The toxicology of phorate was considered by Joint Meetings in
    1977, 1982 and 1983 (Annex 1, FAO/WHO, 1978a, 1983a, 1984). A
    toxicological monograph was prepared by the Joint Meeting in 1977
    (Annex 1, FAO/WHO, 1978b) and a monograph addendum was prepared in
    1982 (Annex 1, FAO/WHO, 1983b). Due to the lack of an appropriate
    delayed-neurotoxicity study, only a temporary acceptable daily intake
    of 0-0.002 mg/kg b.w. was allocated in 1982. In 1983, the required
    study was not available. The meeting extended the temporary ADI,
    however, to 1985. Since then, the required neurotoxicity study and
    additional mutagenicity studies have become available. These studies
    are reviewed in this monograph addendum.



    Toxicological studies

    Special study on Neurotoxicity

         Phorate of 89.5% purity was dissolved in corn oil and orally
    administered by gavage to fasted white leghorn hens (22 to 23 months
    old). The single dose oral LD50 was determined to be 14.2 mg/kg b.w.
    Fifty fasted hens were then atropinized by intramuscular injection of
    10 mg/kg b.w. of atropine sulfate and 1 hour later given single oral
    doses of phorate at a dosage level of 14.2 mg/kg b.w. (test group). An
    additional 15 fasted and atropinized hens were given corn oil only
    (vehicle control group) and 15 more non-atropinized hens were given
    tri-o-tolyl phosphate (TOTP) at a dosage level of 500 mg/kg b.w.
    (positive control group). Twenty-one days later, the dosing was
    repeated for all suviving hens in the test and vehicle control groups
    except that atropine sulfate was administered at a dosage level of
    30 mg/kg b.w. All hens were observed daily for mortality, clinical
    signs and evidence of neurotoxic reactions. Body weights and food
    consumption were recorded every 3 days. Gross necropsies were
    performed on all hens that died during the study and on all hens that
    were sacrificed at termination of the study at 42 days. Hens
    sacrificed at termination of the study were perfused with 10%
    neutralized formalin. Brains, vertebral columns (with spinal cord
    in situ) and the entire lengths of right and left sciatic nerves
    were excised and fixed. Microscopic slides of neural tissue were
    prepared as follows: a sagittal section of the entire brain (corpus
    striatum, cerebellum, pons), a longitudinal and cross section of three
    levels of spinal cord (cervical, thoracic, lumbrosacral) and both
    sagittal and longitudinal sections of the right and left sciatic
    nerves. Sections were stained with hematoxylin and eosin and replicate

    sections with luxol fast blue. The tissues from 10 hens in each of the
    test, vehicle-control and positive-control groups were
    histopathologically examined.

         None of the 15 vehicle-control hens died during the 42-day study.
    All 15 positive-control hens were sacrificed in extremis on day 16
    of the study after clinical signs of neurotoxicity, first observed on
    day 11, progressively became more severe. These signs included
    generalized weakness, ataxia and paralysis of the legs and wings. Of
    the 50 test hens, 27 died within 24 hours of the first dose of phorate
    and 13 more within 24 hours after the second dose. Ten test hens
    survived to termination of the study at 42 days. Vehicle control and
    test hens displayed slight generalized weakness of the limbs, lasting
    about 2 hours, shortly after each treatment with atropine sulfate.
    Test hens had slightly more severe reactions and, in addition, slight
    to moderate ataxia for up to 2 hours after dosing with phorate. No
    clinical signs of delayed neurotoxicity, however, were observed in any
    vehicle-control or test hens. Mean body-weight gains of test hens were
    greater at 0 to 21 days and less at 21 to 42 days than those of
    vehicle-control hens. Mean feed consumption of test hens was less at 0
    to 21 days and greater at 21 to 42 days than that of vehicle-control
    hens. Gross necropsies were negative for any effects attributable to
    the test material.

