2,6-dimethylheptan-4-one

Endpoint summary

Link to relevant study record(s)

Reference

Endpoint:

basic toxicokinetics in vivo

Type of information:

experimental study

Adequacy of study:

key study

Study period:

Experimental Start: 11 Oct 2012, Experimantal Termination: 31 Jan 2012

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Objective of study:

toxicokinetics

Qualifier:

no guideline available

Principles of method if other than guideline:

The objective of this study is to determine the pharmacokinetics of diisobutyl ketone (DIBK), DIBK isomer 4,6-dimethylheptan-2-one, diisobutyl
carbinol (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 AVC) for both ketone and alcohol and their potential
major metabolites would indicate bioequivalence of ketone and alcohol in the male NTac:SD rat.

GLP compliance:

yes

Specific details on test material used for the study:

Diisobutyl carbinol (DIBC)
- Molecular formula (if other than submission substance): C9H20O
- Molecular weight (if other than submission substance): 144.3
- Physical state: Liquid
- Analytical purity: 98%
- Composition of test material, percentage of components: Sum of 2,6-dimethyl-4-heptanol and 4,6-dimethyl-2-heptanol
- Purity test date: 2011
- Lot/batch No.: ZJ1855T3Z1

Diisobutyl Keytone (DIBK)
- Molecular formula (if other than submission substance): C9H18O
- Molecular weight (if other than submission substance): 142.2
- Physical state: Liquid
- Analytical purity: 99%
- Composition of test material, percentage of components: Sum of 2,6-dimethyl-4-heptanol and 4,6-dimethyl-2-heptanol
- Purity test date: 2009
- Lot/batch No.: XA2355T643

Radiolabelling:

no

Species:

rat

Strain:

Sprague-Dawley

Sex:

male

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source: Taconic (Germantown, New York)
- Age at study initiation: 8 to 12 weeks
- Housing: Following administration of gest material by gavages, Groups 1 and 2 for the Probe Study will be housed in the same tubs for acclimation.- Following administration of the test material by gavages, Groups 3 and 4 for the TK study will be housed singly in glass Roth-type cages.
- Diet (e.g. ad libitum): LabDiet certified Roden Diet #5002 in pellet for is provided ad libitum except that access will be restriced approximately
16-hours prior to dosing and will be returned about 4-hours post-dosing.
- Water (e.g. ad libitum): Drinking water obtained from a municipal source is provided ad libitum.
- Housing / Acclimation period: Non-cannulated rates will be acclimated to the building for at least 7-days in solid bottom tubs. Cannulated rates
will be housed individually in glass Roth type metabolism cages on arrival and acclimated for at least 5-days prior to their use in the study.

ENVIRONMENTAL CONDITIONS
- Temperature (°C): 22°C +/- 1°C (and a maxium permissable excursion of +/- 3°C)
- Humidity (%): 40-70%
- Air changes (per hr): 12-15 times per hour
- Photoperiod (hrs dark / hrs light): 12-hour light/dark (on at 6:00 a.m. and off at 6:00 p.m.)

ANIMAL WELFARE
In accordance with the U.S. Department of Agriculture animal welfare regulations, 9 CFR, Subchapter A, Pans 1-4, the animal care and lise activities
required that conduct of the studies be revie\ved and approved by the Institutional Animal Care and Use Committee (IACLJC). The IACUC has
determined that the proposed Activities arc in full accordance with these Final Rules. The IACUC-approved Animal Care and Use Activities to be used for this study are Metabolism 01, Humane Endpoints 01 and Animal ID O1.

CANNULATION
Jugular Vein (Groups 3 and 4 rats only) NTac:SD rats in which the blood/plasma time-course will he determined will be obtained already cannulated
in the jugular vein by the supplier.

Route of administration:

oral: gavage

Vehicle:

other: 0.5% methylcellulose

Details on exposure:

For both the Probe and Time-course phases of this study a single 700mg/Kg dose of DIBK/DIBC will be administed as follows:

VEHICLE
- Justification: Due to the possibe effect (caused by corn oil) on the metabolite identification in plasma samples, 0.5% methylcellulose will be used as
the dose vehicle to reduce the matrix effect.
- Concentration in vehicle: A dose of approximaltely 5mmol/Kg (700mg/Kg) will be administered
- Amount of vehicle (if gavage): The final dose suspension will be administered at a target volume of ~5mL/Kg body weight, via a ball-tipped gavages needle attached to a glass syringe.


