The invention provides methods for enhancing efficiency of prodrugs by specifically engineered enzymes with altered or enhanced activity and broader substrate specificity towards nucleoside analogs used in cancer chemotherapy, and delivering the enzymes to specific target cells in a patient. The invention also provides modified deoxycytidine kinase (dCK) mutants with such enhanced activities. Furthermore, the invention provides antibody-conjugated enzymes, pharmaceutical composition and kit containing the same, that can be specifically delivered to tumor cells.
