Isobutyl methacrylate

Endpoint summary

Description of key information

The NOAE(C)L for systemic effects of n-butyl methacrylate was 120 mg/kg bw/d in a 90-day subchronic oral toxicity study in rats and 1891 ppm (11175 mg/m³) in a 28-day subacute inhalation toxicity study in rats. The NOAE(C)L for local effects was 1832 mg/m³ (310 ppm) in a 28-day subacute inhalation toxicity study in rats. By category read-across these data were regarded as relevant for i-BMA also.

Key value for chemical safety assessment

Repeated dose toxicity: via oral route - systemic effects

Endpoint conclusion

Dose descriptor:

NOAEL

120 mg/kg bw/day

Study duration:

subchronic

Species:

rat

Repeated dose toxicity: inhalation - systemic effects

Endpoint conclusion

Dose descriptor:

NOAEC

1 832 mg/m³

Study duration:

subacute

Species:

rat

Additional information

Data availability

No relevant repeated dose data are available for i-BMA. The required information for the assessment of i-BMA is obtained by category read-across. For n-BMA a combined repeated dose and reproductive toxicity screening study (OECD 422) is available and subchronic toxicity study (OECD 408), both by the oral route. In addition, n-BMA has been tested by inhalation in a subacute inhalation study (OECD 412). A chronic study does not have to be performed, because the substance is rapidly metabolised, and by analogy to MMA ultimately metabolised to carbon dioxide and water. Therefore, there is no concern for lesions due to accumulative toxicity. Since the esters of the category, including i-BMA, are rapidly metabolised with relatively short half-lives in the body it is a strong possibility that the toxicity of the primary hydrolysis products to may contribute to the observed toxicity profile of the parent ester. For this reason repeated dose study data available for the acid and alcohol metabolites are presented in the form of a summary and discussed.

ORAL

n-Butyl methacrylate was studied for oral toxicity in rats in an OECD combined repeat dose and reproductive/developmental toxicity screening test at doses of 0, 30, 100, 300 and 1000 mg/kg/day (Ito, 1998).

Local effects

There were no local effects in the oral cavity or in the intestinal tract – the site of first contact on oral dosing or any other tissue investigated.

Systemic effects

At 1000 mg/kg relative kidney weights were increased. Urinary examination revealed increases in ketone bodies and occult blood, and haematological and blood chemical examinations showed increases in prothrombin time and urea nitrogen and atrophy of the red pulp in the spleen was also observed histopathologically.Absolute and relative weights of the spleen were decreased at a dose of 100 mg/kg or more, and relative kidney weights were increased at a dose of 1000 mg/kg. Histopathological examination revealed atrophy of the splenic red pulp at doses of 100 mg/kg or more. The kidney showed no histopathological abnormalities attributable to the test substance. The NOELs for repeat dose toxicity are considered to be 30 mg/kg/day for males and 300 mg/kg/day for females. This study has been independently reviewed (RSA, 2008). Based on published historical control data from the same test laboratory, effects seen at 100 and 300 mg/kg/d were not statistically significant when compared with historical controls. Hence, the NOAEL of this study is 300 mg/kg/d.

Body weight effects

In both males and females, there were weight gain depression and a decrease in food consumption at a dose of 1000 mg/kg. The NOAEL for body weight effects was 300 mg/kg.

Summary (OECD 422)

In conclusion, the oral administration of n-butyl methacrylate by gavage in an OECD 422 study revealed toxicologically relevant signs of systemic toxicity at the high dose level of 1000 mg/kg bw/day, The NOAEL in this study was 300 mg/kg body weight/day in both males and females.

 

In a more recent study conducted according to OECD TG 408 in compliance with GLP, n-butyl methacrylate was administered daily to male and female Wistar rats by gavage at dose levels of 0, 60, 120 and 360 mg/kg body weight per day over a period of 3 consecutive months (BASF, 2009). Control and high dose groups consisted of 15 animals per sex per group, whereas low and mid dose groups consisted of 10 animals per sex per group. After 3 months of treatment, 10 animals per sex of all dose groups were sacrificed (main groups). The remaining 5 animals per sex of control and high dose groups were maintained for another 28 days without administration of the test substance (recovery groups).

Local effects

There were no local effects in the oral cavity or in the intestinal tract – the site of first contact on oral dosing. On the other hand, multifocal degenerative and regenerative olfactory epithelium of the nasal cavity was observed in high dose (360 mg/kg bw/day) and mid dose (120 mg/kg bw/day) males and females. Multifocal degeneration/regeneration of olfactory epithelium was observed in males (5 out of 10) and females (7 out of 10). At 120 mg/kg bw/day, multifocal degeneration/regeneration of olfactory epithelium was observed in males (4 out of 10) and females (2 out of 10). At 60 mg/kg bw/day, no substance-related adverse findings were observed. However, considering the study design, it is not possible to determine if this effect is of a systemic or local origin. The NOAEL for these effects was 60 mg/kg body weight/day in both males and females. This substance-related effect was completely reversible, as no animal of the recovery group showed any finding in the nose after 28 days after cessation of exposure.

