An antibody-drug conjugate (ADC) has a structure represented by Formula (I): [D2-L2-Cn2yG2-Ab-Sg1G1-L1-D1]x   Formula (I) or a pharmaceutically acceptable salt thereof, wherein Ab is an antibody without glycans (i.e., the protein portion of an antibody); G1 and G2 are glycan moieties, which may be the same or different; Cn1 and Cn2 are conjugation moieties, which may be the same or different; L1 and L2 are linker moieties, which may be the same or different; D1 and D2 are drug units which may be the same or different; and x and y are independently an integer from 0 to 8, provided that x+y≠0.