Development Of An Ihc-Based Detection Method For Studying Indoleamine 2,3-Dioxygenase 1 (Ido1) Expression In Human Cancers.

3043 Background: Indoleamine 2,3-dioxygenase-1 (IDO1) mediates oxidative cleavage of tryptophan, an amino acid essential for cell proliferation and survival. The depletion of tryptophan and the generation of tryptophan metabolites have been shown to suppress immune functions via several cellular mechanisms, allowing tumor escape from host immune surveillance. IDO1 is induced by interferons and TLR agonists and is expressed in a variety of human cancers as well as the invading immune infiltrate and the tumor-draining lymph nodes. High IDO1 expression is significantly associated with more rapid disease progression and poor prognosis in multiple cancer types. Thus, inhibition of IDO1 activity may have therapeutic potential in cancer and furthermore, IDO1 expression could serve as a biomarker for selecting patients that are responsive to an IDO1 inhibitor-based treatment regimen. Methods: We generated and selected an anti-human IDO1 rabbit monoclonal antibody. The specificity of the antibody for IDO1 was confirmed using Western blot analyses of lysates from cells expressing human IDO1, IDO2, or TDO. Results: INCB024360, a novel IDO1-selective inhibitor, is currently being evaluated in clinical trials of cancer patients. To support the clinical development of INCB024360, we developed an immunohistochemistry (IHC)-based method for the detection of IDO1 protein in human tissues. To further evaluate the specificity and reactivity of the antibody against IDO1 in human tissues, an IHC staining protocol was established, standardized and used to stain various human normal and tumor tissues, as well as IDO1-expressing cells. We have examined IDO1 expression in several human tumors including lung, ovarian, melanoma, gastric, and pancreatic cancers. Conclusions: In summary, we have developed and standardized an IHC-based method for the specific detection of IDO1 protein in human tissues. The method will be useful to determine the prevalence of IDO1 expression in a variety of human cancers. Furthermore, incorporating this detection method into ongoing clinical trials may identify patients that are likely responsive to IDO1 inhibitor-based treatment regimens.