N-pentan-2-ylidenehydroxylamine

Endpoint summary

Description of key information

There are no studies available in which the toxicokinetic behaviour of 2 -pentanoen oxime (2 -PO; CAS 623-40-5) was investigated. Blood levels of 2-PO and its metabolite methyl propyl ketone (MPK) were measured after a single inhalation exposure. In addition, the effect of 2-PO on hepatic gene expression using a toxicogenomics approach was examined. In accordance with Annex VIII, Column 1, 8.8.1, of Regulation (EC) 1907/2006 and with ‘Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance’ (ECHA, 2017), an assessment of the toxicokinetic behaviour of the substance 2 -PO was conducted to the extent that can be derived from the relevant available information. 2,5-PO is expected to be absorbed via the oral, dermal and inhaltion route. Absorption factors of 100% are therefore proposed for all routes. The substance is anticipated to be widely distributed through the body, metabolised and excreted via the bile and / or urine. No bioaccumulation potential is expected.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Absorption rate - oral (%):

100

Absorption rate - dermal (%):

100

Absorption rate - inhalation (%):

100

Additional information

Basic toxicokinetics

There are no studies available in which the toxicokinetic behaviour of2-pentanone oxime (2-PO; CAS 623-40-5)was investigated. Blood levels of 2-PO and its metabolite methyl propyl ketone (MPK) were measured after a single inhalation exposure. In addition, the effect of 2-PO on hepatic gene expression using a toxicogenomics approach was examined.

In accordance with Annex VIII, Column 1, 8.8.1, of Regulation (EC) 1907/2006 and with ‘Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance’ (ECHA, 2017), an assessment of the toxicokinetic behaviour of the substance 2-PO was conducted to the extent that can be derived from the relevant available information. This comprises a qualitative assessment of the available substance specific data on physico-chemical and toxicological properties (including blood levels and results of the toxicogenomic analysis) according to the Chapter R.7c Guidance document (ECHA, 2017).

 

Physico-chemical properties

2-POis an organic, mono-constituent substance with a molecular weight of 101.15 g/mol. It is a colourless to pale yellow liquid at 20°C. The water solubility is 34.7 g/L at 19.8°C. The log Pow is 1.43 at 22°C, and the vapour pressure 214 Pa at 20 °C.

Absorption

Absorption is a function of the potential for a substance to diffuse across biological membranes. The most useful parameters providing information on this potential are the molecular weight, the octanol/water partition coefficient (log Pow) value and the water solubility. The log Pow value provides information on the relative solubility of the substance in water and lipids (ECHA, 2017).

 

Oral

In general, molecular weights below 500 and log Pow values between -1 and 4 are favourable for absorption via the gastrointestinal (GI) tract, provided that the substance is sufficiently water soluble (> 1 mg/L). Lipophilic compounds can be taken up by micellar solubilisation by bile salts, but this mechanism may be of particular importance for highly lipophilic compounds (log Pow > 4), in particular for those that are poorly soluble in water (≤ 1 mg/L) as these would otherwise be poorly absorbed. Solids must be dissolved before absorption; the degree depends on the water solubility (ECHA, 2017).

All of the physico-chemical characteristics (log Pow, water solubility, molecular weight and physical state) of2-POindicate high absorption from the GI-tract following oral ingestion.

In an acute oral toxicity study in which rats were administered single doses (175, 550 and 2000 mg/kg bw) of 2-PO, mortality was observed in all rats receiving 2000 mg/kg bw 2-PO (Key, 2009, AOT). Clinical signs were observed in all dose groups. All rats achieved satisfactory body weight gains throughout the study. No macroscopic abnormalities were observed for animals killed on day 15. A pale, discoloured liver was observed for one rat that was found dead on day 2. Decreased liver weights were observed at 550 mg/kg bw, decreased spleen weights at >= 550 mg/kg bw and decreased kidney weights at 2000 mg/kg bw.

In an oral combined repeated dose toxicity and reproductive and developmental toxicity study performed with 2-PO a dose level of 15 mg/kg bw/day can be regarded as LOAEL for general toxicity since adverse effects on the haematopoetic system (affected organs: blood, spleen) were already observed at the lowest dose level of the study (Key, 2012, OECD 422).

In conclusion, all the available information indicate that 2-PO is absorbed via the GI tract. For hazard and risk assessment purposes oral absorption of 2-PO is therefore set to 100%.

