Methacrylic acid, monoester with propane-1,2-diol

Endpoint summary

Key value for chemical safety assessment

Effects on fertility

Description of key information

There is screening-level information on HPMA (combined repeated dose toxicity and reproductive screening study according to OECD 422) available. This is substantially supplemented by reliable data from the reference chemicals which are either metabolites (PG) or metabolite donors (MMA = donor substance of MAA as primary methacrylic metabolite of both category substances).

Regarding fertility, a 2-generation reproductive toxicity study is available on MMA by the oral route. Below others, a continuous breeding study is available for PG (the primary alcoholic metabolite of HPMA) for the oral route. For PG only the most relevant key studies were selected for read across purposes.

 

Read across evaluation according to ECHA’s Read Across Assessment Framework (RAAF)

The metabolism from the category substances to their primary metabolites is well understood. The same is true for the further metabolism pathways of MAA and the alcohol metabolite PG, respectively (see chapter 5.2, ATSDR 1997, NTP 2004b). The endpoint specific “scientific assessment” of the read across is thus “acceptable with at least medium confidence”, see the attached Read Across Justification (2022) and Category document (2019).

Additional information

In the study with HPMA (Furuhashi, 1996), male rats (12/group) were given daily gavage doses of 0 (vehicle), 30, 100, 300 or 1000 mg/kg for 50 days including pre-mating, mating and post-mating intervals. Females (12/group) were administered the same doses for two weeks prior to mating, during mating and gestation up until day 4 of lactation (study duration of approximately 54 days depending upon time to conception). Animals were observed for clinical symptoms of intoxication daily and food consumption and body weight were monitored throughout the study. Blood samples were taken for hematological and clinical chemistry analysis at study termination. Thymus, liver kidney, testes, epididymes, and ovaries were weighed. In addition to these tissues, adrenal gland, brain, heart and spleen were fixed in 10% neutral buffered formalin solution for subsequent staining and histopathological evaluation.

Reproductive observations included observations which permit characterization of male and female fertility and fecundity. These include: number of live births and post implantation loss; number of pups with grossly visible abnormalities, number of runts; number of implantations, corpora lutea, litter size and litter weights. Copulation, fertility, implantation, gestation, live birth, delivery, and viability indices were calculated from the data. Pup sex, body weights and viability at birth through day 4 of lactation were determined; pups were autopsied at this time.

There were no effects of HPMA on any fertility related reproductive index including: estrus frequency, copulation index, number of days to conception, fertility index, length of gestation, number of corpora lutea and gestation index. Further, there were no effects of HEMA on the number of live pups born, birth index, number of dead pups, number of pups born, delivery index, live birth index. No histological changes in male and female reproductive organs were described.

The NOAELs for reproductive/developmental toxicity of HPMA were 1000 mg/kg/d, the highest dose tested, although both paternal and maternal animals evidenced reduced body weight gain, reduced food consumption and liver weight and histopathology (HPMA males only) at this dose level. An extended one generation study or a 2-generation study is not available on any of the hydroxyalkyl methacrylates.

Primary Metabolites

MAA (donor substance: MMA)

Methyl methacrylate (MMA) is the methyl ester of methacrylic acid (MMA) and is rapidly absorbed and metabolised to MAA within the body (see chapter 5.1). It therefore acts as an effective systemic delivery system for MAA avoiding the high local toxicity of MAA due to its acidity. The reference chemical for the methacrylic moiety of HEMA and HPMA, methyl methacrylate, has recently been tested in an OECD TG 416 oral two-generation reproduction toxicity study in rats, in which both, parental and F1 animals were dosed with 0; 50; 150 and 400 mg/kg bw/day (BASF, 2009)

In mid- and high-dose parental animals (150 and 400 mg/kg bw/d) temporary salivation, presumably due to a bad taste of the test substance and associated dose-related intermittent reductions of food consumption were noted. Less significant changes were noted for the F1 generation animals where the effects were limited to the high-dose group and not associated with effects on histopathology or reproductive performance.

