Three enzymes — IDO1, IDO2, and TDO2 — funnel tryptophan into kynurenine, a metabolite that shuts down effector T cells and switches on the immunosuppressive aryl hydrocarbon receptor (AhR). That biology made IDO1 an obvious partner for checkpoint blockade, yet the pivotal ECHO-301 trial showed no benefit from adding epacadostat to pembrolizumab. This review dissects why first-generation IDO1 inhibitors stalled — off-target AhR agonism, incomplete intratumoral kynurenine suppression, and missing pharmacodynamic biomarkers — and maps the next wave of approaches: engineered kynureninases, selective AhR antagonists, and dual IDO1/TDO2 inhibitors.
This review in Frontiers in Immunology — first-authored by Florent Peyraud, with senior authors Alban Bessede (Explicyte) and Prof. Antoine Italiano (Institut Bergonié, University of Bordeaux) — takes stock of a decade of effort to drug the tryptophan–kynurenine–AhR axis in oncology. Explicyte scientists Jean-Philippe Guégan, Dominique Bodet, and Alban Bessede contributed to the conception, analysis, and writing, bringing the group’s immuno-oncology and tumor-microenvironment expertise to a synthesis of the preclinical biology, the clinical-trial record, and the strategies now moving toward the clinic.
The review lays out how IDO1, IDO2, and TDO2 degrade tryptophan to kynurenine, starving tumor-infiltrating T cells through GCN2 activation and mTORC1 suppression while kynurenine engages AhR to expand Tregs, tolerize dendritic cells, and drive PD-1 and PD-L1 expression. Over 90% of dietary tryptophan runs through this pathway, positioning it as a central immunometabolic brake in the tumor microenvironment.
Despite strong preclinical synergy, the phase III ECHO-301/KEYNOTE-252 trial found that epacadostat plus pembrolizumab gave no PFS or OS advantage over pembrolizumab alone in metastatic melanoma (mPFS 4.7 vs 4.9 months, HR 1.00; OS HR 1.13). The review attributes the collapse to underdosing, unmeasured intratumoral kynurenine, and the later finding that epacadostat and navoximod can themselves act as AhR agonists — undercutting their own mechanism.
Recombinant kynureninases degrade extracellular kynurenine regardless of whether IDO1 or TDO2 produced it. In syngeneic melanoma, colon (CT26), and breast (4T1) models, kynureninase combined with checkpoint blockade or a cancer vaccine drove tumor growth inhibition and survival benefit, and outperformed epacadostat plus anti-PD-1 in colon cancer — with a pharmacologically optimized human enzyme advancing toward clinical development.
Because AhR sits downstream of every kynurenine source, selective AhR antagonists could neutralize immunosuppression regardless of ligand origin. Blockers such as BAY-218 increased anti-PD-L1 activity in the CT26 model, and clinical-stage AhR inhibitors including BAY2416964 and IK-175 entered phase I trials in combination with checkpoint inhibitors.
The review argues that next-generation trials must stratify patients by tumor enzyme expression and measure intratumoral — not just serum — kynurenine. It points to dual IDO1/TDO2 inhibitors (RG70099, EPL-1410, CB548, CMG017) and IDO1-targeting vaccines such as IO102/IO103, which with nivolumab reached an 80% overall response rate (43% complete responses) in treatment-naive metastatic melanoma.
The failure of IDO1 inhibitors didn't close the book on tryptophan metabolism — it reframed the problem. The bottleneck is intratumoral kynurenine and AhR signaling, not the enzyme in isolation, which redirects development toward downstream targets and better pharmacodynamic readouts.
For drug developers, the lesson is mechanistic: measure intratumoral kynurenine and screen candidate molecules for off-target AhR agonism before committing to late-stage combinations. Kynureninases and AhR antagonists offer ligand-agnostic ways to release the same immunometabolic brake, and stratifying patients by tumor enzyme expression and TME catabolite levels is now essential to credible trial design. The tryptophan axis remains a live target — but only for programs built on tumor-microenvironment pharmacodynamics rather than serum surrogates.
