Isobutyl acetate

Endpoint summary

Description of key information

Isobutyl acetate is readily absorbed after inhalation exposure. As taken from data on a structural analogue n-butyl acetate, dermal absorption is low. After absorption fransfer from blood into tissues is easy and distribution throughout the body is assumed. Esterases mediate hydrolysis to metabolites isobutanol and acetic acid. This process is fast, with half lifes in blood being comparable to results for the structural analogue n-butyl acetate (i.e. half life less than one minute). Subsequnetly isobutanol is converted to isobutyraldehyde and isobutyric acid, which will ultimately be excreted as CO2.

Additional information

Summary of results and conclusions (details see under toxicokinetics)

 

Isobutyl acetate is rapidly absorbed and distributed in blood during/after inhalation exposure (OPP/ACC, 2004). It is assumed that the same is valid for oral administration. Tissue/blood partition coefficients indicate easy transfer from blood into tissues (Kaneko, 1994). Isobutyl acetate is first metabolized in blood and in tissues by esterases to isobutanol and acetate (Dahl, 1997; OPP/ACC, 2004; Römmelt, 1986). In following steps, isobutanol is enzymatically oxidized to isobutyraldehyde and isobutyric acid by alcohol dehydrogenase and aldehyde dehydrogenase respectively (OPP/ACC, 2004; Römmelt, 1986). Isobutyric acid will subsequently be utilized in intermediary metabolic pathways (tricarboxylic acid cycle) and ultimately excreted as CO2.

 

Accordingly, it has been demonstrated with 14C labeled isobutyric acid (label at C-1), that between 90 and 97% of an orally administered dose was excreted as CO2 in expired air within 48 hours, In urine, 3.2 to 4.6% of radioactivity was excreted. Fecal radioactivity was less than 1% of the dose (DiVincenzo and Hamilton, Toxicol Appl Pharmacol 1978, 47, 609).

 

The levels of ester, alcohol, and acid in blood and clearance rates/half-lifes have been determined more frequently using n-butyl acetate than isobutyl acetate. Combined evidence shows that rates of hydrolysis and clearance from blood are fast and in the same order for both substances.

 

In vitro, half-lifes of 77 and 67 nmol/mg S-9 protein/min have been determined with ethmoturbinate S-9 preparations for isobutyl acetate and n-butyl acetate respectively (Dahl 1987). In human serum samples, in vitro half-lifes of ca. 100 min for isobutyl acetate and ca. 88 min for n-butyl acetate have been determined (Römmelt, 1986). The reaction rate/half-life in vitro of isobutyl acetate can be estimated to be slightly lower/higher (ca. 15%) compared to reaction rates/half-life of n-butyl acetate as would be expected from their in chemical structure. Characteristics in vivo can be concluded not to be substantially different.

 

Half-life and clearance rates from blood in vivo for the ester, alcohol and acid have only been determined for the n-butyl acetate series. n-Butyl acetate is cleared from blood in vivo with half-lifes in a range from as few seconds to some minutes (Essig, 1989; Teeguarden, 2005). Clearance rates/half-lives of butanol and butyric acid are slightly longer in the range of a few to a couple of minutes with butyric acid having the longest half-life.

 

For isobutyl acetate, it is estimated that hydrolytic cleavage in blood and tissues is comparably fast falling in the range of minutes. Reaction rates of alcohol dehydrogenase and aldehyde dehydrogenase for members of the isobutyl series of metabolites will be in the same order as for members of the n-butyl series. This is evidenced by similar concentration patterns of ester, alcohol and acid in blood (OPP/ACC, 2005, Teeguarden, 2005).

 

In conclusion, following absorption, hydrolytic cleavage of the parent ester isobutyl acetate to isobutanol and acetic acid will be fast (in the order of minutes) resulting in higher levels of isobutanol than isobutyl acetate in blood. Clearance of isobutyl acetate from blood is very fast. Thus isobutyl acetate is present in blood only for a limited time.

 

Based on these findings, isobutylalcohol can be considered as intermediate species in the mode of action of isobutyl acetate. Thus isobutanol will be used as supporting substance in the evaluation of the systemic toxicity of isobutyl acetate in cases when data for isobutyl acetate are not available.