The NOAEL for fertility and reproductive performance for the P and F1 parental rats was determined to be 400 mg/kg bw/day, the highest dose tested. The NOAEL for developmental toxicity, in the F1 and F2 progeny, of the test substance was determined to be 400 mg/kg bw/day, the highest dose tested. There were no signs of systemic toxicity other than reduced body weight gain associated with reduced food consumption, presumably due to bad palatability (NOEL 50 mg/kg bw/day for the P and F1 parental rats and LOEL of 150 mg/kg bw/day in the P parental females).

The 2-generation study with MMA provides confidence that the absence of effects seen at in the screening studies with the hydroxyalkyl methacrylates up to 1000 mg/kg/d, the highest dose tested, are a reliable indication for an absence of fertility effects in a higher tier study such as a 2 generation study. For the purpose of the risk assessment, a NOAEL for reproductive toxicity (fertility) of 1000 mg/kg/d is taken forward for hydroxyalkyl methacrylates.

PG

In a NTP continuous breeding study (1985), propylene glycol (PG) was administered to mice in the drinking water at up to 5% (w/v; = 10.1 g/kg/day). This dose had no effect on fertility of either males or females in either the first or second generation. These data are sufficient to conclude that propylene glycol is not a reproductive toxicant in males or females or in their progeny under the conditions of this study. These data are assumed relevant for assessing human hazard. No data on human reproductive toxicity were found.

Following table provides information on the NOAELs of the considered fertility studies of the category substances and metabolites, expressed as applied dosage and as amount of substance (mmol) for comparison reasons: HEMA and HPMA hydrolyses rapidly to MAA and the respective alcohol in a 1→1+1 ratio, see chapter5.2.

Table 5.9.3.1: Summary of NOAELs regarding fertility

NOAEL oral fertility	HEMA MW 130.14	HPMA MW 144.17	MMA MW 100.12	EG MW 62.07	PG MW 76.09
mg/kg bw	1000	1000	400	≥1000^a	≥10,000^b
ca. mmol/kg/d	7.7	6.9	4 (MAA^c)	≥16.1	≥130

^areference: Lambs 1985/ NTP 1984, continuous breeding study in mice

^breference: NTP 1985, continuous breeding study in mice

^cconsidering a rapid hydrolysis of MMA to MAA in the body, see chapter 5.2.

 

Conclusion

The available OECD 422 screening study with HPMA does not show any indication of reproductive effects of HPMA in OECD422 studies up to 1000 mg/kg/d, i.e. up to the threshold of systemic toxicity. Based on a complete absence of the effects in relevant dosages in multiple generation studies with the primary metabolites (MAA/MMA and PG) it can be concluded with high confidence that the parent ester HPMA is unlikely to be a reproductive toxicant in the absence of significant systemic toxicity. For the members of the category of lower alkyl methacrylates, based on studies in experimental animals, there is no evidence of selective toxicity to the reproductive system.

Based on the rapid metabolism and the consistency of the systemic effects between HPMA and its metabolites, which indicate that the metabolite data are relevant for the assessment of the effects of repeated dosing, including reproductive toxicity, the dataset on HPMA is considered to be complete.

 

Effects on developmental toxicity

Description of key information

There is screening-level information on HPMA (combined repeated dose toxicity and reproductive screening study according to OECD 422) available. This is substantially supplemented by reliable data from the reference chemicals which are either metabolites (PG) or metabolite donors (MMA = donor substance of MAA as primary methacrylic metabolite of both category substances).

Regarding developmental toxicity, there are the teratology-related parameters of the OECD 422 studies for HEMA and HPMA. In addition, full sets of developmental toxicity studies according to OECD 414 are available for MMA (rat/ inhalation and rabbit/ oral) and PG (rat and rabbit/ oral). For PG only the most relevant key studies were selected for read across purposes.

