PHOSMET (addendum) JMPR 1998
First draft prepared by
T.C. Marrs
Department of Health
London, United Kingdom
Explanation
Evaluation for acceptable daily intake
Delayed neuropathy
Unscheduled DNA synthesis in vivo
Comments
Toxicological evaluation
References
Explanation
Phosmet was evaluated toxicologically by the Joint Meeting in
1978 (Annex 1, reference 30), when a temporary ADI of 0-0.005 mg/kg bw
was allocated. It was re-evaluated in 1979 (Annex 1, reference 32)
when additional data on teratogenicity became available, and an ADI of
0-0.02 mg/kg bw was established. Further data were made available to
the 1994 JMPR (Annex 1, reference 73), when an ADI of 0-0.01 mg/kg bw
was allocated on the basis of the NOAEL in a multigeneration study of
reproductive toxicity in rats, with a 100-fold safety factor. At that
meeting, further information was requested, namely a further long-term
study in dogs, studies on DNA binding in vivo, and a study of the
potential of phosmet to cause delayed polyneuropathy in hens at an
appropriately high dose, with an estimate of neuropathy target
esterase. The last study was supplied but not the requested study of
repeated dosing in dogs; moreover, a study of unscheduled DNA
synthesis in rat liver in vivo was submitted rather than a study of
DNA binding in vivo. The new studies are reviewed below.
Evaluation for Acceptable Daily Intake
1. Delayed neuropathy
A study of the ability of phosmet to cause delayed polyneuropathy
in Lohmann Brown hens (Gallus gallus domesticus) was conducted with
phosmet of a purity of 97.4%. In a preliminary study, the LD50 of
phosmet given orally was determined in seven groups of 10 birds. It
was estimated to be 580 mg/kg bw (95% confidence interval, 410-770). A
dose of 600 mg/kg bw was therefore given to 24 birds; 12 further birds
received the vehicle (corn oil), and a further 12 were given
tri- ortho-cresyl phosphate at 1 g/kg bw as positive controls. All
birds were injected subcutaneously with atropine sulfate at 20 mg/kg
bw just before treatment, and further injections were given to birds
that showed severe clinical signs. The birds were observed for adverse
clinical signs, ataxia, and effects on body weight. After 48 h, three
birds from the groups receiving phosmet, the vehicle or the positive
control were killed, and brain acetylcholinesterase and brain and
spinal cord neuropathy target esterase activities were estimated.
After 21 days, six birds from each group were sacrificed and fixed by
perfusion, and the head, spinal column, and dissected sciatic and
tibial nerves were taken and stored in 10% buffered formalin. Sections
of fore-, mid-, and hindbrain; upper and lower cervical, mid-thoracic,
and lumbo-sacral spinal cord; and proximal and distal sciatic and
tibial nerves were examined histopathologically.
No ataxia was observed in the birds treated with phosmet, while
8/12 birds treated with tri- ortho-cresyl phosphate developed
clinical ataxia, commencing at 11-18 days. Brain acetylcholinesterase
activity was reduced in birds given phosmet to 37% of the value in
concurrent vehicle controls, but the neuropathy target esterase
activity in both brain and spinal cord was similar to that of
controls. In contrast, in birds given tri- ortho-cresyl phosphate,
the acetylcholinesterase activity was similar to that of vehicle
controls, whereas the neuropathy target esterase activity was markedly
inhibited, being 9.6% of the value for concurrent controls in brain
and 20% that of concurrent controls in spinal cord. Vehicle controls
gained weight normally, while those treated with tri- ortho-cresyl
phosphate lost weight. With phosmet, initial weight loss was followed
by recovery. Histopathological examination showed no evidence of the
characteristic changes of delayed neuropathy in the phosmet-treated or
vehicle control hens, whereas birds that had received
tri- ortho-cresyl phosphate showed minimal axonal degeneration in the
cerebellum and minimal or moderate axonal degeneration at one or more
levels of the spinal cord and in some peripheral nerve sections
(Johnson, 1997).
