Phosphorodithioic acid, mixed O,O-bis(1,3-dimethylbutyl and iso-Pr) esters, zinc salts

Repeated dose toxicity: other routes

Administrative data

Endpoint:

repeated dose toxicity: other route

Remarks:

In vitro mechanistic assay

Type of information:

experimental study

Adequacy of study:

weight of evidence

Study period:

February 20, 2019 - March 1, 2019

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Data source

Reference

Reference Type:

study report

Title:

Unnamed

Year:

2019

Report date:

2019

Materials and methods

Test guideline

Qualifier:

equivalent or similar to guideline

Guideline:

other: In Vitro mechanistic assay

GLP compliance:

no

Limit test:

no

Test material

Test material information

Constituent 1

Reference substance name:

Phosphorodithioic acid, mixed O,O-bis(1,3-dimethylbutyl and iso-Pr) esters, zinc salts

EC Number:

283-392-8

EC Name:

Phosphorodithioic acid, mixed O,O-bis(1,3-dimethylbutyl and iso-Pr) esters, zinc salts

Cas Number:

84605-29-8

Molecular formula:

Not applicable

IUPAC Name:

Phosphorodithioic acid, mixed O,O-bis(1,3-dimethylbutyl and iso-Pr) esters, zinc salts

Specific details on test material used for the study:

The following EC# were tested simultaneously in this in vitro assay:
230-257-6
270-608-0
283-392-8
272-238-5
288-917-4
218-679-9
247-810-2
273-527-9
224-235-5
249-109-7

Vehicle controls
Base Oil 1
Base Oil 2
Base Oil 3
Base Oil 4

Positive Controls
Cisplatin
Diethyl Maleate
Tunicamycin
Alfatoxin B1

Test animals

Species:

other: six validated GFP-based mouse embryonic stem (mES) reporter cell lines

Strain:

other: other: six validated GFP-based mouse embryonic stem (mES) reporter cell

Sex:

not specified

Administration / exposure

Route of administration:

other: in vitro

Vehicle:

other: 4 base oils used for production of ZDDPs

Details on exposure:

50,000 cells/well

Analytical verification of doses or concentrations:

not specified

Duration of treatment / exposure:

24 hours

Frequency of treatment:

1 time

Doses / concentrations

Dose / conc.:

0.062 other: %

Remarks:

0, 0.00391, 0.00781, 0.0156, 0.0313, 0.0625% for this ZDDP
The maximum concentration for the ZDDPs was 0.002 – 0.06% depending on cytotoxicity.
The maximum concentration of base oil was 1%

No. of animals per sex per dose:

50,000 cells / well

Control animals:

yes, concurrent no treatment

Details on study design:

TEST PROTOCOL
The ToxTracker assay requires only standard cell culture facilities and a low-end flow cytometer. The ToxTracker reporter cells are maintained by culturing them in gelatin-coated dishes in the presence of irradiated primary mouse embryonic fibroblasts (MEFs) in mES cell culture medium. During chemical exposures and reporter analysis the ToxTracker cells are cultured in the absence of fibroblasts in mES cell culture medium.

Cytotoxicity testing/dose range finding
For chemical testing, first a dose range finding was performed using wild-type mES cells (strain B4418). Wild type mES cells are exposed to 20 different concentrations of the test substances, with a maximum concentration of 1% (see Table 3). Cytotoxicity is estimated by cell count after 24 h exposure using a flow cytometer and is expressed as percentage of viable cells after 24 h exposure compared to vehicle control exposed cells. From this dose range finding, 5 concentrations are selected.

