Abstract
BACKGROUND AND PURPOSE Pharmacological inhibition of
indoleamine-2,3-dioxygenase 1 activity is now considered to be a
potential therapeutic tool for cancer therapy. However, the anti-cancer
efficacy may be the biggest obstacle for the clinical application of
current IDO1 inhibitors. EXPERIMENTAL APPROACHES HeLa cell-based
IDO1/Kyn assay as well as recombinant IDO1 activity assay were used to
determine the IDO1 enzyme activity. Interaction was examined by
UV-visible spectra, isothermal titration calorimetry assay, cellular
thermal shift assay and co-crystallization. Mouse colon cancer CT26
syngeneic model and azoxymethane/dextran sulfate sodium induced colon
carcinogenesis model were employed to confirm the anti-tumor effect in
vivo. KEY RESULTS B37 effectively and specifically inhibited IDO1 by
targeting its heme-free conformation (apo-IDO1). By competing with heme
for binding to apo-IDO1, B37 potently inhibited IDO1 activity with IC50
for 22 pM in the HeLa cell based assay. X-ray co-crystal structures of
the inhibitor-enzyme complexes showed that unlike the hIDO1-BMS-986205
complex, the B37-hIDO1 complex displayed stronger hydrophobic
interactions, which enhanced its binding affinity measured with ITC.
Accordingly, stronger non-covalent interactions including π stacking and
hydrogen bonds formed between B37 and apo-hIDO1 underlay the
enthalpy-driven force for B37 to bind the enzyme. This binding model
endowed B37 potent anti-tumor efficacy in mouse colon cancer CT26
syngeneic model and azoxymethane/dextran sulfate sodium induced colon
carcinogenesis model by activating the host’s immune system. Moreover,
the combination of B37 with a VEGFR2 inhibitor apatinib synergistically
inhibited tumor growth. CONCLUSIONS AND IMPLICATIONS These results
revealed that B37 may serve as a candidate for apo-IDO1 inhibition
mediated immunotherapy.