Bivalent doxorubicin (DOX)-dipeptides (16a–c) were prepared and conjugated to the monoclonal antibody BR96. The dipeptides are cleaved by lysosomal proteases following internalization of the resulting immunoconjugates. Conjugate 18b demonstrated antigen- specific in vitro tumor cell killing activity (IC50= 0.2 μM) that was equipotent to DOX with a near doubling of drug molecules/MAb. Size exclusion chromatography showed 18b to be ...