Capatina, A. L. L.; Czechowski, T.; Plunkett-Jones, C.; Tonon, T.; Kourtzelis, I.; Lichman, B. R.; Brackenbury, W. J.; Graham, I. A.; Lagos, D.

L-Tryptophan (Trp) is an essential amino acid, catabolised through the kynurenine pathway, which is mediated by the enzymes indoleamine-2,3-dioxygenase 1 (IDO1), IDO2, or Trp-2,3-deoxygenase (TDO). Therapeutic targeting of Trp metabolism could be relevant to several pathologies. In cancer, IDO1 acts as an immune checkpoint suppressing effector T cell function. Yet, direct inhibition of IDO1 has had limited success in clinical trials. Therefore, alternative approaches to Trp metabolism therapeutic targeting are needed. We screened a library of 597 natural products (NPs) or NP derivatives for their effect on kynurenine production in triple negative breast cancer cells. This revealed 24 candidate inhibitors of kynurenine production. Amongst them, artemether, a member of the artemisinin family of anti-malarial drugs, suppressed kynurenine production, likely via an endoperoxide bridge-dependent mechanism. The Euphorbia factor L9 (EFL9) inhibited kynurenine production likely via a C7-benzoylation-dependent mechanism. Neither artemether nor EFL9 affected JAK/STAT signaling or IDO1 levels. Targeted metabolomics analyses demonstrated that artemether suppressed kynurenine production through heme sequestration, a mechanism that would affect all members of the IDO/TDO family of metalloenzymes. EFL9 affected purine and amino acid metabolism and the cellular redox balance. Comparisons to the effects of ouabain, a NP regulator of IDO1 levels, and Linrodostat, a clinically used small molecule IDO1 inhibitor, revealed distinct metabolic profiles, with ouabain and EFL9 showing the largest overlap. Importantly, the kynurenine-suppressing activity of artemether and EFL9 is not cancer cell-specific. Overall, our findings set the foundation for the use of derivatives of artemether or EFL9 as novel Trp metabolism-targeting therapeutics.