Indoleamine 2,3-dioxygenase activity during toremifene therapy for aromatase inhibitor-resistant metastatic breast cancer

We evaluated the clinical significance of indoleamine 2,3-dioxygenase (IDO) activity during toremifene (TOR) therapy for aromatase inhibitor (Al)-resistant metastatic breast cancer. IDO activity can be measured using the tryptophan/kynurenine (Trp/Kyn) ratio. Trp and Kyn were measured using high performance liquid chromatography (HPLC). The response rate of TOR therapy for Al-resistant metastatic breast cancer patients was 21.9%, and the clinical benefit rate was 62.5%. The serum Trp/Kyn ratio was significantly lower in Al-resistant metastatic breast cancer patients with distant metastases than in patients who had local recurrence. During TOR therapy, IDO activity was significantly decreased in the TOR responder group compared to the TOR non-responder group. IDO activity correlated with the number of metastatic lesions treated during TOR therapy. These results suggest that the Trp/Kyn ratio is a useful measurement in evaluating the immunological metastatic status during endocrine therapy.