Activated Carbon - Low Density Skeleton

Endpoint summary

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

basic toxicokinetics

Type of information:

experimental study

Adequacy of study:

key study

Study period:

22 November 2013 - 14 March 2014

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

guideline study

Remarks:

The study was performed in accordance with OECD Guideline 417 and under GLP conditions. The study was assigned a Klimisch 2 rating due to the read-across purpose, in accordance with the ECHA Practical guide #6 on the reporting of read-across in IUCLID. The test is part of an Integrated Test Strategy (ITS) and its results will be used as such.

Justification for type of information:

The justification for read across is attached as a separate document to the "Toxicological information" summary

Reason / purpose for cross-reference:

reference to same study

Objective of study:

absorption

distribution

excretion

Qualifier:

according to guideline

Guideline:

OECD Guideline 417 (Toxicokinetics)

Deviations:

no

GLP compliance:

yes (incl. QA statement)

Radiolabelling:

yes

Species:

rat

Strain:

Wistar

Sex:

male/female

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source: Harlan
- Age at study initiation: 9-10 weeks
- Weight at study initiation: 185 g -206 g (females); 288 g - 313 g (males)
- Housing: conventional (during acclimatization)
- Individual metabolism cages: yes
- Diet (e.g. ad libitum): yes
- Water (e.g. ad libitum): yes
- Acclimation period: at least 5 days

ENVIRONMENTAL CONDITIONS
- Temperature (°C): 22 +/- 2
- Humidity (%): 45% - 65%
- Air changes (per hr): 10
- Photoperiod (hrs dark / hrs light): 12/12

Route of administration:

intratracheal

Vehicle:

physiological saline

Details on exposure:

Route = Intratracheal
The rats received a single intra-tracheal dose of 1.0 mL/kg BW (2.75 mg 14C-carbon particles per kg BW containing a radioactive dose of ca.2.4 MBq/kg BW) using a polycarbonate Penncentury syringe equipped with intra-tracheal aerolizer model 1A-1B.

Duration and frequency of treatment / exposure:

The rats received a single intra-tracheal dose using a polycarbonate Penncentury syringe equipped with intra-tracheal aerolizer model 1A-1B.

Remarks:

Doses / Concentrations:
The concentration was 2.75 mg 14C-carbon particles per mL
The dose volume was 1 mL per kg body weight

No. of animals per sex per dose / concentration:

15 males and 15 females

Control animals:

yes, historical

Details on study design:

Dose selection rationale:
The dose level was 2.75 mg 14C-carbon particles per kg body weight
The dose level was selected based on the 90-day inhalation study in which rats were exposed at 10 mg/m3 at the highest dose (TNO Triskelion Report V20245). This single dose was equivalent to a 5-days intake in the inhalation study.

Details on dosing and sampling:

The study consisted of the following groups:

Group A; dose level 2.75 mg/kg bwt; 5 male + 5 female rats; sacrifice 24 h after treatment
Urine and feces at 0-8h, 8-24h. Cage wash at 24 h.

Group B; dose level 2.75 mg/kg bwt; 5 male + 5 female rats; sacrifice 72 h after treatment
Urine and feces at 0-8h, 8-24h, 24-48 h, 48-72h. Cage wash at 72 h.

Group C; dose level 2.75 mg/kg bwt; 5 male + 5 female rats; sacrifice 168 h after treatment
Urine and feces at 0-8h, 8-24h, 24-48 h, 48-72h, 72-96h, 96-120h, 120-144h, 144-168h. Cage wash at 168 h.
Blood at 30m, 1h, 2h, 4h, 6h, 8h, 24h, 48h, 96h via the tail vein.

All groups:
Organs and tissues: blood/plasma, lungs including trachea up to larynx, trachea-bronchial lymph nodes, liver, kidneys, spleen, testes/ovaria, GI tract and contents, carcass.

The concentration of radioactivity was measured by taking aliquots just before and after application of each group. The homogeneity in the formulations was sufficient. The calculated specific radioactivity was 869.52 KBq/mg and was calculated by dividing the measured radioactivity by the total amount of test substance.

Statistics:

Not applicable

Details on absorption:

Absorption
As a worst case, intratracheal instillation with AC-HDS particles at a dose of 2.75 mg/kg results in a possible systemic exposure of maximally 0.52% of the administered dose. The major part (0.48%) was found in the carcass and is most probably located in the larynx and oral cavity, where it is in transit from the lungs to the intestinal tract. Any AC-HDS present in the larynx/oral cavity will not be systemically absorbed. Therefore, it is plausible to assume that the overall systemic exposure is considerably lower based on the extremely low radioactivity found in several internal organs and the almost absence of radioactivity in the reproduction organs and blood.

