Cytotoxicity of a series of mono- and di-substituted thiourea in freshly isolated rat hepatocytes: a preliminary structure-toxicity relationship study.

The cytotoxicity of a series of 12 mono- and 4 di-substituted thiourea containing compounds in freshly isolated rat hepatocytes was investigated. It was found that thiourea toxicity, as evidenced by an increase in LDH-leakage from the cells, was accompanied by a depletion of intracellular glutathione (GSH). No increase in lipid peroxidation was observed with any of the thiourea. Burimamide and thioperamide, thiourea-containing histamine receptor ligands, were also found to deplete intracellular GSH. A clear structure- toxicity relationship was uncovered among a homologous series of N- phenylalkylthiourea. N-benzylthiourea (BTU) and N-phenylethylthiourea (PETU) were found to be non-toxic at a concentration of 1 mM, while N- phenylpropylthiourea (PPTU) and N-phenylbutylthiourea (PBTU) were found to cause significant LDH-leakage from the cells, accompanied by a depletion of intracellular GSH. This structure-toxicity relationship was further investigated using hepatocytes of differentially induced rats, however, no significantly different results were obtained when using hepatocytes of rats induced with phenobarbital (PB) or β-naphthoflavone (BNF). Oxidation of the thiourea moiety is thought to be the first step in the bioactivation of thiourea containing compounds. The oxidation of thiocholine sulfenic acids, produced by FMO-mediated oxidation of the thiourea moiety, was used to determine whether the compounds examined are substrates for the FMO enzymes in rat liver. No clear relationship was found between cytotoxicity of the mono-substituted thiourea and lipophilicity of the N-substituent, nor with the FMO-mediated oxidation of the thionosulfur atom of the mono-substituted thiourea. It is concluded from this study, that thiourea toxicity in rat hepatocytes is structure-dependent and manifests itself as LDH-leakage and as a depletion of intracellular non-protein sulfhydryls, notably GSH, most likely followed by alkylation of vital macromolecular structures.