Cytotoxicity of dibasic esters (DBE) metabolites in rat nasal explants

Abstract

Dibasic esters (DBE) is a solvent mixture of dimethyl adipate, dimethyl glutarate, and dimethyl succinate which causes a selective degeneration of the nasal olfactory epithelium in rats following a 90-day inhalation exposure. In short-term cultures of rat nasal explants, it has been demonstrated that DBE cytotoxicity is due to a carboxylesterase-mediated activation. In the present study, the putative toxic metabolites of DBE, the monomethyl esters and the dicarboxylic acids, were evaluated in the nasal explant system at concentrations ranging from 10 to 50 mM. Monomethyl adipate (MMA), monomethyl glutarate (MMG), and monomethyl succinate (MMS) induced increases in nasal explant acid phosphatase release, a biochemical index of cytotoxicity. The nasal explant-mediated metabolism of MMA and MMG to their corresponding diacids paralleled the increases in acid phosphatase release. A carboxylesterase inhibitor, bis(p-nitrophenyl)phosphate (BNPP), inhibited both the cytotoxicity and the hydrolysis of MMA and MMG in the nasal explant system. The metabolism and cytotoxicity of MMS was not attenuated as effectively by BNPP pretreatment. Adipate, glutarate, and succinate induced concentration-related increases in cytotoxicity in the nasal explant system. These dicarboxylic acids were neither metabolized nor utilized significantly by the nasal explants. Diacid-induced cytotoxicity was not attenuated by BNPP pretreatment. This study further established the utility of the nasal explant system for evaluating cytotoxicity of organic esters in vitro. It was established that both the monomethyl ester and diacid metabolites are cytotoxic in rat nasal explants. Finally, it was concluded that although both the monomethyl esters and the diacids contribute to the cytotoxic potential of DBE in vitro, it is critical to establish if one or both of these are formed in vivo in order to identify the ultimate toxic metabolite of DBE.