A compound of formula I, or a pharmaceutically acceptable salt or solvate thereof, wherein: R is Cl or CN; R 1 and R 2 are each independently H or D; A is pyridyl, pyrimidinyl or pyrazinyl; R 3 represents one or more substituents, each being independently selected from the group consisting of H, D, halogen, cyano, hydroxyl, trifluoromethyl, methyl optionally substituted by one or more D, methylol, methoxymethyl optionally substituted by one or more D, acetoxymethyl, methoxy optionally substituted by one or more D, methoxyethoxy, amino, methylamino, dimethylamino, t -butylamino, cyclopropylamino, epoxypropylamino, acetylamino, oxetanylamino, methoxymethylamino, methoxyethylamino, methoxypropylamino, ethoxyethylamino, ethoxypropylamino, methoxycarbonylethylamino, methoxycarbonylpropylamino, ethoxycarbonylmethylamino, ethoxycarbonylethylamino, ethoxycarbonylpropylamino, hydroxyethylamino, hydroxypropylamino, hydroxybutylamino, cyclobutylamino, azetidinyl, N,N-dimethylaminomethylamino, N,N-dimethylaminoethylamino, N,N-dimethylaminopropylamino, N,N-dimethylaminobutylamino, t- butoxyacylaminobutylamino, 5-morpholinylmethyl, ethylaminoethylamino, ethylaminopropylamino, ethylaminobutylamino, methoxyethoxyethoxyethylamino, ethoxyethoxyethylamino, t -butoxycarbonylethylamino, t -butoxycarbonylpropylamino, t- butoxycarbonylmethoxyethylamino, carboxylethylamino, carboxylpropylamino, carboxymethoxyethylamino, 6-morpholinylmethyl, 6-(tetrahydrofuranyl)methylamino, methylsulfonylethylamino, (2,2-dimethyl-[1,3]-dioxolan-4-yl)-methylamino, 2,3-dihydroxypropylamino, 2-hydroxypropylamino, trifluoromethylethylamino; R 4 represents one or more substituents, each being independently selected from the group consisting of H, halogen, methyl, methoxy, cyano and trifluoromethyl. R is Cl or CN; R 1 and R 2 are each independently H or D; A is pyridyl, pyrimidinyl or pyrazinyl; R 3 represents one or more substituents, each being independently selected from the group consisting of H, D, halogen, cyano, hydroxyl, trifluoromethyl, methyl optionally substituted by one or more D, methylol, methoxymethyl optionally substituted by one or more D, acetoxymethyl, methoxy optionally substituted by one or more D, methoxyethoxy, amino, methylamino, dimethylamino, t -butylamino, cyclopropylamino, epoxypropylamino, acetylamino, oxetanylamino, methoxymethylamino, methoxyethylamino, methoxypropylamino, ethoxyethylamino, ethoxypropylamino, methoxycarbonylethylamino, methoxycarbonylpropylamino, ethoxycarbonylmethylamino, ethoxycarbonylethylamino, ethoxycarbonylpropylamino, hydroxyethylamino, hydroxypropylamino, hydroxybutylamino, cyclobutylamino, azetidinyl, N,N-dimethylaminomethylamino, N,N-dimethylaminoethylamino, N,N-dimethylaminopropylamino, N,N-dimethylaminobutylamino, t- butoxyacylaminobutylamino, 5-morpholinylmethyl, ethylaminoethylamino, ethylaminopropylamino, ethylaminobutylamino, methoxyethoxyethoxyethylamino, ethoxyethoxyethylamino, t -butoxycarbonylethylamino, t -butoxycarbonylpropylamino, t- butoxycarbonylmethoxyethylamino, carboxylethylamino, carboxylpropylamino, carboxymethoxyethylamino, 6-morpholinylmethyl, 6-(tetrahydrofuranyl)methylamino, methylsulfonylethylamino, (2,2-dimethyl-[1,3]-dioxolan-4-yl)-methylamino, 2,3-dihydroxypropylamino, 2-hydroxypropylamino, trifluoromethylethylamino; R 4 represents one or more substituents, each being independently selected from the group consisting of H, halogen, methyl, methoxy, cyano and trifluoromethyl.