Fmoc-GGFG-Dxd, also known as N-((1S,9S)-9-ethyl-5-fluoro-9-hydroxy-4-methyl-10,13-dioxo-2,3,9,10,13,15-hexahydro-1H,12H-benzo[de]pyrano[3',4':6,7]indolizino[1,2-b]quinolin-1-yl)-2-hydroxyacetamide, is a brown powder. It is a hexacyclic chemical compound analogous to camptothecin with a triplet amino acid chain linked to a terminal Fmoc group. GGFG (glycyl-glycyl-phenylalanyl-glycine peptide could link exatecan derivative (DXd) through cysteine residues. DXd is highly membrane-permeable and exhibited strong bystander killing in-vitro co-culture of HER2 positive (KPL-4) and negative (MDA-MB-468) cells and in mice inoculated with a mixture of HER2 (+) and HER2 (?) cells.