WE CLAIM:
1. A compound of the formula (I) or a salt thereof:
wherein,
X^C-R^orN;
X2:C-R12orN;
R11: i) H, ii) halogen, iii) -CN, or iv) -O-Ci-6 alkyl;
R12: H or halogen;
R1: i) H, ii) Ci-6 alkyl which may be substituted with one or more substituent(s) selected from the group consisting of halogen(s), and pyrazolyl(s), iii) C2-6 alkenyl, or iv) -OR0;
R2: i) Ci-6 alkyl which may be substituted with one or more substituent(s) selected from the group consisting of-OR°(s), halogen(s), -COOR°(s), -CONR21R22(s), phenyl(s) which may be substituted with one or more substituent(s) selected from the G1 group, and heteroaryl(s) which is selected from the group consisting of pyridyl, pyrazolyl, imidazolyl, thiazolyl, thiadiazolyl, thienyl, oxazolyl, isoxazolyl, and triazolyl, wherein, the heteroaryl may be substituted with one or more substituent(s) selected from the G2 group, ii) C2-6 alkenyl, iii) C2-6 alkynyl, iv) -OR0, v) -NR23R24, vi) -COOR0, or vii) phenyl;
R21:H or Ci-6 alkyl;
R22: i) Ci-6 alkyl which may be substituted with one or more phenyl(s), or ii) phenyl;
R23: i) H, or ii) Ci-6 alkyl which may be substituted with one or more -OH(s);
R24: i) Ci-6 alkyl which may be substituted with one or more phenyl(s) which may be substituted with one or more halogen(s), ii) C3-8 cycloalkyl which may be substituted

with one or more Ci-6 alkyl(s), iii) phenyl which may be substituted with one or more halogen(s), or iv) tetrahydropyranyl; or
R1, R2, and a carbon atom bounded by R1 and R2 may interact to form a 4-piperidine ring or 4-tetrahydropyran ring, and the carbon atom bounded by R1 and R2 is a spiro atom and the 4-piperidine ring may be substituted with one or more substituent(s) selected from the group consisting of-S02-(Ci-6 alkyl) and-COOR°;
R3, R4: the same or different each other, i) C1-3 alkyl which may be substituted with one or more substituent(s) selected from the group consisting of halogen(s) and -OH(s) or ii) C2-6 alkenyl which may be substituted with one or more substituent(s) selected from the group consisting of -OH(s) and heteroaryl(s) which is selected from the group consisting of pyrazolyl and thienyl, wherein the heteroaryl may be substituted with one or more C1-6 alkyl(s), or,
R3, R4, and a carbon atom bounded by R3 and R4 may interact to form a 3-oxetane ring and the carbon atom bounded by R3 and R4 is a spiro atom;
R5 : i) H, ii) C1-6 alkyl which may be substituted with one or more -0-(Ci-6 alkyl)(s), iii) -O-C1-6 alkyl, iv) halogen, v) -COO-(Ci-6 alkyl), or vi) C3-8 cycloalkyl;
R6 : i) H, ii) Ci-e alkyl which may be substituted with one or more substituent(s) selected from the group consisting of-0-(Ci-6 alkyl(s) which may be substituted with one or more halogen(s)) and halogen(s), iii) -OH, iv) -0-(Ci-6 alkyl which may be substituted with one or more halogen(s)), v) halogen, vi) -CN, vii) -S-(Ci-6 alkyl), viii) C3-8 cycloalkyl, ix) -NR°R°, or x) C2-6 alkenyl;
G1 group : i) halogen, ii) -COOR0, iii) -CONR°R°, iv) -OH, v) Ci-6 alkyl which may be substituted with one or more substituent(s) selected from the group consisting of -OH(s) and halogen, or vi) -0-(Ci-6 alkyl which may be substituted with one or more substituent(s) selected from the group consisting of -OH(s) and halogen(s));
G2 group: i) halogen, ii) Ci-6 alkyl which may be substituted with one or more substituent(s) selected from the group consisting of -OH(s) and halogen(s) or iii) -CONR°R°;
R°: the same or different each other, H or Ci-6 alkyl, provided that said compound is not methyl l,l-diallyl-3-oxo-2,4-dihydroisoquinoline-4-carboxylate or a salt thereof.
2. The compound or salt of claim 1, wherein