HOMOGENEITY AND STABILITY OF TEST MATERIAL:
- Homogeneity of the dose suspensions may be determined prior to dosing by taking aliquots from 2-3 locations within the container.
- Stability will not necessary as the animals will be dosed within 24 hours of preparation.

Duration and frequency of treatment / exposure:

A single dose will be administered

Remarks:

Doses / Concentrations:
One 700mg/Kg gavages dose will be administered.

No. of animals per sex per dose / concentration:

Probe: 2 Male rats
Time-course: 4 Male rats
Vehicle Control: 1 Male

Control animals:

yes, concurrent vehicle

Positive control reference chemical:

Not applicable

Details on study design:

Diisobulyl ketone (DIBK) and Diisobutyl carbinol (DIBC) are high-production volume chemicals that are widely used in the chemical industry. Due to
the catalytic large-scale production process where acetone was used as a starting material, both DIBC and DIBK contain their corresponding minor
isomers. DiBC contains about 13% of 4,6-dimethylheptan-2-ol and DIBK contains about 13% of 4,6- dimethylheptan-2-one. There are sufficient DIBK toxicity data for supporting DIBK registration (such as REACH registration). In contrast, only limited toxicity data is available for DIBC, which could
hinder the registration for DIBC. To solve the potential registration issue, a Read-cross approach has been proposed, where the ketone DIBK (or DIBK isomer 4,6-dimethylheptan-2-one) toxicity data, in conjunction with new ADME data showing bioequivalence between ketone and alcohol molecules,could be used to develop an adequate REACH registration dossier for the alcohol DIBC (or DlBC isomer 4,6-dimethylheptan-2-o1). The proposed
bioequivalence study is based on the hypothesis that this alcohol-ketone set undergo interconversion in vivo, as has been shown for the analogous compounds alcohol MIBC and ketone MIBK (Gingell et al., 2003). In that study Gingell et al. showed these two compounds are essentially
bioequivalent, with MIBC converting first to MIBK, followed by both materials being metabolized to the common HMP metabolite
(4-hydroxy-4-methyl-2-pentanone). Pharmacokinetic analysis of blood samples, following single oral doses of either MIBK or MIBC showed that the total systemic exposure of the two major metabolites (MIBK +HMP) were comparable, as measured by the sum of the plasma AUC values. Although
no mass balance of administered radiolabel and urinary metabolite profiling \vere conducted in the Gingell's study or other prior metabolism
studies with these compounds (DiVincenzo et aI., 1976; Duguay and Plaa. 1995; Granvil et al., 1994) and some differences in overall
ADME fate of these two test materials may occur. the final result from the Gingell et al. (2003) study showed that metabolic equivalency of
MIBC with MIBK support the conclusion that MIBC will have a similar toxicological profile to that of MIBK, and reduces the need for additional animal
studies (Gingell et al., 2003).

DIBK and DIBC are quite comparable in structure to the related MIBK and MIBC compounds. It would therefore be expected that DIBK and DIBC would undergo similar interconversion in mammals, and also form a common HMP-like metabolite, which would be 6-hydroxy-2,6-dimethyl-4-heptanone (HDH) (Scheme 2). In similar way, the minor isomer of DIBK (4,6-dimethylheptan-2-one) and DIBC (4,6-dimethylheptan-2-o1) will also undergo
similar interconversion in mammals and also form common metabolite 6-hydroxy-4,6-dimethylheptan-2-one. Based on this background
information, a bridging limited ADME study is proposed to confirm this hypothesis via the similar approach as reported in Gingell's publication
(Gingell et al., 2003):

Step 1: Probe Experiment
- 2 Male rates will be adminsitered either DIBC or DIBK, vial oral gavages, at a dose level of 700mg/Kg according to the previouse study on MIBC
and MIBK (Gingell et al., 2003).
- Animals weill be sacrificed at the time of common Cmax previously reported for the higher inter-conversion for both MIBK and MIBC (about 1.5
hours).
- These Cmax plasma samples will be analyzed for the two test materials (DIBK and DIBC) and their corresponding minor isomers
(4,6-dimethylheptan-2-one and 4,6-dimethyiheptan-2-(1), as well as the proposed common metaholites HDH and 6-hydroxy-4,6-
dimethylheptan-2-one (or another major common metabolite).
- If HDH (or another appropriate biomarker metabolite) levels are found comparable to, or higher than DIBK/DIBC, a standard of this metabolite will be synthesized for the subsequent quantitative TK experiment.
- This scenario will be also applied to the minor isomers of both DIBK and DlBC (4.6-dimethylheptan-2-one and 4,6-dimethylheptan-2-o1 ).

