Substituted pyrido-pyridazine derivatives which enhance cognition via the gaba-a receptors

A compound of formula I, or a pharmaceutically acceptable salt thereof:

• wherein:
• X1 represents hydrogen, halogen, C1-6 alkyl, trifluoromethyl or C1-6 alkoxy;
• X2 represents hydrogen or halogen;
• Y represents a chemical bond, an oxygen atom, or a —NH— linkage;
• Z represents an optionally substituted aryl or heteroaryl group;
• R1 represents hydrogen, hydrocarbon, a heterocyclic group, halogen, cyano, trifluoromethyl, nitro, —ORa, —SRa, —SORa, —SO2Ra, —SO2NRaRb, —NRaRb, —NRaCORb, —NRaCO2Rb, —CORa, —CO2Ra, —CONRaRb or —CRa═NORb;
• and
• Ra and Rb independently represent hydrogen, hydrocarbon or a heterocyclic group.

The compound of claim 1 of formula IIA:

• wherein:
• X11 represents hydrogen, fluoro, chloro, methyl, trifluoromethyl or methoxy;
• X12 represents hydrogen or fluoro;
• R11 represents hydrogen, C1-6 alkyl, halo(C1-6)alkyl, dihalo(C1-6)alkyl, hydroxy(C1-6)alkyl, dihydroxy(C1-6)alkyl, C,1-6 alkoxy(C1-6)alkyl, di(C1-6)alkoxy(C1-6)alkyl, cyano(C1-6)alkyl, C2-6 alkoxycarbonyl(C1-6)alkyl, C3-7 cycloalkyl, heteroaryl, C1-6 alkyl-heteroaryl, heteroaryl(C1-6)alkyl, halogen, cyano, trifluoromethyl, formyl, C2-6 alkylcarbonyl, C2-6 alkoxycarbonyl or —CR4═NOR5;
• R4 represents hydrogen or C1-6 alkyl;
• and
• R5 represents hydrogen, C1-6 alkyl, hydroxy(C1-6)alkyl or di(C1-6)alkylamino(C1-6)alkyl.