An imaging agent comprising a compound of Formula: or a pharmaceutically acceptable salt thereof, wherein X is chosen from NR 4, O and S; Y is chosen from CR 4 and N; Z 1, Z 2, Z 3, and Z 4 are independently chosen from CH and N, provided that at least two of Z 1, Z 2, Z 3, and Z 4 are CH; R 1 is chosen from heteroaryl, heterocycloalkenyl, and heterocycloalkyl, each of which is optionally substituted with one or two groups independently chosen from cyano, halo, C 1-6 alkyl optionally substituted with amino, alkylamino, or di(alkyl)amino, C 1-6 alkoxy optionally substituted with C 1-6 alkoxy, optionally substituted amino, haloalkyl, di(alkyl)aminocarbonyl, alkylaminocarbonyl, and aminocarbonyl, or R 1 is phenyl optionally substituted with one or two groups independently chosen from cyano, heteroaryl, halo, phenoxy, benzyloxy, heteroaryl, C 1-6 alkyl optionally substituted with amino, (alkyl)amino, or di(alkyl)amino, C 1-6 alkoxy, optionally substituted amino, di(alkyl)aminocarbonyl, alkylaminocarbonyl, and aminocarbonyl; L 1 is -O- and L 2 is -(CR 7 R 8) m -; L 3 is -CH=CH-, or L 3 is absent; R 2 is pyridin-2-yl or pyridin-3-yl, each of which is optionally substituted with one or two groups independently chosen from C 1-6 alkyl, hydroxyl, C 1-6 alkoxy, C 1-6 alkoxy substituted with amino, (alkyl)amino, (dialkyl)amino, or C 1-6 alkoxy, and C 1-6 alkyl substituted with hydroxyl, C 1-6 alkoxy, amino, (alkyl)amino, or (dialkyl)amino; R 4 is chosen from hydrogen, halo, cyano, and C 1-6 alkyl; R 5 is chosen from C 1-6 alkyl, C 1-6 alkoxy, and halo; R 7 is chosen from hydrogen, hydroxyl, trifluoromethyl, and C 1-6 alkyl; R 8 is chosen from hydrogen and C 1-6 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. X is chosen from NR 4, O and S; Y is chosen from CR 4 and N; Z 1, Z 2, Z 3, and Z 4 are independently chosen from CH and N, provided that at least two of Z 1, Z 2, Z 3, and Z 4 are CH; R 1 is chosen from heteroaryl, heterocycloalkenyl, and heterocycloalkyl, each of which is optionally substituted with one or two groups independently chosen from cyano, halo, C 1-6 alkyl optionally substituted with amino, alkylamino, or di(alkyl)amino, C 1-6 alkoxy optionally substituted with C 1-6 alkoxy, optionally substituted amino, haloalkyl, di(alkyl)aminocarbonyl, alkylaminocarbonyl, and aminocarbonyl, or R 1 is phenyl optionally substituted with one or two groups independently chosen from cyano, heteroaryl, halo, phenoxy, benzyloxy, heteroaryl, C 1-6 alkyl optionally substituted with amino, (alkyl)amino, or di(alkyl)amino, C 1-6 alkoxy, optionally substituted amino, di(alkyl)aminocarbonyl, alkylaminocarbonyl, and aminocarbonyl; L 1 is -O- and L 2 is -(CR 7 R 8) m -; L 3 is -CH=CH-, or L 3 is absent; R 2 is pyridin-2-yl or pyridin-3-yl, each of which is optionally substituted with one or two groups independently chosen from C 1-6 alkyl, hydroxyl, C 1-6 alkoxy, C 1-6 alkoxy substituted with amino, (alkyl)amino, (dialkyl)amino, or C 1-6 alkoxy, and C 1-6 alkyl substituted with hydroxyl, C 1-6 alkoxy, amino, (alkyl)amino, or (dialkyl)amino; R 4 is chosen from hydrogen, halo, cyano, and C 1-6 alkyl; R 5 is chosen from C 1-6 alkyl, C 1-6 alkoxy, and halo; R 7 is chosen from hydrogen, hydroxyl, trifluoromethyl, and C 1-6 alkyl; R 8 is chosen from hydrogen and C 1-6 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.