Inhibition of Transcription in Vitro by Anticancer Active Dirhodium(II) Complexes

The DNA binding and inhibition of transcription in vitro by neutral Rh2(μ-O2CCH3)4 and cationic cis-[Rh2(μ-O2CCH3)2(phen)2]2+ complexes were investigated. The binding constants of the two complexes to calf-thymus DNA were estimated from absorption titrations to be 4.6 × 102 M-1 and 1.7 × 104 M-1 for Rh2(μ-O2CCH3)4 and cis-[Rh2(μ-O2CCH3)2(phen)2]2+, respectively. The shift to higher energies of the low-energy absorption of the complexes upon addition of DNA is consistent with axial binding of the complexes to duplex DNA. The relative concentrations, [complex]/[DNA], of Rh2(μ-O2CCH3)4 and cis-[Rh2(μ-O2CCH3)2(phen)2]2+ at which 50% of the transcription is inhibited (R inh 50), are 0.0031 and 0.0011, respectively. These concentrations are significantly lower than that required for activated cisplatin, cis-[Pt(NH3)2(H2O)2]2+, with R inh 50 = 0.0085 under similar experimental conditions. Upon incubation of cis-[Pt(NH3)2(H2O)2]2+ with the template DNA prior to the addition of the enzyme and nucleobases necessary for the transcription reaction for 30 min at 37 °C, significantly lower concentrations of the complex were required to attain 50% inhibition. In contrast, similar incubation of the DNA with the dirhodium complexes did not result in better transcription inhibition. Experiments designed to elucidate the mechanism of the observed inhibition indicate that, unlike cis-[Pt(NH3)2(H2O)2]2+, Rh2(μ-O2CCH3)4 and cis-[Rh2(μ-O2CCH3)2(phen)2]2+ appear to interact directly with the enzyme T7-RNA polymerase as their mode of inhibition.