Structure-activity relationship of photocytotoxic iron(III) complexes of modified dipyridophenazine ligands

Iron(III) complexes [FeL(B)] (1-5) of a tetradentate trianionic phenolate-based ligand (L) and modified dipyridophenazine bases (B), namely, dipyrido-6,7,8,9-tetrahydrophenazine (dpqC in 1), dipyrido[3,2-a:2',3'-c]phenazine-2-carboxylic acid (dppzc in 2), dipyrido[3,2-a:2',3'-c]phenazine-11-sulfonic acid (dppzs in 3), 7-aminodipyrido[3,2-a:2',3'-c]phenazine (dppza in 4) and benzo[i]dipyrido[3,2-a:2',3'-c]phenazine (dppn in 5), have been synthesized and their photocytotoxic properties studied along with their dipyridophenazine analogue (6). The complexes have a five electron paramagnetic iron(III) center, and the Fe(III)/Fe(II) redox couple appears at about -0.69 V versus SCE in DMF-0.1 M TBAP. The physicochemical data also suggest that the complexes possess similar structural features as that of its parent complex [FeL(dppz)] with FeO3N3 coordination in a distorted octahedral geometry. The DNA-complex and protein-complex interaction studies have revealed that the complexes interact favorably with the biomolecules, the degree of which depends on the nature of the substituents present on the dipyridophenazine ring. Photocleavage of pUC19 DNA by the complexes has been studied using visible light of 476, 530, and 647 nm wavelengths. Mechanistic investigations with inhibitors show formation of HO(•) radicals via a photoredox pathway. Photocytotoxicity study of the complexes in HeLa cells has shown that the dppn complex (5) is highly active in causing cell death in visible light with sub micromolar IC(50) value. The effect of substitutions and the planarity of the phenazine moiety on the cellular uptake are quantified by determining the total cellular iron content using the inductively coupled plasma-optical emission spectrometry (ICP-OES) technique. The cellular uptake increases marginally with an increase in the hydrophobicity of the dipyridophenazine ligands whereas complex 3 with dppzs shows very high uptake. Insights into the cell death mechanism by the dppn complex 5, obtained through DAPI nuclear staining in HeLa cells, reveal a rapid programmed cell death mechanism following photoactivation of complex 5 with visible light. The effect of substituent on the DNA photocleavage activity of the complexes has been rationalized from the theoretical studies.