Systemic effects

At 360 mg/kg bw/day, prothrombin time was significantly prolonged (+10% males; +12% females) and inorganic phosphate (+14% males; +30% females), total bilirubin levels (+17% in males), glucose levels (+13% in females) as well as urea levels (+36% males; 21% females) significantly increased. Calcium (-3% in males and females), globulin levels (-4% in females) and triglyceride levels (-24% in males and females) significantly decreased. Absolute kidney weight (+11% in females), relative kidney weight (+13% males and females) and liver weight (+9% males; +11% females) significantly increased. After the recovery period, phosphate levels (+7% in males) and urea concentrations (+22% in females) were significantly increased. The NOAEL for systemic effects was 120 mg/kg body weight/day in both males and females.

Body weight effects

At 360 mg/kg bw/day, body weight change significantly decreased in male animals from day 77 onwards (-12.1% on days 84 and 91). The NOAEL for body weight effects was 360 mg/kg body weight/day in females and 120 mg/kg body weight/day in males.

Summary (OECD 408)

In conclusion, the oral administration of n-butyl methacrylate by gavage over a period of 3 months with a recovery period of 28 days revealed toxicologically relevant signs of systemic toxicity at the high dose level of 360 mg/kg bw/day, limited to effects on the liver activity (increased liver weight, prolonged prothrombin time, lower serum globulin and triglyceride levels in males and/or females) and kidneys weight (increased absolute weight in females). The NOAEL for systemic toxicity was 120 mg/kg body weight/day in both males and females.

Metabolites

MAA: No data available.

Iso-Butanol: An oral gavage subchronic (90-day)study has also been reported for isobutanol (Toxicology Research Laboratories, 1987). Groups of thirty male and female rats received isobutanol via oral intubation with dose levels of 0, 100, 316, or 1000 mg/kg bw/day for 90 days. Clinical signs related to treatment with 1000 mg/kg dose level included hypoactivity, ataxia, and salivation. Clinical signs of hypoactivity and ataxia were resolved by the 4th week of the study. Slight decreases in feed consumption and body weight gains were noted in the first two weeks and were restricted to the 1000 mg/kg/day group. There were no changes in organ weights or gross or histopathology at any exposure level. The NOAEL was 316 mg/kg bw/day (OECD SIDS, 2004).

DERMAL

There are no relevant dermal repeated dose studies for systemic effects. For assessment purposes the oral data are used.

Metabolites

n-Butanol:In a dermal study, 42 to 55 ml/kg applied to the skin of rabbits each day for 1 to 4 consecutive days resulted in 100% mortality; however, 30 applications of 20 ml/kg over a period of six weeks did not produce any fatalities (Patty, 1982). (OECD SIDS, 2001.

MAA: For methacrylic acid a limited, repeated dose skin irritation study exists, where dilutions of MAA in water (0, 5%/0.56mM) and acetone (0, 5%/0.56mM, 10%/1.12mM, 19%/2.24mM) have been applied (3/wk for 3 weeks) to the skin of male mice.Based on an average body weight of 32 g and an application volume of 100 µl/mouse, the dermal doses correspond to approx. 150, 300 and 600 mg/kg.5% MAA in water produced and vehicle controls produced no irritation while 5% MAA in acetone produced only very slight irritation from the third application and 10 and 20% MAA in acetone produced moderate or severe irritation from the first application. Systemic effects other than clinical signs and changes in body weight have not been assessed.

INHALATION

In an OECD Guideline 412 Repeated Dose 28-day inhalation study, 10 male and 10 female rats were exposed by whole body to 0, 310, 952 and 1891 ppm (0, 1832, 5626, 11175 mg/m³) n-BMA for 6 hr/day, 5 days/week for 4 weeks.

Local effects

Treatment-related effects included lacrimation, eye squinting, and laboured breathing in the 952 and 1891 ppm (5626 and 11175 mg/m³) concentration groups throughout the study. The only treatment-related histopathological finding was localised bilateral degeneration of olfactory epithelium lining the dorsal meatus of the nasal cavity at 952 and 1891 ppm (5626 and 11175 mg/m³) in both sexes. Based on histopathological changes seen in the nasal cavities, the no-observable adverse effect concentration (NOAEC) for local effects was 310 ppm (1832 mg/m³).

