Zinc, O,O-mixed (iso-Bu), (iso-Pr), (pentyl) phosphorodithioate

Genetic toxicity: in vitro

Administrative data

Endpoint:

in vitro gene mutation study in bacteria

Type of information:

read-across based on grouping of substances (category approach)

Adequacy of study:

weight of evidence

Justification for type of information:

REPORTING FORMAT FOR THE CATEGORY APPROACH
Please refer also to the read-across statement attached in section 13

1. HYPOTHESIS FOR THE CATEGORY APPROACH (ENDPOINT LEVEL)
The target and the source substances are structurally similar substances that share the common organometallic core structure consisting of a central zinc metal bonded to four alkyldithiophosphate esters (ligands) by coordinate covalent bonds -Zn[(S2P(OR)2]2. Structural variations between the target and the source substances are related only to the alkyl (R) groups of the alkyldithiophosphate ligands. The substances in this category give thus rise to an (identical) common compound Phosphorodithioic acid moiety that can be released by the breakage of ester bonds and dissociation from the Zinc complex to which the organism would be exposed if the target substance was tested in the toxicity studies. Exposure to the parent compounds (non-transformed constituents) and to the counter alkyl alcohols, possibly released by hydrolysis of P-O bonds – non-common compounds – would not influence the prediction of the (eco)toxicological properties because they are considered to have the same biological targets and to cause the same type of effects through a common underlying mechanism due to the same functional groups (zinc cation, phosphorodithioic cation and aliphatic alcohol anionic moieties). The impurities of the target and the source substances are not expected to impact the prediction because they are identical or, if slightly structural different, belong to the same class of compounds with the same functional groups and their percentages are very low.

2. CATEGORY APPROACH JUSTIFICATION (ENDPOINT LEVEL)
Several ZDDP category members were tested in in vitro bacterial and mammalian gene mutation and in in vivo chromosomal aberration assays (HPV, 2005). Frequencies of reverse mutations in bacteria were not significantly changed after exposure to the zinc dialkyldithiophosphates. All tested substances were negative for mutagenic activity, with and without metabolic activation. In vitro mutation studies in mammalian cells indicate that the zinc dialkyldithiophosphates do not consistently display mutagenic activity in the absence of metabolic activation, however, upon biotransformation, these materials showed mutagenic activity. All test substances were negative for clastogenicity in in vivo assays (HPV, 2005). Positive results in some tests were considered to be governed by zinc, because other organic zinc compounds were mutagenic in similar test systems (HPV, 2005). “However, genotoxicity studies conducted in a variety of test systems have failed to provide unequivocal evidence for mutagenicity of zinc” (ATSDR, 1992, cited in HPV, 2005). “In summary, the weight of evidence supports the conclusion that zinc dialkyldithiophosphates have a low potential genotoxicity, and that these substances do not present a significant risk for mutagenicity or carcinogenicity in humans”.

The overall weight-of-evidence indicates thus that ZDDP category members are non-mutagenic (HPV, 2005). The two source chemicals were also non-mutagenic in the bacterial mutagenicity tests. Since the main constituents of the target substance are structurally similar to the constituents of the source substances with the same functional groups and the alkyl chain lengths of phosphoroditioate moieties are in the range of the established ZDDP category (C3-C12), the same mode of toxicological action is expected for the target and the source substances. The constituents of the target substance do not possess functional groups associated with other mode of actions or toxicity effects. Toxicokinetic behavior of the constituents of the target substance is expected to be essentially the same as that of the source substance. Based on the results of the mutagenicity studies with the source substances and other ZDDP category members it is evident that the structural dissimilarities – the alkyl rests with different chain lengths – did not result in different mutagenic activity. “The findings in bacterial and mammalian cells did not vary in proportion to the alkyl chain length or any other physicochemical parameter” (HPV, 2005). Thus, it is not likely that the presence of structurally dissimilar alerts i.e. alkyl chain rests of the target substance would result in binding to DNA leading to mutations. Therefore, a positive result in bacterial test system is not likely for the target substance. The impurities of the target substance are considered not to contribute to mutagenicity potential because they are also structurally similar to the impurities of the source substances and consist of substances of simple structure without specific mode of action and do not contain functional groups leading to DNA binding and subsequently to mutations. Therefore, it is predicted that the target substance would not possess mutagenic activity if it was tested in a bacterial mutagenicity test.

Materials and methods

Test material

Reference

Name:

Unnamed

Test material form:

liquid

Results and discussion

Test results

Test results 1

Species / strain:

S. typhimurium TA 1535

Metabolic activation:

with and without

Genotoxicity:

negative

Cytotoxicity / choice of top concentrations:

cytotoxicity

Vehicle controls validity:

valid

Untreated negative controls validity:

not specified

Positive controls validity:

valid

Test results 2

Species / strain:

S. typhimurium TA 1537

Metabolic activation:

with and without

Genotoxicity:

negative

Cytotoxicity / choice of top concentrations:

cytotoxicity

Vehicle controls validity:

valid

Untreated negative controls validity:

not specified

Positive controls validity:

valid

Test results 3

Species / strain:

S. typhimurium TA 98

Metabolic activation:

with and without

Genotoxicity:

negative

Cytotoxicity / choice of top concentrations:

cytotoxicity

Vehicle controls validity:

valid

Untreated negative controls validity:

not specified

Positive controls validity:

valid

Test results 4

Species / strain:

S. typhimurium TA 100

Metabolic activation:

with and without

Genotoxicity:

negative

Cytotoxicity / choice of top concentrations:

cytotoxicity

Vehicle controls validity:

valid

Untreated negative controls validity:

not specified

Positive controls validity:

valid

Applicant's summary and conclusion

Conclusions:

negative with metabolic activation
negative without metabolic activation

Executive summary:

The substance was tested in S. typhimurium strains TA 1535, TA 1537, TA 98 and TA 100 similar to OECD Guideline 471 with and without metabolic activation. The substance was negative in all strains tested with and without metabolic activation.