         Histopathological examination of neural tissues from positive-
    control hens revealed treatment-related lesions involving the brain,
    spinal cord, and/or sciatic nerves in all 10 hens. Generally mild to
    moderate axonal degeneration was observed in the brains of 4/10 hens,
    in the spinal cords of 10/10 hens, and in the sciatic nerves of 10/10
    hens. In addition, demyelination was observed in the spinal cords of
    7/10 hens and in the sciatic nerves of 7/10 hens. Schwann cell
    hyperplasia was also observed in the sciatic nerves of 3/10 hens.
    These lesions were compatible with a TOTP-induced delayed neuro-toxic
    response. Minimal to mild focal lesions of axonal degeneration of the
    sciatic nerves were noted in 3/10 test hens. No axonal degeneration
    was noted in any vehicle-control hens. The axonal degeneration
    observed in 3/10 test hens was associated with interstitial
    infiltrations of lymphoid cells, which was also observed in additional
    test and vehicle-control hens. This syndrome, which was distinctly
    different than that observed in the positive-control hens, was
    ascribed to lesions of naturally occurring disease (i.e. Marek's
    disease) and was not considered to be related to the test material.
    Treatment of hens with phorate did not induce clinical or
    histopathological signs indicative of acute delayed neurotoxicity
    (Fletcher, 1984).

    Special studies on mutagenicity

         Technical grade phorate of unspecified purity was tested in the
    mutagenicity studies reported in Table 1.

        Table 1.  Results of mutagenicity assays with phorate

    Study type               Dosage level and/or           Results        Reference

    Bacteria tests

    Reverse mutation,        dosage levels up to 1000      negative       Simmon et al.,
    S. typhimurium,          mg/plate*                                    1977
    strains TA1535,
    TA1537, TA1538,
    & TA100.
    E. coli,
    strain WP2

    Preferential             1 mg (on filter disc)/        negative       Simmon
    toxicity, E. coli,       plate, w/o metabolic                         et al., 1977
    strains p3478,           activation
    B. subtilis,
    strains M45, H17

    Yeast tests

    Mitotic recombination,   5% w/v for 4 hours            negative       Simmon
    S. cerevisiae D3         incubation followed by                       et al., 1977

    Cultured mammalian cells

    Unscheduled DNA          dosage levels up to           negative       Simmon
    synthesis, human         10-3 M*                                      et al., 1977
    (WI-38) cells

    In vivo study

    Dominant lethal,         0, 5, 10, & 20 mg/kg/         negative       Simmon
    male mice                day in the diet for                          et al., 1977
                             7 weeks, followed by
                             weekly matings for 8

    *    without and with metabolic activation

         Phorate did not induce clinical or histophathological signs of
    neurotoxicity in a neurotoxicity study in hens. No evidence of
    mutagenic potential was observed in a series of mutagenicity studies.

         Since all required toxicity studies on phorate have been
    submitted and evaluated, the Meeting estimated an ADI for phorate.



         Rat: 1 ppm in the diet, equivalent to 0.05 mg/kg b.w.

         Dog: 0.01 mg/kg b.w./day


         0-0.0002 mg/kg b.w.


         Observations in man.


    Fletcher, D.W. 42-Day neurotoxicity study with phorate in mature white
    1984      leg horn chickens. Unpublished report from Bio-Life
              Associates, Ltd., BLAL No. 83 DN 103. AC Protocol No.
              981-84-114. Submitted to WHO by American Cyanimid Company.

    Simmon, V.F., Mitchell, A.D. & Jorgenson, T.A. Evaluation of selected
    (1977)    pesticides as chemical mutagens in in vitro and in vivo
              studies. Unpublished report from Stanford Research
              Institute, SRI report No. LSU-3493, for U.S. Environmental
              Protection Agency, EPA report No. EPA-600/1-77-028.
              Submitted to WHO by American Cyanamid Company).

    See Also:
       Toxicological Abbreviations
       Phorate (ICSC)
       Phorate (Pesticide residues in food: 1977 evaluations)
       Phorate (Pesticide residues in food: 1982 evaluations)
       Phorate (Pesticide residues in food: 1984 evaluations)
       Phorate (Pesticide residues in food: 1994 evaluations Part II Toxicology)
       Phorate (Pesticide residues in food: 1996 evaluations Part II Toxicological)