Methotrexate has little visible effect upon the psoriatic lesion when applied in aqueous cream or ointment under polythene occlusive dressings or when infused into the lesion over a period of three days. High concentrations (1%) in aqueous dimethyl sulfoxide when applied under polythene dressings caused pain, purpura, blisters and erosions in some patients. Less than 0.55% of methotrexate applied epicutaneously with occlusion was excreted in the urine over a period of 72 hours, and most of the methotrexate remained on or in the stratum corneum.
Comaish S, Juhlin L. Site of Action of Methotrexate in Psoriasis. Arch Dermatol. 1969;100(1):99-105. doi:10.1001/archderm.1969.01610250105024