The disclosure relates to novel compounds of the formula (I) which are inhibitors of phosphodiesterase type 10A and to their use for the manufacture of a medicament and which thus are suitable for treating or controlling of medical disorders selected from neurological disorders and psychiatric disorders, for ameliorating the symptoms associated with such disorders and for reducing the risk of such disorders. wherein Q is O or S, X1 is N or CH, X2 is N or C-R7 X3 is O, S, -X4=C(R8)-, where C(R8) is bound to the carbon atom which carries R2, or -X5=C(R9)-, where X5 is bound to the carbon atom which carries R2 X4 is N or C-R9 X5 is N Het is selected from optionally substituted phenyl, monocyclic hetaryl and fused bicyclic hetaryl R1 is selected inter alia from hydrogen, halogen, OH, C1-C4-alkyl, trimethylsilyl, C1-C4-alkylsulfanyl, C1-C4-alkoxy-C1-C4-alkyl, C1-C4- alkoxy, C1-C4-alkoxy-C1-C4-alkoxy, the moiety Y1-Cyc1 R2 is selected inter alia from hydrogen, halogen, OH, C1-C4-alkyl, trimethylsilyl, C1-C4-alkoxy-C1-C4-alkyl, C1-C4-alkoxy, C1-C4- alkoxy-C1-C4-alkoxy, C2-C4-alkenyloxy, etc A represents one of the following groups A1, A2, A3, A4 or A5: where * indicates the points of attachment to Het and to the nitrogen atom, respectively and where R3 to R9, R3e, R3f, A&rsquo, Y1 and Cyc are defined in the claims. Exemplary compounds are: 3,7-di(pyridin-4-yl)-5-[2-(quinolin-2-yl)ethyl]thieno[2,3-d]pyridazin-4(5H)-one 8-(furan-3-yl)-2-[2-(quinolin-2-yl)ethyl]phthalazin-1(2H)-one 2-[2-(imidazo[l,2-a]pyridin-2-yl)ethyl]-8-(morpholin-4-yl)phthalazin-l(2H)-one.