Recovery
No study- or test substance-related signs of toxicity or unusual behaviour were observed. Body weights observed at the end of observation were slightly lower than at the start of the treatment. However, the loss was considered secondary to housing in metabolism cages and constant across the groups, and thus not indicative of systemic toxicity. The administered doses were close to the intended doses. Mean recovery of radioactivity in the 6 sub-groups ranged from 89.71% to 112.07%.

Details on distribution in tissues:

Tissue distribution and disposition
After instillation of [14C]activated carbon particles into respiratory tract, radioactivity was mostly found in the respiratory tract (lungs and trachea up to the larynx). In males, 84.41% of the applied dose was found in the respiratory tract at 24 hours post-dosing and this slightly decreased to 79.34% at 168 hours. Similarly, in females, 79.76% of the applied dose was found at 24 hours in the respiratory tract and decreased slightly until the end of the study (76.66% at 168 hours). In addition, radioactivity was also found in other tissues like gastrointestinal tract, trachea-bronchial lymph nodes and carcass. However, the amount of radioactivity in these tissues was low compared to that found in the respiratory tract. The radioactivity found in other tissues measured (such as liver, spleen, and kidneys) was found to be extremely low and in most of the cases was below 0.01% of the administered dose. In the reproduction organs (testes/ovaria) the radioactivity was below 0.001% of the administered dose.
After distribution and equilibration, the disposition of radioactivity in most tissues slightly declined over the course of time (except for liver and spleen), while the appearance of radioactivity in the excreta increased over the course of time. Overall, the high radioactivity found in the respiratory tract indicates that the biological half-life of the activated carbon particles is very large. The fact that the rate of radioactivity disappearance from the respiratory tract resembles the rate of radioactivity appearance in the faeces suggests that most of the activated carbon particles are excreted directly via the gastrointestinal tract rather than being first absorbed in the systemic blood. This suggests that the mucus with the [14C]activated carbon particles is transported by the mucociliary escalator until it reaches the oral cavity where it is swallowed, passed to the stomach and excreted via the intestinal tract.

Details on excretion:

Excretion
Following single dosing with 2.75 mg/kg [14C]-AC-HDS, radioactivity was predominantly excreted in faeces and only a small amount was excreted with urine (the radioactivity in the faeces was a factor of 1000 higher than in the urine). Around 50% of the radioactivity found in the excreta was excreted within the first 48 hours in both genders. It was noted that in some rats the highest excretion of radioactivity was found in the period from 0-24 hours, whereas in others it was found between 24-48 hours post-dosing. The reason for this difference is unknown but might be related to variation in ingestion and/or individual variability in GI –tract mobility.

Key result

Test no.:

#1

Toxicokinetic parameters:

other: The blood concentration of activated carbon particles was very low (around the limit of quantification) and there was no clear uptake and elimination. The data did not allow determination of pharmacokinetic parameters.

Metabolites identified:

not specified

Details on metabolites:

Not applicable

Table 1. Summary of recovery of radioactivity % of applied dose in excreta and tissues of male and female rats after an intratracheal instillation of 14C-AC-HDS (single nominal dose of 2.75 mg/kg).

	Males			Females
Time group	24 h	72 h	168 h	24 h	72 h	168 h
Urine (total)	0.01	0.02	0.03	0.02	0.01	0.03
Faeces (total)	11.31	21.88	30.93	1.21	21.65	30.90
Cage wash	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01
Total excreted	11.32	21.91	30.96	1.23	21.66	30.94
Blood	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01
Organs	84.47	82.25	79.40	79.80	81.90	76.74
GI-tract	5.33	1.52	1.23	7.90	1.43	0.54
Carcass	1.55	0.61	0.48	0.78	0.31	0.20
Total retained tissues	90.94	84.38	81.11	88.48	83.64	77.48
Recovery	102.26	106.28	112.07	89.71	105.30	108.42

Table 2. Summary of % of radioactive dose distributed to organs of male and female rats after an intratracheal instillation of [14C]activated carbon particles (single nominal dose of 2.75 mg/kg).

