Synthesis of new maleimide derivatives of daunorubicin and biological activity of acid labile transferrin conjugates

Maleimide groups were bound to the 3′-amino position of daunorubicin through a benzamide bond or to the 13-keto position through a benzoyl hydrazone or phenylacetyl hydrazone bond. The acid labile transferrin conjugates prepared with the latter two derivatives exhibited high activity in human melanoma cells (MEXF 989) using a clonogenic cell assay comparable to or exceeding that of daunorubicin.