was detected at any time.

MIBK was completely eliminated from the blood of mice within 90 minutes of injection of a single intraperitoneal dose of 5 mmol/kg (Granvil et al., 1994); the blood concentration of 4-hydroxy-4-methyl-2-pentanone peaked at 60 minutes post-dosing and was decreasing at the termination of the study at 90 minutes post-dosing.

In humans, elimination of MIBK from blood following cessation of a 2-hour inhalation exposure with light exercise was biphasic, with a half-life of 11 to 13 minutes during the first 30 minutes post-exposure in subjects exposed to 100 or 200 mg/m3 (Hjelm et al., 1990). The half-life in blood during the second elimination phase (60 and 180 minutes post-exposure) was 59 and 74 minutes in subjects exposed to 100 and 200 mg/m3, respectively. Blood MIBK levels were too low to permit the calculation of blood elimination half-times in subjects exposed to 10 mg/m3. In the eight men studied by Hjelm et al. (1990), about 0.04% of the MIBK dose was excreted in the urine as MIBK within 3 h after a 2-h inhalation exposure to MIBK at 10, 100, or 200 mg/m3(2.4, 24, or 49 ppm). The urinary concentrations of MIBK's metabolites, 4-methyl-2-pentanol and 4-hydroxy-4-methyl-2-pentanone, were below the detection limit of 5 nmol/L at 0.5 or 3 h post-exposure. The total body clearance of MIBK was 1.6 L of blood per hour per kilogram of body weight in these men (Hjelm et al. 1990). However, in another study, Hjelm et al. (1991) found that the total body clearance was 12 L of blood per hour per kilogram of body weight in guinea pigs infused with MIBK intravenously. The reason for the large difference in MIBK's total body clearance between men and guinea pigs is unknown.

Hirota (1991) studied the elimination of MIBK in rats exposed intraperitoneally at 100-300 mg/kg. The major route of MIBK elimination was exhalation via the lungs, which accounted for 41% of the dose. The concentration of MIBK in the exhaled air declined with a half-life of 0.6 h after reaching a maximum at 0.5 h after the injection. Two minor routes of MIBK elimination were urinary excretion of MIBK and 4-methyl-2-pentanol. MIBK in the urine attained a maximum concentration within 3 h of the injection and then it declined with a half-life of 1.8 h. The concentration of 4-methyl-2-pentanol reached its peak within 3-6 h of the injection and then decreased with a half-life of 3.2 h.

 

Additional references:

Brondeau, M.T., M. Ban, P. Bonnet, J.P. Guenier, and J. deCeaurriz. (1989) Acetone compared to other ketones in modifying the hepatotoxicity of inhaled 1,2-dichlorobenzene in rats and mice. Toxicol Letters, 49, 69–78.

Kobusch, A.B., B. Bailey, and P. du Souich. (1987) Enzyme induction by environmental agents: effect on drug kinetics. In: Plaa, G.L., P. du Souich, and S. Erill, eds. Interactions Between Drugs and Chemicals in Industrial Societies. Amsterdam: Elsevier Science Publishers, pp. 29–42.

Sato, A. and T. Nakajima. (1979) Partition coefficients of some aromatic hydrocarbons and ketones in water, blood and oil. Brit J Ind Med, 36, 231–234.

Vézina, M., and G.L. Plaa. (1987) Potentiation by methyl isobutyl ketone of the cholestasis induced in rats by a manganese-bilirubin combination or manganese alone. Toxicol Appl Pharmacol, 91, 477–483.