Diarsenic trioxide

Endpoint summary

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

basic toxicokinetics, other

Type of information:

other: review

Adequacy of study:

weight of evidence

Reliability:

other: Reliable review. In view of the enormous database on arsenic toxicity, this and other reviews are used as the key sources of information for the hazard assessment, instead of repeating as assessment of primary data.

Rationale for reliability incl. deficiencies:

other: see 'Remark'

Remarks:

Comprehensive review and summary on several aspects of arsenic toxicity by a renown scientific organisation. In view of the enormous database on arsenic toxicity, this and other reviews are used as the key sources of information for the hazard assessment, instead of repeating an assessment of primary data.

Principles of method if other than guideline:

Not applicable (comprehensive, reliable review and summary on several aspects of arsenic toxicity by a renown scientific organisation).

Type:

other: see discussion / endpoint summary

Reference 2

Endpoint:

basic toxicokinetics, other

Type of information:

experimental study

Adequacy of study:

weight of evidence

Type:

other:

Results:

Based on the reported findings, an average of 60% of total arsenic intake is assumed to be excreted via urine. For details, please refer to endpoint summary/discussion.

Reference 3

Endpoint:

basic toxicokinetics, other

Type of information:

other: review

Adequacy of study:

weight of evidence

Reliability:

other: Reliable review. In view of the enormous database on arsenic toxicity, this and other reviews are used as the key sources of information for the hazard assessment, instead of repeating as assessment of primary data.

Rationale for reliability incl. deficiencies:

other: see 'Remark'

Remarks:

Comprehensive review and summary on several aspects of arsenic toxicity by a renown scientific organisation. In view of the enormous database on arsenic toxicity, this and other reviews are used as the key sources of information for the hazard assessment, instead of repeating an assessment of primary data.

Principles of method if other than guideline:

Not applicable (comprehensive, reliable review and summary on several aspects of arsenic toxicity by a renown scientific organisation).

Type:

other: see discussion / endpoint summary

Reference 4

Endpoint:

basic toxicokinetics, other

Type of information:

other: PBPK modeling

Adequacy of study:

weight of evidence

Type:

other:

Results:

PB-PK model, please see endpoint summary.

Reference 5

Endpoint:

basic toxicokinetics, other

Type of information:

other: PBPK modeling

Adequacy of study:

weight of evidence

Type:

other:

Results:

PB-PK model, please see endpoint summary.

Reference 6

Endpoint:

dermal absorption in vitro / ex vivo

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Reasonably well-documented publication, conduct acc. to general scientific principles.

Principles of method if other than guideline:

In vivo (Rhesus monkey) and In vitro (human skin) percutaneous absorption of arsenic acid (73As radiolabelled) from soil and water for (comparative reasons) was investigated. This endpoint record focuses on the in vitro data in human skin, because of higher relevance for human health risk assessment. Further, the focus is on the absorption experiments conducted with aqueous solutions (worst-case, as compared to absorption from soil).

GLP compliance:

not specified

Radiolabelling:

yes

Remarks:

73As

Vehicle:

water

Doses:

Water formulations were prepared for comparison to experiments on As absorption from soil. The water load on skin was 5 µL/cm2 skin area. This amount of water is a thin layer of water which covers the skin but does not run off the skin. It is similar to a thin layer of other dermatological doses (cream, ointment).
The low (trace) dose was 0.000024 µg/cm2. The high dose: gave an arsenic skin concentration of 2.1 µg/cm2.

