High IDO1 Expression Is Associated with Poor Outcome in Patients with Anal Cancer Treated with Definitive Chemoradiotherapy

BACKGROUND. This study characterizes the tumor‐immune microenvironment in pretreatment, localized anal squamous cell carcinoma (ASCC), including two markers that have not previously been studied in ASCC: indoleamine 2,3 dioxygenase 1 (IDO1) and human leukocyte antigen (HLA) class I. MATERIALS AND METHODS. Retrospective review identified 63 patients with ASCC receiving definitive chemoradiation between 2005 and 2016 with pretreatment tissue available. Immunohistochemistry was used to quantify cluster of differentiation 8 (CD8), programmed cell death protein 1, programmed death‐ligand 1, HLA class I, and IDO1. Cox proportional hazards models evaluated associations between outcomes and immune markers, controlling for clinical characteristics. RESULTS. With a median follow‐up of 35 months, 3‐year overall survival was 78%. The only marker found to have a robust association with outcome was tumor IDO1. In general, the percentage of tumor cells expressing IDO1 was low (median 1%, interquartile range 0%–20%); however, patients with >50% of tumor cells expressing IDO1 had significantly worse overall survival (hazard ratio [HR] 4.7, p = .007) as well as higher local recurrence (HR 8.6, p = .0005) and distant metastasis (HR 12.7, p = .0002). Tumors with >50% IDO1 were also more likely to have the lowest quartile of CD8 infiltrate (<40 per high‐power field, p = .024). CONCLUSION. ASCC has a diverse immune milieu. Although patients generally do well with standard therapy, IDO1 may serve as a prognostic indicator of poor outcome and could help identify a patient population that might benefit from IDO‐targeted therapies. IMPLICATIONS FOR PRACTICE. After definitive chemoradiation, patients with locally advanced anal cancer may experience significant treatment morbidity and high risk of recurrence. The goal of the current study is to identify novel prognostic factors in the tumor‐immune microenvironment that predict for poor outcomes after definitive chemoradiation. This study characterizes the tumor‐immune microenvironment in pre‐treatment, localized anal squamous cell carcinoma (ASCC), including two markers which have not previously been studied in ASCC: indoleamine 2,3 dioxygenase 1 (IDO1) and HLA class I. With a median follow‐up of 3 years, this study demonstrated that high IDO1 expression is correlated with significantly worse 3‐year overall survival (88% vs. 25%). Whereas recent studies of IDO1 inhibitors have shown mixed results, this study suggests that patients with anal cancer with high IDO1 expression have dismal prognosis and may represent a patient population primed for response to targeted IDO1 inhibition.