 

Toxicokinetics

 

For isobutyl acetate, toxicokinetic data are available on hydrolysis rates in vitro (hydrolysis by esterases in rat ethmoturbinate preparations) (Dahl, 1987), on in vitro hydrolysis in human blood (Römmelt, 1986), on tissue blood partition coefficients in vitro (Kaneko, 1994, and on blood levels in vivo during closed chamber inhalation of rats (OPP/ACC, 2004).

 

In addition, further results on toxicokinetics are presented for n-butyl acetate as supporting substance (Teeguarden, 2005; Groth, 1991). Isobutyl acetate and n-butyl acetate are very similar in structure and chemical properties. Both are acetic acid esters of C-4 alcohols which differ only in branching (isobutyl: branching at C2, n-butyl: linear). According to structure and chemical class, basic properties and chemical behavior are quite similar. Both are metabolized following a similar reaction scheme as will be demonstrated by the following data. Small differences in steric and electronic properties may result in slightly different binding constants and reaction rates. Based on their similarity, it is justified to use n-butyl acetate as supporting substance for isobutyl acetate with respect to toxicokinetics and metabolism.

 

The absorption of isobutyl acetate was only examined in one study. For the inhalation exposure of human subjects (n = 10) with 37 ppm and 103 ppm isobutyl acetate, absorption quotients of 73% and 68% were determined (individual exposure of resting male adults in a closed chamber, absorption quotient is calculated from the ratio of the isobutyl acetate concentrations in exhaled to inhaled air after 90 min of exposure). Further evidence for easy absorption of isobutyl acetate arises from inhalation experiments where isobutyl acetate is found in blood at peak levels 10 min after start of exposure (OPP/ACC, 2004).

Regarding the dermal absorption of Isobutyl Acetate no data have been identified. As there are two studies available regarding the dermal absorption of n-Butyl Acetate which is very similar in structure and chemical properties as described above those studies are used to evaluate the dermal absorption of Isobutyl Acetate.

Absorption of n-butyl acetate after dermal application seems to be low as can be concluded from the permeability constant for n-butyl acetate (1.6 +/- 0.1 g/m2*h or 1.8 +/- 0.1 cm3/m2*h), which was obtained in vitro using human skin (Ursin et al., 1995). Similar results were obtained in vitro using pig-skin (flux = 0.47 +/- 0.07 mg/cm2/h; Rauma, 2009).

 

Specific investigations of the distribution of isobutyl acetate have not been identified. Isobutyl acetate is considered to distribute rapidly with blood over the whole body. Partition coefficients between different tissues and blood have been determined (Kaneko, 1994).

 

 

Table 1  Partition coefficients for rat tissue/blood (calculated as K(tissue/air)/K(blood/air) from experimentally determined K-values)

 

	Liver/blood	Kidney/blood	Brain/blood	Muscle/blood	Fat/blood
isobutyl acetate	5.06	4.08	2.65	2.12	21.3
n-butyl acetate	3.14	2.72	1.85	1.76	17

 

 

Data for n-butyl acetate have been included to demonstrate the similar behavior between the two compounds. Data suggest that isobutyl acetate behaves slightly more lipophilic than n-butyl acetate.

 

The partition coefficients indicate that isobutyl acetate will pass from blood to tissues with a preference for fat tissue.

 

Metabolism of isobutyl acetate is characterized by the hydrolytic cleavage of the parent compound by esterases to isobutanol and acetate/acetic acid. This occurs in different tissues and in blood (Dahl, 1987; Römmelt, 1986). Acetate is transformed to Acetyl coenzyme A and introduced into intermediary metabolic pathways, or it is excreted. Isobutanol is further metabolized to isobutyraldehyde by alcohol dehydrogenase in an oxidative process followed by transformation of isobutyraldehyde to isobutyric acid by aldehyde dehydrogenase via a second oxidation.

 

For isobutyl acetate and n-butyl acetate the presence of the metabolites iso/n-butyl alcohol and iso/n-butyric acid has been verified in various experiments. After exposure of experimental animals or humans to iso- or n-butyl acetate, the respective alcohol and carboxylic acid have been detected in the blood of the test subjects.