Details on in vitro test system (if applicable):

Three separate donor skin sources with three replicates per each experiment were used. Small cells were of the flow-through design with a 1 cm2 surface area. Phosphate-buffered saline at a flow rate of 3.0 mL/hr (reservoir volume) served as receptor fluid. Human cadaver skin was dermatomed to 500 µm and stored refrigerated at 4°C in Eagle's minimum essential medium to preserve skin viability. The skin was used within 5 days.
73As in water formulation was applied with a micropipette to the surface of the skin. Standards for each formulation were made by dissolving 5 µL of formulation in 10 mL of scintillation cocktail. At the end of a 24 h period the system was stopped. The residual water remaining in the cells was collected and analyzed. The skin surface was washed once with liquid soap (50/50 water, v/v: Ivory Liquid, Procter & Gamble. Cincinnati, OH) and twice with 1 mL of distilled water, and the wash solutions were analyzed by scintillation counting. Cells were disassembled. Cell tops were rinsed three times with 1 mL of water. The inner surface of the skin was swabbed with cotton balls and counted. The skin itself was completely solubilized in Soluene 3540 (Packard lnstruments, Downders Grove, IL), and 1 M HCl was added to neutralize the homogenate. The receptor fluid samples from the permeation cells' residual fluid, the skin surface washes, the cotton balls, the glass apparatus, and the skin itself were assayed for 73As content by liquid scintillation counting.

Key result

Time point:

24 h

Dose:

up to 2.1 µg/cm2

Parameter:

percentage

Absorption:

ca. 2 %

Remarks on result:

other: 24 h

Remarks:

in vitro, human skin; result accounts for ca. 1% absorbed As (found in receptor fluid) plus ca. 1% retained in the skin (potentially absorbable)

With water formulation, 0.93 ± 1.1 % of the applied dose accumulated in the receptor fluid and 0.98 ± 0.96% was in skin after surface wash. The soap and water wash accounted for 69.8 ± 16.4%. If in vitro percutaneous absorption is calculated as receptor fluid accumulation plus residual skin concentration (after soap and water wash) then the absorption in human skin is 1.9% from water. It appears from the text of the publication that these detailed results are for the low dose (0.000024 µg/cm2) application only.

However, in the discussion section of the publication, it is mentioned that for the high dose (2.1 µg/cm2), the absorption/penetration was 0.04 µg/cm2, that is also around 2%.

Description of key information

Read across approach

Diarsenic trioxide is readily soluble in water (17.8 g/L at 20°C). Upon dissolution in water, it reacts acidically to trivalent arsenite ions which are not subject to any relevant degree of oxidation for up to 72 hours (Klawonn, 2010). Read-across from toxicological data on inorganic arsenites to diarsenic trioxide is justified without restrictions. However, it is also known that in the human body, inorganic arsenic compounds are converted apart from As(III) also to As(V). Upon becoming systemically available, As(V) is rapidly partly converted to As(III). As(III) species are considered to be more toxic and bioactive than As(V) species. The difference in toxicological potency between As(III) and As(V) cannot be quantified exactly and may vary between routes of exposure and/or type of toxicological effects. Generally, risk assessments are conducted for "inorganic arsenic compounds" as a group, and do not differentiate between various species.

Following a conservative approach, the toxicity of diarsenic trioxide is therefore considered to be determined by the release of soluble inorganic species (trivalent arsenites and pentavalent arsenates) which do not differ substantially in potency and may be interconverted both in the environment and in the body. Consequently, it is justified to apply read across to soluble inorganic arsenic compounds to evaluate its systemic toxicological effects.

Absorption

Oral

Each of the forms of arsenic have different physicochemical properties and bioavailability. Studies in rats, mice and humans indicate that arsenite (As(III)) and arsenate (As(V)) present in drinking water are rapidly and nearly completely (about 95%) absorbed after ingestion (ATSDR, 2007). However, the absorption of ingested inorganic arsenic can vary depending on the solubility of the arsenical compounds (the more water soluble the compound, the greater its absorption), the presence of other food constituents and nutrients in the gastrointestinal tract, and the food matrix itself (EFSA, 2010).

Dermal

In an in vitro human skin permeation study with arsenic acid, a dermal absorption rate of 0.93% was obtained. In addition, 0.98% of the dose were retained in skin after washing (Wester et al., 1993). As a consequence, a conservative dermal absorption rate of maximum 2% from liquid media (0.2% from dry dust exposure according to the default values proposed by HERAG, 2007) may be considered for risk assessment via this route.