	% of radioactive dose
	Males			Females
Time group	24 h	72 h	168 h	24 h	72 h	168 h
Lungs including trachea up to the larynx	84.41	82.19	79.34	79.76	81.84	76.66
Trachea-bronchial lymph nodes	0.050	0.049	0.040	0.027	0.042	0.028
Liver	0.003	0.008	0.013	0.009	0.013	0.039
Kidneys	0.001	0.001	0.001	0.001	0.001	0.001
Spleen	<0.001	<0.001	0.001	0.001	0.001	0.003
Testes	<0.001	<0.001	<0.001
Ovaria				<0.001	<0.001	<0.001

 

Table 3. Summary of concentration of radioactivity in tissues of male and female rats after an intratracheal instillation of [14C]activated carbon particles (single nominal dose of 2.75 mg/kg).

	µg Parent Compound Equivalents/g of tissue
	Males			Females
9Time group	24 h	72 h	168 h	24 h	72 h	168 h
Lungs including trachea up to the larynx	280.50	235.06	226.84	266.03	277.39	293.30
Trachea-bronchial lymph nodes	18.94	16.56	16.59	10.69	13.66	11.60
Liver	<0.01	<0.01	<0.01	<0.01	0.01	0.03
Kidneys	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01
Spleen	<0.01	<0.01	0.01	0.01	0.02	0.04
Testes	<0.01	<0.01	<0.01
Ovaria				<0.01	<0.01	<0.01
GI-tract and contents	1.59	0.39	0.30	2.55	0.39	0.16
Carcass	0.05	0.02	0.02	0.03	0.01	0.01

Conclusions:

A toxicokinetic study was performed in accordance with OECD 417 and under GLP conditions. From the results of the present study it can be concluded as a worst case that intratracheal instillation with AC-HDS particles at a dose of 2.75 mg/kg results in a possible systemic exposure of maximally 0.52% of the administered dose. The major part (0.48%) was found in the carcass and is most probably located in the larynx and oral cavity, where it is in transit from the lungs to the intestinal tract. Any AC-HDS present in the larynx/oral cavity will not be systemically absorbed. It is plausible to assume that the overall systemic exposure is considerably lower based on the extremely low radioactivity found in several internal organs and the almost absence of radioactivity in the reproduction organs and blood. Based on these results it is highly unlikely that exposure to AC-HDS will result in systemic effects.

Executive summary:

An OECD 417 test guideline study was carried out to evaluate the (lack of systemic) absorption and tissue distribution of AC-HDS particles after intratracheal instillation in male and in female Wistar rats at a nominal dose of 2.75 mg/kg. This dose corresponds to approximately 5 times the daily dose in a 90-day inhalation toxicity study with AC-HDS in rats at a dose concentration of 10 mg/m³.

Three groups of male rats (N=5) and three groups of female rats (N=5) received an intratracheal instillation with [14C]-AC-HDS particles and excreta and blood/plasma were collected. Animals were sacrificed at 24, 72 and 168 hours post-dose for collection of organs (lungs including trachea up to the larynx, trachea-bronchial lymph nodes, liver, kidneys, spleen, testes/ovaria), GI tract and its contents and carcass.

The major findings from this study are summarized below:

Excretion. In males as well as in females, the excretion of radioactivity increased over the time of the study (from 1-11% of the administered radioactive dose at 24 hours to 31% of the administered radioactive dose at 168 hours). This excretion occurred mostly with the faeces (up to 31%) and very low radioactivity was found in the urine (up to 0.03%).

Tissue distribution and disposition. Following intratracheal instillation of [14C] AC-HDS particles into respiratory tract, most of the radioactivity was distributed in lungs and trachea (up to the larynx) in males as well as in females. In males, 84.41% of the applied dose was found in the respiratory tract at 24 hours post-dosing and this slightly decreased to 79.34% at 168 hours post-dosing. Similarly, in females, 79.76% of the applied dose was found at 24 hours in the respiratory tract and this slightly decreased to 76.66% at 168 hours. Some radioactivity was also found in other tissues. In either male or female rats, the maximal remaining radioactivity after 168 hours was 1.23% in the gastrointestinal tract, 0.48% in the carcass, 0.04% in the trachea-broncheal lymph-nodes, 0.04% in the liver, 0.003% in the spleen and 0.001% in the kidneys. In the reproductive organs (testes and ovaries) the radioactivity remained below 0.001% if the applied dose. It is assumed that some of the radioactivity is retained in the GI-tract partly due to delayed transit in faeces and partly through continued ingestion of coughed-up particles from the lungs during the 168 h study period. The decrease in lung radioactivity of about 5% in males and about 3% in females supports this assumption. There is no evidence that AC-HSC particles are absorbed through the GI-tract, which is corroborated by the virtual absence of radioactivity in blood and plasma at several time points during the study (blood radioactivity remained around the Limit of Quantification). The relatively high radioactivity in trachea-broncheal lymph nodes can be explained by translocation of macrophages loaded with small numbers of AC-HDS particles from the lungs to the draining lymph nodes. This phenomenon was also observed in the 90-day inhalation toxicity study (TNO Triskelion study V20245). As a worst case, the remaining radioactivity of 0.48% in the carcass may be considered available for systemic absorption, although it is not clear how the radioactivity is distributed over the carcass and whether it is bioavailable. The ca. 0.5% of carcass-radioactivity will most probably be located in the larynx/oral cavity and it seems logical to assume that this AC-HDS is in transit for clearance from the lungs through the GI-tract. The remaining very low systemic absorption of AC-HDS in internal organs was limited to the liver, spleen and kidneys, which in combination accounted for maximally 0.044% of the applied dose of radioactivity. Carcass, liver, spleen and kidneys account for 0.52% of the applied dose. The extremely low disposition in the reproduction organs (testes and ovaries) was less that 0.001% of the applied dose and can be neglected.

