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Endpoint:
basic toxicokinetics in vivo
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
weight of evidence
Justification for type of information:
ANALOGUE APPROACH JUSTIFICATION
Please refer to the attached read across justification in section 13.
Reason / purpose:
read-across source
Type:
absorption
Results:
biological membranes are highly permeable to H2O2: 0.2 cm/min (peroxisomal membranes) and 0.04 cm/min (erythrocyte plasma membrane). This results in rapid absorption from mucuos membranes, skin, and lungs.
Details on absorption:
Toxicokinetic data are scarce, because it seems impossible to measure the fate of exogenous H2O2
as any measurement would interfere with the physiological equilibria. Available qualitative information: rapid absorption leads to increased levels of H2O2
and O2 in adjacent blood vessels, or even bubble formation.
Mucous membranes, small and large bowel, in vivo: bubble formation in draining vessels at concentrations > 1%
rat skin in vivo: elevated levels of H2O2
in excised epidermis within few minutes following application of 5-30% solutions of H2O2
Skin, mucous membranes: bubble formation in blood of patients undergoing irrigation of surgical wounds within minutes (solutions of 30% H2O2 , 5-20 ml).
Details on distribution in tissues:
H2O2 is sufficiently stable to diffuse longer distances and distribute systemically as evidenced by cerebral oxygen embolism following accidental ingestion of 35% hydrogen peroxide. In another case, a child died after ingestion of 230 of 3% hydrogen peroxide solution. Gas emboli in the intestinal lymphatics and pulmonary vasculature, vacuoles inn spleen, kidney, and myocardium were found.
Key result
Test no.:
#1
Transfer type:
blood/brain barrier
Observation:
slight transfer
Remarks:
in humans, accidental ingestion
Key result
Test no.:
#2
Transfer type:
other: across blood vessels into organs
Remarks:
e.g. spleen, kidney, heart
Observation:
distinct transfer
Remarks:
case of fatal accidental ingestion
Details on excretion:
H2O2 is a normal metabolite in aerobic cells. In the rat, the total estimated production is 1.45 µmol/min per 100 rat, with approx. 75% produced in the liver. Most H2O2 is degraded by catalase or glutathione peroxidase, a minor fraction may undergo the Fenton reaction. Only small quantities were found in human exhaled air (0.5 E -08 to 0.5 E -06 M).

H2O2 is sufficiently stable to diffuse longer distances and distribute systemically as evidenced by cerebral oxygen embolism following accidental ingestion of 35% hydrogen peroxide. In another case, a child died after ingestion of 230 of 3% hydrogen peroxide solution. Gas emboli in the intestinal lymphatics and pulmonary vasculature, vacuoles inn spleen, kidney, and myocardium were found.
Metabolites identified:
yes
Details on metabolites:
H2O2 may be reduced to water by Glutathione peroxidase, or water and oxygen by catalase.
In the Fentron reaction, it may also react with free metal ions to give the hydroxyl radical and hydroxal anion.

The results above are for hydrogen peroxide and can be adopted for the target substance, hydrogen peroxide - urea (1:1), because the latter breaks down with water to hydrogen peroxide and urea.

