Enantiopure titanocene complexes – direct evidence for paraptosis in cancer cells

Abstract

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{η-C₅H₄CHEt(2-MeOPh)}₂ (abbreviated Cp^R₂TiCl₂) has been investigated. The in vitro anti-tumour activity of Cp^R₂TiCl₂ is selective for cancer cells; in clonogenic assays, (S,S)-Cp^R₂TiCl₂ was twice as effective at inhibiting colony formation than other stereoisomers after 24 h exposure. HPLC, MS and NMR techniques determined hydrolysis of Cp^R₂TiCl₂; data strongly correlate with soluble [Cp^R₂Ti(OH)(OH₂)]⁺ being the biological trigger. Treatment of cells with Cp^R₂TiCl₂ 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, Cp^R₂TiCl₂ failed to perturb cell cycle dynamics, induce γH2AX foci or evoke apoptosis in MDA-MB-468 and HCT-116 cells.