Conclusion

From the results of the present study it can be concluded as a worst case that intratracheal instillation with AC-HDS particles at a dose of 2.75 mg/kg results in a possible systemic exposure of maximally 0.52% of the administered dose. The major part (0.48%) was found in the carcass and is most probably located in the larynx and oral cavity, where it is in transit from the lungs to the intestinal tract. Any AC-HDS present in the larynx/oral cavity will not be systemically absorbed. It is plausible to assume that the overall systemic exposure is considerably lower based on the extremely low radioactivity found in several internal organs and the almost absence of radioactivity in the reproduction organs and blood. Based on these results it is highly unlikely that exposure to AC-HDS will result in systemic effects.

Reference 2

Endpoint:

basic toxicokinetics

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Justification for type of information:

The justification for read across is attached

Reason / purpose for cross-reference:

read-across source

Details on absorption:

Absorption
As a worst case, intratracheal instillation with AC-HDS particles at a dose of 2.75 mg/kg results in a possible systemic exposure of maximally 0.52% of the administered dose. The major part (0.48%) was found in the carcass and is most probably located in the larynx and oral cavity, where it is in transit from the lungs to the intestinal tract. Any AC-HDS present in the larynx/oral cavity will not be systemically absorbed. Therefore, it is plausible to assume that the overall systemic exposure is considerably lower based on the extremely low radioactivity found in several internal organs and the almost absence of radioactivity in the reproduction organs and blood.

Recovery
No study- or test substance-related signs of toxicity or unusual behaviour were observed. Body weights observed at the end of observation were slightly lower than at the start of the treatment. However, the loss was considered secondary to housing in metabolism cages and constant across the groups, and thus not indicative of systemic toxicity. The administered doses were close to the intended doses. Mean recovery of radioactivity in the 6 sub-groups ranged from 89.71% to 112.07%.

Details on distribution in tissues:

Tissue distribution and disposition
After instillation of [14C]activated carbon particles into respiratory tract, radioactivity was mostly found in the respiratory tract (lungs and trachea up to the larynx). In males, 84.41% of the applied dose was found in the respiratory tract at 24 hours post-dosing and this slightly decreased to 79.34% at 168 hours. Similarly, in females, 79.76% of the applied dose was found at 24 hours in the respiratory tract and decreased slightly until the end of the study (76.66% at 168 hours). In addition, radioactivity was also found in other tissues like gastrointestinal tract, trachea-bronchial lymph nodes and carcass. However, the amount of radioactivity in these tissues was low compared to that found in the respiratory tract. The radioactivity found in other tissues measured (such as liver, spleen, and kidneys) was found to be extremely low and in most of the cases was below 0.01% of the administered dose. In the reproduction organs (testes/ovaria) the radioactivity was below 0.001% of the administered dose.
After distribution and equilibration, the disposition of radioactivity in most tissues slightly declined over the course of time (except for liver and spleen), while the appearance of radioactivity in the excreta increased over the course of time. Overall, the high radioactivity found in the respiratory tract indicates that the biological half-life of the activated carbon particles is very large. The fact that the rate of radioactivity disappearance from the respiratory tract resembles the rate of radioactivity appearance in the faeces suggests that most of the activated carbon particles are excreted directly via the gastrointestinal tract rather than being first absorbed in the systemic blood. This suggests that the mucus with the [14C]activated carbon particles is transported by the mucociliary escalator until it reaches the oral cavity where it is swallowed, passed to the stomach and excreted via the intestinal tract.

