Sponsored jointly by FAO and WHO


    Joint meeting of the
    FAO Panel of Experts on Pesticide Residues
    in Food and the Environment
    and the
    WHO Expert Group on Pesticide Residues
    Rome, 6-15 October 1980



    Cyanofenphos was reviewed by the 1975 Joint Meeting (FAO/WHO, 1976)
    and a temporary ADI of 0-0.005 mg/kg body weight was allocated. 
    Data were available from studies on neurotoxicity, reproduction,
    and teratogenicity as well as a series of acute studies to develop
    basic toxicology data.  Short-term studies in the dog and cow and
    long-term studies in the rat demonstrated an adverse toxicological
    reaction only with respect to cholinesterase inhibition.  No-effect
    levels in rat and dog were based on this parameter.  A lack of
    biochemical data with respect to metabolism was the basis for the
    temporary nature of the ADI.  Studies on absorption, metabolism,
    and excretion were required for a further toxicological

    These studies, as well as additional information on mutagenicity
    and delayed neurotoxicity, were made available to the Meeting and
    are reviewed in this monograph addendum.



    Absorption, distribution, and excretion

    Cyanofenphos, radiolabelled at the 4-cyano moiety, was administered
    orally to both sexes of rats and mice.  In studies with mice,
    cyanofenphos was administered at dose levels of 1/8 and 1/16 LD50
    (LD50=43 mg/kg body weight).  Cyanofenphos was rapidly absorbed,
    and excreted almost entirely within 24 hours.  Maximum distribution
    of radioactivity to tissues and organs was observed within 30
    minutes of treatment.  Excretion was predominantly via urine and
    faeces to the extent of 95% and 5%, respectively, within 72 hours. 
    Within 24 hours, approximately 95% of the administered
    radioactivity had been excreted. There was no 14CO2 expired,
    reflecting stability of the cyano group (Kato and Yamamoto, 1974).

    As noted above with mice, administration of cyanofenphos, as well
    as the individual optical isomers of cyanofenphos, to rats (at 4
    mg/kg) was followed by rapid elimination of the radiolabelled
    chemical in urine and faeces, predominantly within one day.  There
    were no apparent differences with respect to the elimination
    pattern when cyanofenphos (the racemic form) and its isolated
    optical isomers were administered to rats as a single oral dose of
    4 mg/kg body weight (Ohkawa et al.,1977).

    Thus, in both rats and mice, an orally administered dose of
    cyanofenphos was rapidly absorbed, distributed and excreted,
    predominantly within 24 hours.  There was no appreciable

    of the molecule to tissues and organs other than those associated
    with metabolism and excretion.


    The nature of the biotransformation pattern of cyanofenphos in both
    rats and mice following oral administration appears to be very
    similar.  In both species, predominant reactions included oxidation
    of P=S to P=O and oxidative and/or hydrolytic cleavage of the P-O-
    aryl bonds as indicated in Figure 1.

    In addition, P-O-dealkylation reactions were observed.  In mouse
    urine, cyanofenphos (5%), desethyl cyanofenphos (10%), and
    4-cyanofenphenol (15%) were found as unconjugated products.  The
    other urinary metabolites were conjugates (predominantly sulphate
    and glucuronide conjugates of 4-cyanophenol) (Kato and Yamamoto,

    A similar (qualitative) pattern of metabolism of cyanofenphos was
    observed in rats.  When individual isomers and the racemic
    cyanofenphos were administered to rats, there was quantitative
    differences with respect to the urinary excretion.  The excretion
    patterns of free and conjugated 4-cyanophenol suggested that a
    differential metabolism occurred in the rat when the racemic or
    individual optical isomers were administered. Differences in
    conjugation rates of the 4-cyanophenol were reflective of the
    differing metabolic pathways of the optical and racemic

    Further studies with respect to in vitro metabolism using
    isolated microsomes from rat liver clarified some of the
    differences in the metabolic patterns and suggested that oxidation
    of the cyanofenphos was a predominant reaction producing the oxon
    (P=C) analogue and hydrolytic cleavage products.  It was suggested
    that the (+) and the (-) isomers were hydrolysed by different
    subcellular mechanisms.  The stereo-selectivity in metabolism of
    cyanofenphos isomers appears likely to be due to selective
    hydrolysis of (-)-cyanofenphos oxon by an arylesterase, which may
    account for differences noted in the metabolic breakdown of the
    individual optical isomers (Ohkawa et al., 1976).