ToxTracker
The six independent mES reporter cell lines are seeded in gelatin-coated 96-well cell culture plates in 200 μl mES cell medium (50.000 cells per well). 24 h after seeding the cells in the 96-well plates, medium is aspirated and fresh mES cell medium containing 10% fetal calf serum and the diluted chemicals is added to the cells. For each tested sample, five concentrations are tested in 2-fold dilutions. Induction of the GFP reporters was determined after 24 h exposure using a flow cytometer. Only GFP expression in intact single cells was determined. Mean GFP fluorescence was measured and used to calculate GFP reporter induction compared to a vehicle control treatment. Cytotoxicity was estimated by cell count after 24 h exposure using a flow cytometer and was expressed as percentage of intact cells after 24 h exposure compared to vehicle exposed controls. For cytotoxicity assessment in the ToxTracker assay, the relative cell survival for the six different reporter cell lines was averaged. Metabolic activation was included in the ToxTracker assay by addition of S9 liver extract from aroclor1254- induced rats (Moltox). Cells are exposed to five concentrations of the test samples in the presence of 0.25% S9 and required co-factors (RegenSysA+B, Moltox) for 24 h. Positive reference treatments with cisplatin (DNA damage), diethyl maleate (oxidative stress), tunicamycin (unfolded protein response) and aflatoxin B1 (metabolic activation of progenotoxins by S9) were included in all experiments. Solvent concentration was the same in all wells and never exceeded 1% for the base oils. In case auto-fluorescence of the test substances was observed in the dose range finding, wild type mES cells were exposed to the test samples at the same concentrations as used in the ToxTracker. The mean fluorescence caused by the compound was then subtracted from the

ToxTracker results of the respective compound.
This experiment was conducted as a non-GLP study, however general principles to conduct proper scientifically correct in vitro experiments were adhered to, and in particular care was taken for proper handling of test article (stock) solutions to prevent/minimise degradation of the test articles based on instructions/compound information from the sponsor. For all ToxTracker analyses, Toxys strictly follows the Good Cell Culture Practice guidelines from the OECD.

TEST CRITERIA
The ToxTracker assay was considered to have a positive response when a compound induces at least a 2 fold increase in GFP expression in any of the reporters. Activation of the Bscl2-GFP or Rtkn-GFP reporters indicate induction of DNA damage, Srxn1-GFP and Blvrb-GFP indicate induction of cellular oxidative stress and Ddit3-GFP activation is associated with the unfolded protein response. The Btg2-GFP reporter is controlled by the p53 tumor suppressor and is activated by DNA damage but can also be induced by oxidative stress, hypoxia, metabolic stress and apoptosis.

Examinations

Statistics:

DATA ANALYSIS
In order to allow comparison of induction levels of the ToxTracker reporter cell lines for large number of compounds we developed Toxplot, a dedicated data analysis software package. Toxplot imports raw GFP reporter data from the flow cytometer, calculates GFP induction levels and cytotoxicity, performs statistical analysis of the data and hierarchical clustering of the tested compounds, and visualises the data in a heatmap allowing convenient interpretation of obtained test results. ToxPlot software uses agglomerative hierarchical clustering to visualize the ToxTracker data. Agglomerative clustering uses the ‘bottom-up’ approach, which puts each observation in its own cluster and pairs of clusters are merged as one moves up the hierarchy. To compare the induction of the six GFP reporters for a collection of compounds, each with different biological reactivities, dose-response relationships and kinetics, Toxplot calculates for each compound the level of GFP induction for every individual reporter at a specified level of cytotoxicity (typically 10%, 25% and 50%). GFP induction levels are calculated by linear regression between two data points around the specified cytotoxicity level. In case the specified level of cytotoxicity can not be reached at the highest tested compound concentration, Toxplot displays the GFP induction level at this top concentration. In the heatmap, Toxplot clearly marks the compounds that do not induce the selected level of cytotoxicity. Because the cytotoxicity for a compound can vary between the ToxTracker cell lines, calculations of the GFP induction levels of the individual reporters by Toxplot can slightly deviate from the GFP induction and cytotoxicity figures.

Results and discussion

Results of examinations

Clinical signs:

effects observed, treatment-related

Description (incidence and severity):

All ZDDP dosing was limited by cytotoxicity.

