Probes for imaging huntingtin protein

An imaging agent comprising a compound of Formula (I), or a pharmaceutically acceptable salt thereof,

• wherein
• X is chosen from NR4, O and S;
• Y is chosen from CR4 and N;
• Z1, Z2, Z3, and Z4 are independently chosen from CH and N, provided that at least two of Z1, Z2, Z3, and Z4 are CH;
• R1 is chosen from heteroaryl, heterocycloalkenyl, and heterocycloalkyl, each of which is optionally substituted with one or two groups independently chosen from cyano, halo, lower alkyl optionally substituted with amino, alkylamino, or di(alkyl)amino, lower alkoxy optionally substituted with lower alkoxy, optionally substituted amino, haloalkyl, di(alkyl)aminocarbonyl, alkylaminocarbonyl, and aminocarbonyl, or
• R1 is phenyl optionally substituted with one or two groups independently chosen from cyano, heteroaryl, halo, phenoxy, benzyloxy, heteroaryl, lower alkyl optionally substituted with amino, (alkyl)amino, or di(alkyl)amino, lower alkoxy, optionally substituted amino, di(alkyl)aminocarbonyl, alkylaminocarbonyl, and aminocarbonyl;
• L1 is —O— and L2 is —(CR7R8)m— or —(CR7R8)m—O—;
• or
• L1 is —NR3— and L2 is —C(O)— or —(R7R8)m—;
• or
• L1 is —NR3— and L2 is —C(O)(O)(R7R8)m—;
• or
• L1 is —NR3— and L2 is —C(O)(R7R8)m(O)—;
• or
• L1 is —NR3— and L2 is —C(O)(R7R8)m—;
• or
• L1 is —NR3— and L2 is —C(O)CR7═CR8—;
• or
• L1 is —C(O)— and L2 is —NR3;
• or
• L1 is —(R7R8)m— and L2 is —NR3—, —C(O)— or —O—;
• or
• L1 is absent and L2 is absent;
• or
• L1 taken together with L2 is —CH═CH—, —C≡C—, or heterocyclylene;
• L3 is —CH═CH—, or L3 is absent;
• R2 is chosen from heterocycloalkyl, aryl and heteroaryl, each of which is optionally substituted with one or two groups chosen from
• —OC(O)—R6,
• —C(O)O—R6,
• amino,
• halo,
• haloalkyl,
• phenyl,
• heteroaryl,
• cyano,
• (lower alkyl)thio,
• phenoxy,
• phenoxymethyl,
• heteroaryloxy,
• heteroaryloxy substituted with lower alkyl,
• hydroxyl,
• lower alkenyloxy,
• lower alkoxy,
• lower alkoxy substituted with lower alkoxy, amino, (alkyl)amino, (dialkyl)amino, heterocycloalkyl, heteroaryl, or halo,
• lower alkyl, and
• lower alkyl substituted with amino, (alkyl)amino, (dialkyl)amino, hydroxyl or lower alkoxy;
• R3 is chosen from hydrogen and lower alkyl;
• R4 is chosen from hydrogen, halo, cyano, and lower alkyl;
• R5 is chosen from lower alkyl, lower alkoxy, and halo;
• R6 is lower alkyl;
• R7 is chosen from hydrogen, hydroxyl, trifluoromethyl, and lower alkyl;
• R8 is chosen from hydrogen and lower alkyl;
• n is 0 or 1;
• 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.