2. Unscheduled DNA synthesis in vivo
The ability of phosmet (purity, 96.4%) to induce unscheduled DNA
synthesis in male Alpk:APfSD rats was determined by an
autoradiographic technique. Five rats received phosmet orally at a
dose of 32 (two rats) or 50 mg/kg bw (three rats) in corn oil, the
higher dose being the maximum tolerated dose of phosmet in tthis
strain of rat. Hepatocytes were isolated and prepared at 2 and 16 h;
two independent experiments were carried out at each time. Groups of
two animals received the vehicle or dimethylhydrazine dihydrochloride
as a positive control. The mean net nuclear grain counts and the
percentages of cells in repair were recorded. Phosmet did not induce
DNA repair at either dose or time, whereas the positive control
produced marked unscheduled DNA synthesis in comparison with vehicle
controls (Mackay, 1996).
Comments
In the study of delayed polyneuropathy, a dose of 600 mg/kg bw
was given to 24 hens, this dose being greater than the experimentally
determined LD50. There was no evidence that phosmet could produce
clinical signs of delayed polyneuropathy or significantly inhibit
neuropathy target esterase.
The ability of phosmet (96.4% pure) to induce unscheduled DNA
synthesis in the liver of male rats in vivo was determined with
doses of 32 or 50 mg/kg bw, the higher dose being the maximum
tolerated dose of phosmet. Unscheduled DNA synthesis was not observed.
The Meeting noted that the study on DNA binding had not been provided,
but it concluded that no further characterization of mutagenicity was
required. The Meeting considered that a further study in dogs would be
unlikely to affect the overall evaluation.
The ADI of 0-0.01 mg/kg bw allocated by the 1994 JMPR, which was
based on a NOAEL of 1.3 mg/kg bw per day in a multigeneration study in
rats and a safety factor of 100, was confirmed.
An acute RfD of 0.02 mg/kg bw was allocated on the basis of a
NOAEL of 2 mg/kg bw per day in a study of developmental toxicity in
rabbits (fetotoxicity) and a safety factor of 100.
Toxicological evaluation
Levels that have no toxic effect (from 1994 monograph)
Mouse: 25 ppm, equivalent to 4 mg/kg bw per day (two-year
study of carcinogenicity)
Rat: 40 ppm, equal to 1.8 mg/kg bw per day (two-year
study of toxicity and carcinogenicity)
Rat: 20 ppm, equal to 1.3 mg/kg bw per day
(two-generation study of reproductive toxicity)
5 mg/kg bw per day (maternal toxicity in study of
developmental toxicity)
15 mg/kg bw per day (study of developmental
toxicity)
Rabbit: 5 mg/kg bw per day (maternal toxicity in study of
developmental toxicity)
2 mg/kg bw per day (study of developmental
toxicity)
Estimate of acceptable daily intake for humans
0-0.01 mg/kg bw
Estimate of acute reference dose
0.02 mg/kg bw
Studies that would be useful for continued evaluation of the
compound
Further observations in humans
References
Johnson, A.J. (1997) Phosmet acute delayed neurotoxicity study in the
domestic hen. Unpublished report No. CTL/C/3123, dated 23 January
1997, from Huntingdon Life Sciences, Huntingdon, Cambridgeshire,
United Kingdom. Submitted to WHO by Gowan Co., Yuma, Arizona, USA.
Mackay, J.M. (1996) Phosmet: in vivo rat liver unscheduled DNA
synthesis assay. Unpublished report No. CTL/P/5090, dated 23 September
1996, from Central Toxicology Laboratory, Alderley Park, Cheshire,
United Kingdom. Submitted to WHO by Gowan Co., Yuma, Arizona, USA.
WHO (1996) The WHO Recommended Classification of Pesticides by
Hazard and Guidelines to Classification 1996-1997 (WHO/PCS/96.3),
International Programme on Chemical Safety, Geneva.