Mortality:

not examined

Body weight and weight changes:

no effects observed

Food consumption and compound intake (if feeding study):

not examined

Description (incidence and severity):

Fold Activation for Reporter Genes
EC# Genotoxicity Cellular Stress Oxidative Stress Unfolded Protein Response
230-257-6 < 2.0 < 2.0 2.6 2.7
270-608-0 < 2.0 < 2.0 2.6 < 2.0
283-392-8 < 2.0 < 2.0 3.7 3.9
272-238-5 < 2.0 < 2.0 2.6 2.2
288-917-4 < 2.0 < 2.0 2.6 2.8
218-679-9 < 2.0 < 2.0 < 2.0 4.1
247-810-2 < 2.0 < 2.0 3.9 3.0
273-527-9 < 2.0 < 2.0 6.8 3.8
224-235-5 < 2.0 < 2.0 2.2 4.9
249-109-7 < 2.0 < 2.0 4.1 5.8
Base Oil 1 < 2.0 < 2.0 < 2.0 < 2.0
Base Oil 2 < 2.0 < 2.0 < 2.0 < 2.0
Base Oil 3 < 2.0 < 2.0 < 2.0 < 2.0
Base Oil 4 < 2.0 < 2.0 2.2 2.5
Positive Controls
Cisplatin 6.9 4.2 3.2 < 2.0
Diethyl Maleate < 2.0 4.1 31.5 2.0
Tunicamycin < 2.0 < 2.0 < 2.0 9.0
Alfatoxin B1 4.9 3.3 2.2 < 2.0

Applicant's summary and conclusion

Conclusions:

The table above shows the highest fold activation of the reporter genes for each endpoint at compound concentrations that cause 10 – 50% cytotoxicity. The 50% cytotoxicity cut-off was used as there was significantly more variation in the results when the cytotoxicity was > 50%. As there generally were not clear differences in reporter activation between when S9 was used and when it was not used, the worst case value is used for each substance. Since DNA damage and oxidative stress have two reporter genes that are used to measure activation of the pathway, the highest fold induction to give the worst case response is used in the table.

Three of the four base oils did not activate any of reporter genes at all concentrations tested. The final base oil slightly activated the pathways for oxidative stress and unfolded protein response without S9, but the activation was only 2.2 – 2.5 fold. There was no activation of any reporter genes for the fourth base oil when S9 was included in the assay. These results further support the conclusion that the base oils themselves should not contribute to the ZDDP toxicity.

Of the 10 ZDDPs tested, none of the ZDDPs induced a 2-fold response in the reporter genes that measured DNA damage either with or without S9. Therefore, all of the ZDDPs would be considered negative for genotoxicity in this assay. In addition, none of the ZDDPs induced a 2-fold increase in GFP expression for the cellular stress reporter genes either with or without S9.

Of the 10 ZDDPs tested, 9 / 10 activated the oxidative stress pathway. While there were some differences with the fold induction varying from 2.2 – 6.8, all of the ZDDPs induced the pathways significantly less than the positive control diethyl maleate (31.5-fold induction). In addition, 9 / 10 ZDDPs activated the unfolded protein response pathway, causing a 2.2 – 5.8-fold induction. The positive control caused a 9.0-fold induction.

Generally, activation of the different GFP reporters by the positive control compounds is fully compliant with historical data thereby confirming the technical validity of the performed tests.

The key takeaway from these results is that the ZDDPs have very similar modes of action (oxidative stress and unfolded protein response) when causing cellular toxicity. These assays further justify the ZDDP category hypothesis. Further testing using this assay will be completed on all members of the category.

