(S)-IDO1-IN-5 (Example 1B) is an active S-isomer of IDO1-IN-5. (S)-IDO1-IN-5 binds to IDOL with an IC50 value less than 1.5 μΜ. IDO1-IN-5 is a potent, selective and brain penetrated inhibitor of Indoleamine 2,3-Dioxygenase 1 (IDO1) activity, binds to apo-IDO1 lacking heme rather than mature heme-bound IDO1[2].