Step2: Time-course - Collecting plasma samples for TK analysis:
- Once the DlBC, 4,6-dimethylheptan-2-oL DIBK and 4,6-dimethylheptan-2-one presence in the Probe experiment is confirmed, the definitive
TK experiment will be conducted to determine kinetics of the DIBC, 4,6-dimethylheptan-2-01, DIBK 4,6- dimethylheptan-2-one, and possibly
HDH and 6-hydroxy-4,6-dimethylheptan-2-one (or other metabolite) biomarkers.
- A group of 4 male rats, with cannula implanted in the jugular vein, will be administered either DIBK or DIBC, via oral gavage, at a dose level of
700 mg/kg.
- Repetitive bloocl samples will be taken from each rat at 0.08, 0.17, 0.25, 0.5, 1, 2, 3, 6. 12, 24, 48, 72, 96 and 120 hrs post-dosing and centrifuged
to obtain plasma.
- Chemical analysis will then be conducted on individual plasma samples for DlBC, 4,6-dimethylheptan-2-ol, DIBK 4,6-dimethylheptan-2-one and
possibly HDH and 6-hydroxy-4,6-dimethylhcptan-2-one.
- Pharmacokinetic analysis would be conducted on the TK quantitative results, to calculate absorption and eliminalion half-lives of the TK
biomarkers, as well as AUC determination of systemic bioavailabijity.
- The hopeful outcome of this study design would be a conclusion similar to that found for MlBK/MIBC, in which the DIBK/DIBC and
4,6-dimcthylheptan-2-one/4,6-dimethylheptan-2-0l would be found to interconvert in the
rat, with possible formation of comparable levels of a common metabolite.

See Table 1 for details.

Details on dosing and sampling:

Oral Dose Administration
The oral dose will be administered via a ball-tipped gavage needle attached to a glass syringe. Animals from Groups 1-4, will receive a single oral
dose. The volume of dose formulation to be administered to each animal will be calculated based on the body weighr taken on the day of dose
administration.

Statistics:

Descriptive: statistics will be used, i.e., mean ± standard deviation. All calculations in the database will be conducted using Microsoft Excel
(Microsoft Corporation, Redmond,Washington) spreadsheets and databases in full precision mode (15 digits of accuracy). Certain pharmacokinetic parameters will be estimated for blood and urine data, including AUC (area-under-the-curve), Cmax, 1/2Cmax, and elimination rate constants,
using PK Solutions (v.2.0.6., Summit Research Services, Montrose, Colorado).

Preliminary studies:

Not applicable

Details on absorption:

Not applicable

Details on distribution in tissues:

Not applicable

Details on excretion:

Not applicable

Metabolites identified:

yes

Details on metabolites:

DIBC and DIBK share the main major metabolites and are considered to be ‘interconvertable’. DIBC major isomer was metabolised to DIBK major isomer, to DIBK alcohol and to DIBC alcohol.
DIBC minor isomer was metabolised to DIBK minor isomer.

DIBK major isomer was converted to DIBC, DIBC alcohol and to DIBK alcohol. DIBK minor isomer can convert to DIBC minor isomer.

There is evidence that enterohepatic circulation of phase II conjugates occurs.

Bioaccessibility (or Bioavailability) testing results:

Not applicable

The objective of this study was to determine the pharmacokinetics of diisobutyl ketone (DIBK), DIBK isomer (4,6-dimethylheptan-2-one), diisobutyl carbinol (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 these two test materials in the male

NTac:SD rat.

After single oral dose of DIBC (containing DIBC-isomer) or DIBK (containing DIBK-isomer) in male NTac:SD rat (700 mg/kg, nominal), metabolite profiling was initially conducted with plasma samples collected at the estimated time of Cmax plasma concentrations of test material and/or metabolites (1.5 hr post-dosing). In addition to the two test materials, and their isomers, two common metabolites were identified from either test material. These metabolites (DIBC alcohol and DIBK alcohol) were monohydroxylated metabolites of DIBC and DIBK, with the site of hydroxylation at the σ and σ-1 positions, respectively.

