Zinc bis(dibutyldithiocarbamate)

Endpoint summary

Additional information

No information on toxicokinetic behavior of zinc bis(dibutyldithiocarbamate) (ZDBC) is available. However, Article 13 of the REACH legislation states that, in case no appropriate animal studies are available for assessment, information should be generated whenever possible by means other than vertebrate animal tests, i.e. applying alternative methods such as in vitro tests, QSARs, grouping and read-across.

Toxicokinetic data are available on the structural analogue of ZDBC, zinc (bisdimethyldithiocarbamate) (ZDMC). The latter substance is a salt of a homological dialkylcarbamodithioic acid, namely dimethyldithiocarbamic acid, which differs only in the substituents at the amine function in the dithiocarbamate moiety (butyl vs. methyl). Based on the structures of the substances, it is expected that their toxicological behavior shall be governed by the toxicological profiles of the Zn²⁺cation and the respective dithiocarbamate anions. It can also be expected that, based on structural similarity, dimethyl- and dibutyldithiocarbamate anions shall be metabolized via similar pathways, as it is not expected that the difference in the substituents at the amine function of dithiocarbamate moieties shall have a profound difference on the substances reactivity. Therefore it is considered acceptable to derive the data on the toxicokinetic behavior of ZDBC by read-across from ZDMC.

A GLP-compliant guideline toxicokinetic study is available for ZDMC, in which the radiolabelled substance was administered by gavage in carboxymethyl cellulose to groups of male and female Sprague-Dawley rats. Pharmacokinetics, excretion balance and tissue distribution investigations were performed at nominal dose levels of 15 and 150 mg/kg bw, while biliary excretion experiment was conducted using two animals, one dosed at 50 mg/kg bw and the other at 100 mg/kg bw. For one excretion balance study non-radiolabelled ZDMC was administered daily for 14 days. 24 h after receiving the last dose, a single dose of (¹⁴C)-ZDMC was administered.

Absorption

Absorption of ZDMC was relatively slow at both 15 and 150 mg/kg bw with maximal plasma concentrations in the low dose group achieved at 6.8 and 10.4 hours (0.859 and 0.804 μg equiv./g in males and females, respectively), and at 24 hours in both sexes in the high dose group (6.548 and 8.373 μg equiv./g in males and females, respectively). The values for maximal plasma concentrations in the high dose group were ca. 10-fold higher than in the low dose group. The increase in absorption was approximately proportional to the dose, indicating that the absorption was not saturated at least up to 150 mg/kg bw. During the absorption phase, concentrations of radioactivity in blood were similar to those in plasma; however, during the elimination phase, in the group dosed with 150 mg/kg bw, concentrations were ca. 3-fold higher in blood compared to those in plasma, indicating that a binding to blood cells may occur.

Distribution

The distribution of radioactivity after a single oral dose of (¹⁴C)-ZDMC at nominal dose levels of 15 mg/kg bw and 150 mg/kg bw was determined at various time points. Two hours post-administration, in the low-dose group, the highest amounts of the radiolabel were recovered in liver (3.144 and 2.395 μg equiv./g in males and females, respectively), kidneys (2.305 and 1.491 μg equiv./g), fat (2.393 and 2.129 μg equiv./g), spleen (1.209 and 1.126 μg equiv./g) and thyroid (0.373 and 1.106 μg equiv./g), while in the high-dose group, the highest amounts were recovered in liver (17.38 and 13.34 μg equiv./g), fat (10.57 and 10.32 μg equiv./g), kidney (7.486 and 5.986 μg equiv./g), lung (5.792 and 51.09 μg equiv./g), spleen (3.853 and 9.270 μg equiv./g) and thyroid (3.196 and 62.66 μg equiv./g).

Metabolism

The principal route of metabolism for ZDMC was hydrolysis to form carbon disulphide and carbonyl sulphide and the formation of carbon dioxide. These volatile metabolites comprised the majority of the excreted dose (ca. 51%). Urine contained 2-dimethylamine-thiaszolidine carboxylic acid and the S-glucuronide of dimethyldithiocarbamic acid. The latter compound is presumably formed by glutathione conjugation of either the dimethyldithiocarbamic acid or ZDMC directly. The glutathione conjugate would then be catabolised to the cysteine conjugate via the cysteinyl-glycine conjugate, which then cyclised, loosing H₂S, to form 2-dimethylamine-thiazolidine carboxylic acid. Faeces contained tetramethylthiuramdisulfide.

