Variability of diphenhydramine N-glucuronidation in healthy subjects

The H1-antagonist diphenhydramine can undergo direct glucuronidation at its tertiary amino group with formation of a quaternary ammonium glucuronide. The intraindividual variability in the amount of N-glucuronide excretion in urine was investigated in two female volunteers who repeatedly took single doses of 25 mg diphenhydramine hydrochloride without and with concomitant administration of ascorbic acid or ammonium chloride for urine acidification. Another two female and four male subjects underwent single tests without and with additional ascorbic acid. Diphenhydramine N-glucuronide quantities in urine differed significantly among subjects and ranged between 2.7% and 14.8% of the dose within 8 h. Neither ascorbic acid nor ammonium chloride significantly influenced the quantity of N-glucuronide in urine, but ammonium chloride, that in contrast to ascorbic acid proved effective in lowering urinary pH, increased the excretion of the parent drug.