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The thionine dye, methylene blue (MB), is a potent inhibitor of monoamine oxidase (MAO) A, a property that may, at least in part, mediate its antidepressant effects in humans and animals. The central inhibition of MAO-A by MB has also been linked to serotonin toxicity (ST) which may arise when MB is used in combination with serotonergic drugs. Structural analogues and the principal metabolite of MB, azure B, have also been reported to inhibit the MAO enzymes, with all compounds exhibiting specificity for the MAO-A isoform. To expand on the structure-activity relationships (SARs) of MAO inhibition by MB analogues, the present study investigates the human MAO inhibition properties of five MB analogues: neutral red, Nile blue, new methylene blue, cresyl violet and 1,9-dimethyl methylene blue. Similar to MB, these analogues also are specific MAO-A inhibitors with cresyl violet (IC50 = 0.0037 μM), Nile blue (IC50 = 0.0077 μM) and 1,9-dimethyl methylene blue (IC50 = 0.018 μM) exhibiting higher potency inhibition compared to MB (IC50 = 0.07 μM). Nile blue also represents a potent MAO-B inhibitor with an IC50 value of 0.012 μM. From the results it may be concluded that non-thionine MB analogues (e.g. cresyl violet and Nile blue) also may exhibit potent MAO inhibition, a property which should be considered when using these compounds in pharmacological studies. Benzophenoxazines such as cresyl violet and Nile blue are, similar to phenothiazines (e.g. MB), representative of high potency MAO-A inhibitors with a potential risk of ST.MB analogues, cresyl violet and Nile blue, are high potency MAO-A inhibitors.Nile blue also represents a potent MAO-B inhibitor.Potent MAO-A inhibition should alert to potential serotonin toxicity.