Tranylcypromine isomers and Parkinson's disease: new aspects of an old drug.

The tranylcypromine stereoisomers have been investigated in a series of comparative trials in Parkinsons' disease and the results indicate that doses below 3 mg/day, of the (+)-isomer in particular, are effective as adjuvant anti-parkinsonian therapy. Biochemical results, monitoring platelet monoamine oxidase (MAO) activity and plasma concentrations of drugs and phenylethylamine, an MAO substrate, showed such low doses of the (+)-isomer to inhibit MAO without inducing the hypertensive reaction sometimes observed at higher dosage. These findings, along with the observation of substantial pharmacokinetic differences between the two isomers are discussed, particularly in relation to reports on their efficacy in depressive illness.