Solution kinetics of a water-soluble hydrocortisone prodrug: hydrocortisone-21-lysinate.

Hydrocortisone-21-lysinate was synthesized as an amino acid prodrug of hydrocortisone to serve as a substrate for brush border aminopeptidases. This strategy was developed to demonstrate that an improvement in oral absorption could be obtained through reconversion in vivo. The aqueous stability of hydrocortisone-21-lysinate was studied over the pH range 3-8 at 25 degrees C. Reversible acyl migration of the lysine group between the 21- and 17-position hydroxyl groups was observed as well as hydrolysis. The observed half-life for direct hydrolysis of hydrocortisone-21-lysinate is 40 d at pH 3 and 30 min at pH 7. The relative instability at pH 7 is probably due to electrostatic stabilization of the negatively charged tetrahedral intermediate by the protonated amino groups.