2 Urinary Metabolites as Biomarkers of Exposure
2.1 Metabolism of Synthetic Pyrethroids
Chemical structure of pyrethroids in a large extent determines their biotransformation pathways. As esters they are easily hydrolyzed by human carboxylesterases to
form respective alcohol and acidic metabolites. Oxidation by cytochrome P-450 is
the second major reaction of pyrethroids in laboratory animals and humans [1]. Both
oxidation and hydrolysis are the first-phase reactions which are followed by secondphase reactions – conjugation with endogenous substrates. The last process leads to
formation of glucuronides, sulfates, and amino acid conjugates – highly watersoluble metabolites and in some cases lipophilic conjugates with cholesterol, bile
acids, and triglyceride. Hydrophilic metabolites of pyrethroids do not show accumulation in human body and are rapidly and almost completely excreted into urine
within few days after oral exposure. Although pyrethroids undergo both oxidation
and hydrolysis reactions, practically only products of hydrolysis serve as urinary
biomarkers of exposure.
Urine as a major route of elimination of pyrethroid metabolites is thus considered
the most appropriate matrix for the assessment of aggregate exposure. The plasma
half-life for most pyrethroids is shorter than 8 h.
Significant differences occur in respect to cleavage of the ester bond between
trans and cis isomers. Trans isomers of pyrethroids possessing chrysanthemic acid
moiety are hydrolyzed more efficiently than their corresponding cis isomers. Furthermore, cis isomers are more susceptible to oxidative metabolism than trans
isomers [2]. The range of human metabolites identified and used as biomarkers of
exposure to pyrethroids is presented in Table 1.
Several urinary metabolites were identified (Table 1) up-to-date, and they can
serve as a reliable biomarker of exposure. Besides of that, some biomarkers are more
frequently analyzed than others.
The first published methods for the quantitative determination of synthetic
metabolites of pyrethroids in human urine included the metabolites of the most
commonly used pyrethroids, namely, permethrin, cypermethrin, deltamethrin, and
cyfluthrin: cis and trans DCCA, DBCA, 3PBA, and 4F3PBA [3–7].
Of these, 3PBA is unique, because so far, most research is focused on this
biomarker. It is a common metabolite of many pyrethroids, and its concentrations
in urine are usually the highest and detectable in the largest number of samples in the
population. Finally, the highest availability of analytical methods exists for the
determination of this metabolite in the urine; both chromatographic methods and
high-throughput immunological methods are described in the literature.
In addition to 3PBA and the aforementioned metabolites, the remaining ones are
studied less often, although in recent years, several methods have been published
that enable the simultaneous, very sensitive assay of up to eight to nine individual
biomarkers in one chromatographic run [8, 9].
Analytical Methods for Determination Urinary Metabolites of Synthetic. . .
49
2.1 Metabolism of Synthetic Pyrethroids
Chemical structure of pyrethroids in a large extent determines their biotransformation pathways. As esters they are easily hydrolyzed by human carboxylesterases to
form respective alcohol and acidic metabolites. Oxidation by cytochrome P-450 is
the second major reaction of pyrethroids in laboratory animals and humans [1]. Both
oxidation and hydrolysis are the first-phase reactions which are followed by secondphase reactions – conjugation with endogenous substrates. The last process leads to
formation of glucuronides, sulfates, and amino acid conjugates – highly watersoluble metabolites and in some cases lipophilic conjugates with cholesterol, bile
acids, and triglyceride. Hydrophilic metabolites of pyrethroids do not show accumulation in human body and are rapidly and almost completely excreted into urine
within few days after oral exposure. Although pyrethroids undergo both oxidation
and hydrolysis reactions, practically only products of hydrolysis serve as urinary
biomarkers of exposure.
Urine as a major route of elimination of pyrethroid metabolites is thus considered
the most appropriate matrix for the assessment of aggregate exposure. The plasma
half-life for most pyrethroids is shorter than 8 h.
Significant differences occur in respect to cleavage of the ester bond between
trans and cis isomers. Trans isomers of pyrethroids possessing chrysanthemic acid
moiety are hydrolyzed more efficiently than their corresponding cis isomers. Furthermore, cis isomers are more susceptible to oxidative metabolism than trans
isomers [2]. The range of human metabolites identified and used as biomarkers of
exposure to pyrethroids is presented in Table 1.
Several urinary metabolites were identified (Table 1) up-to-date, and they can
serve as a reliable biomarker of exposure. Besides of that, some biomarkers are more
frequently analyzed than others.
The first published methods for the quantitative determination of synthetic
metabolites of pyrethroids in human urine included the metabolites of the most
commonly used pyrethroids, namely, permethrin, cypermethrin, deltamethrin, and
cyfluthrin: cis and trans DCCA, DBCA, 3PBA, and 4F3PBA [3–7].
Of these, 3PBA is unique, because so far, most research is focused on this
biomarker. It is a common metabolite of many pyrethroids, and its concentrations
in urine are usually the highest and detectable in the largest number of samples in the
population. Finally, the highest availability of analytical methods exists for the
determination of this metabolite in the urine; both chromatographic methods and
high-throughput immunological methods are described in the literature.
In addition to 3PBA and the aforementioned metabolites, the remaining ones are
studied less often, although in recent years, several methods have been published
that enable the simultaneous, very sensitive assay of up to eight to nine individual
biomarkers in one chromatographic run [8, 9].
Analytical Methods for Determination Urinary Metabolites of Synthetic. . .
49