Systemic effects

Haematological measurements and clinical chemistry values generally were unaffected by treatment. Increased relative kidney weights were observed at the high concentration (1891 ppm/11175 mg/m³) in both sexes, and slight increases in serum BUN values (resulting in increased BUN: creatinine ratio). In the absence of corresponding histopathological changes this was not regarded as an adverse substance-related effect. The no-observable adverse effect concentration (NOAEC) for systemic effects was 1891 ppm.

Body weight effects

There were no treatment-related effects on body weight or feed consumption, and no deaths occurred.

Metabolites

MAA: In an OECD 413, 90-day vapour inhalation study in Sprague Dawley rats with MAA revealed general toxicity at 350 ppm (1253 mg/m3) in male animals. Local, marginal irritation of the respiratory epithelium in the nasal cavity was observed in two female animals. No changes in sexual organs or sperm mobility and sperm head counts were noted. The NOAEL was 100 ppm (358 mg/m3) for local irritation effects in male and females The NOAEL for systemic effects based upon reduced body weight gain in the presence of reduced feed intake but no other systemic effects was also 100 ppm (358 mg/m3) in male and females.

Iso-Butanol: In a 13-week inhalation studies with Sprague-Dawley rats exposed to 0, 250, 1,000 or 2,500 ppm (0, 760, 3,030, or 7,580 mg/m3) (Branch, et al., 1996). This study included expanded neurotoxicity endpoints (functional observational battery, motor activity, scheduled-control operant behavior, and neuropathology endpoints) as well as the standard parameters for subchronic studies. Intensive investigations of testicular parameters (homogenization-resistant spermatid head counts) were collected at necropsy. The highest exposure concentration (2500 ppm; 7,580 mg/m3) did not have any adverse effects demonstrating a persistent or progressive effect of isobutanol on the central or peripheral nervous system. A slight reduction in responsiveness to external stimuli was noted during exposure. Slight increases (9%) in red blood cell parameters (count, hematocrit, hemoglobin) were noted in female rats exposed to 2500 ppm (7,580 mg/m3) but the slight nature of these findings made them of questionable biological significance. There were no changes in any other parameters. The NOAEL for neurotoxicity was 2,500 ppm (7,580 mg/m3). The NOAEL for repeated-dose toxicity was 1,000 ppm (3,030 mg/m3) (OECD SIDS, 2004).

Human data

There are no relevant studies in humans.

Summary

 

For the assessment of the repeat dose toxicity of LAME esters there are subchronic and chronic (MMA) data available for three members – MMA, nBMA and 2-EHMA, iBMA is discussed by read-across of nBMA.

Methacrylate esters are hydrolysed to the irritant and corrosive MAA metabolite NOAELs for local and systemic effects re relevant to the repeat dose toxicity assessment of these chemicals.

 

Local effects

Local effects only observed at very high doses in oral studies whereas they are the lead effect for the short chain esters in inhalation studies. Local effects in the nose are observed with nBMA but only at high concentrations indicating a trend of reducing toxicity with increasing chain length across the category.

 

Systemic effects

Systemic toxicity is limited to non-specific toxicity, i.e. effects on body weight gain etc., with no specific target organ effects other than organ weight increases unsupported by corresponding changes in histopathology. Furthermore, there appears to be no marked increase in sensitivity for this non-specific toxicity with time. The NOAEL for systemic effects for the three substances (MMA, n-BMA and 2-EHMA) are in the order of 100-200 mg/kg/d although when expressed on a molar basis there is a clear trend of increasing toxicity with ester size across the category.

 

Metabolites

MAA: In a subacute skin irritation study with MAA at concentrations of 5, 10 and 19 %indicates that severe local effects occur at concentrations higher than 5 % in acetone and water (corresponding to approx.150 mg/kg ). In the case of MAA the systemic NOAEL in a 90 day inhalation study was 100ppm (equivalent to approx. 32mg/kg) based upon reduced body weight gain secondary to nasal irritation and reduced uptake of food and in the absence of other systemic effects.

Alcohols: In the repeat dose studies the local effects are observed in the same dose range as the systemic effects, which are higher than the NOAELs of the corresponding esters on a molar basis. There is a trend that the difference is more pronounced with the smaller alcohols.

