p-benzoquinone dioxime

Endpoint summary

Link to relevant study record(s)

Reference

Endpoint:

basic toxicokinetics, other

Remarks:

An assessment of toxicokinetics, based on available data, in accordance with Annex VIII, Section 8.8.1 of Regulation (EC) No 1907/2006

Type of information:

other: Desk-based assessment

Adequacy of study:

key study

Study period:

The study will be available 29-06-2018

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

test procedure in accordance with generally accepted scientific standards and described in sufficient detail

Objective of study:

toxicokinetics

Principles of method if other than guideline:

An assessment of toxicokinetics, based on available data, in accordance with Annex VIII, Section 8.8.1 of Regulation (EC) No 1907/2006

GLP compliance:

no

Specific details on test material used for the study:

Detailed information on the 'test material identity' is provided in the attached document including information on individual constituents.

Details on species / strain selection:

Desk-based assessment.

Details on test animals or test system and environmental conditions:

Desk-based assessment.

Details on exposure:

Desk-based assessment.

Duration and frequency of treatment / exposure:

Desk-based assessment.

No. of animals per sex per dose / concentration:

No animals were used in this desk-based assessment.

Positive control reference chemical:

Desk-based assessment.

Details on study design:

Desk-based assessment.

Details on dosing and sampling:

Not applicable

Statistics:

Not applicable

Preliminary studies:

Not applicable

Details on absorption:

The molecular weight of the substance is considerably low at 138.12 g/mol, similarly the water solubility was 154 mg/L (OECD 105, APPLICANT 2018). The substance is decomposes before melting/boiling at > 237 °C and as such not considered volatile with reported vapour pressure of < 0.0001 Pa at 25 °C (OECD 102, APPLICANT 2018). The n-octanol/water partition coefficient of substance is also considerable low i.e. 0.3 at 35 °C and pH 7.0 (OECD 117, APPLICANT 2018). Furthermore, the substance is not surface active. These physicochemical properties are suggestive of favourable absorption via the oral route. Based on these properties, 100 % oral absorption can be assumed. The low molecular weight of < 200 g/mol, n-octanol/water partition coefficient of 0.3 and the water solubility of 154 mg/L ensure both intracellular and paracellular absorption of the substance is possible resulting in substance delivery into the liver via portal circulation i.e. first pass metabolism. The paracellular absorption of the substance is most favourable route aided by the low molecular weight and the n-octanol/water partition coefficient. This will reduce plasma concentration of the substance as well as plasma half as the substance is expected to go metabolic transformation to its respective metabolites.
Although these properties make uptake from the dermal route possible, based on the surface tension of the substance (refer to IUCLID Section 4.10), dermal absorption would be limited since the transfer of the substances between the stratum corneum and the epidermis would be restricted. For absorption calculation, a default value of 100 % skin absorption according to ECHA (2014) can be applied as conservative approach for risk assessment.
Based on the vapour pressure of < 0.0001 Pa and the fact that the substance is expected to decompose before melting/boiling at > 237 °C, uptake via inhalation route is limited. However, > 90 % of the substance contains respirable particles under 100 µm. This coupled with the low molecular weight and the octanol/water partition coefficient would mean bioavailability via this route is possible. Passive diffusion into pulmonary circulation is possible and a high plasma concentration of the substance as well as plasma half-life is expected since the rate fast pass metabolism in the lung is slower than in the liver.

Details on distribution in tissues:

The substance has physicochemical properties (mainly molecular weight, n-octanol/water partition coefficient, respirable particles and water solubility) which means upon oral and inhalation exposure, both intracellular and paracellular absorption of the substance is possible resulting in substance delivery into the liver via portal circulation i.e. first pass metabolism. As a result, systemic distribution of the parent compound especially via oral route will be limited and therefore resulting into a reduced half-life of the parent compound in blood plasma.
A wide distribution is expected following inhalation exposure since low molecular weight and n-octanol/water partition coefficient would result into passive absorption of the substance into pulmonary circulation resulting in high plasma concentration of the substance as well as plasma half-life since the rate fast pass metabolism in the lung is slower than in the liver. This is supported by the clinical observations in mice and rat following 2 years exposure to the substance. This included mortality, distinct dose-related mean body weight depression observed in both mice and rats and urinary bladder neoplasms in female rats.

