Disposition of [¹⁴C]furan in the male F344 rat

Abstract

In a recently completed 2-yr bioassay, furan was found to induce cholangiocarcinomas at high incidence in rats. The disposition of single and multiple gavage doses of [2,5-14C]furan has been determined in male F344 rats to aid in interpretation of that study. In the 24 h after dosing about 80% of the furan-derived radioactivity was eliminated, primarily via urine and expired air. [14C]Carbon dioxide was a major metabolite, indicating that furan ring opening followed by complete oxidation of at least one of the labeled carbons was a major part of the overall metabolism of furan. Liver contained more furan-derived radioactivity by far than other tissues after 24 h. Approximately 80% of the radioactivity in liver was not extracted by organic solvents and was associated with protein. There was either no binding to DNA or the furan-DNA adduct was not stable to the isolation procedure. Repeated daily administration of [14C]furan resulted in a more or less linear increase in covalent binding through four doses; at this point the amount of nonextractable radioactivity plateaus. Urine contained at least 10 metabolites, again indicating extensive metabolism of the furan ring. From the data obtained in this study it is clear that furan is metabolized to reactive species, apparently primarily in liver, and these intermediates react with protein. The hepatotoxicity resulting from furan exposure may be due to the reaction of furan metabolites with liver macromolecules; the presence of some of these reactive metabolites following chronic exposure to furan may result in cholangiocarcinomas.

Identification of a cis-2-Butene-1,4-dial-derived Glutathione Conjugate in the Urine of Furan-Treated Rats

“…Compound 3 might also be the product resulting from processing of both GSH moieties in the bis-GSH-cis-2-butene-1,4-dial-reaction product (2). Two intermediates in the conversion of 2 to 3 are 2-{3-[S-(glutathionyl)]-1H-pyrrol-1-yl}pentanedioic acid (4) (5). These compounds (3)(4)(5) were prepared by adapting the procedure for preparing the cis-2-butene-1,4-dial-GSH reaction products (1 and 2) (11,12) and characterized by NMR and mass spectrometry.…”

“…Two intermediates in the conversion of 2 to 3 are 2-{3-[S-(glutathionyl)]-1H-pyrrol-1-yl}pentanedioic acid (4) (5). These compounds (3)(4)(5) were prepared by adapting the procedure for preparing the cis-2-butene-1,4-dial-GSH reaction products (1 and 2) (11,12) and characterized by NMR and mass spectrometry. All these standards were then employed to develop chromatographic methods for their separation by capillary LC (Figure 1).…”

“…Cytochrome P-450 inducers increase furan metabolism and its associated toxic effects such as furan derived protein binding, glutathione (GSH) depletion and cytotoxicity (5)(6)(7)(8)(9). Similarly, these toxic effects are reduced in the presence of cytochrome P-450 inhibitors (5)(6)(7)(8)(9). The initial product of microsomal furan oxidation is cis-2-butene-1,4-dial (Scheme 1) (10)(11)(12).…”

“…The urinary and fecal metabolites of furan have not been chemically characterized. However, HPLC analysis of urine from [ 14 C]furan-treated rats indicated the presence of several polar metabolites that were proposed to be GSH or mercapturic acid conjugates of furan metabolites (5).…”

“…Furan is extensively metabolized in rats since only 14% of an 8 mg/kg oral dose of [ 14 C]furan is eliminated unchanged (5). The major metabolite is carbon dioxide, accounting for 26% of the dose.…”