The biliary excretion and enterohepatic circulation of benzo(a)pyrene and its metabolites in the rat

The enterohepatic circulation of benzo(a)pyrene (BP) has been investigated in the rat with a view to determining the availability of potentially toxic metabolities to tissues within this cycle. Some 60% of the dose of [¹⁴C]-BP (3 μmoles kg^?1, i.v.) is excreted in bile in 6 hr, with less than 3% in urine. The biliary metabolities are mainly polar conjugates; only 8% of the ¹⁴C in 2 hr bile samples can be directly extracted into ethyl acetate. However, following hydrolysis by β-glucuronidase some 40% of the ¹⁴C is extractable at pH 7. The extract consisted of polar metabolites (polyhydroxylated and/or conjugated; 37.5%), BP 4,5-diol (16.8%), BP 3,6-quinone (5.9%). 9-hydroxy BP (5.4%) and 3-hydroxy BP (5.3%) as indicated by co-chromatography with authentic standards on reversed phase HPLC, together with several unidentified metabolites. The proximate carcinogen BP 7, 8-diol was not detected. Biliary metabolites of [¹⁴C]-BP undergo enterohepatic recirculation in the rat; following the intraduodenal infusion of bile containing metabolites of [¹⁴C]-BP into bile duct cannulated rats, approximately 20% of the dose is absorbed and excreted in bile in 30 hr, with only 1% in urine. The pattern of metabolites in this bile is very similar to that in bile from rats administered [¹⁴C]-BP i.v. Following a single i.v. dose of [¹⁴C]-BP (3 μmoles kg^?1) to rats with re-entrant bile duct cannulae, which allowed intermittent collection of bile over a period of several days with minimal interference to the enterohepatic circulation, the proximate carcinogen BP 7, 8-diol was detected in recirculating bile. Biliary metabolites of BP, which have recently been shown to be mutagenic, can thus traverse the intestine to undergo enterohepatic circulation in the rat.