A mixture of RR and RS isomers of: (2-(2-methoxy-1-methyl)ethoxy)-1-methylethyl acetate; (2-(2-methoxy-2-methyl)ethoxy)-1-methylethyl acetate; (2-(2-methoxy-2-methyl)ethoxy)-2-methylethyl acetate; (2-(2-methoxy-1-methyl)ethoxy)-2-methylethyl acetate

Endpoint summary

Description of key information

Short description of key information on bioaccumulation potential result: 
An in vitro hydrolysis study with dipropylene glycol methyl ether acetate (DPMA) in rat plasma and liver S9 is available along with a limited probe study to determine absorption, metaboliam and elimination following 4 daily repeated oral gavage administrations at either 329 and 659 mg/kg/day in female NTac:SD rat and another study in Crl:CD(SD) rats following repeated gavage administration over gestation days 19-21 to compare the toxicokinetic profile and systemic exposure of DOWANOL™ DPMA Glycol Ether Acetate and the primary metabolites, DPM (dipropylene glycol monomethyl ether; DPGME) and MPA (2-methoxypropionic acid).
 In addition, data from structurally related substances - propylene glycol methyl ether (PM), propylene glycol methyl ether acetate (PMA) and dipropylene glycol methyl ether (DPM) - is provided. For PM and PMA in vitro hydrolysis studies in rat and human blood and liver samples as well as in vivo blood pharmacokinetic studies (in rats) after intravenous and dermal administration were conducted. For PMA and DPM in vivo metabolism and disposition studies in rats are available.
Further, the in vitro hydrolysis of several glycol ether acetates (Ethylene glycol methyl ether acetate (EGMEAc), Ethylene glycol ethyl ether acetate (EGEEAc), Ethylene glycol butyl ether acetate (EGBEAc) and Propylene glycol methyl ether acetate (PGMEAc)) was investigated in vitro with pooled rat plasma samples. Additional support is available from a published article (Deisinger and Guest, 1989) on the hydrolysis of diethylene glycol butyl ether acetate (DEGBEA) to DEGBE.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Absorption rate - oral (%):

100

Absorption rate - dermal (%):

30

Absorption rate - inhalation (%):

100

Additional information

Propylene glycol methyl ether (PM), propylene glycol methyl ether acetate (PMA), dipropylene glycol methyl ether (DPM) and dipropylene glycol methyl ether acetate (DPMA) are closely related in molecular structure and physicochemical properties and thus, the potential for toxicological effects. They are all liquids with similar boiling points, low to moderate volatility, and high water solubility. Increasing boiling point and vapor pressure are consistent with increasing molecular weight over the series.

Both oral and inhalation absorption rates were set at 100%. For detailed information, refer to read-across justification document for P-series glycol ethers. Dermal absorption rate was based on DPM data and set at 30%, a conservative assumption.

 

As a class, the propylene glycol methyl ethers are rapidly absorbed and distributed throughout the body when introduced by inhalation or oral exposure. Metabolism studies (by oral exposure), conducted with PM, PMA and DPM, support this conclusion. Metabolism of propylene glycol methyl ethers takes place predominantly in the liver by mixed function oxidase which cleaves the ether linkage (O-dealkylation) yielding (mono- or di-) propylene glycol and the alkyl alcohol. These products are further metabolised to water and CO2, with the latter ultimately being excreted in expired air. The parent ether (or intermediate metabolite) may also be conjugated in the liver with glucuronide, sulfate, or glutathione for ultimate excretion, predominantly in the urine. A higher proportion of PM and PMA was eliminated via the lungs as opposed to the urine than the larger molecular weight DPM. While not tested directly, absorption by inhalation exposure also would be expected to be rapid for vapors of propylene glycol methyl ethers and for aerosols that are in the respirable range. Dermal absorption would be expected to be somewhat slower but, once absorbed, subsequent distribution also should be rapid. Most excretion for propylene glycol methyl ethers is via the urine and expired air. A small portion is excreted in the feces.

 

It can be assumed that two steps are involved in the in vivo metabolism of DPMA: ester hydrolysis followed by further metabolism of DPM. The latter is very well investigated in vivo (Miller et al., 1983). Male Fischer 344 rats were given a single oral dose of 14C-DPM. After dosing, expired air, excreta and tissues were analyzed for 14C activity and metabolites in urine were isolated and identified. Approximately 60% of the 14C-DPM was excreted in urine, while 27% was eliminated as 14CO2 within 48 hours after an oral dose of 14C DPM. Less than 3% of the dose was recovered in feces, indicating that the test material was effectively absorbed. DPM, PM, dipropylene glycol, propylene glycol as well as sulfate and glucuronide conjugates of DPM were identified in the urine of animals. Therefore, DPM is apparently metabolized via the same routes to the same general types of metabolites as those for PM.

