DNA binding and photocleavage in vitro by new dirhodium(II) dppz complexes: Correlation to cytotoxicity and photocytotoxicity

Two new dirhodium(II) complexes possessing the intercalating dppz ligand (dppz = dipyrido[3,2-a:2',3'-c]phenazine), cis-[Rh-2(mu-O2CCH3)(2)(dppz)(eta(1)-O2CCH3)(CH3OH)](+) (1) and cis-[Rh-2(mu-O2CCH3)(2)(dPPZ)(2)](2+) (2), were synthesized and characterized as potential agents for photochemotherapy. Various techniques show that 1 binds to DNA through intercalation, although some aggregation of the complex on the DNA surface is also present. In contrast, 2 does not intercalate between the DNA bases; however, strong hypochromic behaviour is observed in the presence of DNA, which can be attributed to intermolecular pi-stacking of 2 enhanced by the polyanion. The apparent DNA binding constants determined using optical titrations are compared to those from dialysis experiments. Both complexes photocleave pUC18 plasmid in vitro under irradiation with visible light (lambda(irr) greater than or equal to 395 nm, 15 min), resulting in the nicked, circular form. Greater photocleavage is observed for 1 relative to 2, which may be due to the ability of 1 to intercalate between the DNA bases. The cytotoxicity toward human skin cells (Hs-27) measured as the concentration at which 50% cell death is recorded, LC50, was found to be 135 +/- 8 muM for 2 in the dark (30 min), which is significantly lower than those of 1 (LC50 = 27 +/- 2 muM) and Rh-2(O2CCH3)(4) (LC50 = 15 +/- 2 muM). Irradiation of cell cultures containing 1 and Rh-2(O2CCH3)(4) with visible light (400-700 nm, 30 min) has little effect on their cytotoxicity, with LC50 values of 21 +/- 3 and 13 +/- 2 muM, respectively. Interestingly, a 3.4-fold increase in the toxicity of 2 is observed when the cell cultures are irradiated (400-700 nm, 30 min), resulting in LC50 = 39 +/- 1 muM. The greater toxicity of 1 compared to 2 in the dark may be related to the ability of the former compound to intercalate between the DNA bases. The lower cytotoxicity of 2, together with its significantly greater photocytotoxicity, makes this complex a potential agent for photodynamic therapy (PDT). These results suggest that intercalation or strong DNA binding may not be a desirable property of a potential PDT agent.