An immunoconjugate in which a phosphate-prodrugged DNA minor groove binding agent of formula (I), where X is a nucleophilically displaceable leaving group, is conjugated to an antibody or an antigen binding fragment of an antibody, and compounds that can be used for making such irnmunoconjugates, and uses of such immunoconjugates.本發明係關於免疫結合物,其中式(I)之經磷酸酯基前藥化之DNA小溝接合劑係結合至抗體或抗體之抗原接合片段,其中X係可經親核性取代之離去基團;及可用於製備此等免疫結合物之化合物;及此等免疫結合物之用途。