Quantitative analysis of DIBC, DIBK, DIBC-isomer, DIBK-isomer, DIBC-alcohol and DIBK alcohol was then conducted for blood samples collected at 5 min to 120 hr after a single oral dose of either DIBC or DIBK (490-635 mg/kg). DIBK, DIBK-isomer, DIBK alcohol and DIBC alcohol were found to be quantifiable from the DIBK test material while DIBC, DIBKisomer, DIBK-alcohol and DIBC alcohol were quantifiable from DIBC oral administration. The current study also showed that DIBC and DIBK, and their minor isomers are well interconverted in rats. Both DIBC and DIBK have similar metabolic pathways to form the major common metabolites (DIBC-alcohol and DIBK-alcohol). DIBC isomer was quantitatively converted to DIBK isomer, with no subsequent metabolites identified at the current dose level. Both sets of major and minor test material isomers are rapidly absorbed

following oral administration, with substantial evidence of enterohepatic recirculation of DIBK, DIBC and several of their metabolites. Quantitatively, DIBC isomer and DIBK isomer have comparable systemic bioavailability, with complete conversion of DIBC isomer to DIBK isomer, resulting in similar blood AUC values for the DIBK isomer arising from either of these minor test materials. In contrast, DIBK was shown to have higher bioavailability (AUC) than DIBC after oral administration. This higher bioavailability may be due to lower absorption of DIBC or a higher rate of Phase II conjugation of DIBC and metabolites, followed by biliary elimination. In summary, DIBK and DIBC, along with the corresponding minor isomers were interconvertible in the male rat, with comparable or lower systemic exposure of DIBC isomers than DIBK isomers.

Conclusions:

Interpretation of results (migrated information): no data
Not applicable

Executive summary:

The objective of this study was to determine the pharmacokinetics of diisobutyl ketone (DIBK), DIBK isomer (4,6-dimethylheptan-2-one), diisobutyl carbinol (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 these two test materials in the male NTac:SD rat.

After single oral dose of DIBC (containing DIBC-isomer) or DIBK (containing DIBK-isomer) in male NTac:SD rat (700 mg/kg, nominal), metabolite profiling was initially conducted with plasma samples collected at the estimated time of Cmax plasma concentrations of test material and/or metabolites (1.5 hr post-dosing). In addition to the two test materials, and their isomers, two common metabolites were identified from either test material. These metabolites (DIBC alcohol and DIBK alcohol) were monohydroxylated metabolites of DIBC and DIBK, with the site of hydroxylation at the σ and σ-1 positions, respectively.

Quantitative analysis of DIBC, DIBK, DIBC-isomer, DIBK-isomer, DIBC-alcohol and DIBK alcohol was then conducted for blood samples collected at 5 min to 120 hr after a single oral dose of either DIBC or DIBK (490-635 mg/kg). DIBK, DIBK-isomer, DIBK alcohol and DIBC alcohol were found to be quantifiable from the DIBK test material while DIBC, DIBKisomer, DIBK-alcohol and DIBC alcohol were quantifiable from DIBC oral administration. The current study also showed that DIBC and DIBK, and their minor isomers are well interconverted in rats. Both DIBC and DIBK have similar metabolic pathways to form the major common metabolites (DIBC-alcohol and DIBK-alcohol). DIBC isomer was quantitatively converted to DIBK isomer, with no subsequent metabolites identified at the current dose level. Both sets of major and minor test material isomers are rapidly absorbed

following oral administration, with substantial evidence of enterohepatic recirculation of DIBK, DIBC and several of their metabolites. Quantitatively, DIBC isomer and DIBK isomer have comparable systemic bioavailability, with complete conversion of DIBC isomer to DIBK isomer, resulting in similar blood AUC values for the DIBK isomer arising from either of these minor test materials. In contrast, DIBK was shown to have higher bioavailability (AUC) than DIBC after oral administration. This higher bioavailability may be due to lower absorption of DIBC or a higher rate of Phase II conjugation of DIBC and metabolites, followed by biliary elimination. In summary, DIBK and DIBC, along with the corresponding minor isomers were interconvertible in the male rat, with comparable or lower systemic exposure of DIBC isomers than DIBK isomers.

Description of key information

A metabolism study for DIBK (in vivo) is available.





  

Data on the metabolism of DIBK and DIBC are available (in vivo). In addition to this there are data available for the category members MIBK and MIBC






 
  

Data on category members DIBC, MIBK and MIBC are also avai

Key value for chemical safety assessment

Bioaccumulation potential:

low 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 (C_max) of 0.644 mmol/L occurring at 0.25 hours [(time to maximum plasma concentration (t_max)] 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 C_maxof 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 (AUC_{0-12 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.