A compound of formula: or a stereoisomer thereof, or a pharmaceutically acceptable salt thereof, wherein R 1 is: (a) (b) wherein ring A is C 3-8 cycloalkyl or 3-7 membered heterocyclyl; the hydroxy moiety is bonded to the A ring either on the carbon atom bonded to the 5H-imidazo[5,1-a]isoindolyl or on a carbon atom adjacent to the carbon atom bonded to the 5H-imidazo[5,1-a]isoindolyl; and each R a is independently a substituent of the A ring or the Bring; (c) wherein ring A and ring B form a spirocyclic system; ring A is C 3-8 cycloalkyl or 3-7 membered heterocyclyl; the hydroxy moiety is bonded to the A ring either on the carbon atom bonded to the 5H-imidazo[5,1-a]isoindolyl or on a carbon atom adjacent to the carbon atom bonded to the 5H-imidazo[5,1-a]isoindolyl; and each R a is independently a substituent of the A ring or the B ring; or (d) wherein, unless stated otherwise above, ring A is C 3-10 cycloalkyl or 3-7 membered heterocyclyl; ring B is C 3-7 cycloalkyl, 3-7 membered heterocyclyl, aryl or heteroaryl; m is 0, 1, 2 or 3; R 4 is OH; each R a is independently selected from oxo, halogen, cyano, nitro, C 1-6 alkyl, -C 1-6 haloalkyl, C 1-6 alkyl-cyano, -OR, -NR 2, -SR, -C OR, -C N 2, -C R, -S R, -S OR, -S N 2, -S 2 R, -S 2 OR, -S 2 N 2, -OC R, -OC OR, -OC N 2, -N C R, -N C OR, and -N C N 2; n is 0 or 1; each R 2 is independently halogen, cyano, C 1-6 alkyl, C 3 cycloalkyl, -C 1-6 haloalkyl, -OR, -NR 2 or -SR; and each R is independently hydrogen, C 1-6 alkyl or C 1-6 haloalkyl. (a) (b) wherein ring A is C 3-8 cycloalkyl or 3-7 membered heterocyclyl; the hydroxy moiety is bonded to the A ring either on the carbon atom bonded to the 5H-imidazo[5,1-a]isoindolyl or on a carbon atom adjacent to the carbon atom bonded to the 5H-imidazo[5,1-a]isoindolyl; and each R a is independently a substituent of the A ring or the Bring; (c) wherein ring A and ring B form a spirocyclic system; ring A is C 3-8 cycloalkyl or 3-7 membered heterocyclyl; the hydroxy moiety is bonded to the A ring either on the carbon atom bonded to the 5H-imidazo[5,1-a]isoindolyl or on a carbon atom adjacent to the carbon atom bonded to the 5H-imidazo[5,1-a]isoindolyl; and each R a is independently a substituent of the A ring or the B ring; or (d) wherein, unless stated otherwise above, ring A is C 3-10 cycloalkyl or 3-7 membered heterocyclyl; ring B is C 3-7 cycloalkyl, 3-7 membered heterocyclyl, aryl or heteroaryl; m is 0, 1, 2 or 3; R 4 is OH; each R a is independently selected from oxo, halogen, cyano, nitro, C 1-6 alkyl, -C 1-6 haloalkyl, C 1-6 alkyl-cyano, -OR, -NR 2, -SR, -C OR, -C N 2, -C R, -S R, -S OR, -S N 2, -S 2 R, -S 2 OR, -S 2 N 2, -OC R, -OC OR, -OC N 2, -N C R, -N C OR, and -N C N 2; n is 0 or 1; each R 2 is independently halogen, cyano, C 1-6 alkyl, C 3 cycloalkyl, -C 1-6 haloalkyl, -OR, -NR 2 or -SR; and each R is independently hydrogen, C 1-6 alkyl or C 1-6 haloalkyl. ring A is C 3-10 cycloalkyl or 3-7 membered heterocyclyl; ring B is C 3-7 cycloalkyl, 3-7 membered heterocyclyl, aryl or heteroaryl; m is 0, 1, 2 or 3; R 4 is OH; each R a is independently selected from oxo, halogen, cyano, nitro, C 1-6 alkyl, -C 1-6 haloalkyl, C 1-6 alkyl-cyano, -OR, -NR 2, -SR, -C OR, -C N 2, -C R, -S R, -S OR, -S N 2, -S 2 R, -S 2 OR, -S 2 N 2, -OC R, -OC OR, -OC N 2, -N C R, -N C OR, and -N C N 2; n is 0 or 1; each R 2 is independently halogen, cyano, C 1-6 alkyl, C 3 cycloalkyl, -C 1-6 haloalkyl, -OR, -NR 2 or -SR; and each R is independently hydrogen, C 1-6 alkyl or C 1-6 haloalkyl.