TOXICOLOGY - ACUTE DIETARY RISK
The twenty-eighth Session of the CCPR raised the issue of the acute toxicity of disulfoton residues and requested the JMPR to derive an acute reference dose.
An ADI of 0-0.0003 mg/kg bw was established for disulfoton by the 1991 Meeting on the basis of an NOAEL of 1 ppm, equal to 0.03 mg/kg bw per day, for the inhibition of brain acetylcholinesterase activity in a two-year study in dogs. This ADI was supported by an NOAEL of 1 ppm, equal to 0.06 mg/kg bw per day, for the inhibition of brain acetylcholinesterase activity in a two-year study in rats.
Disulfoton was not carcinogenic or teratogenic and caused no toxicity other than that associated with acetylcholinesterase inhibition.
Groups of 10 male and 10 female Sprague-Dawley rats, 8-9 weeks old, were given single doses of disulfoton dissolved in polyethylene glycol 400 at 5 ml/kg bw by gavage. The doses were 0, 0.25, 0.75, or 1.5 mg/kg bw for females and 0, 0.25, 1.5, or 5.0 mg/kg bw for males. A functional observational battery and testing of motor activity were carried out 1.5-4 h after treatment. Plasma cholinesterase and erythrocyte acetylcholinesterase activities were determined 24 h after treatment.
Erythrocyte acetylcholinesterase activity was inhibited by 10% in males at the middle dose and 21% in those at the high dose and by 12, 53, and 75% in females at the low, middle and high doses respectively. Plasma cholinesterase activity was inhibited to a similar extent in males but to a lesser extent than that of erythrocyte acetylcholinesterase in females. Clear cholinergic signs were observed in males at 5 mg/kg bw and in females at 1.5 and 0.75 mg/kg bw. The signs appeared on day 0 of dosing but had disappeared by day 3. Functional and motor activity testing showed treatment-related effects at the same doses (Sheets, 1993a). Since cholinesterase activity was not determined when the maximal clinical score was reached, another study was conducted.
Groups of six male and six female fasted Sprague-Dawley rats were given technical-grade disulfoton (purity 99.0%) at doses of 0, 0.25, 0.75 (females only), 1.5, or 5.0 (males only) mg/kg bw by gavage. Cholinesterase activity was determined in the plasma, erythrocytes and brain 3 h after treatment, i.e. approximately at the time of peak clinical signs. Brain acetylcholinesterase activity was inhibited less than that in erythrocytes and plasma. The results are shown in Table 1. The NOAEL for the inhibition of brain acetylcholinesterase activity was 0.25 mg/kg bw in both males and females (Sheets, 1996).
Table 1. Cholinesterase activity 3 h after a single dose of disulfoton1
1 Percentages of activity of the concurrent controls. For plasma and erythrocyte cholinesterase activities similar percentages were obtained when calculated on the basis of pre-exposure activity
|
DOSE (mg/kg bw) |
SEX |
% OF CONTROL CHOLINESTERASE ACTIVITY |
||
|
PLASMA |
ERYTHROCYTES |
BRAIN |
||
|
0.25 |
Female |
96 |
96 |
97 |
|
|
Male |
94 |
93 |
108 |
|
0.75 |
Female |
28 |
55 |
51 |
|
1.50 |
Male |
54 |
40 |
73 |
|
|
Female |
13 |
21 |
38 |
|
5.00 |
Male |
28 |
18 |
42 |
An acute reference dose of 0.003 mg/kg bw was established on the basis of the absence of inhibition of brain acetylcholinesterase activity and clinical signs at 0.25 mg/kg bw in rats treated with a single dose by gavage, applying a 100-fold safety factor.
References
Sheets, L.P. (1993) An acute oral neurotoxicity screening study with technical grade disulfoton (DI-SYSTON) in rats. Unpublished report No. 92-412-OB from Miles Inc., Stilwell, KS, USA. Submitted to WHO by Bayer AG, Wuppertal, Germany.
Sheets, L.P. (1996) Cholinesterase results from an acute oral study with technical grade disulfoton (DI-SYSTON). Summary report No. 96-412-JH from Miles Inc., Stilwell, KS, USA. Submitted to WHO by Bayer AG, Wuppertal, Germany.