R1 is i) H, or ii) Ci-6 alkyl;
R2 is i) Ci-6 alkyl which may be substituted with one or more substituent(s) selected from the group consisting of-OR°(s), halogen(s), -CONR21R22(s), phenyl(s) which may be substituted with one or more substituent(s) selected from the group consisting of halogen(s) and -COOR°(s), and heteroaryl(s) which is selected from the group consisting of pyrazolyl, and triazolyl, ii) C2-6 alkenyl, iii) C2-6 alkynyl, iv) -NR23R24, or v) -COOR0;
R21 is Ci-6 alkyl;
R22 is Ci-6 alkyl;
R23 is Ci-6 alkyl; and
R24 is i) C3-8 cycloalkyl, or ii) phenyl; or
R1, R2, and a carbon atom bounded by R1 and R2 may interact to form a 4-tetrahydropyran ring, and the carbon atom bounded by R1 and R2 is a spiro atom.
3. The compound or salt of claim 1 or 2, wherein
R3 and R4 are the same or different each other, i) C1-3 alkyl which may be substituted with one or more substituent(s) selected from the group consisting of halogen(s) and -OH(s) or ii) C2-6 alkenyl which may be substituted with one or more substituent(s) selected from the group consisting of -OH(s) and pyrazolyl(s) which may be substituted with one or more Ci-6 alkyl(s), or,
R3, R4, and a carbon atom bounded by R3 and R4 may interact to form a 3- oxetane ring and the carbon atom bounded by R3 and R4 is a spiro atom.
4. The compound or salt of any of the preceeding claims, wherein
R5 is i) H, ii) Ci-6 alkyl, iii) -O-C1-6 alkyl, iv) halogen, or v) C3-8 cycloalkyl; and R6 is i) H, ii) Ci-6 alkyl which may be substituted with one or more substituent(s) selected from the group consisting of-0-(Ci-6 alkyl)(s) and halogen(s), iii) -OH, iv) -0-(Ci-6 alkyl which may be substituted with one or more halogen(s)), v) halogen, vi) -CN, vii) -S-C1-6 alkyl, viii) -NR°R°, or ix) C2-6 alkenyl.
5. The compound or salt of any of the preceeding claims, wherein
X'isC-^orN;
X2isC-R12orN;
Ruis i) H, ii) halogen, iii) -CN, or iv) -O-C1-6 alkyl; and

R12 is H.
6. The compound or salt of any of the preceeding claims, wherein
R1 isCi-6alkyl;
R2 is C1-6 alkyl which may be substituted with a -OR0;
R3, R4, and a carbon atom bounded by R3 and R4 interact to form a 3-oxetane ring and the carbon atom bounded by R3 and R4 is a spiro atom as represented by formula (II) below:
R5 is H; and
R6 is i) Ci-6 alkyl, ii) -O-Ci-6 alkyl which is substituted with one to three halogen(s), iii) halogen, or iv) -CN.
7. The compound or salt of any of the preceeding claims, wherein
XMsC-R11;
X2 is C-R12;
R11 is i) H, ii) halogen, iii) -CN, or iv) -O-Ci-6 alkyl; and
R12 is H.
8. The compound or salt of claim 1, which is a compound selected from the
group consisting of
(-)-2-(difluoromethyl)-8-ethyl-8-(2-hydroxyethyl)-6H-spiro[l,6-naphthyridine-5,3'- oxetan]-7(8H)-one,
4,4-diethyl-l ,1 -dimethyl-3-oxo-l ,2,3,4-tetrahydroisoquinoline-6-carbonitrile,
8,8-diethyl-5,5-dimethyl-7-oxo-5,6,7,8-tetrahydro-l,6-naphthyridine-2-carbonitrile,
(-)-6-bromo-4-ethyl-4-(2 -hydroxy ethyl)-1,1 -dimethyl-1,4-dihydroisoquinolin-

