Predictive value of tryptophan/large neutral amino acids ratio to antidepressant response

The molar ratio of total plasma tryptophan to the sum of other large neutral amino acids (viz., valine, isoleucine, leucine, tyrosine, phenylalanine), thought to reflect brain serotonin formation, was estimated in 69 patients with major depression before and after 1 week of treatment with different serotonin reuptake inhibitors, fluvoxamine (n = 28), fluoxetine (n = 10), citalopram (n = 7), mixed serotonin-noradrenaline reuptake inhibitor amitriptyline (n = 11), clomipramine (n = 8) and the preferentially noradrenaline reuptake inhibitor, nortriptyline (n = 5). A significant difference among 'good', 'intermediate' and 'poor' responders for the percentual variation of tryptophan/large neutral amino acids ratio after 1 week of therapy was found, while pretreatment values of tryptophan/large neutral amino acids ratio, basal Hamilton Depression Rating Scale scores and its percentual variation after 1 week of therapy did not show any significant differences. These results suggest that the percentual variation of tryptophan/large neutral amino acids ratio could be a useful tool in predicting drug response.