Details on excretion:

Excretion
Following single dosing with 2.75 mg/kg [14C]-AC-HDS, radioactivity was predominantly excreted in faeces and only a small amount was excreted with urine (the radioactivity in the faeces was a factor of 1000 higher than in the urine). Around 50% of the radioactivity found in the excreta was excreted within the first 48 hours in both genders. It was noted that in some rats the highest excretion of radioactivity was found in the period from 0-24 hours, whereas in others it was found between 24-48 hours post-dosing. The reason for this difference is unknown but might be related to variation in ingestion and/or individual variability in GI –tract mobility.

Key result

Test no.:

#1

Toxicokinetic parameters:

other: The blood concentration of activated carbon particles was very low (around the limit of quantification) and there was no clear uptake and elimination. The data did not allow determination of pharmacokinetic parameters.

Metabolites identified:

not specified

Details on metabolites:

Not applicable

Table 1. Summary of recovery of radioactivity % of applied dose in excreta and tissues of male and female rats after an intratracheal instillation of 14C-AC-HDS (single nominal dose of 2.75 mg/kg).

	Males			Females
Time group	24 h	72 h	168 h	24 h	72 h	168 h
Urine (total)	0.01	0.02	0.03	0.02	0.01	0.03
Faeces (total)	11.31	21.88	30.93	1.21	21.65	30.90
Cage wash	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01
Total excreted	11.32	21.91	30.96	1.23	21.66	30.94
Blood	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01
Organs	84.47	82.25	79.40	79.80	81.90	76.74
GI-tract	5.33	1.52	1.23	7.90	1.43	0.54
Carcass	1.55	0.61	0.48	0.78	0.31	0.20
Total retained tissues	90.94	84.38	81.11	88.48	83.64	77.48
Recovery	102.26	106.28	112.07	89.71	105.30	108.42

Table 2. Summary of % of radioactive dose distributed to organs of male and female rats after an intratracheal instillation of [14C]activated carbon particles (single nominal dose of 2.75 mg/kg).

	% of radioactive dose
	Males			Females
Time group	24 h	72 h	168 h	24 h	72 h	168 h
Lungs including trachea up to the larynx	84.41	82.19	79.34	79.76	81.84	76.66
Trachea-bronchial lymph nodes	0.050	0.049	0.040	0.027	0.042	0.028
Liver	0.003	0.008	0.013	0.009	0.013	0.039
Kidneys	0.001	0.001	0.001	0.001	0.001	0.001
Spleen	<0.001	<0.001	0.001	0.001	0.001	0.003
Testes	<0.001	<0.001	<0.001
Ovaria				<0.001	<0.001	<0.001

 

Table 3. Summary of concentration of radioactivity in tissues of male and female rats after an intratracheal instillation of [14C]activated carbon particles (single nominal dose of 2.75 mg/kg).

	µg Parent Compound Equivalents/g of tissue
	Males			Females
9Time group	24 h	72 h	168 h	24 h	72 h	168 h
Lungs including trachea up to the larynx	280.50	235.06	226.84	266.03	277.39	293.30
Trachea-bronchial lymph nodes	18.94	16.56	16.59	10.69	13.66	11.60
Liver	<0.01	<0.01	<0.01	<0.01	0.01	0.03
Kidneys	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01
Spleen	<0.01	<0.01	0.01	0.01	0.02	0.04
Testes	<0.01	<0.01	<0.01
Ovaria				<0.01	<0.01	<0.01
GI-tract and contents	1.59	0.39	0.30	2.55	0.39	0.16
Carcass	0.05	0.02	0.02	0.03	0.01	0.01

Conclusions:

A toxicokinetic study was performed on AC-HDS, and the results are used for Read across to AC-LDS. The test was performed in accordance with OECD 417 and under GLP conditions. From the results of the present study it can be concluded as a worst case that intratracheal instillation with AC-HDS particles at a dose of 2.75 mg/kg results in a possible systemic exposure of maximally 0.52% of the administered dose. The major part (0.48%) was found in the carcass and is most probably located in the larynx and oral cavity, where it is in transit from the lungs to the intestinal tract. Any AC-HDS present in the larynx/oral cavity will not be systemically absorbed. It is plausible to assume that the overall systemic exposure is considerably lower based on the extremely low radioactivity found in several internal organs and the almost absence of radioactivity in the reproduction organs and blood. Based on these results it is highly unlikely that exposure to AC-HDS will result in systemic effects.