3(2H)-one,
(+)-6-bromo-4-ethyl-4-(2-hydroxyethyl)-1,1 -dimethyl-1,4-dihydroisoquinolin-3(2H)-one,
8,8-diethyl-7-oxo-7,8-dihydro-6H-spiro[l,6-naphthyridine-5,3'-oxetane]-2-
carbonitrile,
8',8,-diethyl-7,-oxo-7,,8,-dihydro-6'H-spiro[oxetane-3,5,-pyrido[3,4-b]pyrazine]-2,-
carbonitrile,
4,4-diethyl-3 -oxo-3,4-dihydro-2H-spiro [isoquinoline-1,3 '-oxetane] -6-carbonitrile, 6-chloro-4,4-dimethyl-2H-spiro[isoquinoline-l,3'-oxetan]-3(4H)-one, 4,4-dimethyl-3 -oxo-3,4-dihydro-2H-spiro [isoquinoline-1,3'-oxetane] -6-
carbonitrile,
2-(difluoromethoxy)-8,8-dimethyl-6H-spiro[l,6-naphthyridine-5,3,-oxetan]-7(8H)
-one,
(+)-6-chloro-4-(2-hydroxyethyl)-4-methyl-2H-spiro [isoquinoline-1,3'-oxetan]-
3(4H)-one,
(-)-6-chloro-4-(2-hydroxyethyl)-4-methyl-2H-spiro [isoquinoline-l,3'-oxetan]-
3(4H)-one, and
(-)-2-(difluoromethoxy)-8-ethyl-8-(2-hydroxyethyl)-6H-spiro[l,6-naphthyridine-5 ,3'-oxetan]-7(8H)-one,
or a salt of said compound.
9. A compound or a salt thereof, which is a compound selected from the group
consisting of
4,4-diethyl-3 -oxo-3,4-dihydro-2H-spiro [isoquinoline-1,3 '-oxetane] -6-carbonitrile, 6-chloro-4,4-dimethyl-2H-spiro[isoquinoline-l,3'-oxetan]-3(4H)-one, 4,4-dimethyl-3 -oxo-3,4-dihydro-2H-spiro [isoquinoline-1,3'-oxetane] -6-
carbonitrile,
(+)-6-chloro-4-(2-hydroxyethyl)-4-methyl-2H-spiro [isoquinoline-l,3'-oxetan]-
3(4H)-one, and
(-)-6-chloro-4-(2-hydroxy ethyl)-4-methyl-2H-spiro [isoquinoline-1,3'-oxetan]-
3(4H)-one,
or a salt of said compound.

10. The compound or salt of claim 1, which is
4,4-diethyl-3 -oxo-3,4-dihydro-2H-spiro [isoquinoline-1,3 '-oxetane] -6-carbonitrile, or a salt thereof.
11. The compound or salt of claim 1, which is
6-chloro-4,4-dimethyl-2H-spiro [isoquinoline-l,3'-oxetan]-3 (4H)-one,
or a salt thereof.
12. The compound or salt of claim 1, which is
4,4-dimethyl-3 -oxo-3,4-dihydro-2H-spiro [isoquinoline- l,3'-oxetane] -6-carbonitrile, or a salt thereof.
13. The compound or salt of claim 1, which is
(+)-6-chloro-4-(2-hydroxyethyl)-4-methyl-2H-spiro [isoquinoline-l,3'-oxetan]-3 (4H)-one,
or a salt thereof.
14. The compound or salt of claim 1, which is
(-)-6-chloro-4-(2-hydroxyethyl)-4-methyl-2H-spiro [isoquinoline-l,3'-oxetan]-3 (4H)-one,
or a salt thereof.
15. A pharmaceutical composition comprising a compound or salt of claim 1 and a pharmaceutically acceptable excipient.
16. A pharmaceutical composition for preventing or treating a disease or condition selected from the group consisting of stress urinary incontinence (SUI), mixed urinary incontinence (MUI) and fecal incontinence comprising a compound or salt of claim 9.
17. A pharmaceutical composition for preventing or treating a disease or condition selected from the group consisting of frailty and sarcopenia comprising a compound or salt of claim 9.
18. A pharmaceutical composition for preventing or treating chronic obstructive

pulmonary disease (COPD) comprising a compound of claim 9, or a salt thereof.
19. A pharmaceutical composition for preventing or treating cachexia syndrome and/or muscle wasting caused by heart failure, cancer, or chronic kidney disease/dialysis comprising a compound or salt of claim 9.
20. A pharmaceutical composition for preventing or treating a disease or condition selected from the group consisting of amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), myasthenia gravis and muscular myopathies comprising a compound or salt of claim 9.
21. A pharmaceutical composition for preventing or treating a disease or condition selected from the group consisting of post-spinal cord injury (SCI) muscle dysfunction and post-stroke muscle dysfunction comprising a compound or salt of claim 9.
22. A pharmaceutical composition for preventing or treating a disease or condition selected from the group consisting of peripheral vascular disease, peripheral arterial disease, rehabilitation-related deficits, metabolic syndrome, obesity, ventilator-induced muscle weakness and chronic fatigue syndrome comprising a compound or salt of claim 9.
23. Use of a compound or salt of claim 9 for the manufacture of a pharmaceutical composition for preventing or treating a disease or condition selected from the group consisting of stress urinary incontinence (SUI), mixed urinary incontinence (MUI) and fecal incontinence.
24. Use of a compound or salt of claim 9 for the manufacture of a pharmaceutical composition for preventing or treating a disease or condition selected from the group consisting of frailty and sarcopenia.
25. Use of a compound or salt of claim 9 for the manufacture of a pharmaceutical composition for preventing or treating chronic obstructive pulmonary disease (COPD).

