Chlorothalonil Biotransformation by Gastrointestinal Microflora:In VitroComparative Approach in Rat, Dog, and Human

Abstract

Chlorothalonil (2,4,5,6-tetrachloroisophthalonitrile) is a broad spectrum contact fungicide used in agriculture and horticulture. The role of digestive microflora in chlorothalonil metabolism was assessed byin vitroincubation of [14C]chlorothalonil with stomach, duodenum, and cecum contents from rat, stomach, duodenum, and colon contents from dog and with human feces and stomach contents using an incubation dose of 500 μg/g of digestive contents. Transformation of chlorothalonil mostly occurred in rat cecum contents, dog colon contents, and human feces, in which unchanged chlorothalonil accounted for 46.7, 29.7, and 22.6% of the radioactivity, respectively. In those incubations, the identified metabolites were: 2,5,6-trichloro-4-methylthioisophthalonitrile (0.8–3% of the radioactivity), 2,5,6-trichloro-4-thioisophthalonitrile (0–7.1%), 3-thia-1-cyano-2,5,6-trichloroisoindolinone (2.6–12.7%), 2,5,6-trichloro-4-hydroxy-isophthalonitrile (0–3.2%), and 2,5,6-trichloroisophthalonitrile (9.9–21.4%). The mercapto metabolites of chlorothalonil, methylated or not, were recovered from rat and dog duodenum contents (0.8 and 0.7%). Only traces of metabolic products were recovered from rat and human stomach contents. The monocysteine conjugate of chlorothalonil was detected from rat duodenum contents and accounted for 0.2% of the substrate. The formation of metabolites decreased when the digestive contents were heated before incubation to inactivate bacteria. Under these conditions, the monomethylthiotrichloroisophthalonitrile formation was suppressed, except in rat and dog duodenum contents (1.5 and 0.5%, respectively). The monothiotrichloroisophthalonitrile accounted for 0.1 to 0.4%. The monohydroxytrichloroisophthalonitrile accounted for 0.5 to 2%, the thia-indolinone metabolite for 3.9 to 8% and the dechlorinated chlorothalonil for 0.3 to 2.8%. These results suggest that intestinal microflora plays a significant role in chlorothalonil metabolism, which implies Cys β-lyases. The formation of methylthio metabolites is discussed, including thiol methylation in digestive contents.