Modulating IDO1 and TDO Inhibition Through Structural Modification of Diaryl Hydroxylamines

Diaryl hydroxylamines have emerged as promising scaffolds for targeting the tryptophan-catabolizing enzymes indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan 2,3-dioxygenase (TDO), key drivers of tumor immune escape. Building on the reported dual IDO1/TDO inhibitory activity of O-((3,5-difluorophenyl)(phenyl)methyl)hydroxylamine, a series of diaryl hydroxylamines and related analogs were designed and synthesized to probe structure-activity relationships governing dioxygenase inhibition. Structural modifications included variation of the aryl framework toward heteroaromatic and highly electron-deficient motifs, amide-linked scaffold elongation, bioisosteric replacement of the hydroxylamine group with thiol or oxime functionalities, and O-benzylhydroxylamine derivatives. Inhibition studies revealed that 1,1'-diaryl hydroxylamines bearing five-membered heterocycles and electron-deficient aryl groups favored micromolar to submicromolar inhibition of both enzymes, whereas scaffold elongation promoted pronounced IDO1 preference and thiol or oxime analogs exhibited diminished potency. Among the compounds evaluated, O-benzylhydroxylamine 28l (IDO1/TDO IC₅₀ = 0.031/2.9 μM) and 1,1'-diaryl hydroxylamine 16j (IDO1/TDO IC₅₀ = 0.18/5.5 μM) displayed the most favorable activity profiles. This SAR study defines key determinants of IDO1 potency while retaining significant TDO inhibition and identifies fluorinated hydroxylamines as potential starting points for future PET imaging probe development.