Synthesis and Stability of Four Maleimide Derivatives of the Anticancer Drug Doxorubicin for the Preparation of Chemoimmunoconjugates

By attaching maleimide groups to anticancer drugs, derivatives are obtained which bind selectively to thiolated carrier proteins. Four maleimide derivatives of the anticancer drug doxorubicin were prepared in which 3-maleimidobenzoic acid or 4-maleimidophenylacetic acid was bound to the 3'-amino position of doxorubicin through a benzoyl or phenylacetyl amide bond (1 or 2) or in which 3-maleimidobenzoic acid hydrazide or 4-maleimidophenylacetic acid hydrazide was bound to the 13-keto position through a benzoyl or phenylacetyl hydrazone bond (3 or 4). The maleimide derivatives of doxorubicin were characterized by means of 13C-NMR spectroscopy, elemental analysis and mass spectrometry. In addition, the stability of the maleimide derivatives 1-4 at pH values of 5.0 and 7.4 was investigated with the aid of HPLC. The amide or hydrazone bond of 1-4 is stable at pH 7.4 whereas the hydrazone bond is acid-sensitive.