RNAScope as a platform to detect IDO1 expression in tumor tissue sections.

Abstract

116 RNAScope is a sensitive, specific platform to detect IDO1 expression in tumor tissue sections. M Pratta, M Rupar, P Waeltz, T Burn, G Hollis, M Covington, M Smith, and R Newton. Incyte Corp. Wilm. DE. Background: Indoleamine 2,3-Dioxygenase 1 (IDO1) catalyzes the primary and rate-limiting step in tryptophan catabolism to generate N-formyl-kynurenine (Kyn). Through a combination of local depletion of tryptophan and an increase in Kyn concentrations, IDO-1 activity can result in the suppression of antitumor immune responses. Because IDO-1 inhibitors are now in the clinic for treatment of multiple tumor types, immunohistological approaches are employed to demonstrate IDO1 expression in tumor biopsies. However, using a commercially available antibody to detect IDO1 by immunohistochemistry (IHC), the level of sensitivity was inadequate. Methods: In order to improve the sensitivity of IDO1 detection, we evaluated in situ hybridization (ISH) using RNAScope technology and digital quantitation by HALO analysis in collaboration with Advanced Cell Diagnostics (ACD). The technology was cross-validated using IDO1 qRT-PCR, Western blot, and activity analysis and compared with standard IHC. We initially evaluated IDO1 expression in HeLa cells stimulated with various concentrations of IFNγ, and then extended the observations using tissue sections from multiple tumor types. Results: In the HeLa cell model, IFNγ induced a time- and concentration-dependent increase of IDO1 at the mRNA, protein, and activity level. Although IDO1 was successfully detected in the HeLa cell samples by IHC, comparison of the platforms indicated IFNγ EC50 values were in strong agreement between RNAScope (193.8 pg/ml) and Western blot analysis (170.8 pg/ml), but was much higher by IHC analysis (2206 pg/ml). A strong positive correlation (*p < 0.0001) between RNAScope and Western blot analysis was observed, suggesting a highly coordinated induction of IDO1 by IFNγ at both the mRNA and protein levels. FFPE tumor tissue from melanoma, HNSCC, bladder, renal, ovarian, and lung cancers visualized by RNAScope all show varying levels of IDO1 expression. Conclusions: These data support the use of RNAScope for the analysis of IDO1 expression in clinical trials.