DC44SMe, a phosphate proagent of cytotoxic DNA alkylator DC44, can be used in the synthesis of antibody-drug conjugate (ADC). DC44SMe exhibits IC 50 s of 2.0 nM, 2.8 nM, and 1.9 nM for Ramos, Namalwa, and HL60/s cancer cells, respectively. DC44SMe can be used for the targeted treatment of cancer.