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

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differences in the percentage actually absorbed, permitting the estimation of an appropriate species
ratio. The flux (i.e. rate of penetration under steady-state conditions) at the lowest, intermediate and
highest concentrations, respectively, was 0.020, 0.455 and 26.0 µg/cm 2 h −1 for the rat skin and 0.002,
0.037 and 0.82 µg/cm 2 h −1 for the human skin. The flux ratios rat : human were therefore about 1 :
10, 1 : 12 and 1 : 32 at the lowest, intermediate and highest concentrations, respectively (Hassler,
2003c). Comparison of the penetration rates at the different doses (concentration ratio, 1 : 25 : 5000),
revealed an increase in the penetration rates of 1 : 23 : 1300 for rat epidermal membranes, while
the penetration rates for human epidermal membranes revealed a ratio of only 1 : 24 : 500 (Hassler,
2003b). These data acquired in vitro are used to arrive at an estimate of dermal absorption partly
because the inter-rat variation observed in the study in vivo was very large (attributed to the irritancy
of the formulation vehicle and for this reason was not considered reliable).
Metabolism
Metabolites were isolated from the urine and faeces of male and female rats given [ 14 C-phenyl]
difenoconazole or [ 14 C-triazole]difenoconazole as single oral dose at 0.5 or 300 mg/kg bw (Figure 1a
and 1b), or 0.5 mg/kg bw after pre-treatment with 14 daily oral doses of unlabelled difenoconazole
at 0.5 mg/kg bw (Craine, 1987a, 1987b).
Balance data for the phenyl and triazole ring labels (Craine 1987a, 1987b) summarized in a report
by Capps et al. (1988) showed that > 97% of the radiolabel in all cases was excreted with > 78%
in all cases eliminated in the faeces. Three main metabolites, A, B and C, were isolated from faeces
and together accounted for an average of 68% of the administered dose. Metabolite B (hydroxy-CGA
169374) was hydroxylated in the outer phenyl ring and spectral analysis showed that that it comprised
two isomers, one with a rearrangement of the chlorine on the outer phenyl ring, attributed to
a mechanism analogous to an NIH shift.1 Metabolite C (CGA 205375) was the hydroxyl product of
cleavage of the dioxolane ring from the difenoconazole molecule and was present only in the faeces
of rats given the higher dose. Metabolite A (hydroxy-CGA 205375) was the outer phenyl-ring hydroxylated
product of metabolite C and comprised two diastereoisomers, as described for metabolite
B. The profile of urinary metabolites was more complex and showed more variability between the two
radiolabelled forms. Free 1,2,4-triazole was identified in male urine, accounting for less than 10% of
the administered dose and represented cleavage of the alkyl bridge between the ring systems. Other
urinary metabolites included metabolite C, its sulfate conjugate, ring-hydroxylated metabolite C and
its sulfate conjugate, as well as an hydroxyacetic acid metabolite of the chlorophenoxy-chlorophenyl
moiety, all of which were present in minor quantities, each representing less than 3% of the administered
dose. One metabolite CGA 189138 (chlorophenoxy-chlorobenzoic acid) was also isolated
from the liver. The difenoconazole molecule was therefore extensively metabolized, although with
limited cleavage of the triazole and dioxolane rings. The extensive biliary elimination observed was
consistent with the major metabolites being of relatively high molecular mass.
The major steps of the metabolism of difenoconazole in the rat were deduced to involve hydrolysis
of the ketal in difenoconazole, resulting in CGA 205375 (1-[2-chloro-4-(4-chloro-phenoxy)-phenyl]-2-
(1,2,4-triazol)-1-yl-ethanol), with the ketone CGA 205374 (1-(2-chloro-4-(4-chloro-phenoxy)-phenyl)-
2-(1,2,4-triazol)-1-yl-ethanone) as a postulated but not identified intermediate, and hydroxylation on the
outer phenyl ring of the parent (hydroxy-CGA 169374) and in CGA 205375 (hydroxy-CGA 205375).
As a minor process, cleavage of the alkyl chain between the triazole and the inner phenyl ring occurs,
resulting in a hydroxyacetic acid (NOA 448731) or CGA 189138 (2-chloro-4-(4-chlorophenoxy)-benzoic
acid) and free triazole. Sulfate conjugates were identified for CGA 205375 and for hydroxy-CGA
205375. The proposed metabolic pathway in mammals is shown in Figure 3 (Capps et al., 1990).
1
An NIH shift is a chemical rearrangement (first described in studies from the United States National Institutes
of Health, NIH) in which a hydrogen atom on an aromatic ring undergoes an intramolecular migration primarily
d uring a hydroxylation reaction.
DIFENOCONAZOLE 201–272 JMPR 2007