Optimization of bioavailability of topical steroids: non-occluded penetration enhancers under thermodynamic control

The non-occluded vasoconstrictor test under thermodynamic control assessed the effect of penetration enhancers on the topical bioavailability of a model steroid betamethasone 17-benzoate, using aqueous dimethylisosorbide (DMI) as a standard solvent. The aqueous potential penetration enhancers used were at 10% steroid saturation i.e. ideally at identical steroid thermodynamic activity. In the vasoconstrictor test, 2-pyrrolidone, N-methyl-2-pyrrolidone, propylene glycol with oleic acid, propylene glycol with azone and dimethylformamide (DMF) increased the steroid bioavailability compared with that from DMI, while propylene glycol alone produced borderline improvement. Azone and oleic acid in combination with DMI or Betnovate cream did not increase the steroid bioavailability indicating the importance of the correct cosolvent. The pyrrolidones established superior stratum corneum reservoirs compared with DMI, the other vehicles being similar to DMI. It was concluded that excepting DMI, the solvents tested were penetration enhancers for the model steroid betamethasone 17-benzoate and are worthy of further study. However, irritant effects may make some of them unacceptable for clinical use.