Abstract
The effect of mixing with other ointment and the ratio of admixture on in vitro release of betamethasone dipropionate (BM-DP) from 4 different model ointment bases were investigated. The bases used included white petrolatum (WP)-purified lanoline bases (I), WP-liquid paraffin bases (II), WP-propylene glycol bases (III) and plastibase (PB) bases (IV). Ointment used for admixture was zinc oxide (10%) ointment (ZO) alone and ointment bases I through IV above. The ratios of admixture were adjusted so that the final concentrations of the mixed ointment were 0, 1/ 4, 2/4 and 3/ 4. The release of BM-DP from the bases was measured by HPLC.As the amount of BM-DP released fitted Higuchi's equation, the release rate constant (K) was calculated by that equation. Constant K of BM-DP from the model ointments mixed with WP bases (I through III) and PB bases (IV) decreased in proportion to the decrease in BMDP levels, but when the concentration of ZO added was 1/ 4 or 2/ 4, the constant K reached the maximum and gradually decreased thereafter, as it was intended for clinical efficacy.From the above, it was suggested that this could be a simple but useful method for determination of compatibility and selection of the proper ratios of mixture of corticosteroid ointment with other bases or ointments.
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