Substituted [1.1.1] bicyclo compounds as indoleamine 2,3-dioxygenase inhibitors

A compound of formula (I), or a pharmaceutically acceptable salt thereof:

• wherein:
• X is selected from a bond and —CH(Ra)—;
• where Ra is selected from hydrogen and C1-6 alkyl;
• Rb is selected from:
• (i) hydrogen and (ii) C1-6 alkyl;
• each occurrence of R1 and R2 is independently selected from:
• (i) hydrogen and (ii) C1-6 alkyl;
• and
• each occurrence of R3 and R4 is independently selected from:
• (i) aryl, and
• (ii) heterocyclyl, wherein the heterocyclyl is selected from furanyl, imidazolyl, isothiazolyl, isoxazolyl, oxadiazolyl, oxazolyl, naphthyridinyl, phthalazinyl, pyranyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrimidinyl, quinazolinyl and a fused bicyclic ring moiety wherein a 6-membered heterocyclic ring comprising 2 nitrogen atoms and a 5-membered carbocyclic ring are connected through two atoms;
• wherein each of the aryl of (i) and heterocyclyl of (ii) is optionally substituted with one to four substituents independently selected from:
• (a) halogen,
• (b) C1-6 alkyl, optionally substituted with one to four halogens, and
• (c) —CN.