Abstract: Methods, compounds and compositions are disclosed for treating amyloidogenic diseases, like Alzheimer's disease and type 2 diabetes, and particularly prion diseases associated with conversion of protease sensitive PrP (PrP-sen) to protease resistant PrP (PrP-res), by administering therapeutically effective amounts of a tetrapyrrole. Particular disclosed tetrapyrroles having this activity include phthalocyanines, deuteroporphyrins, and meso-substituted porphines. Complexes of certain of the pyrroles with metals or metal ions produce compounds that are particularly effective in converting the conversion of PrP-sen to PrP-sen. The treatment of the present invention is particularly suited for preventing or inhibiting the progression of prion related diseases, such as transmissible spongiform encephalopathies.