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Description of key information

The following studies have been submitted to address the immunotoxicity endpoint:
Li, A. P., Dahl, A. R. and Hill, J. O. (1982). In Vitro Cytotoxicity and Genotoxicity of Dibutyltin Dichloride and Dibutylgermanium Dichloride. TOXICOLOGY AND APPLIED PHARMACOLOGY 64, 482-485.
Penninks, A. H. and Seinen, W. (1982). COMPARATIVE TOXICITY OF ALKYLTIN AND ESTERTIN STABILIZERS. Fd Chem. Toxic. Vol. 20. pp. 909 to 916.
Schobel, C. (1991). ZK 22.663: Local dermal tolerance test in rats after a single application and a subsequent rinsing. Report no.: IC 1/91. Report date: 1991-06-13.
Seinen, W., Vos, J. G., van Kreiken, R., Penniks, A., Brands, R. and Hooykaas, H. (1977). Toxicity of Organotin Compounds. III. Suppression of Thymus-Dependent Immunity in Rats by Di-n-Butyltindichloride and Di-n-Octyltindichloride. TOXICOLOGY AND APPLIED PHARMACOLOGY 42, 213-224.
Snoeij, N. J., Penninks, A. H. and Seinen, W. (1988). Dibutyltin and tributyltin compounds induce thymus atrophy in rats due to a selective action on thymic lymphoblasts. Int. J. Immunopharmac. 10: 891-899.
DeWitt et al, 2005, Immune Responses in Sprague–Dawley Rats Exposed to Dibutyltin Dichloride in Drinking Water as Adults, Journal of Immunotoxicology, 2:151–160, 2005.
DeWitt et al, 2005, Developmental Exposure to 1.0 or 2.5 mg/kg of Dibutyltin Dichloride Does Not Impair Immune Function in Sprague-Dawley Rats, Journal of Immunotoxicology, 3:245–252, 2006.
Schobel (1991), Seinen et al (1977), Snoeij et al (1988), De Witt et al (2005) and De Witt et al (2006) have been allocated a Klimisch score of 2 with Seinen et al (1977) and Snoeij et al (1988) considered to be the key studies for this endpoint.

Key value for chemical safety assessment

Additional information

All dibutyltin compounds degrade into dibutyltin and the appropriate ligand (in gastric conditions, dibutyltin dichloride), on this basis, it is possible to read-across between the different dibutyltin compounds to address immunotoxicity endpoints. As no adequate immunotoxicity studies are available on the material itself, this endpoint has been addressed by the submission of studies performed on dibutyltin dichloride and using a read-across approach.

From SPCFC (2004) specific experimental data for Dibutyltin (chloride) indicate immunotoxic effects at doses as low as 1 mg/kg bw in preweaning exposure studies (source unclear). This value is then apparently disregarded, and a LOAEL for DBT is cited as 2.5 mg/kg bw/day. SPCFC also indicate read-across from tributyltin to dibutyltin to be appropriate since the mechanism of action is similar and the immunotoxicity of TBT may be attributable to DBT as a metabolite; an immunotoxicity NOAEL for TBT is identified at 0.025 mg/kg bw/day by repeated dietary exposure. It must be noted that dose spacing in the critical study of TBTO (Wester et al 1988, 1990) is large and the NOAEL correspondingly imprecise. Further, if toxicity of TBTO is even in part due to formation of DBT as the ultimate toxicant, then a NOAEL based on an appropriate study of DBT-Cl is the preferable endpoint.

Modern and robust studies of the immunotoxicity of DBTC in adult rats and in pups exposed during gestation and lactation (DeWitt et al, 2005, 2006) found no repeatable effect on immune function at doses up to 2.5 mg/kg bw/day, although at this dose an effect on bodyweight was reported (possibly due to palatability); 1 mg/kg bw/day was a NOAEL.In a series of other appropriate studies of DBTC (Gaunt et al 1968; Pennincks et al 1982; Osterburg 1993; Waalkens et al 2003) dose levels of approximately 2-2.5 mg/kg/bw/day are an inconsistent NOAEL/LEL; but no effects are detected at 1 mg/kg bw/day or below. EFSA/SPCFC review did not include the significant studies by Osterburg (1993), Waalkens (2003), and DeWitt et al (2005, 2006). Use of a NOAEL at 0.3 mg/kg bw/day form Waalkens (2003) is therefore a robust and precautionary endpoint.

Justification for classification or non-classification

The substance is classified and labelled with T; R48/25 according to Directive 67/548/EEC due to toxicity to the immune system at doses below 50 mg/kg bw/day. Further more according to Regulation (EC) no 1272/2008 the test substance would be classified as a STOT Single Exp. Category 1 based on results obtained in Snoeij, N.J., Penninks, A.H. and Seinen, W. (1988) which indicate 50% reduction of thymus weight following a single oral dose of 18 mg/kg. Signal word: Danger; Hazard statement: H370 Causes damage to thymus.