N⁺-Glucuronidation of aliphatic tertiary amines, a general phenomenon in the metabolism of H₁-antihistamines in humans

Abstract

1. Representative drugs of the various structural classes of H₁ antihistamines were chosen for study. The drugs chosen (class name in parentheses) were chlorpheniramine maleate and pheniramine maleate (alkylamines), diphenhydramine hydrochloride and doxylamine succinate (ethanolamines), pyrilamine maleate and tripelennamine hydro-chloride (ethylenediamines), promethazine hydrochloride (phenothiazine), cyclizine lactate (piperazine) and terfenadine (miscellaneous). In each case oral dose(s) were administered over no more than 6 h to two healthy volunteers and the total urine collected for 36 h.

2. Metabolites from urine were separated by h.p.l.c. and individually collected prior to mass spectrometric analysis in the fast atom bombardment mode. The structure of each metabolite identified as a quaternary ammonium-linked glucuronide metabolite was confirmed by direct comparison of its mass spectrum and chromatographic behaviour with that of a synthetic authentic compound.

3. For eight of the nine drugs studied, metabolism by the N⁺-glucuronidation pathway was observed in each of the volunteers. Terfenadine was the exception.

4. The amount of each N⁺-glucuronide in the urine was estimated by h.p.l.c. analysis. The mean proportion of dose excreted as the metabolite was 14.3%, 6.5% and 4.0% for cyclizine, tripelennamine and diphenhydramine, respectively. Promethazine was the only case where the N⁺-glucuronide accounted for less than 1.0% of the administered dose in both volunteers examined.