Stability of prednisolone and prednisolone acetate in various vehicles used in semi-solid topical preparations

The stability of prednisolone and prednisolone acetate has been investigated at 37 degrees C in seven semi-solid vehicles, including almond oil, beeswax, Cremophor RH 40, Lanette N, and hydrogel formulations of hydroxy-propylcellulose, carbomer and bentonite. Extraction methods and a high-performance liquid chromatographic (HPLC) procedure have been developed in order to assay the content of undegraded steroid at time 0 and after 28 days. Prednisolone acetate showed superior stability compared to prednisolone. Bentonite induced considerable degradation of both steroid compounds, whereas the two absorption bases, Cremophor RH 40 and Lanette N, affected mainly the stability of prednisolone. In the carbomer hydrogel, prednisolone acetate hydrolysed to a small extent during the trituration process. The most effective steroid stability was found in the two lipophilic bases and the hydroxypropylcellulose hydrogel.