Endpoint:
basic toxicokinetics in vivo
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
weight of evidence
Justification for type of information:
ANALOGUE APPROACH JUSTIFICATION
Please refer to the attached read across justification in section 13.
Reason / purpose:
read-across source
Type:
absorption
Results:
oral gavage: maximal plasma levels were reached at Tmax: 1 hour
Type:
distribution
Results:
max tissue concentrations were seen between 30 and 60 min after dosing, comparable to plasma levels. Levels were low in fat and brain, high in kidneys and urinary bladder
Type:
metabolism
Results:
No metabolism apart from that of bacteria of the gut
Type:
excretion
Results:
Rapid excretion. Mostly via urine [>95% (fasted rats) or 54% (nonfasted) in urine at 24 hours following oral gavage] and exhaled air following bacterial hydrolysis [ 3.5% (fasted) and 43% (nonfasted) CO2 with expired air at 24 hours past dosing].
Details on absorption:
The maximum concentration (Cmax) and area under the curve (AUC0-∞) increased proportionally with increasing dose, indicating that urea fits a linear pharmacokinetic model across the wide range of doses tested for both routes of exposure (see Table 3-2 further below). The authors claim that the disposition of exogenous urea is similar to that of endogenous urea and suggests that rats have a sufficiently large capacity for disposition.
Details on distribution in tissues:
Fasting had little effect on the tissue distribution but did produce a slight increase in the overall concentrations. With the exception of the brain and eyeball, the maximum tissue concentrations were recorded 30 minutes to an hour after urea administration (plasma concentration reached Cmax at 30 minutes in both the nonfasted and fasted animals; 1,231 ± 319 and 1,675 ± 938 ng eq/mL, respectively). Excluding the gastrointestinal tract (site of administration), the tissues with the highest radiolabel concentration were the kidney and urinary bladder (~2.5- and 3.2-fold higher than plasma concentrations). Fat and brain had the lowest urea concentrations (225 ± 138 and 263 ± 182 ng eq/mL, respectively) at this time point. Urea concentrations in the remaining tissues were similar to or below that in the plasma. After 24 hours, all tested tissues, with the exception of the large intestine and the Harderian gland, had below detectable levels of radiolabeled urea after 24 hours. At 72 hours, none of the tested tissues had detectable levels of radiolabeled urea.
Key result
Test no.:
#1
Transfer type:
blood/brain barrier
Observation:
slight transfer
Key result
Test no.:
#1
Transfer type:
other:
Observation:
distinct transfer
Remarks:
maximal tissue concentrations were recorded 30 to 60 minutes after treatment. After 24 hours concnentrations were below detectable levels in almost all tested tissues.
Details on excretion:
The results from this analysis are presented in Table 3-3. The total percentage of radiolabel recovered from fasted rats in urine, feces, and expired air 24 hours after dosing was comparable for each route of exposure. In fasted rats, >90% of the radiolabel was in the urine, approximately 4% was in exhaled air, and only around 1% was in feces, with almost all of the radiolabel excreted during the first 24 hours. In nonfasted rats, much less of the administered dose was recovered in urine; >70% after i.v. dosing and only 54% after oral administration. Fecal excretion did not differ much between fasted and nonfasted animals. However, in nonfasted animals, 20% of the administered radioactivity was recovered from exhaled air following i.v. dosing, and almost 43% was recovered following oral administration (Table 3-3). The increased [14C] recovery in expired air and decreased recovery in urine in i.v.-dosed, compared with orally-dosed animals, was attributed to a higher oral absorption rate. Additional sampling up to 96 hours after dosing increased the percentage of total [14C] recovered by <1%, regardless of exposure route or fasting condition.
Key result
Test no.:
#1
Toxicokinetic parameters:
half-life 1st: 2.0 hrs
Remarks:
i.v, fasted, 2 mg/lkg
Key result
Test no.:
#1
Toxicokinetic parameters:
half-life 2nd: 3.5 hrs
Remarks:
i.v, fasted, 2 mg/lkg
Key result
Test no.:
#2
Toxicokinetic parameters:
half-life 1st: 1.7 hrs
Remarks:
i.v, nonfasted, 2 mg/lkg
Key result
Test no.:
#2
Toxicokinetic parameters:
half-life 2nd: 6.2 hrs
Remarks:
i.v, nonfasted, 2 mg/lkg
Key result
Test no.:
#3
Toxicokinetic parameters:
half-life 1st: 1.7 hrs
Remarks:
oral, fasted, 2 mg/lkg
Key result
Test no.:
#3
Toxicokinetic parameters:
half-life 2nd: 3.4 hrs
Remarks:
oral, fasted, 2 mg/lkg
Key result
Test no.:
#4
Toxicokinetic parameters:
half-life 1st: 2.5 hrs
Remarks:
oral, fasted, 2 mg/lkg
Key result
Test no.:
#4
Toxicokinetic parameters:
half-life 2nd: 7.5 hrs
Remarks:
oral, fasted, 2 mg/lkg
Key result
Test no.:
#5
Toxicokinetic parameters:
half-life 1st: 2.0 hrs
Remarks:
oral, non-fasted, 62.5 mg/kg
Key result
Test no.:
#5
Toxicokinetic parameters:
half-life 2nd: 10.7 hrs
Remarks:
oral, non-fasted, 62.5 mg/kg
Key result
Test no.:
#6
Toxicokinetic parameters:
half-life 1st: 2.1 hrs
Remarks:
oral, non-fasted, 250 mg/kg
Key result
Test no.:
#6
Toxicokinetic parameters:
half-life 2nd: 9.0 hrs
Remarks:
oral, non-fasted, 250 mg/kg
Key result
Test no.:
#7
Toxicokinetic parameters:
half-life 1st: 1.9 hrs
Remarks:
oral, non-fasted, 1000 mg/kg
Key result
Test no.:
#7
Toxicokinetic parameters:
half-life 2nd: 8.4 hrs
Remarks:
oral, non-fasted, 1000 mg/kg
Metabolites identified:
yes
Details on metabolites:
Urea is excreted unchanged. Based on the difference of radioactivity in exhaled air from fasted and nonfasted rats (radiolabeled CO2 double as high in nonfasted rats) it was concluded that there was no evidence of urea metabolism other than hydrolysis by bacteria in the gut.

