A heterocyclic derivative of formula I wherein R 1 is H, methyl, ethyl, propyl or isopropyl, optionally substituted with hydroxyl or methoxyl; R 2 is H, C 1-8 alkyl, C 3-8 cycloalkyl or C 3-8 cycloalkylC 1-2 alkyl, said C 1-8 alkyl, C 3-8 cycloalkyl and C 3-8 cycloalkylC 1-2 alkyl being optionally substituted with one or more substituent independently selected from halogen, hydroxyl and methoxy or R 2 is C 6-10 aryl optionally substituted with one or more substituent independently selected from halogen, hydroxy, cyano, C 1-8 alkyl, C 3 - 8 cycloalkyl, C 1-6 alkyloxy and C 3-6 cycloalkyloxy, said C 1-8 alkyl, C 3-8 cycloalkyl, C 1-6 alkyloxy and C 3-6 cycloalkyloxy being optionally substituted with one or more halogens or R 2 is a 5-10 membered heteroaryl ring system comprising a heteroatom selected from N, O and S and optionally substituted with methyl, C 1-6 alkyloxy, halogen or cyano; X 1 is CO or SO 2; X 2 is N; is a substituted arylene or heteroarylene fused to the pyrimidinone at adjacent carbon atoms and is selected from: R 7 is m R 8, wherein Y is O, NR 9 or CR 10 R 11; m is 0 or 1; R 8 is a 6-10 membered bridged or fused multicyclic saturated or partially unsaturated ring system comprising a N(R 12) n moiety and optionally comprising a N(R 13) p moiety, said bridged or fused multicyclic ring system being optionally substituted with methyl or hydroxyl; R 9 is H or C 1-6 alkyl; R 10 and R 11 are independently H or C 1-6 alkyl; R 12 and R 13 are independently H, C 1-6 alkyl or oxo; R 14 is a further optional substituent selected from methyl, halogen and cyano; n is 0 or 1 and p is 0 or 1; or a pharmaceutically acceptable salt or solvate thereof. R 1 is H, methyl, ethyl, propyl or isopropyl, optionally substituted with hydroxyl or methoxyl; R 2 is H, C 1-8 alkyl, C 3-8 cycloalkyl or C 3-8 cycloalkylC 1-2 alkyl, said C 1-8 alkyl, C 3-8 cycloalkyl and C 3-8 cycloalkylC 1-2 alkyl being optionally substituted with one or more substituent independently selected from halogen, hydroxyl and methoxy or R 2 is C 6-10 aryl optionally substituted with one or more substituent independently selected from halogen, hydroxy, cyano, C 1-8 alkyl, C 3 - 8 cycloalkyl, C 1-6 alkyloxy and C 3-6 cycloalkyloxy, said C 1-8 alkyl, C 3-8 cycloalkyl, C 1-6 alkyloxy and C 3-6 cycloalkyloxy being optionally substituted with one or more halogens or R 2 is a 5-10 membered heteroaryl ring system comprising a heteroatom selected from N, O and S and optionally substituted with methyl, C 1-6 alkyloxy, halogen or cyano; X 1 is CO or SO 2; X 2 is N; is a substituted arylene or heteroarylene fused to the pyrimidinone at adjacent carbon atoms and is selected from: R 7 is m R 8, wherein Y is O, NR 9 or CR 10 R 11; m is 0 or 1; R 8 is a 6-10 membered bridged or fused multicyclic saturated or partially unsaturated ring system comprising a N(R 12) n moiety and optionally comprising a N(R 13) p moiety, said bridged or fused multicyclic ring system being optionally substituted with methyl or hydroxyl; R 9 is H or C 1-6 alkyl; R 10 and R 11 are independently H or C 1-6 alkyl; R 12 and R 13 are independently H, C 1-6 alkyl or oxo; R 14 is a further optional substituent selected from methyl, halogen and cyano; n is 0 or 1 and p is 0 or 1;