Luminescent Cycloplatinated Complexes with Biologically Relevant Phosphine Ligands: Optical and Cytotoxic Properties

Two series of neutral luminescent pentafluorophenyl cycloplatinated(II) complexes [Pt(C^N)(C₆F₅)L] [C^N = C-deprotonated 2-phenylpyridine (ppy; a), 2-(2,4-difluorophenylpyridine (dfppy; b)] incorporating dimethyl sulfoxide [L = DMSO for 1 (1a reported by us in ref (14) )] or biocompatible phosphine [L = PPh₂C₆H₄COOH (dpbH; 2), PPh₂C₆H₄CONHCH₂COOMe (dpbGlyOMe; 3), P(C₆H₄SO₃Na)₃ (TPPTS; 4)] ligands have been prepared and characterized and their optical properties studied. Their cytotoxic activities against tumor A549 (lung carcinoma), HeLa (cervix carcinoma), and nontumor NL-20 (lung epithelium) cell lines, as well as the ability to interact with DNA (plasmid pBR322), were evaluated. Complexes 2 exhibit higher cytotoxicity (IC₅₀ 3.89-20.29 μM) than compounds 1 (9.03-20.50 μM), whereas the activities of complexes 3 and 4 are negligible. All cytotoxic complexes show low selective toxicities toward cancer cells. Interestingly, except 1a, these complexes do not show evidence of DNA intercalation. Along the same lines, fluorescence costaining with Hoechst (2,5'-bi-1 H-benzimidazole, 2'-(4-ethoxyphenyl)-5-(4-methyl-1-piperazinyl), a nuclear DNA stain) reveals that all complexes easily internalize, being mainly localized in the cytoplasm. In order to deepen the mechanism of biological action, the effect of the most cytotoxic complex 2b toward the dynamics of tubulin was explored. This complex displays tubulin depolymerization activity, exhibiting more potent inhibition of microtubule formation in A549 than in HeLa cells, in accordance with its higher antiproliferative activity (IC₅₀ 6.98 vs 12.45 μM), placing this complex as a potential antitubulin agent.