Development of simultaneous quantification method of loteprednol etabonate (LE) and its acidic metabolites, and analysis of LE metabolism in rat

Loteprednol etabonate (LE) is a soft corticosteroid with two labile ester bonds at 17α- and 17β-positions. Its corticosteroidal activity disappears upon hydrolysis of either ester bond. Hydrolysis of both ester bonds produces the inactive metabolite, Δ1-cortienic acid (Δ1-CA).The simple high-performance liquid chromatography method using acetic acid gradient was developed for the simultaneous determination of LE and its acidic metabolites.LE was hydrolyzed in rat plasma with a half-life of 9 min. However, LE hydrolysis was undetectable in rat liver and intestine. LE hydrolysis in rat plasma was completely inhibited by paraoxon and bis(p-nitrophenyl) phosphate, thus identifying carboxylesterase as the LE hydrolase. Rat plasma carboxylesterase had a Km of 6.7 μM for LE.In contrast to the disappearance rate of LE in rat plasma, the formation rate of 17α-monoester and Δ1-CA was markedly low, and a main hydrolysate of LE was not detected in rat plasma.The metabolism of LE proceeded via different pathwa...