    FIGURE 1


    Special studies on mutagenicity

    Cyanofenphos was tested for mutagenicity in the Rec-assay using
    B. subtilis (M45 rec- and H 17 wild type) strains to detect
    DNA damage. Dosage levels ranged from 0 to 2,000 g/plate. 
    Positive and negative controls (Mitomycin C and Kanamycin) were
    employed in the study.  It was observed that cyanofenphos had no
    inhibitory effect on the growth of either microbial strain. 
    Cyanofenphos was not mutagenic under the conditions of this assay
    (Moriya et al., 1976).

    Cyanofenphos was examined in the standard "Ames" assay using 5
    strains of S. typhimurium (TA100, TA98, TA1537, and TA1538)
    and E. coli WP-2 (hor and uvr A).  The results of studies,
    with and without metabolic activation, at dosage levels ranging
    from 0 to 5,000 g/plate, were negative.  A positive control using
    either MMS, ENNG, 2-nitrofluorene, or 9-aminoacridine was employed
    and assured the quality of the assay (Kishida et al.) 1980;
    Moriya et al., 1976).

    In a host-mediated assay, administration of cyanofenphos to groups
    of mice (6 male mice/group) at dosage levels of O, 5 or 20
    mg/kg/day for 2 days did not result in an increased mutation rate
    of the Salmonella indicator strain.  The positive control
    (dimethylnitrosamine) gave a significant increase in the number of
    mutants (Moriya et al., 1976).

    Based upon these three bioassays in microbial test systems,
    cyanofenphos is not mutagenic.

    Special studies on delayed neurotoxicity


    Cyanofenphos was orally administered to adult hens in dosage levels
    ranging from 0 to 500 mg/kg body weight.  Hens given dosage levels
    of 100 mg/kg and above received multiple doses of atropine sulphate
    to protect them from cholinergic signs of poisoning.  Depending
    upon the acute oral dose of cyanofenphos, clinical signs of delayed
    neurotoxicity were noted, accompanied by histological evidence for
    myelin and/or axon degeneration (axonpathy).  At dosage levels of
    100 mg/kg, animals exhibited paralytic signs accompanied by
    histological evidence of degeneration in the spinal cord.  In those
    animals surviving higher doses, more prominent signs of ataxia,
    paralysis and histologically-noted disruption in both the spinal
    cord and sciatic nerve were evident.  Ataxia was reported at 10
    mg/kg, but the clinical signs were not accompanied by histologic
    changes in the spinal cord or sciatic nerve.  The ataxic condition
    did not degenerate to a state of paralysis.  At 50 mg/kg, ataxia
    and histopathologic changes were noted; again, paralysis was not
    seen clinically (Abou-Donia and Graham, 1979).

    Published and unpublished studies examining this phenomena have
    confirmed the observation of delayed neuropathy (or axonopathy)
    with cyanofenphos (El-Sebae et al., 1980; Soliman and Curley,
    1980; Soliman et al., 1980), although these studies have
    suggested that the neurotoxic dose inducing neuropathy in hens is
    considerably higher 240mg/kg) than reported above.  Transient
    ataxia was noted at doses of 160 mg/kg and below, but no paralysis.


    Further studies on the delayed neurotoxic potential of cyanofenphos
    were carried out with sheep, a species susceptible to this effect
    (El-Sebae et al., 1979; Soliman, 1980).  As with other
    organophosphates capable of inducing delayed neurotoxicity in hens,
    cyanofenphos was found to induce this effect in adult sheep
    following continuous oral administration (1 mg/kg administered for
    60 days; 2 mg/kg for 45 days; and 4 mg/kg for 30 days).

    Thus, under conditions of these in vivo assays, cyanofenphos
    induced a delayed neurotoxic reaction (axonopathy) in two
    susceptible species following either a single acute or multiple
    oral dose.



    Studies on pharmacokinetics and metabolism with cyanofenphos in
    rats and mice show that cyanofenphos is rapidly absorbed, degraded,
    and excreted in both species.  There is no bioaccumulation and the

    metabolism in animals follows a pattern of degradation noted with
    other organophosphate esters.

    In vivo and in vitro studies to evaluate mutagenicity with
    cyanofenphos are negative.  Additional studies on the
    delayed-neurotoxic potential of cyanofenphos in hens and sheep, two
    species susceptible to the syndrome, have shown that cyanofenphos
    induces a delayed neurotoxic effect similar to that noted with
    certain other organophosphorous esters.  These findings contrast
    with previous studies which reported a negative delayed neurotoxic
    reaction in chickens.  Questions were raised on the possible
    impurities in the technical product which might have contributed to
    this event.  This could not be evaluated with the available data.

