Abstract 4292: In vitro kynurenine modulation by novel dual-acting and selective tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO) inhibitors
Authors: Wise Alan , McGuinness Barry E. , Trewick Sarah C. , Cowley Phillip M. , Bevan Nicola , Szybut Clare , Brown Thomas J. 
Published: August 2015

Introduction: TDO (tryptophan 2,3-dioxygenase) and IDO (indoleamine 2,3-dioxygenase) are structurally distinct enzymes that catalyse the oxidation of tryptophan (Trp) leading to local Trp depletion and formation of immunesuppressive kynurenine pathway (KP) metabolites and dampening of the immune response in the tumour environment. IDO is known to be up-regulated in numerous cancers. Recently, TDO was shown to be up-regulated in various tumour types. Hence, selective IDO inhibitors may have a limited effect in cancers harbouring both TDO and IDO. To test this hypothesis we utilised recently identified proprietary dual-active and highly selective TDO/IDO inhibitors to modulate kynurenine (Kyn) levels in glioma cancer cells expressing either TDO alone or expressing both TDO and IDO. In addition, we measured Kyn modulation in human peripheral blood mononuclear cells (PBMCs) following exposure to dual and selective inhibitors.

Methods: Assays were established to measure the effects of dual-active and highly selective TDO/IDO inhibitors on Kyn production in 1) human A172 glioma cells naturally expressing TDO alone or expressing both TDO and IDO following IDO induction using Interferon γ (IFNΓ) and in 2) freshly isolated human PBMCs treated with IFNΓ.

Results: Dual-active and selective TDO/IDO inhibitors demonstrated distinct Kyn modulating activities in A172 glioma cells expressing either TDO alone or both TDO and IDO. Dual-active molecules fully ablated Kyn levels in both TDO-expressing and TDO/IDO-expressing cells. Crucially, selective inhibitors were unable to fully block Kyn in A172 cells co-expressing both enzymes. As expected TDO-selective inhibitors were highly active in A172 cells expressing TDO alone. IDO-selective inhibitors were largely inactive in A172 cells naturally expressing TDO alone. However, they gained significant activity upon induction of IDO in these cells following IFNΓ exposure. Interestingly, TDO-selective inhibitors were unable to modulate Kyn levels in PBMCs whereas IDO-selective and dual-active molecules were able to potently block Kyn production.

Discussion: These data demonstrate that dual-active TDO/IDO inhibitors are most effective at reducing Kyn levels in cancer cells co-expressing TDO and IDO. In addition, the data suggest that IFNΓ-treated PBMCs primarily express IDO with no measurable Kyn production from TDO. This study demonstrates the utility of applying highly selective and dual active inhibitors of these Trp-catabolizing enzymes as probes for defining the Kyn-producing component from PBMCs. In summary, blocking both TDO and IDO activity with dual-active inhibitors maybe complementary rather than redundant in cancers harbouring both of these enzymes; and IDO appears to be the primary enzyme driving Trp metabolism in PBMCs.

Citation Format: Alan Wise, Barry E. McGuinness, Sarah C. Trewick, Phillip M. Cowley, Nicola Bevan, Clare Szybut, Thomas J. Brown. In vitro kynurenine modulation by novel dual-acting and selective tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO) inhibitors. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4292. doi:10.1158/1538-7445.AM2015-4292