Silicon orthophosphate

Endpoint summary

Additional information

Silicon orthophosphate is highly water soluble (1085 mg/L at 20°C). It completely dissociates into silicon and phosphate ions after systemic uptake (relevant pH ranges 2 -7). The tetravalent silicon ion and the phosphate ion will then react with the media to form different silicon and phosphate species depending on the pH and redox potential of the media. The dissociated silicon ion exists in water predominantly as H4SiO4/Si(OH)4, which is also the main species when silicon dioxide is dissolved in water. Therefore the silicon component of silicon orthophosphate can be covered by data from the read-across substance silicon dioxide, whereas the phosphate component will be covered a read-across from orthophosphoric acid. A study investigating gene mutation in bacteria is available for silicon orthophosphate, but no data for chromosome abberation and gene mutation in mammalian cells.

Gene mutation in bacteria

Mutagenicity of silicon orthophosphate in bacteria was assessed in a GLP-compliant Ames test performed according to OECD 471 (Schreib, 2012) using S. typhimurium strains TA 1535, TA 1537, TA 98, TA 100 and TA102. A plate incorporation assay served as range-finder for the determination of the test concentrations and was considered as a pre-test for toxicity. In the main test, the bacteria were exposed to concentrations of 31.6, 100, 316, 2500 and 5000 µg/plate in a plate incorporation assay and in preincubation experiment in the absence and presence of metabolic activation by rat liver S9-mix in both studies. Cytotoxicity was observed in TA100 in the pre-incubation assay at 5000 µg/plate with metabolic activation. Precipitation was observed from 1000 µg/plate onwards in all strains with and without metabolic acitvation in both experiments. Appropriate positive controls were included into the study design, which gave the expected results. Under the conditions choosen in this test, the test substance is considered not to be mutagenic in bacteria.

Chromosome aberration

The clastogenic potential of silicon dioxide was assessed in a chromosomal aberration test in mammalian cells similar as recommended in OECD guideline 473 (Putman and Morris, 1990). The selection of the concentrations used for the main study was based on the results of a pre-tests: The CHO cells were exposed for 18 h to 38, 75, 150 and 300 µg/mL without metabolic activation and for 2 h to 250, 500, 750 and 1000 µg/mL with metabolic activation and harvested after 18 h (without S9) and 15 h (with S9), respectively. Inhibition of cell proliferation was determined in the pre-test and reported to occur at 30 µg/L (-S9) and 300 µg/L (+S9). There were no biologically and statistically significant increases in numbers of metaphases with aberrations at any exposure duration and at any total culture time, irrespective of metabolic activation, in cells treated with the test substance. The positive controls resulted in clear and statistically significant increases in metaphases with aberrations.

Phosphoric acid is routinely used in the nutrient broths that support cell cultures in the laboratory and as such cells are constantly exposed to inorganic phosphates. In addition, phosphoric acid is also found in the metabolic activation mixture (e.g. S9-mix) which is used in a chromosomal aberration test to determine whether a test chemical can be metabolized within the body to produce a compound that may be genotoxic. The constant exposure of bacteria and cell cultures to these materials suggests that they pose no inherent risk of genotoxicity.

PO43-ions are essential for life and are not considered to be genotoxic or mutagenic in standard test systems. In addition, inorganic phosphate salts are widely used as food ingredients and considered as "generally Recognized as safe" (GRAS; FDA: Food and Drug Administration 1973. GRAS (Generally Recognized as Safe) food ingredients—phosphates. NTIS PB-221-224, FDA, Food and Drug Administration, 1979. Phosphates; Proposed Affirmation of and Deletion From GRAS Status as Direct and Human Food Ingredients. Federal Register 44 (244). 74845-74857, 18 December (1979)). Therefore further testing to determine the genotoxicity or mutagenicity of orthophosphates is not considered to be scientifically justified.

Gene mutation in mammalian cells

The mutagenic potential of silicon dioxide in mammalian cells was assessed by a HPRT-assay according to OECD 476 under GLP-conditions (Sigler and Harbell, 1990). In the pre-test, CHO cells were exposed to 9 test substance concentrations between 0.1 and 500 µg/mL in the absence and presence of metabolic activation. Toxicity was based on colony-forming efficiency. Based on these results, the test substance was tested at 10, 50, 100, 150 and 250 µg/mL without metabolic activation and at 100, 200, 300, 400 and 500 µg/mL with metabolic acitvation in the main test. Cytotoxicity was not pronounced in the mutation test, while a concurrent cytotoxicity test showed considerable inhibition of the cloning efficiency over the same dose ranges. However, the test substance did not induce any statistically significant increases in mutant frequencies at any concentration tested. Therefore, the test substance was not mutagenic in mammalian cells in-vitro. The positive controls gave the expected results.

Phosphoric acid is routinely used in the nutrient broths that support cell cultures in the laboratory and as such cells are constantly exposed to inorganic phosphates. In addition, phosphoric acid is also found in the metabolic activation mixture (e.g. S9-mix) which is used in in vitro gene mutation studies to determine whether a test chemical can be metabolized within the body to produce a compound that may be genotoxic. The constant exposure of bacteria and cell cultures to these materials suggests that they pose no inherent risk of genotoxicity.

PO43-ions are essential for life and are not considered to be genotoxic or mutagenic in standard test systems. In addition, inorganic phosphate salts are widely used as food ingredients and considered as "generally Recognized as safe" (GRAS; FDA: Food and Drug Administration 1973. GRAS (Generally Recognized as Safe) food ingredients—phosphates. NTIS PB-221-224, FDA, Food and Drug Administration, 1979. Phosphates; Proposed Affirmation of and Deletion From GRAS Status as Direct and Human Food Ingredients. Federal Register 44 (244). 74845-74857, 18 December (1979)). Therefore further testing to determine the genotoxicity or mutagenicity of orthophosphates is not considered to be scientifically justified.

In conclusion, silicon orthophosphate is considered not to cause genetic damage, since all in vitro genetic toxicity studies on the substance itself and the read-across substances revealed negative results.

Justification for selection of genetic toxicity endpoint
No study was selected, since all available in vitro genetic toxicity studies were negative. Hazard assessment was conducted by means of data on silicon orthophosphate as well as of read-across from silicon dioxide and orthophosphoric acid.

Short description of key information:
Gene mutation in bacteria (OECD 471, silicon orthophosphate): S. typhimurium TA 1535, TA 1537, TA 98, TA 100 and TA102: negative with and without metabolic activation
Chromosome aberration (OECD 473, silicon dioxide): CHO cells: negative with and without metabolic activation
Gene mutation in mammalain cells (OECD 476, silicon dioxide): CHO cells: negative with and without metabolic activation

Endpoint Conclusion: No adverse effect observed (negative)

Justification for classification or non-classification

The available data on genetic toxicity of the test substance do not meet the criteria for classification according to Regulation (EC) 1272/2008 or Directive 67/548/EEC, and are therefore conclusive but not sufficient for classification.