BackgroundPhotochemotherapy is an effective treatment for psoriasis. The photosensitizer methoxsalen can be applied either orally or topically. When applied topically, the patient is immersed in a bath containing 0.0001% methoxsalen in warm water for 20 min, followed by UVA irradiation (PUVA bath (PUVAb)).Methoxsalen is very toxic and has carcinogenic effects when it is swallowed or gets in contact with mucosa. It must be handled with caution.PurposeTo find an effective and simple method for chemical deactivation of methoxsalen, which could be used in the clinical setting after PUVAb.Material and methodsMaterials: Methoxsalen powder, KOH, Ethanol, NaClO, CHCl3.Samples: 7 solutions of 200 mg of methoxsalen in 10 ml of ethanol.Reagents: 3 solutions of KOH (1 M, 0.5 M, 0.2 M), and 3 of NaClO (5%, 0.5%, 0.05% (v/v)).We mixed 10 ml of each reagent with a different methoxsalen sample and stirred. The seventh sample of methoxsalen was used as a blank.We monitored the chemical reactions with thin layer chromatography at 10 and 30 min. Mobile phase was ethanol 1% in CHCl3. We used UV light to visualise the chromatogram.We performed a qualitative analysis of the remaining methoxsalen in each sample, after acid or alkaline hydrolysis.ResultsThe blank showed that methoxsalen is completely eluted by the mobile phase. In the six samples, we observed that part of the mixture is eluted (methoxsalen), while other compounds are retained in the stationary phase. These may be polar degradation products. The more concentrated the reagents, the smaller the quantity of methoxsalen remaining. The most extensive hydrolysis was seen in the mixture with 1M KOH.ConclusionBoth KOH and HClO hydrolyse methoxsalen. The most efficient reactive was 1M KOH, which hydrolysed almost the whole sample.These results suggest that KOH 1M may be useful to deactivate methoxsalen after PUVAb, although further studies are necessary to characterise degradation products and evaluate their toxicity.References and/or acknowledgementsNo conflict of interest.