A compound of formula: or a pharmaceutically acceptable salt thereof, wherein: R 1, R 2, R 3 and R 4 are independently selected from: (i) H; (ii) C 1-3 alkyl, optionally substituted by: hydroxy, C 1-2 alkoxy, NH 2, phenyl, C 5-6 heteroaryl, C 1-4 alkyl carbamoyl, or acylamido; (iii) C 1-3 alkoxy, optionally substituted by C 3-6 cycloalkyl or by one or more fluoro groups; (iv) C 3-6 cycloalkyl; (v) halo; (vi) COR C, where R C is selected from NR N1 R N2, where R N1 and R N2 are independently selected from H and methyl; (vii) cyano, NH 2, or NO 2; and (viii) phenyl or C 5-6 heteroaryl, optionally substituted by methyl, hydroxy or methoxy; Ar is a phenyl, optionally substituted by one or more groups selected from: (i) C 1-4 alkyl, optionally substituted by hydroxy, C 1-2 alkoxy, NH 2, C 1-4 alkyl carbamoyl, or by one or more fluoro groups; (ii) C 3-6 cycloalkyl; (iii) hydroxy; cyano; NR N3 R N4, where R N3 and R N4 are independently selected from H and methyl; or acylamido; (iv) halo; (v) C 1-3 alkoxy, optionally substituted by hydroxy, C NH 2, C 3-6 cycloalkyl, phenyl, C 5-6 heteroaryl, or by one or more fluoro groups; (vi) phenoxy, optionally substituted by fluoro; (vii) phenyl or C 5-6 heteroaryl; or (viii) SF 5 or SO 2 CH 3; and wherein at least one of R 1, R 2, R 3 and R 4 is not H. R 1, R 2, R 3 and R 4 are independently selected from: (i) H; (ii) C 1-3 alkyl, optionally substituted by: hydroxy, C 1-2 alkoxy, NH 2, phenyl, C 5-6 heteroaryl, C 1-4 alkyl carbamoyl, or acylamido; (iii) C 1-3 alkoxy, optionally substituted by C 3-6 cycloalkyl or by one or more fluoro groups; (iv) C 3-6 cycloalkyl; (v) halo; (vi) COR C, where R C is selected from NR N1 R N2, where R N1 and R N2 are independently selected from H and methyl; (vii) cyano, NH 2, or NO 2; and (viii) phenyl or C 5-6 heteroaryl, optionally substituted by methyl, hydroxy or methoxy; (i) H; (ii) C 1-3 alkyl, optionally substituted by: hydroxy, C 1-2 alkoxy, NH 2, phenyl, C 5-6 heteroaryl, C 1-4 alkyl carbamoyl, or acylamido; hydroxy, C 1-2 alkoxy, NH 2, phenyl, C 5-6 heteroaryl, C 1-4 alkyl carbamoyl, or acylamido; (iii) C 1-3 alkoxy, optionally substituted by C 3-6 cycloalkyl or by one or more fluoro groups; (iv) C 3-6 cycloalkyl; (v) halo; (vi) COR C, where R C is selected from NR N1 R N2, where R N1 and R N2 are independently selected from H and methyl; (vii) cyano, NH 2, or NO 2; and (viii) phenyl or C 5-6 heteroaryl, optionally substituted by methyl, hydroxy or methoxy; Ar is a phenyl, optionally substituted by one or more groups selected from: (i) C 1-4 alkyl, optionally substituted by hydroxy, C 1-2 alkoxy, NH 2, C 1-4 alkyl carbamoyl, or by one or more fluoro groups; (ii) C 3-6 cycloalkyl; (iii) hydroxy; cyano; NR N3 R N4, where R N3 and R N4 are independently selected from H and methyl; or acylamido; (iv) halo; (v) C 1-3 alkoxy, optionally substituted by hydroxy, C NH 2, C 3-6 cycloalkyl, phenyl, C 5-6 heteroaryl, or by one or more fluoro groups; (vi) phenoxy, optionally substituted by fluoro; (vii) phenyl or C 5-6 heteroaryl; or (viii) SF 5 or SO 2 CH 3; (i) C 1-4 alkyl, optionally substituted by hydroxy, C 1-2 alkoxy, NH 2, C 1-4 alkyl carbamoyl, or by one or more fluoro groups; (ii) C 3-6 cycloalkyl; (iii) hydroxy; cyano; NR N3 R N4, where R N3 and R N4 are independently selected from H and methyl; or acylamido; (iv) halo; (v) C 1-3 alkoxy, optionally substituted by hydroxy, C NH 2, C 3-6 cycloalkyl, phenyl, C 5-6 heteroaryl, or by one or more fluoro groups; (vi) phenoxy, optionally substituted by fluoro; (vii) phenyl or C 5-6 heteroaryl; or (viii) SF 5 or SO 2 CH 3; and wherein at least one of R 1, R 2, R 3 and R 4 is not H.