A compound represented by formula: wherein R 1 is selected from 5-chloronicotinoyl, 6-chloronicotinoyl, 2,6-dichloronicotinoyl, 4,6-dicloronicotinoyl, 5,6-dichloronicotinoyl, 6-(trifluoromethyl)nicotinoyl, 3,5-dichloropicolinoyl, 4,6-dichloropicolinoyl, 5-phenylpicolinoyl, 5-(4-chlorophenyl)picolinoyl, 4-(6-chloro-3-pyridinyl)benzoyl, 5-(4-chlorophenyl)thiophene-2-carboxyl, 2,6-dichloroisonicotinoyl, 5-(trifluoromethyl)nicotinoyl, 4-(trifluoromethyl)picolinoyl, 3,5-dibromopicolinoyl, 5-bromonicotinoyl, 2-chloroisonicotinoyl, 2-bromoisonicotinoyl, 4-chloropicolinoyl, 2-(trifluoromethyl)isonicotinoyl, 2,6-dibromoisonicotinoyl, 3,5-dibromopicolinoyl, 5-methylnicotinoyl, 2-fluoroisonicotinoyl, 2-(trifluoromethyl)isonicotinoyl, 5-bromo-3-chloropicolinoyl, 3-chloroisonicotinoyl, 3-chloro-5-(trifluoromethyl)picolinoyl, 3-chloropicolinoyl, 5-chloropicolinoyl, 5-(trifluoromethyl)picolinoyl, 2-chloro-6-methylisonicotinoyl, 2-chloro-6-(trifluoromethyl)nicotinoyl, 6-ethylnicotinoyl, 5-ethylpicolinoyl, 6-chloropicolinoyl, 6-(trifluoromethyl)picolinoyl, 2-(trifluoromethyl)pyrimidine-5-carboxyl, 2-quinoxalinecarboxyl, 1 H -benzimidazole-2-carboxyl, 1-methylindole-2-carboxyl, 6-methyl-imidazo[1,2- a]pyridine-2-carboxyl, benzo[b]thiophene-2-carboxyl, 1-methylindazole-3-carboxyl, 3-quinolinecarboxyl, benzothiazole-6-carboxyl, 1H-indazole-3-carboxyl, quinaldoyl, 1 H -indole-2-carboxyl, 1-methylbenzimidazole-2-carboxyl, 5-chloro-1 -methylindole-2-carboxyl, 5-chloro-1H-indole-2-carboxyl, 5,6-difluoro-1H-indole-2-carboxyl, 3-chlorobezo[b]thiophene-2-carboxyl, 1-methylindole-3-acetyl, 1-methylindole-3-carboxyl, benzo[d]thiazole-2-carboxyl, 6-chlorobenzimidazole-2-carboxyl, benzo[b]thiazole-2-propanoyl, 2-phenylpyrimidine-5-carboxyl, benzooxazole-2-carboxyl, benzo[d]isooxazole-3-carboxyl, 2,5-dibromothiphene-3-carboxyl, 4,5-dibromopyrrole-2-carboxyl, 5-bromothiophene-2-carboxyl, 4,5-dibromofuran-2-carboxyl, 5-phenyl-1,2-oxazole-3-carboxyl, 5-phenyl-1,2,4-oxadiazole-3-carboxyl, 2-phenyl-1 H -imidazole-4-carboxyl, 4,5-dibromothiophene-2-carboxyl, 5-phenyl-1 H -pyrazole-3-carboxyl, 3,5-dibromothiophene-2-carboxyl, 5-(trifluoromethyl)thiophene-2-carboxyl, 3-phenyl-1,2-oxazole-5-carboxyl, 4-bromothiophene-2-carboxyl, 3-chlorothiophene-2-carboxyl, 4 H -thieno[3,2-b]pyrrole-5-carboxyl, 2-bromo-1,3-thiazole-5-carboxyl, benzofuran-2-carboxyl, 4-bromo-1-methylpyrrole-2-carboxyl, 5-(4-chlorophenyl)-1,2-oxazole-3-carboxyl, 5-bromothiophene-3-carboxyl, 4-bromopicolinoyl, 5-bromofuran-3-carboxyl and indole-3-propanoyl; R 3 represents a side chain of leucine; and R 4 represents a side chain of an amino acid selected from alanine, threonine, valine, or 2-aminobutyric acid; or a pharmaceutically