Sodium 2,4-dichlorophenoxyacetate

Endpoint summary

Description of key information

RA-S, CAS 2008-39-1, Key, Charles, 1996, 90 d, rats, RL2 - NAEL 15 mg/kg bw/d 
RA-S, CAS 2008-39-1, Charles, 1996, 90 d, dog, RL2 - NOAEL 1 mg/kg bw/d

Key value for chemical safety assessment

Repeated dose toxicity: via oral route - systemic effects

Endpoint conclusion

Dose descriptor:

NOAEL

15 mg/kg bw/day

Study duration:

subchronic

Species:

rat

Additional information

No repeated dose studies with sodium 2,4-dichlorophenoxyacetate (CAS 2702-72-9) were available. Therefore, data of a structural analogous substance, dimethylammonium 2,4-dichlorophenoxyacetate (CAS 2008-39-1), was used for read-across.

The assessment of systemic effects of salts of 2,4-D is based on data generated with 2,4-D acid. This read across is justified because 2,4-D acid and its salts are toxicologically equivalent once they have entered the system. This is due to the only diffusion-limited rate of reaching the acid-base equilibrium between the protonated acid and its deprotonated salt form in an aqueous environment like the human body. Thus, salts of 2,4-D will exert the same systemic effects as the corresponding acid.

Studies to evaluate the repeated dose toxicity of dimethylammonium 2,4-dichlorophenoxyacetate (CAS 2008-39-1) were conducted according to the EPA-Pesticide Assessment Guidelines, Subdivision F, Hazard Evaluation, Human and Domestic Animals, Series 82-1, 83-1, and 82, revised ed., November, 1984, US Department of Commerce, National Technical Information Service PB86-108958.

In a subchronic repeated dose study conducted in rats (Key, Charles, 1996, 90 d, rats, RL2), 10 male and 10 female Fischer 344 rats were exposed for 90 days to 1, 15, 100 and 300 mg/kg bw/day (acid equivalent). The test substance was mixed to the feed and was available ad libitum during the test period. Animals were observed for signs of overt toxicity, moribundity and mortality twice daily and animal weights, clinical observations, and food consumption were determined weekly. Ophthalmoscopic examinations were conducted prior and at the end of the study. Clinical chemistry, hematology, and urinalysis were performed during weeks 6 and 13. Animals were sacrificed at the end of the study and subjected to gross and microscopic examinations.

Severe body weight gain depressions were observed at the dose level of 300 mg /kg/day with up to 80% less body weight gain for males and up to 88% less body weight gain for females, respectively. 100 mg/kg bw/day (acid equivalent) did not alter the body weight gain or the food consumption of the animals. Effects on the organ weights were observed from 100 mg/kg bw/day (acid equivalent) on, whereas histopathological changes were observed predominantly in animals treated with 300 mg/kg bw/day (acid equivalent). At this concentration, retinal degeneration and cataract formation (females), centrilobular hepatocellular hypertrophy (both sexes), atrophy of the testes, hypertrophy in the zona glomerulosa of the adrenal cortex (both sexes), brush border loss in proximal tubular cells in the kidney (both sexes), and vacuolization of kidney tubular cells (both sexes) were observed. These were considered to be of a slight degree and occurred at a dose that clearly exceeded the maximum tolerated dose (MTD).Therefore a NOEL for body weight and food consumption effects is considered to be 100 mg/kg bw/day (acid equivalent) and a NOEL for organ weights and histopathological changes was 15 mg/kg bw/day (acid equivalent).

Clinical chemistry, hematology, and urinalysis were performed during weeks 6 and 13. Changes in red cell mass, platelet count, T3, and T4 were observed. Blood urea nitrogen (BUN) was increased in the high dose animals (females) whereas the haemoglobin concentration was reduced at this concentration. 100 mg/kg bw/day (acid equivalent) significantly reduced the T3 and T4 levels (females), the red blood cell count (females), and platelet counts (females). The NOEL for hematology and clinical chemistry effects was set to 15 mg/kg bw/day (acid equivalent).

According to the results of this study conducted with rats, an overall NOEL for dimethylammonium 2,4-dichlorophenoxyacetate (CAS 2008-39-1) of 15 mg/kg bw/day (acid equivalent) was determined.