Executive summary:

A toxicokinetic study was performed on AC-HDS, and the results are used for Read across to AC-LDS. An OECD 417 test guideline study was carried out to evaluate the (lack of systemic) absorption and tissue distribution of AC-HDS particles after intratracheal instillation in male and in female Wistar rats at a nominal dose of 2.75 mg/kg. This dose corresponds to approximately 5 times the daily dose in a 90-day inhalation toxicity study with AC-HDS in rats at a dose concentration of 10 mg/m³.

Three groups of male rats (N=5) and three groups of female rats (N=5) received an intratracheal instillation with [14C]-AC-HDS particles and excreta and blood/plasma were collected. Animals were sacrificed at 24, 72 and 168 hours post-dose for collection of organs (lungs including trachea up to the larynx, trachea-bronchial lymph nodes, liver, kidneys, spleen, testes/ovaria), GI tract and its contents and carcass.

The major findings from this study are summarized below:

Excretion. In males as well as in females, the excretion of radioactivity increased over the time of the study (from 1-11% of the administered radioactive dose at 24 hours to 31% of the administered radioactive dose at 168 hours). This excretion occurred mostly with the faeces (up to 31%) and very low radioactivity was found in the urine (up to 0.03%).

Tissue distribution and disposition. Following intratracheal instillation of [14C] AC-HDS particles into respiratory tract, most of the radioactivity was distributed in lungs and trachea (up to the larynx) in males as well as in females. In males, 84.41% of the applied dose was found in the respiratory tract at 24 hours post-dosing and this slightly decreased to 79.34% at 168 hours post-dosing. Similarly, in females, 79.76% of the applied dose was found at 24 hours in the respiratory tract and this slightly decreased to 76.66% at 168 hours. Some radioactivity was also found in other tissues. In either male or female rats, the maximal remaining radioactivity after 168 hours was 1.23% in the gastrointestinal tract, 0.48% in the carcass, 0.04% in the trachea-broncheal lymph-nodes, 0.04% in the liver, 0.003% in the spleen and 0.001% in the kidneys. In the reproductive organs (testes and ovaries) the radioactivity remained below 0.001% if the applied dose. It is assumed that some of the radioactivity is retained in the GI-tract partly due to delayed transit in faeces and partly through continued ingestion of coughed-up particles from the lungs during the 168 h study period. The decrease in lung radioactivity of about 5% in males and about 3% in females supports this assumption. There is no evidence that AC-HSC particles are absorbed through the GI-tract, which is corroborated by the virtual absence of radioactivity in blood and plasma at several time points during the study (blood radioactivity remained around the Limit of Quantification). The relatively high radioactivity in trachea-broncheal lymph nodes can be explained by translocation of macrophages loaded with small numbers of AC-HDS particles from the lungs to the draining lymph nodes. This phenomenon was also observed in the 90-day inhalation toxicity study (TNO Triskelion study V20245). As a worst case, the remaining radioactivity of 0.48% in the carcass may be considered available for systemic absorption, although it is not clear how the radioactivity is distributed over the carcass and whether it is bioavailable. The ca. 0.5% of carcass-radioactivity will most probably be located in the larynx/oral cavity and it seems logical to assume that this AC-HDS is in transit for clearance from the lungs through the GI-tract. The remaining very low systemic absorption of AC-HDS in internal organs was limited to the liver, spleen and kidneys, which in combination accounted for maximally 0.044% of the applied dose of radioactivity. Carcass, liver, spleen and kidneys account for 0.52% of the applied dose. The extremely low disposition in the reproduction organs (testes and ovaries) was less that 0.001% of the applied dose and can be neglected.

Conclusion

From the results of the present study it can be concluded as a worst case that intratracheal instillation with AC-HDS particles at a dose of 2.75 mg/kg results in a possible systemic exposure of maximally 0.52% of the administered dose. The major part (0.48%) was found in the carcass and is most probably located in the larynx and oral cavity, where it is in transit from the lungs to the intestinal tract. Any AC-HDS present in the larynx/oral cavity will not be systemically absorbed. It is plausible to assume that the overall systemic exposure is considerably lower based on the extremely low radioactivity found in several internal organs and the almost absence of radioactivity in the reproduction organs and blood. Based on these results it is highly unlikely that exposure to AC-HDS will result in systemic effects.

