Long-term stability of 8-methoxypsoralen in ointments for topical PUVA therapy ('cream-PUVBA')

8-Methoxypsoralen (8-MOP) is an established photochemotherapeutic agent for PUVA therapy. Recently, a so-called 'cream-PUVA' modality was introduced into therapy of psoriasis and other dermatoses. Little is known, however, about the stability of 8-MOP in ointments used for the topical application of this compound. Therefore, we investigated the long-term stability of 8-MOP in three different ointments, Unguentum Gordes(TM), Cold Cream Naturel(TM) and a water containing gel on the basis of Carbopol 940. All three ointments were prepared with 8-MOP concentrations of 0.05 and 0.005%, and stored over 12 weeks at room temperature (19-20 degrees C) and at 5 degrees C. 8-MOP concentrations were measured at days 1. 8, 15, 29, 57 and 88 after preparation by thin-layer chromatography (TLC). The ointments were dissolved in an organic solvent, 10 mu l were transferred onto the TLC plate and the chromatograms were developed first in toluene and then in toluene/ethyl acetate 2:1 v/v to resolve 8-MOP from the ointment constituents. the peak heights of 8-MOP were used for quantitation. The intraday variabilities are <3% for Unguentum Cordes and Cold Cream Natural and <6% for the Carbopol 940 gel. The interday variabilities were <6.3% in all cases. In Unguentum Cordes and Cold Cream Naturel the concentrations of 8-MOP remain stable, but in Unguentum Cordes the emulsion began to break up after 8 weeks. In the Carbopol gel, only about 40% of the nominal concentrations of 8-MOP were found and they decrease significantly at storage at 5 degrees C. We conclude that the Carbopol gel seems to be unsuitable for PUVA therapy, whereas Cold Cream Naturel shows the best results.