Executive summary:

Ten of the ZDDPs demonstrated similar biological activity using six in vitro Toxys ToxTracker mechanistic assays that measure four specific mechanisms of cellular toxicity. The ToxTracker assay is a panel of six validated GFP-based mouse embryonic stem (mES) reporter cell lines that can be used to identify the biological reactivity and potential carcinogenic properties of newly developed compounds in a single test. The assay monitors activation of cellular signalling pathways using specific biomarkers for detection of the biological reactivity of compounds. The activation of these biomarker genes is monitored using generated green fluorescent (GFP) mES reporter cell lines. ToxTracker consists of a panel of six different mES GFP reporter cell lines representing four distinct biological responses: general cellular stress, DNA damage, oxidative stress and the unfolded protein response.

 

Based on an initial test with 64 compounds, the assay has been able to correctly predict the mode of action for ~97% of the chemicals testing. The assay is currently undergoing OECD validation.

 

For the initial assay, 10 ZDDPs and 4 base oils were tested for cytotoxicity and mode of action. All samples were tested with and without S9 activation to simulate metabolism. Positive reference treatments with cisplatin (DNA damage), diethyl maleate (oxidative stress), tunicamycin (unfolded protein response) and aflatoxin B1 (metabolic activation of progenotoxins by S9) were included in all experiments. Solvent concentration was the same in all wells and never exceeded 1% for the base oils. The ToxTracker assay was considered to have a positive response when a compound induces at least a 2-fold increase in GFP expression in any of the reporters. Only GFP inductions at compound concentrations that showed < 75% cytotoxicity are used for the ToxTracker analysis. To compare the induction of the six GFP reporters for a collection of compounds, each with different biological reactivities, dose-response relationships and kinetics, Toxys calculates for each compound the level of GFP induction for every individual reporter at a specified level of cytotoxicity (typically 10%, 25% and 50%).

 

Based on the results of the assay, none of the four base oils exhibited any cytotoxicity, so all base oils were tested up to 1%. All of the ZDDPs exhibited cytotoxicity, so the maximum concentration for the ZDDPs was 0.002 – 0.06%. At least 5 serial dilutions were tested for each substance, and the assays were repeated in triplicate.

 

The highest fold activation of the reporter genes for each endpoint at compound concentrations that cause 10 – 50% cytotoxicity was reported. The 50% cytotoxicity cut-off was used as there was significantly more variation in the results when the cytotoxicity was > 50%. As there generally were not clear differences in reporter activation between when S9 was used and when it was not used, the worst case value is used for each substance. Since DNA damage and oxidative stress have two reporter genes that are used to measure activation of the pathway, the highest fold induction to give the worst case response is used in the table.

 

Three of the four base oils did not activate any of reporter genes at all concentrations tested. The final base oil slightly activated the pathways for oxidative stress and unfolded protein response without S9, but the activation was only 2.2 – 2.5 fold. There was no activation of any reporter genes for the fourth base oil when S9 was included in the assay. These results further support the conclusion that the base oils themselves should not contribute to the ZDDP toxicity.

 

Of the 10 ZDDPs tested, none of the ZDDPs induced a 2-fold response in the reporter genes that measured DNA damage either with or without S9. Therefore, all of the ZDDPs would be considered negative for genotoxicity in this assay. In addition, none of the ZDDPs induced a 2-fold increase in GFP expression for the cellular stress reporter genes either with or without S9.

 

Of the 10 ZDDPs tested, 9 / 10 activated the oxidative stress pathway. While there were some differences with the fold induction varying from 2.2 – 6.8, all of the ZDDPs induced the pathways significantly less than the positive control diethyl maleate (31.5-fold induction). In addition, 9 / 10 ZDDPs activated the unfolded protein response pathway, causing a 2.2 – 5.8-fold induction. The positive control caused a 9.0-fold induction.

 

Generally, activation of the different GFP reporters by the control compounds is fully compliant with historical data thereby confirming the technical validity of the performed tests.

 

The key takeaway from these results is that the ZDDPs have very similar modes of action (oxidative stress and unfolded protein response) when causing cellular toxicity. These assays further justify the ZDDP category hypothesis. Further testing using this assay will be completed on all members of the category.