Probes for imaging huntingtin protein

An imaging agent comprising a compound of Formula I,

• or a pharmaceutically acceptable salt thereof, wherein
• L1 is —CH═CH— or L1 is absent;
• R1 is chosen from phenyl or heteroaryl, each of which is optionally substituted with one, two, or three groups independently chosen from
• cyano,
• halo,
• heteroaryl,
• lower alkyl,
• lower alkyl substituted with one or two substituents independently chosen from lower alkoxy substituted with heteroaryl,
• —C(O)O-lower alkyl,
• hydroxyl,
• lower alkynyloxy,
• lower alkoxy, and
• lower alkoxy substituted with one or two substituents independently chosen from
• halo,
• heterocycloalkyl,
• heteroaryl,
• heteroaryl substituted with lower alkoxy,
• optionally substituted amino,
• alkyl substituted with heteroaryl, and
• alkyl substituted with heteroaryl substituted with lower alkoxy;
• or
• R1 is phenyl substituted with two groups, which taken together with the carbon atoms, to which they are bonded form a heterocycloalkenyl ring wherein said phenyl is further optionally substituted with a substituent chosen from
• halo,
• heteroaryl, and
• optionally substituted amino;
• L2 is —N(R4)— or L2 is absent;
• R2 is chosen from
• hydrogen,
• lower alkyl, and
• lower alkyl substituted with lower alkoxy, amino, (alkyl)amino, (dialkyl)amino, or hydroxy;
• for each occurrence, R3 is independently chosen from
• halo,
• cyano,
• lower alkoxy,
• lower alkyl optionally substituted with amino, (alkyl)amino, or di(alkyl)amino, and ethynyl optionally substituted with tri(alkyl)silyl;
• R4 is chosen from hydrogen and lower alkyl;
• and
• m is 0, 1, or 2,
• wherein the compound of Formula I, or a pharmaceutically acceptable salt thereof, is labeled with one or more positron-emitting radionuclides.