Details on excretion:

Based on the absorption, distribution and potential metabolic pathways highlighted coupled with available systemic data, the substance will most likely be excreted via bile and urine. Absorption via paracellular and intracellular pathways would most likely be excreted via urine as supported by the changes observed in the kidney such as urinary bladder neoplasms. Although the neoplasm was sex and species specific effect confined in rats following sub/chronic exposure to substance it is nonetheless and indication of bladder role in the elimination of the substance.
Based on the physicochemical property of the substance which ensure first past metabolism /biotransformation, the elimination of the substance is assumed to be rapid. As the substance is expected to become more polar following metabolism and the potential for bioaccumulation is to be expected to very low.

Details on metabolites:

Metabolism following oral and inhalation administration is mainly through phase I and II enzymes in the liver and lung. The main phase I reactions would most like be an oxidative reaction followed by hydrolysis reaction, this would result into production of benzoquinone and nitro derivative and potentially reactive radical species. The benzoquinone is expected to undergo redox cycling by NAD(P)H-quinone oxioreductase or carbonyl reductase forming hydroquinone, semiquinione and reactive oxygen species (ROS). Phase II reaction would potentially involve conjugation of hydroquinone with sulphate and glucuronic acid by the enzymatic activity of sufotransferase and glucuronosyl transferase, respectively. In addition to this, hydroquinone can also undergo redox cycling forming semiquinione which can be further conjugated with glutathione under the influence of glutathione s-transferase and the glutathione conjugate can be further metabolized to cysteine conjugates and mercapturic acids by enzymes such as N-acetyltransferease. The semiquinione formed can also undergo further redox cycling in presence of molecular oxygen, resulting in ROS, including superoxide anion (O2.-) and H2O2. The generation of ROS is supported by the neoplasm observed in the kidney, liver, lung and urinary bladder, elevated ROS are known to initiate cancer.
The substance is expected to undergo redox cycling reaction by NAD(P)H-quinone oxidoreductase or carbonyl reductase producing metabolic conjugates and reactive oxygen species, the conjugates are erected via urine and bile while the ROS can bind into protein triggering cascade of events such DNA strand break. However, the substance did not induce any DNA damage in unscheduled DNA synthesis – OECD 486 nor was it mutagenic in micronucleus studies conducted according to OECD 474. The substance and or its metabolites could be considered as non-genotoxic carcinogens acting through cell proliferation as demonstrated by the positive results of renal carcinoma - OECD 453. Based on data obtained on cancer bioassays, an increase in proliferative response caused by chemicals/metabolites induced cytotoxicity which appears to play a role in the initiation of neoplasia [reference: Waris G and Ahsan H (2006)]. Since the renal carcinoma is only observed in rat of both sexes, the effect can be considered accumulation of α2u-globulin, which is known to have some association with renal carcinoma. The study did not indicate any measurement of α2u-globulin and we cannot rule out its association with the observed effects.

Executive summary:

A desk-based assessment of the basic toxicokinetics of the substance, in accordance with Regulation (EC) 1907/2006: Annex VIII - Section 8.8.1:

The molecular weight of all the constituents is low i.e. < 200 g/mol, with a corresponding n-octanol/water partition coefficient in the range of > 4.17 and < 4.25, water solubility (range) of 1 – 100 mg/L and vapour pressure of 19.62 Pa at 25 °C, are suggestive of favourable absorption via all routes of administration. Oral absorption from the gastro-intestinal tract (GI tract) and respiratory absorption via the respiratory tract epithelium: of the substance is mainly via passive diffusion (paracellular pathway) through the intercellular junction pores into portal circulation. With delivery into the liver ensuring first pass metabolism which means that the concentration of the parent substance is reduced before reaching systemic circulation as demonstrated by the hepatocyte vacuolation and the no observed adverse effects following repeated exposure in rats. Furthermore, the irritation potential of the substance may also aid absorption by causing damage to the membrane resulting in high systemic bioavailability of the substance. The substance is considered volatile based on its vapour pressure, therefore exposure via inhalation route is possible. Depending on the inhaled particle size, larger particles deposited within the upper airways which may be largely cleared via the digestive system whereas particles deposited in the lung may be more likely to dissolve and may be absorbed through the lung. Based on the physicochemical properties of the substance passive diffusion is the main route of absorption into the epithelia cells and potentially entering systemic circulation. The physicochemical properties indicate the substance will demonstrate moderate dermal absorption.