 

Read across evaluation according to ECHA’s Read Across Assessment Framework (RAAF)

The metabolism from the category substances to their primary metabolites is well understood. The same is true for the further metabolism pathways of MAA and the alcohol metabolite PG, respectively (see chapter 5.2, ATSDR 1997, NTP 2004b). The endpoint specific “scientific assessment” of the read across is thus “acceptable with at least medium confidence”.

Additional information

In the screening study with HPMA (Furuhashi, 1996), male rats (12/group) were given daily gavage doses of 0 (vehicle), 30, 100, 300 or 1000 mg/kg for 50 days including pre-mating, mating and post-mating intervals. Females (12/group) were administered the same doses for two weeks prior to mating, during mating and gestation up until day 4 of lactation (study duration of approximately 54 days depending upon time to conception). Animals were observed for clinical symptoms of intoxication daily and food consumption and body weight were monitored throughout the study. Blood samples were taken for hematological and clinical chemistry analysis at study termination. Thymus, liver kidney, testes, epididymes, and ovaries were weighed. In addition to these tissues,adrenal gland, brain, heart and spleen were fixed in 10% neutral buffered formalin solution for subsequent staining and histopathological evaluation.

Reproductive observations included observations which permit characterization of male and female fertility and fecundity. These include:number of live births and post implantation loss; number of pups with grossly visible abnormalities, number of runts; number of implantations,corpora lutea, litter size and litter weights. Copulation, fertility, implantation, gestation, live birth, delivery, and viability indices were calculated from the data. Pup sex, body weights and viability at birth through day 4 of lactation were determined; pups were autopsied at this time.

There were no effects of HPMA on any reproductive index. This includes reproductive parameters related to developmental toxicity including: number of live pups born, birth index, number of dead pups, number of pups born, delivery index, live birth index.

The NOAELs for developmental toxicity of HPMA were 1000 mg/kg/d, the highest dose tested, although both paternal and maternal animals evidenced reduced body weight gain, reduced food consumption and liver weight and histopathology (HPMA males only) at this dose level. Developmental toxicity studies are not available on any of the hydroxyalkyl methacrylates.

 

Primary Metabolites

MAA (donor substance: MMA)

The reference chemicals for the methacrylic moiety of HEMA and HPMA, MMA and MAA, have been tested in a series of developmental toxicity studies on rats and rabbits.

Methacrylic acid (MAA), the common metabolite for all the esters, was tested in groups of 19-25 pregnant female rats (whole-body inhalation exposure for 6 hr/day, during days 6 to 20 of gestation), at 0, 50, 100, 200, and 300 ppm (0, 179, 358, 716 and 1076 mg/m³) and produced no embryo- or fetal lethality, nor fetal malformations after exposure with MAA at any concentration, despite overt maternal toxicity (decreased body weight and feed consumption) at 300 ppm (1076 mg/m³). The NOAEL for developmental toxicity was considered 300 ppm (1076 mg/m³) MAA (Saillenfait et al., 1999).

In a developmental toxicity study according to OECD 414, MMA was administered by inhalation exposure to 99, 304, 1178, and 2028 ppm (412, 1285, 4900, 8436 mg/m³ (Solomon et al., 1993). No relevant maternal, treatment-related effects were noted at any concentration tested. No embryo of foetal toxicity was evident and no increase in the incidence in the malformations or variations was noted at exposure levels up to and including 2028 ppm. In this study the NOAEC for developmental toxicity was 2028 ppm (8436 mg/m³). For the assessment of the methacrylate part, the MMA study is regarded to be more relevant due to the wider dose range.

In addition, another study with MMA has been performed, an oral OECD 414 study in rabbits at 50, 150 , and 405 mg/kg/d. The no observed adverse effect level (NOAEL) for prenatal developmental toxicity is 450 mg/kg bw/d. No adverse foetal findings of toxicological relevance were evident at any dose, even in the presence of maternal toxicity (Schneider, 2009). MMA is not a selective teratogen.

