Metabolism of an oral tryptophan load. II: Effect of pretreatment with the putative tryptophan pyrrolase inhibitors nicotinamide or allopurinol.

Abstract

1 The effect of seven days administration of either allopurinol (300 mg daily) or nicotinamide (500 mg twice daily) on the metabolism of an oral L‐tryptophan load (50 mg/kg) has been investigated. 2 Administration of either drug failed to alter the plasma total or free tryptophan or plasma kynurenine curve. Nor was the urinary excretion of tryptophan, kynurenine, 5‐hydroxyindoleacetic acid or indole acetic acid influenced. 3 Allopurinol pretreatment did increase the volume of distribution of tryptophan. 4 These data suggest that allopurinol and nicotinamide are unlikely to be of value as tryptophan pyrrolase inhibitors in vivo and therefore would not increase the therapeutic effect of L‐tryptophan when it is given to treat depressive illness. 1980 The British Pharmacological Society