Inhalation

Deposition and subsequent absorption of arsenic powder in the lungs may be expected to be dependent on particle size: respirable particles (0.1 -1 µm) are carried further into the lungs where they are likely to be absorbed quickly, whereas larger particles will be translocated to the gut. However, based on the high oral bioavailability of soluble As(III) substances, complete systemic bioavailability after inhalation exposure can be assumed for conservative risk assessment purposes.

Distribution

In the bloodstream, arsenic is distributed between the plasma and the erythrocytes, in which it is bound to haemoglobin. Arsenite and arsenate ions are readily transported into cells: arsenite by aquaglycoporins 7 and 9 which normally transport water and glycerol, and arsenate by phosphate transporters. In most species, residue levels following uptake are initially elevated in liver, kidney, spleen and lung, but several weeks later arsenic is translocated to hair, nails and skin. Residual levels in animals tended to be higher for arsenite than arsenate. Arsenic is readily methylated in the body and methylarsonate is the predominant metabolite in kidneys, whereas dimethylarsinate is the predominant metabolite in lungs. It is worthy of note that rats differ from most mammalian species by accumulating arsenic in erythrocytes, likely by the binding of trivalent arsenic species to cysteine components of haemoglobin, with the binding affinity of trivalent arsenic species to red blood cells estimated to be 15-30-fold higher in rats than in humans (EFSA, 2010).

Placental transfer

Arsenic readily passes through the placenta of mammals and humans, resulting in similar exposure levels both in fetuses and mothers. Inorganic arsenic as well as its methylated metabolites (methylarsonate and dimethylarsinate) pass through the placenta. In newborn babies of women exposed to arsenic via drinking water in Argentina, essentially all arsenic in plasma and urine was in the form of dimethylarsinate, suggesting that it is mainly this metabolite that reaches the foetal circulation in late gestational phases. The metabolic methylation of arsenic via a one-carbon metabolism increases in women during pregnancy, which is why the human foetus is likely to be exposed to more inorganic arsenic and methylarsonate in early gestation (EFSA, 2010).

Transfer via mother’s milk

In contrast to the rapid transfer of arsenic to the foetus, very little arsenic is excreted in breast milk. Argentinian women exposed to about 200 μg/L arsenic in their drinking water showed very low excretion in breast milk (ca. 3 μg/L). A study in Bangladesh indicated very low arsenic concentrations in breast-milk samples (median 1 μg/kg; range 0.25-19 μg/kg) despite high arsenic exposures from drinking water (about 50 μg/L). It is assumed that the small amounts of arsenic passing to milk are almost entirely inorganic, with efficient maternal methylation of arsenic likely to protect against excretion via breast milk (EFSA, 2010).

Metabolism

In most mammalian species including humans, inorganic arsenicals are extensively biotransformed and excreted mainly as their metabolites. In oxygen-rich environments, inorganic arsenic is present primarily as arsenate (As(V)), which is the most common form in drinking water. Arsenite (As(III)) can also be present, particularly under anaerobic conditions (Tsuji et al., 2019). Arsenate enters the cell via the phosphate carrier system and can be biotransformed enzymatically to arsenite via glutathione reductase. In mammals, arsenite directly undergoes extensive oxidative methylation in the liver. There are considerable differences between species and between individuals of a same species in arsenic biotransformation (EFSA, 2010; Tsuji et al., 2019). Most studied animals are more efficient in methylating arsenic to dimethylarsinate (DMA) than humans, except primates which have been shown not to methylate arsenic at all. Rats, the most common standard testing species, differ significantly from most other mammals by accumulating arsenic in erythrocytes (ATSDR, 2007).

