Metabolism, pharmacokinetics and toxicology of the neonicotinoid insecticides in mice

There are three types of neonicotinoid insecticides with chloropyridinylmethyl (imidacloprid or IMI, thiacloprid or THI, acetamiprid or ACE and nitenpyram or NIT), chlorothiazolylmethyl (thiamethoxam or TMX and clothianidin or CLO) or tetrahydrofuranylmethyl (dinotefuran or DIN) substituents. In this investigation, the neonicotinoid compounds were individually administered intraperitoneally to mice at 10 mg/kg (for IMI) or 20 mg/kg (the others) for analysis of brain, liver and plasma at 15 to 240 min and the 0-24 h urine by HPLC/DAD, LC/MSD and LC/MS/MS. Maximum levels of the parent compounds in brain were 11-17 ppm (NIT, TMX, THI and CLO), 6 ppm (IMI and DIN) and 3 ppm (ACE). Urinary excretion of the parent compound was greatest with NIT and IMI. Each of the chloropyridinyl compounds was cleaved to the same eight urinary metabolites derived from the chloropyridinylmethyl moiety. IMI gave nitrosoguanidine, nitroguanidine, aminoguanidine and desnitro, olefin, methyltriazinone, hydroxy- and dihydroxyimidazole derivatives. NIT metabolism involved N-demethylation, conversion to a cyano derivative and oxidation at the nitromethylene carbon to the carboxylic acid. THI yielded olefin, descyano, descyano olefin, amide and hydroxythiazolidine derivatives and a ring-opened and methylated THI sulfoxide. ACE formed N-desmethyl, acetamide, amide, chloropyridinylmethylamine and N-methylchloropyridinylmethylamine derivatives. Thirty-seven metabolites of TMX, its desmethyl metabolite TMX-dm, CLO and CLO-dm are identified by comparison with synthetic standards or their structures are proposed by molecular weights and 35Cl:37Cl ratios often supplemented by previous reports or sequence studies in which intermediates are readministered. A facile reaction sequence involves TMX → TMX-dm or CLO → CLO-dm. CLO-dm, reported to be a contributor to TMX hepatocarcinogenesis in mice, is unexpectedly remethylated in part to CLO in brain. Chlorothiazolylcarboxaldehyde from oxidative cleavage of TMX and CLO is quite persistent in brain, liver and particularly plasma compared with chloropyridinylcarboxaldehyde and tetrahydrofuranylcarboxaldehyde from the other neonicotinoids. DIN metabolism involves nitro reduction, N-demethylation, N-methylene hydroxylation and amine cleavage, and tetrahydrofuranylmethyl hydroxylation at the 2-, 4- and 5-positions giving 29 tentatively-identified metabolites. The diversity of biodegradable sites and multiple pathways insures against parent compound accumulation but provides intermediates reported to be active as nicotinic agonists and inducible nitric oxide synthase inhibitors.