In a recent study, the metabolic conversion of dipropylene glycol methyl ether (DPM) to 2-methoxypropionic acid (2-MPA) was examined in three female CD rats and three female New Zealand White rabbits (ACC, 2006). 2-MPA was found as a measurable metabolite from both species. The relative formation of 2-MPA was highest in the rat, comprising 5.8 -12.2% of the administered dose. In contrast, substantially lower levels of 2-MPA were found in the urine of the rabbit (1.5-2.4% of administered dose). The rate of MPA formation appears to be dose proportional in the rabbit, and more than dose proportional in the rat. Excretion of 2-MPA was substantially slower in the rabbit than the rat, with calculated half-lives of 17.4-20.9 hours and 7.5-8.1 hours, respectively.

 

The tissue distribution depends on the hydrolysis rate and there is evidence that hydrolysis takes place in gut, liver and blood.

 

In a recent study, the in vitro hydrolysis rate of DPMA has been determined in rat whole blood and rat liver S9 fraction under physiological conditions and compared with its shorter-chain analog, PMA. Additionally, tissue:plasma partition coefficients of the plasma protein unbound DPMA and its possible metabolite (DPM) and PMA were estimated in situ for comparison. Hydrolysis of PMA was very rapid, both from the rat whole blood and rat liver S9 fraction without any dose dependency. Hydrolysis of both of the major isomers of DPMA was faster than PMA in whole blood. However, hydrolysis of DPMA from the rat liver S9 fractions was slower than that of PMA. Consistent with the physiochemical properties of glycol ether and acetate (high water soluble neutral molecules), plasma binding was estimated to be only 7-21%, which resulted in the tissue:plasma partition coefficient of ≤0.80 in all tissues for DPMA, DPM and PMA. Similarly, all three test materials had low volume of distribution.

 

On the basis of these results, DPMA is expected to rapidly hydrolyze once absorbed in vivo. The overall rate of hydrolysis of DPMA in biological systems is expected to be similar to that of PMA, which is supported by the similarities in log Kow, plasma protein binding, tissue:plasma partition coefficients and volume of distribution. Although, DPMA was slowly hydrolyzed by the liver S9, the overall impact of this slow hydrolysis will likely be limited due to its low tissue partition coefficient and restriction of the absorbed DPMA mostly to the blood, where it is rapidly hydrolyzed.

In another recently conducted limited probe study, the absorption, metabolism, and elimination (AME) of DOWANOL™ DPMA glycol ether acetate was evaluated in female NTac:SD rats following 4 daily repeated oral gavage administrations at 329 and 659 mg/kg/day. Time-course blood samples from the 2 animals dosed with DOWANOL™ DPMA glycol ether acetate were analyzed to determine concentrations of DPMA and the metabolites – DPM and 2-MPA. The results indicated that DOWANOL™ DPMA glycol ether acetate was rapidly absorbed without any apparent lag time based on blood concentrations of DPMA, DPM and MPA. Blood concentrations declined rapidly during the elimination phase (t½ = ~1.5-3.5 hours) for both metabolites. Kinetic parameters could not be calculated for parent DPMA due to a limited number of samples with quantifiable levels of DPMA.

 

However a comparison of estimated AUC for DPMA vs. the sum of DPM isomers plus MPA, show that ≤2% of the parent test material was systemically bioavailable. DPMA total systemic exposure, was dose proportional between the low and high dose groups. Thus, in summary, administered DOWANOL™ DPMA glycol ether acetate was rapidly absorbed and highly metabolized to DPM-1,2, DPM-2,2 isomers and 2-MPA.

 

In another probe repeated gavage administration developmetal toxicity study, groups of Crl:CD(SD) rats were orally administered by gavage DOWANOL™ DPMA Glycol Ether Acetate during gestation days 19-21 and blood samples were collected to compare the toxicokinetic profile and systemic exposure of DOWANOL™ DPMA Glycol Ether Acetate and the primary metabolites, DPM (dipropylene glycol monomethyl ether; DPGME) and MPA (2-methoxypropionic acid), for the purposes of read-across between these structurally similar molecules. Results from this study were used to aid dose level selection for a subsequent definitive DOWANOL™ DPMA Glycol Ether Acetategavage developmental toxicity study in rats.

Blood samples were collected from all dams at 7:10 a.m. (just after dosing) on GD 19, and then at 7 a.m. (just prior to dosing), at 10 a.m. and at 3 p.m. on GD 20 for calculation of daily systemic exposure (AUC24h). Blood samples were also collected at a single time point from all dams and their fetuses (pooled for each litter) at the time of terminal sacrifice (~ 9 a.m.) on GD 21. 

In summary, administered DOWANOL™ DPMA glycol ether acetate was rapidly absorbed and highly metabolized to DPM and MPA. DOWANOL™DPMA was present at concentrations above the analytical lower limit of quantitation (LLQ) in only a few blood samples from dams and fetuses. These were primarily in the higher dose groups, and only early after dosing. This was consistent with the rapid absorption and short elimination half-life of DOWANOL™DPMA. In contrast, DPM and MPA were present at quantifiable levels (≥ LLQ) in most of the blood samples from exposed animals. DPM was the major metabolite and was present in blood at all time points in adult females and fetuses, except just prior to dosing, consistent with the rapid formation and relatively short elimination half-life of DPM. MPA was present in blood at all time points in adults and fetuses at levels ~7-20% those of DPM, consistent with the slower formation and slightly longer elimination half-life of MPA. Fetus blood concentrations of DPM and MPA were generally equivalent to, or slightly higher than, dam blood concentrations. Blood levels of all analytes were dose-proportional, where measurable.

