Glucuronidation as a transient intermediate metabolic step in the elimination of (-)-carbovir: Identification of (-)-carbovir-5'-O-glucuronide in rat bile

Carbocyclic 2',3'-didehydo-2',3'-dideoxy-guanosine [(-)-CBV] is a potent and selective inhibitor of the human immunodeficiency virus. The formation of the (-)-CBV-5'-O-glucuronide has been reported to be species-specific and stereoselective with rats forming little of the metabolite after administration of (-)-CBV. A series of studies of (-)-CBV disposition in bile duct-cannulated rats and in the in situ perfused rat liver were conducted. Based on differential hydrolysis with β-glucuronidase, UV, and tandem MS, the 5'-O-glucuronide was unequivocally identified as the major metabolite of (-)-CBV in rat bile. In the bile duct-cannulated rats receiving ¹⁴C-(-)- CBV, 15.8 ± 4.8% of the radioactivity was recovered in bile after ≥4 hr. The 5'-O-glucuronide accounted for 75% of the biliary radioactivity. In the in situ perfused rat liver, ~20.5% of the radioactivity appeared in the bile. Of this (-)-CBV-derived radioactivity, ~70% was due to the glucuronide, 13% to unchanged (-)-CBV, and 17% to an unidentified metabolite. These results suggested that biliary excretion was an important source of radioactivity in the feces. A plausible explanation for the lack of excretion of the conjugate in the feces of rats is intestinal degradation of the conjugate catalyzed by the gut microflora. This hypothesis was supported by the demonstration that rat cecal contents efficiently hydrolyzed (-)-CBV-5'- O-glucuronide in bile to (-)-CBV under anaerobic conditions.