Probes for imaging huntingtin protein

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

• wherein
• 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 aryl and heteroaryl, and heterocycloalkenyl, each of which is optionally substituted with one or two groups independently chosen from alkynyl, heteroaryl, cyano, optionally substituted amino, halo, lower alkyl, and lower alkyl substituted with optionally substituted amino;
• L1 is chosen from O and NR4;
• R4 is chosen from hydrogen and lower alkyl;
• L2 is (CH2)m where m is 0, 1, or 2;
• and
• R2 is chosen from hydrogen, aryl, aryl substituted with hydroxyl or lower alkoxy, heteroaryl, and heteroaryl substituted with hydroxyl or lower alkoxy,
• R5 is chosen from lower alkyl, lower alkoxy, halo, and oxo (as a substituent on the heterocycloalkyl ring);
• and
• n is 0 or 1;
• wherein the compound of Formula I, or a pharmaceutically acceptable salt there of, is labeled with one or more positron-emitting radionuclides.