Description of key information

Background

The Activated carbon substances Activated Carbon - High Density Skeleton and Activated Carbon - Low Density Skeleton can be described as porous amorphous high surface area substances composed largely of elemental carbon. Both Activated Carbon - High Density Skeleton (AC-HDS) and Activated Carbon - Low Density Skeleton (AC-LDS) consist of >80% carbon. They are registered within a grouping approach, and are mentioned as ‘activated carbon’ in this toxicokinetic assessment.

 

Information from physicochemical properties and other studies from the dossier.

The available physicochemical properties and toxicology studies can provide useful information regarding the behaviour of ‘activated carbon’ in the body (based on the REACH Guidance on Information Requirements and Chemical Safety Assessment Chapter R.7c Endpoint Specific Guidance). ‘Activated carbon’ substances are solid at room temperature, have a water solubility <0.1 mg/L (Harlan report # C80385 and # C80306, AC-HDS and AC-LDS resp.) and they have an expected melting point > 1000 degrees C. They are not highly flammable (Harlan report # C80396 and # C80317, AC-HDS and AC-LDS resp.). The particle size of AC-HDS ranges from approximately 0.5 to >0.5 µm (Harlan report # C80418) and the particle size of AC-LDS ranges from approximately 0.3 to > 0.3 um.

Absorption via the oral, inhalation, and dermal route is not expected based on these phys/chem parameters, as ‘activated carbon’ is not soluble in water or in organic solvents. After chronic exposure via the inhalatory route (Gross and Nau,1967) activated carbon dust is found predominantly in the alveoli of rats, mice and guinea pigs. The dust is generally found in relatively dense masses which fill the air spaces completely or partially. The alveolar membrane adjacent to the dust is frequently altered. The alterations consist of slight thickening of alveolar walls by proliferation and enlargement of cells lining the alveolar surface. Focal chronic interstitial pneumonitis is also not uncommonly found in the tissue between dust deposits, but this was also apparent in control animals. These findings indicate that activated carbon is an “inert” dust.

In a sub-chronic 90 -day inhalation study the lungs of rats exposed to 3, 7 or 10 mg/m3 of AC-HDS all showed accumulation of black particles in the lungs. Most of the rats also showed translocation of AC particles to the trachealbroncheal lymph nodes. Most of the activated carbon particles were present in alveolar macrophages.A few macrophages with particles were present in the alveolar walls. Small numbers of particles were present as free particles, mostly in the alveoli, sometimes also in the bronchiolar lumen. In the tracheobronchial lymph nodes the activated carbon particles was present as free particles as well as within macrophages. In general, the number of particles in the lungs and tracheobronchial lymph nodes was in proportion to the exposure levels. Significant systemic uptake via the inhalatory route is not expected. In general the substance size will hamper passing membranes as the substance consists of particles with sizes ranging from approximately 0.5 to > 0.5 µm. Therefore AC-HDS is not expected to pass cell membranes to become accessible for enzymatic attack and thus enzymatic oxidation.

Studies on kinetics in the dossier

The systemic absorption and tissue distribution of radiolabeled AC-HDS after intratracheal instillation was studied in rats.An OECD 417 test guideline study was carried out to evaluate the (lack of systemic) absorption and tissue distribution of AC-HDS particles after intratracheal instillation in male and in female Wistar rats at a nominal dose of 2.75 mg/kg. This dose corresponds to approximately 5 times the daily dose in a 90-day inhalation toxicity study with AC-HDS in rats at a dose concentration of 10 mg/m³.

Three groups of male rats (N=5) and three groups of female rats (N=5) received an intratracheal instillation with [14C]-AC-HDS particles and excreta and blood/plasma were collected. Animals were sacrificed at 24, 72 and 168 hours post-dose for collection of organs (lungs including trachea up to the larynx, trachea-bronchial lymph nodes, liver, kidneys, spleen, testes/ovaria), GI tract and its contents and carcass. The major findings from this study are summarized below: Excretion. In males as well as in females, the excretion of radioactivity increased over the time of the study (from 1-11% of the administered radioactive dose at 24 hours to 31% of the administered radioactive dose at 168 hours). This excretion occurred mostly with the faeces (up to 31%) and very low radioactivity was found in the urine (up to 0.03%).

