Structure Property Relationships for Dirhodium Antitumor Active Compounds: Reactions with Biomolecules and In Cellulo Studies

The molecular characteristics that affect the activity of various dirhodium complexes are reported. The importance of the axial position in the action of dirhodium compounds was studied. Three dirhodium complexes with increasing number of accessible axial coordination sites were synthesized and characterized. In cis-[Rh2(u-OAc)2(np)2]2+ (np = 1,8- naphthyridine) both axial sites are available for coordination, whereas for cis-[Rh2(u-OAc)2(np)(pynp)]+2 (pynp = 2-(2-pyridyl)1,8-naphthyridine) and cis-[Rh2(u-OAc)2(pynp)2]+2 the pyridyl arm on the ligand pynp blocks one and two axial sites, respectively. The availability of the axial positions affects the in vitro and in cellulo activity of these complexes demonstrating that open axial coordination sites are necessary for biological activity. The inhibitory activity of derivatives of dirhodium-dppz complexes (dppz = dipyrido[3,2-a:2',3'-c]phenazine) has also been investigated. The dppz derivatives included compounds with electron-withdrawing (Cl, CN, and NO2) as well as electro-donating (MeO and Me) substituents. These compounds inhibit transcription of T7-RNA polymerase by reducing accessible cysteine residues. The activity correlates with the electron withdrawing character of the substituent on the dppz ligand. Density functional theory (DFT) calculations reveal that the lowest unoccupied molecular orbitals (LUMOs) in the series are ligand-based pi* orbitals localized on the phenazine ring. These complexes represent the first family of dirhodium complexes whose inhibitory ability can be tuned by controlling their redox properties. The effect of the presence of diimine ligands in the dirhodium core in both in vitro and in cellulo activity is discussed. The presence of one diimine ligand allows for dual binding, intercalation and covalent, as observed by melting temperature and relative viscosity measurements, as well as electrophoretic mobility shift assay (EMSA). The mono-substituted dirhodium complexes are effective against HeLa and COLO-316 cell lines, with [Rh2(u-O2CCH3)2(n1-O2CCH3)(dppz)]+ being the most effective compound of the series. Results of the comet assay indicate that all of the monosubstituted complexes studied damage nuclear DNA, although in different degrees. The cytotoxic effect of these complexes is not affected by the presence of glutathione. The addition of the second diimine ligand hinders the ability of the complexes to damage DNA. The bis-substituted complexes are also slightly less cytotoxic than their mono-substituted congeners. Thus, the number of equatorial positions occupied by diimine ligands play a critical role in the mechanism of cytotoxicity of dirhodium(II,II) complexes. Finally, the results also demonstrate that improving the internalization of the dirhodium complexes can be achieved by co-incubation with cell penetrating peptides. This work provides a foundation for the preparation of new and more effective dirhodium complexes