Toxicology and carcinogenesis studies of tetrakis(hydroxymethyl)phosphonium sulfate (THPS) and tetrakis(hydroxymethyl)phosphonium chloride (THPC) were conducted because of the widespread use of these chemicals as flame retardants in cotton fabrics. THPS was available as a 72% aqueous solution and THPC as a 75% aqueous solution. Short-term gavage studies with a range of doses were conducted first to identify toxic effects and affected sites and to determine doses for the 2-year studies. The doses selected for the 14-day studies ranged from 12.5 to 200 mg/kg THPS for rats and mice, 9.4 to 150 mg/kg THPC for rats, and 18.8 to 300 mg/kg THPC for mice. Mortality and reduction in body weight gain occurred at the two highest doses in the 14-day studies. There was hind limb paralysis in some rats and mice dosed at the highest concentrations of THPS and THPC.
In the 13-week studies, doses of THPS ranged from 5 to 60 mg/kg in rats and from 2 to 180 mg/kg in mice; doses of THPC ranged from 3.75 to 60 mg/kg in rats and from 1.5 to 135 mg/kg in mice. Mortality and reduction in body weight gain occurred at the two higher doses for both sexes and species. Vascular degeneration of hepatocytes or hepatocellular necrosis was a common histopathologic finding. Hind limb paralysis was noted in rats and mice receiving the highest dose of THPC, and axonal degeneration, characterized by swollen axon sheaths, missing or fragmented axons, and some proliferation of neurolemma cells, was observed in rats. These lesions were found in the sciatic nerve, dorsal roots of the caudal spinal nerves, and the tracts of the spinal cord, particularly in the dorsal column of the lumbar cord.
Two-year studies were conducted in F344/N rats by administering 0, 5, or 10 mg/kg THPS or 0, 3.75, or 7.5 mg/kg THPC in deionized water by gavage to groups of 49 or 50 animals of each sex, 5 days per week for 103 or 104 weeks. Groups of 49 or 50 B6C3F1 mice were administered 0, 5, or 10 mg/kg THPS (each sex), 0, 7.5, or 15 mg/kg THPC (males), or 0, 15, or 30 mg/kg THPC (females).
Survival of male rats was reduced for the low dose (after week 102) and the high dose (after week 67) groups given THPS compared with that of the vehicle controls; survival at terminal kill was as follows: vehicle control, 28/50; low dose, 13/50; high dose, 16/50. Survival of the high dose group of female rats given THPC was lower after week 70 than that of the vehicle controls (survival at terminal kill: 37/50; 34/50; 21/50). Mean body weights of rats dosed with THPS or THPC were comparable to those of the vehicle controls. There was no difference in survival or mean body weights between the vehicle controls and mice dosed with either THPS or THPC. No neurotoxicity or any other signs of clinical toxicity were observed.
A nonneoplastic effect common to 13-week and 2-year exposure to THPS or THPC was an increase in the incidence of hepatocellular lesions, primarily cytoplasmic vacuolization. The incidences of this lesion in the two-year studies were dose related for all studies except for the mice receiving THPS. Other lesions observed included focal hyperplasia of the adrenal medulla in high dose male mice given THPS and follicular cell hyperplasia of the thyroid gland in high dose female mice given THPC. The increased incidences of hematopoietic system lesions observed in these studies were not considered biologically related to chemical exposure because the increases were marginal, no dose-response relationship was observed, and the incidences of these lesions are highly variable in untreated rats and mice.
The incidences of mononuclear cell leukemia in low dose male rats administered THPS or THPC were somewhat greater than those in the vehicle controls (THPS: 30/50; 36/50; 20/50; THPC: 19/50; 25/50; 16/50). Low dose male mice administered THPS had an increased incidence of malignant lymphomas when compared with vehicle controls (2/50; 9/50; 0/50). These marginal increases in the incidences of hematopoietic system tumors were not considered related to chemical exposure, since they were significant only by the life table tests and were not dose related.
THPC demonstrated no mutagenic activity in Salmonella typhimurium strains TA98, TA100, TA1535, or TA1537 with or without metabolic activation. Both THPS and THPC induced forward mutations in mouse lymphoma L5178Y cells without metabolic activation; neither was tested in the presence of S9. THPC increased the frequency of sister-chromatid exchanges and chromosomal aberrations in Chinese hamster ovary cells in the presence and absence of exogenous metabolic activation.
An audit of the experimental data was conducted for the 2-year studies of THPS and THPC. No discrepancies were found that influenced the final interpretations.
Under the conditions of these 2-year gavage studies, there was no evidence of carcinogenicity of THPS in either sex of F344/N rats or B6C3F1 mice given 5 or 10 mg/kg. There was no evidence of carcinogenicity of THPC in either sex of F344/N rats given 3.75 or 7.5 mg/kg, in male B6C3F1 mice given 7.5 or 15 mg/kg, or in female B6C3F1 mice given 15 or 30 mg/kg.