QUANTITATIVE VASOCONSTRICTOR ASSAY FOR TOPICAL CORTICOSTEROIDS - THE PUZZLING CASE OF FLUOCINOLONE ACETONIDE

Background: The validity of the vasoconstrictor test has been questioned because of the apparent nonequivalence of some generic corticosteroid products. Objective: Our purpose was to determine the vasoconstrictor activity of a generic 0.025% fluocinolone acetonide ointment by two different methods, dilution by volume and dilution by area of application. Methods: Vasoconstrictor activity on a visual 4-point scale was measured in 20 volunteers. Topical fluocinolone acetonide ointment (0.025%) was tested by comparing dilution by volume (1:1, 1:3, and 1:7) in petrolatum and dilution by area of application. Fluocinolone release from the diluted ointments was also measured with an in vitro method. Results: Dilution by area gave the expected results, a decreasing average vasoconstrictor score; but dilution by volume did not. Both the 1:1 and 1:3 dilutions inexplicably showed greater activity than the full-strength product; this finding was similarly reflected in the in vitro studies that showed a greater rate of drug release from the 1:1 and 1:3 dilutions. Conclusion: Dilution by area provides a useful quantitative assay for comparative vasoconstrictor activity.