Fluocinolone acetonide

Methanol and acetonitrile and formic acid 99.9% (LC-MS grade) and water (HPLC gradient grade) were supplied from VWR (VWR International PBI Srl, Milan, Italy). Methylprednisolone, dexamethasone, prednisolone, fluocinolone acetonide, flumetasone, prednisone, triamcinolone, triamcinolone acetonide, beclomethasone, clobetasol propionate, dexamethasone D4, Methylprednisolone D2, and prednisolone D6 (purity > 98%) were purchased from Sigma-Aldrich (Milan, Italy). A 1000 mg L⁻¹ stock solution was made by dissolving the standard in methanol. From this solution, a 10 mg L⁻¹ work solution was made by dilution in methanol.

Four topical corticosteroids were investigated in these studies. The APIs were chosen based on their availability. Clobetasol propionate, mometasone furoate, and fluocinolone acetonide were purchased from Sigma^® Aldrich (Kappelweg, Schnelldorf, Germany) and halcinonide was obtained as a gift from Ranbaxy^®, (Mumbai, India). The solutions were freshly prepared immediately before use in each study and kept away from direct sunlight and at ambient room temperature not exceeding 25 °C. A standard molar concentration (0.0025 M) was chosen for all the respective topical corticosteroids. A 0.0025 M concentration was used to approximate a 0.1% topical corticosteroid solution for each compound. Based on solubility of the APIs in various solvents, analytical grade propylene glycol (MINEMA Chemicals (Pty) Ltd., Clockwork Road, Gauteng, South Africa) was chosen as the appropriate solvent to obtain a standard concentration of 0.0025 M for each of the APIs used. All the APIs were readily soluble in propylene glycol following sonication.