Particle size of dexamethasone incorporated in ointment and its behaviour during penetration in the stratum corneum of living skin.

The particle size of dexamethasone was microscopically determined in polarized light in two emulsion ointments: a standard water-in-old formula (aqueous wool alcohol ointment), and a standard oil-in-water formula (aqueous hydrophilic ointment), both containing 0.023% dexamethasone. The distribution of dexamethasone was on average smaller and finer in the water-in-oil emulsion than in the oil-in-water emulsion. In a further series of experiments we studied by means of the adhesive tape-stripping method the proportions of the particle size and counted the total number of particles in various layers of the stratum corneum in vivo. The difference between the two emulsion types once again proved to be significant, but only as far as the epidermal barrier. A new, uniform particle size spectrum was formed at this level, having a maximum value of 24 mum, which, irrespective of the type of ointment, allows a release of the desamethasone in the live epidermis and dermis. The original particle size distribution is thus qualitatively insignificant in so far as the therapeutic effect of dexamethasone below the barrier zone is concerned. Since the particle size has a strongly distorted frequency distribution, with amarked shift to the left, only about 0.5% of the particles in the w/o emulsion and only about 2% in the o/w emulsion are in fact larger than 56mum, the maximum value measured lying just below the surface of the horny layer. Any loss during further penetration is probably negligible.