ABSTRACT
The chemical stability of a range of corticosteroid ointments diluted with Compound Zinc Paste B.P. was studied by high-performance liquid chromatography (HPLC). Betamethasone dipropionate degraded by first-order kinetics at both 25 degrees C and 32 degrees C. At 25 degrees C the mean first-order reaction rate constant was 9.58 x 10(-3)/day, with a t90 (time to reach 90% of the original concentration) of 11.0 days. At 32 degrees C the mean first-order reaction rate constant was 2.42 x 10(-2)/day, with a t90 of 4.4 days. Of the other steroids studied, Fluocinolone acetonide was the least stable, with 33.8% remaining after a 7-day storage period at 25 degrees C. Betamethasone-17-valerate and fluocinonide were of intermediate stability between betamethasone dipropionate and fluocinolone acetonide. None of the steroid ointments studied had adequate stability in Compound Zinc Paste B.P. to allow extemporaneous dilution with this base.
