[Indoleamine 2,3-Dioxygenase Activity during I131-Radioisotope Therapy for Metastatic Thyroid Cancer].

We evaluated the clinical significance of indoleamine 2,3-dioxygenase (IDO) during I131-radioisotope therapy for metastatic thyroid cancer. The IDO activity can be measured by the tryptophan (Trp)/kynurenine (Kyn) ratio. Trp and Kyn were measured by using high performance liquid chromatography (HPLC). The serum Trp/Kyn levels of were lower in thyroid cancer patients with lung metastases than in patients without metastases. The IDO activity increased after the development of lung metastases from thyroid cancer. The IDO activity was correlated with the number of metastatic lesions during I131-radioisotope therapy. These results suggest that measurement of the Trp/Kyn ratio is useful in the evaluation of the immunological metastatic status during I131-radioisotope therapy.