First draft prepared by
    D.J. Clegg, Carp, Ontario, Canada


         Chlorpyrifos-methyl was evaluated by previous Joint Meetings in
    1975 and 1991 (Annex 1, references 24 and 62). An ADI of 0.01 mg/kg
    bw allocated in 1975 was reduced to 0.001 mg/kg bw in 1991. The
    present Meeting re-evaluated the recently-conducted long-term
    dietary study in rats on which the ADI of 0.001 mg/kg bw was based
    in 1991. The major interest was the vacuolation of the  zona
     fasciculata of the adrenal gland.


    Long-term toxicity/carcinogenicity studies


         Five groups of 50 Fischer 344 rats (5-6 weeks old)/sex/dose
    were fed chlorpyrifos-methyl (average purity 95.4  0.8%) at dietary
    concentrations of 0, 1, 2, 20 or 1000 ppm for 2 years, equivalent to
    0.05, 0.1, 1 or 50 mg/kg bw/day. Ten additional rats/sex/dose were
    administered the same diets for 53 weeks prior to sacrifice.
    Analyses of the diet at 9 time intervals during the study showed
    dietary concentrations to be acceptable. A single set of analyses
    confirmed adequate homogeneity and stability in the diet for up to
    24 days. Rats housed singly with food and water available  ad
     libitum, were observed daily, with a detailed examination weekly,
    throughout the study. Animal weights were recorded weekly for 13
    weeks and then once every 4 weeks. Feed intake, using 20
    rats/sex/dose followed the same time pattern as body-weight
    measurement. Blood samples (from fasted animals) for clinical
    chemistry and haematology and urine for urinalyses were collected at
    26, 52-53, 78, and 104 weeks. Brain cholinesterase was measured in
    half brain samples in all interim sacrifice rates, and in 10
    rats/sex/dose at terminal sacrifice. Organ weights (recorded only in
    scheduled sacrifices) were determined in fasted rats at necropsy.
    Histopathology was performed on 53 tissues in all rats receiving 0
    and 50 mg/kg bw/day for 2 years and on all rats dying or terminated
    in moribund condition. At 0.05, 0.1 and 1 mg/kg bw/day,
    histopathological examination was limited to adrenal glands, brain,
    epididimides, kidneys, liver, lungs, pituitary gland, spinal cord,
    sciatic nerve, testes, tibial nerve and gross lesions in rats
    sacrificed at term.

         Mortality was not affected. Body-weight reduction
    (statistically significant but less than 10%) was detected at 50
    mg/kg bw/day in both sexes. Food intake was slightly reduced in
    males at 50 mg/kg bw/day. Clinical signs were comparable in all
    groups. Haematological parameters (PCV, Hb, RBC counts, WBC total
    and differential counts and platelet counts) showed no differences
    attributable to chlorpyrifos-methyl; neither did urinalyses
    parameters (pH, protein, glucose, ketones, bilirubin, urobilinogen
    or sediment macroscopic examination). Clinical chemistry values
    (alanine transaminase, albumin, aspartate transaminase, alkaline
    phosphatase, glucose, phosphorus, total protein, triglycerides,
    blood urea nitrogen, chloride, sodium and potassium) were
    sporadically significantly different, but results were inconsistent,
    except for depressed levels of ALT and AST in males at 6, 12 and 18
    months (but not at 24 months) and in females at 6 and 12 months
    only. Cholinesterase inhibition was noted in plasma at 1 and 50
    mg/kg bw/day in both sexes. Erythrocyte and brain cholinesterase
    activities were depressed at 50 mg/kg bw/day in both sexes.

    Incidence of gross lesions were comparable in all groups. Organ
    absolute and relative weights were comparable in all groups except
    for the adrenal gland. After deletion of adrenal glands bearing
    tumours (i.e. where the organ weight was likely to be abnormal) both
    absolute and relative adrenal weights were increased in both sexes
    at 50 mg/kg bw/day at interim (1 year) and terminal (2 year)
    sacrifice. At interim sacrifice, both sexes in the 50 mg/kg bw/day
    group showed 10/10 rats with moderate vacuolation, consistent with
    lipid accumulation in the  zona fasciculata of the adrenal. At
    terminal sacrifice, in males, the incidence of moderate vacuolation
    was 49/50, 1/50, 0/50, and 0/50 at 50, 1, 0.1 and 0.05 mg/kg bw/day,
    respectively. Slight vacuolation was noted in 1/50, 10/50, 9/50,
    5/50 and 3/50 at 50, 1, 0.1, 0.05 and 0 mg/kg bw/day. In the
    females, adrenal  zona fasciculata vacuolation was noted in 30/50,
    0/50, 0/50, 0/50 and 0/50 as being moderate, and in 19/50, 0/50,
    1/50, 2/50 and 2/50 as being slight at 50, 1, 0.1, 0.05 and 0 mg/kg
    bw/day, respectively.

