Tin zinc oxide (SnZnO3)

Endpoint summary

Description of key information

The toxicokinetics have been assessed based upon the results of the available toxicology studies and the commonly known properties of the elements of which the substance is composed.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

The substance is a zinc salt of a tin complex. Absorption is not precluded by molecular weight for a salt. No SAR predictions can be made, other than that the toxicity and ADME are likely to be that of zinc and tin ions. The substance is stable to hydrolysis but exposure will be to the ions. The white crystalline powder may be potentially respirable, so inhalation exposure could be relevant.

Absorption

Single and repeated dose oral toxicity studies showed no toxic effects and, hence, provide no evidence of absorption. An acute inhalation toxicity study, however, showed mild, transient symptoms. These effects may have been associated with a localised response in the respiratory tract and may not be indicative of systemic absorption. Passive diffusion across membranes only occurs for unionised forms but active transport systems exist for certain ionic species. In particular, zinc is an essential trace element and is absorbed from the gastro-intestinal tract at rates dependent on nutritional status. Tin is known to be poorly absorbed. The low solubility of this substance in water and fat suggests poor absorption and deposition at the dosage site. Dermal absorption would not be expected.

Distribution

There is no experimental evidence to indicate distribution which, in any event, would be dependent on absorption. Although the log Pow is not necessarily indicative of low bioaccumulation for a salt the low fat solubility does suggest this conclusion. Zinc and tin are both widely distributed following absorption. Zinc is regulated by normal homeostatic mechanisms and two thirds of plasma zinc is protein bound. Tin is non cumulative except following injection of high doses.

Metabolism

Metabolism is also dependent on absorption and there is no evidence, either from the 28-day oral toxicity study or the in-vitro Ames test, of any change in toxicity effected by metabolising enzymes. Zinc is an essential trace element involved in numerous biological functions and as a co-factor for several enzymes. Absorbed zinc would be expected to enter the normal metabolic pool.

Excretion

Both zinc and tin are almost entirely excreted in faeces with only very limited urinary excretion. It is unlikely that any significant proportion of this water insoluble substance would be excreted in urine following exposure by ingestion or inhalation. Elimination via the lungs in expired air would not occur for this non-volatile substance.