 

 

Table 2  Blood levels of ester, alcohol, and acid after inhalation exposure to isobutyl acetate of n-butyl acetate

 

	Acetate		Alcohol		Acid
	Peak level	Termination	Peak level	Termination	Peak level	Termination
Closed chamber inhalation 2000 ppm (nominal), rat (OPP/ACC, 2004) Isobutyl-series  [µM]	10 min 31.5	90 min 7.6	20 min 101.6	90 min 10.6	20 min 12.52	90 min 6.2
Closed chamber inhalation 1400 ppm (nominal), rat (Teeguarden, 2005) n-Butyl-series  [µM]	20 min 16.3	120 min 4.2	20 min 158	120 min 11	nd	nd
(tracheal intubation) 1021 ppm (measured), rat (Groth, 1991) n-Butyl-series  [µM]	36 min 34	300 min 20	36 min 65	300 min 48	-	-
Chamber inhalation 38.4 ppm, human (Römmelt, 1986) n-butyl-series  [µM]	average 0.13 (0.10 - 0.16)		average ca. 0.16 (0.12 - 0.19)		-

 

nd = not detectable

 -  = not analyzed

 

 

In none of the experiments, the respective aldehyde was detected. It seems that the kinetics of the reactions involved are in favor (comparatively fast) for the formation of the acid thus preventing the appearance of the aldehyde in blood.

 

It is obvious, that the metabolic cleavage of the parent acetate is fast resulting in higher levels of alcohol in blood compared to ester concentrations (ca. 2- to 10-fold).

 

For isobutyl acetate and n-butyl acetate half lifes in human serum in vitro (Römmelt, 1986) and reactions rates in tissue homogenates (Dahl, 1987) have been determined.

 

Rates of hydrolysis with rat ethmoturbinate S-9 preparations and half-lifes in human and rat blood/serum of isobutyl acetate and n-butyl acetate

 

 

Table 3  Hydrolysis rates of S-9 preparation from nasal cavity tissue and half-lifes in human serum/blood in vitro

 

	Hydrolysis rate [nmol/mg S-9 protein/min]	Half-life (in vitro) [min]
	(Dahl, 1987)	(Römmelt, 1986)		(Essig, 1989)
	S-9 preparation from ethmoturbinates	Human serum TS 1.29 mM	Human serum TS 6.46 mM	Human blood	Rat blood
Isobutyl acetate	67	100	165	-	-
n-Butyl acetate	77	88	100	12	4

 

 - = not examined

 

 

With other substrates (pentyl acetate and phenyl acetate) it has been demonstrated that liver homogenate exhibits the highest hydrolysis rate out of the tested rat tissues (liver, lung, trachea, maxilloturbinates, ethmoturbinates). For rabbit and hamster, ethmoturbinate and maxilloturbinate S-9 preparations showed hydrolysis rates similar to or even higher than liver S-9 preparations (Dahl, 1987). It can be concluded that acetate esters are already hydrolyzed to a considerable degree in the nasal cavity during inhalation exposure.

 

Half-lifes in vivo have been determined only for n-butyl acetate in rats either after iv bolus injection of 0.282 mmol/kg bw via the femoral vein (Teeguarden, 2005) or after a 1 hour inhalation exposure to 7000 ppm (Essig, 1989) via a tracheal cannula.

 

After iv bolus injection, the blood concentration time course of n-butyl acetate, butanol, and butyric acid was observed. Data indicate at least biphasic clearance characteristics for all three substances. The 1 hour inhalation exposure resulted in nearly constant blood levels of n-butyl acetate within 1 min (140 µM). The hydrolysis product butanol amounted to 480 µM within 40 min. Clearance of n-butyl acetate was very fast (complete disappearance within 1 min after termination of exposure). For the elimination of butanol from blood, a half-life of 5 min was determined. Butyric acid could not be detected.

 

 

Table 4  Half-lifes (clearance) of n-butyl acetate and metabolites in vivo in rats after iv bolus injection of n-butyl acetate or inhalation exposure via tracheal intubation

 

	iv Bolus injection (Teegarden, 2005)		Tracheal intubation (Essig, 1989)
	Half-live 1st [min]	Half live 2nd [min]	Half-live [min]
n-Butyl acetate	ca. 0.33	ca. 2	< 1
n-Butanol	ca. 1.5	ca. 3.5	5
n-Butyric acid	ca.1.5	ca. 10	nd

 

nd = not detectable

 

 

Data on excretion of isobutyl acetate have not been identified. But from the data on metabolism, it can be concluded that isobutyl acetate is rapidly and completely cleaved into isobutanol and acetate followed by secondary metabolism of isobutanol to isobutyric acid and acetate to acetyl Coenzyme A. Excretion products will result from secondary metabolic pathways.