Platelet and plasma amine oxidase inhibition and urinary amine excretion changes during phenelzine treatment

The treatment of depressed patients with the monoamine oxidase (MAO)-inhibiting antidepressant, phenelzine, was found to be accompanied by an 82 per cent reduction in platelet MAO activity and a 91 per cent reduction in plasma amine oxidase activity. Maximum reductions in enzyme activity occurred after 7 to 14 days of treatment; recovery of enzyme activity posttreatment required an average of 14 days for the platelet MAO and greater than 6 weeks for the plasma enzyme. Marked elevations in urinary tryptamine excretion occurred in conjunction with MAO inhibition during phenelzine treatment, but the magnitidue of urinary tryptamine change in individual patients was not closely correlated with the magnitude of enzyme activity reduction. Changes in platelet serotonin content serotonin content and in urinary catecholamine metabolites also occurred in response to phenelzine treatment.