Enantiopure titanocene complexes - direct evidence for paraptosis in cancer cells

Tolerated by normal tissues, anti-cancer therapies based on titanium compounds are limited by low efficacy/selectivity and lack of understanding of their mode(s) of action. In vitro antitumour activity and mode of cell death incurred by enantiopure TiCl2{eta-C5H4CHEt(2-MeOPh)}(2) (abbreviated (Cp2TiCl2)-Ti-R) has been investigated. The in vitro anti-tumour activity of (Cp2TiCl2)-Ti-R is selective for cancer cells; in clonogenic assays, (S, S)-(Cp2TiCl2)-Ti-R was twice as effective at inhibiting colony formation than other stereoisomers after 24 h exposure. HPLC, MS and NMR techniques determined hydrolysis of (Cp2TiCl2)-Ti-R; data strongly correlate with soluble [(Cp2Ti)-Ti-R(OH)(OH2)](+) being the biological trigger. Treatment of cells with (Cp2TiCl2)-Ti-R provoked extensive cytoplasmic vacuolization, endoplasmic reticulum (ER) swelling and activation of MAPKinase signal transduction, consistent with ligand-induced paraptosis, type III cell death, which is morphologically distinct from, and independent of apoptosis. Indeed, distinct from cisplatin, (Cp2TiCl2)-Ti-R failed to perturb cell cycle dynamics, induce gamma H2AX foci or evoke apoptosis in MDA-MB-468 and HCT-116 cells.