Tissue distribution and disposition. Following intratracheal instillation of [14C] AC-HDS particles into respiratory tract, most of the radioactivity was distributed in lungs and trachea (up to the larynx) in males as well as in females. In males, 84.41% of the applied dose was found in the respiratory tract at 24 hours post-dosing and this slightly decreased to 79.34% at 168 hours post-dosing. Similarly, in females, 79.76% of the applied dose was found at 24 hours in the respiratory tract and this slightly decreased to 76.66% at 168 hours. Some radioactivity was also found in other tissues. In either male or female rats, the maximal remaining radioactivity after 168 hours was 1.23% in the gastrointestinal tract, 0.48% in the carcass, 0.04% in the trachea-broncheal lymph-nodes, 0.04% in the liver, 0.003% in the spleen and 0.001% in the kidneys. In the reproduction organs (testes and ovaries) the radioactivity remained below 0.001% if the applied dose. It is assumed that some of the radioactivity is retained in the GI-tract partly due to delayed transit in faeces and partly through continued ingestion of coughed-up particles from the lungs during the 168 h study period. The decrease in lung radioactivity of about 5% in males and about 3% in females supports this assumption. There is no evidence that AC-HSC particles are absorbed through the GI-tract, which is corroborated by the virtual absence of radioactivity in blood and plasma at several time points during the study (blood radioactivity remained around the Limit of Quantification). The relatively high radioactivity in trachea-broncheal lymph nodes can be explained by translocation of macrophages loaded with small numbers of AC-HDS particles from the lungs to the draining lymph nodes. This phenomenon was also observed in the 90 -day inhalation toxicity study (TNO Triskelion study V20245). As a worst case, the remaining radioactivity of 0.48% in the carcass may be considered available for systemic absorption, although it is not clear how the radioactivity is distributed over the carcass and whether it is bioavailable. The ca. 0.5% of carcass-radioactivity will most probably be located in the larynx/oral cavity and it seems logical to assume that this AC-HDS is in transit for clearance from the lungs through the GI-tract. The remaining very low systemic absorption of AC-HDS in internal organs was limited to the liver, spleen and kidneys, which in combination accounted for maximally 0.044% of the applied dose of radioactivity. Carcass, liver, spleen and kidneys account for 0.52% of the applied dose. The extremely low disposition in the reproduction organs (testes and ovaries) was less that 0.001% of the applied dose and can be neglected.

Other information

Activated carbons have a well-known therapeutic purpose based on its adsorbing properties due to its highly developed internal pore structure. It is used in the treatment of acute poisoning and the treatment of acute diarrhea. The beneficial effect of activated carbons is achieved by interruption of the enteroenteric and, in some cases, the enterohepatic and enterogastric circulation of toxins and bacteria, and, in addition, by adsorbtion of any unabsorbed toxins and bacteria still present in the gut. The mechanism of this use is based on the ability of activated carbons to adsorb other substances, and on their inertness, resulting in the excretion of activated carbon together with the toxin or bacteria, preventing absorption into the blood. (see for example the Position Paper on Single-dose Activated Charcoal (2005), and the Position Statement and Practice Guidelines on the use of Multi-dose Activated Charcoal in the treatment of acute poisoning (1999), both by the American Academy of Clinical Toxicology and European Association of Poison Centres and Clinical Toxicologists and published in the Journal “Clinical Toxicology” (43, pp 61-87 and 37, pp 731-751, respectively)).

 

Conclusions

From the results of the absorption/distribution study it can be concluded as a worst case that intratracheal instillation with AC-HDS particles at a dose of 2.75 mg/kg results in a possible systemic exposure of maximally 0.52% of the administered dose. The major part (0.48%) was found in the carcass and is most probably located in the larynx and oral cavity, where it is in transit from the lungs to the intestinal tract. Any AC-HDS present in the larynx/oral cavity will not be systemically absorbed. It is plausible to assume that the overall systemic exposure is considerably lower based on the extremely low radioactivity found in several internal organs and the almost absence of radioactivity in the reproduction organs and blood.

Based on the physical and chemical properties of activated carbons, the absence of systemic effects in toxicological studies and the therapeutic use of activated carbons as adsorbing agents for the treatment of acute poisoning and acute diarrhea, it can be expected that Activated Carbon - High Density Skeleton is not absorbed via the oral or dermal route. Moreover, based on the results of the intratracheal absorption/distribution study and the absence of systemic effects in the 90-day inhalation toxicity study, it is estimated that about 0.5% of the applied dose of Activated Carbon - High Density Skeleton is available for absorbtion via the intratracheal/inhalation route. Due to the extremely low radioactivity found in organs and blood, it is estimated that only 0.02% of the applied dose reaches the internal organs, most of this radioactivity concerning translocation of small numbers of particles to the tracheobroncheal lymph nodes. The observed radioactivity in the reproductive organs (ovaries and testes) was less than 0.001% of the applied dose. Based on these results it is highly unlikely that exposure to 'activated carbon' will result in systemic effects.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential