Pesticide residues in food — 2007: Toxicological ... - ipcs inchem

450
time, while the concentration in the adrenal appeared to have reached a plateau, and that in the liver
appeared to be declining. Although urine and faeces were collected from one group after 28 days, no
data were provided in the study report (Hemmings, 1991a).
In an another study with repeated doses, groups of five male and five female Crl:CD(SD)BR
rats were given 14 daily doses of unlabelled pyrimethanil at 10 mg/kg bw per day by gavage. They
were then given a single oral dose of [U-phenyl ring 14 C]pyrimethanil (purity, 99.00% or greater) at
10 mg/kg bw by gavage in an aqueous solution of 1% (w/v) gum tragacanth. Treated rats were placed
in all-glass metabolism cages. Urine, faeces and cage wash were collected at 6 h and 24 h after dosing
in cooled containers to prevent bacterial degradation of the metabolites present. At necropsy (24
h after dosing with the radiolabel) the following tissues/organs were removed for analysis: adrenals,
blood, bone, brain, residual carcass, eyes, gastrointestinal tract, heart, kidneys, liver, lungs, muscle,
ovaries, renal fat, spleen and testes. Plasma was prepared from the blood. Tissues were either combusted
or solubilized. All samples were analysed for radioactivity by LSC.
Pyrimethanil was rapidly excreted, with 91% of the administered dose being excreted within
24 h. The major route of elimination was via the urine that, together with the cage wash, accounted
for 72% of the administered dose. There were no sex differences in the rate or route of elimination.
Tissue concentrations of residue at necropsy were low, with concentrations in most tissues being below
the limits of quantification. Quantifiable residues were only found in the liver (0.30 and 0.44 mg
equivalent/kg for male and females, respectively), kidney (0.13 and 0.17 mg equivalent/kg for males
and females, respectively) and blood (0.044 and 0.058 mg equivalent/kg for males and females, respectively).
The excretion profile and magnitude of the tissue residues were similar to those seen 24
h after a single oral dose of pyrimethanil at 11.8 mg/kg bw (Hemmings, 1993).
In a study of metabolite identification, the excreta from rats at the lower dose (11.8 mg/kg bw)
and higher dose (800 mg/kg bw) were collected at 24, 48, 72 and 96 h after dosing and subjected to
metabolic identification. The excreta from rats receiving the repeated dose at 10 mg/kg bw for 14
days was also subjected to metabolic identification. The proposed metabolic scheme of pyrimethanil
in rats is shown in Figure 2. The excreta contained more than 95% and 63–67% of the administered
dose after a single dose at 11.8 mg/kg bw or 800 mg/kg bw, respectively, in the first 24 h. Approximately
89–90% of the administered dose was recovered in the excreta in the first 24 h after repeated
dosing at 10 mg/kg bw for 14 days. Many metabolites of pyrimethanil were excreted as glucuronide
or sulfate conjugates that could be released after incubation with Helix pomatia juice, a mixture of
beta D-glucuronidase and aryl sulfatase obtained from snails. Only low concentrations of unchanged
pyrimethanil were detected in faeces. The main pathways of metabolism involved aromatic oxidation
to form phenols in either or both rings, which were then excreted as glucuronide and/or sulfate
conjugates. After deconjugation to release the aglycones, the major metabolite peak was identified
as 4-(4,6-dimethylpyrimidin-2-ylamino)phenol (SN 614 276, which was also detected as a sulfate
conjugate). Aromatic oxidation on the pyrimidine ring produced 2-anilino-4, 6-dimethylpyrimidin-
5-ol (SN 614 277), and oxidation on both rings gave 2-(4-hydroxyanilino)-4, 6-dimethylpyrimidin-
5-ol (SN 615 244). A minor metabolic pathway resulted in oxidation of the methyl group on the
pyrimidine ring to produce an alcohol with the proposed structure of 2-anilino-6-methylpyrimidine-
4-methanol (SX 614 278). This was present in trace amounts in the excreta, but was further oxidized
to the corresponding phenol, 2-(4-hyroxyanilino)-6-methylpyrimidine-4-methanol (SN 614 800).
The mass spectra of the polar metabolites did not contain any recognizable fragments. The polar
fraction was therefore collected and treated with diazomethane to methylate any acidic protons. The
resulting mixture then separated on HPLC into four peaks, which were collected and examined by
mass spectrometry. One of the peaks contained the diphenol SN 615 224, two other peaks contained
a monomethylated derivative of SN 615 224, and the final peak contained the pyrimidine p henol
PYRIMETHANIL 445–486 JMPR 2007