 

 

Another subchronic repeated dose study was conducted in male and female beagle dogs (Charles, 1996, 90 d, dog, RL2). 4 male and 4 female animals were exposed for 90 days to 1.0, 3.75 and 7.5 mg/kg bw/day (acid equivalent) day of the test substance (based on acid equivalents). The test substance was mixed to the feed and was available ad libitum during the test period. Animals were observed for signs of overt toxicity, moribundity and mortality twice daily and animal weights, clinical observations, and food consumption were determined weekly. Ophthalmoscopic examinations were conducted prior and at the end of the study. Clinical chemistry, hematology, and urine-analysis were performed prior to treatment and during weeks 4 and 13. Animals were sacrificed at the end of the study and a complete necropsy was performed and organs were weighted and subjected to gross and microscopic examinations.

No treatment related mortalities were observed and there were no treatment related effects of compound administration on any of the hematology or urinalyses parameters during the treatment period. The highest dose group of both sexes showed lower body weight gains compared to the controls. In addition, the food consumption was reduced (approximately 15%) compared to the controls. 12 of the 17 clinical chemistry parameters evaluated were unaffected by treatment. 4 parameters were consistently increased (BUN, creatinine and alanine aminotransferases and aspartate aminotransferase) whereas the alkaline phosphatase value was decreased. These changes generally followed a dose response with only minor effects, if any seen at 1.0 mg/kg bw/day (acid equivalent).

The testicular weight ratios (testes-to-body weight and testes-to-brain weight) were a bit lower versus the concurrent control at the dose level of 7.5 mg/kg bw/day (acid equivalent). No other differences in organ weights were considered to be treatment related. There was a minimal increase in the average severity of perivascular chronic active inflammation in the liver of the dogs at 7.5 mg/kg bw/day (acid equivalent) when compared to control dogs. This finding was observed in both sexes. The severity increase resulted from observations of slight-to-moderate severity grading in one to three dogs. No microscopic change was noted in the dog’s kidneys that correlated with increases in the mean values for blood urea nitrogen and creatinine concentration.

The NOAEL for Body weight gain was determined to be 3.75 mg/kg bw/day (acid equivalent), a NOAEL for clinical chemistry parameters could not be determined. For liver histology and testes weight, a NOAEL of 3.75 mg/kg bw/day (acid equivalent) was determined.

According to the results of this study in beagle dogs, an overall NOEL for dimethylammonium 2,4-dichlorophenoxyacetate (CAS 2008-39-1) of 1 mg/kg bw/day (acid equivalent) was determined.

 

 

It was shown that there is an early saturation effect in the excretion of 2,4-D in dogs. Clearance of 2,4-D is clearly dose-dependent and with higher doses, the excretion of unmetabilised 2,4-D changes and nearly a hundred percent of the dose is excreted as metabolites. This reflects the limited capacity of dogs to excrete a wide range of organic acids via the kidneys. Likely mechanisms responsible for this decreased clearance include saturation of renal secretion and increased renal tubule re-absorption. Therefore, based on scientific evidence, it would seem reasonable not to use the toxicity data generated in dogs but rather the data obtained from rodent experiments for risk evaluation (For further details see chapter Toxicokinetics, metabolism and distribution).

The rat study resulted in an overall NOEL of 15 mg/kg bw/day (acid equivalent) and a LOAEL of 100 mg/kg bw/day (acid equivalent). Changes in the clinical chemistry and haematologic parameters were observed but no histopathologic correlation was found at this dose. Histological changes observed in the liver, kidney, testes and adrenal gland were observed predominantly in the highest dose (300 mg/kg bw/day (acid equivalent)) effects and these effects observed were considered to be of a slight degree. Taking into account the clear dose dependency of the effects and the minor markedness of the effects observed at 100 mg/kg bw/day (acid equivalent), it is scientifically justified to define this dose as overall LOAEL and to use it for classification.

 

There are no other repeated dose studies available. Although other routes of exposure are possible due to the intended use, the oral exposure is considered a reasonable worst case assumption and therefore, no other studies are needed for risk evaluation. The molecular weight of sodium 2,4-dichlorophenoxyacetate is only 10% higher than that of 2,4-D acid. Thus, no MW correction is applied to the point of departure. 

 

Justification for classification or non-classification

According to the DSD (67/548/EEC) and CLP (EC No. 1272/2008) criteria for classification and labelling of dangerous substances sodium 2,4-dichlorophenoxyacetate (CAS No. 2702-72-9) is not classified as toxic after repeated dose administration.