acceptable salt thereof. R 1 is selected from 5-chloronicotinoyl, 6-chloronicotinoyl, 2,6-dichloronicotinoyl, 4,6-dicloronicotinoyl, 5,6-dichloronicotinoyl, 6-(trifluoromethyl)nicotinoyl, 3,5-dichloropicolinoyl, 4,6-dichloropicolinoyl, 5-phenylpicolinoyl, 5-(4-chlorophenyl)picolinoyl, 4-(6-chloro-3-pyridinyl)benzoyl, 5-(4-chlorophenyl)thiophene-2-carboxyl, 2,6-dichloroisonicotinoyl, 5-(trifluoromethyl)nicotinoyl, 4-(trifluoromethyl)picolinoyl, 3,5-dibromopicolinoyl, 5-bromonicotinoyl, 2-chloroisonicotinoyl, 2-bromoisonicotinoyl, 4-chloropicolinoyl, 2-(trifluoromethyl)isonicotinoyl, 2,6-dibromoisonicotinoyl, 3,5-dibromopicolinoyl, 5-methylnicotinoyl, 2-fluoroisonicotinoyl, 2-(trifluoromethyl)isonicotinoyl, 5-bromo-3-chloropicolinoyl, 3-chloroisonicotinoyl, 3-chloro-5-(trifluoromethyl)picolinoyl, 3-chloropicolinoyl, 5-chloropicolinoyl, 5-(trifluoromethyl)picolinoyl, 2-chloro-6-methylisonicotinoyl, 2-chloro-6-(trifluoromethyl)nicotinoyl, 6-ethylnicotinoyl, 5-ethylpicolinoyl, 6-chloropicolinoyl, 6-(trifluoromethyl)picolinoyl, 2-(trifluoromethyl)pyrimidine-5-carboxyl, 2-quinoxalinecarboxyl, 1 H -benzimidazole-2-carboxyl, 1-methylindole-2-carboxyl, 6-methyl-imidazo[1,2- a]pyridine-2-carboxyl, benzo[b]thiophene-2-carboxyl, 1-methylindazole-3-carboxyl, 3-quinolinecarboxyl, benzothiazole-6-carboxyl, 1H-indazole-3-carboxyl, quinaldoyl, 1 H -indole-2-carboxyl, 1-methylbenzimidazole-2-carboxyl, 5-chloro-1 -methylindole-2-carboxyl, 5-chloro-1H-indole-2-carboxyl, 5,6-difluoro-1H-indole-2-carboxyl, 3-chlorobezo[b]thiophene-2-carboxyl, 1-methylindole-3-acetyl, 1-methylindole-3-carboxyl, benzo[d]thiazole-2-carboxyl, 6-chlorobenzimidazole-2-carboxyl, benzo[b]thiazole-2-propanoyl, 2-phenylpyrimidine-5-carboxyl, benzooxazole-2-carboxyl, benzo[d]isooxazole-3-carboxyl, 2,5-dibromothiphene-3-carboxyl, 4,5-dibromopyrrole-2-carboxyl, 5-bromothiophene-2-carboxyl, 4,5-dibromofuran-2-carboxyl, 5-phenyl-1,2-oxazole-3-carboxyl, 5-phenyl-1,2,4-oxadiazole-3-carboxyl, 2-phenyl-1 H -imidazole-4-carboxyl, 4,5-dibromothiophene-2-carboxyl, 5-phenyl-1 H -pyrazole-3-carboxyl, 3,5-dibromothiophene-2-carboxyl, 5-(trifluoromethyl)thiophene-2-carboxyl, 3-phenyl-1,2-oxazole-5-carboxyl, 4-bromothiophene-2-carboxyl, 3-chlorothiophene-2-carboxyl, 4 H -thieno[3,2-b]pyrrole-5-carboxyl, 2-bromo-1,3-thiazole-5-carboxyl, benzofuran-2-carboxyl, 4-bromo-1-methylpyrrole-2-carboxyl, 5-(4-chlorophenyl)-1,2-oxazole-3-carboxyl, 5-bromothiophene-3-carboxyl, 4-bromopicolinoyl, 5-bromofuran-3-carboxyl and indole-3-propanoyl; R 3 represents a side chain of leucine; and R 4 represents a side chain of an amino acid selected from alanine, threonine, valine, or 2-aminobutyric acid; or a pharmaceutically acceptable salt thereof.