STUDIES ON THE STEREOSELECTIVE INTERNAL ACYL MIGRATION OF KETOPROFEN GLUCURONIDES USING C-13 LABELING AND NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY

Internal acyl migration reactions of drug 1 beta-O-acyl glucuronides a re of interest because of their possible role in covalent binding to s erum proteins and consequent allergic reactions as well as their influ ence on drug disposition. An approach using C-13 labeling and nuclear magnetic resonance (NMR) spectroscopy has been used to investigate in situ the kinetics of acyl migration and hydrolysis of 1 beta-O-acyl gl ucuronides of enantiomeric ketoprofens (KPs) in phosphate buffer solut ions at 37 degrees C. Apparent first-order degradation of the 1 beta-O -acyl glucuronide labeled in the ester carbonyl carbon and the sequent ial appearance of 2-, 3-, and 4-O-acyl isomers as both alpha- and beta -anomeric forms were observed for each enantiomer, All of the position al isomers and anomers were characterized using two-dimensional NMR sp ectroscopy (heteronuclear multiple bond correlation, correlated spectr oscopy, totally correlated spectroscopy) of the reaction mixtures. The overall degradation rate constants (hr(-1)) of (R)- and (S)-KP glucur onides were 1.07 +/- 0.154 and 0.55 +/- 0.034, respectively. To evalua te in detail the stereoselective reactivity, a kinetic model describin g the rearrangement reactions was constructed, and the kinetics were s imulated using a theoretical approach. Only the acyl migration, 1 beta -->2 beta, was found to have significant stereoselectivity, The rate c onstants (hr(-1)) for 1 beta-->2 beta migration of (R)- and (S)-KP glu curonides were 1.04 +/- 0.158 and 0.52 +/- 0.029, respectively. The re sults may suggest that (R)-KP glucuronide could be more susceptible to covalent binding to proteins via acyl migration than the correspondin g antipode. This stereoselective reactivity may be responsible for the stereoselective pharmacokinetics of KP, The direct approach using C-1 3 labeling and NMR spectroscopy could also provide insight into the re activities of other labile drug acyl glucuronides and their isomeric g lucuronides.