Bicyclic heterocycle derivatives having selective bace1 inhibitory activity

A compound of Formula (IA), (IB) or (IC):

• -A1- is alkylene optionally substituted with one or more halogen;
• R1 is a hydrogen atom, halogen, alkyl, haloalkyl or amino;
• R2 is substituted or unsubstituted alkyl;
• R3 and R4 are each independently a hydrogen atom, halogen, alkyl or haloalkyl;
• R5 is a hydrogen atom or halogen;
• A4 is N or CR6 wherein R6 is a hydrogen atom, halogen or substituted or unsubstituted alkyl;
• A6 is CR18 or N;
• R18 is a hydrogen atom;
• A4 and A6 are not simultaneously both N;
• A5 is NR7 or CR8R9;
• R7 is substituted or unsubstituted alkyl;
• R8 and R9 are each independently a hydrogen atom, halogen, alkyl or haloalkyl;
• R2, R3R4, R8 and R9 may be any one of (i) to (iv):
• (i) R2 and one of R3 and R4 may be taken together with the carbon atoms to which they are attached to form a carbocycle or a heterocycle;
• (ii) one of R3 and R4 and one of R8 and R9 may form alkylene wherein each carbon atom in the alkylene may be replaced with an oxygen atom or a nitrogen atom;
• the carbon atom(s) in the alkylene is each independently substituted with one or more group(s) selected from Ra;
• and the nitrogen atom(s) in the alkylene is each substituted with one or more group(s) selected from Rb;
• Ra is a hydrogen atom, halogen, hydroxy, cyano, or substituted or unsubstituted alkyl;
• Rb is a hydrogen atom or substituted or unsubstituted alkyl;
• (iii) R3 and R4 may be taken together with the carbon atom to which they are attached to form a carbocycle or a heterocycle;
• (iv) R8 and R9 may be taken together with the carbon atom to which they are attached to form a carbocycle or a heterocycle;
• R14 is each independently alkyl optionally substituted with one or more group(s) selected from halogen, cyano, alkyloxy, haloalkyloxy, and non-aromatic carbocyclyl;
• or heteroaryl optionally substituted with one or more alkyl;
• two R14s attached to a same carbon atom may be taken together with the carbon atom to which they are attached to form a 3- to 5-membered non-aromatic carbocycle optionally substituted with one or more group(s) selected from halogen, alkyl and haloalkyl;
• t is an integer from 0 to 3;
• R15 is alkyl optionally substituted with one or more group(s) selected from halogen;
• and
• R16 is substituted or unsubstituted alkyl or non-aromatic carbocyclyl;
• or a pharmaceutically acceptable salt thereof.