Interferon-alpha influences tryptophan metabolism without inducing psychiatric side effects

Background: Interferon-α (IFN-α) treatment is often associated with psychiatric side effects and has been found to lower the amount of tryptophan (TRP) available to the brain. The alterations in tryptophan metabolism might underlie the psychiatric side effects during treatment with IFN-α. Methods: In this study, 43 oncology patients treated with IFN-α were included. In order to study de novo depressions, depressed patients at baseline were excluded. Psychiatric evaluation comprising clinical judgment combined with a structured psychiatric interview and observer-based and self-report rating scales was performed at baseline and at 4 weeks, 8 weeks and 6 months after the start of treatment with IFN-α, and in the case of emerging psychopathology. Blood samples were drawn at the same evaluation times and assessed for concentrations of TRP, large neutral amino acids, kynurenine, 5-hydroxyindole acetic acid, neopterin and biopterin. Results: During treatment with IFN-α, several alterations in laboratory parameters occurred that were consistent with an increased degradation of peripheral TRP. Psychometric ratings revealed hardly any psychiatric changes. No consistent associations were found between changes in the laboratory assessments determined and the diverse psychiatric measures. Conclusion: In this study, IFN-α was found to alter TRP metabolism without inducing psychiatric side effects. Therefore, a possible relationship between TRP metabolism and depression was not substantiated by this study.