The mammalian metabolism of R-(-)-m-synephrine

The metabolism of R-(-)-m-synephrine (administered orally and by inhalation in man and intraperitoneally in rats) was studied quantitatively by a gas chromatography-mass spectrometry-selected ion monitoring (g.c.-m.s.-s.i.m.) method using deuterated internal standards. When m-synephrine hydrochloride was administered orally to humans in normal dosage regimens four main metabolites were excreted in urine: (i) unconjugated m-hydroxymandelic acid (MHMA, 30%), (ii) m-hydroxyphenylglycol (MHPG) sulphate (6%), (iii) m-synephrine sulphate (47%) and (iv) m-synephrine glucuronide (12%). The comparable figures after inhalation of the drug were 24, 6, 56 and 5%. Intraperitoneal injection of m-synephrine into rats gave: unconjugated MHMA (5%), MHPG sulphate (35%), unconjugated m-synephrine (7%) and conjugates of m-synephrine (9%:4% as the glucuronide and 5% as the sulphate).