26. Use of a compound or salt of claim 9 for the manufacture of a pharmaceutical composition for preventing or treating cachexia syndrome and/or muscle wasting caused by heart failure, cancer, or chronic kidney disease/dialysis.
27. Use of a compound or salt of claim 9 for the manufacture of a pharmaceutical composition for preventing or treating a disease or condition selected from the group consisting of amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA) and myasthenia gravis, muscular myopathies.
28. Use of a compound or salt of claim 9 for the manufacture of a pharmaceutical composition for preventing or treating a disease or condition selected from the group consisting of post-spinal cord injury (SCI) muscle dysfunction and post-stroke muscle dysfunction.
29. Use of a compound or salt of claim 9 for the manufacture of a pharmaceutical composition for preventing or treating a disease or condition selected from the group consisting of peripheral vascular disease, peripheral arterial disease, rehabilitation-related deficits, metabolic syndrome, obesity, ventilator-induced muscle weakness and chronic fatigue syndrome.
30. Use of a compound or salt of claim 9 for preventing or treating a disease or condition selected from the group consisting of stress urinary incontinence (SUI), mixed urinary incontinence (MUI) and fecal incontinence.
31. Use of a compound or salt of claim 9 for preventing or treating a disease or condition selected from the group consisting of frailty and sarcopenia.
32. Use of a compound or salt of claim 9 for preventing or treating chronic obstructive pulmonary disease (COPD).
33. Use of a compound or salt of claim 9 for preventing or treating cachexia syndrome and/or muscle wasting caused by heart failure, cancer, or chronic kidney

disease/dialysis.
34. Use of a compound or salt of claim 9 for preventing or treating a disease or condition selected from the group consisting of amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), myasthenia gravis and muscular myopathies.
35. Use of a compound or salt of claim 9 for preventing or treating a disease or condition selected from the group consisting of post-spinal cord injury (SCI) muscle dysfunction and post-stroke muscle dysfunction.
36. Use of a compound or salt of claim 9 for preventing or treating a disease or condition selected from the group consisting of peripheral vascular disease, peripheral arterial disease, rehabilitation-related deficits, metabolic syndrome, obesity, ventilator-induced muscle weakness and chronic fatigue syndrome.
37. A method for preventing or treating a disease or condition selected from the group consisting of stress urinary incontinence (SUI), mixed urinary incontinence (MUI) and fecal incontinence, comprising administering to a subject an effective amount of the compound or salt of claim 9.
38. A method for preventing or treating a disease or condition selected from the group consisting of frailty and sarcopenia, comprising administering to a subject an effective amount of the compound or salt of claim 9.
39. A method for preventing or treating chronic obstructive pulmonary disease (COPD), comprising administering to a subject an effective amount of the compound or salt of claim 9.
40. A method for preventing or treating cachexia syndrome and/or muscle wasting caused by heart failure, cancer, or chronic kidney disease/dialysis, comprising administering to a subject an effective amount of the compound or salt of claim 9.
41. A method for preventing or treating a disease or condition selected from the

group consisting of amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), myasthenia gravis and muscular myopathies, comprising administering to a subject an effective amount of the compound or salt of claim 9.
42. A method for preventing or treating a disease or condition selected from the group consisting of post-spinal cord injury (SCI) muscle dysfunction and post-stroke muscle dysfunction, comprising administering to a subject an effective amount of the compound or salt of claim 9.
43. A method for preventing or treating a disease or condition selected from the group consisting of peripheral vascular disease, peripheral arterial disease, rehabilitation-related deficits, metabolic syndrome, obesity, ventilator-induced muscle weakness and chronic fatigue syndrome, comprising administering to a subject an effective amount of the compound or salt of claim 9.
44. A compound or salt of claim 9 for use in the prevention or treatment of a disease or condition selected from the group consisting of stress urinary incontinence (SUI), mixed urinary incontinence (MUI) and fecal incontinence.
45. A compound or salt of claim 9 for use in the prevention or treatment of a disease or condition selected from the group consisting of frailty and sarcopenia.
46. A compound or salt of claim 9 for use in the prevention or treatment of chronic obstructive pulmonary disease (COPD).
47. A compound or salt of claim 9 for use in the prevention or treatment of cachexia syndrome and/or muscle wasting caused by heart failure, cancer, or chronic kidney disease/dialysis.
48. A compound or salt of claim 9 for use in the prevention or treatment of a disease or condition selected from the group consisting of amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), myasthenia gravis and muscular myopathies.

49. A compound or salt of claim 9 for use in the prevention or treatment of a disease or condition selected from the group consisting of post-spinal cord injury (SCI) muscle dysfunction and post-stroke muscle dysfunction.
50. A compound or salt of claim 9 for use in the prevention or treatment of a disease or condition selected from the group consisting of peripheral vascular disease, peripheral arterial disease, rehabilitation-related deficits, metabolic syndrome, obesity, ventilator-induced muscle weakness and chronic fatigue syndrome.