An antibody-drug conjugate (ADC) or a pharmaceutically acceptable salt thereof, is provided. The ADC includes a tetravalent monoclonal antibody conjugated to a cytotoxic drug by a chemical linker. The monoclonal antibody includes two long chains and four short chains. Each of the long chains includes a first segment and a second segment, the first segment located proximal to the N-terminus of the long chain, and the second segment located proximal to the C-terminus of the long chain. Each of the first segment and second segment pairs with one of the short chains to form an antigen-binding fragment domain, therefore forming four antigen-binding fragment domains having specificity toward the same antigen. Pharmaceutical composition including the ADC and methods of treating diseases using the compositions are also provided.