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 i nto therapy of psoriasis and other dermatoses. Little is known, however, ab out 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 ointmen ts were prepared with 8-MOP concentrations of 0.05 and 0.005%, and stored o ver 12 weeks at room temperature (19-20 degrees C) and at 5 degrees C. 8-MO P concentrations were measured at days 1. 8, 15, 29, 57 and 88 after prepar ation by thin-layer chromatography (TLC). The ointments were dissolved in a n organic solvent, 10 mu l were transferred onto the TLC plate and the chro matograms 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 Ungue ntum Cordes the emulsion began to break up after 8 weeks. In the Carbopol g el, only about 40% of the nominal concentrations of 8-MOP were found and th ey decrease significantly at storage at 5 degrees C. We conclude that the C arbopol gel seems to be unsuitable for PUVA therapy, whereas Cold Cream Nat urel shows the best results.