 

Thus, it is appropriate to conclude that dipropylene glycol methyl ether acetate behaves in a similar way to the parent glycol ether due to the rapid conversion.

Discussion on bioaccumulation potential result:

In a GLP-study, the in vitro hydrolysis rate of DPMA has been determined in rat whole blood and rat liver S9 fraction under physiological conditions and compared with its shorter-chain analog, PMA. Additionally, tissue:plasma partition coefficients of the plasma protein unbound DPMA and its possible metabolite (DPM) and PMA were estimated in situ for comparison.

 

Hydrolysis of PMA was very rapid, both from the rat whole blood and rat liver S9 fraction without any dose dependency resulting in half life times between 11 and 18 min. Hydrolysis of both of the major isomers of DPMA was faster (t½ DPMA-A = 10-12 min; DPMA-B = 13 -17 min) than PMA in whole blood. However, hydrolysis of DPMA from the rat liver S9 fractions was 3- to 5-fold slower (t½ DPMA-A = 40-50; DPMA-B = 58-82 min) than that of PMA. Consistent with the physiochemical properties of glycol ether and acetate (high water soluble neutral molecules), plasma binding was estimated to be only 7-21%, which resulted in the tissue:plasma partition coefficient of ≤0.80 in all tissues for DPMA, DPM and PMA. Similarly, all three test materials had low volume of distribution.

 

On the basis of these results, DPMA is expected to rapidly hydrolyze once absorbed in vivo. The overall rate of hydrolysis of DPMA in biological systems is expected to be similar to that of PMA, which is supported by the similarities in log Kow, plasma protein binding, tissue:plasma partition coefficients and volume of distribution. Although, DPMA was slowly hydrolyzed by the liver S9, the overall impact of this slow hydrolysis will likely be limited due to its low tissue partition coefficient and restriction of the absorbed DPMA mostly to the blood, where it is rapidly hydrolyzed. However it is important to note the more rapid hydrolysis of acetate esters that occur in vivo vs. in vitro.

 

In a published study with an analogue, PMA, Domoradzki et al. (2003) showed a 10-fold faster hydrolysis of the parent acetate ester of PM when this test material was given to ratsin vivothan fromin vitroincubations with blood (t_1/2= 1.6-2.3 minin vivovs. 15-16 minin vitro) (see table below). In the same fashion, Gargas et al. (2000) found no detectable acetate ester of ethylene glycol ethyl ether (EGEEA) immediately after inhalation exposure to rats (t_1/2< 1 min), while Hoffmann and Jäckh (BASF 1985) calculated a longer 10 min half life for EGEEAin vitroin plasma. Finally, Gallaher and Loomis (1975) studied the metabolic fate of ethyl acetate in both intact animals and via incubations with rat blood. These authors determined anin vitrohalf life of 65-70 min for ethyl acetatein vitro, while thein vivohalf life was substantially faster, at 5 -10 min. 

 

Test Material	In Vitro t1/2(min)	In Vivo t1/2(min)	Reference
PMA	15-16	1.6 - 2.3	Domoradzki 2003
EGEEA	10	< 1.0	BASF 1985 Gargas 2000
Ethyl Acetate	65-70	5-10	Gallaher 1975

 

The rationale for faster metabolism of these acetate esters in the whole animal has been well reviewed. It is known that there are numerous sites of esterase activity in the body, including blood, liver, skin, nasal mucosa, heart, muscle, adipose tissue and kidney (Stott 1985, McCracken 1993, Satoh 1998). Evaluation of esterase activity from any single tissue source, such as liver or blood, would therefore underestimate metabolic fate that occurs in vivo. Several groups have reviewed predictions of in vivo kinetics from in vitro liver metabolism data as well (Riley 2005, Hallifax 2010) and found thatin vitroclearance measurements for a variety of metabolic pathways can also substantially under predictin vivorates, in some cases more than 20-fold.  

 

The relative partitioning of any trace levels of DPMA to tissues, prior to hydrolysis, would be expected to be comparable to levels seen in the blood. Both the Volume of Distribution (Vd) and tissue:plasma partition coefficients are predicted to be approximately 1 for the highly water soluble DPMA isomers. As a result, this compound is expected to reside entirely in the water fraction and freely exchange between the blood and tissues (Lemke 2007) therefore not accumulating in any tissue.   

 

In summary, it is highly likely that DPMA would undergo a high rate of hydrolysis of the acetate ester in an intact mammalian system. Rapid hydrolysis has been shown for several acetate esters (Table above) at the high doses routinely used in guideline toxicity studies. Since this group of enzymes follows classic Michaelis-Menten kinetics (Stott and McKenna, 1985), it would be expected that hydrolysis rates would be maximal at low exposure levels. As such, any potential human exposure to DPMA would result in rapid hydrolysis to DPM.