Discussion

Local effects

The available repeat dose toxicological data on LAME esters are consistent with the findings from acute studies indicating that local effects at the site of entry are predominantly due to a combination of the irritant action of the parent ester and the acid metabolite MAA. In this regard there appears to be a general trend of decreasing irritancy across the category with increasing molecular weight. In the case of the dermal route of administration, and to a lesser extent oral administration, this trend is likely to be due to the measured decrease in rate of skin absorption as a result of the lower hydrophilicity higher molecular weight of the larger category members. In addition to toxicokinetics differences, toxicodynamic differences, such as the longer half-life of the larger esters, may contribute to a lesser degree to this observed trend.  In the case of Inhalation the primary irritation potential of the esters is less than the acid which more greatly affects the anterior region of the nose where the concentration is greatest. In contrast, the esters are extracted more efficiently in posterior (olfactory) region of the nasal cavity where they are subsequently hydrolysed by non-specific carboxylesterases. This localised effect is the result of TK and TD factors (very efficient extraction from the airstream combined with localised high levels of carboxylesterase activity in the Bowman’s glands) which favour localised high concentration of the metabolites and tissue damage due to the acid metabolite. In the case of MMA, the ester for which the most extensive inhalation data is available, there appears to be little or no decrease in NOAEC from acute exposures of 6 hrs to two years. In terms of the NOAEC for the development of this lesion one might expect a lowering of the NOAEC with increasing ester size. Data available for MMA and n-BMA tend to support this hypothesis as well does the acute inhalation data for MMA, EMA, n-BMA, and i-BMA.

 

Severe local effects due to the corrosive acid metabolite are not observed by the dermal and oral routes as absorption is slower and esterase activity within the barrier tissue lower and more diffuse.

 

Systemic effects

In general the LAME only show general signs of repeat-dose toxicity with no Specific Target Organ Toxicity (STOT). The lead systemic effect is reduced body weight gain and other organs systems are only involved at high doses. There is some evidence of a trend of increasing toxicity with increasing ester size across the category, particularly if the NOAELs are expressed on a molar basis, but this is probably only representing increased metabolic burden rather than any specific effects.

 

Metabolites

This picture of overall general toxicity with lack of specific STOT is consistent with the toxicity profiles of the primary metabolites, MAA and the respective alcohols.

MAA: MAA appears not to have any specific STOT within the range of doses that have been studied and higher systemic levels cannot be achieved due to the severity of the local effects. Indeed the systemic burden of MAA is perhaps the greatest when it is delivered in the form of the methyl ester, for which no STOT is observed.

Alcohols

In general the alcohols are metabolised relatively rapidly with a significant portion the more volatile ones being excreted un-metabolised in urine and/or exhaled air. Since the NOAELs for the alcohols are higher than the NOAELs for the respective ester in comparable studies any high dose substance-specific alcohol STOT will not be expressed in the relevant toxicity range of the esters.

In the case of ethanol its widespread consumption over many years has led to the development of an extremely large toxicological database that indicates that it is generally of low toxicity and unlikely to contribute to the observed systemic toxicity profile of the parent ethyl ester. In the case of methanol, there is no STOT of noteworthiness within the toxicological dose range relevant to the parent ester MMA. For the butanols, the repeat dose NOAELs for BuOH and iBuOH are very comparable and there is no marked difference toxicity profile despite the not sharing the same last metabolic step. Finally, 2-Ethylhexanol does not demonstrate any STOT around the LOAEL as liver effects only occur at very high doses outside the dose range relevant to the parent ester.

 

Confidence in the category and justification of interpolation

There are reliable repeat dose studies for the extremes of the category (MMA and 2-EHMA) and one intermediate (n-BMA). The lower alkyl methacrylate category represents a closely related group of chemical structures that share common chemical reactivity with consistent trends in key physical chemical as well as toxicokinetic and toxicodynamic properties.

In the case of iBMA the local and systemic no-effect levels have been derived by read-across to nBMA. Read across between these esters is justified on the basis that the branched butyl ester (i-BMA) is a close structural analogue of the linear n-BMA ester sharing a common, and comparable level of, chemical reactivity (Michael addition) consistent with their comparable mammalian- and eco-toxicity profiles. It is recognised that in some chemistries branching of the alkyl moiety does increase toxicity. However, in the case of the n- and iso-butyl esters it is known that their chemical reactivity (predominantly Michael addition) is not greatly affected by presence of the branched methyl group is sufficiently distant from the reactive center that it does not cause steric hindrance. Furthermore, the half-life of n- and iso-butyl esters is very comparable and short. Furthermore, neither butanol demonstrates any STOT that needs to be considered.

On this basis there is a high degree of confidence in the interpolation and read-across to fill the data gaps for i-BMA.

Repeated dose toxicity: inhalation - systemic effects (target organ) respiratory: nose

Justification for classification or non-classification

According to the available data and CLP criteria for classification, no classification is warranted for specific target organ toxicity — repeated exposure. Based on the available information, the potential of isobutyl methacrylate for systemic toxicity after repeated dosing is low and no severe, irreversible organ effects have been observed in the relevant dose range. Hazards based on local effects were covered by the classification for the irritation potential on skin and respiratory tract (see chapter 5.3). Therefore, no additional classification is considered as justified.