 

Dermal

The dermal uptake of liquids and substances in solution is higher than that of dry particulates, since dry particulates need to dissolve into the surface moisture of the skin before uptake can begin. Molecular weights below 100 g/mol favour dermal uptake, while for those above 500 g/mol the molecule may be too large. Dermal uptake is anticipated to be low if the water solubility is < 1 mg/L; low to moderate if it is between 1 - 100 mg/L; and moderate to high if it is between 100 - 10000 mg/L. Log Pow values in the range of 1 to 4 (values between 2 and 3 are optimal) are favourable for dermal absorption, in particular if the water solubility is high. For substances with a log Pow above 4, the rate of penetration may be limited by the rate of transfer between the stratum corneum and the epidermis, but uptake into the stratum corneum will be high. Log Pow values above 6 reduce the uptake into the stratum corneum and decrease the rate of transfer from the stratum corneum to the epidermis, thus limiting dermal absorption (ECHA, 2017).

2-PO has a molecular weight of 101.15 g/mol, and is a liquid with a high water solubility of 34.7 g/L. The log Pow is 1.43 at 22°C. Based on these physico-chemical properties, dermal absorption is anticipated.

If a substance shows skin irritating or corrosive properties, damage to the skin surface may enhance penetration. If the substance has been identified as a skin sensitizer then some uptake must have occurred although it may only have been a small fraction of the applied dose (ECHA, 2017).

The available data for 2-PO (Key, 2019, OECD 439 ) provide no indications for skin irritating effects. The result of the skin sensitisation tests (Buehler and LLNA) performed with 2-PO were negative (Key, 2009, Buehler; Supporting, 2009, LLNA). Therefore, penetration of the substance due to skin damage is not expected.

Based on a QSAR calculated dermal absorption a value of 131.13 µg/cm²/h was predicted for 2-PO (Dermwin v.2.02), supporting that the dermal absorption of 2-PO is high.

Overall, based on the available information, the dermal absorption potential of 2-PO is predicted to be high. A worst-case approach should be applied for risk assessment and the dermal absorption of 2-PO should be regarded as 100%.

 

Inhalation

Substances that can be inhaled include gases, vapours, liquid aerosols (both liquid substances and solid substances in solution) and finely divided powders/dusts. To be readily soluble in blood, a gas or vapour must be soluble in water and increasing water solubility would increase the amount absorbed per breath. However, the gas or vapour must also be sufficiently lipophilic to cross the alveolar and capillary membranes. Therefore, a moderate log P value (between -1 and 4) would be favourable for absorption. Generally, liquids, solids in solution and water-soluble dusts would readily diffuse/dissolve into the mucus lining the respiratory tract. Lipophilic substances (log P > 0) would then have the potential to be absorbed directly across the respiratory tract epithelium (ECHA, 2017).

2-PO is a liquid with a vapour pressure of 214 Pa at 20°C, and therefore low volatility. The high water solubility of 34.7 g/L is an indication that 2-PO vapours may be retained within the mucus, but the moderate log Pow value of 1.43 at 22°C favours 2-PO for absorption directly across the respiratory tract epithilium by passive diffusion. The substance may also be available for inhalatory absorption after inhalation of aerosols, if the substance is sprayed (e.g. as a formulated product). In humans, particles with aerodynamic diameters below 100μm have the potential to be inhaled.

Also the available data on repeated inhalation toxicity indicate absorption of 2-PO via inhalation.

In a sub-acute inhalation toxicity study (Key, 2014, RDT 14 days) male rats exposed to 2-PO concentrations (overall mean ± standard deviation) of 149.3 (± 1.9) and 298.9 (± 3.1) ppm (mid and high-dose groups) for 6 h/day, 5 days/week, during 2 consecutive weeks showed higher spleen weights (absolute and relative to body weight) compared to control group animals. In addition, higher weight of the ovaries (absolute and relative to body weight) in females of the mid-dose group compared to control group animals was observed.