         An increased incidence of adrenal vacuolation was observed in
    both sexes at 50 mg/kg bw/day. This increase was probably compound-
    related. At lower doses, percentage incidence of adrenal  zona
     fasciculata vacuolation, consistent with lipid accumulation were,
    in males, 22%, 18%, 10% and 6% and in females, 0%, 2%, 4% and 4% at
    1, 0.1, 0.05 and 0 mg/kg bw/day. Historical control data from 5
    contemporary studies from the same laboratory and in the same strain
    of rat showed 40%, 10%, 42%, 0% and 20% incidence of vacuolation of
    the adrenal  zona fasciculata in males, and 26%, 0%, 10%, 4% and 0%
    in females. Thus incidences observed in the current study at 1 mg/kg
    bw/day and below are well within historical control data ranges, and
    are unlikely to be compound-related. The NOAEL for the study was 1
    mg/kg bw/day (Barna Lloyd  et al., 1991).

    Observations in humans

         Fourteen male volunteers were divided into two treatment groups
    of five men each and a control group of four men. Chlorpyrifos-
    methyl was administered by gelatin capsule in a single daily dose of
    0, 0.03 or 0.1 mg/kg bw/day for four weeks. Plasma and erythrocyte
    cholinesterase activities were not depressed at levels tested.
    Haematology, blood chemistry, urinalysis, blood pressure, pulse rate
    and ophthalmology were not affected by treatment. The NOAEL was the
    highest dose tested, 0.1 mg/kg bw/day (Coulston  et al., 1975).


         The two-year dietary study in rats, which utilized dietary
    doses of 0, 0.05, 0.1, 1 or 50 mg chlorpyrifos-methyl/kg bw/day, did
    not show any carcinogenic potential of the compound.

         On histopathological examination, no correlation was found
    between the incidence of vacuolation of the  zona fasciculata of
    the adrenal gland and of other organs. The incidence of vacuolation
    of the  zona fasciculata at all dose levels except the high dose
    was within the range of occurrence noted in contemporary rat studies
    performed in the same laboratory. The NOAEL for the study was
    therefore interpreted as being 1 mg/kg bw/day.

         The 1975 Joint Meeting reviewed a human study in which five
    males/test group were given single doses of 0, 0.03, or 0.1 mg
    chlorpyrifos-methyl/kg bw/day for four weeks. Test groups were
    comparable to a control group of four males with respect to plasma
    and erythrocyte cholinesterase activity, haematology, blood
    chemistry, blood pressure, pulse rate and ophthalmology. The NOAEL
    was the highest dose tested, 0.1 mg/kg bw/day.

         The ADI allocated by the 1991 Joint Meeting was based on the
    changes in rat adrenal pathology which were interpreted as showing a
    NOAEL of 0.1 mg/kg bw/day, to which a 100-fold safety factor was
    applied. With the revision of the NOAEL with respect to the rat
    adrenal, the present Meeting allocated an ADI based on the human
    data (NOAEL 0.1 mg/kg bw/day) using a 10-fold safety factor. This
    ADI is supported by the NOAEL in studies in rats (1 mg/kg bw/day)
    using a 100-fold safety factor.


    Level causing no toxicological effects

         Mouse:    50 ppm, equal to 3.9 mg/kg bw/day (78-week study)

         Rat:      1 mg/kg bw/day (two-year feeding study)

         Human:    0.1 mg/kg bw/day (four-week study).

    Estimate of acceptable daily intake for humans

         0-0.01 mg/kg bw.

    Studies which will provide information valuable in the contimued
    evaluation of the compound

         Further observations in humans.


    Barna-Lloyd, T., Szabo, J.R. & Davis, N.L. (1991) Chlorpyrifos-
    methyl (Reldon R) rat chronic dietary toxicity/oncogenicity study.
    Unpublished report TXT: K-046193-020 from Dow Chemical, Texas, USA.
    Submitted to WHO by Dow Elanco, Indianapolis, USA.

    Coulston, F., Rosenblum, I. & Griffin, T.B. (1975) Study of
    chlorpyrifos-methyl in human volunteers. Unpublished report from
    Institute of Comparative and Human Toxicollogy, Albany Medical
    College and International Centre of Environmental Safety, Holloman
    AFB, New Mexico. Submitted to WHO by Dow Chemical Company, Midland,
    Michigan, USA.

    See Also:
       Toxicological Abbreviations
       Chlorpyrifos-methyl (WHO Pesticide Residues Series 5)
       Chlorpyrifos-methyl (Pesticide residues in food: 1979 evaluations)
       Chlorpyrifos-methyl (Pesticide residues in food: 1991 evaluations Part II Toxicology)