title: Characterization of phentermine and related compounds as monoamine oxidase (MAO) inhibitors.
abstract: Phentermine was shown in the 1970s to inhibit the metabolism of serotonin by monoamine oxidase (MAO), but never was labeled as an MAO inhibitor; hence, it was widely used in combination with fenfluramine, and continues to be used, in violation of their labels, with other serotonin uptake blockers. We examined the effects of phentermine and several other unlabeled MAO inhibitors on MAO activities in rat lung, brain, and liver, and also the interactions of such drugs when administered together. Rat tissues were assayed for MAO-A and -B, using serotonin and beta-phenylethylamine as substrates. Phentermine inhibited serotonin-metabolizing (MAO-A) activity in all three tissues with K(i) values of 85-88 microM. These potencies were similar to those of the antidepressant MAO inhibitors iproniazid and moclobemide. When phentermine was mixed with other unlabeled reversible MAO inhibitors (e.g. pseudoephedrine, ephedrine, norephedrine; estradiol benzoate), the degree of MAO inhibition was additive. The cardiac valvular lesions and primary pulmonary hypertension that have been reported to be associated with fenfluramine-phentermine use may have resulted from the intermittent concurrent blockage of both serotonin uptake and metabolism.
authors.affiliation: Department of Pharmacology and Clinical Pharmacology, Uludag University Medical School, Bursa, Turkey.
authors.forename: I H
authors.initials: IH
authors.lastname: Ulus
authors.forename: T J
authors.initials: TJ
authors.lastname: Maher
authors.forename: R J
authors.initials: RJ
authors.lastname: Wurtman
mesh_terms: D000818:Animals; D000928:Antidepressive Agents / Q000494:pharmacology; D001921:Brain / Q000187:drug effects / Q000201:enzymology; D003913:Dextroamphetamine / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D004809:Ephedrine / Q000494:pharmacology; D007490:Iproniazid / Q000494:pharmacology; D008099:Liver / Q000187:drug effects / Q000201:enzymology; D008168:Lung / Q000187:drug effects / Q000201:enzymology; D008297:Male; D020912:Moclobemide / Q000494:pharmacology; D008995:Monoamine Oxidase / Q000187:drug effects / Q000378:metabolism*; D008996:Monoamine Oxidase Inhibitors / Q000737:chemistry / Q000494:pharmacology*; D010645:Phentermine / Q000031:analogs & derivatives / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley
chemical_list: D000928:Antidepressive Agents; D008996:Monoamine Oxidase Inhibitors; D010645:Phentermine; D007490:Iproniazid; D008995:Monoamine Oxidase; D004809:Ephedrine; D020912:Moclobemide; D003913:Dextroamphetamine