Excretion

The tissue retention and excretion of radioactivity was determined after single and multiple oral doses of (¹⁴C)-ZDMC at nominal dose levels of 15 and 150 mg/kg bw. In addition, for one excretion balance study non-radiolabelled ZDMC was administered daily for 14 days and twenty-four hours after receiving the last dose, a single dose of (¹⁴C)-ZDMC was administered. In the low dose group, 63.27 and 64.41% of total dose was recovered over 168 hours in males and females, respectively, from which 3.068 and 3.238% were recovered in faeces. In the high dose group, the recovery over 168 hours was 75.88 and 76.46% in males and females, respectively, out of which 4.574 and 2.844% were recovered in faeces. In the repeated administration group, the recovery was 74.09 and 84.93% in males and females, respectively, with 3.112 and 4.145 % recovered in faeces. In the biliary excretion study, following a single administration of¹⁴C-ZDMC to 2 male animals at nominal dose levels of 50 and 100 mg/kg bw, 2.2% and 1.9% was excreted in bile, 16.9% and 9.6% in urine and 17 and 3.1% in faeces, respectively. The majority (ca. 51%) of the administetered dose was excreted as volatile metabolites CS₂, COS or CO₂. The remaining dose was excreted in urine (ca. 10.9-20.8%) and faeces (ca. 4%), with virtually none via bile. Excretion was rapid and essentially complete within 24 hours.

 

Application of the available data for ZDBC

Based on the available results, a comparable toxicokinetic behavior is expected for ZDBC. Based on the higher molecular weight of ZDBC, it is expected that its absorption from the gut shall not be faster than that of ZDMC. For ZDMC, at least 60% oral absorption was determined based on the available toxicokinetic study. The default value of 50% oral absorption is recommended in the Chapter R.8 of REACH Guidance on information requirements and chemical safety assessment in case route-to-route extrapolation needs to be performed. As the oral absorption of ZDBC is expected not to exceed the oral absorption of ZDMC, and taking into account that the lower oral absorption represents a worst-case approach in case of route-to-route extrapolation (as less substance becomes systemically available), using the precautionary principle a value of 50% oral absorption shall be used for DNEL derivation for ZDBC. However, if the read-across is applied and the DNELs are calculated based on the results of the studies with the structural analogue ZDMC, a value of 60% for oral absorption is applied.

The distribution of ZDBC, similarly to ZDMC, is expected to occur primarily to organs of metabolism and excretion (liver, lung, kidney) and vascularised tissues (spleen, thyroid, adrenals), as well as body fat. The metabolism is expected to occur via a hydrolysis of a parent compound into the respective acid, which either undergoes a transformation to CS₂further oxidized into CO₂, or a conjugation with glucuronic acid or GSH.

Dermal absorption

No data on dermal absorption of ZDBC are available. Dermal penetration of its structural analogue ZDMC was studied in vitro in a GLP-compliant guideline study, using human cadaver skin. 6.4μl of two test susbtance preparations with nominal concentrations of 1.55 and 643 mg/ml was applied to 0.64 cm²of human skin for 6 hours under occlusive conditions (4 -6 diffusion cells per experiment). The total applied amounts of the test substance corresponded to 0.016 and 6.4 mg/cm²skin. The total recovery of radioactivity in two experiments was 91.75% and 94.12%, respectively. 87.82% and 93.95% were recovered upon the skin swab after 6 hours exposure in the low-dose and high-dose experiments, respectively, while 0.39% and 0.2% penetrated through skin into the receptor fluid and 1.84% and 0.04% were recovered in stratum corneum. The total percentage of absorption through skin was thus determined to correspond to 2.23% and 0.1% in low-dose and high-dose experiments, respectively. It should be added that the amount applied in the high dose experiment is approximately in the range of 1-5 mg/cm²skin, recommended by OECD Guideline 428 for solid substances.

The approximate dermal absorption of ZDBC can also be calculated based on its (water) solubility. Solid substances will only penetrate the skin in (aqueous) solution. Therefore, skin absorption can only occur through the water that penetrates the skin and the maximum skin absorption is defined by the maximum water solubility of the salts and the maximum amount of water that can penetrate the skin.

 The maximum amount of water that can penetrate the skin is determined to be 17 µl per 1 cm²per 24 hours (Ten Berge, W. A simple dermal absorption model: derivation and application. Chemosphere. 2009 Jun; 75(11):1440-5), which equals 6 µl per cm²per 8 hours.

ZDBC is virtually insoluble in water (ca. 1 mg/l (= 10^-3µg/µl).

Since 6 µl of water can maximally penetrate 1 cm²of skin per 8 hours, 6 x 10^-3= 0.006 µg of hydrolysed salt may penetrate 1 cm²of skin per 8 hours. In an in vitro skin absorption experiment (according to OECD guideline 428), the application should mimic human exposure, normally 1-5 mg/cm²(1000-5000 µg/cm²). Thus, in case the skin penetration of ZBEC would be experimentally be determined according to OECD guideline 428 using 5 mg/cm²as exposure condition, a skin penetration of 1.2 × 10^-4% (0.006/5000) would be observed maximally. Therefore a value of 0.1% dermal absorption, observed in the study with ZDMC, is considered to be a worst-case approach and shall be used for DNEL derivation.