Stereoselective disposition of naproxen glucuronide in the rat.

The disposition of R- and S-naproxen glucuronides were investigated after intravenous administration (approximately 1.5 mg/kg) to normal male Sprague-Dawley rats and to rats pretreated with phenylmethylsulfonylfluoride, an inhibitor of esterases. The relative stability of the two glucuronides also was measured in vitro. Both diastereomers were hydrolyzed rapidly in vivo, liberating naproxen, but R-naproxen glucuronide was hydrolyzed faster than the corresponding S-diastereomer. This difference resulted in a larger plasma AUC(Nap):AUC(Nap-G) ratio for the R-glucuronide. There was, however, no marked difference in the apparent clearance of the R- and S-diastereomers. Administration of phenylmethylsulfonylfluoride had no significant effect on the disposition of the two diastereomers. In 0.15 M phosphate buffer (ph 7.4) at 37 degree C, the fastest degradation process for both diastereomers in vitro was acyl migration. Our results show that R-naproxen glucuronide is more labile than S-naproxen glucuronide in vivo and in vitro, and suggest that hydrolysis, rather than biliary excretion, is the major process leading to elimination of R-naproxen glucuronide in vivo in the rat. These results demonstrate that the rat may in certain situations be an inadequate model for studying the disposition of acyl glucuronides and that the metabolic disposition, and possibly toxicities, of diastereomeric metabolites of chiral drugs can be quite different even when the individual diastereomers have similar apparent clearances.

Stereoselective reversible binding properties of the glucuronide conjugates of fenoprofen enantiomers to human serum albumin.

The stereoselective binding of fenoprofen enantiomers and fenoprofen glucuronide diastereomers to human serum albumin (HSA) was investigated using an ultrafiltration method. Fenoprofen glucuronides exhibit a considerable and stereoselective affinity to HSA, although less than seen for the parent drug. The (R)-glucuronide shows a higher affinity to HSA than the (S)-diastereomer. With the enantiomers, no significant difference could be detected. Diazepam and probenecid reduced the binding of the glucuronides, as well as that of the fenoprofen enantiomers. These results suggest that parent drug and its glucuronide metabolites occupy the same binding region on the albumin molecule. Both fenoprofen enantiomers, as well as racemic fenoprofen, are capable of reducing the extent of reversibly bound fenoprofen glucuronide.

Stereoselective binding properties of naproxen glucuronide diastereomers to proteins.

The stability of naproxen glucuronide (NAP-G) diastereomers was investigated in buffer, 0.3% and 3% human serum albumin (HSA) solutions, and human plasma. R-NAP-G was found to be less stable in phosphate buffer than its S-diastereomer, whereas incubation media containing protein in general increased the degradation rate of NAP-G but also caused a change of the stereoselective stability where the R-NAP-G was more stable than S-NAP-G. Reversible binding of NAP-Gs to HSA (0.3%) was investigated and compared with the corresponding properties of naproxen (NAP) enantiomers. NAP-G diastereomers exhibited a considerable and stereoselective affinity to HSA, although less than that observed for the NAP enantiomers. In vitro irreversible binding of NAP-Gs to HSA, human and rat plasma proteins was also investigated. Irreversible binding was higher for R-NAP-G (50 microM) than for S-NAP-G (50 microM) in all incubation media. This stereoselective difference was observed with HSA containing medium as well as in rat and human plasma. Incubation with unconjugated NAP did not lead to irreversible binding. Preincubation of HSA with acetylsalicylic acid (approximately 11 mM) and glucuronic acid (50 mM) decreased the extent of irreversible binding suggesting involvement of lysine residues for covalent binding. Preincubation with S-NAP also decreased the irreversible binding yield.