The results above are for the source substance, urea, and may be used in a read-across approach for the target substance

Description of key information

No data were located for the target substance, Hydrogen peroxide - urea (1:1), but as this substance dissolves breaks down with water to the two components, hydrogen peroxide and urea, in equimolar proportions, data for the breakdown products were used in a read-across approach. No bioaccumulation potential is expected for the source substances as well as for the target substance.

Key value for chemical safety assessment

Bioaccumulation potential:
no bioaccumulation potential
Absorption rate - oral (%):
100
Absorption rate - dermal (%):
50

Additional information

No data were located for the target substance, Hydrogen peroxide - urea (1:1), but as this substance dissolves breaks down with water to the two components, hydrogen peroxide and urea, in equimolar proportions, data for the breakdown products may be used in a read-across approach.

Hydrogen peroxide

Hydrogen peroxide is a normal metabolite in aerobic cells, it is a by-product of reactions in mitochondria, microsomes and peroxisomes, and in oxidative burst of immuno cells. The total estimated production in the rat is 1.45 µmol/min per 100 g rat. Cellular concentration is regulated at 1xE-09 and 1xE-09 M, especially by glutathione peroxidase and catalase.

The EU Risk assessment for hydrogen peroxide summarises available ADME studies. H2O2 is a small, unloaded molecule that can easily cross biological membranes; the permeability of membranes for hydrogen peroxide is comparable to that for water. As a consequence, hydrogen peroxide is readily absorbed via all routes of exposure. As an oxidant. It may react with organic molecules, but is sufficiently stable to diffuse, enter adjacent blood vessels, and distribute throughout the body. Terminally, most of hydrogen peroxide is degraded at low levels by glutathione peroxidase [equation 1] or by catalase [equation 2].

H2O2+ 2 GSH  > 2 H2O + 2 GSSG [equation 1]

2 H2O2   > 2 H2O + O2  [equation 2]

Further, hydrogen peroxide may react with free metal ions like iron or copper in the Fenton reaction [3] to give the highly reactive hydroxyl radical that reacts with organic molecules in the nearest proximity.

H2O2 + Fe2+/ Cu+   >  OH. + OH- + Fe3+/ Cu2+ [equation 3]

As a consequence, only trace amounts are found in human exhaled air.

 

The toxicity of hydrogen peroxide is associated with the oxidation of organic molecules at the point of contact (e.g. skin corrosion) and, after absorption and distribution, at distant locations, the generation of the highly reactive hydroxyl radical, and embolism due to the formation of large amounts of oxygen. One mL of 30% H2O2 yields approximately 100 mL of oxygen, and oxygen bubbles in blood vessels and organs (brain, spleen, kidneys, hart muscle) were found in animals and humans after accidental ingestion.

Though absorption and degradation are known to occur within few minutes, no precise toxicokinetic data are known to exist, primarily due to analytical difficulties, e.g. to discriminate between endogenous and external hydrogen peroxide (ECB, 2003).

 

This information is considered to be suitable for assessment and can be used in a read across approach for hydrogen peroxide – urea (1:1).

Urea

Urea is formed during normal physiological processes primarily in the liver in the so-called ornithine cycle for the removal of nitrogen from the body. Nitrogen, present as ammonia, is a deamination product of amino acids. Urea blood levels are approx. 200-400 mg/L, and 25 -50 g are excreted per day with the urine.

 

The results from studies of exogenously administered urea illustrate that the route of administration has little effect on the distribution, metabolism, or excretion of urea. In animal studies, maximum plasma and tissue concentrations were achieved 30 minutes to 1 hour after intravenous or oral gavage dosing. Excluding the gastrointestinal tract, the kidneys and urinary bladder tended to show the highest urea concentrations. The uptake of [14C]-urea and distribution from plasma into the lateral ventricular choroid plexus of rats was shown to be much slower than in skeletal muscle. There was little evidence that exogenously administered urea undergoes any metabolic transformation in humans or animals other than hydrolysis by bacteria in the gut. Studies also showed that urea was primarily eliminated via the urine within 24 hours after dosing.

 

This information is considered to be suitable for assessment and can be used in a read across approach for hydrogen peroxide – urea (1:1).

Conclusion

The oral uptake of hydrogen peroxide – urea (1:1) was considered to be 100%. The dermal uptake is considered to be 50 % of the oral uptake. The water solubility of hydrogen peroxide – urea (1:1) and also of the break down products H2O2 and urea is very high (> 10.000 mg/L). According to the “Guidance on information requirements and chemical safety assessment Chapter R.8: Characterisation of dose [concentration]-response for human health” pulished by ECHA (2012) substances with a water solubility > 10.000 mg/L are expect to pass the lipid rich environment of the stratum corneum only to a minor content. Therefore, for risk assessment purposes the dermal uptake is considered to be 50 % of the oral uptake.