acetic acid 3S-{5R-(3'-{4S-acetoxy-1-[2S-(2S-methylaminopropionylamino)butyryl]pyrrolidin-2R-ylmethyl}-6,6'-difluoro-1H,1'H-[2,2']biindol-3-ylmethyl)-1-[2S-(2S-methylaminopropionylamino)butyryl]pyrrolidin}-3-yl ester