LY3381916 binds to newly synthesized apo-IDO1 lacking heme, but does not inhibit mature heme-bound IDO1. Protein x-ray crystallography confirms that LY3381916 binds to apo-IDO1 where it occupies the heme-binding pocket of IDO1. Modeling of the pre-clinical PK/PD relationship suggests QD dosing of LY3381916 will maintain greater than 90% inhibition over 24 hours. In addition, due to the favorable properties of the drug, significant central nervous system (CNS) penetration has been measured for LY3381916.