Initial monoamine oxidase-A inhibition by moclobemide does not predict the therapeutic response in patients with major depression. A double blind, randomized study.

It is generally accepted that the clinical efficacy of monoamine oxidase inhibitors (MAOI) is related to inhibition of this enzyme. In order to evaluate the predictive ability of monoamine oxidase-A inhibition for therapeutic efficacy, the start of treatment effects of moclobemide, a selective, reversible monoamine oxidase-A inhibitor, on plasma concentrations of monoamines and monoamine metabolites were determined. The plasma levels of 3,4-dihydroxyphenylglycol (DHPG, deaminated metabolite of noradrenaline), 5-hydroxyindoleacetic acid (5-HIAA, deaminated metabolite of serotonin), 3,4-dihydroxyphenylacetic acid and homovanillic acid (DOPAC and HVA, deaminated metabolites of dopamine), L-dihydroxyphenylalanine (L-dopa) and noradrenaline were investigated and related to treatment outcome. This was a randomized double blind parallel group study in 47 patients with criteria of major depression according to DSM III R. Moclobemide 300 mg/day, 450 mg/day or 600 mg/day was administered continuously for 6 weeks. Plasma concentrations of monoamine metabolites and monoamines were determined just before treatment by moclobemide, 4 h after the first dose, 24 h after the first dose, before the first dose on day 7, and 4 h after the first dose, on day 7. Each moclobemide dose improved depression as measured by MADRS (Montgomery-Asberg Depression Rating scale) but there was no difference between the three doses. Moclobemide dose-dependently reduced plasma concentration of DHPG, L-dopa and HVA. No dose-dependent treatment effect was observed for plasma 5-HIAA, noradrenaline and DOPAC. The clinical outcome as defined by the final MADRS score was not related to any start of treatment changes in plasma monoamine metabolites reflecting inhibition of MAO-A. It is concluded that monoamine oxidase-A inhibition at the beginning of the treatment does not predict clinical outcome.