Electrochemical Analysis and Inhibition Assay of Immune-Modulating Enzyme, Indoleamine 2,3-Dioxygenase

<b>Background:</b> An accurate and rapid analysis of human indoleamine 2,3-dioxygenase (hIDO) is crucial for the development of anticancer pharmaceuticals because of the role of hIDO in promoting tumoral immune escape. However, the conventional assay of hIDO is limited by interference from reductants, which are used to reduce the heme iron to begin the hIDO catalytic reaction. <b>Methods:</b> A direct electrochemical method was applied to drive the hIDO reaction. <b>Results:</b> The nanostructured gold electrode enabled the electrochemical reduction of the heme iron of hIDO1. In the presence of substrates (tryptophan and oxygen), a bioelectrocatalytic current was observed, confirming an electrochemically driven hIDO reaction. A well-known inhibitor of hIDO, epacadostat, hindered this catalytic signal according to its concentration, demonstrating the rapid evaluation of its inhibition activity for the hIDO reaction. Through an in silico study using the proposed electrochemical assay system, we discovered a strong inhibitor candidate with a half-maximal inhibitory concentration of 10 nM. <b>Conclusions:</b> An accurate and rapid assay system in drug discovery for hIDO and kynureine pathway-targeted immunotherapy has been developed.