A compound of Formula I: or a pharmaceutically acceptable salt thereof, wherein: R 1 is a substituent selected from the group consisting of (C 3 -C 8)cycloalkyl, (4- to 10-membered)heterocycloalkyl, (C 6 -C 10)aryl, and (5- to 10-membered)heteroaryl, wherein the (C 3 -C 8)cycloalkyl, (4- to 10-membered)heterocycloalkyl, (C 6 -C 10)aryl and (5- to 10-membered)heteroaryl are optionally substituted with one to five R 5; R 2 and R 3 are each independently selected from the group consisting of hydrogen, optionally substituted (C 1 -C 6)alkyl, (C 3 -C 8)cycloalkyl, (4- to 10-membered)heterocycloalkyl, (C 6 -C 10)aryl, and (5- to 10-membered)heteroaryl, wherein the (C 3 -C 8)cycloalkyl, (4- to 10-membered)heterocycloalkyl, (C 6 -C 10)aryl, and (5- to 10-membered)heteroaryl are optionally substituted with one to five R 6; or R 2 and R 3 taken together with the nitrogen to which they are attached form a (4- to 10-membered)heterocycloalkyl or a (5- to 10-membered)heteroaryl, wherein the (4- to 10-membered)heterocycloalkyl and (5- to 10-membered)heteroaryl are optionally substituted with one to five R 6; when present, R 4, at each occurrence, is independently selected from the group consisting of halogen, cyano, hydroxy, -SF 5, nitro, optionally substituted (C 1 -C 6)alkyl, optionally substituted (C 2 -C 6)alkenyl, optionally substituted (C 2 -C 6)alkynyl, optionally substituted (C 1 -C 6)alkylthio, optionally substituted (C 1 -C 6)alkoxy, -N(R 7)(R 8), -N(R 7)(C= R 8), -C(=O)N(R 7)(R 8), -O-C(=O)-N(R 7)(R 8), -C(=O)-R, and -C(=O)-OR 7; when present, R 5 and R 6, at each occurrence, are independently selected from the group consisting of halogen, oxo, cyano, hydroxy, -SF 5, nitro, optionally substituted (C 1 -C 6)alkyl, optionally substituted (C 2 -C 6)alkenyl, optionally substituted (C 2 -C 6)alkynyl, optionally substituted (C 1 -C 6)alkylthio, optionally substituted (C 1 -C 6)alkoxy, -N(R 7)(R 8), -N(R 7)(C= R 8), -C(=O)N(R 7)(R 8), -O-C(=O)-N(R 7)(R 8), -C(=O)-R 7, and -C(=O)-OR 7; R 7 and R 8 at each occurrence are independently selected from the group consisting of hydrogen and (C 1 -C 6)alkyl; and a is represented by an integer selected from 0, 1, 2 or 3. R 1 is a substituent selected from the group consisting of (C 3 -C 8)cycloalkyl, (4- to 10-membered)heterocycloalkyl, (C 6 -C 10)aryl, and (5- to 10-membered)heteroaryl, wherein the (C 3 -C 8)cycloalkyl, (4- to 10-membered)heterocycloalkyl, (C 6 -C 10)aryl and (5- to 10-membered)heteroaryl are optionally substituted with one to five R 5; R 2 and R 3 are each independently selected from the group consisting of hydrogen, optionally substituted (C 1 -C 6)alkyl, (C 3 -C 8)cycloalkyl, (4- to 10-membered)heterocycloalkyl, (C 6 -C 10)aryl, and (5- to 10-membered)heteroaryl, wherein the (C 3 -C 8)cycloalkyl, (4- to 10-membered)heterocycloalkyl, (C 6 -C 10)aryl, and (5- to 10-membered)heteroaryl are optionally substituted with one to five R 6; or R 2 and R 3 taken together with the nitrogen to which they are attached form a (4- to 10-membered)heterocycloalkyl or a (5- to 10-membered)heteroaryl, wherein the (4- to 10-membered)heterocycloalkyl and (5- to 10-membered)heteroaryl are optionally substituted with one to five R 6; when present, R 4, at each occurrence, is independently selected from the group consisting of halogen, cyano, hydroxy, -SF 5, nitro, optionally substituted (C 1 -C 6)alkyl, optionally substituted (C 2 -C 6)alkenyl, optionally substituted (C 2 -C 6)alkynyl, optionally substituted (C 1 -C 6)alkylthio, optionally substituted (C 1 -C 6)alkoxy, -N(R 7)(R 8), -N(R 7)(C= R 8), -C(=O)N(R 7)(R 8), -O-C(=O)-N(R 7)(R 8), -C(=O)-R, and -C(=O)-OR 7; when present, R 5 and R 6, at each occurrence, are independently selected from the group consisting of halogen, oxo, cyano, hydroxy, -SF 5, nitro, optionally substituted (C 1 -C 6)alkyl, optionally substituted (C 2 -C 6)alkenyl, optionally substituted (C 2 -C 6)alkynyl, optionally substituted (C 1 -C 6)alkylthio, optionally substituted (C 1 -C 6)alkoxy, -N(R 7)(R 8), -N(R 7)(C= R 8), -C(=O)N(R 7)(R 8), -O-C(=O)-N(R 7)(R 8), -C(=O)-R 7, and -C(=O)-OR 7; R 7 and R 8 at each occurrence are independently selected from the group consisting of hydrogen and (C 1 -C 6)alkyl; and a is represented by an integer selected from 0, 1, 2 or 3.