Condensed heterocyclic compound

1. A compound represented by the formula (I-i):

wherein

R 1 is a hydrogen atom or an optionally substituted C 1-6 alkyl group,

X 1 is CH or a nitrogen atom,

ring A optionally has a substituent,

X 2 is CR 2 or a nitrogen atom,

X 3 is CR 3 or a nitrogen atom,

X 4 is CR 4 or a nitrogen atom,

X 5 is CR 5 or a nitrogen atom,

R 2 , R 3 , R 4 and R 5 are each independently a hydrogen atom or a substituent,

ring B is an optionally substituted 5- or 6-membered nitrogen-containing heterocycle (said nitrogen-containing heterocycle optionally further forms a fused ring with a ring other than ring C 1 ),

Z 5 is ═N— or —N═,

ring C 1 is an optionally substituted 5-membered ring,

Z 1 , Z 3 ′ and Z 4 ′ are each independently a ring C 1 -constituting atom selected from a carbon atom and a nitrogen atom,

ring D is an optionally substituted 5- or 6-membered ring, and

is a single bond or a double bond,

or a salt thereof.

2. A compound represented by the formula (I-ii):

wherein

R 1 is a hydrogen atom or an optionally substituted C 1-6 alkyl group,

X 1 is CH or a nitrogen atom,

ring A optionally has a substituent,

X 2 is CR 2 or a nitrogen atom,

X 3 is CR 3 or a nitrogen atom,

X 4 is CR 4 or a nitrogen atom,

X 5 is CR 5 or a nitrogen atom,

R 2 , R 3 , R 4 and R 5 are each independently a hydrogen atom or a substituent, ring B is an optionally substituted 5- or 6-membered nitrogen-containing heterocycle (said nitrogen-containing heterocycle optionally further forms a fused ring with a ring other than ring C 2 ),

Z 5 is ═N— or —N═,

ring C 2 is an optionally substituted 5-membered ring,

Y is a ring C 2 -constituting atom selected from a carbon atom and a nitrogen atom,

Z 1 is a ring C 2 -constituting atom selected from a carbon atom and a nitrogen atom,

Z 3 ″ is a ring C 2 -constituting atom selected from a carbon atom, a nitrogen atom, a sulfur atom and an oxygen atom,

Z 4 ″ is —O—, —S—, —N═, —NR 6 ′—, —CR 7 ′═, —SO—, —SO 2 — or an optionally substituted methylene group (excluding a carbonyl group),

R 6 ′ and R 7 ′ are each independently a hydrogen atom or a substituent, and

is a single bond or a double bond,

or a salt thereof.

3. A compound represented by the formula (I-iii):

wherein

R 1 is a hydrogen atom or an optionally substituted C 1-6 alkyl group,

X 1 is CH or a nitrogen atom,

ring A optionally has a substituent,

X 2 is CR 2 or a nitrogen atom,

X 3 is CR 3 or a nitrogen atom,

X 4 is CR 4 or a nitrogen atom,

X 5 is CR 5 or a nitrogen atom,

R 2 , R 3 , R 4 and R 5 are each independently a hydrogen atom or a substituent, ring B is an optionally substituted, 5- or 6-membered nitrogen-containing heterocycle (said nitrogen-containing heterocycle optionally further forms a fused ring with a ring other than ring C 3 ),

Z 5 is ═N— or —N═,

ring C 3 is an optionally substituted 6-membered ring,

Y is a ring C 3 -constituting atom selected from a carbon atom and a nitrogen atom,

Z 1 is a ring C 3 -constituting atom selected from a carbon atom and a nitrogen atom,

Z 2 , Z 3 and Z 4 are each independently a ring C 3 -constituting atom selected from a carbon atom, a nitrogen atom, a sulfur atom and an oxygen atom, and

is a single bond or a double bond,

or Z 3 and Z 4 are optionally joined to form ring D, and

ring D is an optionally substituted 5- or 6-membered ring,

or a salt thereof.

4. The compound according to claim 2 , wherein Z 4 ″ is —O—, —S—, —N═, —NR 6 ′—, —CR 7′ ═, —SO—, —SO 2 — or —CR 8 R 9 —(R 8 and R 9 are each independently a hydrogen atom or a substituent), or a salt thereof.

5. The compound according to any one of claims 1 to 3 , wherein X 2 , X 3 , X 4 and X 5 are each CH, or a salt thereof.

6. The compound according to claim 1 , which is represented by the formula (I-i)′:

wherein each symbol is as defined in claim 1 ,

or a salt thereof.

7. 9-(4-{[3-(2-Methoxyethyl)-3H-imidazo[4,5-b]pyridin-2-yl]oxy}phenyl)-9H-imidazo[1′,2′:1,2]imidazo[4,5-b]pyridine or a salt thereof.

8. 5-Methyl-9-{4-[(3-methyl-3H-imidazo[4,5-b]pyridin-2-yl)oxy]phenyl}-9H-imidazo[1′,2′:1,2]imidazo[4,5-b]pyridine or a salt thereof.

9. A medicament comprising a compound according to any one of claims 1 to 3 , or a salt thereof.

10. The medicament according to claim 9 , which is a phosphodiesterase 10A inhibitor.

11. The medicament according to claim 9 , which is an agent for the treatment of schizophrenia.

12. A method for the treatment of schizophrenia in a mammal, comprising administering an effective amount of a compound according to any one of claims 1 to 3 , or a salt thereof, to the mammal.

13. A compound according to any one of claims 1 to 3 , or a salt thereof, for the use in the treatment of schizophrenia.