There is also a sub-chronic inhalation toxicity study (Key, 2016, RDT 90 days) in which rats were exposed by nose-only inhalation exposure to 0, 49.7 (± 0.7), 149 (± 4) or 301 (± 15) ppm 2-PO for 6 h/day, 5 days/week over a 13-week period (65 exposure days). Exposure to the test substance resulted in slight changes in clinical pathology parameters in animals sacrificed at the end of the exposure period, e.g. decreased haemoglobin concentration in the high-dosed males, increased percentage of reticulocytes in blood of males in the mid- and high-dose group and increased plasma bilirubin levels in males and females of the highest dose level. All changes were fully reversible after the 4-week recovery period. A few statistically significant differences in coagulation parameters between animals of mid and high concentration groups versus unexposed controls (increased number of thrombocytes in males, increased prothrombin time in females) were observed. These findings were observed in one sex only (or were even reversed in the other sex), the magnitude of the changes was limited, and all changes were fully reversible within the 4-week recovery period. In the absence of any corroborative histopathological changes or significant alterations in associated parameters, a transient increase in relative weight of the kidneys (in males and females of the high concentration), liver (in males of the mid and high concentration) and spleen (in females of the high concentration group; +13.4%) were found at the end of the exposure period – but no longer at the end of the recovery period. Taken into account the multiple effects in the blood and spleen related to haemolytic anaemia observed in the oral OECD 422 with 2-PO and the classification for STOT RE Cat. 2 (Key, 2012), the described effects of this repeated dose inhalation toxicity study referring to clinical / haematological parameters and spleen weights were regarded as adverse in an overall worst-case assessment, despite the fact that they all were transient and reversible. Therefore, the medium concentration of 149 ppm (corresponding to 615.4 mg/m3) was set as No-Observed-Adverse-Effect Concentration (NOAEC) for local and systemic toxicity. Analysis of blood samples (Supporting, 2015, TK) taken in the final stage of the exposure phase from animals of the high concentration recovery group indicated that 2-PO and its metabolite methyl propyl ketone (MPK) reached the blood upon inhalation exposure. Blood samples for analysis were collected prior to exposure, at 2, 4, and 6 h during the exposure and 6 h following termination of the exposure. Both detectable levels of 2-PO and MPK were present following 2 h of exposure (average 10.2 and 1.3 mg/kg blood, respectively) and concentrations increased during the 6 h exposure period and rapidly declined following exposure such that detectable levels [LOD (limit of detection) = 2 mg/kg blood] were no longer present 6 h after the termination of the exposure.Please refer to the technical dossier, section 7.1 Toxicokinetics for more details.

Overall, based on the available information, the inhalation absorption potential of 2-PO is predicted to be high and there is no hint that inhalation exposure is higher than the oral exposure. Therefore, for risk assessment the absorption of 2-PO via inhalation should be regarded as 100%

Conclusion:

Based on the available information it is concluded that 2-PO is expected to be absorbed after oral, dermal and inhalation exposure.

Distribution and Accumulation

Distribution of a compound within the body depends on the physico-chemical properties of the substance, especially the molecular weight, the lipophilic character and the water solubility. In general, the smaller the molecule, the wider is the distribution. If the molecule is lipophilic, it is likely to distribute into cells and the intracellular concentration may be higher than extracellular concentration, particularly in fatty tissues (ECHA, 2017).

Distribution of 2-PO is possible, indicated by its low molecular weight and high water solubility. This is also supported by the different available acute, subacute and subchronic oral and inhalation toxicity studiesin which effects were seen in different organs (e.g. blood, spleen, kidney). As the log Pow of 2-PO is 1.43 at 22°C, 2-PO could have the potential to distribute into cells and accumulate within the body unless there is a high metabolic clearance.Analysis of blood samples taken in the final stage of the exposure phase in a sub-chronic repeated dose inhalation toxicity study (Supporting, 2015, TK) from animals of the high concentration recovery group showed that after the end of exposure, concentrations of 2-PO and its metabolite methyl propyl ketone (MPK) rapidly decreased and were no longer detectable 6 h after exposure, indicating rapid clearance from the blood and no accumulation of 2-PO in the body under the experimental conditions(please refer to the technical dossier, section 7.1 Toxicokinetics for more details).

 

Metabolism and Excretion

There are no data on the metabolism of 2-PO available.As discussed above, 2-PO is distributed in the body and thus, reaches the liver, which is the organ with the greatest capacity for metabolism, and probably also the kidney, where excretion processes leading to excretion via the urine take place.A potential metabolite was already mentioned, the hydrolysis product MPK. In a toxicogenomic study (Supporting, 2017, TG) in which male and female Sprague-Dawley rats were exposed to 2-PO by inhalation at concentrations of 50 ppm, 150 ppm and 300 ppm for 6 h/day, 5 days/week, over a 90-day period (please refer to the technical dossier, section 7.9. Specific investigations for more details), whole genome expression in the liver was examined. In male rats exposure to 2-PO at 150 ppm or 300 ppm, resulted in dose-dependent increases in the expression of xenobiotic metabolizing enzymes (e.g. GSTs, UGTs). Thus,conjugation of the hydroxyl groups with sulphate or glucuronic acid occurs, leading to water-soluble metabolites which could be excreted via the bile and / or urine.

In an in vitro chromosomal aberration test (Key, 2013, CA) 2-PO caused an increase in the frequency of structural chromosome aberrations, in the presence of S9 mix following 3 h treatment. Thus gave hints on reactive metabolites. However, all other genotoxicity tests (both in vitro and in vivo) resulted in negative results leading to the assumption that this reactive metabolite has no relevance in vivo.

References

ECHA (2017). Guidance on information requirements and chemical safety assessment, Chapter R.7c: Endpoint specific guidance.