The substance has physicochemical properties which may favour wide distribution depending in the route of exposure. Wide distribution is limited via oral/gastro intestinal absorption since the substance is of small molecular weight and can move through first pass metabolic pathway, meaning that the parent constituents are converted to respective metabolites before distribution and overall systemic circulation. This means that the distribution of the parent compound systemically is reduced and the half-life of the parent compound in blood plasma is also reduced. Pulmonary absorption could potentially result into a wide distribution with the heart as the first point and then general circulatory system and subsequently entering the liver before the parent compound is transformed to its respective metabolites. Although skin absorption is expected, the distribution of the substance is reduced compared with oral absorption and distribution.

Metabolism following oral and inhalation administration is mainly through phase I and II enzymes in the liver and lung. The main phase I reactions would most like be an oxidative reaction followed by hydrolysis reaction, this would result into production of benzoquinone and nitro derivative and potentially reactive radical species.  The benzoquinone is expected to undergo redox cycling by NAD(P)H-quinone oxioreductase or carbonyl reductase forming hydroquinone, semiquinione and reactive oxygen species (ROS). Phase II reaction would potentially involve conjugation of hydroquinone with sulphate and glucuronic acid by the enzymatic activity of sufotransferase and glucuronosyl transferase, respectively. In addition to this, hydroquinone can also undergo redox cycling forming semiquinione which can be further conjugated with glutathione under the influence of glutathione s-transferase and the glutathione conjugate can be further metabolized to cysteine conjugates and mercapturic acids by enzymes such as N-acetyltransferease. The semiquinione formed can also undergo further redox cycling in presence of molecular oxygen, resulting in ROS, including superoxide anion (O2.-) and H2O2. The generation of ROS is supported by the neoplasm observed in the kidney, liver, lung and urinary bladder, elevated ROS are known to initiate cancer. The substance is expected to undergo redox cycling reaction by NAD(P)H-quinone oxidoreductase or carbonyl reductase producing metabolic conjugates and reactive oxygen species, the conjugates are erected via urine and bile while the ROS can bind into protein triggering cascade of events such DNA strand break. However, the substance did not induce any DNA damage in unscheduled DNA synthesis – OECD 486 nor was it mutagenic in micronucleus studies conducted according to OECD 474. The substance and or its metabolites could be considered as non-genotoxic carcinogens acting through cell proliferation as demonstrated by the positive results of renal carcinoma - OECD 453. Based on data obtained on cancer bioassays, an increase in proliferative response caused by chemicals/metabolites induced cytotoxicity which appears to play a role in the initiation of neoplasia [reference: Waris G and Ahsan H (2006)]. Since the renal carcinoma is only observed in rat of both sexes, the effect can be considered accumulation of α2u-globulin, which is known to have some association with renal carcinoma. The study did not indicate any measurement of α2u-globulin and we cannot rule out its association with the observed effects.

The substance has physicochemical properties (mainly molecular weight, n-octanol/water partition coefficient, respirable particles and water solubility) which means upon oral and inhalation exposure, both intracellular and paracellular absorption of the substance is possible resulting in substance delivery into the liver via portal circulation i.e. first pass metabolism. As a result, systemic distribution of the parent compound especially via oral route will be limited and therefore resulting into a reduced half-life of the parent compound in blood plasma. A wide distribution is expected following inhalation exposure since low molecular weight and n-octanol/water partition coefficient would result into passive absorption of the substance into pulmonary circulation resulting in high plasma concentration of the substance as well as plasma half-life since the rate fast pass metabolism in the lung is slower than in the liver. This is supported by the clinical observations in mice and rat following 2 years exposure to the substance. This included mortality, distinct dose-related mean body weight depression observed in both mice and rats and urinary bladder neoplasms in female rats.

Description of key information

Toxicokinetics Assessment: no bioaccumulation potential; desk-based assessment in accordance with Regulation (EC) 1907/2006: Annex VIII, Section 8.8.1, 2018

Key value for chemical safety assessment

Bioaccumulation potential:

low bioaccumulation potential

Absorption rate - oral (%):

100

Absorption rate - dermal (%):

100

Absorption rate - inhalation (%):

100

Additional information

Basictoxicokinetics (expert assessment, 2018): The substance possesses physico-chemical properties which are favourable for ADME. Based on the log Pow of < 4.5 and BCF < 2,000, bioaccumulation is not significant and elimination is expected to be rapid.

References:

1. ECHA Guidance on Information Requirements and Chemical Safety Assessment (Chapter R.7c: Endpoint Specific Guidance, June 2017)