The screening data for HPMA and the additional data from developmental toxicity studies with MMA, MAA and PG provides confidence that the absence of effects seen at 1000 mg/kg/d in the screening study with HPMA is a reliable indication for an absence of developmental toxicity of HPMA. For the purpose of the risk assessment, a NOAEL for reproductive toxicity (developmental toxicity) of 1000 mg/kg/d is taken forward for HPMA.

 

PG (from HPMA)

NTP-CERHR published a comprehensive evaluation on the developmental potential of propylene glycol (2004). The Expert Panel summarized“Prenatal developmental toxicity was assessed in CD-1 mice (gd 6–15), Wistar rats (gd 6–15), Golden hamsters (gd 6–10), and Dutch-belted rabbits (gd 6–18) by daily oral intubation. Neither developmental nor maternal toxicity was detected at the highest dose used in each of these studies (mice – 1.6 g/kg bw/day or 10.4 g/kg bw/day; rats – 1.6 g/kg bw/day; hamsters – 1.55 g/kg bw/day; rabbits – 1.23 g/kg bw/day). These data are sufficient to conclude that propylene glycol is not a developmental toxicant in these species under these treatment conditions. These data are assumed relevant for assessing human hazard. No human developmental toxicity data were identified.”Besides the mentioned rabbit study as representative for effects on a non-rodent species (FDA 1973), a drinking water study in the mouse was selected for presentation in the IUCLID dataset of HPMA (Bushy Run Research 1993) for the reason of species sensitivity to the analogous metabolite ethylene glycol for HEMA. In this study, no developmental toxicity was observed in dosages up to 10 mL/kg/d (= 10.4 g/kg/d), while maternal effects in doses ≥ 5 ml/kg/d (= 5.2 g/kg/d; increase in water consumption) was considered as physiological response and not as adverse effect. 

 

The following table provides information on the NOAELs of the considered developmental studies of the category substances and metabolites, expressed as applied dosage and as amount of substance (mmol) for comparison reasons: HEMA and HPMA hydrolyses rapidly to MAA and the respective alcohol in a 1->1+1 ratio, see chapter5.2.

 

Table 5.9.3.2: Summary of NOAELs/ NOAECs regarding developmental toxicity

NOAEL/ NOAEC developmental	route	HEMA MW 130.14	HPMA MW 144.17	MMA MW 100.12	EG MW 62.07	PG MW 76.09
mg/m³	inhalation			8436
mg/kg bw				3206
ca. mmol/kg/d				32 (MAA^a)
mg/kg bw	oral	1000	1000	450	≥ 1000^b	≥1230^c
ca. mmol/kg/d		7.7	6.9	4.5 (MAA^a)	≥ 16.1	≥ 16.2

^aconsidering a rapid hydrolysis of MMA to MAA in the body, see chapter 5.2.

^breference: Maronpot 1983, selected for the reason of lowest NOAEL after continuous dosing (human relevant according to NTP-CERHR)

^creference: NTP 1973, selected for the reason of lowest NOAEL

 

Conclusion

In summary, it can be concluded with high confidence that HPMA is not a developmental toxicant based upon a complete absence of effects in non-maternally toxic dosages in a relevant dose range in studies with rodent and non-rodent species with the primary metabolites MAA(MMA) and PG. The read-across to the metabolite data is justified by the short half-life of HPMA and the fact that the metabolites – in summary – possess the same modes of action.

From a read across perspective, there is high confidence that data generated on HEMA would act as a conservative surrogate for HPMA. Nevertheless, with reliable read-across studies for all endpoints in all relevant species the dataset for HPAM is considered to be complete.

Justification for classification or non-classification

For HPMA (and HEMA), the members of the category of lower alkyl methacrylates, based on studies in experimental animals, there is no evidence of selective toxicity to the reproductive system - neither in the studies in HPMA itself, nor in the read-across studies in the metabolites.