    Studies have shown sheep to be extremely susceptible to
    cyanofenphos-induced delayed neurotoxicity.  Longer-term,
    subchronic studies in this species have failed to show a definitive
    no-effect level.

    The Meeting considered that the delayed neurotoxicity is another
    toxicological property and based its considerations of a no-effect
    level on cholinesterase level depression in rat and dog.

    As a result of these considerations, a temporary ADI was
    reaffirmed. Further work to define fully a no-effect level in
    sheep, or another appropriate species, for delayed neurotoxicity is
    required to evaluate fully the toxicological profile for this

    Level causing no toxicological effect

    Rat: 10 mg/kg in the diet equivalent to 0.5 mg/kg bw/day.
    Dog: 30 mg/kg in the diet equivalent to 0.75 mg/kg bw/day.

    Estimation of a temporary acceptable daily intake for man

    0-0.001 mg/kg bw/day.


    Required (by 1983)

    Further subchronic studies with sheep (or another appropriate,
    susceptible mammalian species) to determine no-effect level with
    respect to delayed neurotoxicity.


    1. Observations in man including cholinesterase studies.
    2. A further long-term study.
    3. Observations in man primarily exposed through high-level
    occupational conditions to monitor for clinical signs of delayed
    neurotoxicity (axonopathy).


    Abou-Donia, M.B. and Graham, D.G. Delayed neurotoxicity of a single
    oral dose of O-ethyl 0-4-cyanophenyl phosphonothioate in the hen.
    Neurotoxicol. 1: 449-466.

    El-Sebae, A.H., Othman, M.A.S., Hammam, S.M., Tantawy, G. and
    Soliman, S.A. Delayed neurotoxicity of cyanofenphos in chickens. J.
    Environ. Sci. Health B15(3): 267-285.

    El-Sebae, A.H., Soliman, S.A. and Ahmed, N.S. Delayed neuropathy in
    sheep by the phosphonothioate insecticide cyanofenphos. J. Environ.
    Sci. Health, B14(3): 247-263.

    Kato, S. and Yamamoto, I. Metabolism of Surecide,
    O-(4-cyanophenyl)phenylphosphonothioate in mice. (1974) Unpublished
    report from Sumitomo Chemical Co., Ltd., submitted to the World
    Health Organization by Sumitomo Chemical Co., Ltd.

    Kishida, F., Suzuki, H. and Miyamoto, J. Studies on mutagenicity of
    Surecide in bacterial system. (1980) Unpublished report from
    Sumitomo Chemical Co., Ltd., submitted to the World Health
    Organization by Sumitomo Chemical Co., Ltd.

    Moriya, M., Watanabe, Y. and Shirasu, Y. Mutagenicity of Surecide
    in bacterial test systems. (1976) Unpublished report from the
    Institute of Environmental Toxicology (Japan), submitted to the
    World Health Organization by Sumitomo Chemical Co. Ltd.

    Ohkawa, H., Mikami, N. and Miyamato, J. Stereoselectivity in
    metabolism of the optical isomers of cyanofenphos (O-p-cyanophenyl
    O-ethyl phenylphosphonothioate) in rats and liver microsomes.
    Agric. Biol. Chem. 41: 369-376.

    Soliman, A. (1980) Unpublished studies from the Health Effects
    Research Laboratory, U.S. Environmental Protection Agency,
    submitted as a personal communication to the World Health
    Organization by the U.S. Environmental Protection Agency.

    Soliman, S.A. and Curley, A. In vivo inhibition of chicken
    brain neurotoxic esterase by leptophos and cyanofenphos as
    determined by a new direct method. (1980) Unpublished studies from
    the Health Effects Research Laboratory, U.S. Environmental
    Protection Agency submitted to the World Health Organization by the
    U.S. Environmental Protection Agency.

    Soliman, S.A., Curley, A., Farmer, J. and Novak, R. In vivo
    inhibition of chicken brain acetylcholinesterase and neurotoxic
    esterase in relation to the delayed neurotoxicity of leptophos and
    cyanofenphos. (1980) Unpublished studies for the Health Effects
    Research Laboratory, U.S. Environmental Protection Agency submitted
    to the World Health Organization by the U.S. Environmental
    Protection Agency.


    See Also:
       Toxicological Abbreviations
       Cyanofenphos (WHO Pesticide Residues Series 5)
       Cyanofenphos (Pesticide residues in food: 1982 evaluations)