To a mixture of 203 mg of methyl 4-{2-[4-({[2-({3-[7-methyl-3,6-dioxo-10-(pentan-3-yl)-2,5-dioxa-7,10-diazaundecan-11-yl]benzoyl}amino)-4,5,6,7-tetrahydro-1-benzothiophen-3-yl]carbonyl}amino)phenyl]ethyl}benzoate, 4.0 mL of methanol and 4.0 mL of THF was added 1.5 mL of a 1.0 M aqueous sodium hydroxide solution at room temperature, followed by stirring for 14 hours at 60° C. 1.5 mL of 1.0 M hydrochloric acid was added to the reaction mixture under ice cooling, followed by concentration under reduced pressure. A mixed solvent (methanol/chloroform=15/85) was added to the residue, the insoluble material was filtered, and the filtrate was concentrated under reduced pressure. The residue was purified by silica gel column chromatography (15% methanol/chloroform). A mixed solvent (acetonitrile/water=90/10) was added to the obtained product, 0.5 mL of 1.0 M hydrochloric acid was added thereto under ice cooling, and the solvent was removed under reduced pressure. The residue was subjected to lyophilization, thereby obtaining 186 mg of 4-(2-{4-[({2-[(3-{[(2-{[(carboxymethoxy)carbonyl](methyl)amino}ethyl)(pentan-3-yl)amino]methyl}benzoyl)amino]-4,5,6,7-tetrahydro-1-benzothiophen-3-yl}carbonyl)amino]phenyl}ethyl)benzoic acid hydrochloride.
