Studies on the stability of the anticancer-active [N,N′-bis(salicylidene)-1,2-phenylenediamine] chloridoiron(III) complex under pharmacological-like conditions

Metal complexes of substituted N,N'-bis(salicylidene)ethylenediamine ligands are interesting metallo drugs for chemotherapy. Our lead [N,N'-bis(salicylidene)-1,2-phenylenediamine]chloridoiron(III) [Fe(III)salopheneCl] showed excellent in vitro activity against tumor cells sensitive but also resistant to common antitumor drugs. For the analysis of the biological results, information about the stability in various solvents at defined biological pH values is necessary. Separation of the complex and its ligand, the most likely degradation product, was feasible by HPLC using the Chromolith (R) HighResolution RP-18e column (100 x 4.6 mm, Merck) and an eluent of acetonitrile/25 mM phosphate buffer pH 3 (25/75). [Fe(III)salopheneCl] was stable over 3.5 h in acetonitrile, ethanol and in the presence of Deferoxamine. Stability also remained under pharmacological-like conditions in acetonitrile/water mixtures (80/20) with NaCl (0.9%) or phosphate buffered saline (pH 5.0 or 7.4). These findings indicate that the intact complex and no degradation products caused the intracellular effects. Mixtures with HCl, HNO3, or H2SO4 (0.1 N each), however, led to a fast release of the ligand, excluding peroral application without enteric coating. For parenteral application, a pharmaceutical formulation with 10% ethanol and 10% polyethylene glycol 300 in 0.9% NaCl solution was developed, in which [Fe(III)salopheneCl] was stable for at least 24 h.