Excretion

Arsenic and its metabolites are readily excreted predominantly via urine but to some extent also via bile. Urinary excretion rates of 80% up to 61 hours following oral doses and 30-80% in 4-5 days following parenteral doses have been measured in humans. Although rats tend to excrete arsenic and metabolites preferentially into bile, the major route of excretion of arsenic compounds in most mammalian species and humans is via urine, and dimethylarsinate is the primary urinary metabolite. In contrast to most other mammals, humans excrete appreciable amounts of methylarsonate in urine. The composition of urinary arsenic metabolites varies from person to person and has been interpreted to reflect arsenic methylation efficiency, with a typical profile of urinary arsenic metabolites consisting of 10-30% inorganic arsenic, 10-20% methylarsonate and 60-70% dimethylarsinate (EFSA, 2010; ATSDR, 2007).

Buchet et al. (1981) studied the urinary elimination of arsenic metabolites in volunteers who ingested a single oral dose of arsenic (500 µg As) either as sodium arsenite (SA), monomethylarsonate (MMA) or cacodylate (DMA). The excretion rate increased in the order SA < DMA < MMA. After 4 days, the arsenic excreted in urine corresponded to 46, 78, and 75% of the ingested doses, respectively. Based on these findings, an average of 60% of total arsenic intake is assumed to be excreted via urine.

PBPK model for arsenic

A physiologically based pharmacokinetic (PBPK) model for exposure to inorganic arsenic in hamsters, rabbits and humans has been developed (Mann, 1996a and b). The model in its present state simulates three routes of exposure to inorganic arsenic: oral intake, intravenous injection and intratracheal instillation. It describes the tissue concentrations and the urinary and faecal excretions of the four arsenic metabolites: inorganic As(III) and As(V), methylarsonic acid and dimethylarsinic acid. The model consists of five tissue compartments, chosen according to arsenic affinities: liver, kidneys, lungs, skin and others. The model is based on physiological parameters which were scaled according to body weight. When physiological parameters were not available, the data for the model were obtained by fitting (tissue affinity, absorption rate and metabolic rate constants). The excretion of arsenic metabolites in urine and faeces are simulated well with the model for both species. This toxicokinetic model for the oral exposure route was validated using data on urinary excretion after repeated oral exposure to As(III) as well as after exposure to inorganic As via drinking water. Absorption by inhalation was validated using data on urinary excretion after occupational exposure to arsenic trioxide dust and fumes. In both cases, the model gave satisfactory results for urinary excretion of the four As metabolites. The PBPK model was also used in the description of the effects on the kinetics of exposure via different routes and for the simulation of various realistic exposure scenarios. The data presented substantiated the assumption that the systemic availability after ingestion is comparable to that after inhalation.

Another PBPK model was developed by El-Masri and Kenyon (2008) to predict levels of arsenic and its metabolites in human tissue and urine after oral exposure. The model accounted for the fate and transport of inorganic As(III) and As(V), as well as mono- and demethylated arsenical metabolites in humans. The model was evaluated against two datasets for arsenic-exposed populations from Bangladesh and the US. The evaluations showed its adequacy and usefulness for oral exposure reconstructions in human health risk assessment, particularly in individuals exposed to relatively low levels of arsenic in water or food (El-Masri et al., 2018).

​Additional references

El-Masri HA and Kenyon EM (2008). Development of a human physiologically based pharmacokinetic (PBPK) model for inorganic arsenic and its mono- and demethylated metabolites. J. Pharmacokinet. Pharmacodyn. 35(1), 31-68.

El-Masri HA et al. (2018). Evaluation of a physiologically based pharmacokinetic (PBPK) model for inorganic arsenic exposure using data from two diverse human populations. Environmental Health Perspectives 126(7), 1-9.

HERAG (2007). Health risk assessment Guidance for metals. https://www.icmm.com/website/publications/pdfs/chemicals-management/herag/herag-fs1-2007.pdf .

Tsuji JS et al. (2019). Dose-response for assessing the cancer risk of inorganic arsenic in drinking water: the scientific basis for use as a threshold. Critical Reviews in Toxicology. https://doi.org/10.1080/10408444.2019.1573804.

Key value for chemical safety assessment

Bioaccumulation potential:

low bioaccumulation potential

Absorption rate - oral (%):

100

Absorption rate - dermal (%):

2

Absorption rate - inhalation (%):

100