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The stabilizing effects of alpha-, beta-, gammar- and dimethyl-beta-cyclodextrins (alpha-, beta-, gammar- and DM-beta-CyDs) on the degradation of hydrocortisone 17-buryrate (HC-17B) in aqueous solution was investigated. HC-17B underwent a facile hydroxide ion-catalyzed rearrangement to the less active 21-butyrate ester by the apparent first-order kinetics, and maximum stability of HC-17B was obtained at around pH 4.0. The stability of HC-17B was increased by inclusion complexation with alpha-gammar- and DM-beta-CyD int the ph range of 2.0-8.0 examined, whereas beta-CyD accelerated the degradation of HC-17B at pH higher than 5.0 The effects of ionic strength, solvent, temperature and CyD concentration were also investigated. Stability constants and apparent degradation rate constants of HC-17B-gammar-CyD and HC-17B-DM-beta-CyD complexes were determined kinetically on the basis of 1 : 1 complexation. The results suggested that the inclusion complexation with gammar-CyD or DM-beta-Cyd was most useful means to enhance the stability of the steroid.
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